Hygroscopic biological matter in plants, fungi and bacteria make up a large fraction of Earth’s biomass¹. Although metabolically inert, these water-responsive materials exchange water with the environment and actuate movement^2,3,4,5 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^{8,9,10,11,12,13}. 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^16,17,18. 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.

植物、真菌和细菌中的吸湿性生物物质占地球生物量的很大一部分¹ 。尽管代谢惰性，这些水响应材料可与环境交换水并驱动运动^2,3,4,5并激发了技术用途^6,7 。尽管化学成分各不相同，但多个生命领域的吸湿性生物材料表现出相似的机械行为，包括尺寸和刚度随相对湿度的变化^{8,9,10,11,12,13} 。在这里，我们报告了对常见土壤细菌的吸湿孢子^14,15的原子力显微镜测量，并发展了一种捕获观察到的平衡、非平衡和水响应机械行为的理论，发现这些行为是由水合力控制的^{16, 17,18} 。我们基于水合力的理论解释了水传输的极度减慢，并成功预测了强非线性弹性以及不同于玻璃态和多孔弹性行为的机械性能转变。这些结果表明，水不仅赋予生物物质流动性，而且还可以通过水合力控制宏观性质，并产生具有不寻常性质的“水合固体”。很大一部分生物物质可能属于这一类独特的固体物质。