When patients are immobilized in bed, friction between the skin and the bed covers may damage the skin, particularly at contact points where there is a bony prominence like the anklebone or heel. Damaged skin may break down, forming an open sore (pressure ulcer). As pressure ulcers develop, they often lead to infections of the soft tissues and bones, complicating care and prolonging hospitalization. Pressure ulcers are difficult to treat. Until recently, the only treatments available were local wound care (such as washing the ulcer with salt water or applying cleaning enzymes and protective coatings) or surgery. Recently, researchers have found that certain tissues of the body produce substances that promote healing by stimulating the growth of cells to replace those that have been damaged or lost. The first of these stimulating substances to be discovered was called nerve growth factor because it caused nerve cells to grow into damaged tissues. When researchers studied nerve growth factor further, they found that it also stimulated skin cells and other cells that helped repair damaged tissue.