Hydration solids.

Hygroscopic biological matter in plants, fungi and bacteria make up a large fraction of Earth's biomass 1 . Although metabolically inert, these water-responsive materials exchange water with the environment and actuate movement 2-5 and have inspired technological uses 6,7 . Despite the variety in chemical composition, hygroscopic biological materials across multiple kingdoms of life exhibit similar mechanical behaviours including changes in size and stiffness with relative humidity 8-13 . Here we report atomic force microscopy measurements on the hygroscopic spores 14,15 of a common soil bacterium and develop a theory that captures the observed equilibrium, non-equilibrium and water-responsive mechanical behaviours, finding that these are controlled by the hydration force 16-18 . Our theory based on the hydration force 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 behaviours. These results indicate that water not only endows biological matter with fluidity but also can-through the hydration force-control macroscopic properties and give rise to a 'hydration solid' with unusual properties. A large fraction of biological matter could belong to this distinct class of solid matter.