Published: Jan 1985
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Compared to standard silicone elastomers, polyurethanes offer enhanced mechanical properties, improved durability, and reduced wall thickness for pacing leads. These thinner-wall polyurethane leads can be used in smaller vessels, or will allow multiple leads to be introduced into a single vein.
Recent reports have shown that pacing leads made with Pellethane 2363-80A exhibit shallow surface fissuring; some pacemaker or lead constructions have failed, particularly at areas of chronic mechanical stress. Several mechanisms have been proposed as the cause of surface fissuring: protein absorption with associated swelling, leaching of low molecular weight substances to the surface, or lipid absorption. However, there is little evidence supporting any of these mechanisms.
Our studies suggest that polyether-based polyurethanes are susceptible to in vivo oxidation of the polyether chain. In this chain, the most susceptible group is the -CH2 group in the alpha position to the ether oxygen, which undergoes peroxidation, free radical dissociation, and, eventually, chain cleavage, possibly leading to a reduction in molecular weight averages. Attenuated total reflectance infrared (ATR-IR) studies at the surface of the fissured polyurethanes have shown the presence of oxidative byproducts such as hydroxyl end groups (-OH).
Based on these preliminary results, we hypothesize that progressive surface degradation is caused, in part, by stress-induced oxidation of the polyether macroglycol used in the synthesis of polyurethane elastomers. Our hypothesis has been reinforced by experimental evidence that surface cracking can be significantly reduced, if not eliminated, by using higher-durometer polyurethanes, because these polymers contain fewer polyether macroglycol chains in the molecular backbone.
polyurethanes, pacing leads, surface fissuring, stress cracking, stress-induced oxidation, implant materials, biological degradation
Director of Biomaterials Research, Thermo Electron Corporation, Waltham, MA
Pacesetter Systems, Inc., Sylmar, CA