Fractal Electrodes for Interfacing Neurons to Retinal Implants

Abstract

With life expectancy on the rise, age-related ailments are a significant strain on the welfare of individuals and the economy. Progress is being made towards combating the leading cause of unavoidable blindness, age-related macular degeneration (AMD). AMD affects ten million Americans and costs the world economy $343 billion annually. Retinal implants promise to restore sight by replacing the eye's damaged photoreceptors with electronic photodiodes. Clinical trials succeed at restoring some vision, but are limited by the stimulating electrodes. We study the electrode-neuron interface with a focus on the geometrical dependence of the electrode.

The functionality of neurons is intimately connected to their branching and curving shape, described by fractal geometry. We examine the morphology of neurons using fractal analysis. The results inform our electrode designs, which are fabricated using top-down lithographic and bottom-up self-assembly techniques.

A novel technique for fabricating a fractal electrode is presented. Heating and cooling a film of poly(methyl methacrylate) on a SiO2 substrate causes fractal structures to form on the surface. The geometry of the structures is temperature dependent, producing crystalline branches at lower temperatures and diffusion-limited aggregates at higher temperatures. Subsequent deposition of antimony nanoclusters shows preferred diffusion to the fractal surface features.

The dependence of a photodiode's performance on its top contact geometry is explored using modified nodal analysis. The results reinforce the need to balance a low mean semiconductor-metal separation distance with an adequate contact width for low resistance, all while maximizing light input. Future designs will benefit from the spatial voltage maps produced by the simulation.

The electric field emanating from an electrode is also dependent on the geometry of the electrode. The Faraday cage effect is exploited to achieve similar electric field responses to traditional electrode shapes.

A preliminary study of neural adhesion to SU-8 fractal electrodes is promising. The neuron grows along the electrode even at 90° turns.

The role the fractal geometry plays in neuron and electrode functionality is shown to be significant. Continued study of, and experimentation with, new electrode designs is sure to produce exciting possibilities in the future.

This dissertation includes previously unpublished co-authored material.