Characterization of Outer Insulation in Long-Term-Implanted Leads

Success of pacemakers and implantable cardioverter defibrillators may be limited by premature lead failure. Lead insulation polymers, such as polyurethane (PU) and polydimethylsiloxane (PDMS), are reported to degrade over time in vivo. PU is known to undergo oxidation, whereas PDMS undergoes surface hydrolysis. Previous studies have characterized polymer degradation in vitro, in animals or in short-term human study; however, complex effects of the biochemical and mechanical environment on the lead insulation can only be fully understood by evaluating long-term-implanted leads. Therefore, we established a retrieval program to systematically characterize the chemical and surface changes in 37 of 104 retrieved pacing and defibrillator leads, implanted for ≥5 yr. Fourier transform infrared (FTIR) spectroscopy was used for chemical analysis, and a scanning electron microscope was used for surface degradation evaluation. PDMS leads were investigated for changes in the ratio of Si-O-Si to Si-C peaks, whereas PU degradation was evaluated by changes in ether (C-O-C), carbonyl (C=O), methylene (C-H), and amino (C-N/N-H) peaks. Under SEM, PDMS showed enhanced roughness but no statistical increase in Si-O-Si bonds. PU showed uniform cracking throughout the lead body and statistical changes in each of the oxidation indicative peaks. Overall, both polymers showed surface changes in the physiological environment, but PU was the only material to show chemical changes. This work is a large-scale characterization study on long-term-implanted leads that confirmed PU oxidation but not hydrolysis of PDMS in vivo. It provides important insight for manufacturers when making design improvements and for surgeons when making decisions about lead implantation.