Hydration Solids and the Hygroelastic Theory*

Hygroscopic biological matter accounts for a large fraction of biological matter on Earth. These water-responsive materials have diverse chemical compositions, but they also share some common physical characteristics like changes in size and stiffness with relative humidity. Based on atomic force microscopy measurements on the hygroscopic spores of a common soil bacterium, we present the hygroelastic theory that provides a microscopic physical basis not just for the mechanical behaviors common to hygroscopic biological matter but for many equilibrium and non-equilibrium mechanical behaviors, finding that these are controlled by the hydration force (1). Furthermore, the hygroelastic theory explains an extreme slowdown of water transport and successfully predicts a strong nonlinear elasticity and a transition in mechanical properties that differs from glassy and poroelastic behaviors. These results indicate a class of matter that we termed hydration solids. Due to the simplicity of the underlying assumptions, many types of biological matter could belong to this distinct class of solid matter.

 

[1] Harrellson, S.G. et al. Hydration solids. Nature 619, 500–505 (2023).