Acute and chronic behavior of stimulating electrodes.

One of the most commonly practiced applications of functional electrical stimulation of neuronal systems is the auditory prosthesis, which activates either the peripheral auditory nerve or the central nervous system. When a stimulating electrode is placed in a biological environment, the contact between the stimulating site material and the surrounding media forms a complex electrode-electrolyte interface. Understanding the behavior of this interface and finding proper characterization methods are essential to the design of effective, reliable and safe chronic functional electrical stimulation devices. In this research effort, the behavior of the interface is investigated in-vitro by cyclic voltammetric tests, pulse tests, impedance tests, and small dynamic range tests. It has been found that both the interface capacitance and interface impedance are dependent on the time rate of change of the electrode voltage. This observation leads to a small dynamic range interface impedance model. The electrode's in-vivo chronic behavior was investigated by analyzing the long-term impedance drift of implanted cochlear electrodes and cochlear nucleus electrodes. The pre- and post-stimulation impedance drift observed during chronic electrical stimulation was recorded and analyzed statistically. The results suggest that the impedance drift is caused by a post-stimulation masking effect which is not likely coupled to any electrochemical reactions. The tissue reaction to the stimulating electrode was investigated by post-stimulation electrode evaluation and histological tissue studies. It was found that charge injection processes induce tissue growth. On the other hand, the typical thresholds for neural activation were far below the electrochemical damage threshold. Electrical current fields generated by bipolar Medwire electrodes and bipolar thin-film electrodes were evaluated with finite element analysis (FEA). The simulated results showed that the current density profiles generated by these electrodes were highly localized, which was suitable for differential neuron activation. However, highly concentrated current densities occurred along the two adjacent sides of each stimulating site in the case of bipolar electrodes. In the case of thin film electrodes, multiple peaks occurred along the boundary of the stimulating site. These phenomena suggested that the circular site geometry was not optimal for stimulating electrodes on a planar structure. An iterative procedure was developed to optimize the stimulating site geometry.Ph.D.Biological SciencesBiophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128608/2/9034546.pd