Due to its high biocompatibility and its common presence in the extracellular matrix of tissues, hyaluronan is gaining popularity as a biomaterial scaffold in tissue engineering research. In particular, a number of research groups have found hyaluronan's properties for tissue engineering and regenerative medicine are significantly improved with cross-linking, producing a hydrogel. The pioneering work on crosslinked hyaluronan derivatives was initiated by a small research group headed by Prof. Aurelio Romeo in the late 1980s. Crosslinking allows a researcher to form a desired shape, as well as to deliver therapeutic molecules, into a host. Hyaluronan can be crosslinked by attaching thiols (trade names: Extracel, HyStem), methacrylates, hexadecylamides (trade name: Hymovis), and tyramines (trade name: Corgel). Hyaluronan can also be crosslinked directly with formaldehyde (trade name: Hylan-A) or with divinylsulfone (trade name: Hylan-B).