Development of Implantable Electrodes Based on Polymer Derived Ceramics

Abstract

The invention of heart pacemakers in the 50's aims to help patients to overcome heart strokes, which represent the first cause of death worldwide. In Europe and in the USA, more than 3000 pacemakers are implanted every day. Despite the constant evolution of the pacemaker system, the lifetime of the implants is limited to several years. State of the art pacemaker electrodes suffer from pacing signal inhibition related to fibrotic tissue growth. This thesis describes the development of novel electrically conductive ceramic based materials, which promise to be more bio-inert than the currently used metals electrodes. Several Polymer Derived Ceramic (PDC) materials (Ceraset, SMP10 and SU8) have been investigated. In order to induce electrical conductance our project partner at EMPA doped the ceramics by an increase in carbon contents. However, for SU8 this step was not necessary. Photolithography and moulding were the two main microfabrication methods used for shaping the PDC materials. Moulding showed to be the most suitable method for shaping Ceraset and SMP10 materials. Thanks to the optimization work of the mould coatings, the main advantage of moulding consists in the possibility to release the PDC from the mould prior the sintering at high temperature. The release can be further facilitated when using elastomeric mould. SU8 was shaped by means of photolithography on top of a water soluble Dextran release layer and released before sintering. The materials were characterized at each fabrication step. The material composition, surface analysis and hardness of the densified ceramic samples have been measured. Furthermore an electrochemical characterization was performed in PBS and 1M KCl solution, by means of cyclic voltammetry and electrochemical impedance spectrometry. The measurements of the new materials show comparable values and behaviours to standard Pt metal electrodes. As the electrodes are meant to be implanted, it was necessary to investigate their cytotoxicity and in vitro biocompatibility for non-conductive and conductive compositions. Low cytotoxicity levels comparable to the state of the art positive reference implant materials have been assessed. To elucidate the fibrosis inhibition, long-term implantation tests (1 week to 6 months) have been carried out without the presence of electric signals. The fibrosis level turned out to be low for the new materials and comparable the reference materials. The pacing functionality of the new electrodes has been experimentally assessed as follows: (a) In vitro pacing of isolated cardiomyocytes by applying an electric potential of 20-40 V between two electrodes. The cell contraction was quantified by correlating the pacing signal with the hemodynamic parameters by means of video recording and image processing; (b) In vivo and ex vivo tests on a rat model were conducted by implanting SU8 micro-electrodes into peripheral muscles and in an isolated and perfused heart, respectively. The analysis confirmed that both muscle tissues were successfully actuated by using typical signal parameters. In conclusion, this thesis presents the successful development, optimisation and fabrication of pacemaker electrodes with new materials, which possess properties that are comparable to state of the art platinum electrode. This work provides a selection of materials for further characterization of long term implanted electrodes, studying the influence of an electric pacing signal on the fibrosis.