Polymer Packages for Active Implants: With Application of Liquid Crystal Polymer and Polyetheretherketone to the Packaging of a Fully Implantable Optogenetic Stimulation System

Abstract

Active medical implants are constantly immersed in the body’s fluids, operate at near 38 °C, and managing the risks associated with moisture ingress and failure of the electronics is a major part of the testing process. Although titanium can based protection systems are well established, they bring high expense, and may not support miniaturisation relating to radiofrequency (RF) and power transfer capabilities. This thesis presents the development of liquid crystal polymer (LCP) and polyetheretherketone (PEEK) as cost effective, RF compatible packaging materials for chronic animal studies. The lifetime of a package was defined as the duration before the internal relative humidity (RH) reaches 63 %, above which the risk of corrosion induced failure increases. The RH change is governed by the package material’s moisture transport properties which are material specific and is dependent on the environmental conditions. The moisture transport properties of package materials, permeability and diffusion coefficients, were obtained through gravimetric methods at 38 °C in 0.9 % saline solution to simulate an implant environment. A model was used to predict the internal RH increase over time. Confidence in the model came from validation experiments which measured the humidity of packaged optogenetic devices over 120 days. A PEEK package was designed with desiccant added to the internal volume to a level intended to maintain humidity below 63 % for 105 days. A telemetry system recorded the internal RH which reached the threshold within 2 % of the predicted value, at 107 days. Injection moulded LCP and PEEK packages without desiccant were predicted to have lifetimes of 68 days and 8.5 days, respectively. The modelled lifetime was supported by experimental data over 28 days. This study demonstrated that LCP and PEEK can be used to design implant packages with lifetimes in the order of months and even years, suitable for many applications.