1 00:00:00,030 --> 00:00:02,530 NARRATOR: The following content is provided under a Creative 2 00:00:02,530 --> 00:00:04,000 Commons license. 3 00:00:04,000 --> 00:00:06,330 Your support will help MIT OpenCourseWare 4 00:00:06,330 --> 00:00:10,690 continue to offer high-quality educational resources for free. 5 00:00:10,690 --> 00:00:13,300 To make a donation or view additional materials 6 00:00:13,300 --> 00:00:17,025 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,025 --> 00:00:17,650 at ocw.mit.edu. 8 00:00:26,176 --> 00:00:27,800 LORNA GIBSON: So last time, we finished 9 00:00:27,800 --> 00:00:29,780 talking about trabecular bone. 10 00:00:29,780 --> 00:00:31,680 And what I wanted to talk about this week 11 00:00:31,680 --> 00:00:33,782 was tissue engineering scaffolds. 12 00:00:33,782 --> 00:00:35,240 So the idea with tissue engineering 13 00:00:35,240 --> 00:00:39,730 is you want to be able to repair damaged or diseased tissue. 14 00:00:39,730 --> 00:00:42,190 And typically, that's done by regenerating the tissue 15 00:00:42,190 --> 00:00:43,120 in some way. 16 00:00:43,120 --> 00:00:46,520 So in your body, many types of cells, like maybe not blood 17 00:00:46,520 --> 00:00:49,940 cells, but most types of cells for sort of structural tissues 18 00:00:49,940 --> 00:00:52,200 are attached to an extracellular matrix. 19 00:00:52,200 --> 00:00:54,870 And this is sort of a schematic of the extracellular matrix 20 00:00:54,870 --> 00:00:55,460 here. 21 00:00:55,460 --> 00:00:57,960 And the composition depends on the type of tissue 22 00:00:57,960 --> 00:00:59,030 that's involved. 23 00:00:59,030 --> 00:01:02,890 For example, in skin, it would be collagen and something 24 00:01:02,890 --> 00:01:06,050 called glycosaminoglycans and elastin. 25 00:01:06,050 --> 00:01:09,120 In bone, it would be collagen and a mineral-- a calcium 26 00:01:09,120 --> 00:01:11,550 phosphate mineral, hydroxyapatite. 27 00:01:11,550 --> 00:01:13,640 So the composition varies. 28 00:01:13,640 --> 00:01:15,780 But the idea with the tissue engineering scaffold 29 00:01:15,780 --> 00:01:18,240 is that you want to make a material that essentially 30 00:01:18,240 --> 00:01:21,270 substitutes for the extracellular matrix. 31 00:01:21,270 --> 00:01:23,240 And it does so on a sort of temporary basis. 32 00:01:23,240 --> 00:01:25,430 So the idea is you put something in the body. 33 00:01:25,430 --> 00:01:28,930 The cells attach to that, whatever scaffold you put in. 34 00:01:28,930 --> 00:01:30,820 And the scaffold has to be made in such a way 35 00:01:30,820 --> 00:01:33,440 that the material-- that the cells, they 36 00:01:33,440 --> 00:01:34,460 can migrate through it. 37 00:01:34,460 --> 00:01:35,390 They can attach to it. 38 00:01:35,390 --> 00:01:36,450 They can differentiate. 39 00:01:36,450 --> 00:01:38,160 They can proliferate. 40 00:01:38,160 --> 00:01:40,420 So the cells can do all their normal function. 41 00:01:40,420 --> 00:01:42,040 And the idea is that as the cells are 42 00:01:42,040 --> 00:01:44,000 doing their normal function, they then 43 00:01:44,000 --> 00:01:46,710 secrete the natural extracellular matrix. 44 00:01:46,710 --> 00:01:49,640 And the engineered thing that you put in is resorbed. 45 00:01:49,640 --> 00:01:52,510 So there has to be a balance between the rate at which 46 00:01:52,510 --> 00:01:56,260 the scaffold you've made resorbs and the rate at which the cells 47 00:01:56,260 --> 00:02:00,510 are depositing the new native extracellular matrix. 48 00:02:00,510 --> 00:02:03,320 So that's one of the key things about this. 49 00:02:03,320 --> 00:02:07,680 So this is just an example of sort of a schematic 50 00:02:07,680 --> 00:02:09,460 of an extracellular matrix. 51 00:02:09,460 --> 00:02:12,330 In this case here, there's collagen fibers. 52 00:02:12,330 --> 00:02:15,030 So these guys here collagen fibers. 53 00:02:15,030 --> 00:02:18,780 And these kind of hairy-looking things are proteoglycans. 54 00:02:18,780 --> 00:02:21,550 So they have a core of protein with sugars kind 55 00:02:21,550 --> 00:02:22,880 of hanging off of them. 56 00:02:22,880 --> 00:02:26,400 And there's different kinds of GAGs, they're called, 57 00:02:26,400 --> 00:02:28,120 that hang off of them. 58 00:02:28,120 --> 00:02:29,850 So one's chondroitin sulfate. 59 00:02:29,850 --> 00:02:32,930 The CS here stands for Chondroitin Sulfate. 60 00:02:32,930 --> 00:02:36,326 There's one called dermatan sulfate. 61 00:02:36,326 --> 00:02:37,950 And there's one called heparin sulfate. 62 00:02:37,950 --> 00:02:41,410 So there's different of these glycosaminoglycans. 63 00:02:41,410 --> 00:02:43,400 So let me write down some of this, 64 00:02:43,400 --> 00:02:44,960 and then we'll kind of get into this. 65 00:02:44,960 --> 00:02:46,501 So what I wanted to do today was show 66 00:02:46,501 --> 00:02:50,090 you some examples of tissue engineering scaffolds. 67 00:02:50,090 --> 00:02:52,717 Show you some of the sort of design requirements. 68 00:02:52,717 --> 00:02:55,050 So you have to have, obviously, a material that's porous 69 00:02:55,050 --> 00:02:56,300 so the cells can get in there. 70 00:02:56,300 --> 00:02:59,822 And that's where the cellular solids comes in. 71 00:02:59,822 --> 00:03:01,280 So we'll talk about some scaffolds, 72 00:03:01,280 --> 00:03:03,800 some of the sort of design requirements for them. 73 00:03:03,800 --> 00:03:05,600 And also, we'll talk a little bit 74 00:03:05,600 --> 00:03:07,420 about processing of the scaffolds 75 00:03:07,420 --> 00:03:09,570 and mechanical properties of the scaffolds. 76 00:03:09,570 --> 00:03:12,260 So I'm hoping I can finish most of that today. 77 00:03:12,260 --> 00:03:14,220 And then next time, I have a little case study 78 00:03:14,220 --> 00:03:16,460 on osteochondral scaffolds. 79 00:03:16,460 --> 00:03:17,480 So osteo means bone. 80 00:03:17,480 --> 00:03:19,150 And the chondral means cartilage. 81 00:03:19,150 --> 00:03:21,070 So this was sort of a two-layer scaffold 82 00:03:21,070 --> 00:03:22,974 that we developed in collaboration 83 00:03:22,974 --> 00:03:25,390 with some other people at MIT and some people at Cambridge 84 00:03:25,390 --> 00:03:26,870 University. 85 00:03:26,870 --> 00:03:29,990 And it went from a research thing to a startup thing 86 00:03:29,990 --> 00:03:31,650 and being used clinically. 87 00:03:31,650 --> 00:03:34,640 So I was going to talk about that Wednesday. 88 00:03:34,640 --> 00:03:35,180 OK. 89 00:03:35,180 --> 00:03:38,346 So let me just get started here. 90 00:03:58,340 --> 00:04:02,670 So the goal of tissue engineering 91 00:04:02,670 --> 00:04:05,390 is to regenerate diseased or damaged tissues. 92 00:04:22,600 --> 00:04:24,530 And in the body, the cells attach 93 00:04:24,530 --> 00:04:26,570 to the extracellular matrix. 94 00:04:35,010 --> 00:04:36,585 And that's sometimes called the ECM. 95 00:04:41,720 --> 00:04:43,960 Sometimes, the scaffolds are called scaffolds. 96 00:04:43,960 --> 00:04:45,550 And sometimes, they're called matrix 97 00:04:45,550 --> 00:04:48,740 because the extracellular matrix is called matrix. 98 00:04:57,350 --> 00:05:12,190 So then, the composition of the ECM depends on the tissue, 99 00:05:12,190 --> 00:05:15,447 but it usually involves some sort of structural protein. 100 00:05:15,447 --> 00:05:17,030 So something like collagen or elastin. 101 00:05:32,300 --> 00:05:35,100 It also typically involves some sort adhesive proteins. 102 00:05:42,150 --> 00:05:45,823 So something like fibronectin or laminin. 103 00:05:51,000 --> 00:05:53,130 And it involves these proteoglycans, 104 00:05:53,130 --> 00:05:58,158 which are the core of protein with a sugar hanging off them. 105 00:05:58,158 --> 00:05:58,658 Whoops. 106 00:06:40,010 --> 00:06:43,020 And the sugars are typically glycosaminoglycans. 107 00:06:51,520 --> 00:06:55,650 And for short, people call them GAGs, just because it's easier 108 00:06:55,650 --> 00:06:56,201 to say. 109 00:06:59,530 --> 00:07:03,043 So some examples of the GAGs are chondroitin sulfate. 110 00:07:06,650 --> 00:07:08,900 And we're going to talk more about that a little later 111 00:07:08,900 --> 00:07:11,260 because that's one that we've used 112 00:07:11,260 --> 00:07:14,110 in making collagen-based scaffolds with [INAUDIBLE]. 113 00:07:37,250 --> 00:07:40,890 So for example, if you look at the composition 114 00:07:40,890 --> 00:07:44,960 of extracellular matrix in something like cartilage, 115 00:07:44,960 --> 00:07:51,760 it has a collagen component and a hyaluronic acid component 116 00:07:51,760 --> 00:07:52,630 and some GAGs. 117 00:08:06,630 --> 00:08:08,710 And this hyaluronic acid is a proteoglycan. 118 00:08:14,890 --> 00:08:19,608 If you look at bone, extracellular matrix in bone, 119 00:08:19,608 --> 00:08:21,733 it's made up mostly of collagen and hydroxyapatite. 120 00:08:32,610 --> 00:08:38,111 And if you look at skin, skin is made up of collagen, elastin, 121 00:08:38,111 --> 00:08:38,860 and proteoglycans. 122 00:08:50,342 --> 00:08:51,800 And the idea is that the cells have 123 00:08:51,800 --> 00:08:53,920 to be attached to this extracellular matrix 124 00:08:53,920 --> 00:08:55,610 in order to function. 125 00:08:55,610 --> 00:08:59,030 Or, they have to be attached to this or to other cells in most 126 00:08:59,030 --> 00:08:59,530 cases. 127 00:09:39,980 --> 00:09:43,100 So next week, I'm going to talk a bit about cell mechanics. 128 00:09:43,100 --> 00:09:45,480 And we have some video where we watch 129 00:09:45,480 --> 00:09:49,050 a cell deforming a scaffold. 130 00:09:49,050 --> 00:09:50,880 And I don't have videos to show you, 131 00:09:50,880 --> 00:09:53,540 but I had a student who took videos of cells migrating along 132 00:09:53,540 --> 00:09:56,300 with scaffolds. 133 00:09:56,300 --> 00:09:58,320 We'll look at how the stiffness of the scaffold 134 00:09:58,320 --> 00:10:00,361 affects how the cells migrate along the scaffold. 135 00:10:06,140 --> 00:10:09,910 So that's kind of the native ECM in the body. 136 00:10:09,910 --> 00:10:11,860 And the idea with tissue engineering 137 00:10:11,860 --> 00:10:14,290 is that you want to make a scaffold that's 138 00:10:14,290 --> 00:10:16,520 porous that mimics the extracellular 139 00:10:16,520 --> 00:10:17,720 matrix in the body. 140 00:10:17,720 --> 00:10:20,110 So, say, there's some tissue that's damaged. 141 00:10:20,110 --> 00:10:22,030 Say there's damaged cartilage. 142 00:10:22,030 --> 00:10:25,050 You want to provide sort of an extracellular matrix 143 00:10:25,050 --> 00:10:26,880 that's a sort of synthetic thing that's 144 00:10:26,880 --> 00:10:30,090 going to provide the same function as the ECM 145 00:10:30,090 --> 00:10:31,150 in the native tissue. 146 00:10:58,320 --> 00:11:01,000 So people have been working on scaffolds for regenerating 147 00:11:01,000 --> 00:11:02,410 all sorts of different tissues. 148 00:11:02,410 --> 00:11:04,190 And probably, the most successful one 149 00:11:04,190 --> 00:11:06,530 has been used to regenerate skin. 150 00:11:06,530 --> 00:11:08,030 And there's been scaffolds available 151 00:11:08,030 --> 00:11:11,550 for regenerating skin for probably almost 20 years now. 152 00:11:11,550 --> 00:11:13,230 And one of the first ones was developed 153 00:11:13,230 --> 00:11:15,150 by Professor Yannas in mechanical engineering 154 00:11:15,150 --> 00:11:15,960 here at MIT. 155 00:11:15,960 --> 00:11:19,200 And it's actually still sold by a company called Integra. 156 00:11:50,050 --> 00:11:51,950 But this research to develop scaffolds 157 00:11:51,950 --> 00:11:55,270 for lots of different tissues, orthopedic tissues, things 158 00:11:55,270 --> 00:11:59,480 like bone and cartilage, cardiovascular tissues, 159 00:11:59,480 --> 00:12:02,600 nerve-- like Professor Yannas works on peripheral nerve 160 00:12:02,600 --> 00:12:03,680 these days. 161 00:12:03,680 --> 00:12:05,820 People have looked at trying to make scaffolds 162 00:12:05,820 --> 00:12:08,430 for gastrointestinal tissues. 163 00:12:08,430 --> 00:12:10,040 So all sorts of different tissues. 164 00:12:10,040 --> 00:12:12,240 And at MIT, there's quite a lot of interest in this. 165 00:12:12,240 --> 00:12:13,320 There's a lot of people working on it. 166 00:12:13,320 --> 00:12:14,530 So Bob Langer works on it. 167 00:12:14,530 --> 00:12:15,570 Linda Griffith. 168 00:12:15,570 --> 00:12:17,070 Sangeeta Bhatia. 169 00:12:17,070 --> 00:12:19,963 There's really quite a number of people at MIT who work on it. 170 00:13:18,930 --> 00:13:21,200 Those are just some of the people at MIT. 171 00:13:21,200 --> 00:13:23,380 So the idea is in the body, the cells 172 00:13:23,380 --> 00:13:26,700 are constantly resorbing the extracellular matrix 173 00:13:26,700 --> 00:13:28,050 and depositing more. 174 00:13:28,050 --> 00:13:29,850 So if you think about bone, for example. 175 00:13:29,850 --> 00:13:32,760 Remember we said bone grows in response to load? 176 00:13:32,760 --> 00:13:36,290 Even in healthy bone, the bone is constantly being resorbed 177 00:13:36,290 --> 00:13:37,610 and deposited. 178 00:13:37,610 --> 00:13:40,170 And in normal bone, the rate of resorption 179 00:13:40,170 --> 00:13:42,874 and the rate of deposition is roughly the same. 180 00:13:42,874 --> 00:13:45,040 When people get osteoporosis, the thing that happens 181 00:13:45,040 --> 00:13:47,080 is that that balance gets out of whack. 182 00:13:47,080 --> 00:13:50,070 And so it's not being deposited at the same rate 183 00:13:50,070 --> 00:13:50,990 it's being resorbed. 184 00:14:10,190 --> 00:14:12,800 And the idea with the tissue engineering scaffolds 185 00:14:12,800 --> 00:14:15,460 is that they degrade over time. 186 00:14:15,460 --> 00:14:18,200 And that the cells that were attached to them 187 00:14:18,200 --> 00:14:20,440 are forming their own extracellular matrix. 188 00:14:20,440 --> 00:14:22,240 So there's kind of a balancing act 189 00:14:22,240 --> 00:14:25,784 between the cells depositing the native ECM 190 00:14:25,784 --> 00:14:27,450 and the tissue engineering scaffold that 191 00:14:27,450 --> 00:14:32,490 was provided, say, by the clinician being resorbed. 192 00:14:32,490 --> 00:14:35,170 So the scaffolds are actually designed to degrade. 193 00:14:35,170 --> 00:14:36,930 And controlling that degradation rate 194 00:14:36,930 --> 00:14:39,190 is one of the design parameters of the scaffolds. 195 00:15:34,280 --> 00:15:34,780 OK. 196 00:15:34,780 --> 00:15:38,470 So that's kind of the overall, kind of big picture. 197 00:15:38,470 --> 00:15:40,140 And what I wanted to talk about next 198 00:15:40,140 --> 00:15:42,896 was some design requirements for the scaffolds. 199 00:15:42,896 --> 00:15:44,270 So if you think about it, there's 200 00:15:44,270 --> 00:15:47,700 different sort of ways you can think about what 201 00:15:47,700 --> 00:15:48,620 the requirements are. 202 00:15:48,620 --> 00:15:50,867 So you have to make the scaffold out of some solid. 203 00:15:50,867 --> 00:15:52,700 And there's some requirements for the solid. 204 00:15:52,700 --> 00:15:55,100 So obviously, you want a solid that's biocompatible. 205 00:15:55,100 --> 00:15:57,750 That's one kind of main requirement. 206 00:15:57,750 --> 00:16:00,220 Another requirement is that not only the solid has 207 00:16:00,220 --> 00:16:04,220 to be biocompatible, but when the solid decomposes, 208 00:16:04,220 --> 00:16:06,740 if it decomposes into other components, 209 00:16:06,740 --> 00:16:08,390 they have to be biocompatible too. 210 00:16:08,390 --> 00:16:11,840 So you don't want the solid to degrade during this resorption 211 00:16:11,840 --> 00:16:13,760 process into toxic components. 212 00:16:13,760 --> 00:16:15,267 That would be a bad idea. 213 00:16:15,267 --> 00:16:16,850 And then, the other thing is the solid 214 00:16:16,850 --> 00:16:19,730 itself has to promote cell attachment, and cell 215 00:16:19,730 --> 00:16:22,630 proliferation, and cell migration, all 216 00:16:22,630 --> 00:16:24,090 these kinds of things. 217 00:16:24,090 --> 00:16:26,460 So we're going to talk about some different materials 218 00:16:26,460 --> 00:16:27,210 for the scaffolds. 219 00:16:27,210 --> 00:16:30,110 And some of them are sort of native proteins. 220 00:16:30,110 --> 00:16:32,530 So some of them are things like collagen. Collagen already 221 00:16:32,530 --> 00:16:35,110 has binding sites for cells to attach to it. 222 00:16:35,110 --> 00:16:39,050 Obviously, it's one of the proteins in the native ECM. 223 00:16:39,050 --> 00:16:41,849 There's also a number of synthetic polymers you can use. 224 00:16:41,849 --> 00:16:43,390 And with the synthetic polymers, they 225 00:16:43,390 --> 00:16:46,420 don't have natural binding sites for the cells. 226 00:16:46,420 --> 00:16:48,582 And so you have to coat them with something else. 227 00:16:48,582 --> 00:16:50,915 So you have to coat them with, say, adhesive proteins so 228 00:16:50,915 --> 00:16:53,150 that the cells will attach to them. 229 00:16:53,150 --> 00:16:56,776 So we're going to talk about the requirements for the solid. 230 00:16:56,776 --> 00:16:58,400 Then, you make the solid into some sort 231 00:16:58,400 --> 00:16:59,650 of porous, foamy thing. 232 00:16:59,650 --> 00:17:01,260 I think I have some slides here. 233 00:17:01,260 --> 00:17:04,430 Here's an example of a collagen GAG scaffold. 234 00:17:04,430 --> 00:17:08,200 And this is one of the ones that is made in Yannas' lab. 235 00:17:08,200 --> 00:17:10,119 And you can see it looks a lot like a foam. 236 00:17:10,119 --> 00:17:11,790 It's very, very porous. 237 00:17:11,790 --> 00:17:13,710 And there's some requirements for the sort 238 00:17:13,710 --> 00:17:16,800 of cellular structure, the foamy structure of the scaffold 239 00:17:16,800 --> 00:17:17,839 as well. 240 00:17:17,839 --> 00:17:20,819 So typically, you want interconnected pores 241 00:17:20,819 --> 00:17:23,849 so it's easy for the cells to migrate in. 242 00:17:23,849 --> 00:17:26,964 Typically, you want pores to be within a certain range. 243 00:17:26,964 --> 00:17:28,630 It turns out if the pores are too small, 244 00:17:28,630 --> 00:17:30,550 it makes it difficult for the cells to get in. 245 00:17:30,550 --> 00:17:33,530 Sometimes, they can by eating away at the material. 246 00:17:33,530 --> 00:17:36,550 But typically, the pores-- you want them to be bigger 247 00:17:36,550 --> 00:17:37,700 than a certain size. 248 00:17:37,700 --> 00:17:40,100 You also want them to be smaller than a certain size 249 00:17:40,100 --> 00:17:43,580 because how much specific surface area, how much surface 250 00:17:43,580 --> 00:17:46,840 area per unit volume you've got, depends on the pore size. 251 00:17:46,840 --> 00:17:49,170 The smaller the pores, the more surface area 252 00:17:49,170 --> 00:17:50,789 per unit volume you have. 253 00:17:50,789 --> 00:17:52,330 And then, the number of binding sites 254 00:17:52,330 --> 00:17:53,950 you have for cells to attach to it 255 00:17:53,950 --> 00:17:56,080 depends on that specific surface area. 256 00:17:56,080 --> 00:17:58,880 I'm going to write all this down, so I'll do that. 257 00:17:58,880 --> 00:18:02,344 So there's requirements for sort of the pore structure. 258 00:18:02,344 --> 00:18:04,010 And then, there's also some requirements 259 00:18:04,010 --> 00:18:05,325 for the whole scaffold itself. 260 00:18:05,325 --> 00:18:06,700 So for instance, it's got to have 261 00:18:06,700 --> 00:18:08,924 some minimal mechanical integrity. 262 00:18:08,924 --> 00:18:10,840 So there's sort of some requirements for that. 263 00:18:10,840 --> 00:18:12,820 So there's requirements for the solid. 264 00:18:12,820 --> 00:18:15,890 There's requirements for the sort of cellular structure, 265 00:18:15,890 --> 00:18:16,920 the porous structure. 266 00:18:16,920 --> 00:18:19,530 And there's requirements for the overall scaffold. 267 00:18:19,530 --> 00:18:21,973 So let me write some of these things down. 268 00:18:40,870 --> 00:18:42,710 So there's some requirements for the solid. 269 00:18:42,710 --> 00:18:43,876 So it must be biocompatible. 270 00:18:53,670 --> 00:18:57,284 It must also promote cell attachment and proliferation 271 00:18:57,284 --> 00:18:58,200 in the cell functions. 272 00:19:29,470 --> 00:19:31,620 And then it must degrade into nontoxic components. 273 00:20:22,540 --> 00:20:26,080 So there's some requirements for the cellular structure, too. 274 00:20:26,080 --> 00:20:28,690 And what you want to have is a large volume fraction 275 00:20:28,690 --> 00:20:30,250 of interconnected pores. 276 00:20:30,250 --> 00:20:33,080 And so you want that to facilitate the cell migration. 277 00:20:33,080 --> 00:20:35,785 And also, the transport of nutrients into the cells. 278 00:22:39,320 --> 00:22:44,762 So you also want the pore size to be within a critical range. 279 00:22:44,762 --> 00:22:46,470 So you need the pores bigger than a lower 280 00:22:46,470 --> 00:22:48,979 limit so the cells can migrate through easily, 281 00:22:48,979 --> 00:22:50,020 can kind of get in there. 282 00:22:50,020 --> 00:22:52,640 And you want the pore size to be less than an upper limit 283 00:22:52,640 --> 00:22:55,080 to have enough surface area to have enough binding sites 284 00:22:55,080 --> 00:22:56,644 to actually attach cells. 285 00:22:56,644 --> 00:22:58,060 And for different tissues, there's 286 00:22:58,060 --> 00:22:59,980 different critical ranges of the pore size. 287 00:23:58,930 --> 00:24:02,620 So for example, for skin they found 288 00:24:02,620 --> 00:24:07,650 that you want to have a pore size between about 20 microns 289 00:24:07,650 --> 00:24:10,170 and about 150 microns. 290 00:24:12,690 --> 00:24:18,375 And for bone, the pore sizes that people tend to use 291 00:24:18,375 --> 00:24:22,075 are between about 100 and 500 microns. 292 00:24:25,140 --> 00:24:26,900 So there's the pore size. 293 00:24:26,900 --> 00:24:29,500 And one other feature is that the pore geometry should 294 00:24:29,500 --> 00:24:31,870 be conducive for the cell type. 295 00:24:31,870 --> 00:24:35,190 So lots of cells are somewhat equiaxed. 296 00:24:35,190 --> 00:24:36,814 Maybe a little elongated. 297 00:24:36,814 --> 00:24:38,730 But if you look at something like nerve cells, 298 00:24:38,730 --> 00:24:40,896 like peripheral nerve, they're incredibly elongated. 299 00:24:40,896 --> 00:24:43,380 So you want to have pores in the scaffold that 300 00:24:43,380 --> 00:24:44,580 are also very elongated. 301 00:25:33,680 --> 00:25:35,650 And then for the overall scaffold, 302 00:25:35,650 --> 00:25:38,320 it needs to have some mechanical integrity. 303 00:25:38,320 --> 00:25:39,396 You guys OK? 304 00:25:39,396 --> 00:25:39,895 Yeah? 305 00:25:39,895 --> 00:25:42,380 We're good. 306 00:25:42,380 --> 00:25:43,090 Oh, achy. 307 00:25:43,090 --> 00:25:43,950 Yeah. 308 00:25:43,950 --> 00:25:47,300 I know. 309 00:25:47,300 --> 00:25:49,600 So you want to have some overall mechanical integrity. 310 00:25:49,600 --> 00:25:52,090 I mean, the thing has to be put into the body in surgery. 311 00:25:52,090 --> 00:25:53,610 And people are going to be pushing and poking at it. 312 00:25:53,610 --> 00:25:56,500 And so it has to have some just overall mechanical integrity. 313 00:25:56,500 --> 00:26:01,480 Also, it turns out that if you put stem cells into scaffolds, 314 00:26:01,480 --> 00:26:03,259 the types of cells they differentiate into 315 00:26:03,259 --> 00:26:05,300 depends in part on the stiffness of the scaffold. 316 00:26:05,300 --> 00:26:06,675 So you want to be able to control 317 00:26:06,675 --> 00:26:07,940 the stiffness of the scaffold. 318 00:26:07,940 --> 00:26:09,565 AUDIENCE: How do they do this research? 319 00:26:09,565 --> 00:26:11,320 Do they use animals? 320 00:26:11,320 --> 00:26:12,439 Or they do it in vitro? 321 00:26:12,439 --> 00:26:13,230 LORNA GIBSON: Yeah. 322 00:26:13,230 --> 00:26:15,350 So the question is, how do they do the research? 323 00:26:15,350 --> 00:26:18,240 So they do a sort of series of different things. 324 00:26:18,240 --> 00:26:21,210 So at one level, you could have the scaffold 325 00:26:21,210 --> 00:26:22,700 and you'd put cells on it. 326 00:26:22,700 --> 00:26:24,930 Say you're making a bone scaffold. 327 00:26:24,930 --> 00:26:26,430 You'd put osteocytes onto it. 328 00:26:26,430 --> 00:26:28,890 So one level, you just put cells onto it. 329 00:26:28,890 --> 00:26:31,950 And you want to see, are the cells attaching? 330 00:26:31,950 --> 00:26:33,220 Are they dying? 331 00:26:33,220 --> 00:26:34,230 Are they proliferating? 332 00:26:34,230 --> 00:26:36,060 So sometimes, what people will do 333 00:26:36,060 --> 00:26:38,630 is put the-- they'll seed a certain number of cells 334 00:26:38,630 --> 00:26:39,380 at a certain time. 335 00:26:39,380 --> 00:26:40,730 Say, time 0. 336 00:26:40,730 --> 00:26:44,040 Then, they'll look at how many cells are attached 337 00:26:44,040 --> 00:26:45,930 at 24 hours or 48 hours. 338 00:26:45,930 --> 00:26:48,640 And you kind of see the cell attachment. 339 00:26:48,640 --> 00:26:51,610 You can measure relatively easily. 340 00:26:51,610 --> 00:26:54,130 Another thing people do is animal studies. 341 00:26:54,130 --> 00:26:56,650 So for instance, Yannas does research 342 00:26:56,650 --> 00:26:58,390 on peripheral nerves and scaffolds 343 00:26:58,390 --> 00:26:59,670 for peripheral nerves. 344 00:26:59,670 --> 00:27:02,920 And they cut a piece out of the sciatic nerve of rats. 345 00:27:02,920 --> 00:27:04,690 So obviously, they have a surgeon 346 00:27:04,690 --> 00:27:06,200 and do a surgery thing with it. 347 00:27:06,200 --> 00:27:07,950 You can't just kind of do this in the lab. 348 00:27:07,950 --> 00:27:10,830 You've got to get permissions and stuff to do it. 349 00:27:10,830 --> 00:27:12,837 And then, they put in the scaffold. 350 00:27:12,837 --> 00:27:14,420 And the scaffold's actually in a tube. 351 00:27:14,420 --> 00:27:16,474 And so they put the two stumps of the nerve end 352 00:27:16,474 --> 00:27:18,390 at either end of the tube, and then the tube's 353 00:27:18,390 --> 00:27:20,080 filled with a sort of porous scaffold 354 00:27:20,080 --> 00:27:21,710 that we're talking about here. 355 00:27:21,710 --> 00:27:24,820 And then, they wait some period of time. 356 00:27:24,820 --> 00:27:29,480 And they take video of rat running, things like that. 357 00:27:29,480 --> 00:27:32,270 They then sacrifice the rat and they do histology. 358 00:27:32,270 --> 00:27:34,220 And they look at the sort of cross-sections 359 00:27:34,220 --> 00:27:37,300 and see what it looks like. 360 00:27:37,300 --> 00:27:39,140 And so I'm going to talk next time 361 00:27:39,140 --> 00:27:41,890 about this osteochondral scaffold we worked on. 362 00:27:41,890 --> 00:27:43,740 So we did cell studies. 363 00:27:43,740 --> 00:27:44,850 We did goat studies. 364 00:27:44,850 --> 00:27:46,540 We put into goat knees. 365 00:27:46,540 --> 00:27:48,700 There was a longer term sheep study. 366 00:27:48,700 --> 00:27:51,350 And then, the student who's in Cambridge, England, 367 00:27:51,350 --> 00:27:54,910 started up a company and he ended up 368 00:27:54,910 --> 00:27:58,090 getting approval in Europe to start clinical trials. 369 00:27:58,090 --> 00:28:00,280 And then he worked with an orthopedic surgeon who 370 00:28:00,280 --> 00:28:01,840 started putting it in people. 371 00:28:01,840 --> 00:28:04,470 But typically, they're looking at cells, looking at animals 372 00:28:04,470 --> 00:28:07,650 before you get to the people stage. 373 00:28:07,650 --> 00:28:09,400 And one of the things people do when 374 00:28:09,400 --> 00:28:11,627 they're making these scaffolds is 375 00:28:11,627 --> 00:28:13,210 you want to use materials that already 376 00:28:13,210 --> 00:28:15,560 have some sort of regulatory approval. 377 00:28:15,560 --> 00:28:18,094 So say FDA approval or approval in Europe. 378 00:28:18,094 --> 00:28:20,510 So typically, people don't start with a brand new material 379 00:28:20,510 --> 00:28:21,150 from scratch. 380 00:28:21,150 --> 00:28:22,941 Because to get approval for that would just 381 00:28:22,941 --> 00:28:25,340 take a very long time. 382 00:28:25,340 --> 00:28:28,310 So typically, people start with-- the solid material 383 00:28:28,310 --> 00:28:32,116 is already approved for some other sort of use. 384 00:28:32,116 --> 00:28:33,490 OK. 385 00:28:33,490 --> 00:28:35,690 So one requirement for the overall scaffold 386 00:28:35,690 --> 00:28:40,711 is it has to have sufficient mechanical integrity. 387 00:28:40,711 --> 00:28:41,210 Sufficient. 388 00:29:08,810 --> 00:29:11,770 And then also, as I mentioned, the stiffness of the scaffold 389 00:29:11,770 --> 00:29:14,060 can affect differentiation of cells. 390 00:29:24,540 --> 00:29:26,780 And the other thing that is really 391 00:29:26,780 --> 00:29:28,460 a factor for the overall scaffold 392 00:29:28,460 --> 00:29:30,600 is you want to control the rate of degradation 393 00:29:30,600 --> 00:29:31,700 of the overall scaffold. 394 00:29:31,700 --> 00:29:34,060 So you want that rate to be matched 395 00:29:34,060 --> 00:29:37,460 to the rate at which the new tissue is forming. 396 00:29:37,460 --> 00:29:39,750 So it has to degrade at a controllable rate. 397 00:30:44,381 --> 00:30:44,880 OK. 398 00:31:26,310 --> 00:31:29,160 So I want to talk about the materials that people use. 399 00:31:29,160 --> 00:31:31,800 And you can kind of break them down into a few classes. 400 00:31:31,800 --> 00:31:34,190 So one class is natural polymers. 401 00:31:34,190 --> 00:31:35,600 So things like collage. 402 00:31:35,600 --> 00:31:38,140 So you can get collagen. And that's an example up 403 00:31:38,140 --> 00:31:40,620 there of a scaffold that's made with collagen. 404 00:31:40,620 --> 00:31:44,200 Another class of materials is synthetic biomaterials. 405 00:31:44,200 --> 00:31:47,114 And if you've had stitches or surgery, 406 00:31:47,114 --> 00:31:49,030 you may know that some of the sutures they use 407 00:31:49,030 --> 00:31:49,812 are resorbable. 408 00:31:49,812 --> 00:31:51,270 So some of those polymers that they 409 00:31:51,270 --> 00:31:55,200 use for resorbable sutures are also used for tissue 410 00:31:55,200 --> 00:31:56,610 engineering scaffold. 411 00:31:56,610 --> 00:31:59,610 And then, there's also hydrogels that people use as well. 412 00:31:59,610 --> 00:32:01,580 So those are probably the three main groups 413 00:32:01,580 --> 00:32:06,250 are sort of natural polymers, synthetic biopolymers, 414 00:32:06,250 --> 00:32:07,850 and I guess the hydrogels are sort 415 00:32:07,850 --> 00:32:09,750 of a subset of the sympathetic biopolymers. 416 00:32:18,570 --> 00:32:23,930 So collagen is probably the most common kind of natural polymer 417 00:32:23,930 --> 00:32:25,500 that's used. 418 00:32:25,500 --> 00:32:27,200 They also use GAGs. 419 00:32:27,200 --> 00:32:29,900 And this scaffold up here is made 420 00:32:29,900 --> 00:32:33,070 by making a coprecipitative collagen with a GAG chondroitin 421 00:32:33,070 --> 00:32:34,280 sulfate. 422 00:32:34,280 --> 00:32:35,460 People also use alginate. 423 00:32:38,860 --> 00:32:40,664 I think one of the project groups-- 424 00:32:40,664 --> 00:32:43,080 you guys are going to make some sort of alginate scaffold, 425 00:32:43,080 --> 00:32:43,910 right? 426 00:32:43,910 --> 00:32:44,430 No? 427 00:32:44,430 --> 00:32:45,500 [INAUDIBLE] foamy thing? 428 00:32:45,500 --> 00:32:46,800 Yeah. 429 00:32:46,800 --> 00:32:50,730 And people also use something called chitosan, 430 00:32:50,730 --> 00:32:52,840 which is a derivative of chiton, which 431 00:32:52,840 --> 00:32:56,360 is what's in the exoskeleton of insects and things 432 00:32:56,360 --> 00:32:57,370 like lobsters. 433 00:32:57,370 --> 00:33:00,690 So those would be all examples of natural polymers 434 00:33:00,690 --> 00:33:03,247 that can be used and people have tried. 435 00:33:03,247 --> 00:33:05,080 I'm going to talk a bit more about collagen, 436 00:33:05,080 --> 00:33:07,180 just because it's the most common one. 437 00:33:07,180 --> 00:33:10,390 So collagen is a major component in the natural extracellular 438 00:33:10,390 --> 00:33:11,520 matrix. 439 00:33:11,520 --> 00:33:14,830 And not surprisingly, it has binding sites for cells 440 00:33:14,830 --> 00:33:15,660 to attach to it. 441 00:33:15,660 --> 00:33:18,260 So if you use that, that kind of takes care of that issue. 442 00:33:21,591 --> 00:33:22,590 Let me put it down here. 443 00:33:38,780 --> 00:33:41,730 So collagen exists in many types of tissues. 444 00:33:41,730 --> 00:33:43,460 Exists in skin. 445 00:33:43,460 --> 00:33:48,310 Exists in bone, cartilage, ligament, tendon-- cartilage. 446 00:33:56,420 --> 00:33:58,260 So it's very common. 447 00:33:58,260 --> 00:34:03,470 It has surface binding sites for cells. 448 00:34:32,360 --> 00:34:34,630 It has a relatively low Young's modulus. 449 00:34:39,550 --> 00:34:43,110 So the Young's modulus is a little less than a gigapascal. 450 00:34:49,090 --> 00:34:52,000 But you can increase the modulus by either cross-linking 451 00:34:52,000 --> 00:34:55,363 or by using it in conjunction with some synthetic polymers. 452 00:35:21,770 --> 00:35:23,780 And I'm going to talk a little bit about how you 453 00:35:23,780 --> 00:35:25,890 make these scaffolds up here. 454 00:35:25,890 --> 00:35:28,680 And the first step in making those scaffolds 455 00:35:28,680 --> 00:35:30,760 is you put the collagen in acetic acid, 456 00:35:30,760 --> 00:35:32,610 and then you add the glycosaminoglycan 457 00:35:32,610 --> 00:35:34,430 and it forms a coprecipitate. 458 00:35:34,430 --> 00:35:36,620 And the fact that it forms a coprecipitate 459 00:35:36,620 --> 00:35:39,912 with the glycosaminoglycan, the GAG, 460 00:35:39,912 --> 00:35:41,870 means that you can use a freeze-drying process. 461 00:35:41,870 --> 00:35:42,994 And that's how that's made. 462 00:36:15,330 --> 00:36:17,520 Collagen is one option. 463 00:36:17,520 --> 00:36:20,523 So then synthetic biopolymers is another option. 464 00:36:26,960 --> 00:36:29,070 And as I said, typically they use the materials 465 00:36:29,070 --> 00:36:31,030 that are used for resorbable sutures. 466 00:36:48,920 --> 00:36:50,350 So there's several of those. 467 00:36:50,350 --> 00:36:53,490 There's something called PGA. 468 00:36:53,490 --> 00:36:55,160 That's polyglycolic acid. 469 00:37:02,400 --> 00:37:04,490 And something called PLA. 470 00:37:04,490 --> 00:37:05,835 That's polylactic acid. 471 00:37:12,090 --> 00:37:14,660 And then you can combine those two and make something 472 00:37:14,660 --> 00:37:18,380 called PLGA polylactic co-glycolic acid. 473 00:37:32,700 --> 00:37:35,174 And you can control the degradation rate 474 00:37:35,174 --> 00:37:37,340 of these things by controlling the molecular weight. 475 00:37:37,340 --> 00:37:39,830 And in this case, you can also control it 476 00:37:39,830 --> 00:37:42,510 by controlling how much of each of those things you put in. 477 00:38:10,731 --> 00:38:12,730 And there's another one called polycaprolactone. 478 00:38:24,050 --> 00:38:28,490 So those are several synthetic biopolymers that people use. 479 00:38:28,490 --> 00:38:29,990 There's lots of different materials, 480 00:38:29,990 --> 00:38:31,531 but these are just some typical ones. 481 00:39:07,010 --> 00:39:09,130 And then another class are hydrogels, 482 00:39:09,130 --> 00:39:11,020 which are produced by cross-linking water 483 00:39:11,020 --> 00:39:13,857 soluble polymers to form an insoluble network. 484 00:39:45,190 --> 00:39:47,372 And those are typically used for soft tissues. 485 00:39:53,670 --> 00:39:56,940 Sometimes, they're used for things like cartilage. 486 00:39:56,940 --> 00:39:59,642 And again, there's a few different materials 487 00:39:59,642 --> 00:40:00,600 that are commonly used. 488 00:40:00,600 --> 00:40:02,586 One's PEG, Polyethylene Glycol. 489 00:40:11,600 --> 00:40:15,565 One's PVA, Polyvinyl Alcohol. 490 00:40:22,760 --> 00:40:27,130 And another one's PAA, Polyacrylic Acid. 491 00:40:35,240 --> 00:40:37,080 So for these synthetic polymers, there's 492 00:40:37,080 --> 00:40:39,179 many different processing techniques available. 493 00:40:39,179 --> 00:40:40,720 And I'll talk a little bit about some 494 00:40:40,720 --> 00:40:41,940 of the processing techniques. 495 00:40:41,940 --> 00:40:44,370 But one of the limitations is they don't 496 00:40:44,370 --> 00:40:45,820 have natural binding sites. 497 00:40:45,820 --> 00:40:50,290 And you have to coat them with some sort of binding agent, 498 00:40:50,290 --> 00:40:53,080 like an adhesive protein, to get the cells to attach to them. 499 00:41:31,650 --> 00:41:33,900 And then, as I mentioned before, you have to make sure 500 00:41:33,900 --> 00:41:35,316 that whatever material you choose, 501 00:41:35,316 --> 00:41:37,490 if it's a synthetic material that when it degrades, 502 00:41:37,490 --> 00:41:39,950 it's not toxic to the cells. 503 00:41:39,950 --> 00:41:43,330 Because you don't want to have some sort of toxic reaction 504 00:41:43,330 --> 00:41:46,200 or inflammation. 505 00:41:46,200 --> 00:41:46,940 OK. 506 00:41:46,940 --> 00:41:50,670 So there's a couple more things about materials. 507 00:41:50,670 --> 00:41:53,780 So those are all polymer-based materials. 508 00:41:53,780 --> 00:41:56,770 When people are trying to make scaffolds for bone tissue 509 00:41:56,770 --> 00:41:59,917 engineering, they also include a calcium phosphate mineral. 510 00:41:59,917 --> 00:42:02,250 And there's different versions of the calcium phosphate. 511 00:42:02,250 --> 00:42:04,250 So they can include-- you can buy, for instance, 512 00:42:04,250 --> 00:42:06,630 hydroxy powders now. 513 00:42:06,630 --> 00:42:08,230 There's another calcium phosphate 514 00:42:08,230 --> 00:42:12,040 called octacalcium phosphate, which will, with water, turn 515 00:42:12,040 --> 00:42:13,710 into hydroxyapatite. 516 00:42:13,710 --> 00:42:16,410 So typically, there is this mineral, some sort of calcium 517 00:42:16,410 --> 00:42:18,239 phosphate, is combined with either collagen 518 00:42:18,239 --> 00:42:20,030 or with one of these synthetic biopolymers. 519 00:43:29,630 --> 00:43:31,240 And one other option is something 520 00:43:31,240 --> 00:43:33,450 called an acellular scaffold. 521 00:43:33,450 --> 00:43:37,120 And what that is they take some natural tissue 522 00:43:37,120 --> 00:43:39,082 and they remove all the cell material from it. 523 00:43:39,082 --> 00:43:41,540 And so when they remove all the cell material, what they're 524 00:43:41,540 --> 00:43:43,510 left with is the native ECM. 525 00:43:43,510 --> 00:43:47,280 And that's called an acellular scaffold. 526 00:43:47,280 --> 00:43:52,960 So it's a native ECM with all the cell matter removed. 527 00:44:02,660 --> 00:44:04,900 And they remove the cells by-- they 528 00:44:04,900 --> 00:44:09,340 can use sort of a physical agitation or chemical, 529 00:44:09,340 --> 00:44:11,034 or using enzymatic methods. 530 00:44:11,034 --> 00:44:13,200 Using something like trypsin to get rid of the cell. 531 00:44:13,200 --> 00:44:16,321 So there's ways that they can get rid of the cells. 532 00:44:16,321 --> 00:44:16,820 OK. 533 00:44:16,820 --> 00:44:18,280 So are we good so far? 534 00:44:18,280 --> 00:44:20,660 So there are some requirements for what materials we 535 00:44:20,660 --> 00:44:22,670 kind of use, what the cell structure should be, 536 00:44:22,670 --> 00:44:25,242 and these are some examples of typical materials. 537 00:44:25,242 --> 00:44:27,450 So I wanted to talk a little bit about the processing 538 00:44:27,450 --> 00:44:30,599 of the materials. 539 00:44:30,599 --> 00:44:32,390 Let me wait until people catch up a little. 540 00:44:39,942 --> 00:44:43,760 Oh, and I have some scaffolds I was going to pass around. 541 00:44:43,760 --> 00:44:48,210 So this big sheet is a piece of the collagen GAG scaffold 542 00:44:48,210 --> 00:44:50,530 that I showed a minute ago. 543 00:44:50,530 --> 00:44:53,230 And then this little piece is a mineralized version of that. 544 00:44:53,230 --> 00:44:56,960 So this has the collagen plus calcium phosphate 545 00:44:56,960 --> 00:44:58,370 plus hydroxyapatite in it. 546 00:45:03,955 --> 00:45:04,650 OK. 547 00:45:04,650 --> 00:45:08,100 So this slide shows some examples of different scaffolds 548 00:45:08,100 --> 00:45:09,201 that people have made. 549 00:45:09,201 --> 00:45:10,700 And I was going to talk a little bit 550 00:45:10,700 --> 00:45:11,867 about some of these methods. 551 00:45:11,867 --> 00:45:13,783 And why don't I talk about them, and then I'll 552 00:45:13,783 --> 00:45:15,120 write some notes on the board. 553 00:45:15,120 --> 00:45:18,730 So this top one here on the top left, that's the collagen GAG 554 00:45:18,730 --> 00:45:20,750 scaffold that's made in Yannas' group. 555 00:45:20,750 --> 00:45:23,130 And that's made by a freeze-drying process. 556 00:45:23,130 --> 00:45:25,280 So you put the collagen in acetic acid, 557 00:45:25,280 --> 00:45:26,630 then you put in the GAG. 558 00:45:26,630 --> 00:45:29,300 The GAG and the collagen form a coprecipitate. 559 00:45:29,300 --> 00:45:30,774 And then you can freeze that. 560 00:45:30,774 --> 00:45:32,190 And if you freeze it, what happens 561 00:45:32,190 --> 00:45:34,880 is-- it's just like if you freeze saltwater. 562 00:45:34,880 --> 00:45:37,525 The water freezes. 563 00:45:37,525 --> 00:45:39,320 The pure water freezes. 564 00:45:39,320 --> 00:45:42,630 And you've got increasingly higher brine content 565 00:45:42,630 --> 00:45:45,910 in the bit in between the water grains, or in between the ice. 566 00:45:45,910 --> 00:45:49,160 So you get the sort of solid ice forming. 567 00:45:49,160 --> 00:45:51,250 And the collagen and the GAG are kind of 568 00:45:51,250 --> 00:45:54,677 squeezed into the interstitial bits between the ice crystals. 569 00:45:54,677 --> 00:45:56,260 And then if you sublimate the ice off, 570 00:45:56,260 --> 00:45:58,660 you're left with this porous kind of structure 571 00:45:58,660 --> 00:46:00,150 that looks like a foam. 572 00:46:00,150 --> 00:46:02,680 So that's made by a freeze-drying process. 573 00:46:02,680 --> 00:46:05,150 And I'll go over it in a bit more detail when 574 00:46:05,150 --> 00:46:06,820 I write the notes on the board. 575 00:46:06,820 --> 00:46:09,050 You could also foam some of these polymers. 576 00:46:09,050 --> 00:46:10,190 So just blowing a gas. 577 00:46:10,190 --> 00:46:13,737 The same way you can blow a gas through engineering foam. 578 00:46:13,737 --> 00:46:15,820 You do the same thing with some of these polymers. 579 00:46:15,820 --> 00:46:17,520 This one's made by foaming. 580 00:46:17,520 --> 00:46:19,640 You can have a fugitive phase process. 581 00:46:19,640 --> 00:46:22,310 So this is made by salt leaching, the second row 582 00:46:22,310 --> 00:46:23,332 on the left there. 583 00:46:23,332 --> 00:46:25,540 So you could imagine you could take a polymer powder. 584 00:46:25,540 --> 00:46:27,331 You could mix it with salt. You can sort of 585 00:46:27,331 --> 00:46:28,910 mix them up, combine them together. 586 00:46:28,910 --> 00:46:31,320 You heat it up to get the polymer to melt and to sort 587 00:46:31,320 --> 00:46:34,262 of form a connected mass. 588 00:46:34,262 --> 00:46:35,970 And then you leech out the salt. And then 589 00:46:35,970 --> 00:46:37,840 you get pores where the salt was. 590 00:46:37,840 --> 00:46:40,670 This one here is made by an electrospinning process. 591 00:46:40,670 --> 00:46:41,620 So you have a nozzle. 592 00:46:41,620 --> 00:46:44,809 You feed the polymer through the nozzle. 593 00:46:44,809 --> 00:46:46,350 Then you have plates that are charged 594 00:46:46,350 --> 00:46:49,650 and you get fibers forming and kind of scattered 595 00:46:49,650 --> 00:46:52,720 in different directions by the electrospinning process. 596 00:46:52,720 --> 00:46:56,930 Then, this one here represents scaffolds 597 00:46:56,930 --> 00:47:02,980 that are made by things like 3D printing, selective laser 598 00:47:02,980 --> 00:47:03,560 centering. 599 00:47:03,560 --> 00:47:07,720 You can have laser-sensitive polymer. 600 00:47:07,720 --> 00:47:10,610 And you can produce scaffolds that way. 601 00:47:10,610 --> 00:47:14,280 I think the geometry of this one matches some part in the body. 602 00:47:14,280 --> 00:47:16,750 I think it was a knee or something like that. 603 00:47:16,750 --> 00:47:18,510 And then, these two examples down here 604 00:47:18,510 --> 00:47:19,950 are the acellular scaffolds. 605 00:47:19,950 --> 00:47:22,200 That's what I was talking about at the very end there. 606 00:47:22,200 --> 00:47:25,250 So those are from porcine pork heart tissue. 607 00:47:25,250 --> 00:47:26,980 You know, pig heart tissue. 608 00:47:26,980 --> 00:47:29,480 And those are mostly elastin. 609 00:47:29,480 --> 00:47:32,330 And they've had all the cell matter removed from them. 610 00:47:32,330 --> 00:47:32,950 OK. 611 00:47:32,950 --> 00:47:35,990 So you can kind of see that these synthetic scaffolds here 612 00:47:35,990 --> 00:47:39,980 have a structure that's not so different from these native ECM 613 00:47:39,980 --> 00:47:41,810 scaffolds down here. 614 00:47:41,810 --> 00:47:42,690 OK. 615 00:47:42,690 --> 00:47:46,300 So let me write some of the things about the processing 616 00:47:46,300 --> 00:47:48,996 on the board. 617 00:47:48,996 --> 00:47:50,630 Let me rub this off. 618 00:47:50,630 --> 00:47:51,720 Start over here. 619 00:48:35,380 --> 00:48:41,515 So these freeze-dried scaffolds are used for skin regeneration. 620 00:49:03,170 --> 00:49:05,620 And I think I have some more slides here. 621 00:49:05,620 --> 00:49:07,610 So it's kind of a two-step process. 622 00:49:07,610 --> 00:49:10,700 In the first step, you make what they call a slurry. 623 00:49:10,700 --> 00:49:13,710 So you make the slurry by taking the collagen. 624 00:49:13,710 --> 00:49:17,570 And for skin, I think you want type 1 collagen. Yeah, skin 625 00:49:17,570 --> 00:49:19,790 is type 1 collagen. There's different types. 626 00:49:19,790 --> 00:49:23,580 You put it in acetic acid, and then you add the GAG. 627 00:49:23,580 --> 00:49:26,690 And we use chondroitin 6-sulfate is just the particular GAG 628 00:49:26,690 --> 00:49:27,820 that we use. 629 00:49:27,820 --> 00:49:29,870 And one of the things that the acid does 630 00:49:29,870 --> 00:49:32,430 is that it swells the collagen. And collagen 631 00:49:32,430 --> 00:49:34,220 has a sort of periodic structure in it, 632 00:49:34,220 --> 00:49:35,930 sort of periodic banding. 633 00:49:35,930 --> 00:49:38,880 And the acid destroys that periodic banding structure. 634 00:49:38,880 --> 00:49:42,820 And that helps increase the resistance 635 00:49:42,820 --> 00:49:45,100 to having some host immune response. 636 00:49:45,100 --> 00:49:48,092 So that you remove the immunological markers 637 00:49:48,092 --> 00:49:50,300 and it makes it less likely that the scaffold's going 638 00:49:50,300 --> 00:49:53,570 to get rejected by the body. 639 00:49:53,570 --> 00:49:57,000 So then when you put the GAG in, you form a coprecipitate. 640 00:49:57,000 --> 00:49:59,190 So this next step just shows kind 641 00:49:59,190 --> 00:50:00,980 of mixing the whole thing up. 642 00:50:00,980 --> 00:50:03,880 And then you've got kind of a little slurry 643 00:50:03,880 --> 00:50:06,120 that you can store. 644 00:50:06,120 --> 00:50:08,320 And then, this is the freeze-drying step here. 645 00:50:08,320 --> 00:50:11,170 So you put the slurry, the suspension, into a pan. 646 00:50:11,170 --> 00:50:13,902 Kind of just like a cookie sheet, really. 647 00:50:13,902 --> 00:50:14,860 And then you freeze it. 648 00:50:14,860 --> 00:50:16,770 So if you think of this phase diagram 649 00:50:16,770 --> 00:50:19,070 here, where you have temperature and pressure. 650 00:50:19,070 --> 00:50:21,290 So here we have liquid, solid, and vapor. 651 00:50:21,290 --> 00:50:24,240 So if you start off at this point here, you freeze it. 652 00:50:24,240 --> 00:50:26,920 So you've reduced the temperature. 653 00:50:26,920 --> 00:50:28,640 So that forms the ice. 654 00:50:28,640 --> 00:50:32,170 And the ice is surrounded by the collagen and the GAG fibers. 655 00:50:32,170 --> 00:50:34,002 And then if you do the sublimation step, 656 00:50:34,002 --> 00:50:36,460 you reduce the pressure and increase the temperature a bit. 657 00:50:36,460 --> 00:50:38,660 And then you get over to the vapor end of the world. 658 00:50:38,660 --> 00:50:42,586 And then you're left with this porous scaffold. 659 00:50:42,586 --> 00:50:43,460 And then, let me see. 660 00:50:43,460 --> 00:50:45,790 Let me do one more step here. 661 00:50:45,790 --> 00:50:48,290 And then you can control the size of the pores 662 00:50:48,290 --> 00:50:50,380 by controlling the freezing temperature. 663 00:50:50,380 --> 00:50:53,410 So the size of the pores is exactly 664 00:50:53,410 --> 00:50:56,160 related to the size of the ice grains that are forming. 665 00:50:56,160 --> 00:50:58,720 And the faster it freezes, the smaller the grains 666 00:50:58,720 --> 00:50:59,660 are going to be. 667 00:50:59,660 --> 00:51:02,140 And then, the smaller your pore size are going to be. 668 00:51:02,140 --> 00:51:03,455 So you can control that. 669 00:51:11,210 --> 00:51:13,920 The type 1 collagen is mixed with acetic acid. 670 00:51:22,410 --> 00:51:30,070 And it then swells the collagen and disrupts periodic banding. 671 00:51:38,025 --> 00:51:40,890 And it removes immunological markers. 672 00:52:16,870 --> 00:52:19,400 And then you add the GAG, the chondroitin 6-sulfate. 673 00:52:34,370 --> 00:52:37,427 And then that cross links with the collagen 674 00:52:37,427 --> 00:52:38,510 and forms a coprecipitate. 675 00:52:52,230 --> 00:52:54,877 And then you can freeze dry that to get the porous scaffold. 676 00:53:06,930 --> 00:53:09,640 And typically, the relative densities of these scaffolds 677 00:53:09,640 --> 00:53:11,930 is very low. 678 00:53:11,930 --> 00:53:14,555 So typically, they're 0.5% dense. 679 00:53:14,555 --> 00:53:16,640 The relative density is 0.005. 680 00:53:16,640 --> 00:53:19,560 So they're 99.5% air and the rest of it's 681 00:53:19,560 --> 00:53:21,690 the collagen and the GAG. 682 00:53:21,690 --> 00:53:27,096 And the pore sizes are typically between about 100 and 150 683 00:53:27,096 --> 00:53:27,595 microns. 684 00:53:30,930 --> 00:53:33,000 And Yannas uses the same scaffolds 685 00:53:33,000 --> 00:53:34,250 for the nerve regeneration. 686 00:53:34,250 --> 00:53:35,880 And he uses a directional cooling. 687 00:53:35,880 --> 00:53:37,960 And that then elongates the pores, 688 00:53:37,960 --> 00:53:40,830 so that-- the idea is that they elongate so the nerves kind 689 00:53:40,830 --> 00:53:42,070 of grow along that length. 690 00:54:05,830 --> 00:54:06,870 So that's one way. 691 00:54:06,870 --> 00:54:12,230 Another way is leaching a fugitive phase. 692 00:54:21,890 --> 00:54:22,440 So let's see. 693 00:54:22,440 --> 00:54:23,961 I think-- yep. 694 00:54:23,961 --> 00:54:24,460 Here we go. 695 00:54:24,460 --> 00:54:25,760 Back to there. 696 00:54:25,760 --> 00:54:30,030 So if you look at the one on the second row on the left, 697 00:54:30,030 --> 00:54:32,490 that's done by using salt as the fugitive phase. 698 00:54:32,490 --> 00:54:33,490 People use other things. 699 00:54:33,490 --> 00:54:34,800 You can use wax. 700 00:54:34,800 --> 00:54:36,417 Paraffin wax works as well. 701 00:54:36,417 --> 00:54:38,000 So it doesn't have to be salt. There's 702 00:54:38,000 --> 00:54:39,208 different things you can use. 703 00:54:59,050 --> 00:55:02,350 So you combine a powder of the polymer 704 00:55:02,350 --> 00:55:12,210 with your fugitive phase Say, salt. 705 00:55:12,210 --> 00:55:16,830 Then, you heat it up to get the polymer to bind. 706 00:55:16,830 --> 00:55:35,070 And then you leach out the salt. So you can control the porosity 707 00:55:35,070 --> 00:55:37,220 by the volume fraction of the fugitive phase, 708 00:55:37,220 --> 00:55:39,730 and then the pore size by the size of whatever 709 00:55:39,730 --> 00:55:40,976 the fugitive phase is. 710 00:56:51,520 --> 00:56:53,420 Another technique is electrospinning. 711 00:56:53,420 --> 00:56:57,000 The idea is you produce fibers from a polymer solution 712 00:56:57,000 --> 00:56:58,460 that you extrude through a nozzle. 713 00:56:58,460 --> 00:57:01,750 And then you apply a voltage across some plates 714 00:57:01,750 --> 00:57:02,840 to spin the fibers. 715 00:58:07,490 --> 00:58:10,940 And then you get a network of these fibers. 716 00:58:10,940 --> 00:58:12,816 And typically, their micron-scale diameter. 717 00:58:37,349 --> 00:58:39,140 And the last method I'm going to talk about 718 00:58:39,140 --> 00:58:40,390 is rapid prototyping. 719 00:58:40,390 --> 00:58:42,730 So you can think of using 3D printing. 720 00:58:42,730 --> 00:58:47,940 Or you could use selective laser centering or stereo lithography 721 00:58:47,940 --> 00:58:50,742 using a photo-sensitive polymer. 722 00:58:50,742 --> 00:58:52,450 So the idea is-- you know how this works. 723 00:58:52,450 --> 00:58:55,540 You just build up layers of solid, one layer at a time. 724 00:59:27,070 --> 00:59:29,380 And then you can make complex geometries with that. 725 00:59:29,380 --> 00:59:30,713 So that's one of the advantages. 726 00:59:42,270 --> 00:59:45,800 If you wanted to make a part to fit a particular place 727 00:59:45,800 --> 00:59:48,310 in the tissue, then it's convenient 728 00:59:48,310 --> 00:59:50,530 that you can control the geometry of the whole part. 729 01:00:27,590 --> 01:00:28,540 OK. 730 01:00:28,540 --> 01:00:31,730 So that just kind of summarizes very briefly, 731 01:00:31,730 --> 01:00:34,000 kind of how the tissue engineering scaffolds 732 01:00:34,000 --> 01:00:36,390 are meant to work, what kinds of materials people 733 01:00:36,390 --> 01:00:39,140 make them from, and a few of the processes. 734 01:00:39,140 --> 01:00:40,710 And there's many, many processes. 735 01:00:40,710 --> 01:00:43,392 These are just sort of a few common processes. 736 01:00:43,392 --> 01:00:45,850 I wanted to talk a little bit about the mechanical behavior 737 01:00:45,850 --> 01:00:47,990 because that's kind of what I do. 738 01:00:47,990 --> 01:00:50,765 And so this next plot just shows a stress strain curve 739 01:00:50,765 --> 01:00:53,636 in compression for a collagen GAG scaffold. 740 01:00:53,636 --> 01:00:55,010 And I'm hoping that by now you're 741 01:00:55,010 --> 01:00:57,070 getting the idea all of these cellular materials 742 01:00:57,070 --> 01:00:58,892 have this kind of shape of a curve. 743 01:00:58,892 --> 01:01:01,350 So there's the same kind of linear elastic regime, and then 744 01:01:01,350 --> 01:01:02,840 a collapse plateau. 745 01:01:02,840 --> 01:01:05,220 These collagen things, as you can imagine just 746 01:01:05,220 --> 01:01:06,970 pressing them in your hand, they fail 747 01:01:06,970 --> 01:01:09,060 by buckling, by inelastic buckling. 748 01:01:09,060 --> 01:01:11,050 And then there's a densification regime. 749 01:01:11,050 --> 01:01:15,600 So they look like all the other kinds of curves that we've got. 750 01:01:15,600 --> 01:01:18,100 One of the things we've done in the modeling 751 01:01:18,100 --> 01:01:20,100 is typically in the model, we want 752 01:01:20,100 --> 01:01:22,000 to be able to calculate or measure 753 01:01:22,000 --> 01:01:25,650 the modulus of the solid or the strength of the solid 754 01:01:25,650 --> 01:01:27,200 from which the thing's made. 755 01:01:27,200 --> 01:01:30,750 So [? Brendan ?] Harley was one of my PhD students. 756 01:01:30,750 --> 01:01:33,660 And he took a little microscope, cut a little strut. 757 01:01:33,660 --> 01:01:34,870 The struts are very small. 758 01:01:34,870 --> 01:01:36,350 The pore size is 100 microns. 759 01:01:36,350 --> 01:01:39,040 So the struts are on that order. 760 01:01:39,040 --> 01:01:42,420 He glued one end of the strut to a glass slide, 761 01:01:42,420 --> 01:01:44,950 and then he used an AFM probe to do a little bending test 762 01:01:44,950 --> 01:01:47,090 on that little strut. 763 01:01:47,090 --> 01:01:49,404 If he could measure the deflection, he knew the length. 764 01:01:49,404 --> 01:01:50,820 He knew the geometry of the strut. 765 01:01:50,820 --> 01:01:53,111 He could figure out what the modulus was for the solid. 766 01:01:53,111 --> 01:01:55,160 So he backs out what the modulus is. 767 01:01:55,160 --> 01:01:58,120 So he did these measurements on a dry strut. 768 01:01:58,120 --> 01:02:02,380 And then by comparing the overall modulus 769 01:02:02,380 --> 01:02:04,850 of a dry scaffold with a wet scaffold, 770 01:02:04,850 --> 01:02:08,180 he estimated what the modulus of the wet scaffold 771 01:02:08,180 --> 01:02:09,590 or the wet strut would be. 772 01:02:09,590 --> 01:02:11,850 So the modulus of the dry collagen GAG 773 01:02:11,850 --> 01:02:14,190 was 672 megapascals. 774 01:02:14,190 --> 01:02:15,910 A little less than a gigapascal. 775 01:02:15,910 --> 01:02:17,190 And wet it was about 5. 776 01:02:17,190 --> 01:02:20,600 So there's a huge difference between the wet and the dry. 777 01:02:20,600 --> 01:02:21,100 OK. 778 01:02:21,100 --> 01:02:23,440 So let me just write a few notes about that. 779 01:02:52,810 --> 01:02:54,832 So in compression, there's the three regimes 780 01:02:54,832 --> 01:02:56,706 that we see for all these cellular materials. 781 01:03:00,650 --> 01:03:04,380 And so you can estimate the modulus by using the model 782 01:03:04,380 --> 01:03:06,780 we have for the foam for the modulus. 783 01:03:06,780 --> 01:03:10,050 The modulus of the foam divided by the modulus of the solid 784 01:03:10,050 --> 01:03:13,340 goes as the relative density squared. 785 01:03:13,340 --> 01:03:16,670 And that's related to bending in the cell wall. 786 01:03:16,670 --> 01:03:20,330 And then, the collapse plateau is related to elastic buckling. 787 01:03:20,330 --> 01:03:22,945 And so that's equal to some constant times 788 01:03:22,945 --> 01:03:27,450 E of the solid times the relative density squared. 789 01:03:27,450 --> 01:03:29,160 And that's related to elastic buckling. 790 01:03:38,030 --> 01:03:41,940 And then we measured E of the solid doing this little AFM 791 01:03:41,940 --> 01:03:42,990 beam bending test. 792 01:04:14,270 --> 01:04:14,770 OK. 793 01:04:23,940 --> 01:04:26,800 And for one of these low-density scaffolds, 794 01:04:26,800 --> 01:04:28,250 we measured the modulus. 795 01:04:28,250 --> 01:04:29,940 We measured the buckling strength. 796 01:04:29,940 --> 01:04:33,960 And we got pretty good agreement by using these equations here. 797 01:04:33,960 --> 01:04:39,480 And the good agreement was if this constant was 0.2. 798 01:04:39,480 --> 01:04:41,610 That was the strain, that buckling. 799 01:04:41,610 --> 01:04:42,316 Yeah. 800 01:04:42,316 --> 01:04:46,316 AUDIENCE: So what does it mean to be wet in here? 801 01:04:46,316 --> 01:04:50,520 And why is it so much lower? 802 01:04:50,520 --> 01:04:52,850 LORNA GIBSON: Well, we make the scaffolds 803 01:04:52,850 --> 01:04:54,160 by this freeze-drying thing. 804 01:04:54,160 --> 01:04:56,504 And like this thing I passed around, that was dry. 805 01:04:56,504 --> 01:04:57,670 We just immerse it in water. 806 01:04:57,670 --> 01:04:58,890 We just put it in water. 807 01:04:58,890 --> 01:05:01,880 And then, he does the test. 808 01:05:01,880 --> 01:05:03,920 So he does the test on the whole scaffold dry, 809 01:05:03,920 --> 01:05:06,549 and then he does the test on the whole scaffold wet. 810 01:05:06,549 --> 01:05:09,090 And I assume there's some sort of bonding that gets disrupted 811 01:05:09,090 --> 01:05:10,210 by having the water. 812 01:05:10,210 --> 01:05:12,010 I don't know the details of how that works, 813 01:05:12,010 --> 01:05:13,843 but there must be some change in the bonding 814 01:05:13,843 --> 01:05:15,070 to make that happen. 815 01:05:17,620 --> 01:05:18,705 So let's see. 816 01:05:18,705 --> 01:05:19,582 I'm trying to see. 817 01:05:19,582 --> 01:05:20,790 What else should we do today? 818 01:05:20,790 --> 01:05:21,290 Oh, yeah. 819 01:05:21,290 --> 01:05:25,860 So we get pretty good agreement with this sort 820 01:05:25,860 --> 01:05:27,520 of simple foamy model. 821 01:05:27,520 --> 01:05:28,930 One of the things we did find was 822 01:05:28,930 --> 01:05:32,010 that when we tested higher-density scaffolds, 823 01:05:32,010 --> 01:05:33,560 the agreement wasn't so good. 824 01:05:33,560 --> 01:05:36,260 And I think this was because when you get higher density, 825 01:05:36,260 --> 01:05:40,060 it's just hard to get the collagen GAG mixture to mix in 826 01:05:40,060 --> 01:05:41,090 with the acetic acid. 827 01:05:41,090 --> 01:05:43,500 And so we ended up getting inhomogeneous scaffolds 828 01:05:43,500 --> 01:05:45,270 with sort of large voids in them. 829 01:05:45,270 --> 01:05:46,940 So in order for the modeling to work, 830 01:05:46,940 --> 01:05:49,880 you have to have a scaffold that's relatively homogeneous. 831 01:05:49,880 --> 01:05:53,110 You don't have kind of big defects in it. 832 01:05:53,110 --> 01:05:56,400 I think that's all I'm going to say about that. 833 01:05:56,400 --> 01:06:00,611 I have one more slide here. 834 01:06:00,611 --> 01:06:02,610 So those are sort of three-dimensional scaffolds 835 01:06:02,610 --> 01:06:03,620 we've been talking about. 836 01:06:03,620 --> 01:06:04,786 Sort of foam-like scaffolds. 837 01:06:04,786 --> 01:06:07,070 People have also made honeycomb-like scaffolds 838 01:06:07,070 --> 01:06:07,980 as well. 839 01:06:07,980 --> 01:06:09,510 And this just shows some examples 840 01:06:09,510 --> 01:06:12,170 of some honeycomb-type scaffolds. 841 01:06:12,170 --> 01:06:17,575 So this one here, I think was made in Sangeeta Bhatia's lab. 842 01:06:17,575 --> 01:06:19,480 You can sort of think of it as a hexagon, 843 01:06:19,480 --> 01:06:22,180 but it's also kind of triangulated as well. 844 01:06:22,180 --> 01:06:24,660 These two here were made by George Engelmayr. 845 01:06:24,660 --> 01:06:27,910 He worked with Bob Langer at one point. 846 01:06:27,910 --> 01:06:30,940 And these two scaffolds here, they're 847 01:06:30,940 --> 01:06:33,690 both sort of rectangular cells. 848 01:06:33,690 --> 01:06:35,850 And they were designed to look at how 849 01:06:35,850 --> 01:06:38,650 the cell geometry or the pore geometry 850 01:06:38,650 --> 01:06:42,117 affected the sort of morphology of the cells that attached. 851 01:06:42,117 --> 01:06:43,950 So if you have different, say, [? porous, ?] 852 01:06:43,950 --> 01:06:46,120 you get different morphology in the cells 853 01:06:46,120 --> 01:06:47,550 that you're trying to attach. 854 01:06:47,550 --> 01:06:50,720 And then, George Engelmayr also made these scaffolds here. 855 01:06:50,720 --> 01:06:53,300 And those were designed to be anisotropic and have 856 01:06:53,300 --> 01:06:56,130 different mechanical properties in different directions. 857 01:06:56,130 --> 01:06:57,590 And I think what they had done was 858 01:06:57,590 --> 01:07:00,640 try to match the anisotropy in the mechanical properties 859 01:07:00,640 --> 01:07:03,990 to anisotropy in heart tissue, in cardiac tissue. 860 01:07:03,990 --> 01:07:07,375 So these are some examples of honeycomb-type scaffolds. 861 01:07:07,375 --> 01:07:09,500 So let me just write down a few things about those. 862 01:07:41,220 --> 01:07:44,440 So I think these were more-- obviously, 863 01:07:44,440 --> 01:07:46,780 the scaffolds are used-- the ultimate goal 864 01:07:46,780 --> 01:07:47,960 is to use them clinically. 865 01:07:47,960 --> 01:07:49,920 But sometimes, people make scaffolds just 866 01:07:49,920 --> 01:07:51,211 to study cell behavior. 867 01:07:51,211 --> 01:07:52,960 And some of these, I think, were made just 868 01:07:52,960 --> 01:07:54,793 to study how the cells would behave on them. 869 01:07:54,793 --> 01:07:57,153 So they're sort of idealized to do that. 870 01:08:21,310 --> 01:08:22,462 So that triangulated. 871 01:08:22,462 --> 01:08:24,540 It kind of looks like a hexagon, but there's also 872 01:08:24,540 --> 01:08:25,815 sort of triangles in there. 873 01:08:30,012 --> 01:08:31,470 I think the thing they were looking 874 01:08:31,470 --> 01:08:36,834 at with that was transport of nutrients to cells. 875 01:08:55,450 --> 01:08:58,370 And from a mechanical point of view, if it's triangulated 876 01:08:58,370 --> 01:09:04,250 you'd expect, say, the modulus of that to go linearly 877 01:09:04,250 --> 01:09:06,670 with how much solid there is there. 878 01:09:24,600 --> 01:09:26,939 So the rectangular honeycomb and the diamond shaped 879 01:09:26,939 --> 01:09:30,479 pores, they were used to study the effect of pore geometry 880 01:09:30,479 --> 01:09:31,624 on the cell orientation. 881 01:09:31,624 --> 01:09:32,540 They used fibroblasts. 882 01:09:57,800 --> 01:10:01,010 And I'm going to call it the accordion-like honeycomb. 883 01:10:07,450 --> 01:10:09,780 That's mechanically anisotropic. 884 01:10:09,780 --> 01:10:13,705 And the mechanical anisotropy is matched to the cardiac tissue. 885 01:10:36,795 --> 01:10:37,770 OK. 886 01:10:37,770 --> 01:10:40,110 So I think I'm going to stop there for today. 887 01:10:40,110 --> 01:10:41,817 That's sort of the end of this part. 888 01:10:41,817 --> 01:10:43,400 And next time, I'm going to talk about 889 01:10:43,400 --> 01:10:44,960 the osteochondral scaffolds. 890 01:10:44,960 --> 01:10:46,918 I don't think it really makes sense to start it 891 01:10:46,918 --> 01:10:48,000 for two or three minutes. 892 01:10:48,000 --> 01:10:49,350 So I'll start that tomorrow. 893 01:10:49,350 --> 01:10:50,330 Or Wednesday. 894 01:10:50,330 --> 01:10:52,870 And we should be able to finish that on Wednesday. 895 01:10:52,870 --> 01:10:57,160 So one of the things that these honeycomb-type scaffolds kind 896 01:10:57,160 --> 01:10:58,950 of suggests is that the scaffolds are 897 01:10:58,950 --> 01:11:02,160 used both to try to regenerate tissue 898 01:11:02,160 --> 01:11:04,210 in the body in clinical applications, 899 01:11:04,210 --> 01:11:06,780 but they're also used as sort of an environment for cells, 900 01:11:06,780 --> 01:11:08,739 in order to study cell behavior. 901 01:11:08,739 --> 01:11:10,280 So next time, I'm going to talk about 902 01:11:10,280 --> 01:11:11,549 an osteochondral scaffold. 903 01:11:11,549 --> 01:11:13,840 And the idea with that was to try to use it clinically. 904 01:11:13,840 --> 01:11:16,830 But next week, I'm going to talk about cell mechanics a bit. 905 01:11:16,830 --> 01:11:18,690 And when people study cell mechanics, 906 01:11:18,690 --> 01:11:21,920 or look at the mechanics of biological cells, not 907 01:11:21,920 --> 01:11:24,120 the cellular structures. 908 01:11:24,120 --> 01:11:28,510 So when people look at trying to study how cells behave, 909 01:11:28,510 --> 01:11:30,260 they need some environment to put them on. 910 01:11:30,260 --> 01:11:33,930 Typically, people started by just using flat 2D substrates. 911 01:11:33,930 --> 01:11:36,909 But the flat 2D substrates are kind of easy to study, 912 01:11:36,909 --> 01:11:39,200 but they don't really represent the tissue in the body. 913 01:11:39,200 --> 01:11:42,140 And so people are now using tissue engineering scaffold 914 01:11:42,140 --> 01:11:44,680 as an environment that they can control 915 01:11:44,680 --> 01:11:46,240 to study how cells behave. 916 01:11:46,240 --> 01:11:49,150 So they study cell attachment, cell proliferation, 917 01:11:49,150 --> 01:11:51,760 cell migration, and cell differentiation 918 01:11:51,760 --> 01:11:53,210 all by using the scaffolds as kind 919 01:11:53,210 --> 01:11:54,890 of a controlled environment. 920 01:11:54,890 --> 01:11:56,800 So we'll talk about that, probably not-- 921 01:11:56,800 --> 01:11:58,675 I don't know if we'll get to it on Wednesday. 922 01:11:58,675 --> 01:12:00,120 But we might start it on Wednesday 923 01:12:00,120 --> 01:12:01,710 and finish it next week. 924 01:12:01,710 --> 01:12:03,260 OK?