Collagen is the most abundant protein in humans and animals; it makes up more than 30% of the total protein mass by some estimates. Collagen molecules assemble to form protein fibers that are used as versatile building materials for the extracellular matrix and connective tissues such as skin, tendon, cartilage, and bone. In these tissues, collagen is organized in ways that give each tissue its unique mechanical properties. For example, in skin, collagen is organized into bundles of fibers that are interwoven with elastic fibers to give skin its mechanical toughness and resiliency. The bone is another tissue that shows the versatility of collagen as a building material: collagen forms a composite with mineral crystals, and while the inorganic phase contributes to the stiffness and strength of bone, the collagen phase adds flexibility and ductility (Lakes 1993).