Fractal Retinal Implants Could Restore People’s Sight

From the gecko’s sticky feet to the sophisticated sniffing ability of dogs, nature often provides inspiration for new materials and technologies. Recently, nature has inspired something that could help many people see life a little more clearly; in research recently published in the journal Scientific Reports, researchers from the University of Oregon show that fractal-inspired retinal implants could be the first viable approach to helping people with retinal diseases regain sight to the point where they can navigate without assistance.

(Top): Side-view of a retinal implant. A signal passes through the retinal neurons (pink) and reaches a photodiode (dark blue). The photodiode creates an electrical signal based on the voltage difference between its two electrodes (grey). The yellow area is an insulated region that separates the two electrodes. (Bottom-left) Top view of the traditional design featuring a square inner electrode. (Bottom-right) Top view of the proposed fractal electrode. The dashed white lines show the bounding perimeters.
Image Credit: W. J. Watterson, W.J. et al. Fractal Electrodes as a Generic Interface for Stimulating Neurons, Scientific Reports 7, Article number: 6717 (2017), doi:10.1038/s41598-017-06762-3. (CC BY 4.0).

The eye is a pretty complicated structure that’s home to a lot of great physics, but in short, it works like this: Light from your surroundings enters the eye. The light is focused as it travels through the lens, and it forms an image on your retina. Your retina contains millions of light-sensitive cells known as rods and cones. When hit by a photon, these cells stimulate nerve impulses that travel to the brain where they are interpreted. The retina is the link between light and how we see our surroundings.

The retina is the surface at the back of the eye, where an image of our surroundings is transformed into electrochemical impulses that make their way to the brain along the optic nerve.

Retinal diseases like macular degeneration and retinitis pigmentosa cause the rods and cones in the retina to degenerate. Consider a healthy eye and an eye with retinal damage. Even though the exact same amount of light can enter both eyes and focus perfectly on the retina, the damaged retina has a hard time translating that light into the appropriate nerve impulses. Depending on the type and location of the damage, this can mean difficulty seeing at night, loss of visual acuity (sharpness), loss of peripheral vision, and even losing the ability to see straight ahead.

Research shows that the nerve cells that carry signals to the brain in patients with macular degeneration and retinitis pigmentosa often work just fine—they just need to be properly stimulated. One exciting approach to addressing this problem, therefore, involves surgically implanting a device into the retina that converts light signals into electrical signals and stimulates neurons.

Current implants under consideration consist of a silicon wafer that contains many tiny photodiodes, devices that convert light into electrical signals, integrated into an array of electrodes that use this electricity to stimulate nearby neurons. Unfortunately, clinical trials of current designs aren’t showing the kind of vision improvements that researchers (and, more importantly, patients) hope for.

In this new research, William Watterson, Rick Montgomery, and Richard Taylor explored a possible new design for the electrodes on retinal implants, a design based on fractal geometry. Fractals are patterns that repeat on different scales, like the way a broccoli floret closely resembles a head of broccoli. Fractals appear throughout nature—in seashells, rivers, snowflakes, blood vessels, the cytoskeleton of living cells, and in the structure of neurons.

The fractal structure of broccoli is particularly evident in the “Romanesco” variety.
Image Credit: Wiki user AVM (CC BY-SA 3.0)

Current implants use traditionally shaped electrodes, like squares. However, the Oregon team realized that fractal structures have properties that could be a good match for what’s required in an implant electrode, and that this type of design might interface better with neurons.

To explore this idea, they turned to computer simulations. The team began by modeling simple retinal implants consisting of two electrodes, an inner electrode and an outer electrode. The inner electrode, the one at the heart of the design, had the shape of either a square, grid, or fractal pattern. These virtual electrodes were immersed in retinal tissue.

First, the researchers looked at how electrical charge (like that from a photodiode) spread out through each electrode. The simulations showed that the fractal electrode stored more electricity than the other two shapes and generated a uniform electric field. Since it stored more electricity, the fractal electrode was able to generate a stronger electric field that reached farther outward than the other two designs, stimulating more neurons.

To explore this in detail, the researchers placed nine neurons directly above each inner electrode. Then, they ran the simulation and compared the point at which all nine neurons were stimulated for each design. The results show that the fractal electrode stimulated all of its neurons at a lower voltage than the grid electrode, and at a much lower voltage than the square electrode. In other words, the fractal design was by far more efficient at stimulating neurons than current designs.

Practically, this suggests that replacing the square electrodes on retinal implants with fractal electrodes could significantly increase the sharpness, or resolution, with which patients can see. Not to 20/20 vision, but perhaps to a point where people that experienced significant loss can once again navigate rooms and streets without assistance. The results also suggest that fractal electrodes may be useful for stimulating neurons in other areas of the body. Fractal implants are still a long way from being listed as options on hospital brochures, but it seems like this nature-inspired idea is well worth pursuing.

Kendra Redmond

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