There are a number of non-invasive head cooling caps and helmets designed to target cooling at the brain. A hypothermia cap is typically made of a synthetic material such as neoprene, silicone, or polyurethane and filled with a cooling agent such as ice or gel which is either cooled to a very cold temperature, , before application or continuously cooled by an auxiliary control unit. Their most notable uses are in preventing or reducing alopecia in chemotherapy, and for preventing cerebral palsy in babies born with hypoxic ischemic encephalopathy. In the continuously cooled iteration, coolant is cooled with the aid of a compressor and pumped through the cooling cap. Circulation is regulated by means of valves and temperature sensors in the cap. If the temperature deviates or if other errors are detected, an alarm system is activated. The frozen iteration involves continuous application of caps filled with Crylon gel cooled to  to the scalp before, during and after intravenous chemotherapy. As the caps warm on the head, multiple cooled caps must be kept on hand and applied every 20 to 30 minutes.