Michael J. Wiggins

Biodegradation of polyether polyurethane inner insulation in bipolar pacemaker leads

Several bipolar coaxial pacemaker leads, composed of an outer silicone rubber insulation and an inner polyether polyurethane (PEU) insulation, which were explanted due to clinical evidence of electrical dysfunction, were analyzed in this study. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to determine the cause of failure. Attenuated total reflectance-Fourier transform infrared microscopy (ATR-FTIR) was used to analyze the PEU insulation for chemical degradation. In all leads, the silicone rubber outer insulation showed no signs of physical damage. Physical damage to the inner PEU insulation was the source of electrical dysfunction. Cracks through the PEU compromised the insulation between the inner and outer conductor coils in the lead. It was observed with SEM that these cracks originated on the outer surface of the inner insulation and progressed inward. ATR-FTIR analysis showed that the PEU had chemically degraded via oxidation of the ether soft segment. Furthermore, it was revealed that chemical degradation was more advanced on the outer surface of the PEU. It was hypothesized that hydrogen peroxide permeated through the outer silicone insulation and decomposed into hydroxyl radicals that caused the chemical degradation of PEU. The metal in the outer conductor coil catalyzed the decomposition of the hydrogen peroxide. Chemical degradation of the PEU could also have been catalyzed by metal ions created from the corrosion of the metal in the outer conductor coil by hydrogen peroxide. Physical damage probably occurred in regions of the leads that were subjected to a higher hydrogen peroxide concentration from inflammatory cells and high degrees and rates of strain due to intercorporeal movement, including, but not limited to, cardiac movement. Chemical degradation and physical damage probably had a synergistic affect on failure of the insulation, in that as chemical degradation proceeded, the polymer surface became brittle and more susceptible to physical damage. As physical damage proceeded, cracks propagated into the unaffected bulk, exposing it to oxidants.

Role of oxygen in biodegradation of poly(etherurethane urea) elastomers

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.

Recent Advances in Biomedical Polyurethane Biostability and Biodegradation

This paper summarizes our recent efforts to better understand the effects of antioxidants, the effects of strain-state, mechanistic studies of soft segment cleavage by reactive oxygen radicals, and the effects of different soft segment chemistries on the biostability/biodegradation of polyether polyurethanes (PEUUs). In vivo cage implant system studies and in vitro cobalt ion/hydrogen peroxide studies have been carried out on PEUUs and the polymers have been analysed by attenuated total reflectance and Fourier transform infrared (ATR-FTIR) spectroscopy, and scanning electron microscopic (SEM) characterization of the PEUU surfaces. The natural antioxidant, vitamin E, has been shown to inhibit biodegradation and enhance biostability of PEUUs. Studies of the effect of stress state on PEUU biodegradation demonstrate that stress can inhibit biodegradation. While polyether soft segments may be cleaved by the presence of reactive oxygen radicals, the presence of oxygen has a profound effect in accelerating biodegradation. The biodegradation of polyurethanes may be inhibited by substituting different chemistries such as polydimethylsiloxanes, polycarbonates, and hydrocarbon soft segments for the polyether soft segments. To safely utilize polyurethanes in long-term biomedical devices, the biodegradation mechanisms of polyurethane elastomers must be fully understood and subsequently prevented.

Vitamin E as an antioxidant for poly(etherurethane urea): In vivo studies

Poly(etherurethane) elastomers are useful materials in medical devices because of their mechanical properties and biocompatibility. However, it is necessary to stabilize these elastomers against the oxidation of their ether soft segments. Synthetic antioxidants such as Santowhite and Irganox are often satisfactory; however, particularly for biomedical applications, it was of interest to test the natural antioxidant vitamin E in poly(etherurethane urea) (PEUU) elastomers in vivo. The alpha-tocopherol form of vitamin E was added to PEUU at 5% by weight. Biaxially strained PEUU specimens with and without vitamin E were tested in vivo in the cage implant system. The influence of vitamin E on PEUU biostability was analyzed by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and scanning electron microscopic (SEM) characterization of the PEUU surface. ATR-FTIR results showed that vitamin E prevented chemical degradation of the PEUU surface up to 5 weeks implantation, and at 10 weeks 82% of the ether remained. In contrast, without an antioxidant, only 18% of the ether remained after 10 weeks. No surface pitting or cracking was observed by SEM on PEUU with vitamin E; PEUU without antioxidant ruptured owing to extensive pitting and cracking. It was concluded that the antioxidant properties of vitamin E prevented oxidation of strained PEUU elastomers in vivo. The influence of vitamin E on PEUU biocompatibility was characterized by exudate leukocyte counts, density of leukocytes adherent to the PEUU, and morphology of adherent leukocytes. These results indicated decreased leukocyte counts in the exudate and less active adherent cells on the PEUU with vitamin E compared to PEUU without antioxidant. A proposed cell-polymer feedback system demonstrates how vitamin E improves both biostability and biocompatibility of PEUU elastomers in vivo.

Comparison of two antioxidants for poly(etherurethane urea) in an accelerated in vitro biodegradation system

Vitamin E (+/-alpha-tocopherol) was recently investigated as an antioxidant for implanted poly(etherurethane urea) (PEUU) elastomers. In that work, vitamin E prevented chemical degradation of biaxially strained PEUU up to 5 weeks implantation, and prevented pitting and cracking of the PEUU surface for the duration of the 10-week cage implant study. The promising results of the in vivo studies motivated a detailed comparison of vitamin E with Santowhite, the standard antioxidant used in PEUU elastomers. To evaluate vitamin E and Santowhite as antioxidants in PEUU, an accelerated in vitro treatment system was used that mimics the in vivo degradation of PEUUs. Vitamin E was even more effective than Santowhite in preventing pitting and cracking to the biaxially strained PEUU elastomers. The inhibition of ether oxidation was greater with vitamin E than with Santowhite when compared by equivalent concentrations and molar concentrations, respectively. It is hypothesized that the increased effectiveness of vitamin E in this system, compared to Santowhite, is due to differences in antioxidant mechanism(s). Vitamin E is more efficient in preventing PEUU oxidation than Santowhite because its phenoxy radical is more stable and it can terminate more than one chain per vitamin E molecule.

The effect of strain state on the biostability of a poly(etherurethane urea) elastomer.

The effect of deformation state on degradation of a PEUU without added stabilizers was examined in an oxidative environment that simulates the in vivo biodegradation of the polymer. Polymer tubes were stressed uniaxially and biaxially over glass mandrels and treated in 20% hydrogen peroxide/0.1 M cobalt chloride solution for 12 days at 37 degrees C. The amount of degradation was determined from the ATR-FTIR peak height of the amorphous aliphatic ether absorbance at 1110 cm-1. If a uniaxial stress was applied, degradation was inhibited and the amount of surface ether remaining after treatment increased linearly with strain. If the stress was biaxial, the amount of degradation was not reduced unless the strain was greater than 200%. Decreased degradation correlated with the amount of soft-segment orientation. The decreased degradation rate was attributed to compaction of the polyether phase by orientation, which resulted in lower permeability to oxidative agents, particularly oxygen. Macroscopic damage was confined to a thin peeling surface layer if the stress was uniaxial. In comparison, biaxially stressed PEUU ruptured.