The base material for the tissue-contacting surface can be any of a wide variety of materials including suitable metals such as titanium or stainless steel or a suitable polymer such as polyolefins, such as, polyethylene and polypropylene, polyisobutylene and ethylene-alphaolefin copolymers; silicone polymers such as polydimethylsiloxane; acrylic polymers and copolymers, such as polyacrylate, poly