Multi-Carrier Wireless Link for Implantable Biomedical Devices
An inductive link between two magnetically-coupled coils that constitute a transformer is the most common method to wirelessly transmit power and data to implantable biomedical devices that have relatively high power consumption such as neuromuscular stimulators, cochlear implants, and visual prostheses. Neuroprostheses that substitute sensory functions also need sizeable amounts of real-time data to interface with a large number of neurons by means of tens to hundreds of stimulating sites that are driven simultaneously through multiple parallel channels. The wireless link should be robust enough not to be affected by patient’s motion artifacts or minor coils misalignments. A back telemetry link is also needed for implant power regulation, stimulating sites impedance measurement, and recording the neural response for accurate electrode placement and stimulation parameter adjustments.
Therefore, high power transmission efficiency, high data transmission bandwidth, magnetic coupling insensitivity, and back telemetry are the major wireless link requirements in the design and implementation of high performance implantable biomedical devices. While these requirements are individually attainable, they have not been achieved concurrently with traditional techniques. The reason is that there are conflicting constraints involved in achieving high performance in two or more of the above system requirements.
The wireless link operating frequency, also known as the carrier frequency, is one of the most important parameters of an inductive link, which affects all other system specifications. Traditionally, a single carrier frequency has been used for (1) inductive power transmission, (2) forward data transmission from outside to the implanted device, and (3) back telemetry from the implanted device outward. In this research we are using three carrier signals at three different frequencies and amplitude levels: (a) low-frequency high-amplitude (fP < 1MHz) for power transmission, (b) medium-frequency medium-amplitude (fFD ~ 50MHz) for forward data link, and (c) high-frequency low-amplitude (fBT > 400MHz) for back telemetry. These frequencies are optimal for the above three major functions and we can effectively isolate many of the competing parameters in the design of a wireless link. Therefore, we expect to achieve a high performance in all of the aforementioned system requirements.
The research presented here is aimed at developing a robust, power-efficient, wideband, bidirectional wireless link using multiple carrier frequencies. The new link will be utilized in development of a prototype neuroprosthetic testbed for a visual prosthesis. The prototype neuroprosthesis will be tested in vitro to evaluate the multi-frequency wireless link performance in the tissue environment. Then it will be used in short term in vivo experiments.
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