Developed by four engineering students at MIT, Wristify makes you feel cooler by reducing the skin temperature of the wrist by just a few fractions of a degree per second, over a few seconds. The engineers are still working on finding the optimal cooling cycle, but so far the best appears to be to cool the wrist by 0.4°C per second for five seconds, and then turn off for 10 seconds. With this pulse cooling, the students say that wearers perceive whole-body cooling of a few degrees Celsius. If needed, Wristify can also reverse its operation to provide heating instead. 

The device is essentially a copper alloy heat sink that uses thermoelectric cooling to reduce the skin temperature of the wrist. Thermoelectric cooling is governed by the Peltier effect, which describes heating or cooling caused by electric current flowing across a junction of two different conductors. As current traverses these junctions, one side heats up while the other side cools down. If you have a series of these junctions, and keep a heat sink on the hot side, you can have a very effective heat pump. Peltier coolers are not efficient, but they do have the advantage of having no moving parts or circulating fluids. All you need is a Peltier heating/cooling element, a heat sink and a battery. The main advantage of a personal cooling solution is that it is much more efficient than cooling an entire building. The team says that adjusting the temperature of a single building by 1°C can consume 100 kilowatt-hours of electricity per month. Wristify, on the other hand, can be powered for eight hours by a small lithium battery.