The document discusses renewable energy generation from evaporating water using hygroscopic materials like bacterial spores. It describes how nanoscale confinement of water in bacterial spores induces large pressure changes during wetting and drying, allowing the design of evaporation-driven engines. Two types of engines are presented - an oscillatory engine that uses expanding and contracting hygroscopic artificial muscles (HYDRAs) to alternately open and close shutters, and a rotary engine consisting of HYDRAs attached to concentric rings that rotates when their centers of mass shift due to differing curvature changes. Both engines are able to power small devices like LEDs and drive miniature cars. Challenges include the slow rate of evaporation, while future