Tissue expansion is a medical procedure used to grow extra skin for reconstructive purposes. It involves placing a special balloon-like device, called an expander, under the skin. Over time, the expander is gradually filled with saline, causing the skin to stretch and grow. Once enough skin has been generated, the expander is removed, and the new skin can be used to cover areas affected by injury, surgery, or congenital defects. This technique is commonly used in plastic surgery and can help improve the appearance and function of the affected area.