Prototyping a Basis for the Next Generation of Retinal Prostheses

Current approaches to prosthetic sight involve connecting an grid of electrodes implanted in the retina to an external camera. The electrodes stimulate the production of glowing phosphenes, a crude visual representation of what the camera sees. It is simple and in no way real sight, but a great improvement over being completely blind. Researchers here present work on the next generation of technology for artificial substitutes to replace natural vision, removing the camera and shifting towards more miniaturized electronics:

Researchers have developed a new type of retinal prosthesis that brings research a step closer to restoring the ability of neurons in the retina to respond to light. The new prosthesis relies on two groundbreaking technologies. One consists of arrays of silicon nanowires that simultaneously sense light and electrically stimulate the retina accordingly. The nanowires give the prosthesis higher resolution than anything achieved by other devices - closer to the dense spacing of photoreceptors in the human retina. The other breakthrough is a wireless device that can transmit power and data to the nanowires over the same wireless link at record speed and energy efficiency. One of the main differences between the researchers' prototype and existing retinal prostheses is that the new system does not require a vision sensor outside of the eye to capture a visual scene and then transform it into alternating signals to sequentially stimulate retinal neurons. Instead, the silicon nanowires mimic the retina's light-sensing cones and rods to directly stimulate retinal cells. Nanowires are bundled into a grid of electrodes, directly activated by light and powered by a single wireless electrical signal.

The power provided to the nanowires from the single wireless electrical signal gives the light-activated electrodes their high sensitivity while also controlling the timing of stimulation. Power is delivered from outside the body to the implant through an inductive powering telemetry system. The device is highly energy efficient because it minimizes energy losses in wireless power and data transmission and in the stimulation process, recycling electrostatic energy circulating within the inductive resonant tank, and between capacitance on the electrodes and the resonant tank. Up to 90 percent of the energy transmitted is actually delivered and used for stimulation, which means less RF wireless power emitting radiation in the transmission, and less heating of the surrounding tissue from dissipated power.

For proof-of-concept, the researchers inserted the wirelessly powered nanowire array beneath a transgenic rat retina with rhodopsin P23H knock-in retinal degeneration. The degenerated retina interfaced in vitro with a microelectrode array for recording extracellular neural action potentials (electrical "spikes" from neural activity). The bipolar neurons fired action potentials preferentially when the prosthesis was exposed to a combination of light and electrical potential - and were silent when either light or electrical bias was absent, confirming the light-activated and voltage-controlled responsivity of the nanowire array.



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