I’ve got this great undergraduate student in my lab who is (at least for now) exhibiting all the traits of the ideal student any professor would love to have around: he volunteers ~10+ hours per week here, if no one is around, he finds things to do to teach himself new skills, he reads journal articles on his own accord that are aligned with interests of the lab, AND he’s creative! Lately, he has been trying to tackle a rather substantial challenge I presented to the lab: designing a force plate that can measure forces under granular media.
OK. Here’s the context: I used force plates to measure how hard and in which directions an animal is pushing when it steps on the ground. By combining this with measured movements of the animal that I quantify by analyzing synchronized high-speed video, I can calculate how much power the muscles must produce around each joint in order for the animal to move the way it does. This technology has been commonly used since the mid-1980s or earlier, for locomotion on flat, homogeneously hard surfaces. While we know a lot about how animals move across these types of “lab environment” surfaces, we know far less about movement over natural surfaces that may shift or change squishiness, orientation, etc. with each step.
Granular media, such as sand, therefore, is a particularly interesting material to me, as it actually makes state changes as an animal moves. For example, when sand is sitting undisturbed, it resembles a solid. Yet, when an animal strikes the surface and strokes through it with its foot, the sand actually becomes a fluid for a short while. Any sand that is kicked up during the step is actually acting like a gas! With all this in mind, measuring forces on sand can be a rather challenging conundrum.
In a meeting with my undergraduate student last week, he presented a design to me that involved peppering a surface with a grid of lumps that each could sense fluid movement. Little did he know, what he was showing me was something holding remarkable resemblance to hair cells, sensory receptors that are found in our ears AND in the lateral line system of fishes!
Ever wonder how a school of hundreds of fish manages to… school with such regularity and neat, synchronized prowess? The next time you catch that rainbow trout, sunfish, sailfish, or whatever strikes your fancy, take a close look at the side of their body and you’ll see a series of dashes and dots that run from the rear side of the gill margin all the down to the base of the tail. These little holes mark the external opening to the lateral line, the pressure-sensing secret for fishes.
Within these pores are receptors called neuromasts. Neuromasts look a little like a thimble placed open side down on a table top. Each neuromast is made up of a group of cells called hair cells, named because they (grossly) resemble a Leprocaun troll doll, with different length hair bundles mounted on top.
The entire neuromast is covered in a gelatinous glob, forming a cupula. Deflection of these hair bundles due to changes in fluid flow causes the production of a receptor potential. Deflection direction also stimulates the production of different types and magnitudes of potentials, enabling the fish to determine the direction of the flow. Changes in fluid flow direction or pressure can be due to an underwater obstacle, a neighboring fish, or even a predator, enabling a fish to respond seemingly magically while the approaching object is still far away.
It turns out that my student (I guess not so surprisingly) was not the first person to think of using the neuromast as a biomimetic sensor: a group of scientists at the University of Illinois, Northwestern University, and Institut fur Zoologie have developed nano hair cells they call ALL (for artificial lateral line) that can localize the position of a crayfish placed in a tank. Take a look at their paper, published recently in Bioinspiration and Biomimetics. Call it bias or whatever you want, I am still excited to see where (if anywhere) my student will take this biomimetic idea of his. Let me know if you have any input on how he can potentially use this to invent a new type of force plate technology, and I’ll put you in touch with my student to try to make this a reality!