Elastic Energy Storage in Biological Materials: Internal Stresses and Their Functionality.

In the biological world, materials are often heterogeneous and anisotropic, comprising components with very different elastic properties. The resulting structures are exposed to force generation by chemo-mechanical energy conversion-such as water absorption, phase separation, or crystallization. Such phenomena may result in strain misfits that generate internal stresses that store elastic energies, which turn out to be extremely useful for enabling functions such as shape change, locomotion, or predation. However, the significance of elastic energy storage has received little attention. In this review, by considering examples of a broad spectrum of biological materials spanning shape-morphing plant seed pods, smart appendages of crustaceans, ballistic tongues, and damage-tolerant mineralized tissues, the fundamental aspects involved in the generation, storage, and release of internally generated elastic energies are surveyed. These have major implications for functions such as strengthening and toughening, shape morphing and actuation, ballistic movements, tensegrity, and bending stabilization. How Phenomena such as atomic or protein incorporation into minerals, conformational changes of proteins, phase transformation, and osmotic pressure are manipulated in the biological world to generate function by storing elastic energy are described. Such "elastic energy batteries" provide efficient performance and evolutionarily adapted functionality through a smart, structure-based energy management.