Brittle stars are known for the high flexibility of their arms, a characteristic required for locomotion, food grasping, and for holding onto a great diversity of substrates. Their high agility is facilitated by the numerous discrete skeletal elements (ossicles) running through the center of each arm and embedded in the skin. This study investigated how ossicle morphology and skeletal organization affect arm flexibility of three brittle star species.

A high-throughput workflow that combines behavioral studies with parametric modeling, additive manufacturing, micro-computed tomography (micro-CT), scanning electron microscopy (SEM), and finite element simulations provides a fundamental understanding of 3D structure-kinematic relationships in these skeletal systems. First, micro-computed tomography (micro-CT) studies were performed on small sections of the arms, that allowed the individual ossicles to be virtually segmented.

The virtual ossicle models constructed from the micro-CT and segmentation-based workflow were then 3D printed. The tangible 3D-printed models allowed for the quick identification of mechanical interferences in these complex linkage systems.

Using Rhinoceros and Grasshopper, we developed parametric models that provided critical insight into the complex interaction between ossicles and achievable range of movement. The script was developed from data observed in the micro-CT scans and interactions with the 3D printed models. The SEM imaging studies allowed us to directly compare the predicted zones of contact between adjacent ossicles during arm bending with ossicle porosity gradients.

Team: Lara Tomholt, Larry Friesen, Daniel Berdichevsky, Matheus Fernandes, Christoph Pierre, Robert Wood, James Weaver

Harvard Graduate School of Design
Harvard John A. Paulson School of Engineering and Applied Sciences
Wyss Institute for Biologically Inspired Engineering at Harvard University
Santa Barbara City College
University of California, Santa Barbara

L. Tomholt, L.J. Friesen, D. Berdichevsky, M.C. Fernandes, C. Pierre, R.J. Wood, J.C. Weaver (2020) The structural origins of brittle star arm kinematics: An integrated tomographic, additive manufacturing, and parametric modeling-based approach. Journal of Structural Biology, 211 (1), 107481.

Media coverage
Press release by Harvard School of Engineering and Applied Sciences