Energy harvesting technologies that are engineered to miniature sizes, while still increasing the power delivered to wireless electronics, (1, 2) portable devices, stretchable electronics, (3) and implantable biosensors, (4, 5) are strongly desired. Piezoelectric nanowire- and nanofiber-based generators have potential uses for powering such devices through a conversion of mechanical energy into electrical energy. (6) However, the piezoelectric voltage constant of the semiconductor piezoelectric nanowires in the recently reported piezoelectric nanogenerators (7-12) is lower than that of lead zirconate titanate (PZT) nanomaterials. Here we report a piezoelectric nanogenerator based on PZT nanofibers. The PZT nanofibers, with a diameter and length of approximately 60 nm and 500 microm, were aligned on interdigitated electrodes of platinum fine wires and packaged using a soft polymer on a silicon substrate. The measured output voltage and power under periodic stress application to the soft polymer was 1.63 V and 0.03 microW, respectively.

Evaporation is a ubiquitous phenomenon in the natural environment and a dominant form of energy transfer in the Earth's climate. Engineered systems rarely, if ever, use evaporation as a source of energy, despite myriad examples of such adaptations in the biological world. Here, we report evaporation-driven engines that can power common tasks like locomotion and electricity generation. These engines start and run autonomously when placed at air–water interfaces. They generate rotary and piston-like linear motion using specially designed, biologically based artificial muscles responsive to moisture fluctuations. Using these engines, we demonstrate an electricity generator that rests on water while harvesting its evaporation to power a light source, and a miniature car (weighing 0.1 kg) that moves forward as the water in the car evaporates. Evaporation-driven engines may find applications in powering robotic systems, sensors, devices and machinery that function in the natural environment.

Materials that respond mechanically to external chemical stimuli have wide--ranging applications in biomedical devices, adaptive architectural systems, robotics, and energy harvesting 1 . Synthesis and design principles inspired by biological systems have led to materials with capabilities for highly controlled and complex shape change 2 , oscillations 3 , fluid transport 4 , and homeostasis 5 . Despite the enhanced control over material behavior, the effectiveness of synthetic stimuli--responsive materials in generating work has been limited when compared to mechanical actuators 6 . Biological organisms with structures responsive to water gradients could potentially offer a solution for the limited work density of stimuli--responsive materials. Because water--responsive biological structures accomplish vital tasks like ascent of sap 7,8 , dispersal and self--burial of seeds 9,10 , they could possibly exhibit high energy densities and serve as building blocks of stimuli--responsive materials effective in generating work. Furthermore, biological nature of these materials offers the possibility of improving their characteristics through genetic mutations 11,12 . Here we report the discovery that the response of the spores of Bacillus to water potential gradients exhibit energy densities more than 10 MJ/m 3 , exceeding best synthetic water--responsive materials by 1000--fold 13,14 . We also identified a mutant spore form that nearly doubles the energy density relative to its wild type, highlighting the possibility for further improvements with genetic engineering of spores. We found that spores can self--assemble into dense, submicron--thick monolayers on substrates like silicon microcantilevers and elastomer sheets, creating bio--hybrid hygromorph actuators 15 . The spore monolayers forming these hygromorphs exhibited high--energy density and rapid response to changing water potentials. As an application of the strong mechanical response of spores, we have built an energy harvesting device that can remotely generate electrical power from an evaporating body of water. These results demonstrate that spores have a significant potential as building blocks of stimuli--responsive materials with dramatically enhanced capabilities for energy harvesting, storage, and actuation of robotic devices.Bacillus spores are dormant cells that can withstand harsh environmental conditions for long periods of time and still maintain biological functionality 16 (Fig. 1a,b). Despite their dormancy, spores are dynamic structures. For example, Bacillus spores respond to changes in relative humidity (RH) by expanding and shrinking and changing their diameter by as much as 12% 17--19 . We have used an atomic force microscope (AFM) based experiment (Fig. 2c) to determine the energy density of individual spores as they respond to changes in RH. By adjusting force and RH, we have created a thermodynamic cycle, in which individual spores go through four stages illustrated in Fig. 1d. In stage I, the spores rest at low RH (~20%). In stage II,...

About 50% of the solar energy absorbed at the Earth’s surface drives evaporation, fueling the water cycle that affects various renewable energy resources, such as wind and hydropower. Recent advances demonstrate our nascent ability to convert evaporation energy into work, yet there is little understanding about the potential of this resource. Here we study the energy available from natural evaporation to predict the potential of this ubiquitous resource. We find that natural evaporation from open water surfaces could provide power densities comparable to current wind and solar technologies while cutting evaporative water losses by nearly half. We estimate up to 325 GW of power is potentially available in the United States. Strikingly, water’s large heat capacity is sufficient to control power output by storing excess energy when demand is low, thus reducing intermittency and improving reliability. Our findings motivate the improvement of materials and devices that convert energy from evaporation.