Poly(carbonate urethane) and poly(ether urethane) biodegradation: In vivo studies

Abstract

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.

In vivo biostability of polymeric spine implants: retrieval analyses from a United States investigational device exemption study

“…Biostability of the retrieved Dynesys spacers was suggested in the IR results, particularly due to an absence of peaks at 1,650 and 1,174 cm -1 (Fig. 6), which would be attributed to the biodegradation process and products if present [8]. Table 3 shows that any as-characterized chemical changes were minor and limited to the PCU surfaces at the 5% systematic detection limit of the FTIR spectrometer and that nearly no changes occurred below the surface of PCU spacers.…”

“…Christenson et al [8] reported that PCU 80A was susceptible to biodegradation after implantation for 20 weeks in a rat model. In comparison with the PCU material used in Dynesys, PCU 80A has a different hard-soft segment ratio and molecular weights.…”

“…No new peaks at 1,650 or 1,174 cm -1 were present (Fig. 6b), where the 1,650 cm -1 peak was attributed to a potential degradation product of aromatic amine in hard segments [8], and the 1,174 cm -1 peak was attributed to branched ether peak due to a potential crosslinking during the biodegradation process in soft segments [15]. The spectra of 100 lm below the surface and in the bulk (not shown) were nearly identical to the controls.…”

“…1 The Dynesys TM system is composed of pedicle screws and set screws, universal spacers and cords different positions were taken at each depth. At each position, the PCU sample was placed on the ATR cell to detect surface chemistry in a thin surface region (1-5 lm) [8,10]. Because the surface chemistry of polyurethanes may vary in the open-air environment, [11] spectra of the bulk spacer samples were recorded immediately after cutting.…”

“…Nevertheless, the biostability of PCU and PET may have been a concern; the literature [8,9] has reported biodegradation of PCU material at 20 weeks in an animal model and long-term in vivo chemical changes of PET.…”