Scaffolds may also be constructed from natural materials: in particular different derivatives of the extracellular matrix have been studied to evaluate their ability to support cell growth. Protein based materials - such as collagen, or fibrin, and polysaccharidic materials- like chitosan or glycosaminoglycans (GAGs), have all proved suitable in terms of cell compatibility. Among GAGs, hyaluronic acid, possibly in combination with cross linking agents (e.g. glutaraldehyde, water-soluble carbodiimide, etc.), is one of the possible choices as scaffold material. Another form of scaffold is decellularized tissue. This is a process where chemicals are used to extracts cells from tissues, leaving just the extracellular matrix. This has the benefit of a foully formed matrix specific to the desired tissue type. However, the decellurized scaffold may present immune problems with future introduced cells.