Certain medical grades of silicone rubber have been found to be very useful for implantation in the extremely demanding environments encountered inside man's body. These materials are used in plastic surgery as replacement for soft tissue and cartilage. They are also useful as prostheses, tubes or devices that are used in contact with various body fluids, such as bile, stomach acids, urine and blood. Fairly recently there have been a few instances of failures of artificial heart valves prostheses, which have been caused by an absorption of fatty materials into silicone rubber components of these prostheses. These failures are a matter of great concern, and efforts are being made to study this problem with the hope of finding some solution.