Materials that respond mechanically to external chemical stimuli have applications in biomedical devices, adaptive architectural systems, robotics and energy harvesting1,2,3,4,5. 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 m−3, 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.
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This work is supported by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under award no. DE-SC0007999 (spore energy density measurements), the Rowland Junior Fellows Program (bio-hybrid hygromorph actuators), and the Wyss Institute for Biologically Inspired Engineering (hygrovoltaic generators). The authors thank D. E. Ingber for encouragement and comments on the manuscript, C. P. Stokes for help with the experimental set-up for rapidly switching the humidity levels surrounding the spores, D. Bell for help with electron microscopy and J. M. Sungur for prototype device manufacturing.
A patent application has been filed by Harvard University. US provisional application no. 61/415,902: ‘Bacterial spore based energy system’.
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Chen, X., Mahadevan, L., Driks, A. et al. Bacillus spores as building blocks for stimuli-responsive materials and nanogenerators. Nature Nanotech 9, 137–141 (2014). https://doi.org/10.1038/nnano.2013.290
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