WEBVTT 00:00:00.000 --> 00:00:04.955 align:middle line:84% [SQUEAKING] [RUSTLING] [CLICKING] 00:00:04.955 --> 00:00:16.360 align:middle line:90% 00:00:16.360 --> 00:00:17.000 align:middle line:90% JUEJUN HU: OK. 00:00:17.000 --> 00:00:21.360 align:middle line:84% So as I said, this is not the topic I proposed. 00:00:21.360 --> 00:00:22.680 align:middle line:90% You proposed it. 00:00:22.680 --> 00:00:26.280 align:middle line:84% So this is my 3071 final project. 00:00:26.280 --> 00:00:28.740 align:middle line:84% Now you get a chance to hear the presentation and grade it. 00:00:28.740 --> 00:00:31.280 align:middle line:90% 00:00:31.280 --> 00:00:32.040 align:middle line:90% All right. 00:00:32.040 --> 00:00:33.790 align:middle line:84% I also just also want to mention that this 00:00:33.790 --> 00:00:36.482 align:middle line:84% is a topic that's not in my area of expertise, obviously. 00:00:36.482 --> 00:00:37.940 align:middle line:84% It's actually, in fact, pretty far. 00:00:37.940 --> 00:00:39.168 align:middle line:90% But it's good. 00:00:39.168 --> 00:00:40.960 align:middle line:84% I actually get to learn a lot of new stuff. 00:00:40.960 --> 00:00:44.440 align:middle line:84% So that's part of the good thing I sometimes say about academia. 00:00:44.440 --> 00:00:48.120 align:middle line:84% People actually pay you to learn new things. 00:00:48.120 --> 00:00:48.740 align:middle line:90% All right. 00:00:48.740 --> 00:00:51.000 align:middle line:84% So we are talking about biomineralization. 00:00:51.000 --> 00:00:53.617 align:middle line:84% Specifically, I'm going to talk a lot about what 00:00:53.617 --> 00:00:56.200 align:middle line:84% the role amorphous material has played in that kind of process 00:00:56.200 --> 00:00:56.700 align:middle line:90% here. 00:00:56.700 --> 00:00:58.348 align:middle line:90% Some references. 00:00:58.348 --> 00:01:00.140 align:middle line:84% Obviously, you can always find these books. 00:01:00.140 --> 00:01:00.960 align:middle line:90% Read a bit more. 00:01:00.960 --> 00:01:04.720 align:middle line:84% These are some of the ones I referenced in the process here. 00:01:04.720 --> 00:01:06.740 align:middle line:84% This is the outline of the lecture today. 00:01:06.740 --> 00:01:09.560 align:middle line:84% I will start out by giving you a brief introduction 00:01:09.560 --> 00:01:11.520 align:middle line:90% of biomineralization. 00:01:11.520 --> 00:01:14.200 align:middle line:84% I will then delve into a few specific examples 00:01:14.200 --> 00:01:17.640 align:middle line:84% about biomineralization and talk about some of the processes 00:01:17.640 --> 00:01:18.800 align:middle line:90% there. 00:01:18.800 --> 00:01:21.680 align:middle line:84% Then I'll move on to talk about some common features that you 00:01:21.680 --> 00:01:23.600 align:middle line:90% will see in biomineralization. 00:01:23.600 --> 00:01:26.560 align:middle line:84% In particular, if you look from a structural perspective, 00:01:26.560 --> 00:01:31.220 align:middle line:84% a lot of these materials form in this process. 00:01:31.220 --> 00:01:34.560 align:middle line:84% They all have a hierarchical microstructure. 00:01:34.560 --> 00:01:36.400 align:middle line:84% So what I specifically mean by that 00:01:36.400 --> 00:01:39.080 align:middle line:84% is that there are different length scales. 00:01:39.080 --> 00:01:42.620 align:middle line:84% And at different length scales, from macroscopic to mesoscopic, 00:01:42.620 --> 00:01:45.720 align:middle line:84% microscopic to nanoscale, you have different kind 00:01:45.720 --> 00:01:48.120 align:middle line:84% of structure entities that forms this kind 00:01:48.120 --> 00:01:50.740 align:middle line:90% of hierarchical microstructure. 00:01:50.740 --> 00:01:51.760 align:middle line:90% We'll go into that. 00:01:51.760 --> 00:01:53.480 align:middle line:84% That's a very common feature in this kind 00:01:53.480 --> 00:01:56.000 align:middle line:90% of biomineralized materials. 00:01:56.000 --> 00:01:58.810 align:middle line:84% And in order to form this kind of hierarchical microstructure, 00:01:58.810 --> 00:02:00.750 align:middle line:84% turns out the common way of doing 00:02:00.750 --> 00:02:03.230 align:middle line:84% deposition and crystallization, the common material growth 00:02:03.230 --> 00:02:05.540 align:middle line:84% techniques, they do not really apply. 00:02:05.540 --> 00:02:07.790 align:middle line:84% And that's actually where amorphous material comes in. 00:02:07.790 --> 00:02:11.030 align:middle line:84% What you will see is that in a lot of these materials, 00:02:11.030 --> 00:02:13.730 align:middle line:84% instead of directly forming the final microstructure, 00:02:13.730 --> 00:02:15.970 align:middle line:84% they always go through an intermediate step 00:02:15.970 --> 00:02:18.110 align:middle line:84% by having this kind of transient amorphous phase 00:02:18.110 --> 00:02:19.790 align:middle line:84% or transient amorphous precursors 00:02:19.790 --> 00:02:22.390 align:middle line:84% and then crystallize it into the final crystalline form. 00:02:22.390 --> 00:02:25.110 align:middle line:84% So amorphous material does play a very big role 00:02:25.110 --> 00:02:25.985 align:middle line:90% in biomineralization. 00:02:25.985 --> 00:02:27.360 align:middle line:84% So you picked a good topic that's 00:02:27.360 --> 00:02:28.590 align:middle line:90% very relevant to this class. 00:02:28.590 --> 00:02:30.290 align:middle line:84% And in the end, I'm going to just talk 00:02:30.290 --> 00:02:31.290 align:middle line:90% about some applications. 00:02:31.290 --> 00:02:33.750 align:middle line:84% Basically, some bioinspired different kind 00:02:33.750 --> 00:02:38.670 align:middle line:84% of applications leveraging the principles of biomineralization. 00:02:38.670 --> 00:02:41.370 align:middle line:90% So what is biomineralization? 00:02:41.370 --> 00:02:42.870 align:middle line:84% Now, specifically, what we are going 00:02:42.870 --> 00:02:44.328 align:middle line:84% to focus on in this lecture is what 00:02:44.328 --> 00:02:47.870 align:middle line:84% we call the biologically controlled mineralization. 00:02:47.870 --> 00:02:49.790 align:middle line:84% If you look at literature, people 00:02:49.790 --> 00:02:51.630 align:middle line:84% like to categorize biomineralization 00:02:51.630 --> 00:02:53.270 align:middle line:90% into two main classes. 00:02:53.270 --> 00:02:56.110 align:middle line:84% You have so-called biologically induced mineralization. 00:02:56.110 --> 00:02:58.870 align:middle line:84% You also have biologically controlled mineralization. 00:02:58.870 --> 00:03:01.710 align:middle line:84% So for biologically induced mineralization, 00:03:01.710 --> 00:03:04.790 align:middle line:84% the organism does not play an active role 00:03:04.790 --> 00:03:07.610 align:middle line:84% in trying to control essentially growth steps of the materials. 00:03:07.610 --> 00:03:11.190 align:middle line:84% What they did is they essentially facilitate or create 00:03:11.190 --> 00:03:15.030 align:middle line:84% the microenvironment that then triggers mineralization. 00:03:15.030 --> 00:03:16.770 align:middle line:84% So the example will be some bacteria. 00:03:16.770 --> 00:03:19.550 align:middle line:84% For example, their metabolic products 00:03:19.550 --> 00:03:21.750 align:middle line:84% becomes essentially the essential ingredient 00:03:21.750 --> 00:03:24.150 align:middle line:84% for subsequent mineral precipitation. 00:03:24.150 --> 00:03:26.895 align:middle line:84% But in that case, the bacteria has no active role 00:03:26.895 --> 00:03:28.270 align:middle line:84% in controlling the microstructure 00:03:28.270 --> 00:03:29.970 align:middle line:90% of those kind of precipitation. 00:03:29.970 --> 00:03:31.550 align:middle line:84% So that's quite different in the case 00:03:31.550 --> 00:03:33.550 align:middle line:84% of biologically controlled mineralization, which 00:03:33.550 --> 00:03:36.350 align:middle line:84% is the focus of today, where the organism actually 00:03:36.350 --> 00:03:40.830 align:middle line:84% play a very active role in shaping the final product. 00:03:40.830 --> 00:03:43.790 align:middle line:84% We'll go through that in the next few slides. 00:03:43.790 --> 00:03:45.230 align:middle line:90% So definition here. 00:03:45.230 --> 00:03:47.750 align:middle line:84% As I said, it's basically the formation of inorganic solids 00:03:47.750 --> 00:03:49.430 align:middle line:90% by living organisms. 00:03:49.430 --> 00:03:51.670 align:middle line:84% But when we say inorganic solids here, 00:03:51.670 --> 00:03:54.670 align:middle line:84% it turns out usually it's a composite. 00:03:54.670 --> 00:03:58.520 align:middle line:84% It's a composite that contains usually organic matrix and then 00:03:58.520 --> 00:04:02.820 align:middle line:84% and inorganic phase, which comes from this precipitation process. 00:04:02.820 --> 00:04:06.800 align:middle line:84% So I'm giving you a few examples here. 00:04:06.800 --> 00:04:09.280 align:middle line:84% This is where your first proposal, talking about glass 00:04:09.280 --> 00:04:09.780 align:middle line:90% sponge. 00:04:09.780 --> 00:04:12.260 align:middle line:84% I'm going to discuss about glass sponge a little bit. 00:04:12.260 --> 00:04:14.840 align:middle line:84% But you also see other examples, even in us. 00:04:14.840 --> 00:04:15.520 align:middle line:90% We have teeth. 00:04:15.520 --> 00:04:16.339 align:middle line:90% We have bones. 00:04:16.339 --> 00:04:19.620 align:middle line:84% That's very typical products of biomineralization. 00:04:19.620 --> 00:04:22.940 align:middle line:84% So now, let me go through a few examples talking 00:04:22.940 --> 00:04:24.420 align:middle line:90% about biomineralization. 00:04:24.420 --> 00:04:27.620 align:middle line:90% The first one here, seashells. 00:04:27.620 --> 00:04:29.780 align:middle line:84% I actually have a little YouTube video here, 00:04:29.780 --> 00:04:32.680 align:middle line:84% which I believe talk about the material structure very well. 00:04:32.680 --> 00:04:33.900 align:middle line:90% [VIDEO PLAYBACK] 00:04:33.900 --> 00:04:36.360 align:middle line:84% - But durability goes right along with this. 00:04:36.360 --> 00:04:38.140 align:middle line:90% You see, we have-- 00:04:38.140 --> 00:04:39.820 align:middle line:90% JUEJUN HU: There we go. 00:04:39.820 --> 00:04:44.020 align:middle line:84% - Thanks to Curiosity Stream for supporting PBS Digital Studios. 00:04:44.020 --> 00:04:48.260 align:middle line:84% Seashells and chalk are both made of calcium carbonate. 00:04:48.260 --> 00:04:50.840 align:middle line:84% By itself, the mineral is brittle, crumbly, 00:04:50.840 --> 00:04:53.280 align:middle line:84% and soft enough to write out a homework assignment. 00:04:53.280 --> 00:04:56.040 align:middle line:84% Seashells, on the other hand, are wonderfully resilient. 00:04:56.040 --> 00:04:57.800 align:middle line:84% They're tough, they resist breaking, 00:04:57.800 --> 00:05:00.760 align:middle line:84% and they definitely can't be used to doodle on the sidewalk, 00:05:00.760 --> 00:05:03.620 align:middle line:84% but they're still primarily CaCO3. 00:05:03.620 --> 00:05:05.980 align:middle line:84% So what makes seashells so different? 00:05:05.980 --> 00:05:10.200 align:middle line:84% How can this and this be made of the same stuff? 00:05:10.200 --> 00:05:12.820 align:middle line:90% 00:05:12.820 --> 00:05:15.580 align:middle line:84% We're talking about the mineral chalk, by the way. 00:05:15.580 --> 00:05:17.260 align:middle line:84% Sidewalk and classroom chalk are usually 00:05:17.260 --> 00:05:20.780 align:middle line:84% made from a different mineral, known as gypsum, but we digress. 00:05:20.780 --> 00:05:22.780 align:middle line:84% Both seashells and mineral chalk are 00:05:22.780 --> 00:05:24.600 align:middle line:90% the product of living things. 00:05:24.600 --> 00:05:26.100 align:middle line:84% Those chalk deposits were generally 00:05:26.100 --> 00:05:28.340 align:middle line:84% formed from the gradual buildup and compression 00:05:28.340 --> 00:05:31.980 align:middle line:84% of single-celled ocean organisms with calcified shells. 00:05:31.980 --> 00:05:34.660 align:middle line:84% But bivalve mollusks, like oysters and mussels, 00:05:34.660 --> 00:05:37.000 align:middle line:84% are much more sophisticated shell builders. 00:05:37.000 --> 00:05:39.740 align:middle line:84% When we use that mineral chalk, it's long dead, 00:05:39.740 --> 00:05:41.940 align:middle line:84% and all that's left is the crumbly stuff. 00:05:41.940 --> 00:05:45.300 align:middle line:84% But these mollusks are pulling dissolved carbonate and calcium 00:05:45.300 --> 00:05:49.260 align:middle line:84% from the ocean for some pretty fancy biological architecture. 00:05:49.260 --> 00:05:53.450 align:middle line:84% Seashells contain 95% or more calcium carbonate. 00:05:53.450 --> 00:05:56.370 align:middle line:84% The rest is mostly protein and a bit of sugar. 00:05:56.370 --> 00:05:59.450 align:middle line:84% But a little biochemistry goes a long way. 00:05:59.450 --> 00:06:02.690 align:middle line:84% Calcium carbonate can take on a variety of forms. 00:06:02.690 --> 00:06:05.030 align:middle line:84% Even though its chemical formula never changes, 00:06:05.030 --> 00:06:08.330 align:middle line:84% its crystal structure, which is the 3D arrangement of its atoms 00:06:08.330 --> 00:06:10.355 align:middle line:90% in space, does change. 00:06:10.355 --> 00:06:11.730 align:middle line:84% The two forms we care about right 00:06:11.730 --> 00:06:14.610 align:middle line:90% now are calcite and aragonite. 00:06:14.610 --> 00:06:17.930 align:middle line:84% Calcite tends to be more stable than aragonite, but many bivalve 00:06:17.930 --> 00:06:19.650 align:middle line:90% shells contain a layer of each-- 00:06:19.650 --> 00:06:21.170 align:middle line:84% calcite on the outside of the shell 00:06:21.170 --> 00:06:23.210 align:middle line:90% and aragonite on the inside. 00:06:23.210 --> 00:06:26.010 align:middle line:84% One benefit to layering is that the crystal structures are 00:06:26.010 --> 00:06:28.210 align:middle line:90% oriented differently in space. 00:06:28.210 --> 00:06:30.530 align:middle line:84% If the calcite layer fractures, that fracture 00:06:30.530 --> 00:06:33.550 align:middle line:84% will probably stop when it hits the aragonite layer 00:06:33.550 --> 00:06:35.890 align:middle line:84% because aragonite doesn't break the same way. 00:06:35.890 --> 00:06:39.050 align:middle line:84% But that alone isn't nearly enough to make seashells so much 00:06:39.050 --> 00:06:40.330 align:middle line:90% tougher than chalk. 00:06:40.330 --> 00:06:42.890 align:middle line:84% We told you mollusks were amazing builders. 00:06:42.890 --> 00:06:45.610 align:middle line:84% The aragonite layer in a shell like an oyster's is called 00:06:45.610 --> 00:06:48.210 align:middle line:90% nacre, or mother of pearl. 00:06:48.210 --> 00:06:50.610 align:middle line:84% Here's how scientists have speculated mollusks actually 00:06:50.610 --> 00:06:51.810 align:middle line:90% make the stuff. 00:06:51.810 --> 00:06:55.410 align:middle line:84% Nacre is made up of layers of aragonite crystals separated 00:06:55.410 --> 00:06:57.610 align:middle line:90% by very thin layers of protein. 00:06:57.610 --> 00:07:01.010 align:middle line:84% Those layers are anchored by a chain of sugars called chitin. 00:07:01.010 --> 00:07:03.450 align:middle line:84% Mono and disaccharides, the kind of sugars we eat, 00:07:03.450 --> 00:07:06.090 align:middle line:84% are water soluble and not good building materials 00:07:06.090 --> 00:07:06.833 align:middle line:90% by themselves. 00:07:06.833 --> 00:07:08.500 align:middle line:84% Try to build a castle out of sugar cubes 00:07:08.500 --> 00:07:10.170 align:middle line:90% and see how long it lasts. 00:07:10.170 --> 00:07:12.490 align:middle line:84% But chain them together in long strings called 00:07:12.490 --> 00:07:16.210 align:middle line:84% polysaccharides, and they can become tough and fibrous. 00:07:16.210 --> 00:07:18.770 align:middle line:84% Chitin is one such polysaccharide. 00:07:18.770 --> 00:07:20.510 align:middle line:84% Proteins attached to the chitin layer 00:07:20.510 --> 00:07:21.970 align:middle line:84% give the calcium carbonate crystals 00:07:21.970 --> 00:07:23.770 align:middle line:90% a place to start growing. 00:07:23.770 --> 00:07:25.650 align:middle line:84% Those proteins have charged regions 00:07:25.650 --> 00:07:27.090 align:middle line:90% that attract the minerals. 00:07:27.090 --> 00:07:29.110 align:middle line:84% Because of how those regions are spaced, 00:07:29.110 --> 00:07:31.010 align:middle line:84% the crystals can grow in regular chunks 00:07:31.010 --> 00:07:33.410 align:middle line:84% until they get the next layer and meet each other 00:07:33.410 --> 00:07:34.570 align:middle line:90% at the edges. 00:07:34.570 --> 00:07:37.710 align:middle line:84% That's not everything the proteins and polysaccharides do. 00:07:37.710 --> 00:07:39.530 align:middle line:84% Shell formation requires a whole army 00:07:39.530 --> 00:07:42.090 align:middle line:84% of complex biological molecules, and scientists are still 00:07:42.090 --> 00:07:44.250 align:middle line:84% trying to understand how it all goes down. 00:07:44.250 --> 00:07:48.220 align:middle line:84% Nacre is literally thousands of times tougher than chalk, 00:07:48.220 --> 00:07:49.970 align:middle line:84% thanks to the layers of protein and chitin 00:07:49.970 --> 00:07:52.090 align:middle line:84% and the precise structural control. 00:07:52.090 --> 00:07:54.030 align:middle line:84% And then the oysters go on to make pearls. 00:07:54.030 --> 00:07:55.330 align:middle line:90% They're so talented. 00:07:55.330 --> 00:07:57.370 align:middle line:84% What all that means is that mollusks don't just 00:07:57.370 --> 00:07:58.970 align:middle line:90% wear these minerals as a hat. 00:07:58.970 --> 00:08:02.250 align:middle line:84% They control how the shell is made and what kind of crystal 00:08:02.250 --> 00:08:04.730 align:middle line:84% forms, whether calcite or aragonite. 00:08:04.730 --> 00:08:07.250 align:middle line:84% They can make all sorts of neat shell shapes 00:08:07.250 --> 00:08:09.610 align:middle line:84% that don't look anything like a stick of chalk. 00:08:09.610 --> 00:08:11.890 align:middle line:84% And they can even remodel their shells 00:08:11.890 --> 00:08:13.290 align:middle line:90% throughout their lifetime. 00:08:13.290 --> 00:08:15.490 align:middle line:84% And that's why you can't make sidewalk doodles 00:08:15.490 --> 00:08:18.970 align:middle line:84% with a seashell, which does work out in the animal's favor 00:08:18.970 --> 00:08:19.850 align:middle line:90% after all. 00:08:19.850 --> 00:08:22.450 align:middle line:84% Thank you to Curiosity Stream for supporting PBS Digital 00:08:22.450 --> 00:08:23.310 align:middle line:90% Studios. 00:08:23.310 --> 00:08:23.893 align:middle line:90% [END PLAYBACK] 00:08:23.893 --> 00:08:26.450 align:middle line:84% JUEJUN HU: You get to see a little bit about what 00:08:26.450 --> 00:08:28.010 align:middle line:90% seashells are composed of. 00:08:28.010 --> 00:08:30.210 align:middle line:84% So as the video says, basically, it's 00:08:30.210 --> 00:08:31.970 align:middle line:90% similarly a composite material. 00:08:31.970 --> 00:08:35.190 align:middle line:84% You have a mineral phase, which is either 00:08:35.190 --> 00:08:36.750 align:middle line:90% the calcite or aragonite. 00:08:36.750 --> 00:08:41.210 align:middle line:84% These are two polymorphs of calcium carbonate. 00:08:41.210 --> 00:08:44.290 align:middle line:84% And there's also some organic materials 00:08:44.290 --> 00:08:47.810 align:middle line:84% like chitin, the polysaccharide, as well as some proteins. 00:08:47.810 --> 00:08:53.280 align:middle line:84% So the matrix proteins, they actually 00:08:53.280 --> 00:08:55.960 align:middle line:84% play a very important role because scientists 00:08:55.960 --> 00:08:58.240 align:middle line:84% have performed studies looking into essentially how 00:08:58.240 --> 00:09:01.720 align:middle line:84% these proteins can actually suppress or promote 00:09:01.720 --> 00:09:04.160 align:middle line:84% the formation of one polymorph over the other. 00:09:04.160 --> 00:09:05.760 align:middle line:84% Some proteins will actually induce 00:09:05.760 --> 00:09:07.783 align:middle line:84% precipitation, for example, of only aragonite, 00:09:07.783 --> 00:09:08.700 align:middle line:90% but not the other one. 00:09:08.700 --> 00:09:12.580 align:middle line:84% And the reason they need these two kind of polymorphs, 00:09:12.580 --> 00:09:14.560 align:middle line:84% if you look at what I'm showing you here, 00:09:14.560 --> 00:09:18.040 align:middle line:84% this is basically what the two polymorphs look like. 00:09:18.040 --> 00:09:19.803 align:middle line:90% This is basically abalone shell. 00:09:19.803 --> 00:09:21.720 align:middle line:84% So if you look at the inside of abalone shell, 00:09:21.720 --> 00:09:24.986 align:middle line:90% this is basically nacre. 00:09:24.986 --> 00:09:30.360 align:middle line:84% It has this kind of prismatic color with a smooth surface. 00:09:30.360 --> 00:09:32.160 align:middle line:90% This is actually a nacre phase. 00:09:32.160 --> 00:09:34.540 align:middle line:84% Outside is what they call the prismatic phase. 00:09:34.540 --> 00:09:36.000 align:middle line:84% But this is basically the calcite. 00:09:36.000 --> 00:09:39.200 align:middle line:84% So inside is aragonite, outside is calcite. 00:09:39.200 --> 00:09:40.600 align:middle line:84% They have different complementary 00:09:40.600 --> 00:09:41.880 align:middle line:90% mechanical properties. 00:09:41.880 --> 00:09:45.200 align:middle line:84% Calcite tends to be very resistant to puncture, 00:09:45.200 --> 00:09:46.400 align:middle line:90% to puncture damage. 00:09:46.400 --> 00:09:49.960 align:middle line:84% And they also help to suppress any kind of crack propagation. 00:09:49.960 --> 00:09:54.080 align:middle line:84% Whereas nacre, because of the interlayer organic phase, 00:09:54.080 --> 00:09:57.080 align:middle line:84% is actually a lot more ductile and compliant 00:09:57.080 --> 00:09:59.100 align:middle line:90% compared to calcite. 00:09:59.100 --> 00:10:01.720 align:middle line:84% So this combination of materials will 00:10:01.720 --> 00:10:05.600 align:middle line:84% give a very excellent mechanical toughness of the seashells. 00:10:05.600 --> 00:10:08.120 align:middle line:90% 00:10:08.120 --> 00:10:08.720 align:middle line:90% OK. 00:10:08.720 --> 00:10:11.200 align:middle line:84% So one more thing I also want to mention here 00:10:11.200 --> 00:10:16.520 align:middle line:84% is that these mollusks, they do not directly 00:10:16.520 --> 00:10:18.520 align:middle line:90% form this crystalline phase. 00:10:18.520 --> 00:10:21.880 align:middle line:84% What they will do, before they form the crystalline phase, 00:10:21.880 --> 00:10:25.640 align:middle line:84% is they always go through a Amorphous Calcium 00:10:25.640 --> 00:10:27.600 align:middle line:90% Carbonate or ACC. 00:10:27.600 --> 00:10:30.840 align:middle line:84% This is one of the transient amorphous precursors 00:10:30.840 --> 00:10:32.520 align:middle line:90% we're going to be talking about. 00:10:32.520 --> 00:10:35.280 align:middle line:84% Now, this is a very common strategy 00:10:35.280 --> 00:10:37.200 align:middle line:90% for this kind of organisms. 00:10:37.200 --> 00:10:39.857 align:middle line:84% In biomineralization, they almost always 00:10:39.857 --> 00:10:41.440 align:middle line:84% go through amorphous phase before they 00:10:41.440 --> 00:10:42.640 align:middle line:90% go to crystalline phase. 00:10:42.640 --> 00:10:44.960 align:middle line:84% There are a few reasons for that. 00:10:44.960 --> 00:10:49.430 align:middle line:84% From a genetic perspective, the amorphous structure 00:10:49.430 --> 00:10:53.150 align:middle line:84% may look a lot similar to the liquid phase that 00:10:53.150 --> 00:10:54.790 align:middle line:90% come from solution. 00:10:54.790 --> 00:10:57.470 align:middle line:84% So this structural similarity, what it also means 00:10:57.470 --> 00:11:00.510 align:middle line:84% is that the interface energy between the amorphous phase 00:11:00.510 --> 00:11:03.350 align:middle line:84% and the solution phase tends to be a lot smaller compared 00:11:03.350 --> 00:11:06.710 align:middle line:84% to the crystalline phase and the solution phase. 00:11:06.710 --> 00:11:08.910 align:middle line:84% So lower interface energy essentially 00:11:08.910 --> 00:11:12.310 align:middle line:84% means there's a lower energy barrier to nucleation. 00:11:12.310 --> 00:11:16.690 align:middle line:84% So kinetically, it can be more favorable to form the ACC first, 00:11:16.690 --> 00:11:20.740 align:middle line:84% and then go to crystalline phase. 00:11:20.740 --> 00:11:24.430 align:middle line:84% Also, we'll be talking about this process a little bit more 00:11:24.430 --> 00:11:25.510 align:middle line:90% later. 00:11:25.510 --> 00:11:28.520 align:middle line:84% But typically, when they actually precipitate out 00:11:28.520 --> 00:11:30.270 align:middle line:84% this kind of amorphous phase, it comes out 00:11:30.270 --> 00:11:35.350 align:middle line:84% in the form of very small clusters, nanoscale clusters. 00:11:35.350 --> 00:11:37.870 align:middle line:84% So these nanoscale amorphous clusters, 00:11:37.870 --> 00:11:39.650 align:middle line:84% they reside in the solution phase. 00:11:39.650 --> 00:11:42.530 align:middle line:84% So it's basically like a liquid with these amorphous clusters. 00:11:42.530 --> 00:11:46.270 align:middle line:84% It's very easy for this phase to essentially infiltrate 00:11:46.270 --> 00:11:52.030 align:middle line:84% into usually the porous structure of organic scaffold. 00:11:52.030 --> 00:11:54.270 align:middle line:84% And then these little part of clusters 00:11:54.270 --> 00:11:56.470 align:middle line:90% then attach to the scaffold. 00:11:56.470 --> 00:11:59.330 align:middle line:84% And this will be a pretty quick process. 00:11:59.330 --> 00:12:02.650 align:middle line:84% This kind of infiltration and precipitation, 00:12:02.650 --> 00:12:06.030 align:middle line:84% it can take place within a few seconds to minutes. 00:12:06.030 --> 00:12:08.630 align:middle line:84% And this is actually advantageous 00:12:08.630 --> 00:12:12.310 align:middle line:84% because, in lots of cases, the organisms, or the animal, 00:12:12.310 --> 00:12:14.990 align:middle line:84% basically, want to have essentially 00:12:14.990 --> 00:12:17.218 align:middle line:84% the biomineralized products quickly 00:12:17.218 --> 00:12:19.010 align:middle line:84% to adapt to different kind of environments. 00:12:19.010 --> 00:12:20.910 align:middle line:84% We are going to talk about crustaceans. 00:12:20.910 --> 00:12:23.030 align:middle line:84% For example, crustaceans, when they molt, 00:12:23.030 --> 00:12:25.570 align:middle line:84% they essentially shed their old shell, put on a new one. 00:12:25.570 --> 00:12:28.310 align:middle line:84% They need to have that new shell quickly, 00:12:28.310 --> 00:12:30.910 align:middle line:84% within a matter of days, a couple of days, 00:12:30.910 --> 00:12:33.110 align:middle line:84% or even shorter, just to protect themselves. 00:12:33.110 --> 00:12:35.970 align:middle line:84% So having this amorphous phase is actually very convenient. 00:12:35.970 --> 00:12:37.990 align:middle line:84% This allows this very fast deposition 00:12:37.990 --> 00:12:42.190 align:middle line:84% of the amorphous materials to form essentially the prototype 00:12:42.190 --> 00:12:43.750 align:middle line:90% of the shell. 00:12:43.750 --> 00:12:46.910 align:middle line:84% And then after that, the actual phase transformation, 00:12:46.910 --> 00:12:48.450 align:middle line:84% going from amorphous crystalline, 00:12:48.450 --> 00:12:50.390 align:middle line:84% this tend to be a slower process. 00:12:50.390 --> 00:12:52.670 align:middle line:90% This can take hours or days. 00:12:52.670 --> 00:12:55.950 align:middle line:84% And in the process, the kinetics is basically 00:12:55.950 --> 00:12:58.590 align:middle line:84% controlled by a lot of the organic molecules. 00:12:58.590 --> 00:13:00.750 align:middle line:84% They're called inhibitory organic molecules. 00:13:00.750 --> 00:13:03.570 align:middle line:84% They essentially suppress the crystallization 00:13:03.570 --> 00:13:07.190 align:middle line:84% so that the animal can actually control 00:13:07.190 --> 00:13:09.790 align:middle line:84% what is the kinetics of the phase transformation. 00:13:09.790 --> 00:13:12.870 align:middle line:84% And that's actually useful because, in lots of cases, 00:13:12.870 --> 00:13:15.470 align:middle line:84% in order to form this kind of hierarchical microstructure, 00:13:15.470 --> 00:13:17.830 align:middle line:84% the organic template will then dictate 00:13:17.830 --> 00:13:20.550 align:middle line:84% what is the orientation and alignment of the resulting 00:13:20.550 --> 00:13:21.950 align:middle line:90% crystals. 00:13:21.950 --> 00:13:24.950 align:middle line:84% If you do that very quickly-- from kinetics, 00:13:24.950 --> 00:13:27.710 align:middle line:84% you know that it becomes very difficult to actually control 00:13:27.710 --> 00:13:30.678 align:middle line:84% the orientation of those crystals-- 00:13:30.678 --> 00:13:32.970 align:middle line:84% essentially, you get a lot of random atomic attachment. 00:13:32.970 --> 00:13:34.990 align:middle line:84% You get highly polycrystalline solids 00:13:34.990 --> 00:13:36.830 align:middle line:84% with a random orientation of grains. 00:13:36.830 --> 00:13:40.390 align:middle line:84% It's sort of like growing microepitaxy. 00:13:40.390 --> 00:13:42.800 align:middle line:84% In order to get to this kind of microepitaxy process, 00:13:42.800 --> 00:13:44.500 align:middle line:84% you need to control the atomic detachment, the phase 00:13:44.500 --> 00:13:46.417 align:middle line:84% transformation, so that it doesn't go too fast 00:13:46.417 --> 00:13:47.860 align:middle line:90% and screw up everything. 00:13:47.860 --> 00:13:50.793 align:middle line:84% So that's why having this kind of amorphous phase 00:13:50.793 --> 00:13:51.960 align:middle line:90% is actually quite important. 00:13:51.960 --> 00:13:56.060 align:middle line:84% And controlling the kinetics of its precipitation and growth 00:13:56.060 --> 00:13:57.200 align:middle line:90% is also quite important. 00:13:57.200 --> 00:13:59.785 align:middle line:90% 00:13:59.785 --> 00:14:01.160 align:middle line:84% So this is basically one example. 00:14:01.160 --> 00:14:04.060 align:middle line:84% We're going to see that in other cases as well. 00:14:04.060 --> 00:14:06.000 align:middle line:84% So the second example I'm going to talk about, 00:14:06.000 --> 00:14:09.880 align:middle line:84% these are the exoskeleton of crustaceans. 00:14:09.880 --> 00:14:13.660 align:middle line:84% So I'm talking about basically crabs, shrimps, lobsters, 00:14:13.660 --> 00:14:16.460 align:middle line:90% crayfish. 00:14:16.460 --> 00:14:18.020 align:middle line:90% In terms of composition-- 00:14:18.020 --> 00:14:20.460 align:middle line:84% oh, by the way, I forgot one thing about the chemical 00:14:20.460 --> 00:14:21.200 align:middle line:90% reaction here. 00:14:21.200 --> 00:14:22.260 align:middle line:90% I should mention this. 00:14:22.260 --> 00:14:25.820 align:middle line:84% This is a chemical reaction for the seashells' formation. 00:14:25.820 --> 00:14:28.700 align:middle line:84% Basically, the mollusks, they were taking calcium 00:14:28.700 --> 00:14:29.960 align:middle line:90% from the ocean. 00:14:29.960 --> 00:14:33.300 align:middle line:84% There's actually lots of calcium in the dissolved solution 00:14:33.300 --> 00:14:34.360 align:middle line:90% form in the seawater. 00:14:34.360 --> 00:14:39.620 align:middle line:84% They're also taking this ion here, HCO3. 00:14:39.620 --> 00:14:43.100 align:middle line:84% It basically come from dissolved carbon dioxide in seawater. 00:14:43.100 --> 00:14:47.300 align:middle line:84% Then they can react, form the ACC, precipitate out, 00:14:47.300 --> 00:14:50.540 align:middle line:84% and that's how you actually form the seashell materials. 00:14:50.540 --> 00:14:53.860 align:middle line:84% It's actually a very similar chemical reaction 00:14:53.860 --> 00:14:59.915 align:middle line:84% when it also comes to the crustaceans that 00:14:59.915 --> 00:15:01.040 align:middle line:90% actually live in the ocean. 00:15:01.040 --> 00:15:04.780 align:middle line:84% They basically use the same kind of chemical reaction. 00:15:04.780 --> 00:15:07.680 align:middle line:84% So in terms of composition of the exoskeleton, 00:15:07.680 --> 00:15:10.540 align:middle line:84% it's also quite similar to seashells. 00:15:10.540 --> 00:15:13.240 align:middle line:84% You similarly have chitin, the polysaccharides. 00:15:13.240 --> 00:15:14.300 align:middle line:90% You get some proteins. 00:15:14.300 --> 00:15:15.520 align:middle line:90% You get minerals. 00:15:15.520 --> 00:15:20.180 align:middle line:84% The minerals are mostly ACC and calcite. 00:15:20.180 --> 00:15:22.680 align:middle line:84% Depending on different species, it's a mix of the two. 00:15:22.680 --> 00:15:25.220 align:middle line:90% 00:15:25.220 --> 00:15:28.260 align:middle line:84% Now let's take a look at actual structure of the exoskeleton. 00:15:28.260 --> 00:15:29.940 align:middle line:84% It's one typical example of this kind 00:15:29.940 --> 00:15:31.460 align:middle line:90% of hierarchical architecture. 00:15:31.460 --> 00:15:34.700 align:middle line:84% So if you look at the shell-- let's say a lobster shell-- 00:15:34.700 --> 00:15:38.060 align:middle line:84% there are actually multiple layers of the shell. 00:15:38.060 --> 00:15:42.170 align:middle line:90% You have so-called exocuticle. 00:15:42.170 --> 00:15:45.490 align:middle line:90% You get the smaller ones. 00:15:45.490 --> 00:15:47.670 align:middle line:84% The outer layer is called epicuticle. 00:15:47.670 --> 00:15:50.370 align:middle line:84% It's basically a layer that has essentially a wax-like feeling 00:15:50.370 --> 00:15:51.490 align:middle line:90% on the surface. 00:15:51.490 --> 00:15:52.850 align:middle line:90% And then you get exocuticle. 00:15:52.850 --> 00:15:54.330 align:middle line:90% And then you get endocuticle. 00:15:54.330 --> 00:15:55.550 align:middle line:90% So different layers there. 00:15:55.550 --> 00:15:58.690 align:middle line:84% Now if you look closely into the microstructure there, 00:15:58.690 --> 00:16:01.090 align:middle line:84% they all consist of essentially these kind 00:16:01.090 --> 00:16:04.970 align:middle line:90% of basically little fibers. 00:16:04.970 --> 00:16:06.950 align:middle line:84% The fibers are essentially made of chitin, 00:16:06.950 --> 00:16:09.270 align:middle line:84% but it's a chitin that's heavily mineralized. 00:16:09.270 --> 00:16:13.010 align:middle line:84% So chitin decorated with calcite crystals. 00:16:13.010 --> 00:16:17.610 align:middle line:84% So these fibers, if you look at how they're actually arranged, 00:16:17.610 --> 00:16:20.170 align:middle line:84% they form this kind of a multilayer structure 00:16:20.170 --> 00:16:20.998 align:middle line:90% between the layers. 00:16:20.998 --> 00:16:23.290 align:middle line:84% The orientation of fibers actually rotated with respect 00:16:23.290 --> 00:16:25.150 align:middle line:90% to each other. 00:16:25.150 --> 00:16:30.285 align:middle line:84% People call this helicoidal Bouligand stacking. 00:16:30.285 --> 00:16:31.910 align:middle line:84% I don't know where the name comes from, 00:16:31.910 --> 00:16:35.090 align:middle line:84% but basically, this actually has a pretty important implication 00:16:35.090 --> 00:16:36.190 align:middle line:90% of mechanical properties. 00:16:36.190 --> 00:16:39.528 align:middle line:84% The reason you have different orientations here, 00:16:39.528 --> 00:16:41.570 align:middle line:84% one is that suppose there's some cracks developed 00:16:41.570 --> 00:16:42.470 align:middle line:90% in one of the layers. 00:16:42.470 --> 00:16:45.765 align:middle line:84% Then, because of the rotation of the orientation 00:16:45.765 --> 00:16:47.390 align:middle line:84% of the fibers between different layers, 00:16:47.390 --> 00:16:48.930 align:middle line:84% it means that when the crack tries 00:16:48.930 --> 00:16:52.370 align:middle line:84% to propagate between layers, it also has to rotate, 00:16:52.370 --> 00:16:54.810 align:middle line:84% change orientation, and then essentially 00:16:54.810 --> 00:16:57.450 align:middle line:84% also have to branch out in the process there. 00:16:57.450 --> 00:16:59.670 align:middle line:84% So because of this kind of configuration, 00:16:59.670 --> 00:17:01.810 align:middle line:84% it means that as the crack propagates, 00:17:01.810 --> 00:17:04.650 align:middle line:84% the area of the cracked surface has 00:17:04.650 --> 00:17:07.730 align:middle line:84% to expand as it goes through multiple layers. 00:17:07.730 --> 00:17:10.410 align:middle line:84% Now that means you have to overcome more surface energy 00:17:10.410 --> 00:17:11.450 align:middle line:90% barrier. 00:17:11.450 --> 00:17:14.930 align:middle line:84% So it effectively helps you to absorb a lot of the energy. 00:17:14.930 --> 00:17:18.970 align:middle line:84% And absorbing energy corresponds to an enhanced toughness 00:17:18.970 --> 00:17:21.530 align:middle line:90% of the material. 00:17:21.530 --> 00:17:24.682 align:middle line:84% Also, if you just look at the chitin fiber by itself, 00:17:24.682 --> 00:17:26.849 align:middle line:84% it obviously has this kind of anisotropic mechanical 00:17:26.849 --> 00:17:29.490 align:middle line:84% properties-- stronger along the direction and then weaker 00:17:29.490 --> 00:17:31.110 align:middle line:90% in the parallel direction. 00:17:31.110 --> 00:17:33.850 align:middle line:84% But by having this kind of stacking with a rotating 00:17:33.850 --> 00:17:36.768 align:middle line:84% orientation, it also eventually creates isotropic material 00:17:36.768 --> 00:17:39.060 align:middle line:84% that has uniform properties along different directions, 00:17:39.060 --> 00:17:40.720 align:middle line:90% which will be useful. 00:17:40.720 --> 00:17:43.800 align:middle line:84% And finally, the mixture of organic phase 00:17:43.800 --> 00:17:46.560 align:middle line:84% in the organic phase is also what made the shells actually 00:17:46.560 --> 00:17:49.380 align:middle line:84% highly flexible, despite the fact that it's made of calcite, 00:17:49.380 --> 00:17:51.080 align:middle line:90% which is a pretty hard mineral. 00:17:51.080 --> 00:17:54.520 align:middle line:84% Now, one thing I have also found out 00:17:54.520 --> 00:17:56.480 align:middle line:84% while I was looking for the information 00:17:56.480 --> 00:17:59.500 align:middle line:84% about crustacean exoskeleton formation 00:17:59.500 --> 00:18:00.980 align:middle line:90% is the process called molting. 00:18:00.980 --> 00:18:04.060 align:middle line:84% So molting just means that, for example, 00:18:04.060 --> 00:18:06.880 align:middle line:84% crabs, as they grow bigger, they need to essentially shed away 00:18:06.880 --> 00:18:09.380 align:middle line:84% the old shell and actually put on a new one 00:18:09.380 --> 00:18:12.520 align:middle line:84% so that they can accommodate their growing size. 00:18:12.520 --> 00:18:15.400 align:middle line:84% In this process here, as the crab, for example, 00:18:15.400 --> 00:18:18.600 align:middle line:84% dumps its old shell, the old shell contains lots of calcium. 00:18:18.600 --> 00:18:21.668 align:middle line:84% So one priority for the animal is 00:18:21.668 --> 00:18:23.960 align:middle line:84% that it needs to be able to essentially accumulate lots 00:18:23.960 --> 00:18:26.780 align:middle line:84% of calcium and then deposit them to form new shells, 00:18:26.780 --> 00:18:29.680 align:middle line:84% as I said, within a short period of time, just to protect itself. 00:18:29.680 --> 00:18:31.960 align:middle line:84% So this comes with the question-- 00:18:31.960 --> 00:18:34.640 align:middle line:84% how do they actually get this large amount of calcium 00:18:34.640 --> 00:18:37.160 align:middle line:90% within a short period of time? 00:18:37.160 --> 00:18:40.220 align:middle line:84% For animals living in the ocean, like lobsters, 00:18:40.220 --> 00:18:43.380 align:middle line:84% this is much less of an issue because, as I said, 00:18:43.380 --> 00:18:46.840 align:middle line:84% ocean water, seawater, actually has a high concentration 00:18:46.840 --> 00:18:48.100 align:middle line:90% of calcium in itself. 00:18:48.100 --> 00:18:50.360 align:middle line:84% So it's easier for them to get it right. 00:18:50.360 --> 00:18:53.940 align:middle line:84% But for terrestrial and freshwater crustaceans, 00:18:53.940 --> 00:18:56.920 align:middle line:84% for example, like crayfish, it does not 00:18:56.920 --> 00:18:59.120 align:middle line:84% have access to this huge reservoir 00:18:59.120 --> 00:19:01.360 align:middle line:90% of solution-phase calcium ion. 00:19:01.360 --> 00:19:06.320 align:middle line:84% So what they would do is, in fact, before they actually molt, 00:19:06.320 --> 00:19:10.200 align:middle line:84% they will start to reabsorb calcium in the exoskeleton 00:19:10.200 --> 00:19:13.440 align:middle line:84% before they actually molt. And in that process, 00:19:13.440 --> 00:19:15.300 align:middle line:84% they have to store the calcium somewhere. 00:19:15.300 --> 00:19:19.680 align:middle line:84% So they store it in the form of a so-called gastrolith. 00:19:19.680 --> 00:19:22.500 align:middle line:90% Have you heard of the term? 00:19:22.500 --> 00:19:25.560 align:middle line:84% It's basically like little stones, technically. 00:19:25.560 --> 00:19:29.600 align:middle line:84% They form little stones in their stomach. 00:19:29.600 --> 00:19:32.135 align:middle line:84% I don't know if you have eaten crayfish. 00:19:32.135 --> 00:19:32.760 align:middle line:90% AUDIENCE: Yeah. 00:19:32.760 --> 00:19:33.343 align:middle line:90% JUEJUN HU: OK. 00:19:33.343 --> 00:19:34.715 align:middle line:90% AUDIENCE: Do I eat the stones? 00:19:34.715 --> 00:19:35.590 align:middle line:90% JUEJUN HU: Sometimes. 00:19:35.590 --> 00:19:36.673 align:middle line:90% Only sometimes you see it. 00:19:36.673 --> 00:19:39.820 align:middle line:84% It's right before they molt you get those kind of stones. 00:19:39.820 --> 00:19:42.240 align:middle line:84% So the stomach of crayfish is actually 00:19:42.240 --> 00:19:43.300 align:middle line:90% very close to their head. 00:19:43.300 --> 00:19:46.328 align:middle line:84% It's right behind their head, which you don't eat the head. 00:19:46.328 --> 00:19:48.620 align:middle line:84% But you may find stones in there if you look carefully. 00:19:48.620 --> 00:19:50.820 align:middle line:84% The stones can get actually pretty big. 00:19:50.820 --> 00:19:51.852 align:middle line:90% You see the size here. 00:19:51.852 --> 00:19:54.060 align:middle line:84% This is actually-- sorry, they're scaled a bit small. 00:19:54.060 --> 00:19:55.360 align:middle line:90% This is 5 millimeters. 00:19:55.360 --> 00:19:58.400 align:middle line:90% So they can be this big. 00:19:58.400 --> 00:19:59.280 align:middle line:90% Yeah. 00:19:59.280 --> 00:20:04.120 align:middle line:84% And they primarily consist of ACC, Amorphous Calcium 00:20:04.120 --> 00:20:05.960 align:middle line:90% Carbonate. 00:20:05.960 --> 00:20:08.700 align:middle line:84% Now the reason they put in ACC is because, as I said, 00:20:08.700 --> 00:20:12.467 align:middle line:84% ACC is easy to deposit from the solution phase. 00:20:12.467 --> 00:20:14.800 align:middle line:84% And when they need to return it into the solution phase, 00:20:14.800 --> 00:20:18.280 align:middle line:84% to return it to a body fluid to build the exoskeleton, 00:20:18.280 --> 00:20:22.720 align:middle line:84% the dissolution process can also be very efficient and very fast. 00:20:22.720 --> 00:20:24.866 align:middle line:84% So they essentially use this amorphous phase sort 00:20:24.866 --> 00:20:28.800 align:middle line:84% of as a reservoir that's easily accessible 00:20:28.800 --> 00:20:30.920 align:middle line:84% and can be expediently transformed 00:20:30.920 --> 00:20:32.988 align:middle line:84% into the calcium in the body fluid. 00:20:32.988 --> 00:20:35.530 align:middle line:84% Then they can transport it to actually build the exoskeleton. 00:20:35.530 --> 00:20:38.510 align:middle line:90% 00:20:38.510 --> 00:20:43.527 align:middle line:84% So essentially, this is a diagram 00:20:43.527 --> 00:20:45.110 align:middle line:84% that actually shows the microstructure 00:20:45.110 --> 00:20:46.742 align:middle line:90% of this exoskeleton. 00:20:46.742 --> 00:20:48.950 align:middle line:84% This is what it looks like in terms of cross-section. 00:20:48.950 --> 00:20:52.070 align:middle line:84% And this last one here, this panel, 00:20:52.070 --> 00:20:56.250 align:middle line:84% shows the microstructure after you etch away the ACC phase. 00:20:56.250 --> 00:21:00.590 align:middle line:84% So what's left behind here is basically the organic scaffold. 00:21:00.590 --> 00:21:02.630 align:middle line:84% So it's basically an organic scaffold 00:21:02.630 --> 00:21:06.750 align:middle line:84% with lots of ACC essentially infiltrated in there. 00:21:06.750 --> 00:21:08.550 align:middle line:84% This is how these animals actually 00:21:08.550 --> 00:21:11.390 align:middle line:84% were able to very quickly build the exoskeleton, 00:21:11.390 --> 00:21:12.630 align:middle line:90% with this kind of storage. 00:21:12.630 --> 00:21:15.510 align:middle line:84% AUDIENCE: And they're taking it from their environment 00:21:15.510 --> 00:21:17.590 align:middle line:90% or they're already-- 00:21:17.590 --> 00:21:18.850 align:middle line:90% JUEJUN HU: Well, both. 00:21:18.850 --> 00:21:20.190 align:middle line:84% They essentially accumulate it essentially 00:21:20.190 --> 00:21:21.065 align:middle line:90% from the environment. 00:21:21.065 --> 00:21:22.538 align:middle line:84% That's kind of the primary source. 00:21:22.538 --> 00:21:23.330 align:middle line:90% They have to do it. 00:21:23.330 --> 00:21:25.430 align:middle line:84% But then, as I said, before they molt, 00:21:25.430 --> 00:21:29.123 align:middle line:84% they also reabsorb the calcium from their own exoskeleton. 00:21:29.123 --> 00:21:30.123 align:middle line:90% AUDIENCE: Kind of like-- 00:21:30.123 --> 00:21:31.270 align:middle line:90% OK. 00:21:31.270 --> 00:21:32.270 align:middle line:90% JUEJUN HU: Yeah. 00:21:32.270 --> 00:21:34.190 align:middle line:90% That's actually what they do. 00:21:34.190 --> 00:21:37.070 align:middle line:84% And so that essentially minimizes the calcium loss 00:21:37.070 --> 00:21:39.670 align:middle line:84% when they actually molt. So they recycle it. 00:21:39.670 --> 00:21:42.150 align:middle line:84% But you only see the gastrolith, as I said, 00:21:42.150 --> 00:21:45.070 align:middle line:84% in terrestrial or freshwater crustaceans. 00:21:45.070 --> 00:21:46.410 align:middle line:90% You don't see that in lobsters. 00:21:46.410 --> 00:21:48.590 align:middle line:84% You never see in lobsters because they have lots 00:21:48.590 --> 00:21:50.390 align:middle line:90% of calcium at their disposal. 00:21:50.390 --> 00:21:52.930 align:middle line:90% 00:21:52.930 --> 00:21:54.990 align:middle line:84% So next time, if you try crayfish, 00:21:54.990 --> 00:21:57.490 align:middle line:84% look out for this kind of a gastrolith. 00:21:57.490 --> 00:22:00.150 align:middle line:90% 00:22:00.150 --> 00:22:00.990 align:middle line:90% All right. 00:22:00.990 --> 00:22:06.230 align:middle line:84% Now let's come back and talk about ourselves. 00:22:06.230 --> 00:22:10.070 align:middle line:84% So vertebrates like us also always go 00:22:10.070 --> 00:22:11.710 align:middle line:84% through a biomineralization process 00:22:11.710 --> 00:22:14.290 align:middle line:90% in order to form bone and teeth. 00:22:14.290 --> 00:22:17.710 align:middle line:84% Now, for bone and teeth, one thing 00:22:17.710 --> 00:22:19.390 align:middle line:84% that's quite different from seashells 00:22:19.390 --> 00:22:21.170 align:middle line:84% is their composition is different. 00:22:21.170 --> 00:22:23.490 align:middle line:84% We do not rely on calcium carbonate. 00:22:23.490 --> 00:22:27.630 align:middle line:84% We rely on a different kind of mineral, essentially 00:22:27.630 --> 00:22:29.820 align:middle line:90% a calcium phosphate. 00:22:29.820 --> 00:22:31.380 align:middle line:90% That's the main difference. 00:22:31.380 --> 00:22:34.520 align:middle line:84% So if you look closely into either the bone or teeth, 00:22:34.520 --> 00:22:36.523 align:middle line:90% they have similar compositions. 00:22:36.523 --> 00:22:38.440 align:middle line:84% They always consist of three different phases. 00:22:38.440 --> 00:22:41.620 align:middle line:84% Here you have a mineral phase, which is basically taking 00:22:41.620 --> 00:22:44.200 align:middle line:90% the form of hydroxyapatite. 00:22:44.200 --> 00:22:50.140 align:middle line:84% Essentially, it's a hydrolyzed form of calcium phosphate. 00:22:50.140 --> 00:22:53.100 align:middle line:84% You then also have an organic matrix. 00:22:53.100 --> 00:22:58.260 align:middle line:84% Now, mainly, it's actually collagen. 00:22:58.260 --> 00:23:00.622 align:middle line:90% And some proteins, of course. 00:23:00.622 --> 00:23:02.080 align:middle line:84% And then finally, some water phase. 00:23:02.080 --> 00:23:04.538 align:middle line:84% The water phase is basically just to provide the hydration. 00:23:04.538 --> 00:23:07.100 align:middle line:84% So the specific composition really 00:23:07.100 --> 00:23:09.020 align:middle line:84% depends on the specific kind of tissue there. 00:23:09.020 --> 00:23:11.300 align:middle line:84% If you're talking about bone, you typically 00:23:11.300 --> 00:23:14.380 align:middle line:84% have anywhere from 50% to 60% of the mineral phase 00:23:14.380 --> 00:23:16.860 align:middle line:84% and then maybe 20% to 30% organic phase, the rest 00:23:16.860 --> 00:23:18.880 align:middle line:90% being the solution phase. 00:23:18.880 --> 00:23:26.060 align:middle line:84% If it's dentin, which is basically the core of the teeth, 00:23:26.060 --> 00:23:29.280 align:middle line:84% dentin has about 70% mineral, maybe 20% organic phase. 00:23:29.280 --> 00:23:30.380 align:middle line:90% So it's more like bone. 00:23:30.380 --> 00:23:32.620 align:middle line:84% But for the surface layer of teeth, 00:23:32.620 --> 00:23:36.660 align:middle line:84% which is the white outside, the white shiny piece, 00:23:36.660 --> 00:23:38.820 align:middle line:90% that's called the enamel. 00:23:38.820 --> 00:23:41.760 align:middle line:84% The enamel is pretty much just the mineral phase. 00:23:41.760 --> 00:23:43.440 align:middle line:90% It's a very dense mineral phase. 00:23:43.440 --> 00:23:47.080 align:middle line:84% It's about 95% calcium phosphate. 00:23:47.080 --> 00:23:50.383 align:middle line:84% And then just a little bit of organic proteins. 00:23:50.383 --> 00:23:52.300 align:middle line:84% So the composition also depends on what tissue 00:23:52.300 --> 00:23:53.180 align:middle line:90% you're looking for. 00:23:53.180 --> 00:23:58.675 align:middle line:84% Now enamel is actually quite an interesting part of the tissue 00:23:58.675 --> 00:24:00.300 align:middle line:84% that I actually want to talk a bit more 00:24:00.300 --> 00:24:01.842 align:middle line:90% about in subsequent slides. 00:24:01.842 --> 00:24:03.300 align:middle line:84% If you look at enamel here, it also 00:24:03.300 --> 00:24:05.540 align:middle line:84% has this kind of hierarchical architecture here. 00:24:05.540 --> 00:24:09.100 align:middle line:84% It actually consists of tiny little rods called enamel rods. 00:24:09.100 --> 00:24:13.780 align:middle line:84% And enamel rods basically consists of mineralized collagen 00:24:13.780 --> 00:24:16.000 align:middle line:90% fibers. 00:24:16.000 --> 00:24:18.560 align:middle line:84% They stack together in a regular array to form the enamel. 00:24:18.560 --> 00:24:19.600 align:middle line:84% If you look at it under a microscope, 00:24:19.600 --> 00:24:21.892 align:middle line:84% you see these tiny little rods arranged in more or less 00:24:21.892 --> 00:24:22.920 align:middle line:90% a hexagonal lattice. 00:24:22.920 --> 00:24:25.920 align:middle line:90% 00:24:25.920 --> 00:24:29.410 align:middle line:90% Now, how are those formed? 00:24:29.410 --> 00:24:33.610 align:middle line:84% How do we humans, for example, form these kind of minerals? 00:24:33.610 --> 00:24:36.610 align:middle line:90% Again, amorphous phase. 00:24:36.610 --> 00:24:38.190 align:middle line:90% We still use the precursor. 00:24:38.190 --> 00:24:41.590 align:middle line:84% In this case, it's Amorphous Calcium Phosphate, or ACP. 00:24:41.590 --> 00:24:44.110 align:middle line:90% 00:24:44.110 --> 00:24:47.890 align:middle line:84% Now, the reason ACP is used is very similar to why 00:24:47.890 --> 00:24:50.290 align:middle line:90% other animals use ACC. 00:24:50.290 --> 00:24:52.570 align:middle line:84% It reduces also interfacial energy. 00:24:52.570 --> 00:24:54.810 align:middle line:84% It also provides the shape flexibility 00:24:54.810 --> 00:24:57.530 align:middle line:84% because, again, these kind of little clusters in the solution 00:24:57.530 --> 00:25:00.810 align:middle line:84% phase can very easily infiltrate into the tissues, bone tissues, 00:25:00.810 --> 00:25:03.210 align:middle line:84% precipitate on them, and eventually transform 00:25:03.210 --> 00:25:08.170 align:middle line:84% into the crystalline form of hydroxyapatite. 00:25:08.170 --> 00:25:13.410 align:middle line:84% So one example of a mineralization process 00:25:13.410 --> 00:25:16.210 align:middle line:84% that's actually actively ongoing, even 00:25:16.210 --> 00:25:19.703 align:middle line:84% after you form the bone and teeth, is the enamel. 00:25:19.703 --> 00:25:21.870 align:middle line:84% Because enamel is actually a surface layer of teeth, 00:25:21.870 --> 00:25:24.370 align:middle line:84% it's always in interaction with the microenvironment 00:25:24.370 --> 00:25:25.810 align:middle line:90% in the mouth. 00:25:25.810 --> 00:25:31.470 align:middle line:84% So for example, if you eat some sugary food which feed bacteria, 00:25:31.470 --> 00:25:34.570 align:middle line:84% the bacteria's metabolic products tend to be acidic, 00:25:34.570 --> 00:25:37.878 align:middle line:84% or you drink lemon juice or whatever, that's also acidic, 00:25:37.878 --> 00:25:40.170 align:middle line:84% then it can actually push the chemical equilibrium goes 00:25:40.170 --> 00:25:41.070 align:middle line:90% the other way around. 00:25:41.070 --> 00:25:44.450 align:middle line:84% You start to cause dissolution of the hydroxyapatite 00:25:44.450 --> 00:25:45.910 align:middle line:90% or the enamel layer. 00:25:45.910 --> 00:25:47.310 align:middle line:90% Go back into the saliva. 00:25:47.310 --> 00:25:49.110 align:middle line:84% So essentially you lose the mineralization. 00:25:49.110 --> 00:25:50.970 align:middle line:84% That's how you actually damage the teeth. 00:25:50.970 --> 00:25:54.210 align:middle line:84% You can actually also reverse the process 00:25:54.210 --> 00:25:56.530 align:middle line:84% by going to a more basic solution. 00:25:56.530 --> 00:25:59.770 align:middle line:84% And you supply it with sufficient amount of calcium ion 00:25:59.770 --> 00:26:01.750 align:middle line:90% as well as the phosphate ion. 00:26:01.750 --> 00:26:03.450 align:middle line:84% If you do that, they can actually 00:26:03.450 --> 00:26:05.650 align:middle line:84% go through a so-called remineralization process 00:26:05.650 --> 00:26:08.730 align:middle line:84% and repair any damage to the enamel layer. 00:26:08.730 --> 00:26:11.048 align:middle line:84% I will go through that later when 00:26:11.048 --> 00:26:13.590 align:middle line:84% we talk about the applications of this kind of process there. 00:26:13.590 --> 00:26:16.010 align:middle line:84% But that's a biomineralization process 00:26:16.010 --> 00:26:19.490 align:middle line:84% that's actually reversible and is actually ongoing always. 00:26:19.490 --> 00:26:24.070 align:middle line:90% 00:26:24.070 --> 00:26:25.715 align:middle line:90% All right. 00:26:25.715 --> 00:26:27.090 align:middle line:84% Now, next example, we are talking 00:26:27.090 --> 00:26:30.970 align:middle line:84% about different class of minerals now, silica or glass. 00:26:30.970 --> 00:26:32.890 align:middle line:84% So there are quite a few different kinds 00:26:32.890 --> 00:26:37.970 align:middle line:84% of microorganisms that can actually 00:26:37.970 --> 00:26:41.130 align:middle line:90% mineralize SiO2, silica. 00:26:41.130 --> 00:26:46.490 align:middle line:84% The two most notable examples is diatoms and glass sponge. 00:26:46.490 --> 00:26:48.870 align:middle line:84% So you obviously know glass sponge. 00:26:48.870 --> 00:26:51.050 align:middle line:84% That's how you propose the lecture topic. 00:26:51.050 --> 00:26:55.050 align:middle line:84% Diatoms, these are essentially single-cell plankton 00:26:55.050 --> 00:26:56.970 align:middle line:90% that live in the ocean. 00:26:56.970 --> 00:27:02.930 align:middle line:84% And they are able to essentially convert the silica dissolved 00:27:02.930 --> 00:27:05.610 align:middle line:90% in seawater into a glass. 00:27:05.610 --> 00:27:07.810 align:middle line:84% And for diatoms, in fact, what they will do 00:27:07.810 --> 00:27:09.830 align:middle line:84% is, essentially, similar to glass sponge, 00:27:09.830 --> 00:27:12.370 align:middle line:84% they basically build the skeleton, or basically 00:27:12.370 --> 00:27:14.890 align:middle line:84% their houses they live in, out of glass, 00:27:14.890 --> 00:27:16.410 align:middle line:90% which is quite remarkable. 00:27:16.410 --> 00:27:20.210 align:middle line:84% So I have a little YouTube video here 00:27:20.210 --> 00:27:22.550 align:middle line:84% that essentially discusses about this process. 00:27:22.550 --> 00:27:25.216 align:middle line:90% So let me see. 00:27:25.216 --> 00:27:27.320 align:middle line:90% [VIDEO PLAYBACK] 00:27:27.320 --> 00:27:30.600 align:middle line:84% - The odds are high that you'll run into a diatom. 00:27:30.600 --> 00:27:33.760 align:middle line:84% They're both abundant and easy to spot because of the shells 00:27:33.760 --> 00:27:36.120 align:middle line:90% they encase themselves in. 00:27:36.120 --> 00:27:39.000 align:middle line:84% The results are beautiful, exacting 00:27:39.000 --> 00:27:46.560 align:middle line:84% geometries that create a living kaleidoscope in the microcosmos. 00:27:46.560 --> 00:27:49.760 align:middle line:84% And even if you lived your entire life 00:27:49.760 --> 00:27:54.160 align:middle line:84% without ever seeing a diatom, without ever hearing the word 00:27:54.160 --> 00:27:57.080 align:middle line:84% diatom, you would still be living 00:27:57.080 --> 00:28:01.200 align:middle line:84% a life that's shaped by them, all the way down to the oxygen 00:28:01.200 --> 00:28:04.040 align:middle line:84% you breathe, thanks in no small part 00:28:04.040 --> 00:28:08.840 align:middle line:84% to their outsized contribution to the world's photosynthesis. 00:28:08.840 --> 00:28:12.320 align:middle line:84% But what remains captivating about diatoms is, 00:28:12.320 --> 00:28:16.000 align:middle line:84% well, the thing that remains even when the diatom itself 00:28:16.000 --> 00:28:17.240 align:middle line:90% is gone-- 00:28:17.240 --> 00:28:20.480 align:middle line:84% the hard silica shell they once lived in, also 00:28:20.480 --> 00:28:23.400 align:middle line:90% known as the frustule. 00:28:23.400 --> 00:28:25.200 align:middle line:84% We've talked about frustules a bit 00:28:25.200 --> 00:28:27.400 align:middle line:84% before and how they've given diatoms 00:28:27.400 --> 00:28:29.600 align:middle line:90% the nickname jewels of the sea. 00:28:29.600 --> 00:28:32.840 align:middle line:84% And today, we're going to dive a bit more into how 00:28:32.840 --> 00:28:38.960 align:middle line:84% those frustules are made and the role they play in our world. 00:28:38.960 --> 00:28:43.320 align:middle line:84% Diatoms are the architects of their own glass houses, 00:28:43.320 --> 00:28:47.120 align:middle line:84% and the scale of this project is immense. 00:28:47.120 --> 00:28:51.320 align:middle line:84% The silica shells aren't just a smooth, uniform exterior. 00:28:51.320 --> 00:28:54.340 align:middle line:84% There are pores and elongated channels, 00:28:54.340 --> 00:28:57.280 align:middle line:84% creating ornate microscopic patterns 00:28:57.280 --> 00:29:00.560 align:middle line:84% that the organism depends on to survive. 00:29:00.560 --> 00:29:04.039 align:middle line:90% [MUSIC PLAYING] 00:29:04.039 --> 00:29:07.520 align:middle line:90% 00:29:07.520 --> 00:29:11.400 align:middle line:84% The source of diatoms' building blocks is geology. 00:29:11.400 --> 00:29:14.300 align:middle line:84% As rocks and land get weathered and eroded, 00:29:14.300 --> 00:29:16.600 align:middle line:84% silica enters the water, where it dissolves 00:29:16.600 --> 00:29:20.310 align:middle line:84% into silicic acid, which can then diffuse into the diatom 00:29:20.310 --> 00:29:22.470 align:middle line:90% and get converted into silica. 00:29:22.470 --> 00:29:25.850 align:middle line:84% But if diffusion isn't bringing in enough material, 00:29:25.850 --> 00:29:29.590 align:middle line:84% the diatoms can also rely on silicon transporter proteins 00:29:29.590 --> 00:29:32.750 align:middle line:84% in their membranes to gather more. 00:29:32.750 --> 00:29:35.190 align:middle line:84% Inside the diatom is a special area 00:29:35.190 --> 00:29:37.830 align:middle line:84% called the silica deposition vesicle, 00:29:37.830 --> 00:29:40.710 align:middle line:84% and it's there that the silica bits get linked together 00:29:40.710 --> 00:29:43.950 align:middle line:84% to form bigger and bigger pieces. 00:29:43.950 --> 00:29:46.830 align:middle line:84% But while we know that this is where the silica walls get 00:29:46.830 --> 00:29:49.550 align:middle line:84% put together, the chemistry and biology 00:29:49.550 --> 00:29:53.230 align:middle line:84% of how they get put together is still unclear. 00:29:53.230 --> 00:29:56.611 align:middle line:90% [MUSIC PLAYING] 00:29:56.611 --> 00:30:04.830 align:middle line:90% 00:30:04.830 --> 00:30:08.610 align:middle line:84% We know that the silica can itself form a gel-like network, 00:30:08.610 --> 00:30:11.630 align:middle line:84% so that could make up a key assembly step. 00:30:11.630 --> 00:30:14.490 align:middle line:84% But there may also be other organic molecules, 00:30:14.490 --> 00:30:17.110 align:middle line:84% like proteins, that bind to the silica and help 00:30:17.110 --> 00:30:19.990 align:middle line:90% create the final structure. 00:30:19.990 --> 00:30:23.650 align:middle line:84% While scientists are still untangling those mechanisms, 00:30:23.650 --> 00:30:26.270 align:middle line:84% they have observed the way that the silica deposition 00:30:26.270 --> 00:30:28.970 align:middle line:84% vesicle and other structures in the cell, 00:30:28.970 --> 00:30:31.270 align:middle line:84% including the mitochondria, can help 00:30:31.270 --> 00:30:36.390 align:middle line:84% to mold the shape of the final silica structure. 00:30:36.390 --> 00:30:39.570 align:middle line:84% For example, elongated symmetric diatoms, 00:30:39.570 --> 00:30:41.910 align:middle line:84% also called pennate diatoms, often 00:30:41.910 --> 00:30:44.030 align:middle line:90% have a slit called the raphe. 00:30:44.030 --> 00:30:46.770 align:middle line:84% The slit helps the diatom move around, 00:30:46.770 --> 00:30:49.590 align:middle line:84% serving as an opening through which they can secrete 00:30:49.590 --> 00:30:53.070 align:middle line:90% a viscous fluid to glide on. 00:30:53.070 --> 00:30:56.733 align:middle line:84% And one of the ways that diatoms make the raphe is by physically 00:30:56.733 --> 00:30:58.150 align:middle line:84% molding it through the arrangement 00:30:58.150 --> 00:31:00.350 align:middle line:90% of their microtubules. 00:31:00.350 --> 00:31:03.612 align:middle line:90% [MUSIC PLAYING] 00:31:03.612 --> 00:31:05.950 align:middle line:90% 00:31:05.950 --> 00:31:10.790 align:middle line:84% Many diatoms are non-motile, meaning they don't move around, 00:31:10.790 --> 00:31:13.870 align:middle line:84% and the lack of visible movement from the outside 00:31:13.870 --> 00:31:16.230 align:middle line:84% is striking when you consider all 00:31:16.230 --> 00:31:19.550 align:middle line:84% of the activity that has gone on inside of them 00:31:19.550 --> 00:31:23.430 align:middle line:90% to make those frustules happen. 00:31:23.430 --> 00:31:25.830 align:middle line:84% Like we said earlier, they're doing more than 00:31:25.830 --> 00:31:28.950 align:middle line:84% creating a smooth, transparent surface. 00:31:28.950 --> 00:31:31.990 align:middle line:84% They're creating a molded, ornate structure 00:31:31.990 --> 00:31:36.870 align:middle line:84% personalized to their own shape, like a house couture. 00:31:36.870 --> 00:31:40.630 align:middle line:84% And not only is this process a lot of work, 00:31:40.630 --> 00:31:44.830 align:middle line:84% it renders diatoms dependent on silica, a chemical 00:31:44.830 --> 00:31:47.830 align:middle line:90% they can't make for themselves. 00:31:47.830 --> 00:31:50.670 align:middle line:90% So why do it? 00:31:50.670 --> 00:31:54.790 align:middle line:84% Well, for one, it's cheaper than both making and assembling 00:31:54.790 --> 00:31:56.670 align:middle line:90% your own materials. 00:31:56.670 --> 00:32:01.510 align:middle line:84% Silica is already out there, so why not take advantage of it? 00:32:01.510 --> 00:32:04.830 align:middle line:84% One study found that the materials in diatoms' frustules 00:32:04.830 --> 00:32:08.250 align:middle line:84% might help the organism take in carbon dioxide, 00:32:08.250 --> 00:32:11.470 align:middle line:84% improving their photosynthetic capabilities. 00:32:11.470 --> 00:32:14.823 align:middle line:90% [MUSIC PLAYING] 00:32:14.823 --> 00:32:16.740 align:middle line:90% 00:32:16.740 --> 00:32:19.220 align:middle line:84% These advantages seem to translate over 00:32:19.220 --> 00:32:21.980 align:middle line:84% into the immense capacity of diatoms 00:32:21.980 --> 00:32:27.140 align:middle line:84% to grow and dominate over other algae that share their waters. 00:32:27.140 --> 00:32:30.380 align:middle line:84% But taking over a body of water is all fun and games 00:32:30.380 --> 00:32:32.560 align:middle line:90% until the silica runs out. 00:32:32.560 --> 00:32:35.180 align:middle line:90% 00:32:35.180 --> 00:32:37.140 align:middle line:84% Diatoms are known for their ability 00:32:37.140 --> 00:32:40.860 align:middle line:84% to form large, visible blooms, whether in your local lake 00:32:40.860 --> 00:32:42.780 align:middle line:90% or the Arctic Ocean. 00:32:42.780 --> 00:32:45.140 align:middle line:84% But scientists studying those Arctic blooms 00:32:45.140 --> 00:32:48.940 align:middle line:84% found that as the diatom population expanded, 00:32:48.940 --> 00:32:52.540 align:middle line:84% they also depleted their waters of silicic acid, 00:32:52.540 --> 00:32:54.500 align:middle line:84% and that in turn halted their growth 00:32:54.500 --> 00:32:57.800 align:middle line:84% and began to increase the number of dead diatoms. 00:32:57.800 --> 00:33:00.860 align:middle line:90% 00:33:00.860 --> 00:33:04.780 align:middle line:84% And both of these things, the lack of growth and the dying, 00:33:04.780 --> 00:33:08.340 align:middle line:84% have another impact on diatom populations-- 00:33:08.340 --> 00:33:09.940 align:middle line:90% sinking. 00:33:09.940 --> 00:33:13.300 align:middle line:84% Healthy growing diatoms can up their buoyancy to keep them 00:33:13.300 --> 00:33:17.180 align:middle line:84% afloat, while dying cells cannot. 00:33:17.180 --> 00:33:20.980 align:middle line:84% It might be sad to imagine those blooms dying and descending 00:33:20.980 --> 00:33:24.060 align:middle line:84% to lower depths, the ornate jewels collapsing 00:33:24.060 --> 00:33:26.140 align:middle line:90% like a microbial Empire. 00:33:26.140 --> 00:33:31.060 align:middle line:84% But nature doesn't trade in inconsequential tragedies. 00:33:31.060 --> 00:33:33.780 align:middle line:84% The death of diatoms leaves behind frustules 00:33:33.780 --> 00:33:37.980 align:middle line:84% like this one, which means it leaves behind silica. 00:33:37.980 --> 00:33:41.500 align:middle line:84% Some of that silica can be recycled by other diatoms. 00:33:41.500 --> 00:33:43.500 align:middle line:84% But over time, it will eventually 00:33:43.500 --> 00:33:49.020 align:middle line:84% descend further and further down until it gathers on sediments. 00:33:49.020 --> 00:33:51.940 align:middle line:84% When a diatom takes in silicic acid, 00:33:51.940 --> 00:33:55.620 align:middle line:84% it's taking in the remnants of a geological process-- 00:33:55.620 --> 00:33:59.220 align:middle line:84% the weathering and erosion that brought the silicon to water 00:33:59.220 --> 00:34:01.020 align:middle line:90% in the first place. 00:34:01.020 --> 00:34:03.820 align:middle line:84% And then, through the construction of its shell, 00:34:03.820 --> 00:34:07.260 align:middle line:84% it recycles and transforms the dissolved silicon 00:34:07.260 --> 00:34:11.020 align:middle line:84% into a form that can become geological once again. 00:34:11.020 --> 00:34:13.170 align:middle line:84% And how that silica gets used again 00:34:13.170 --> 00:34:17.449 align:middle line:84% can vary, but as always with the diatom, your chances of running 00:34:17.449 --> 00:34:19.850 align:middle line:90% into it again are high. 00:34:19.850 --> 00:34:23.610 align:middle line:84% Scientists found that dust from a diatom shell-rich sediment 00:34:23.610 --> 00:34:28.010 align:middle line:84% in Chad might travel as far as the Amazon. 00:34:28.010 --> 00:34:29.969 align:middle line:84% The diatom itself may not be making 00:34:29.969 --> 00:34:34.370 align:middle line:84% the trip, but the microcosmos and the things built in it 00:34:34.370 --> 00:34:38.000 align:middle line:90% have a way of sticking around. 00:34:38.000 --> 00:34:38.910 align:middle line:90% [END PLAYBACK] 00:34:38.910 --> 00:34:40.730 align:middle line:90% 00:34:40.730 --> 00:34:44.610 align:middle line:84% JUEJUN HU: So you see, these amazing creatures 00:34:44.610 --> 00:34:47.710 align:middle line:84% that can make these highly delicate silica structures. 00:34:47.710 --> 00:34:49.489 align:middle line:84% In fact, in the first slide, I showed you 00:34:49.489 --> 00:34:51.090 align:middle line:84% some photos of this kind of structures 00:34:51.090 --> 00:34:53.050 align:middle line:90% that were made by diatoms. 00:34:53.050 --> 00:34:55.435 align:middle line:84% You see the geometric variety of the structure 00:34:55.435 --> 00:34:56.810 align:middle line:84% they were able to make, this kind 00:34:56.810 --> 00:34:58.570 align:middle line:84% of highly irregular structure there. 00:34:58.570 --> 00:35:02.450 align:middle line:84% And also, I want to mention that the silica shell they made 00:35:02.450 --> 00:35:04.090 align:middle line:84% is actually not just a solid piece. 00:35:04.090 --> 00:35:05.715 align:middle line:84% It's actually a highly porous structure 00:35:05.715 --> 00:35:07.598 align:middle line:84% with very large surface-to-volume ratio. 00:35:07.598 --> 00:35:09.390 align:middle line:84% So the porous structure is actually useful. 00:35:09.390 --> 00:35:10.848 align:middle line:84% It helps them to essentially-- they 00:35:10.848 --> 00:35:14.170 align:middle line:84% need to exchange change with the surrounding environment. 00:35:14.170 --> 00:35:15.653 align:middle line:84% So that's why they actually make it 00:35:15.653 --> 00:35:17.070 align:middle line:84% into the kind of porous structure. 00:35:17.070 --> 00:35:18.630 align:middle line:84% Again, the hierarchical structure. 00:35:18.630 --> 00:35:23.130 align:middle line:84% So we'll see later on that how this kind of structure 00:35:23.130 --> 00:35:25.970 align:middle line:84% can be useful for practical applications, leveraging 00:35:25.970 --> 00:35:29.210 align:middle line:84% the fact that they have this kind of nanoporous architecture 00:35:29.210 --> 00:35:30.530 align:middle line:90% there. 00:35:30.530 --> 00:35:34.270 align:middle line:84% Now, in terms of how the silica shell is formed, 00:35:34.270 --> 00:35:37.650 align:middle line:84% similarly, it's also a composite with organic phase 00:35:37.650 --> 00:35:39.770 align:middle line:84% and the mineral amorphous silica. 00:35:39.770 --> 00:35:43.110 align:middle line:84% And the organic molecules also play similar roles. 00:35:43.110 --> 00:35:45.810 align:middle line:84% It serves as a scaffold that catalyzes 00:35:45.810 --> 00:35:51.450 align:middle line:84% the precipitation of silica and also guides its microstructure. 00:35:51.450 --> 00:35:53.170 align:middle line:84% So in the case of diatoms, they actually 00:35:53.170 --> 00:35:55.570 align:middle line:90% use a protein called silaffins. 00:35:55.570 --> 00:35:58.090 align:middle line:84% So silaffins basically can actually 00:35:58.090 --> 00:36:02.410 align:middle line:84% trigger precipitation of silica from surrounding 00:36:02.410 --> 00:36:05.770 align:middle line:84% silicic solutions and then they can actually 00:36:05.770 --> 00:36:07.930 align:middle line:84% interact with other molecules to actually form 00:36:07.930 --> 00:36:10.840 align:middle line:84% organized nanostructures that form the intricate structure 00:36:10.840 --> 00:36:14.120 align:middle line:90% of the glass house for diatoms. 00:36:14.120 --> 00:36:17.620 align:middle line:84% For glass sponges, they develop their own chemistry, 00:36:17.620 --> 00:36:18.220 align:middle line:90% interestingly. 00:36:18.220 --> 00:36:19.760 align:middle line:84% All the silica-grown animals, they all 00:36:19.760 --> 00:36:21.552 align:middle line:84% have different chemistry to actually do it. 00:36:21.552 --> 00:36:23.400 align:middle line:84% For glass sponge, they use silicatein, 00:36:23.400 --> 00:36:26.840 align:middle line:84% which is actually an enzyme that catalyzes 00:36:26.840 --> 00:36:28.260 align:middle line:90% the precipitation of silica. 00:36:28.260 --> 00:36:32.200 align:middle line:84% So similar biochemistry process, but with very different 00:36:32.200 --> 00:36:35.730 align:middle line:90% specific molecules involved. 00:36:35.730 --> 00:36:38.100 align:middle line:84% Now, because you talk about glass sponge, 00:36:38.100 --> 00:36:40.600 align:middle line:84% I'm going to discuss a little bit more about glass sponges. 00:36:40.600 --> 00:36:44.120 align:middle line:84% So glass sponge is actually quite interesting 00:36:44.120 --> 00:36:48.640 align:middle line:84% because they have these highly hierarchical structures 00:36:48.640 --> 00:36:50.400 align:middle line:90% in their glass skeleton there. 00:36:50.400 --> 00:36:54.000 align:middle line:84% So I quote this figure from the science paper 00:36:54.000 --> 00:36:56.520 align:middle line:84% here, which gives a very nice description about how 00:36:56.520 --> 00:36:59.400 align:middle line:84% the structural hierarchy are distributed in silica structure 00:36:59.400 --> 00:36:59.920 align:middle line:90% there. 00:36:59.920 --> 00:37:04.680 align:middle line:84% So macroscopically this is sort of like a cage-like structure 00:37:04.680 --> 00:37:07.180 align:middle line:84% that was precipitated by one of these glass sponges. 00:37:07.180 --> 00:37:10.200 align:middle line:84% Now if you look closely, you see that you 00:37:10.200 --> 00:37:13.320 align:middle line:84% have this kind of square lattices 00:37:13.320 --> 00:37:17.640 align:middle line:84% that's actually reinforced by these diagonal struts 00:37:17.640 --> 00:37:19.280 align:middle line:90% or diagonal rods. 00:37:19.280 --> 00:37:21.452 align:middle line:84% So later on, we'll see that scientists actually 00:37:21.452 --> 00:37:22.660 align:middle line:90% study this kind of structure. 00:37:22.660 --> 00:37:24.035 align:middle line:84% They figure that this is actually 00:37:24.035 --> 00:37:26.240 align:middle line:84% an optimized configuration that give it 00:37:26.240 --> 00:37:29.480 align:middle line:84% the maximum resistance against buckling deformation 00:37:29.480 --> 00:37:31.600 align:middle line:84% with the minimum amount of material, 00:37:31.600 --> 00:37:34.310 align:middle line:90% which is quite remarkable. 00:37:34.310 --> 00:37:37.000 align:middle line:84% Again, we're going to talk about that in a few slides. 00:37:37.000 --> 00:37:41.120 align:middle line:84% But if you look closely, basically, this silica fibers 00:37:41.120 --> 00:37:43.660 align:middle line:84% actually has core-shell structure. 00:37:43.660 --> 00:37:45.880 align:middle line:84% So you can see these are actually silica fibers. 00:37:45.880 --> 00:37:47.020 align:middle line:90% It's a bundle. 00:37:47.020 --> 00:37:50.280 align:middle line:84% And then, within bundle, you have this essentially fibrils. 00:37:50.280 --> 00:37:52.920 align:middle line:90% Each fibril is actually smaller. 00:37:52.920 --> 00:37:54.840 align:middle line:84% And then they have a core-shell structure. 00:37:54.840 --> 00:37:57.320 align:middle line:84% With the core, it basically contains more protein 00:37:57.320 --> 00:38:01.360 align:middle line:84% or the organic phase, while the outer layers, these 00:38:01.360 --> 00:38:04.200 align:middle line:84% are primarily essentially amorphous silica. 00:38:04.200 --> 00:38:07.280 align:middle line:84% In between them, they are bound together 00:38:07.280 --> 00:38:11.680 align:middle line:84% by these very thin, organic interlayers. 00:38:11.680 --> 00:38:13.655 align:middle line:84% So there are some interesting implications 00:38:13.655 --> 00:38:14.780 align:middle line:90% for this kind of structure. 00:38:14.780 --> 00:38:16.560 align:middle line:84% One is that this actually helps to improve 00:38:16.560 --> 00:38:17.980 align:middle line:90% the mechanical robustness. 00:38:17.980 --> 00:38:18.480 align:middle line:90% Same thing. 00:38:18.480 --> 00:38:20.720 align:middle line:84% Because you are combining essentially flexible-- 00:38:20.720 --> 00:38:22.700 align:middle line:84% it's sort of like a laminated glass. 00:38:22.700 --> 00:38:24.760 align:middle line:84% It's one good example of naturally occurring 00:38:24.760 --> 00:38:25.540 align:middle line:90% laminated glass. 00:38:25.540 --> 00:38:27.873 align:middle line:84% You have organic phase binding different kinds of silica 00:38:27.873 --> 00:38:29.400 align:middle line:90% plates together. 00:38:29.400 --> 00:38:32.440 align:middle line:84% Also, it also means that there's actually 00:38:32.440 --> 00:38:37.320 align:middle line:84% a refractive index gradient from the core to go to the outside 00:38:37.320 --> 00:38:38.440 align:middle line:90% there. 00:38:38.440 --> 00:38:41.140 align:middle line:84% It turns out, the core tends to have a higher refractive index. 00:38:41.140 --> 00:38:43.180 align:middle line:84% The cladding tends to have low refractive index. 00:38:43.180 --> 00:38:47.975 align:middle line:84% So effectively, you have a fiber-optic waveguide. 00:38:47.975 --> 00:38:49.100 align:middle line:90% People actually look at it. 00:38:49.100 --> 00:38:51.160 align:middle line:90% This is actually a little photo. 00:38:51.160 --> 00:38:53.600 align:middle line:84% These glass sponges can grow these kind of fibers 00:38:53.600 --> 00:38:57.200 align:middle line:90% to centimeters in length. 00:38:57.200 --> 00:38:59.960 align:middle line:84% And people have actually measured optical loss 00:38:59.960 --> 00:39:02.060 align:middle line:84% by sending light from one end of this fiber 00:39:02.060 --> 00:39:03.480 align:middle line:84% and then measure how much light actually escapes 00:39:03.480 --> 00:39:04.380 align:middle line:90% from the other end of fiber. 00:39:04.380 --> 00:39:05.420 align:middle line:84% You can actually see this picture here. 00:39:05.420 --> 00:39:06.220 align:middle line:90% This is actually a fiber. 00:39:06.220 --> 00:39:07.870 align:middle line:84% It's glowing because there's actually light 00:39:07.870 --> 00:39:09.930 align:middle line:84% scattered from the surface as they propagate inside. 00:39:09.930 --> 00:39:12.110 align:middle line:84% And they were able to measure loss that's actually 00:39:12.110 --> 00:39:14.293 align:middle line:84% less than 1 dB per centimeter, which is better 00:39:14.293 --> 00:39:15.710 align:middle line:84% than some of the waveguides people 00:39:15.710 --> 00:39:17.750 align:middle line:84% actually made, for example, on chip. 00:39:17.750 --> 00:39:19.110 align:middle line:90% That's quite remarkable. 00:39:19.110 --> 00:39:21.010 align:middle line:84% Now if you look even more closely, 00:39:21.010 --> 00:39:22.690 align:middle line:84% look at one of these silica layers, 00:39:22.690 --> 00:39:24.990 align:middle line:84% it's, again, not a solid piece of silica glass. 00:39:24.990 --> 00:39:27.670 align:middle line:84% Instead, you see this kind of granular structure, 00:39:27.670 --> 00:39:30.370 align:middle line:84% indicating that when this layer is actually formed, 00:39:30.370 --> 00:39:32.870 align:middle line:84% it's also formed through attachment 00:39:32.870 --> 00:39:36.630 align:middle line:84% of silica nanoparticles, which is a very typical process 00:39:36.630 --> 00:39:40.270 align:middle line:90% in biomineralization. 00:39:40.270 --> 00:39:42.430 align:middle line:84% So this serves as a good introduction 00:39:42.430 --> 00:39:45.550 align:middle line:84% into some of the interesting kinetic processes 00:39:45.550 --> 00:39:50.190 align:middle line:84% that help to shape the unique microstructure 00:39:50.190 --> 00:39:52.710 align:middle line:90% of the biominerals. 00:39:52.710 --> 00:39:56.230 align:middle line:84% So what I'm going to do in this next section here, 00:39:56.230 --> 00:39:58.510 align:middle line:84% I will essentially be giving you a few-- introducing 00:39:58.510 --> 00:40:01.190 align:middle line:84% a few general features about the kinetic processes 00:40:01.190 --> 00:40:04.330 align:middle line:84% in microstructure evolution of these kind of biominerals. 00:40:04.330 --> 00:40:07.750 align:middle line:84% So first thing is that, as we have mentioned, 00:40:07.750 --> 00:40:09.870 align:middle line:84% the formation of this kind of biominerals 00:40:09.870 --> 00:40:13.950 align:middle line:84% almost always involves a transient amorphous phase. 00:40:13.950 --> 00:40:17.170 align:middle line:84% So this can be summarized with this quick phrase here. 00:40:17.170 --> 00:40:21.830 align:middle line:84% It's always amorphous first, crystalline later. 00:40:21.830 --> 00:40:24.470 align:middle line:84% The reason this amorphous phase is desirable 00:40:24.470 --> 00:40:27.270 align:middle line:84% primarily is it helps to lower the nucleation barrier. 00:40:27.270 --> 00:40:30.230 align:middle line:84% So it's a lot easier to form from solution phase. 00:40:30.230 --> 00:40:33.330 align:middle line:84% This actually is dictated by the so-called Ostwald's step rule. 00:40:33.330 --> 00:40:36.070 align:middle line:84% I don't know if you talk about it in your other classes, 00:40:36.070 --> 00:40:39.110 align:middle line:84% but essentially what it says is that 00:40:39.110 --> 00:40:42.590 align:middle line:84% in a non-equilibrium process, when you're 00:40:42.590 --> 00:40:46.270 align:middle line:84% trying to precipitate some solid phase from a solution, what 00:40:46.270 --> 00:40:49.790 align:middle line:84% usually precipitates out tends to be thermodynamically least 00:40:49.790 --> 00:40:50.850 align:middle line:90% stable phase. 00:40:50.850 --> 00:40:52.810 align:middle line:90% It's not the most stable phase. 00:40:52.810 --> 00:40:54.672 align:middle line:84% Because part of the argument there 00:40:54.672 --> 00:40:56.630 align:middle line:84% is that the thermodynamically most stable phase 00:40:56.630 --> 00:41:00.390 align:middle line:84% tends to be structurally most dissimilar 00:41:00.390 --> 00:41:02.270 align:middle line:90% compared to the solution phase. 00:41:02.270 --> 00:41:04.140 align:middle line:84% So the kinetic barrier is actually high. 00:41:04.140 --> 00:41:07.020 align:middle line:84% And that's why there's always an intermediate phase that's 00:41:07.020 --> 00:41:09.860 align:middle line:84% thermodynamically unstable, but kinetically expedient 00:41:09.860 --> 00:41:12.460 align:middle line:90% to form, guide the process. 00:41:12.460 --> 00:41:14.140 align:middle line:84% So in this case, it's actually usually 00:41:14.140 --> 00:41:16.060 align:middle line:84% the amorphous phase that actually forms first, 00:41:16.060 --> 00:41:18.060 align:middle line:84% consistent with the Ostwald's step rule. 00:41:18.060 --> 00:41:21.720 align:middle line:84% So it helps to reduce interfacial energy. 00:41:21.720 --> 00:41:22.900 align:middle line:90% I mentioned that. 00:41:22.900 --> 00:41:25.000 align:middle line:84% And also, these organic matrices, 00:41:25.000 --> 00:41:28.740 align:middle line:84% they also provide preferential nucleation sites. 00:41:28.740 --> 00:41:32.100 align:middle line:84% That's how they actually guide the microstructural assembly 00:41:32.100 --> 00:41:35.660 align:middle line:84% in this kind of precipitation process. 00:41:35.660 --> 00:41:41.420 align:middle line:84% And then subsequently-- So when you actually 00:41:41.420 --> 00:41:43.140 align:middle line:84% grow these minerals, you always start out 00:41:43.140 --> 00:41:45.360 align:middle line:84% with a solution-phase infiltration deposition. 00:41:45.360 --> 00:41:47.760 align:middle line:84% As I said, this is a kinetically fast process. 00:41:47.760 --> 00:41:49.620 align:middle line:84% Takes seconds, maybe two minutes. 00:41:49.620 --> 00:41:52.740 align:middle line:84% The subsequent crystallization is a lot slower. 00:41:52.740 --> 00:41:53.440 align:middle line:90% It's kinetic. 00:41:53.440 --> 00:41:56.140 align:middle line:84% It's basically controlled by these organic molecules that 00:41:56.140 --> 00:42:00.860 align:middle line:84% serve as inhibitory agents that control the kinetics 00:42:00.860 --> 00:42:03.020 align:middle line:90% of the crystal growth process. 00:42:03.020 --> 00:42:07.380 align:middle line:84% And that's why you can actually vary-- these organisms can 00:42:07.380 --> 00:42:10.060 align:middle line:84% control the orientation and spatial arrangement 00:42:10.060 --> 00:42:12.420 align:middle line:84% of those crystals when they actually form. 00:42:12.420 --> 00:42:17.620 align:middle line:84% So this is one example of the so-called non-classical 00:42:17.620 --> 00:42:19.780 align:middle line:90% crystallization process. 00:42:19.780 --> 00:42:21.380 align:middle line:84% I have this schematic here that's 00:42:21.380 --> 00:42:23.620 align:middle line:84% trying to illustrate the difference 00:42:23.620 --> 00:42:26.580 align:middle line:84% of this non-classical crystallization 00:42:26.580 --> 00:42:27.920 align:middle line:90% with classical crystallization. 00:42:27.920 --> 00:42:30.340 align:middle line:84% So for classical crystallization process, 00:42:30.340 --> 00:42:32.800 align:middle line:84% this is what you have learned in your kinetics class. 00:42:32.800 --> 00:42:36.940 align:middle line:84% You have a supersaturation of ions in the solution. 00:42:36.940 --> 00:42:40.540 align:middle line:84% Then, once you have a little nuclei 00:42:40.540 --> 00:42:42.860 align:middle line:84% or you have little nucleus, then these ions 00:42:42.860 --> 00:42:45.333 align:middle line:84% can attach to the surface essentially ion by ion. 00:42:45.333 --> 00:42:47.000 align:middle line:84% And then you build up the crystal there. 00:42:47.000 --> 00:42:49.140 align:middle line:84% That's the classical nucleation process. 00:42:49.140 --> 00:42:52.900 align:middle line:84% Now, in bioorganisms, usually this kind of process 00:42:52.900 --> 00:42:54.460 align:middle line:90% is not what actually happens. 00:42:54.460 --> 00:42:57.020 align:middle line:84% What happens is that these ions will start out 00:42:57.020 --> 00:42:59.980 align:middle line:84% by forming these little amorphous clusters. 00:42:59.980 --> 00:43:02.570 align:middle line:84% Instead of attaching to a solid ion by ion, 00:43:02.570 --> 00:43:04.310 align:middle line:90% its actually cluster by cluster. 00:43:04.310 --> 00:43:06.978 align:middle line:90% 00:43:06.978 --> 00:43:08.770 align:middle line:84% And then that's eventually how you actually 00:43:08.770 --> 00:43:09.970 align:middle line:90% form the bulk crystal. 00:43:09.970 --> 00:43:13.750 align:middle line:84% So this is actually one good example here in this figure. 00:43:13.750 --> 00:43:15.970 align:middle line:84% This schematically shows how the ACC actually 00:43:15.970 --> 00:43:19.208 align:middle line:84% attaches to the surface in the form of clusters. 00:43:19.208 --> 00:43:21.250 align:middle line:84% And that's why if, you look at the microstructure 00:43:21.250 --> 00:43:22.810 align:middle line:84% for a lot of the biominerals, you 00:43:22.810 --> 00:43:27.570 align:middle line:84% tend to see this essentially nanoparticle formation 00:43:27.570 --> 00:43:31.130 align:middle line:84% in the solid phase there because of the kinetic pathway. 00:43:31.130 --> 00:43:34.850 align:middle line:84% Now, one example to actually illustrate 00:43:34.850 --> 00:43:39.970 align:middle line:84% this kind of kinetic pathway here, 00:43:39.970 --> 00:43:43.810 align:middle line:84% I'm showing you, in this case, this is actually a sea urchin. 00:43:43.810 --> 00:43:46.530 align:middle line:84% Basically the spikes of the sea urchin. 00:43:46.530 --> 00:43:49.810 align:middle line:84% The spikes are basically calcites, calcium carbonate. 00:43:49.810 --> 00:43:55.290 align:middle line:90% 00:43:55.290 --> 00:44:00.830 align:middle line:84% Now, calcium carbonate is actually in the solution phase. 00:44:00.830 --> 00:44:03.570 align:middle line:84% If it's supersaturated, it can crystallize 00:44:03.570 --> 00:44:06.170 align:middle line:90% within a matter of seconds. 00:44:06.170 --> 00:44:12.070 align:middle line:84% So in order to control the kinetics of the amorphous phase 00:44:12.070 --> 00:44:15.410 align:middle line:84% so that you can actually control the microstructure, 00:44:15.410 --> 00:44:17.543 align:middle line:84% different organisms, like sea urchin, 00:44:17.543 --> 00:44:19.210 align:middle line:84% they have developed their own strategies 00:44:19.210 --> 00:44:23.290 align:middle line:84% how to regulate the kinetics of crystallization. 00:44:23.290 --> 00:44:25.830 align:middle line:84% There can be different chemical processes that's involved. 00:44:25.830 --> 00:44:28.390 align:middle line:84% For example, you can have organic macromolecules, 00:44:28.390 --> 00:44:31.010 align:middle line:84% whether it's protein or polysaccharides. 00:44:31.010 --> 00:44:33.890 align:middle line:84% And the way these organic molecules essentially slows down 00:44:33.890 --> 00:44:36.970 align:middle line:84% the kinetics is by essentially they attach to the surface 00:44:36.970 --> 00:44:39.010 align:middle line:90% and block nucleation sites. 00:44:39.010 --> 00:44:41.490 align:middle line:84% So that way, it can slow down the crystallization. 00:44:41.490 --> 00:44:47.050 align:middle line:84% You can also use small ions, like magnesium, in ACC. 00:44:47.050 --> 00:44:49.170 align:middle line:84% Magnesium works by essentially distorting 00:44:49.170 --> 00:44:50.890 align:middle line:84% the local coordination environment 00:44:50.890 --> 00:44:54.290 align:middle line:84% and making nucleation more difficult. 00:44:54.290 --> 00:44:56.030 align:middle line:90% Water and hydration shells. 00:44:56.030 --> 00:44:58.283 align:middle line:84% This is also another commonly used strategy here. 00:44:58.283 --> 00:45:00.950 align:middle line:84% By essentially forming this kind of liquid-like hydration shell, 00:45:00.950 --> 00:45:03.088 align:middle line:84% it basically creates a kinetic barrier 00:45:03.088 --> 00:45:05.130 align:middle line:84% to prevent the growth of the crystals, which also 00:45:05.130 --> 00:45:07.050 align:middle line:90% helps to control the kinetics. 00:45:07.050 --> 00:45:12.530 align:middle line:84% And then finally, geometric confinement also plays a role. 00:45:12.530 --> 00:45:14.083 align:middle line:84% Lots of the organic scaffolds, they 00:45:14.083 --> 00:45:15.750 align:middle line:84% have this kind of nanoporous structures. 00:45:15.750 --> 00:45:18.125 align:middle line:84% So it means that the actual space that allows for crystal 00:45:18.125 --> 00:45:19.370 align:middle line:90% growth is highly constrained. 00:45:19.370 --> 00:45:22.050 align:middle line:84% So this prevents the formation of large randomly oriented 00:45:22.050 --> 00:45:25.090 align:middle line:84% crystals and is essentially used as a template 00:45:25.090 --> 00:45:28.190 align:middle line:84% to guide the crystal assembly in the formation process. 00:45:28.190 --> 00:45:32.410 align:middle line:84% So one example, as I said, is this kind of sea urchins here. 00:45:32.410 --> 00:45:34.590 align:middle line:84% So let me explain what they actually did. 00:45:34.590 --> 00:45:39.450 align:middle line:84% So basically, essentially, the scientists 00:45:39.450 --> 00:45:40.730 align:middle line:90% collected these urchins. 00:45:40.730 --> 00:45:49.850 align:middle line:84% They intentionally cut the little spines or the sharp ends 00:45:49.850 --> 00:45:51.610 align:middle line:90% of these little spines. 00:45:51.610 --> 00:45:55.370 align:middle line:84% So then the sea urchin actually goes ahead and repairs it. 00:45:55.370 --> 00:45:58.160 align:middle line:84% And they look at what happens during this kind of repairing 00:45:58.160 --> 00:45:59.560 align:middle line:90% process. 00:45:59.560 --> 00:46:02.840 align:middle line:84% So what I have noticed here is that this is actually 00:46:02.840 --> 00:46:07.620 align:middle line:84% an example of one of these spicules that was actually cut. 00:46:07.620 --> 00:46:10.200 align:middle line:84% And after a few days, you notice that this 00:46:10.200 --> 00:46:12.440 align:middle line:84% is what the surface actually looks like. 00:46:12.440 --> 00:46:15.360 align:middle line:84% This liquid-like structure, this is actually 00:46:15.360 --> 00:46:17.620 align:middle line:90% amorphous calcium carbonate. 00:46:17.620 --> 00:46:20.400 align:middle line:90% This is basically ACC formation. 00:46:20.400 --> 00:46:22.480 align:middle line:84% And they use this kind of liquid phase 00:46:22.480 --> 00:46:25.840 align:middle line:84% to first infiltrate the structure 00:46:25.840 --> 00:46:28.160 align:middle line:84% and then transform it into crystalline phase. 00:46:28.160 --> 00:46:31.800 align:middle line:84% Now, to stabilize this ACC, they actually 00:46:31.800 --> 00:46:34.960 align:middle line:84% combine all the three strategies I'm talking about here. 00:46:34.960 --> 00:46:39.280 align:middle line:84% It's actually a magnesium-rich hydrated ACC, also stabilized 00:46:39.280 --> 00:46:41.408 align:middle line:90% with a specific kind of protein. 00:46:41.408 --> 00:46:43.700 align:middle line:84% So that's how they actually control the kinetics there. 00:46:43.700 --> 00:46:47.298 align:middle line:84% And as this kind of molecules, they dehydrate, 00:46:47.298 --> 00:46:48.840 align:middle line:84% this usually causes the precipitation 00:46:48.840 --> 00:46:52.000 align:middle line:84% of the crystalline phase and transformation. 00:46:52.000 --> 00:46:55.320 align:middle line:84% What's also remarkable is that by controlling 00:46:55.320 --> 00:47:00.040 align:middle line:84% the kinetics, the spicules that were formed, 00:47:00.040 --> 00:47:03.120 align:middle line:84% each of the spicules is actually a single crystal calcium 00:47:03.120 --> 00:47:07.640 align:middle line:84% carbonate, which is quite remarkable because naturally 00:47:07.640 --> 00:47:09.800 align:middle line:84% occurring process usually don't lead 00:47:09.800 --> 00:47:11.900 align:middle line:84% to this kind of single crystal minerals. 00:47:11.900 --> 00:47:13.440 align:middle line:84% In most cases, it's polycrystalline. 00:47:13.440 --> 00:47:15.140 align:middle line:84% If you want to get single crystalline, 00:47:15.140 --> 00:47:17.100 align:middle line:84% you need to introduce in epitaxial process. 00:47:17.100 --> 00:47:20.300 align:middle line:84% You basically have to do this transformation very, 00:47:20.300 --> 00:47:21.520 align:middle line:90% very slowly. 00:47:21.520 --> 00:47:23.240 align:middle line:84% That's what you learn from kinetics. 00:47:23.240 --> 00:47:24.920 align:middle line:84% If you have lots of molecules, you dump all the molecules 00:47:24.920 --> 00:47:26.360 align:middle line:84% into the surface, they are just going 00:47:26.360 --> 00:47:28.240 align:middle line:84% to randomly form essentially randomly oriented 00:47:28.240 --> 00:47:29.260 align:middle line:90% polycrystalline material. 00:47:29.260 --> 00:47:30.968 align:middle line:84% If you want to get the crystalline phase, 00:47:30.968 --> 00:47:33.720 align:middle line:84% you need to ensure that all the atoms essentially slowly 00:47:33.720 --> 00:47:37.200 align:middle line:84% move, find the lowest energy position, which corresponds 00:47:37.200 --> 00:47:40.160 align:middle line:84% to atomic lattice sites, and then they can form actually 00:47:40.160 --> 00:47:41.220 align:middle line:90% the single crystal. 00:47:41.220 --> 00:47:43.000 align:middle line:84% So the whole idea here is that you 00:47:43.000 --> 00:47:45.640 align:middle line:84% want to be able to suppress the crystallization 00:47:45.640 --> 00:47:47.400 align:middle line:84% kinetics, to the extent that it's slow 00:47:47.400 --> 00:47:52.080 align:middle line:84% enough so that you can have this templated epitaxial process. 00:47:52.080 --> 00:47:53.960 align:middle line:84% So there's something that people actually 00:47:53.960 --> 00:47:57.150 align:middle line:84% have verified using diffraction and TEM to find that this indeed 00:47:57.150 --> 00:47:59.990 align:middle line:84% is actually a single crystal calcium carbonate. 00:47:59.990 --> 00:48:01.630 align:middle line:90% OK. 00:48:01.630 --> 00:48:02.290 align:middle line:90% All right. 00:48:02.290 --> 00:48:07.270 align:middle line:84% So that's about the kinetics and the involvement 00:48:07.270 --> 00:48:09.750 align:middle line:84% of amorphous transient phases in the formation 00:48:09.750 --> 00:48:11.370 align:middle line:90% of the biominerals. 00:48:11.370 --> 00:48:14.390 align:middle line:84% So another structural feature I just want to highlight here 00:48:14.390 --> 00:48:17.070 align:middle line:84% is that this kind of process also 00:48:17.070 --> 00:48:20.910 align:middle line:84% enables the formation of hierarchical microstructures. 00:48:20.910 --> 00:48:24.310 align:middle line:84% The typical kinetic process, as I mentioned here, 00:48:24.310 --> 00:48:27.030 align:middle line:84% is that they involve the formation of amorphous precursor 00:48:27.030 --> 00:48:27.790 align:middle line:90% phase. 00:48:27.790 --> 00:48:30.990 align:middle line:84% You also have a mesoscale scaffold that mostly 00:48:30.990 --> 00:48:33.190 align:middle line:90% consists of organic materials. 00:48:33.190 --> 00:48:35.270 align:middle line:84% The organic materials, they get mineralized 00:48:35.270 --> 00:48:37.970 align:middle line:84% because of infiltration of the amorphous precursors. 00:48:37.970 --> 00:48:39.470 align:middle line:84% And then over time, they then slowly 00:48:39.470 --> 00:48:41.750 align:middle line:84% transform into the final crystalline structures 00:48:41.750 --> 00:48:44.425 align:middle line:84% with the nanocrystals essentially being-- 00:48:44.425 --> 00:48:46.550 align:middle line:84% their arrangement, their alignment, and orientation 00:48:46.550 --> 00:48:52.550 align:middle line:84% being controlled by the local organic matrix. 00:48:52.550 --> 00:48:56.630 align:middle line:84% So one example for this kind of hierarchical structure 00:48:56.630 --> 00:49:00.350 align:middle line:84% is basically bone, like human bone. 00:49:00.350 --> 00:49:03.270 align:middle line:84% So this diagram shows the hierarchical structure 00:49:03.270 --> 00:49:04.792 align:middle line:90% of the human cortical bone. 00:49:04.792 --> 00:49:06.750 align:middle line:84% So cortical bone is basically the surface layer 00:49:06.750 --> 00:49:10.110 align:middle line:84% of bone, which is the most dense. 00:49:10.110 --> 00:49:13.750 align:middle line:84% And that's why it's also called the compact bone. 00:49:13.750 --> 00:49:15.670 align:middle line:84% So if you look at the structure here, 00:49:15.670 --> 00:49:19.770 align:middle line:84% the basic structural unit is this kind of collagen molecules. 00:49:19.770 --> 00:49:23.590 align:middle line:84% The collagen molecules have this kind of long-chain form. 00:49:23.590 --> 00:49:25.630 align:middle line:84% And then these long-chain organic molecules, 00:49:25.630 --> 00:49:26.930 align:middle line:90% they basically get mineralized. 00:49:26.930 --> 00:49:29.470 align:middle line:84% A lot of the hydroxyapatite crystals 00:49:29.470 --> 00:49:32.610 align:middle line:84% attach to this kind of long fibril 00:49:32.610 --> 00:49:35.710 align:middle line:84% to form, essentially, a mineralized composite. 00:49:35.710 --> 00:49:38.510 align:middle line:84% And then these kind of fibrils then stack together 00:49:38.510 --> 00:49:40.270 align:middle line:90% to form the collagen fibers. 00:49:40.270 --> 00:49:43.070 align:middle line:84% These collagen fibers then subsequently 00:49:43.070 --> 00:49:45.090 align:middle line:84% assemble into this kind of structure here. 00:49:45.090 --> 00:49:50.110 align:middle line:84% Essentially it's sort of like core-shell filaments 00:49:50.110 --> 00:49:53.070 align:middle line:84% where, within each core-shell filaments, 00:49:53.070 --> 00:49:55.910 align:middle line:90% this is called the osteons. 00:49:55.910 --> 00:49:59.510 align:middle line:84% The osteons, they typically have a size of maybe a couple hundred 00:49:59.510 --> 00:50:02.330 align:middle line:90% microns in terms of diameter. 00:50:02.330 --> 00:50:07.250 align:middle line:84% And in the osteon, the inside basically is a hollow core. 00:50:07.250 --> 00:50:10.310 align:middle line:84% That's where the blood vessels and nerves run through. 00:50:10.310 --> 00:50:13.470 align:middle line:84% The outside layers is this collagen 00:50:13.470 --> 00:50:16.350 align:middle line:84% decorated with hydroxyapatite that give you 00:50:16.350 --> 00:50:18.120 align:middle line:90% the mechanical strength. 00:50:18.120 --> 00:50:19.370 align:middle line:90% So that's the basic structure. 00:50:19.370 --> 00:50:20.287 align:middle line:90% You need to know both. 00:50:20.287 --> 00:50:22.453 align:middle line:84% But you can see that you have this kind of hierarchy 00:50:22.453 --> 00:50:24.830 align:middle line:84% here, structural hierarchy here, from microscale, which 00:50:24.830 --> 00:50:30.590 align:middle line:84% is the basic osteon, then go to essentially the submicron scale, 00:50:30.590 --> 00:50:32.230 align:middle line:84% where you have the collagen fibers, 00:50:32.230 --> 00:50:33.870 align:middle line:84% and eventually you go to even smaller, 00:50:33.870 --> 00:50:38.310 align:middle line:84% go to essentially the individual mineralized strands of collagen 00:50:38.310 --> 00:50:39.150 align:middle line:90% fibrils. 00:50:39.150 --> 00:50:41.683 align:middle line:84% So this is a very typical characteristic 00:50:41.683 --> 00:50:43.350 align:middle line:84% of a lot of biominerals having this kind 00:50:43.350 --> 00:50:46.230 align:middle line:90% of structural hierarchy there. 00:50:46.230 --> 00:50:49.095 align:middle line:84% And amorphous materials, again, plays a very important role 00:50:49.095 --> 00:50:51.220 align:middle line:84% in forming this kind of hierarchical microstructure 00:50:51.220 --> 00:50:53.077 align:middle line:90% here. 00:50:53.077 --> 00:50:55.160 align:middle line:84% I probably don't have to talk too much about this. 00:50:55.160 --> 00:50:56.740 align:middle line:84% I have already mentioned about that, 00:50:56.740 --> 00:50:58.198 align:middle line:84% that it helps to essentially create 00:50:58.198 --> 00:51:00.260 align:middle line:84% this kind of a structural flexibility 00:51:00.260 --> 00:51:03.060 align:middle line:84% by this infiltration deposition process. 00:51:03.060 --> 00:51:07.240 align:middle line:84% Also, by slowing down the crystallization kinetics, 00:51:07.240 --> 00:51:10.060 align:middle line:84% it avoids the formation of large, randomly oriented 00:51:10.060 --> 00:51:12.820 align:middle line:84% crystals, which may actually cause a lot of internal stress 00:51:12.820 --> 00:51:13.700 align:middle line:90% and cracking. 00:51:13.700 --> 00:51:16.980 align:middle line:84% So it helps to decrease the brittleness 00:51:16.980 --> 00:51:19.100 align:middle line:84% of the composite in the formation process 00:51:19.100 --> 00:51:21.060 align:middle line:84% and ensure its structural integrity. 00:51:21.060 --> 00:51:25.803 align:middle line:90% And then kinetics control. 00:51:25.803 --> 00:51:27.220 align:middle line:84% The organisms can also essentially 00:51:27.220 --> 00:51:29.340 align:middle line:84% control the amorphous formation by essentially 00:51:29.340 --> 00:51:32.420 align:middle line:84% using these inhibitory molecules to control kinetics 00:51:32.420 --> 00:51:36.380 align:middle line:84% and integrate them into hierarchical microstructure. 00:51:36.380 --> 00:51:37.180 align:middle line:90% All right. 00:51:37.180 --> 00:51:41.180 align:middle line:84% So that's basically about the kinetics of this kind 00:51:41.180 --> 00:51:42.720 align:middle line:90% of biomineralized process. 00:51:42.720 --> 00:51:44.470 align:middle line:84% Any questions about that before we move on 00:51:44.470 --> 00:51:47.340 align:middle line:84% to talk about how people learn from the processes? 00:51:47.340 --> 00:51:49.220 align:middle line:84% AUDIENCE: Well, maybe this will be related. 00:51:49.220 --> 00:51:54.860 align:middle line:84% For the sponges, the amorphous silica, is that the same-- 00:51:54.860 --> 00:51:57.640 align:middle line:84% I guess you said it was used for waveguides. 00:51:57.640 --> 00:52:01.780 align:middle line:84% Is it the same structure as amorphous silica that we-- 00:52:01.780 --> 00:52:02.740 align:middle line:90% JUEJUN HU: Not exactly. 00:52:02.740 --> 00:52:04.380 align:middle line:90% AUDIENCE: --hydrated? 00:52:04.380 --> 00:52:06.540 align:middle line:90% I tried to look at it. 00:52:06.540 --> 00:52:07.832 align:middle line:90% JUEJUN HU: It contains protein. 00:52:07.832 --> 00:52:08.957 align:middle line:90% That's the main difference. 00:52:08.957 --> 00:52:10.060 align:middle line:90% It's a composite material. 00:52:10.060 --> 00:52:10.560 align:middle line:90% Yeah. 00:52:10.560 --> 00:52:11.435 align:middle line:90% It's not pure silica. 00:52:11.435 --> 00:52:11.935 align:middle line:90% Yeah. 00:52:11.935 --> 00:52:13.940 align:middle line:84% It always contains a little bit of protein. 00:52:13.940 --> 00:52:15.873 align:middle line:84% AUDIENCE: It still worked well enough? 00:52:15.873 --> 00:52:16.540 align:middle line:90% JUEJUN HU: Yeah. 00:52:16.540 --> 00:52:19.380 align:middle line:84% I mean, protein itself is pretty transparent to 00:52:19.380 --> 00:52:20.660 align:middle line:90% certain wavelengths of light. 00:52:20.660 --> 00:52:24.100 align:middle line:84% So that's why it can work as basically a fiber-optic cable, 00:52:24.100 --> 00:52:24.800 align:middle line:90% basically. 00:52:24.800 --> 00:52:25.300 align:middle line:90% Yeah. 00:52:25.300 --> 00:52:26.760 align:middle line:90% But it's essentially silica. 00:52:26.760 --> 00:52:29.620 align:middle line:84% But when they die, essentially, the organic molecules, 00:52:29.620 --> 00:52:30.658 align:middle line:90% they decompose. 00:52:30.658 --> 00:52:32.700 align:middle line:84% So what's really left behind becomes pure silica, 00:52:32.700 --> 00:52:36.540 align:middle line:84% essentially, in a more porous form, in nanoporous form. 00:52:36.540 --> 00:52:37.140 align:middle line:90% Yeah. 00:52:37.140 --> 00:52:40.040 align:middle line:84% AUDIENCE: So that maybe would be better? 00:52:40.040 --> 00:52:42.180 align:middle line:90% Well, I don't know. 00:52:42.180 --> 00:52:49.450 align:middle line:84% And then how are they turning things in this kinetic control 00:52:49.450 --> 00:52:52.432 align:middle line:84% so that you don't get a polycrystalline thing? 00:52:52.432 --> 00:52:54.890 align:middle line:84% JUEJUN HU: For silica, it's easy because silica practically 00:52:54.890 --> 00:52:57.050 align:middle line:90% almost never crystallizes. 00:52:57.050 --> 00:52:59.170 align:middle line:90% So that's quite straightforward. 00:52:59.170 --> 00:53:01.370 align:middle line:84% You need to inhibit the crystallization 00:53:01.370 --> 00:53:05.370 align:middle line:84% mostly for calcium carbonate and calcium phosphate. 00:53:05.370 --> 00:53:09.450 align:middle line:84% AUDIENCE: So they're purposefully 00:53:09.450 --> 00:53:10.570 align:middle line:90% slowing everything down? 00:53:10.570 --> 00:53:16.330 align:middle line:84% Because every process-- it felt like every process 00:53:16.330 --> 00:53:19.130 align:middle line:84% we talked about this semester takes place 00:53:19.130 --> 00:53:21.290 align:middle line:84% at some sort of annealing temperature 00:53:21.290 --> 00:53:23.290 align:middle line:90% or something like that. 00:53:23.290 --> 00:53:28.370 align:middle line:90% And high temperatures. 00:53:28.370 --> 00:53:29.990 align:middle line:84% JUEJUN HU: In fact, bio organisms, 00:53:29.990 --> 00:53:32.790 align:middle line:84% as I said, purposefully, they want to slow it down. 00:53:32.790 --> 00:53:33.010 align:middle line:90% AUDIENCE: Slow it down. 00:53:33.010 --> 00:53:33.510 align:middle line:90% OK. 00:53:33.510 --> 00:53:35.730 align:middle line:90% So the issue is that it's-- 00:53:35.730 --> 00:53:38.230 align:middle line:84% JUEJUN HU: The issue is not that they cannot crystallize it. 00:53:38.230 --> 00:53:40.110 align:middle line:84% It's usually they want to do it slowly. 00:53:40.110 --> 00:53:40.610 align:middle line:90% Yeah. 00:53:40.610 --> 00:53:42.510 align:middle line:84% So that's a strategy I talk about, 00:53:42.510 --> 00:53:45.930 align:middle line:84% for example, for the crustacean, when they're molting. 00:53:45.930 --> 00:53:47.970 align:middle line:84% The idea always is they want amorphous phase 00:53:47.970 --> 00:53:50.610 align:middle line:84% to form essentially a raw shape quickly, 00:53:50.610 --> 00:53:53.050 align:middle line:84% but then they slowly try to essentially mold 00:53:53.050 --> 00:53:55.508 align:middle line:84% the microstructure into the desired geometry. 00:53:55.508 --> 00:53:56.050 align:middle line:90% AUDIENCE: OK. 00:53:56.050 --> 00:53:56.717 align:middle line:90% JUEJUN HU: Yeah. 00:53:56.717 --> 00:53:57.930 align:middle line:90% And that always takes time. 00:53:57.930 --> 00:54:01.130 align:middle line:84% Whether it's the spicules for sea urchin 00:54:01.130 --> 00:54:04.770 align:middle line:84% or it's the exoskeleton for crustaceans, 00:54:04.770 --> 00:54:07.530 align:middle line:84% they all have this specific microstructure. 00:54:07.530 --> 00:54:12.030 align:middle line:84% If you allow the crystallization to happen normally, on pace, 00:54:12.030 --> 00:54:14.110 align:middle line:84% then you always get randomly oriented crystals 00:54:14.110 --> 00:54:16.610 align:middle line:84% that does not follow this hierarchical microstructure. 00:54:16.610 --> 00:54:20.370 align:middle line:84% They need a templated process, such that the crystals only 00:54:20.370 --> 00:54:22.570 align:middle line:90% attached to the side they want. 00:54:22.570 --> 00:54:25.950 align:middle line:84% And to do that, you need to do it slowly. 00:54:25.950 --> 00:54:26.450 align:middle line:90% Yeah. 00:54:26.450 --> 00:54:28.510 align:middle line:90% To ensure the-- 00:54:28.510 --> 00:54:32.730 align:middle line:84% AUDIENCE: I guess I was surprised that it's kinetically 00:54:32.730 --> 00:54:35.382 align:middle line:90% favorable to crystallize even-- 00:54:35.382 --> 00:54:36.590 align:middle line:90% JUEJUN HU: It's kinetically-- 00:54:36.590 --> 00:54:39.215 align:middle line:84% I mean, it's thermodynamically always favorable to crystallize. 00:54:39.215 --> 00:54:41.690 align:middle line:84% But it won't kinetically slow it down, that process. 00:54:41.690 --> 00:54:45.452 align:middle line:84% And kinetically, by forming this amorphous phase first, 00:54:45.452 --> 00:54:47.160 align:middle line:84% that gives them a lot of control in terms 00:54:47.160 --> 00:54:50.180 align:middle line:84% of geometry and the controlled crystallization pathways. 00:54:50.180 --> 00:54:51.160 align:middle line:90% AUDIENCE: OK. 00:54:51.160 --> 00:54:51.800 align:middle line:90% OK. 00:54:51.800 --> 00:54:53.413 align:middle line:90% I'm back on track. 00:54:53.413 --> 00:54:54.080 align:middle line:90% JUEJUN HU: Yeah. 00:54:54.080 --> 00:54:56.680 align:middle line:90% So that's why. 00:54:56.680 --> 00:54:57.748 align:middle line:90% OK? 00:54:57.748 --> 00:54:58.540 align:middle line:90% AUDIENCE: All good. 00:54:58.540 --> 00:54:58.940 align:middle line:90% JUEJUN HU: All right. 00:54:58.940 --> 00:55:00.140 align:middle line:90% Now just a few examples. 00:55:00.140 --> 00:55:01.040 align:middle line:84% I mean, this slide I actually quoted 00:55:01.040 --> 00:55:02.780 align:middle line:84% from one of the previous lectures. 00:55:02.780 --> 00:55:06.800 align:middle line:84% This is essentially a bioinspired nacre-like 00:55:06.800 --> 00:55:07.675 align:middle line:90% composite. 00:55:07.675 --> 00:55:09.800 align:middle line:84% You have essentially these kind of glass platelets. 00:55:09.800 --> 00:55:11.617 align:middle line:84% In between, you put the polymer layers. 00:55:11.617 --> 00:55:13.200 align:middle line:84% So when you actually try to deform it, 00:55:13.200 --> 00:55:16.880 align:middle line:84% essentially the glass will slide with respect 00:55:16.880 --> 00:55:19.840 align:middle line:84% to each other because of the interpolymeric layers. 00:55:19.840 --> 00:55:21.840 align:middle line:84% And that actually helps to improve the toughness 00:55:21.840 --> 00:55:24.040 align:middle line:90% of these kind of glass plates. 00:55:24.040 --> 00:55:26.440 align:middle line:84% So that's just one example how these kind 00:55:26.440 --> 00:55:29.680 align:middle line:84% of hierarchical structures can help boost the strength 00:55:29.680 --> 00:55:32.360 align:middle line:90% of manmade structures. 00:55:32.360 --> 00:55:35.840 align:middle line:84% Now, as I promised, I'm going to talk about glass sponge. 00:55:35.840 --> 00:55:40.680 align:middle line:84% So this is another example where we humans can 00:55:40.680 --> 00:55:42.680 align:middle line:84% learn from essentially nature to create 00:55:42.680 --> 00:55:45.840 align:middle line:90% a mechanically robust design. 00:55:45.840 --> 00:55:50.200 align:middle line:84% So this is basically the silica skeleton, glass skeleton, 00:55:50.200 --> 00:55:53.480 align:middle line:90% of a specific kind of a sponge. 00:55:53.480 --> 00:55:55.257 align:middle line:84% I will not pretend that I know Latin. 00:55:55.257 --> 00:55:57.840 align:middle line:84% I have no idea how to pronounce it, but it's a kind of sponge, 00:55:57.840 --> 00:55:59.400 align:middle line:90% basically. 00:55:59.400 --> 00:56:02.680 align:middle line:84% So as I said, it has this kind of very unique arrangement 00:56:02.680 --> 00:56:04.320 align:middle line:90% of the silica skeleton here. 00:56:04.320 --> 00:56:06.320 align:middle line:84% You have essentially a square lattice 00:56:06.320 --> 00:56:10.680 align:middle line:84% that's essentially reinforced by these kind of diagonal rods. 00:56:10.680 --> 00:56:13.320 align:middle line:84% So there are actually two sets of diagonal rods 00:56:13.320 --> 00:56:14.220 align:middle line:90% if you look at it. 00:56:14.220 --> 00:56:17.960 align:middle line:84% So what the scientists did, they try 00:56:17.960 --> 00:56:20.480 align:middle line:84% to model this structure here with a set 00:56:20.480 --> 00:56:21.540 align:middle line:90% of geometric parameters. 00:56:21.540 --> 00:56:23.740 align:middle line:84% So you have, for example, the lattice period. 00:56:23.740 --> 00:56:26.180 align:middle line:84% You have the width of the square lattice. 00:56:26.180 --> 00:56:27.840 align:middle line:84% You have the width of the struct. 00:56:27.840 --> 00:56:29.640 align:middle line:90% All those kind of things. 00:56:29.640 --> 00:56:33.240 align:middle line:84% What they do is then they put it into a model. 00:56:33.240 --> 00:56:36.880 align:middle line:84% They try to optimize all those parameters to maximize 00:56:36.880 --> 00:56:41.720 align:middle line:84% the resistance of the structure against buckling deformation. 00:56:41.720 --> 00:56:44.640 align:middle line:84% They also look into several reference structures. 00:56:44.640 --> 00:56:46.160 align:middle line:90% These are alternative designs. 00:56:46.160 --> 00:56:47.920 align:middle line:84% For all the alternative designs, you also 00:56:47.920 --> 00:56:49.520 align:middle line:84% have a few set of geometric parameters 00:56:49.520 --> 00:56:51.620 align:middle line:84% that characterize essentially the arrangement. 00:56:51.620 --> 00:56:55.480 align:middle line:84% So similarly, they optimize all those parameters to maximize 00:56:55.480 --> 00:56:57.640 align:middle line:90% its buckling resistance. 00:56:57.640 --> 00:57:00.440 align:middle line:84% And what ends up happening here, when you look at experiment, 00:57:00.440 --> 00:57:03.840 align:middle line:84% this structure that's bioinspired by sponges, 00:57:03.840 --> 00:57:06.480 align:middle line:84% exhibits the highest buckling resistance 00:57:06.480 --> 00:57:09.440 align:middle line:84% compared to all the other optimized structures. 00:57:09.440 --> 00:57:13.440 align:middle line:84% What they also found out is that the geometric parameters coming 00:57:13.440 --> 00:57:16.720 align:middle line:84% from the natural sponge is actually 00:57:16.720 --> 00:57:19.600 align:middle line:84% very close to what people optimized using 00:57:19.600 --> 00:57:22.665 align:middle line:90% finite-element simulation. 00:57:22.665 --> 00:57:24.040 align:middle line:84% So in other words, the constraint 00:57:24.040 --> 00:57:26.082 align:middle line:84% they are putting about here, the figure of merit, 00:57:26.082 --> 00:57:27.780 align:middle line:84% is the maximum buckling resistance 00:57:27.780 --> 00:57:31.040 align:middle line:84% with the same amount of total material, total silica. 00:57:31.040 --> 00:57:34.180 align:middle line:84% So it essentially means that these glass sponges, 00:57:34.180 --> 00:57:36.400 align:middle line:84% they are really ingenious architects. 00:57:36.400 --> 00:57:38.100 align:middle line:84% They were able to find out this design, 00:57:38.100 --> 00:57:39.620 align:middle line:84% use the minimum amount of silica, 00:57:39.620 --> 00:57:41.670 align:middle line:84% it gives the maximum mechanical strength. 00:57:41.670 --> 00:57:45.830 align:middle line:84% They're very close to optimal, within a few percent. 00:57:45.830 --> 00:57:48.270 align:middle line:84% So to me that's quite remarkable, 00:57:48.270 --> 00:57:51.410 align:middle line:84% how nature were able to get to this kind of optimized design. 00:57:51.410 --> 00:57:54.305 align:middle line:90% 00:57:54.305 --> 00:57:55.430 align:middle line:90% So that's one example here. 00:57:55.430 --> 00:57:57.670 align:middle line:84% We're talking about mechanical properties. 00:57:57.670 --> 00:57:59.450 align:middle line:84% Now let's take a look at another example. 00:57:59.450 --> 00:58:01.250 align:middle line:84% In this case, let's look at diatoms. 00:58:01.250 --> 00:58:04.030 align:middle line:90% 00:58:04.030 --> 00:58:07.990 align:middle line:84% So the silica shell of diatoms, as I mentioned, 00:58:07.990 --> 00:58:11.910 align:middle line:84% they have this kind of nanoporous structure here. 00:58:11.910 --> 00:58:15.070 align:middle line:84% So this nanoporous structure and also the fact that 00:58:15.070 --> 00:58:17.370 align:middle line:84% it's basically silica-- it's very abundant, 00:58:17.370 --> 00:58:19.190 align:middle line:90% it's biocompatible-- 00:58:19.190 --> 00:58:21.830 align:middle line:84% has attracted lots of attention for applications 00:58:21.830 --> 00:58:23.630 align:middle line:90% like sensing or drug delivery. 00:58:23.630 --> 00:58:27.430 align:middle line:84% So this is actually one example of a single-molecule sensor 00:58:27.430 --> 00:58:30.652 align:middle line:84% people were able to develop using essentially this diatom. 00:58:30.652 --> 00:58:32.110 align:middle line:84% So the technique they're using here 00:58:32.110 --> 00:58:37.650 align:middle line:84% is called SERS, Surface Enhanced Raman Spectroscopy. 00:58:37.650 --> 00:58:39.538 align:middle line:84% It's basically using Raman scattering. 00:58:39.538 --> 00:58:40.830 align:middle line:90% We talk about Raman scattering. 00:58:40.830 --> 00:58:42.788 align:middle line:84% Different molecules, they essentially 00:58:42.788 --> 00:58:44.330 align:middle line:84% have different kind of phonon energy, 00:58:44.330 --> 00:58:45.630 align:middle line:90% different vibrational energy. 00:58:45.630 --> 00:58:48.590 align:middle line:84% And that means that if you essentially program, excite them 00:58:48.590 --> 00:58:51.590 align:middle line:84% using a laser, they're going to emit different frequencies 00:58:51.590 --> 00:58:52.890 align:middle line:90% of Raman-scattered signals. 00:58:52.890 --> 00:58:55.190 align:middle line:84% From that, they serve as a molecular signature 00:58:55.190 --> 00:58:57.430 align:middle line:90% to identify different species. 00:58:57.430 --> 00:58:59.850 align:middle line:84% Downside, which I did not talk about previously, 00:58:59.850 --> 00:59:02.310 align:middle line:84% is that Raman is actually a very weak process. 00:59:02.310 --> 00:59:06.430 align:middle line:84% Compared to Rayleigh scattering, Raman-scattering cross-section 00:59:06.430 --> 00:59:10.210 align:middle line:84% is billions of times weaker compared to Rayleigh scattering. 00:59:10.210 --> 00:59:14.110 align:middle line:84% So normally, Raman tends to be quite an insensitive technique. 00:59:14.110 --> 00:59:15.990 align:middle line:84% And that's why, in fact, Raman spectroscopy 00:59:15.990 --> 00:59:18.830 align:middle line:84% was not even used when Raman discovered Raman scattering. 00:59:18.830 --> 00:59:20.710 align:middle line:84% Nobody cared about it because it's so weak, 00:59:20.710 --> 00:59:25.170 align:middle line:84% until '60s, '70s, when people actually develop lasers. 00:59:25.170 --> 00:59:27.670 align:middle line:84% They now have a very powerful light source that can actually 00:59:27.670 --> 00:59:29.170 align:middle line:90% excite enough Raman photons. 00:59:29.170 --> 00:59:31.070 align:middle line:84% That's why it actually became useful. 00:59:31.070 --> 00:59:35.030 align:middle line:84% Now, that said, if you want to detect a single molecule using 00:59:35.030 --> 00:59:38.020 align:middle line:84% Raman, you can imagine it can be quite difficult. 00:59:38.020 --> 00:59:40.500 align:middle line:84% So the way it works is to essentially create 00:59:40.500 --> 00:59:43.660 align:middle line:84% these kind of microstructures, where you can concentrate 00:59:43.660 --> 00:59:46.060 align:middle line:84% the optical field into a very small regime 00:59:46.060 --> 00:59:47.717 align:middle line:84% to get into high enough photon density 00:59:47.717 --> 00:59:49.800 align:middle line:84% so that you can excite enough number of molecules. 00:59:49.800 --> 00:59:51.300 align:middle line:90% That's the basic idea of SERS. 00:59:51.300 --> 00:59:56.220 align:middle line:84% So in this case, this kind of porous structure of the silica, 00:59:56.220 --> 00:59:59.240 align:middle line:84% biosilica, produced by diatom, actually is very useful here. 00:59:59.240 --> 01:00:03.060 align:middle line:84% It sort of acts optically as a photonic crystal. 01:00:03.060 --> 01:00:05.340 align:middle line:84% So when it's actually decorated with these metallic 01:00:05.340 --> 01:00:07.700 align:middle line:84% nanoparticles, they can actually interact 01:00:07.700 --> 01:00:10.180 align:middle line:84% with the metallic nanoparticles to cause essentially 01:00:10.180 --> 01:00:13.820 align:middle line:84% a plasmonic enhancement also with the photonic crystal 01:00:13.820 --> 01:00:14.580 align:middle line:90% effect. 01:00:14.580 --> 01:00:17.980 align:middle line:84% And that produces a high density of what are called hotspots. 01:00:17.980 --> 01:00:19.772 align:middle line:84% So hotspots is the terminology used in SERS 01:00:19.772 --> 01:00:21.313 align:middle line:84% to essentially describe regions where 01:00:21.313 --> 01:00:23.460 align:middle line:84% you have very high local electromagnetic field. 01:00:23.460 --> 01:00:25.553 align:middle line:84% In those regions, if you put a molecule there, 01:00:25.553 --> 01:00:27.720 align:middle line:84% it's going to generate very strong Raman scattering. 01:00:27.720 --> 01:00:29.303 align:middle line:84% That's how you can actually detect it. 01:00:29.303 --> 01:00:31.500 align:middle line:84% So this kind of porous structure helps 01:00:31.500 --> 01:00:34.660 align:middle line:84% you to amplify the density of these hotspots 01:00:34.660 --> 01:00:36.780 align:middle line:84% through basically a photonic crystal effect. 01:00:36.780 --> 01:00:40.080 align:middle line:84% It also helps to have large surface area. 01:00:40.080 --> 01:00:42.620 align:middle line:84% You are able to absorb lots of these metallic nanoparticles 01:00:42.620 --> 01:00:43.920 align:middle line:90% to have plasmonic enhancement. 01:00:43.920 --> 01:00:47.900 align:middle line:84% So optically, the nanoporous structure is quite useful. 01:00:47.900 --> 01:00:50.900 align:middle line:90% So that's one part. 01:00:50.900 --> 01:00:52.740 align:middle line:84% What's really ingenious about this study 01:00:52.740 --> 01:00:56.220 align:middle line:84% is that they also use the fact that silica is silica. 01:00:56.220 --> 01:00:58.940 align:middle line:90% So it has hydrophilic surface. 01:00:58.940 --> 01:01:01.500 align:middle line:84% It also has this kind of nanoporous geometry. 01:01:01.500 --> 01:01:05.280 align:middle line:84% So it means that it actually wets solution really well. 01:01:05.280 --> 01:01:07.980 align:middle line:90% 01:01:07.980 --> 01:01:10.380 align:middle line:84% So imagine you put a droplet onto one 01:01:10.380 --> 01:01:13.620 align:middle line:84% of these substrates that contain this kind of diatoms, biosilica. 01:01:13.620 --> 01:01:19.460 align:middle line:84% Then as the solution dries, this diatom, the silica, biosilica, 01:01:19.460 --> 01:01:23.300 align:middle line:84% essentially pulls the drying liquid droplets towards itself. 01:01:23.300 --> 01:01:25.100 align:middle line:84% In the process, anything that's originally 01:01:25.100 --> 01:01:26.808 align:middle line:84% in the solution phase, all the molecules, 01:01:26.808 --> 01:01:28.500 align:middle line:84% essentially get concentrated down 01:01:28.500 --> 01:01:32.180 align:middle line:84% into the surface of the silica shell here. 01:01:32.180 --> 01:01:34.620 align:middle line:84% So this also creates very good spatial overlap 01:01:34.620 --> 01:01:37.290 align:middle line:84% between the molecules originally in the solution 01:01:37.290 --> 01:01:42.890 align:middle line:84% phase to the hotspots, the SERS hotspots on the surface there. 01:01:42.890 --> 01:01:45.010 align:middle line:84% So that's why they were able to actually detect 01:01:45.010 --> 01:01:47.670 align:middle line:84% single molecules by using this kind of architecture here. 01:01:47.670 --> 01:01:52.130 align:middle line:84% It's like a self-concentrating mechanism in the solution 01:01:52.130 --> 01:01:56.410 align:middle line:84% and also provides optical enhancement. 01:01:56.410 --> 01:01:58.863 align:middle line:84% AUDIENCE: Can you repeat how the optical enhancement-- 01:01:58.863 --> 01:02:00.530 align:middle line:84% JUEJUN HU: Yeah, the optical enhancement 01:02:00.530 --> 01:02:01.862 align:middle line:90% happens basically through-- 01:02:01.862 --> 01:02:04.570 align:middle line:84% so what they did is they have to essentially decorate the surface 01:02:04.570 --> 01:02:06.130 align:middle line:90% with silver nanoparticles. 01:02:06.130 --> 01:02:09.318 align:middle line:84% So silver nanoparticles provide plasmonic enhancement. 01:02:09.318 --> 01:02:11.610 align:middle line:84% Essentially, because of oscillations of free electrons, 01:02:11.610 --> 01:02:13.530 align:middle line:84% it creates a very high electromagnetic field 01:02:13.530 --> 01:02:14.570 align:middle line:90% at the surface. 01:02:14.570 --> 01:02:17.290 align:middle line:84% So by having this kind of porous nanostructure, 01:02:17.290 --> 01:02:19.370 align:middle line:84% you can have essentially many attachment sites 01:02:19.370 --> 01:02:23.610 align:middle line:84% for the silver nanoparticles, which helps in that regard. 01:02:23.610 --> 01:02:25.367 align:middle line:90% Itself is also photonic crystal. 01:02:25.367 --> 01:02:27.450 align:middle line:84% So that helps essentially also locally concentrate 01:02:27.450 --> 01:02:28.970 align:middle line:90% electromagnetic field. 01:02:28.970 --> 01:02:30.650 align:middle line:84% And the whole game about SERS is just 01:02:30.650 --> 01:02:33.050 align:middle line:84% try to concentrate electromagnetic field 01:02:33.050 --> 01:02:35.290 align:middle line:90% to get enhanced scattering. 01:02:35.290 --> 01:02:38.270 align:middle line:84% So basically, that's why they call it double enhancement. 01:02:38.270 --> 01:02:41.210 align:middle line:84% You have essentially the fluid enhancement, 01:02:41.210 --> 01:02:43.230 align:middle line:84% essentially pulling the molecule towards it. 01:02:43.230 --> 01:02:45.890 align:middle line:84% Also, optically, it helps to boost the signal. 01:02:45.890 --> 01:02:48.290 align:middle line:84% That's why I think it's quite an interesting study there. 01:02:48.290 --> 01:02:48.790 align:middle line:90% Yeah. 01:02:48.790 --> 01:03:06.730 align:middle line:90% 01:03:06.730 --> 01:03:08.090 align:middle line:90% All right. 01:03:08.090 --> 01:03:13.170 align:middle line:84% Next example for this kind of nanoporous structure. 01:03:13.170 --> 01:03:16.290 align:middle line:84% Because it has very large surface-to-volume ratio, 01:03:16.290 --> 01:03:19.090 align:middle line:84% you can imagine that they can also be used for drug delivery, 01:03:19.090 --> 01:03:23.170 align:middle line:84% to transport drug molecules to biological tissues. 01:03:23.170 --> 01:03:25.350 align:middle line:84% So in this particular study I'm quoting here, 01:03:25.350 --> 01:03:26.970 align:middle line:84% you can see this is actually a diatom, biosilica, 01:03:26.970 --> 01:03:27.570 align:middle line:90% they're using. 01:03:27.570 --> 01:03:33.010 align:middle line:84% You can see it's a highly porous structure. 01:03:33.010 --> 01:03:35.690 align:middle line:84% So what they did is they actually genetically engineered 01:03:35.690 --> 01:03:36.810 align:middle line:90% the diatom. 01:03:36.810 --> 01:03:40.200 align:middle line:84% So when they build the silica shell, the silica shell, 01:03:40.200 --> 01:03:42.450 align:middle line:84% the surface is actually decorated with a specific kind 01:03:42.450 --> 01:03:43.290 align:middle line:90% of protein. 01:03:43.290 --> 01:03:46.650 align:middle line:84% And this protein actually binds to antibody 01:03:46.650 --> 01:03:50.010 align:middle line:84% that's present on the surface of the targeted cells, targeted 01:03:50.010 --> 01:03:50.910 align:middle line:90% biological cells. 01:03:50.910 --> 01:03:53.530 align:middle line:84% They want to essentially deliver the drugs specifically 01:03:53.530 --> 01:03:56.150 align:middle line:84% to only one kind of biological cells. 01:03:56.150 --> 01:03:58.650 align:middle line:84% So that's why they biological engineer, genetically engineer 01:03:58.650 --> 01:04:00.730 align:middle line:90% the biosilica, the diatom. 01:04:00.730 --> 01:04:02.250 align:middle line:84% So in this plot here, this basically 01:04:02.250 --> 01:04:05.330 align:middle line:84% shows essentially the surface-binding capabilities. 01:04:05.330 --> 01:04:07.710 align:middle line:84% This one, WT, this is basically a wild type. 01:04:07.710 --> 01:04:10.470 align:middle line:84% This is ungenetically modified diatom. 01:04:10.470 --> 01:04:11.630 align:middle line:90% It doesn't really bind. 01:04:11.630 --> 01:04:13.410 align:middle line:84% It only has some physical adsorption 01:04:13.410 --> 01:04:16.110 align:middle line:90% to the kind of proteins. 01:04:16.110 --> 01:04:18.710 align:middle line:84% But if it's genetically modified, these four strands, 01:04:18.710 --> 01:04:21.730 align:middle line:84% you can see that they exhibit much stronger surface affinity 01:04:21.730 --> 01:04:24.342 align:middle line:90% to the antibodies. 01:04:24.342 --> 01:04:26.050 align:middle line:84% So they can selectively bind to the cell. 01:04:26.050 --> 01:04:30.690 align:middle line:84% Also, this porous surface also means that they can actually 01:04:30.690 --> 01:04:33.000 align:middle line:84% be essentially-- they can electrostatically 01:04:33.000 --> 01:04:36.180 align:middle line:84% attract to the hydrophobic drug molecules with a larger 01:04:36.180 --> 01:04:37.180 align:middle line:90% surface-to-volume ratio. 01:04:37.180 --> 01:04:38.980 align:middle line:84% They can also carry lots of drug molecules. 01:04:38.980 --> 01:04:42.640 align:middle line:84% So then they can then use this kind of little biosilica 01:04:42.640 --> 01:04:44.840 align:middle line:84% as a little vehicle here to transport 01:04:44.840 --> 01:04:48.120 align:middle line:84% the drug molecules in a site-specific manner 01:04:48.120 --> 01:04:49.240 align:middle line:90% to target the cells. 01:04:49.240 --> 01:04:51.612 align:middle line:84% So this is actually the image here. 01:04:51.612 --> 01:04:53.320 align:middle line:84% This is a fluorescent image of the cells. 01:04:53.320 --> 01:04:56.400 align:middle line:84% So the blue part is basically the nuclei. 01:04:56.400 --> 01:04:58.840 align:middle line:84% The red part is the surrounding region there. 01:04:58.840 --> 01:05:00.960 align:middle line:84% And on the surface of the cells, you actually 01:05:00.960 --> 01:05:05.460 align:middle line:84% see this little silica, little vehicles. 01:05:05.460 --> 01:05:08.320 align:middle line:84% They serve as vehicles that eventually attach to the surface 01:05:08.320 --> 01:05:11.120 align:middle line:84% and deliver the drug molecules specifically 01:05:11.120 --> 01:05:12.600 align:middle line:90% to that kind of cell there. 01:05:12.600 --> 01:05:18.320 align:middle line:84% So another example of using these kind of diatom structures. 01:05:18.320 --> 01:05:22.880 align:middle line:84% Now, last part I want to quickly mention here 01:05:22.880 --> 01:05:27.560 align:middle line:84% is kind of a material synthetic strategy 01:05:27.560 --> 01:05:32.200 align:middle line:84% that people have developed by copying what the nature is 01:05:32.200 --> 01:05:33.620 align:middle line:90% doing in biomineralization. 01:05:33.620 --> 01:05:36.160 align:middle line:84% It's often called a Polymer-Induced Liquid 01:05:36.160 --> 01:05:38.480 align:middle line:90% Precursor, PILP. 01:05:38.480 --> 01:05:41.480 align:middle line:84% The PILP process is really a direct replica 01:05:41.480 --> 01:05:44.640 align:middle line:84% of the natural biomineralization process. 01:05:44.640 --> 01:05:46.680 align:middle line:84% You start out by adding these polymers that 01:05:46.680 --> 01:05:49.480 align:middle line:84% contains ionic species in a solution that's already 01:05:49.480 --> 01:05:51.520 align:middle line:90% supersaturated with minerals. 01:05:51.520 --> 01:05:54.640 align:middle line:84% In this case, because the solution 01:05:54.640 --> 01:05:56.460 align:middle line:84% is supersaturated with mineral ions, 01:05:56.460 --> 01:05:59.720 align:middle line:84% the ions then electrostatically attract to the polymer chain. 01:05:59.720 --> 01:06:02.200 align:middle line:84% And then they can then assemble into essentially 01:06:02.200 --> 01:06:05.360 align:middle line:84% these nanoclusters that contains amorphous mineral 01:06:05.360 --> 01:06:07.160 align:middle line:90% with the polymer matrix. 01:06:07.160 --> 01:06:08.840 align:middle line:90% So these are the nanoclusters. 01:06:08.840 --> 01:06:12.600 align:middle line:84% These clusters, people use them to infiltrate whatever 01:06:12.600 --> 01:06:14.200 align:middle line:90% template they have made. 01:06:14.200 --> 01:06:17.800 align:middle line:84% And in the end, this template, you then 01:06:17.800 --> 01:06:19.480 align:middle line:84% essentially trigger the crystallization 01:06:19.480 --> 01:06:21.180 align:middle line:90% of those nanoclusters. 01:06:21.180 --> 01:06:22.760 align:middle line:84% So then you form the material that 01:06:22.760 --> 01:06:27.000 align:middle line:84% follows whatever template you have set for the crystallization 01:06:27.000 --> 01:06:27.740 align:middle line:90% process. 01:06:27.740 --> 01:06:31.230 align:middle line:84% So as you can tell, this is very, very similar 01:06:31.230 --> 01:06:34.390 align:middle line:84% to the biomineralization process. 01:06:34.390 --> 01:06:37.630 align:middle line:84% Some examples about the application of this kind of PILP 01:06:37.630 --> 01:06:39.452 align:middle line:90% process. 01:06:39.452 --> 01:06:40.910 align:middle line:84% This is an example where people are 01:06:40.910 --> 01:06:46.070 align:middle line:84% trying to develop essentially a bone-like microstructure 01:06:46.070 --> 01:06:48.110 align:middle line:84% for a composite that contains essentially 01:06:48.110 --> 01:06:52.310 align:middle line:84% collagen with essentially mineralized hydroxyapatite 01:06:52.310 --> 01:06:53.450 align:middle line:90% that's decorated. 01:06:53.450 --> 01:06:55.930 align:middle line:90% So create artificial bone. 01:06:55.930 --> 01:06:57.510 align:middle line:84% That's what people are trying to do. 01:06:57.510 --> 01:07:00.570 align:middle line:84% So what's shown here in this plot here, 01:07:00.570 --> 01:07:03.410 align:middle line:84% this is basically they call the collagen sponge. 01:07:03.410 --> 01:07:05.630 align:middle line:84% It's basically a porous structure containing 01:07:05.630 --> 01:07:07.710 align:middle line:90% many collagen fibers. 01:07:07.710 --> 01:07:13.290 align:middle line:84% And if you look at the SEM, this is what the sponge looks like. 01:07:13.290 --> 01:07:16.350 align:middle line:84% You can see this kind of fibrous structures. 01:07:16.350 --> 01:07:18.910 align:middle line:84% Now, if you directly use a conventional crystallization 01:07:18.910 --> 01:07:21.110 align:middle line:84% deposition process, this is what you end up getting. 01:07:21.110 --> 01:07:22.990 align:middle line:90% You see these little spheres? 01:07:22.990 --> 01:07:25.150 align:middle line:84% These are essentially a calcium carbonate 01:07:25.150 --> 01:07:27.630 align:middle line:84% that crystallizes and form little crystals that 01:07:27.630 --> 01:07:29.430 align:middle line:84% directly then attach to the surface there. 01:07:29.430 --> 01:07:31.510 align:middle line:84% So this is not a desirable microstructure. 01:07:31.510 --> 01:07:37.990 align:middle line:84% If you use the polymer-induced liquid precursor approach, 01:07:37.990 --> 01:07:41.070 align:middle line:84% then you get these kind of nanoclusters 01:07:41.070 --> 01:07:45.170 align:middle line:84% that then attach to the surface of the collagen fibers here. 01:07:45.170 --> 01:07:46.830 align:middle line:84% And you get this kind of microstructure 01:07:46.830 --> 01:07:49.430 align:middle line:84% that's actually very similar to what you get in the bone. 01:07:49.430 --> 01:07:52.350 align:middle line:84% What's also interesting is that they actually found out 01:07:52.350 --> 01:07:54.730 align:middle line:84% that the nanocrystals attached to fiber, 01:07:54.730 --> 01:07:57.710 align:middle line:84% they are all aligned crystallographically. 01:07:57.710 --> 01:08:01.710 align:middle line:84% The long axis of the fibers, they all align with 011 01:08:01.710 --> 01:08:05.327 align:middle line:84% direction of the calcite crystals that was formed, 01:08:05.327 --> 01:08:06.410 align:middle line:90% which is quite remarkable. 01:08:06.410 --> 01:08:08.785 align:middle line:84% This is actually the same as what people observe actually 01:08:08.785 --> 01:08:10.150 align:middle line:90% in human bone. 01:08:10.150 --> 01:08:10.695 align:middle line:90% Same idea. 01:08:10.695 --> 01:08:12.070 align:middle line:84% The crystals crystallographically 01:08:12.070 --> 01:08:13.650 align:middle line:90% are aligned along the fibers. 01:08:13.650 --> 01:08:20.410 align:middle line:90% 01:08:20.410 --> 01:08:23.350 align:middle line:84% So one example about how we can learn from nature to create 01:08:23.350 --> 01:08:27.590 align:middle line:84% microstructures that resembles natural biominerals. 01:08:27.590 --> 01:08:33.910 align:middle line:84% Now, another example, this is about enamels, the teeth. 01:08:33.910 --> 01:08:38.830 align:middle line:84% So as I mentioned, if you eat sugary foods, 01:08:38.830 --> 01:08:42.282 align:middle line:84% if you drink acidic liquids, you can 01:08:42.282 --> 01:08:44.490 align:middle line:84% push the chemical equilibrium to the other direction. 01:08:44.490 --> 01:08:48.710 align:middle line:84% You start to get what's called demineralization of the enamel. 01:08:48.710 --> 01:08:52.750 align:middle line:84% And what shows up is this kind of a so-called white spot 01:08:52.750 --> 01:08:54.122 align:middle line:90% lesion. 01:08:54.122 --> 01:08:55.330 align:middle line:90% This is actually one example. 01:08:55.330 --> 01:08:58.750 align:middle line:84% You see these little white spots here, like right here? 01:08:58.750 --> 01:09:01.470 align:middle line:84% So this is actually indication of the initial phase 01:09:01.470 --> 01:09:02.450 align:middle line:90% of demineralization. 01:09:02.450 --> 01:09:04.950 align:middle line:84% You are essentially extracting the hydroxyapatite 01:09:04.950 --> 01:09:09.630 align:middle line:84% from the surface, leaving behind the porous enamel, which then, 01:09:09.630 --> 01:09:11.143 align:middle line:90% of course that's bad. 01:09:11.143 --> 01:09:13.310 align:middle line:84% Later on, you're going to expose essentially dentin. 01:09:13.310 --> 01:09:17.149 align:middle line:84% And then you have basically a cavity. 01:09:17.149 --> 01:09:22.510 align:middle line:84% So as I mentioned, good thing is that this process is actually 01:09:22.510 --> 01:09:23.590 align:middle line:90% reversible. 01:09:23.590 --> 01:09:29.060 align:middle line:84% So if you supply it more basic solution, 01:09:29.060 --> 01:09:33.720 align:middle line:84% you supply enough amount of essentially calcium phosphate, 01:09:33.720 --> 01:09:35.220 align:middle line:90% you can reverse the process. 01:09:35.220 --> 01:09:37.925 align:middle line:84% The problem though, however, is that in order 01:09:37.925 --> 01:09:39.300 align:middle line:84% to form the microstructure that's 01:09:39.300 --> 01:09:40.920 align:middle line:84% similar to what you have in enamel, 01:09:40.920 --> 01:09:44.899 align:middle line:84% you need to have a transient amorphous phase, ACP. 01:09:44.899 --> 01:09:47.207 align:middle line:84% So ACP itself is also not stable. 01:09:47.207 --> 01:09:49.540 align:middle line:84% It will crystallize quickly, within a matter of minutes. 01:09:49.540 --> 01:09:51.617 align:middle line:84% So that's why people have developed 01:09:51.617 --> 01:09:53.700 align:middle line:84% different kinds of strategies, even different kind 01:09:53.700 --> 01:09:58.100 align:middle line:84% of molecules, to essentially try to stabilize the ACP molecules 01:09:58.100 --> 01:09:59.080 align:middle line:90% in the solution phase. 01:09:59.080 --> 01:10:01.200 align:middle line:84% I'm not going to go through the details here. 01:10:01.200 --> 01:10:03.220 align:middle line:84% There are many options people have tried. 01:10:03.220 --> 01:10:06.020 align:middle line:84% One, probably the most commonly used option, 01:10:06.020 --> 01:10:07.860 align:middle line:84% is to use a kind of molecule called 01:10:07.860 --> 01:10:12.160 align:middle line:90% a Casein Phosphopeptide, CPP. 01:10:12.160 --> 01:10:14.260 align:middle line:84% CPP is basically a polypeptide where 01:10:14.260 --> 01:10:18.860 align:middle line:84% it has a certain arrangement in its polypeptide chain 01:10:18.860 --> 01:10:22.200 align:middle line:84% that can selectively attract the ions 01:10:22.200 --> 01:10:24.180 align:middle line:84% and then prevent the crystallization. 01:10:24.180 --> 01:10:29.100 align:middle line:84% So that's why this kind of complex, 01:10:29.100 --> 01:10:36.300 align:middle line:84% called CPP, ACP, if you do a Google search, 01:10:36.300 --> 01:10:38.820 align:middle line:84% you'll find lots of dental products 01:10:38.820 --> 01:10:40.520 align:middle line:84% that contain this kind of complex there. 01:10:40.520 --> 01:10:41.020 align:middle line:90% CPP. 01:10:41.020 --> 01:10:41.520 align:middle line:90% ACP. 01:10:41.520 --> 01:10:47.140 align:middle line:84% You find toothpaste, like dental or tooth mousse 01:10:47.140 --> 01:10:49.640 align:middle line:84% that you can put on your teeth that contain 01:10:49.640 --> 01:10:50.640 align:middle line:90% these kind of molecules. 01:10:50.640 --> 01:10:52.015 align:middle line:84% And there are also lots of papers 01:10:52.015 --> 01:10:53.780 align:middle line:84% that study that, indeed, clinically, they 01:10:53.780 --> 01:10:57.780 align:middle line:84% have been verified to be useful in catalyzing 01:10:57.780 --> 01:11:00.955 align:middle line:84% so-called the remineralization process. 01:11:00.955 --> 01:11:03.580 align:middle line:84% So remineralization essentially means that they can essentially 01:11:03.580 --> 01:11:07.020 align:middle line:84% repair damages, something like white spot lesions, 01:11:07.020 --> 01:11:10.980 align:middle line:84% by essentially redepositing this calcium 01:11:10.980 --> 01:11:12.360 align:middle line:90% phosphate onto the teeth. 01:11:12.360 --> 01:11:14.562 align:middle line:84% So this is actually the one example 01:11:14.562 --> 01:11:16.020 align:middle line:84% I quoted from this paper that shows 01:11:16.020 --> 01:11:18.820 align:middle line:84% one example of this kind of remineralization process. 01:11:18.820 --> 01:11:20.620 align:middle line:90% They do not use CPP. 01:11:20.620 --> 01:11:23.730 align:middle line:84% They use another molecule called PCBAA. 01:11:23.730 --> 01:11:24.722 align:middle line:90% I'm not a biochemist. 01:11:24.722 --> 01:11:26.930 align:middle line:84% I'm not going to tell you what exactly a molecule is. 01:11:26.930 --> 01:11:31.250 align:middle line:84% But what they claim is that this PCBAA serves two functions. 01:11:31.250 --> 01:11:34.530 align:middle line:84% On one hand, it helps to slow down the crystallization 01:11:34.530 --> 01:11:38.030 align:middle line:84% kinetics of the calcium phosphate, 01:11:38.030 --> 01:11:40.210 align:middle line:84% and they help with the remineralization process. 01:11:40.210 --> 01:11:42.050 align:middle line:84% They also have this added function 01:11:42.050 --> 01:11:44.710 align:middle line:84% that they can prevent the formation of biofilms. 01:11:44.710 --> 01:11:47.210 align:middle line:84% So the idea is that if you think about the surface of tooth, 01:11:47.210 --> 01:11:48.430 align:middle line:90% there are lots of bacteria. 01:11:48.430 --> 01:11:51.450 align:middle line:84% These bacteria, their metabolic activities 01:11:51.450 --> 01:11:53.570 align:middle line:84% form this kind of biofilm that covers 01:11:53.570 --> 01:11:56.650 align:middle line:84% the surface of the tooth, which prevents attachment 01:11:56.650 --> 01:12:00.810 align:middle line:84% of the calcium phosphate clusters. 01:12:00.810 --> 01:12:04.370 align:middle line:84% So if you want to have a sustainable, robust 01:12:04.370 --> 01:12:06.090 align:middle line:84% remineralization process, you need 01:12:06.090 --> 01:12:08.290 align:middle line:84% to inhibit the formation of biofilms. 01:12:08.290 --> 01:12:10.930 align:middle line:84% So that's what they found out, that this molecule also 01:12:10.930 --> 01:12:14.290 align:middle line:84% helps to inhibit the formation of the biofilm to facilitate 01:12:14.290 --> 01:12:16.330 align:middle line:90% the remineralization process. 01:12:16.330 --> 01:12:18.170 align:middle line:90% And yeah. 01:12:18.170 --> 01:12:19.450 align:middle line:90% So that's why. 01:12:19.450 --> 01:12:21.150 align:middle line:90% This could be useful here. 01:12:21.150 --> 01:12:21.930 align:middle line:90% All right. 01:12:21.930 --> 01:12:25.650 align:middle line:90% So OK. 01:12:25.650 --> 01:12:28.530 align:middle line:84% I think we are running out of time towards the end of class 01:12:28.530 --> 01:12:29.030 align:middle line:90% here. 01:12:29.030 --> 01:12:31.480 align:middle line:84% This is the last slide I quoted from a review paper that 01:12:31.480 --> 01:12:33.730 align:middle line:84% tells you that there are lots of exciting applications 01:12:33.730 --> 01:12:38.370 align:middle line:84% of biomimetic mineralization process 01:12:38.370 --> 01:12:41.090 align:middle line:84% that have yet to be explored, with a lot 01:12:41.090 --> 01:12:42.570 align:middle line:90% of exciting prospects. 01:12:42.570 --> 01:12:44.110 align:middle line:90% So I'm going to stop here. 01:12:44.110 --> 01:12:45.770 align:middle line:90% Any questions? 01:12:45.770 --> 01:12:47.583 align:middle line:84% How do you grade my final presentation? 01:12:47.583 --> 01:12:48.250 align:middle line:90% AUDIENCE: Great. 01:12:48.250 --> 01:12:51.330 align:middle line:90% 01:12:51.330 --> 01:12:51.830 align:middle line:90% Yeah. 01:12:51.830 --> 01:12:52.330 align:middle line:90% Thank you. 01:12:52.330 --> 01:12:53.330 align:middle line:90% This is cool. 01:12:53.330 --> 01:12:54.290 align:middle line:90% JUEJUN HU: OK, great. 01:12:54.290 --> 01:12:54.850 align:middle line:90% Yeah. 01:12:54.850 --> 01:12:57.410 align:middle line:84% So you should get to the same level for final presentation. 01:12:57.410 --> 01:12:59.007 align:middle line:90% AUDIENCE: Oh, yeah. 01:12:59.007 --> 01:12:59.590 align:middle line:90% JUEJUN HU: No. 01:12:59.590 --> 01:13:00.770 align:middle line:90% It's fine. 01:13:00.770 --> 01:13:02.910 align:middle line:84% But anyway, if you have any questions, let me know. 01:13:02.910 --> 01:13:04.670 align:middle line:84% I think this is a quite exciting process. 01:13:04.670 --> 01:13:07.400 align:middle line:84% Well, I got to learn a lot myself. 01:13:07.400 --> 01:14:18.000 align:middle line:90%