But what these techniques allow for is for you to dissect the problem. So it allows you to look at different orientation effects, it allows you to come into the tissue in this orientation and then it allows you to take a different cross-section and come out of it from these sides. So you look at different effects of orientation. And the comment being if I just tried it to load the whole structure instead of taking cross-sections or are looking at very tiny parameters, a globally averaged, and so I can’t look at the role of orientation of a tubule, I can’t look at what happens to dentin by itself is maybe you changed a drug, something that didn’t come up with Professor Ritchie’s lecture, he talked a little bit about pharmaceutical treatment. There is a lot of work in bone right now, where if you look at pharmaceutical treatment, question is what happens to the quality of the bone? And again if you try to macroscopically characterize the mechanical properties you might miss out what goes on at the microstructural levels. So that’s where combinations of imaging with things like micro CT coupled with nanoindentation and modeling become very powerful tools.
So some of the classic mechanical testing protocols that we learned in E45 that worked really well for steels, well for engineering materials probably miss we have these highly complicated hierarchical structures. But that’s a challenge, how do we learn from biology and also it teaches us because it lets us ask the question of how can we better design engineering materials to give us this type of wear resistance and this type of fracture resistance? Most of us would love to have tooling materials that would have the same wear resistance as enamel, right? We put diamond like coatings on carbide or carbide treated steel to get better machining properties and reality is we don’t even come close to what we get out of biology. So there’s a lot — it works both ways, we can learn from tissues to understand disease but we can also learn from healthy tissues to better engineer materials.
That’s a bizarre looking plot, huh? So the concept of what I was trying to teach here, we should probably have a lecture on nanoindentation one day but the idea is that we would come in with an indentured tip into this structure. It would make an indent into the material and we can get low displacement behaviors. So we can monitor load and displacement and from that we can back out of unloading stiffness, and we can get a representation of the elastic modulus. And so the nice thing about doing to set nanoindentation scales is we can change out that tip geometry to be anything from a diamond pyramid tip on nanometer length scale all the way to a spherical tip that span several microns. So you can start to probe out with this technique nanometer length scales all the way to micron or hundreds of micron length scales, that becomes important because then you can start to see well, what are the cellular contributions, what are the trabecular orientation contributions, what are all the sub-structural contributions? So there’s not one biological tissue that isn’t built on hierarchy of these constituents. And so if you really want to get clear about the mechanics you have to start asking the deeper question because if we just go to this idea of well, we will make a little dogbone and we will load it up and get stress strain behavior, well that’s fine but it globally averages everything. And so we miss on all the constituent elements.
And same thing if we were to do a little compression test, we could still get a compressive modulus, we could still get compressive yield but we’re globally averaging everything that goes on. So we have no way to deal with size scales in that context.
So again this is — if you’re interested in that type of work in dental, this is a good group to watch for. The Marshall group has done a lot of work on looking at nanoindentation and how it plays a role on the basic mechanical properties and then recently they’ve coupled as I said with Professor Ritchie’s group looking at how the microstructure plays a role in fracture mechanics. So it’s a good combination, always when you can get your biological groups together with your mechanics groups. And hopefully you’re trying to learn out as we go through the course, right? There are some benefits to both, there’s benefits of understanding biology and structure and there’s benefits to really understanding mechanics and if you can bring the two together you’ve got a lot of power.
Biology and structure
Okay. So again just different constituents, you’ve got enamel, so again very unique material and offering wear resistance. So it’s our hardest substance in the body, it’s calcium phosphate salt type crystals, they are large hexagonal type structures. Again nanometer length scales, so again that’s that whole nanostructured material, dentin composed largely of type 1 collagen fibrils, so again you’ve got a lot of fibrous tissue and you’ve got it blended with nanocrystalline apatite mineral. And then as Rob said very similar to bone in its microstructure. So that’s another thing you want to think about, every time we’re studying these materials we want to ask ourselves where can we learn from, so the literature from orthopedics offers us a lot of insight to dental and vice versa.
Dentin, the dentinal tubules radiate from the pulp. So we saw that in that one image. So we get radiation of these tubules and again they’re just marching around in a radial orientation. So they’re taking different orientations depending on where we are whether they’re tying into the jaw, whether they’re supporting below the enamel. The pulp is again almost like our bone marrow, it’s what provides for us a lot of the elements of vascularization and blood supply, it’s innervation, it’s a very important structure and so we talk about root canals, you get a lot of work into what happens at the pulp level.