The exact structure of a vertebra depends on which species you are looking at and where along the vertebral column you happen to be looking. Because we were using the caudal vertebrae of sharks as our biological target (Figures 6.2 and 6.3), let me describe them.[121] Compared to those of bony fishes, the vertebrae of sharks are relatively simple, lacking neural and hemal spines (Figure 6.4). The neural and hemal arches are not fused to the centra, and those arches form their own small-diameter columns that span the intervertebral joints.
FIGURE 6.4. The vertebral column of sharks. These portions of the vertebral column are from the region between the end of the abdominal cavity and the beginning of the caudal fin.
To make a vertebral column Justin and Tom figured out how to sew the vertebrae together using long horse hairs glued to the perimeter of each element. Because the hairs serve the same function as the intervertebral ligaments of real vertebral columns, we called this model 1 the ligament-linked artificial vertebral column (Figure 6.3). Between the vertebrae they injected gelatin, like the marshmallow in the middle of a camper’s s’more. Once the gelatin firmed up, the whole column was bathed in a chemical fixative. This fixative cross-linked the gelatin, making the molecular collagen into a lattice that was both stiffer than the raw gelatin and resistant to degradation. This may sound familiar: the hydrogels that we made from gelatin and cross-linked were our artificial notochords that functioned as the axial skeleton in Tadro3.
Our second marine platoon worked at Vassar and was led by Kira Irving, a major in our neuroscience and behavior program at the time. Kira had been part of the Tadro3 team, along with Keon Combie, a major in biochemistry, and Virginia Engel and Gianna McArthur, both majors in biology. This group also got help from Kurt Bantilan, another major in neuroscience and behavior, and Carl Bertsche, Vassar’s resident expert in machining.
In response to the problem of holding together a chain of rigid and flexible elements, Kira’s team came up with a solution quite different from that of Tom’s (Figure 6.5). Instead of using horse hairs as ligaments, they used thin plastic coffee stirrers as neural and hemal arches running along the top and bottom of the column, spanning each joint and preventing dislocation. This was a shark-like solution (see Figure 6.4), and it gave us the ability to explore something unusuaclass="underline" having very long intervertebral joints. We call this model 2 the arch-linked artificial vertebral column.
Both model 1 and model 2 gave us three structures to evolve: (1) the length of the centrum, (2) the angle of the concave joint surface on the centrum, and (3) the length of the intervertebral joint. In this respect, they were equivalent models. Where they differed dramatically, however, was in how they operated mechanically when we bent them. Model 1, the ligament-linked vertebral column, appeared to be dominated by the mechanical properties of the horse hairs. Doug Pringle, a gifted mechanical engineer in Tom’s lab, helped Justin perform bending tests on the biomimetic vertebral columns. Instead of stretching the horse hairs on one side (the convex side) and squishing the joint material on the other side (concave side) during bending, they noticed that the horse hairs were stiff enough that they weren’t allowing much stretching. As a result, the model 1 column bent by compressing one side, buckling locally at each joint. If you can picture this geometry in your head, then if one side of the column compresses while the other stays the same length, then the whole structure bends and shortens.
FIGURE 6.5. Biomimetic vertebral column, model 2. Using the vertebral columns of sharks as our biological target, Kira worked with Virginia, Gianna, and Keon to design a column that was stabilized by neural and hemal arches that spanned the vertebral column. Carl built molds that allowed our team to glue coffee stirrers onto centra in a repeatable process. The molds were then injected with gelatin, which, once gelled, was chemically cross-linked.
Shortening of the column didn’t happen in model 2, the arch-linked vertebral column, because it used the coffee stirrers to hold the length of the column constant along its midline. During bending we saw compression of the concave side of the joint and elongation of the convex side. This looked good—at first. However, on the elongated side of the bend, we could sometimes see a separation of the hydrogel material from the face of the rigid vertebra. When this occurred it meant that the bending properties of the joint were being controlled only by the hydrogel on the compression side and the neural and hemal spines along the midline. To keep the joint attached to the vertebra, we found that we could put in a little bit of cyanoacrylate glue.
Both models of the vertebral column are beautiful examples of biomimetic design (not that I’m biased or anything). They capture the composite nature of real vertebral columns, creating a serial column of rigid and flexible material.[122] They both use a collagen-derived hydrogel for the viscoelastic intervertebral joint. They both have vertebral centra with the cup-shaped joint surfaces that we see in sharks. In addition, even though the two kinds of columns bend in different ways, their stiffness is in the same range as the stiffness we find in the vertebral columns of sharks. In order to fine-tune our biomimetic designs, we continue to explore the mechanical behavior of both the biomimetic vertebral columns and the shark vertebral columns under the expert guidance of Dr. Marianne Porter, a postdoctoral researcher in my laboratory.
The biomimetic design from Tom’s lab was the preliminary winner. We thought we could solve the problem of the too-stiff horse hairs by using either fewer hairs or a different kind of material for a ligament. In addition, the arch-linked design, by virtue of its arches, had bending stiffness even when no joint was present. As with the horse hairs, we could solve that problem by reducing the stiffness of the arches. With the arch design we were also having some problems making the vertebrae and, as mentioned, keeping the joint attached to the vertebrae. The rapid prototyping, because it was computer controlled, made centra more repeatably than our hand-milled process. In addition, the horse hairs, by enforcing bending by buckling, never let the vertebrae unleash the joint material.
Thus, we began with the ligament-linked model 1. Because we were performing the robotic experiments at Vassar, we needed to train our students how to make the model 1 biomimetic vertebral columns. Because Tom and I had been meeting for research every fall at the Mount Desert Island Biological Laboratory in Salisbury Cove, Maine, we decided to use our time there to transfer the production technology. Keon and Virginia came with me to learn the task. Keon, always very keen on new techniques, volunteered to be the first trainee. Despite Tom’s patience and Keon’s prowess as an experimentalist, the manufacturing of the ligament-linked columns was proving to be slow at best, and a mess at worst. Imagine having to align seven small objects, keep them equally spaced, and then glue long fibers to their outsides. We built little rigs to help align and hold all the parts, but still, on most days we found that Keon soon became one with the vertebral column. Virginia and I were even worse. Our collective failure made us appreciate the abilities of Justin, who made the series of original model 1 vertebral columns down in Tom’s lab in Florida. However, Justin, working on his own PhD project, was unavailable for full-time work in our vertebral column factory at Vassar.
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In fishes the vertebral column is seen to have two main sections, the precaudal and the caudal. The precaudal section is what we haughty mammals might be tempted to call “abdominal” because each vertebra is associated with ribs and an underlying visceral cavity. The caudal section comes after (posterior) the precaudal and, in bony fishes, stops at the caudal fin. In sharks, skates, and rays, however, the vertebral column continues all the way up to the tip of the upper lobe of the caudal fin. Thus, in these cartilaginous fishes it’s not clear where the “caudal” vertebrae stop—at the anterior or posterior margin of the caudal fin? Although I’m sure that this is a fascinating topic for most of you, we have to stop. When I say “caudal” here I mean from the last precaudal vertebra to the anterior margin of the caudal fin.
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Kurt and Keon presented our work on biomimetic vertebral columns at the Annual Meeting of the Society for Integrative and Comparative Biology: K. Bantilan, K. Combie, J. Schaeffer, D. Pringle, J. H. Long Jr., and T. Koob, “Building Biomimetic Backbones: Modeling Axial Skeleton Morphospace,”