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tissue engineering The creation of synthetic or semisynthetic tissue that can be used instead of human tissue in surgery. Different kinds of tissue have been developed or are currently being researched, including skin, bone, cartilage, cornea, and spinal tissue, using various combinations of biopolymers, cultured cells, and growth factors. For example, the first such product to gain approval for clinical use was a form of artificial skin consisting of a thin sheet of collagen gel infiltrated with two layers of cultured human cells – keratinocytes on the outer surface to form the ‘epidermis’, and fibroblasts on the inner surface to form the ‘dermis’. More rigid tissues, such as synthetic bone and cartilage, are typically based on a biopolymer scaffold, which is seeded with cultured bone cells or cartilage cells to secrete the natural tissue material. The scaffold can be inserted in situ, for example at a fracture site, or used to construct a new body part entirely in vitro. For successful colonization and growth by cultured cells or the body's own cells, the scaffold is treated with appropriate growth factors; for example, bone cells require a substance called bone morphogenetic protein.