In vitro biostability of cardiac pacemaker lead insulations under static mechanical loading

Abstract

Patients with cardiac arrhythmias are currently treated with conventional transvenous rhythm implants. Complications are frequently associated with intracardiac implanted leads primary insulated by silicone and polyurethane. However, experiences show that polyurethanes in particular are susceptible to various degradation mechanisms including hydrolysis, environmental stress cracking (ESC) and metal ion induced oxidation (MIO). In vivo, pacemaker leads are exposed to a complex thermal, chemical, mechanical and biological loading. The current study focusses on in vitro analyses to assess the biostability of Pellethane 2363 55D and the silicone MED 4765 as cardiac pacemaker lead insulations. Degradation processes were simulated in vitro in a load-free state and under static mechanical loading by subjecting a coaxial lead design to bending radii from 3 mm to 19 mm. Physiological environmental conditions were mimicked using a physiological saline solution and a tempered oxidative solution. Surface morphological and thermal analyses were performed before and after in vitro testing by means of scanning electron microscopy (SEM) and differential scanning calorimetry. Melting temperatures of silicones around 40°C were measured, before and after in vitro testing, respectively. Pellethane insulation layers had two endothermal melting regions at 100°C and 170°C before and a third melting region at 45°C after in vitro testing. The additional melting peak may indicate a change of thermal material properties due to degradation. SEM images showed degradation phenomena similar to in vivo studies, varying in severity and depending on the bending radius. Thus, the relevance of mechanical loading for in vitro replication of clinically relevant lead insulation degradation was demonstrated.

Keywords

• cardiac pacemaker lead
• insulation materials
• polyurethane
• biostability
• mechanical bending.