BRC-BIO: Foot Morphology for Substrates with Varied Compliance

Almost every surface that animals evolved to run on, such as soil, leaf litter, sand, and snow, is soft or flowing. However, describing the precise force responses from these materials in order to understand the dynamics of running is extremely challenging because small changes to the grains, how the material is packed, or how the foot hits the ground can lead to huge differences in the behavior of the ground.

Furthermore, animals rarely do the same exact motion twice on exactly the same kind of ground, making the empirical data about these interactions very noisy. Two ways of conducting controlled experiments that can produce cleaner empirical data about the foot-ground interactions include using robotic models and simulating the behavior of the ground using mathematical and numerical models.

Such experiments can inform our understanding of how and why animals have evolved the diversity of feet that we see in the animal kingdom. Understanding how animals might have evolved to handle the different kinds of flowing and soft substrates that they encounter in their natural environments could lead to breakthroughs in legged robot locomotion and suggest better designs for shoes and prosthetics.

In addition, this project will support the development of an interdisciplinary undergraduate course in using robotics to address biological questions and will engage numerous students in authentic research at the interface of computer science and biology.

This project will develop and use an open-source and low-cost legged robot platform to stand in for an animal, and systematically vary foot features to answer questions about the evolution of different foot structures found in lizards. The focus on lizards stems from the diversity of foot shapes in this group and previous results suggesting that sand specialist lizards, which often have long, thin toes, may have evolved the separation of these toes to be 3-5 times the width of the most common particles in the environment to which they are adapted.

Discrete element method simulations, which are simulations modeling the forces acting on individual grains in a granular media, suggest that this may be because a spacing of 3-5 particle diameters leads to a virtual webbing effect in which the granular media reacts to a foot as if it has more surface area than it actually does. A primary goal of this project is to determine the effects of foot shape on the force response from the ground compared with simple surface area using a combination of discrete element method simulations and experiments jumping a physical robot on homogeneous granular media.

The project will also investigate another potential benefit of toes for locomotion on granular media: removing the foot from the ground. Long, thin toes, as are often seen in sand specialist species, can collapse down quickly for easy extraction from the ground. Larger, including more webbed feet, might drag more granular media up during extraction, slowing down the foot.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.