Hydrocollator packs are filled with one of a few different heat-retaining materials or substances. They can contain thermal beads, gel, silica, or even mediums like rice or corn. Regardless of the pack’s consistency, they are all made to absorb and hold heat when placed in a hydrocollator. Once sufficiently warm, hydrocollator packs can be placed on various areas of the body to speed healing or relieve pain and stiffness.