One of the hurdles for making enduring, adaptable, and agile robots is managing the robots’ internal temperature. If the high-torque-density motors and exothermic engines that power a robot overheat, the robot will cease to operate. This is a particular issue for soft robots made of synthetic materials. While more flexible, they hold their heat unlike metals, which dissipate heat quickly. An internal cooling technology, such as a fan, may not be much help because it would take up space inside the robot and add weight.
Researchers have created a soft robot muscle that can regulate its temperature through sweating. They created the necessary nanopolymer materials for sweating via a 3D printing technique called multi-material stereolithography, which uses light to cure resin into predesigned shapes. Mixtures of nanoparticles and polymeric materials allow control of the viscosity of the fluids.
The researchers fabricated fingerlike actuators composed of two hydrogel materials that can retain water and respond to temperature — in effect, smart sponges. The base layer, made of poly-N-isopropylacrylamide, reacts to temperatures above 30 °C (86 °F) by shrinking, which squeezes water up into a top layer of polyacrylamide that is perforated with micron-sized pores. These pores are sensitive to the same temperature range and automatically dilate to release the “sweat,” then close when the temperature drops below 30 °C.
The evaporation of this water reduces the actuator’s surface temperature by 21 °C within 30 seconds, a cooling process that is approximately three times more efficient than in humans. The actuators are able to cool off roughly six times faster when exposed to wind from a fan. Sensors or other components are not needed to control the sweating rate. When the local temperature rises above the transition, the pores simply open and close on their own.
The team incorporated the actuator fingers into a robot hand that could grab and lift objects and realized that autonomous sweating not only cooled the hand but lowered the temperature of the object as well. While the lubrication could make a robot hand slippery, modifications to the hydrogel texture could compensate by improving the hand’s grip, much like wrinkles in skin.
One disadvantage of the technology is that it can hinder a robot’s mobility. There is also a need for the robots to replenish their water supply, which has led the team to envision soft robots that will someday not only perspire like mammals but drink like them, too. The ability of a robot to secrete fluids could also lead to methods for absorbing nutrients, catalyzing reactions, removing contaminants, and coating the robot’s surface with a protective layer.