Robot sweat regulates temperature, key for extreme conditions

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, who led the project.

The team's paper, "Autonomic Perspiration in 3D Printed Hydrogel Actuators," published in Science Robotics.

One of the hurdles for making enduring, adaptable and agile robots is managing the robots' internal temperature, according to Shepherd. 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, which are 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.

So Shepherd's team took inspiration from the natural cooling system that exists in mammals: sweating.

"The ability to perspire is one of the most remarkable features of humans," said co-lead author T.J. 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 a 3D-printing technique called multi-material stereolithography, which uses light to cure resin into predesigned shapes.

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 researchers found. The actuators are able to cool off roughly six times faster when exposed to wind from a fan.

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 Shepherd to envision soft robots that will someday not only perspire like mammals, but drink like them, too.