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A reversible sterilisation process for men and women has been patented by Jeffrey Callister, William Tremulis and Denise Harges of the US (WO 98/26737). It consists of a small plug of inert plastics, whose outer surface is coated with a growth-promoting enzyme. The plug is deformed by stretching it until it is slightly wider than its natural shape. The deformed plug is inserted using a catheter into a man鈥檚 vas deferens or a women鈥檚 Fallopian tube and left there for around a week, until tissue grows into the surface and binds with it. The plug is then heated by a warm fluid, which makes it revert to its natural shape, closing the tube and blocking the passage of eggs or sperm. A removable core can be pulled or screwed out to reverse the process.