"Clinically, exposure of electrical components has been shown to result in electrical abnormalities, high-voltage failures and death . It was recently reported that the E2A insulation on a cardiac lead deteriorated after a 5 year human implant . PDMS-urethane polymers undergo hydrolysis upon exposure to water . "
[Show abstract][Hide abstract] ABSTRACT: Segmented polyurethane multiblock polymers containing polydimethylsiloxane and polyether soft segments form tough and easily processed thermoplastic elastomers (PDMS-urethanes). Two commercially available examples, PurSil 35 (denoted as P35) and Elast-Eon E2A (denoted as E2A), were evaluated for abrasion and fatigue resistance after immersion in 85 °C buffered water for up to 80 weeks. We previously reported that water exposure in these experiments resulted in a molar mass reduction, where the kinetics of the hydrolysis reaction is supported by a straight forward Arrhenius analysis over a range of accelerated temperatures (37-85 °C). We also showed that the ultimate tensile properties of P35 and E2A were significantly compromised when the molar mass was reduced. Here, we show that the reduction in molar mass also correlated with a reduction in both the abrasion and fatigue resistance. The instantaneous wear rate of both P35 and E2A, when exposed to the reciprocating motion of an ethylene tetrafluoroethylene (ETFE) jacketed cable, increased with the inverse of the number averaged molar mass (1/Mn). Both materials showed a change in the wear surface when the number-averaged molar mass was reduced to ≈16 kg/mole, where a smooth wear surface transitioned to a 'spalling-like' pattern, leaving the wear surface with ≈0.3 mm cracks that propagated beyond the contact surface. The fatigue crack growth rate for P35 and E2A also increased in proportion to 1/Mn, after the molar mass was reduced below a critical value of ≈30 kg/mole. Interestingly, this critical molar mass coincided with that at which the single cycle stress-strain response changed from strain hardening to strain softening. The changes in both abrasion and fatigue resistance, key predictors for long term reliability of cardiac leads, after exposure of this class of PDMS-urethanes to water suggests that these materials are susceptible to mechanical compromise in vivo.
[Show abstract][Hide abstract] ABSTRACT: A 55-year-old male patient presented after a single shock caused by oversensing of isolated nonphysiologic signals on both the distal HV and pace-sense channels. No other abnormalities were found. He subsequently returned complaining of device "vibration" and his St. Jude implantable defibrillator (ICD; St. Jude Medical, St. Paul, MN, USA) was found to be in VVI backup mode and could not be interrogated. Direct testing in the electrophysiology lab showed normal lead impedances and thresholds with an inability to reproduce the abnormal signals. Detailed cine fluoroscopy of the leads found no abnormalities. A new ICD was connected and successfully delivered a 20-joule shock but failed to deliver a maximum output (39-joule) shock. The new ICD was again found to be in backup mode. A new Endotak Reliance G lead (Boston Scientific, Natick, MA, USA) was implanted and a maximum-output shock was successful using a new Fortify DR ICD. This case likely represents a Durata lead insulation defect in the form of an inside-out abrasion under the distal HV coil. Increased awareness of this defect is warranted, particularly since routine interrogation and submaximum-output shocks may fail to detect the problem.
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