Materials that respond mechanically to external chemical
stimuli have applications in biomedical devices, adaptive architectural systems, robotics and energy harvesting1–5. Inspired by
biological systems, stimuli-responsive materials have been
created that can oscillate2, transport fluid3, provide homeostasis4 and undergo complex changes in shape5. However, the
effectiveness of synthetic stimuli-responsive materials in generating work is limited when compared with mechanical actuators6. Here, we show that the mechanical response of Bacillus
spores to water gradients exhibits an energy density of more
than 10 MJ m23
, which is two orders of magnitude higher
than synthetic water-responsive materials7,8. We also identified
mutations that can approximately double the energy density of
the spores and found that they can self-assemble into dense,
submicrometre-thick monolayers on substrates such as silicon
microcantilevers and elastomer sheets, creating bio-hybrid
hygromorph actuators9,10. To illustrate the potential applications of the spores, we used them to build an energy-harvesting device that can remotely generate electrical power from an
evaporating body of water