 Tissue engineering, also called regenerative medicine, refers to the attempt to create functional human tissue from cells in a laboratory. Its ultimate goal is to be a cure, not merely a treatment, by repairing or replacing tissues and organs that fail due to disease, genetic errors, congenital abnormalities, or traumatic injury. Tissue engineering relies on four important factors. The right cells to do the job, the right environment, such as a scaffold, to support the cells, the right biomolecules, like growth factors to make those cells healthy and productive, and physical and mechanical forces to influence the development of the cells. The cells can be directly harvested from the target organ, developed from precursor, or stem cells, or taken from lines grown in the lab, all ideally from the patient as that limits problems with rejection. The supporting structures can be derived from donor tissue, or from natural or synthetic polymers made to order for their strength or endurance. Biomolecules can be added directly, or coaxed from the cells that take up residence on the scaffold. Sometimes the scaffolds dissolve over time, but others remain to provide support to the organ. Some examples of tissues and organs that have already been successfully tissue engineered and implanted in humans include bladders, small arteries, skin grafts, cartilage, and even a full trachea.