Biostability of polyurethane elastomers: Pacemaker leads, effect of fatigue, and novel antioxidant stabilizers

The failure mechanism of bipolar coaxial pacemaker leads was investigated with infrared (IR) microspectroscopy and scanning electron microscopy (SEM). Upon dissection of several failed leads, the poly(etherurethane) (PEU) inner insulation showed highly localized regions of environmental stress cracking (ESC) which were likely responsible for lead failure. Based on IR and SEM evidence, it was proposed that relatively stable hydrogen peroxide produced by adherent cells permeated through the outer silicone sheath and upon contact with the outer conducting coil decomposed into reactive oxygen radicals. These radicals then oxidized the PEU.; A method utilizing expansion of a diaphragm-type film specimen was developed to study in vitro biodegradation of poly(etherurethane urea) (PEUU) under conditions of dynamic loading (fatigue). The geometry of deformation allowed uniaxial and biaxial stress to be tested simultaneously. Dynamic loading did not affect the rate of degradation relative to unstressed and constant stress (creep) controls in regions of the film that experienced primarily uniaxial fatigue, however degradation was accelerated in regions that experienced almost equibiaxial fatigue. It was concluded that the combination of dynamic loading and biaxial tensile strain accelerated oxidative degradation in this system. The rate of degradation was highly strain rate dependent. Chemical degradation produced a brittle surface layer that was marked by numerous pits and dimples. Physical damage of the surface in the form of cracking occurred only in fatigue experiments. Cracking was not observed in unstressed or creep experiments. Cracks initiated at dimples produced by chemical degradation, and propagated in a direction that was determined by the strain state.; Two different approaches to control PU biodegradation were tested. Both approaches attempted to modify the surface of the PU, instead of the bulk. The first approach used synthetic molecules covalently bound to the surface of a PU to act as catalytic antioxidants. The second approach was to modify the surface to prevent cell adhesion. The challenge in using these approaches is controlling the stability and uniformity of surface modification.