U.S. patent application 20150272775 (applicants Eric Fayez Chebab, et al, Mountain View, Calif.):
"Thermal packs adapted for use in a muscle stimulation system, comprising an external housing with a first material therein and at least one phase change element therein. ... In preferable embodiments, the thermal packs also contain small packets or pods of a second material (which in this disclosure may be referred to as a phase change material). In preferable embodiments this phase change material will be a liquid at room temperature and have a freezing point warmer than that of the gel material used."