U.S. patent application 20190059619 (applicant Aspen Surgical Products Inc., Caledonia, Mich.):
"Disclosed is a self-heating personal covering that includes a first layer and a second layer that is parallel to and opposite of the first layer, and at least a portion of the second layer is fastened to at least a portion of the first layer. The self-heating personal covering also includes at least one heat source positioned between the first layer and the second layer. The at least one heat source is held in a prescribed position by the first layer and the second layer, and includes a material that undergoes an exothermic reaction upon exposure to oxygen. ... [At] least one heat distributing structure is a layer of phase change material. The layer of phase change material may be positioned between the at least one heat source and the second layer."