In order to achieve the challenge of developing an invisible clinical visual neuroprosthesis, it will be necessary to use a wireless link by which power and video signals are delivered to the implanted neural interface. The design constraints of such a wireless link are such that they are only recently becoming possible due to recent advances in integrated circuit technology. The wireless link could be mediated by radiofrequency or by light, and recent work on integrated optical systems and cellphone technologies are making these approaches tenable. The telemetry link should be bidirectional so the implanted electronics can inform the external electronics of the need for more or less power. In order to minimize excessive power requirements of the transmitting electronics, it is expected that the distance between the transmitter and receiver will be on the order of a centimeter. This suggests that the receiving coil will likely be implanted under the scalp (for a cortical prosthesis) or within the eye for a retinal based system. The coupling of power between transmitters and receivers can be achieved with very high efficiencies over such short distances. One possible design would be the use of transmitting coils built into the image-encoding eyeglasses. Such a design will have the advantage that registering the transmitting coil over the implanted receiving coil can be readily accomplished and will not vary significantly during the course of normal daily activities of the wearer. Further, the external electronics would consist only of the eyeglasses, signal-processing electronics, and the telemetry circuitry.
In order to ensure that the telemetry system does not deliver damaging levels of heat and radiation to the tissues near the implanted electronics, the frequency of the radiated power must be limited and the efficiencies of the implanted electronics must be very high. Further, the implanted receiver must be very small and have ultra-high reliability, considering that the implanted electronics will be immersed in the corrosive environment of the human body, possibly for decades. Finally, the bandwidth of the telemetered video signals will depend upon a number of factors. As the number of electrodes in the implanted neural interface increases, the bandwidth of the transmitted signals increases. If the signal-processing electronics use clever encoding strategies, the bandwidth of the video signals can be significantly reduced. It is clear that all these design considerations present the design engineer with very significant challenges.
An experimental wireless power and video signal telemetry link has been designed and fabricated by Dr. Phillip Troyk at the Indiana Institute of Technology. This system is intended to be used in a cortical visual prosthesis. The system has a modular design so that failure of any individual components will not cause failure of other modules. The system has also been designed to consume little power and to facilitate the interconnection of neural interfaces.
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