Autonomic perspiration in 3D-printed 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 hydrogel-based actuator that can maintain stable body temperatures via autonomic perspiration. Using multimaterial stereolithography, we three-dimensionally print finger-like fluidic elastomer actuators having a poly-N-isopropylacrylamide (PNIPAm) body capped with a microporous (similar to 200 micrometers) polyacrylamide (PAAm) dorsal layer. The chemomechanical response of these hydrogel materials is such that, at low temperatures (<30 degrees C), the pores are sufficiently closed to allow for pressurization and actuation, whereas at elevated temperatures (>30 degrees C), the pores dilate to enable localized perspiration in the hydraulic actuator. Such sweating actuators exhibit a 600% enhancement in cooling rate (i.e., 39.1 degrees C minute(-1)) over similar non-sweating 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 (similar to 107 watts kilogram(-1)) greatly exceeds the evaporative cooling capacity found in the best animal systems (similar to 35 watts kilogram(-1)) at the cost of a temporary decrease in actuation efficiency.