The gel inside ice packs is typically made of a substance called sodium polyacrylate, which has a high water content and can absorb many times its own weight in water. This allows the gel to remain pliable and flexible, even when frozen, and helps it to effectively transfer cold temperature to the surface it is applied to. Other ingredients may be added to the gel, such as glycerin or other moisturizing agents, to prevent dryness or cracking.