Biologically inspired architectures for evaporation-driven active materials

The overarching goal of the proposed research is to further investigate biologically inspired architectures for evaporation-driven active materials. Both experimental and theoretical work will be conducted to understand the coupling between nanoscale hydration-driven energy conversion and the macroscale response of these materials. The proposed research is based on the Principal Investigator~s recent discovery that when bacterial spores (i.e. dry, dormant bacteria) shrink and swell with changing humidity, they push and pull other objects forcefully. In fact, they exhibit energy densities in mechanical actuation higher than all state of the art actuator materials and more than two orders of magnitude higher than synthetic water-responsive polymers. Because bacterial spores maximize work done for a given amount of water absorbed and released, they can convert energy from evaporation efficiently. Therefore, the Principal Investigator used bacterial spores as the fundamental building block to develop macroscopic, hygroscopically-driven, artificial muscles. He then used those macroscopic, hygroscopically-driven, artificial muscles to develop two novel devices that derive power directly from evaporation: a floating, oscillatory engine that generates electricity causing a light to flash, and a rotary engine that drives a miniature car. The Principal Investigator has predicted key characteristics of evaporation-based power based on theoretical modeling. He estimates evaporation power density output to be 1-9 W/m2, which he states is comparable to reported total area power densities for United States wind (2.90 W/m2) and photovoltaic (8.06 W/m2). Further, he estimates an annual water savings of 1-6 mmH2O/day. Finally, he calculates from simulations a maximum power density generation of 2.4, 4.9, an 8.1 W/m2 with capacity factors of 72%, 82%, and 69% (at 90% of maximum generation) for New York, Texas, and California test locations, respectively; surpassing those of U.S. wind (31%) and photovoltaic(21%) installations over the past 5 years. Complete resurrection of bacterial spores would make any energy harvesting device using spore power less efficient. Spores come to life when there is an appropriate trigger like certain chemicals or nutrients, but water by itself is not a trigger. Additionally, the bacteria could be engineered to make it even less likely they would come to life. The proposed research aims to (1) improve the power output of evaporation-driven active materials; (2) investigate ~super-expansion~ phenomenon with spore-inspired materials for evaporation-driven active liquids; (3) develop theoretical models to determine the limitations imposed by heat and moisture transport in the environment on evaporation-driven active materials, and (4) construct evaporation driven active materials and devices that exhibit higher work and power density, specifically printable curling hygroscopically-driven artificial muscles and a floating oscillatory engine with a horizontally moving shutter mechanism that can have energy outputs comparable with solar panels at similar size. The proposed effort, if successful, is expected to yield advanced materials that can respond strongly to external stimuli by creating extreme actuation pressures (exceeding 100 MPa) and work densities approaching 20 MJ/m3. Such materials would likely find applications in manufacturing and in robotic systems. In addition, the effort would lead to novel energy harvesting devices that can function over the ocean surface, be easily deployed, and cost substantially less than other energy harvesting methods like wind and solar power.