Explaining the mechanical properties of hygroscopic bacterial spores using water nanoconfinement*

Nanoconfined water can exhibit unusual properties. Water filled nanopores are ubiquitous in biological materials, but the effects of water nanoconfinement on their mechanical properties is not well understood. Recently, bacterial spores have been used as hygroscopic actuators for energy harvesting and soft robotics applications. Here we show the coupling of the mechanical properties and water nanoconfinement in Bacillus Subtilis spores can be understood by using a simple statistical mechanical model. Using nanomechanical experiments to probe the stiffness of the spore over four frequency decades, we found that the effective elastic modulus increases by up to an order of magnitude, far larger than common models of water transport predict. Force-Indentation experiments reveal a strain-dependent elastic modulus which can be understood using our statistical mechanical model. This work could be used to build intuitive understanding of the mechanical properties of nanoporous hygroscopic materials and could be used as the basis for the development of materials with frequency dependent mechanical properties.