Nature provides a multitude of examples of multifunctional structural materials in which trade-offs are imposed by conflicting functional requirements. One such example is the biomineralized armor of the chiton Acanthopleura granulata, which incorporates an integrated sensory system that includes hundreds of eyes with aragonite-based lenses. We use optical experiments to demonstrate that these microscopic lenses are able to form images. Light scattering by the polycrystalline lenses is minimized by the use of relatively large, crystallographically aligned grains. Multiscale mechanical testing reveals that as the size, complexity, and functionality of the integrated sensory elements increase, the local mechanical performance of the armor decreases. However, A. granulata has evolved several strategies to compensate for its mechanical vulnerabilities to form a multipurpose system with co-optimized optical and structural functions.
We gratefully acknowledge support from the U.S. Army Research Office through the Massachusetts Institute of Technology (MIT) Institute for Soldier Nanotechnologies (contract W911NF-07-D- 0004) and the National Security Science and Engineering Faculty Fellowship Program (N00244-09-1-0064). This work made use of the Materials Research Science and Engineering Center Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-08-19762. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357. M.K. and J.A. gratefully acknowledge support by the NSF Designing Materials to Revolutionize and Engineer our Future program (DMR 1533985). M.K. thanks the Alexander von Humboldt Foundation for a Feodor Lynen Research Fellowship and gratefully acknowledges financial support from the MIT Department of Mechanical Engineering. D.I.S. gratefully acknowledges support by the NSF (DEB-1354831). We thank A. Schwartzman, Y. Zhang, and S. Chen for their technical assistance and E. Belmonte and B. Anseeuw for providing photographs of A. granulata. D.I.S. collected and identified chiton specimens. M.J.C. and X.X. performed synchrotron experiments. M.J.C. and L.L. processed and analyzed data from synchrotron experiments. L.L. performed electron microscopy studies and mechanical tests with data analysis. M.J.C., G.T.E., M.K., and L.L. performed optical measurements and data analysis. M.K. wrote the ray-tracing program. M.J.C and M.K. performed ray-trace simulations. All authors interpreted results. L.L. and M.J.C. prepared figures, tables, and movies and wrote the draft manuscript. C.O. and J.A. supervised the project. All authors revised the manuscript for submission.