Energy Harvesting across Temporal Temperature Gradients using Vaporization Energy harvesting is an attractive alternative to carrying onboard power for mobile robots, especially for long duration missions. While solar is a powerful option, alternatives are needed for situations where direct sunlight is unavailable. One intriguing concept was proposed in the 17th century to power clocks: energy harvesting based on temporal, rather than spatial, temperature gradients, using a low boiling point fluid that vaporizes at ambient temperatures. This concept has many strengths: it offers allinone energy harvesting and storage; direct highforce and large displacement mechanical output, eliminating the need for a motor; and temporal gradients are ubiquitous, due to diurnal thermal fluctuations. The challenge for robotic applications, however, is to create large enough amounts of work in a small enough package to power a mobile device while using a nontoxic and readily available working fluid. Here we present a simple, lowcost energy harvesting actuator, powered by the vaporization of butane and isobutane, with an isobaric energy density of up to 38000Jm3 (i.e. energy extracted per total volume expansion) each time the temperature fluctuates 13.1xc2xb0C, enough to power a small car to drive 10m. Two principles enable this: i) precompression of the working fluid, allowing us to tune the boiling point and choose among many nontoxic fluids that do more work than noncompressed fluids; and ii) a constant force profile of the return springs, allowing more work than a linear spring. We present a simple model of the actuator and experimental results characterizing its behavior. Our work lays the foundation for energy harvesting across temporal temperature gradients using vaporization as a viable option for powering mobile robots.

Autonomic perspiration in 3Dprinted hydrogel actuators In both biological and engineered systems, functioning at peak power output for prolonged periods of time requires thermoregulation. Here, we report a soft hydrogelbased actuator that can maintain stable body temperatures via autonomic perspiration. Using multimaterial stereolithography, we threedimensionally print fingerlike fluidic elastomer actuators having a polyNisopropylacrylamide (PNIPAm) body capped with a microporous (200 micrometers) polyacrylamide (PAAm) dorsal layer. The chemomechanical response of these hydrogel materials is such that, at low temperatures ( 30xc2xb0C), the pores dilate to enable localized perspiration in the hydraulic actuator. Such sweating actuators exhibit a 600% enhancement in cooling rate (i.e., 39.1xc2xb0C minutexe2x88x921) over similar nonsweating devices. Combining multiple finger actuators into a single device yields soft robotic grippers capable of both mechanically and thermally manipulating various heated objects. The measured thermoregulatory performance of these sweating actuators (107 watts kilogramxe2x88x921) greatly exceeds the evaporative cooling capacity found in the best animal systems (35 watts kilogramxe2x88x921) at the cost of a temporary decrease in actuation efficiency.