Examples of biocompatible polymers from which porous sheets can be formed include polyesters, such as poly(ethylene terephthalate), polylactide, polyglycolide, and copolymers thereof; fluorinated polymers, such as polytetrafluoroethylene (PTFE), expanded PTFE (ePTFE) and poly(vinylidene fluoride); polysiloxanes, including polydimethyl siloxane; and polyurethanes, including polyetherurethanes, poly