Relative thrombogenicity of polydimethylsiloxane and silicone rubber constituents

Medical grade silicone rubber (MGSR) is composed of polydimethylsiloxane (PDMS) as well as silica filler, oxidation products from the curing process, and other components. In a test that excludes air-blood interfaces, PDMS radiation cured under nitrogen has a whole blood clotting time 22% longer than MGSR. Curing the PDMS under oxidizing conditions maintains a 10% prolongation, but addition of silica filler to the PDMS returns the clotting time to that of MGSR. Extracting MGSR with solvents other than water appreciably lowers the clotting time. These results indicate the "pure" PDMS has an intrinsically high thromboresistance. Thrombogenicity is increased by the use of silica filler and oxidizing cure, as in MGSR.