| Electronic retinal prostheses aim to create the perception of vision for the millions of patients afflicted by retinal degenerative diseases. The retinal prosthesis stimulates remaining retinal neurons with a microelectrode array. Sight restoration demands a significantly more advanced prosthesis than previous devices that augment or replace neural function, termed "neural interfaces", such as cochlear implants or deep brain stimulators. Achieving a visual acuity greater than legal blindness, 20/200 vision, requires electrode densities exceeding 400 pixels/mm2, corresponding to electrode diameters smaller than 25 mum. Electrode-neuron separations of greater than the electrode size require larger electric fields that can lead to tissue hyperthermia, irreversible electrochemical reactions on electrodes, and increased collateral stimulation of nearby neurons. Thus, the primary premise of this dissertation is that close proximity between cells and electrodes is critical to the operation of a high-resolution prosthesis. To achieve cellular proximity, the implant must suppress reactive tissue mechanisms, such as encapsulating layers and glial hypertrophy. Chronic retinal exposure to pulsed electric fields risks damaging the neural cells and surrounding connective tissues. Understanding these effects is required for the safe and efficient operation of a retinal prosthetic.;The presented work includes the design of a high-resolution retinal prosthesis that allows for parallel delivery of information to thousands of pixels while maintaining the natural connection between eye movements and the visual scene. I include models of electric fields for various electrode configurations, and measurements of the extracellular stimulation thresholds of retinal neurons in vitro. Additionally, I determine thresholds of damage from electrical stimulation for neural and connective tissues as a function of electrode size, pulse duration, and number of exposures. To address the problem of proximity, I evaluate the biocompatibility of a variety of coatings, and have found that certain 3-dimensional microstructures prompt the retina to migrate into very close proximity to the implant with its neural circuitry largely intact. This effect allows for maintaining proximity between electrodes and cells over the whole implant. Finally, a flexible stimulating array was developed, fabricated and implanted to evaluate the retina-prosthesis interface in vivo . |
