Investigation of electrochemical properties and performance of stimulation/sensing electrodes for pacemaker applications

Abstract: People suffering from certain types of arrhythmia may benefit from the implantation of a cardiac pacemaker. Pacemakers artificially stimulate the heart by applying short electrical pulses to the cardiac tissue to restore and maintain a steady heart rhythm. By adjusting the pulse delivery rate the heart is stimulated to beat at desired pace. The stimulation pulses are transferred from the pacemaker to the heart via an electrode, which is implanted into the cardiac tissue. Additionally, the electrode must also sense the cardiac response and transfer those signals back to the electronics in the pacemaker for processing. The communication between the electrode and the tissue takes place on the electrode/electrolyte (tissue) interface. This interface serves as the contact point where the electronic current in the electrode is converted to ionic currents capable to operate in the body. The stimulation/sensing signals are transferred across the interface via three electrochemical mechanisms: i) non-faradaic charging/discharging of the electrochemical double layer, ii) reversible and iii) irreversible faradaic reactions. It is necessary to study the contribution of each mechanism to the total charge transferred to evaluate the pacing/sensing performance of the pacemaker electrode.In this thesis, the electrochemical properties and performance of stimulation/sensing electrodes for pacemaker applications have been investigated by electrochemical impedance spectroscopy, cyclic voltammetry and transient electrochemical techniques. All measurements were performed in synthetic body fluid with buffer capacity. Complementary surface analysis was performed with scanning electron microscopy, energy dispersive spectroscopy and X-ray photoelectron spectroscopy.The results reveal different interfacial behaviour and stability for electrode materials such as Pt, TiN, porous carbon, conducting oxides (RuO2 and IrO2 and mixed oxides) and porous Nb2O5 oxide. The influence of the charge/discharge rate on the electrode characteristics also has been evaluated. Although the rough and porous electrodes provide a high interfacial capacitance, the maximum capacitance cannot be fully employed at high charge/discharge rates because only a small part of the effective surface area is accessible. The benefit of pseudo-capacitive material properties on charge delivery was observed. However, these materials suffer similar limitations at high charge/discharge rate and, hence, are only utilising the surface bound pseudo-capacitive sites. Porous Nb2O5 electrodes were investigated to study the performance of capacitor electrodes. These electrodes predominantly deliver the charge via reversible non-faradaic mechanisms and hence do not produce irreversible by-products. They can deliver very high potential pulses while maintaining high impedance and, thus, charge lost by faradaic currents are kept low. By producing Nb oxide by plasma electrolysis oxidation a porous surface structure is obtained which has the potential to provide a biocompatible interface for cell adherence and growth.This thesis covers a multidisciplinary area. In an attempt to connect diverse fields, such as electrophysiology, materials science and electrochemistry, the first chapters have been attributed to explaining fundamental aspects of the respective fields. This thesis also reviews the current opinion of pacing and sensing theory, with special focus on some areas where detailed explanation is needed for the fundamental nature of electrostimulation/sensing.