Fractal Electrodes as a Generic Interface for Stimulating Neurons

Abstract

The prospect of replacing damaged body parts with artificial implants is being transformed from science fiction to science fact through the increasing application of electronics to interface with human neurons in the limbs, the brain, and the retina. We propose bio-inspired electronics which adopt the fractal geometry of the neurons they interface with. Our focus is on retinal implants, although performance improvements will be generic to many neuronal types. The key component is a multifunctional electrode; light passes through this electrode into a photodiode which charges the electrode. Its electric field then stimulates the neurons. A fractal electrode might increase both light transmission and neuron proximity compared to conventional Euclidean electrodes. These advantages are negated if the fractal’s field is less effective at stimulating neurons. We present simulations demonstrating how an interplay of fractal properties generates enhanced stimulation; the electrode voltage necessary to stimulate all neighboring neurons is over 50% less for fractal than Euclidean electrodes. This smaller voltage can be achieved by a single diode compared to three diodes required for the Euclidean electrode’s higher voltage. This will allow patients, for the first time, to see with the visual acuity necessary for navigating rooms and streets.

Fractal solar panels: Optimizing aesthetic and electrical performances

“…Accordingly, each exposed emitter pixel (i.e. not located under the electrode) was modeled as a constant current source of I = 5 x 10 -6 A and excluded any diode behavior [40]. This current was then pictured as flowing up through the electrode via a network of nodes connected by resistive elements (Fig 2, inset).…”

“…Nature's fractals have previously served as bio-inspiration to enhance performances in diverse applications from wind barriers [38] to capacitors [39]. Most relevant for solar panels, fractal electrodes have been shown to out-perform Euclidean electrodes in simulations of retinal implants which use photodiodes to restore human vision [40][41]. This raises the possibility that fractal electrodes in solar panel photodiodes might surpass bus-bars aesthetically and electrically.…”

Modeling the Improved Visual Acuity Using Photodiode Based Retinal Implants Featuring Fractal Electrodes

“…Recently, we proposed using fractal inner electrodes, featuring branched patterns that repeat at increasingly fine size scales (Figure 1C ), as the ideal solution to this problem (Watterson et al, 2017 ). The sidewalls of the repeating branches generate a large surface area, and hence capacitance, while the gaps between the branches allow the light to pass through.…”

“…The sidewalls of the repeating branches generate a large surface area, and hence capacitance, while the gaps between the branches allow the light to pass through. We also proposed that this fractal design might offer additional favorable properties, including enhanced neural stimulation due to the close proximity of the neurons to the electrode (due to the fractal's surface texture promoting neural adhesion), favorable optical properties (including extraordinary transmission whereby the transmitted light intensity is greater than that expected from a simple pixel count of the photodiode's exposed area), and an increase in mechanical flexibility (which could be exploited to facilitate less obtrusive surgery and also to allow implants to conform to the curved surface at the back of the eye) (Watterson et al, 2017 ).…”

“…To quantify the impact of the fractal electrode's enhanced capacitance, we previously modeled a 20 μm-wide fractal and simulated the stimulation of retinal neurons when a bias, V , was applied to the electrode (Watterson et al, 2017 ). We found that all neighboring neurons (i.e., all neurons immediately above the electrode) were stimulated by 0.32 V for the fractal electrode while the equivalent square electrode required 0.9 V. Significantly, this fractal bias lies within the maximum voltage (i.e., the open-circuit voltage of 0.6 V) that a silicon photodiode can generate.…”