In vivo testing of a biostable polyurethane.

At present all the commercially available "medical grade" urethane elastomers exhibit a phenomenon known as environmental stress cracking (ESC). This phenomenon is characterized by surface microcracking when the elastomer is elongated while in vivo. The degree of strain that is required to initiate microcracking varies from composition to composition. It has been found that harder compounds generally tend to have a higher strain threshold than corresponding softer ones. We theorized that this degradation occurs when certain enzymes (present only in vivo) attack and break down the ether linkages that link the polymer molecules together. Those elastomers that contain more ether linkages (such as the softer compositions) appear to microcrack more easily than elastomers with fewer ether linkages (such as the harder ones). The molecular composition of ChronoFlex urethane has been chosen so that the finished elastomer will be free of ether linkages; thus, it is expected to be immune from environmental stress cracking.