Hydration-induced reversible deformation of biological materials

The influx and efflux of water in biological structures actuates reversible deformation and recovery processes that are crucial for mechanical functions in plants and animals. These processes utilize various mechanochemical mechanisms: swelling directed by the arrangement of cellulosic microfibrils in a bilayer construct, which generates different deformation patterns; lignification gradients; hierarchical foam-like inner structures, some of which also include swelling by hygroscopic cellulose inner cell layer; turgor pressure, which is activated by osmosis and acts at the cellular level, generating reversible motions. In this Review, we present representatives of each of these four mechanisms: pine cones, wheat awns, the twisted opening of Bauhinia pods and the seed of the stork’s bill; the resurrection plant; ice plant seed capsules and carrotwood seed pod; the wilting and redressing of plant stems. Natural polymeric materials produced by animals can also exhibit hydration-driven shape and strength recovery: bird feathers and hair are prime examples. Spider silk — a non-keratinous biopolymer — also exhibits humidity-driven reversible deformation. After describing these animal-based mechanisms, we outline bioinspired applications to actuate multifunctional and biocompatible smart materials, and indicate future directions of research with potential for new bioinspired designs. The influx and efflux of water in biological structures leads to reversible deformation, which has important functions in plants (for example, in seed protection and dispersal) and animals (for example, in the recovery of the strength and shape of feathers, and for reversible changes in silk and hair). Here the authors review the main hydration-induced deformation mechanisms and highlight applications inspired by these processes.