Biological materials have exceptional mechanical properties in comparison to synthetic or engineered materials. Wood has a strength (the ability to carry a load) per unit weight that is comparable to that of the strongest steels. Seashell, bone, antler, and keratin have a toughness (the ability to resist fracture) more than 10 times greater than engineering ceramics. And mature bamboo stalks or culms have slenderness ratios (the ratio of length to diameter) that are remarkable even by standards of modern engineering. Natural materials are optimized to fulfill the complex biological and mechanical requirements posed by the way plants and animals function. Frequently, these requirements are mechanical in nature. For example, tree trunks and branches as well as animal legs and wings support or propel the organism. In doing so, their structural materials (wood and bone) are highly stressed. Beyond this, living natural materials are unique in that they can self-repair and adapt to changes during their lifetime (bone) or during the cycle of a year (wood), modifying their properties according to the requirements placed on them.