Tissue expansion is a technique used by plastic and restorative surgeons to cause the body to grow additional skin, bone or other tissues. Skin expansion is a common surgical procedure to grow extra skin through controlled mechanical overstretch.It creates skin that matches the color, texture, and thickness of the surrounding tissue, while minimizing scars and risk of rejection. When skin is stretched beyond its physiological limit mechanotransduction pathways are activated. This leads to cell growth as well as to the formation of new cells. In some cases, this may be accomplished by the implantation of inflatable balloons under the skin (see figure). By far the most common method, the surgeon inserts the inflatable expander beneath the skin and periodically, over weeks or months, injects a saline solution to slowly stretch the overlaying skin. The growth of tissue is permanent, but will retract to some degree when the expander is removed.
Info taken from Wikipedia