Biological Engineering Principles

The level of control that organisms exercise over the materials properties of structural inorganic biomaterials is unparalleled in modern engineering. Even more tantalizing is the organisms’ ability to form multifunctional materials that are optimized to perform structural, optical, mechanical and other functions – almost unrelated from the engineering point of view. 

Our studies suggest that these properties originate from a sophisticated structural design achieved by the interplay between inorganic minerals and organic biological macromolecules. Our research is aimed at studying biological composite materials and understanding how biology arranges simple minerals and polymers into complex architectures. 

Among organisms that attract our attention are: deep-sea sponges that construct damage-resistant structural glasses; brittlestars with skeletons optimized for both mechanical and optical performance; a slimy biofilm that turns out to be one of the best liquid-repellent materials known.

Often nature’s solutions to engineering problems are so different from our conventional ways of thinking that the most fruitful way to investigate them is not immediately obvious.

We are therefore engaged in a continuous dialogue: we study the biological material itself to begin to understand its underlying principles, adapt these concepts to design a bio-inspired architecture, and then apply insights from the designed system to guide further investigation of the biological system.

Kumar K, Liu J, Christianson C, Ali M, Tolley MT, Aizenberg J, Ingber DE, Weaver JC, Bertoldi K. A Biologically Inspired, Functionally Graded End Effector for Soft Robotics Applications. Soft Robotics. 2017;4 (4) :317-323.Abstract

Soft robotic actuators offer many advantages over their rigid counterparts, but they often are unable to apply highly localized point loads. In contrast, many invertebrates have not only evolved extremely strong ‘‘hybrid appendages’’ that are composed of rigid ends that can grasp, puncture, and anchor into solid substrates, but they also are compliant and resilient, owing to the functionally graded architecture that integrates rigid termini with their flexible and highly extensible soft musculatures. Inspired by the design principles of these natural hybrid appendages, we demonstrate a synthetic hybrid end effector for soft-bodied robots that exhibits excellent piercing abilities. Through the incorporation of functionally graded interfaces, this design strategy minimizes stress concentrations at the junctions adjoining the fully rigid and soft components and optimizes the bending stiffness to effectively penetrate objects without interfacial failure under shear and compressive loading re- gimes. In this composite architecture, the radially aligned tooth-like elements apply balanced loads to maximize puncturing ability, resulting in the coordinated fracture of an object of interest.

Li L, Connors MJ, Kolle M, England GT, Speiser DI, Xiao X, Aizenberg J, Ortiz C. Multifunctionality of chiton biomineralized armor with an integrated visual system. Science. 2015;350 (6263) :952-956.Abstract
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.
Monn MA, Weaver JC, Zhang T, Aizenberg J, Kesari H. New functional insights into the internal architecture of the laminated anchor spicules of Euplectella aspergillum. Proc. Nat. Acad. Sci. 2015;112 (16) :4976-4981.Abstract
To adapt to a wide range of physically demanding environmental conditions, biological systems have evolved a diverse variety of robust skeletal architectures. One such example, Euplectella aspergillum, is a sediment-dwelling marine sponge that is anchored into the sea floor by a flexible holdfast apparatus consisting of thousands of anchor spicules (long, hair-like glassy fibers). Each spicule is covered with recurved barbs and has an internal architecture consisting of a solid core of silica surrounded by an assembly of coaxial silica cylinders, each of which is separated by a thin organic layer. The thickness of each silica cylinder progressively decreases from the spicule’s core to its periphery, which we hypothesize is an adaptation for redistributing internal stresses, thus increasing the overall strength of each spicule. To evaluate this hypothesis, we created a spicule structural mechanics model, in which we fixed the radii of the silica cylinders such that the force transmitted from the surface barbs to the remainder of the skeletal system was maximized. Compared with measurements of these parameters in the native sponge spicules, our modeling results correlate remarkably well, highlighting the beneficial nature of this elastically heterogeneous lamellar design strategy. The structural principles obtained from this study thus provide potential design insights for the fabrication of high-strength beams for load-bearing applications through the modification of their internal architecture, rather than their external geometry.
Li L, Kolle S, Weaver JC, Ortiz C, Aizenberg J, Kolle M. A highly conspicuous mineralized composite photonic architecture in the translucent shell of the blue-rayed limpet. Nat. Commun. 2015;6 :6322.Abstract
Many species rely on diverse selections of entirely organic photonic structures for the manipulation of light and the display of striking colours. Here we report the discovery of a mineralized hierarchical photonic architecture embedded within the translucent shell of the blue-rayed limpet Patella pellucida. The bright colour of the limpet’s stripes originates from light interference in a periodically layered zig-zag architecture of crystallographically co-oriented calcite lamellae. Beneath the photonic multilayer, a disordered array of light-absorbing particles provides contrast for the blue colour. This unique mineralized manifestation of a synergy of two distinct optical elements at specific locations within the continuum of the limpet’s translucent protective shell ensures the vivid shine of the blue stripes, which can be perceived under water from a wide range of viewing angles. The stripes’ reflection band coincides with the spectral range of minimal light absorption in sea water, raising intriguing questions regarding their functional significance.
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