Several strategies have been used to increase the biostability of medical-grade polyurethanes while maintaining biocompatibility and mechanical properties. One approach is to chemically modify or replace the susceptible soft segment. Currently, poly(carbonate urethanes) (PCUs) are being evaluated as a replacement of poly(ether urethanes) (PEUs) in medical devices because of the increased oxidative stability of the polycarbonate soft segment. Preliminary in vivo and in vitro studies have reported improved biostability of PCUs over PEUs. Although several studies have reported evidence of in vitro degradation of these new polyurethanes, there has been no evidence of significant in vivo degradation that validates a degradation mechanism. In this study, the effect of soft segment chemistry on the phase morphology, mechanical properties, and in vivo response of commercial-grade PEU and PCU elastomers was examined. Results from dynamic mechanical testing and infrared spectroscopy suggested that the phase separation was better in PCU as compared with PEU. In addition, the higher modulus and reduced ultimate elongation of PCU was attributed to the reduced flexibility of the polycarbonate soft segment. Following material characterization, the in vivo biostability and biocompatibility of PEU and PCU were studied using a subcutaneous cage implant protocol. The results from the cage implant study and cell culture experiments indicated that monocytes adhere, differentiate, and fuse to form foreign body giant cells on both polyurethanes. It is now generally accepted that the reactive oxygen species released by these adherent macrophages and foreign body giant cells initiate PEU biodegradation. Attenuated total reflectance-Fourier transform infrared analysis of explanted samples provided evidence of chain scission and crosslinking in both polyurethanes. This indicated that the PCU was also susceptible to biodegradation by agents released from adherent cells. These results reinforce the need to evaluate and understand the biodegradation mechanisms of PCUs.

As part of ongoing studies in polyurethane biostability and biodegradation, we have investigated an in vitro system to test strained poly(etherurethane urea) (PEUU). Recently, we utilized this system to reproduce in vivo stress cracking in strained Pellethane. In this study, strained PEUU was tested to determine whether it degrades through a common mechanism with Pellethane and to further examine the steps involved in this degradation. Biaxially strained PEUU elastomers were treated with an alpha 2-macroglobulin (alpha 2-Mac) protein solution followed by an oxidative H2O2/CoCl2 treatment. Characterization of the strained PEUU specimens was performed with attenuated total reflectance-Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), electron spectroscopy for chemical analysis, and contact angle analysis. The results from these characterization techniques provide conclusive evidence that biodegradation of PEUU and Pellethane occurs through a common mechanism. Chemical changes to the PEUU include cleavage of the polyether soft segments and urethane linkages, leaving the hard segment domains unaffected. SEM analysis shows that this chain cleavage leads to the development of severe pitting and cracking of the PEUU surface. In addition, the in vitro degradation accurately reproduces the in vivo degradation chemically and physically. This result verifies that the primary species responsible for biodegradation of PEUUs, in vivo, are hydroxyl and/or hydroperoxide radicals. alpha 2-Mac pretreatment increases the rate of degradation compared to direct treatment in H2O2/CoCl2. As the PEUU soft segment chains are cleaved, the degradation products are extracted into the treatment solution or environment. Finally, a new biodegradation mechanism of PEUUs is presented that involves crosslinking of the polyether soft segments.

It is generally accepted that biodegradation of poly(etheruethane urea) (PEUU) involves oxidation of the polyether segments on the surface where leukocytes are adhered. The influence of dissolved oxygen, which is known to control oxidation of polymers in more traditional environments, was explored in this study. Specimens treated in vitro with hydrogen peroxide-cobalt chloride for 12 days exhibited a brittle, degraded surface layer about 10 microm thick. Attenuated total reflectance-Fourier transform infrared spectroscopy of the surface revealed that the ether absorbance at 1110 cm(-1) gradually decreased with in vitro treatment time to 30% of its initial value after 12 days. In contrast, 6 days in vitro followed by 6 days in air produced a decrease to 12% of the initial volume. Therefore, removing a specimen from the in vitro solution after 6 days and exposing it to air for the remainder of the 12 days actually resulted in more oxidation than leaving it in the in vitro solution for the entire 12 days. These results suggest that PEUU degrades by an autooxidation mechanism sustained by oxygen. By successfully modeling the depth of the surface degraded layer with a diffusion-reaction model, it was demonstrated that PEUU biodegradation is controlled by diffusion of oxygen into the polymer.

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