Collagen is a major type of protein in mammals, with 25% to 35% of all protein content. People can find collagen in various body parts, including skins, bones, teeth, muscles, tendons, ligaments, cartilage, and other connective tissues. Depending on the types, collagens can play different roles. Type I collagen is a strong, long, thin fibril densely packed by collagen molecules. It can boost cell attachment and provide structural support. Type II collagen is a main collagenous component of cartilage, with relatively lower mineralization. Because of its loose molecule binding structure, type II collagen has better flexibility to bear mechanical stress and avoid bone friction. Up to now, there are sixteen collagens in the human body at least.