Robots are becoming more human-like every day: now they can sweat.

Thomas Wallin at Cornell University in New York and his colleagues have created soft robotic grippers that are capable of sweating to cool down.

The grippers are capable of a cooling capacity of 107 watts per kilogram, making them more efficient sweaters than mammals. By comparison, humans and horses have a maximum cooling capacity around 35 watts per kilogram.

Each gripper consists of three finger-like parts that bend simultaneously to grasp small objects. The 3D-printed grippers are made from hydrogels, polymer materials which can store large amounts of water. Each finger is made from an underlayer with an internal channel to let fluid flow and is capped with a surface layer containing micropores.

At cold temperatures, the pores close. At temperatures higher than 30°C, the surface layer expands, dilating the pores and enabling pressurised fluid from the underlayer to sweat out. The material responds spontaneously to temperature changes without the need for external sensors.

“Sweating takes advantage of evaporative water loss to rapidly dissipate heat,” said Wallin in a press briefing yesterday. Unlike convection or radiation, sweating lowers the temperature of a body below that of its environment, he said.

“When the local temperature rose above the transition, the pores would simply open and close on their own,” said Wallin.

When blown by wind from a fan, the sweating robots cooled at a rate of 39.1°C per minute, about six times faster than similar devices that are unable to sweat.

The technique could be used to help untethered robots operate for long periods of time without overheating, said team-member Robert Shepherd, also at Cornwell.

However, there is currently no means for the robot to replenish its fluid stores after sweating. This means “the robots that operate via the autonomous sweating that we’ve created would have to also be able to drink”, said Shepherd.