Investigation of Wear Mechanisms in Silicone Sleeved Implantable Cardiac Device Leads using an In Vitro Approach

Highlights

• An accelerated test protocol was developed to investigate lead-on-device wear of cardiac device leads.

• A factorial study of applied load, fluid protein and silica enrichment yielded representative wear scars.

• Liberated silica particles from the silicone matrix appear to impact wear scar appearance but not size.

Abstract

Cardiac implantable electronic devices (CIED), such as pacemakers and defibrillators, have a titanium casing which holds the electronics, connected to insulated leads which deliver therapeutic pulses to regulate heart activity. CIED lifetimes are often limited by wear of their polymer components, as on the surface of the elastomeric insulation of the leads, which is frequently silicone. Insulation wear during regular activity can yield patient discomfort or surgical complications upon replacement. Little is known about the wear mechanisms of these silicone materials in situ, but it is known that wear occurs between the leads and either the titanium casing, other leads, or bodily tissue. This study investigated titanium-on-silicone wear of lead insulation used in CIEDs. Surgically retrieved silicone insulated leads showed unusual wear scars that were polished and smooth. The goal of this study was twofold: replicate the unique wear scar with a testing apparatus, and determine wear mechanisms of the silicone insulation. Silicone cardiac leads were obtained from the manufacturer and an apparatus was constructed to simulate in-body conditions while accelerating the wear process. Three key parameters were chosen to investigate the wear mechanisms of this system: load, environmental fluid, and third-body abrasive. A factorial matrix with two replications was used to test these variables. Wear scars were examined using white light profilometry, optical microscopy, and scanning electron microscopy (SEM). An analysis-of-variance (ANOVA) showed that all test factors did not significantly affect the size and depth of the wear scars, but revealed key mechanisms that could affect other known wear configurations such as lead-on-lead wear.

Introduction

Pacemakers and cardioverter-defibrillators are cardiac biomedical devices that treat patients with disorders related to frequency and/or stability of the heart beating. In general, these cardiac implantable electronic devices (CIEDs) generate an electrical signal that stimulates cardiac muscles to facilitate normal heart rhythms. The devices are typically composed of a titanium enclosure, which houses the battery and electronic monitoring system, and multiple electrical lead wires depending on the required treatment. Though there are some variations in surgical placement, most such configurations employ an approach of implanting the titanium housing under the skin in the general vicinity of the heart, while the lead wires are run from the device through hard and soft tissue, and are ultimately transvenously inserted into the appropriate chamber in the heart [1,2]. While their exact structure can differ, CIED lead wires are generally composed of layers of insulating material which maintain electrical isolation of the multiple helical metal conductors from the in vivo environment and from other lead wires. Typically, the outer insulation layer – or sleeve – is either a polyurethane or silicone elastomer; these materials are both sufficiently biocompatible and durable to handle the internal chemical environment and mechanical stresses of the body [3]. As with any engineered system, failure of the lead wires can occur, however little is known about the specific tribological issues of the lead wires in situ. Lead wire failure here is defined as the loss of function of the lead wire that results in the CIED not able to fully perform the necessary therapy or any alteration of the lead wire that would result in danger to the patient.

Potential lead wire failure, via tribological mechanisms or otherwise, poses a critical problem for patient safety and reliability, but there is not a long reported history of research specifically focused on tribological performance. To give a sense of the scope of this issue, Kleeman, et al. studied devices implanted between 1992 and 2005, and reported that the failure rate of leads was as high as 15% after 5 years [4]. This does not indicate that the devices are not reliable, rather it suggests that the leads are subject to an aggressive in vivo environment and often require replacement during the lifetime of the device. Of the various causes of lead failure in CIED, one particular mechanism that can cause loss of device function, or in other cases complete device failure, is lead wire sleeve wear. It is likely that some amount of wear along the length of the sleeves is ubiquitous and typically does not affect safety, however, extreme enough wear can lead to deleterious consequences such as an electrical short [5], loss of sensing [6], or improper charge delivery leading to discomfort and pain [7]. In rare cases, some catastrophic outcomes are hypothesized to be caused by wear products, such as cardiac infection [8], or severe complications during lead extraction [9].

There are four primary zones of tribological concern in implanted CIEDs that can be readily identified, due to the design of the devices as well as their surgical placement. Firstly, the sleeves of two or more leads can be in intermittent sliding contact with each other, producing a lead-on‑lead (elastomer on elastomer) wear mode. Secondly, leads may come into sliding contact with hard or soft tissues at multiple locations along their length [10]. Thirdly, the internal layers of the lead can experience sliding contact due to mechanical flexing of the lead during bodily motion [10]. Finally – and the topic of this investigation – is the contact that occurs between the lead sleeve and the edges of the metallic enclosure of the control module.

The wear performance of the sleeve material is likely a factor in all these modes [11]. Of the two primary elastomers used for the sleeves, polyurethane and its co-polymers are generally more wear resistant but subject to in vivo degradation. Silicone, which generally exhibits greater chemical stability, is less resistant to abrasion and generally has higher friction [1]. Several wear modes of elastomers exhibited in non-medical applications have been reported. In some materials, viscoelastic rolling produces characteristic ridges in the wear zone [12,13]. As the wear cycles accumulate, the ridges can detach and form wear debris via a fatigue-driven process [14]. This is in addition to fatigue cracking which can be initiated in the elastomer at high localized loading conditions [15]. Due to the placement of the lead, there is also the potential for more complex wear mechanisms involving viscoelastic creep, persistence of wear products at the interface, as well as possible lubricating action of biological proteins in the cardiac environment [[16], [17], [18], [19]]. Finally, the silicone elastomer itself is reinforced with a hard phase, usually silica particles, to enhance mechanical properties. While the addition of this hard particulate raises concerns regarding abrasive damage, they are necessary in order for silicone to be a mechanically viable in most applications. Wear scars of retrieved silicone leads often have a shiny appearance which is indicative of third-body abrasion analogous to an aqueous polishing process [20].

This study focused on developing a baseline understanding the wear mechanisms and key parameters involved in the wear of silicone lead sleeves against the edges of the titanium device enclosure, and employed an experimental setup which was designed to simulate in vivo conditions. A better understanding of specific mechanisms can then be used to confirm or refute various tribological hypotheses regarding specific wear modes. Silicone sleeved leads were provided by the manufacturer for this testing. Three testing parameters were studied with regards to their impact on sleeve wear: applied normal load, testing fluid, and presence of a third-body abrasive. The study followed a factorial experimental design, and results were analyzed using both frequentist and Bayesian methods. Wear scars were examined using white light profilometry, optical microscopy, and scanning electron microscopy (SEM). The details and conclusions of this study are reported in this paper.