From IEEE Spectrum:
The researchers, from the University of Tokyo’s JSK Lab led by Professor Masayuki Inaba, were trying to figure out how to add a cooling system to their 1.7-meter tall, 56-kilogram musculoskeletal humanoid named Kengoro (who joins Kojiro and Kenshiro as part of the JSK robot family). Kengoro is already stuffed to the brim with structural components, circuit boards, gears, and 108 motors (!), and there was simply no room to add active water cooling with tubes and a radiator and fans. The researchers started looking at how they could make better use of Kengoro’s existing components, and they came up with the idea of using the robot’s skeletal structure (its metal frame) as a coolant-delivery system.
The approach goes way beyond just running water channels through the frame and circulating water through them, since that wouldn’t have solved the problem of needing to place a radiator in there somewhere. The researchers instead decided to try a passive technique, allowing the water to seep out through the frame around the motors to cool them evaporatively. In other words, Kengoro sweats.