Collagen is the most prevalent protein in the connective tissue of animals and constitutes approximately 25% of total body protein in vertebrates. The molecular subunit of collagen, called tropocollagen, is a rigid rod with a molecular weight ~300 kDa. Tropocollagen self assembles in to larger structural units called fibrils or fibers. While there are several types of collagen, these larger units all share the characteristic triple helical structure but have variations in the length of the non-helical and helical sections and variations in the number of carbohydrates attached to the helical fraction.
There are four commonly used types of collagen, and here we will follow the nomenclature proposed by Kauzmann (1959). Type I collagen is commonly found in skin, bone and tendons while type II is common in cartilage. Type III collagen is prevalent in the vasculature structures (i.e. blood vessel walls) and can also be found in small amounts in skin. Type IV collagen is unique from other types in that is it largely non-helical and does not form fibrils. Collagen type IV is found in the basement membrane that separates epithelial and mesodermal tissues. Gelatin is an irreversibly denatured, hydrolyzed form of collagen that is commonly used in pharmaceuticals, cosmetics, and foodstuffs. Most medical grade collagen and gelatin are obtained from closely regulated bovine and porcine tissue. However, human collagen can be obtained by recombinant methodology, isolation from tissues such as lung and placenta, or from human cell culture.
Collagen is used as a biomaterial because of its widespread abundance in nature and ease of modification. Collagen is composed of some 20 amino acids each providing pendant side groups available for chemical modification. The prevalence of hydroxyl, amine, and carboxylic acid groups allow simple crosslinking between and within collagen units. Crosslinking collagen can decelerate the degradation of implanted materials, and is thought to reduce immunogenicity in xenografted collagen materials. Self-crosslinking of collagen via interchain peptide bonds can be induced by dehydration when exposed to high temperatures (i.e. >105 ºC). Dialdehydes are also commonly used to crosslink collagen and have been long used in the leather industry as tanning agents. Exposure to glutaraldehyde produces collagen with firm covalent crosslinks and elastic behavior.
Numerous commercial products have been developed from collagen and collagen based technologies. A few examples are dermal fillers Cosmoderm and Zyderm, the burn/wound treatment Dermagraft, and the sponge-like Gelfoam.