text: Biological hydrogels are derived from preexisting components of body tissues such as collagen, hyaluronic acid (HA), or fibrin. Collagen, HA, and fibrin are components that occur naturally in the extracellular matrix of mammals. Collagen is the main structural component in tissues and it already contains cell-signaling domains that can promote cell growth. In order to mechanically enhance collagen into a hydrogel, it must be chemically crosslinked, crosslinked using UV light or temperature, or mixed with other polymers. Collagen hydrogels would be nontoxic and biocompatible.