Soft robot keeps cool by replicating sweating

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The human body’s ability to regulate temperature through sweating has inspired engineers to replicate the process in soft robot muscle.

A Cornell team made a 3D-printed hand with hydraulically controlled fingers that can cool itself by sweating (Image: Cornell University)

This form of thermal management is a basic building block for enabling untethered, high-powered robots to operate for long periods of time without overheating, according to Rob Shepherd, associate professor of mechanical and aerospace engineering at Cornell University, who led the project. The team's paper is published in Science Robotics.

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According to Shepherd, a hurdle for making enduring, adaptable and agile robots is managing their internal temperature. If the high-torque density motors and exothermic engines that power a robot overheat, the robot will cease to operate.

Soft robots have the advantage of flexibility, but the synthetic materials they are made of retain heat. Internal cooling technology, such as a fan, would take up space inside the robot and add weight.

"The ability to perspire is one of the most remarkable features of humans," said co-lead author TJ Wallin, a research scientist at Facebook Reality Labs. "Sweating takes advantage of evaporated water loss to rapidly dissipate heat and can cool below the ambient environmental temperature...So as is often the case, biology provided an excellent guide for us as engineers."

Shepherd's team partnered with the lab of Cornell engineering professor Emmanuel Giannelis to create the necessary nanopolymer materials for sweating via multi-material stereolithography, which uses light to cure resin into predesigned shapes.

The researchers are said to have fabricated finger-like actuators composed of two hydrogel materials that retain water and respond to temperature. The base layer, made of poly-N-isopropylacrylamide, reacts to temperatures above 30oC (86oF) by shrinking, which squeezes water up into a top layer of perforated polyacrylamide. According to Cornell, these pores are sensitive to the same temperature range and automatically dilate to release the ‘sweat,’ then close when the temperature drops below 30oC.

The evaporation of this water reduces the actuator's surface temperature by 21oC within 30 seconds, a cooling process that is approximately three times more efficient than in humans, the researchers found. The actuators are able to cool off roughly six times faster when exposed to a fan.

One disadvantage of the technology is that it can hinder a robot's mobility, and there is also a need for the robots to replenish their water supply. This has led Shepherd to envision soft robots that will one day drink as well as perspire.