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W Liu, R Bashirullah, G Lazzi, M Humayun, J Weiland; A Smart Bi-Directional Telemetry for Retinal Prosthesis . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4469.
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Purpose: In order to provide power efficient solutions for the retinal prosthesis, we use back-telemetry to optimize the overall quality of the link in the new bi-directional telemetry with a separate data carrier. Via detectable changes of internal unit, the primary unit obtains information on the instantaneous power level necessary to the internal unit and possible malfunctions of electrodes. It is operated in dual frequency to minimize tissue absorption while maximizing power transmission efficiency and forward telemetry data rates. Methods: The RF transceiver is operated at 2.5 GHz whereas the power and back-telemetry links are at 135kHz (ISM band). The clock/data recovery will run at the desired data rates between 2-10 MHz. It could thus accommodate the increasing number of electrodes via forward link and the status report of prosthesis via reversed link. A DBPSK receiver is chosen to achieve the target data rates. The inherent constant-envelope modulation provides headroom for the design of highly efficient power amplifiers in the RF transmitter. Additional power savings is achieved by the close proximity of the RF link. Results: The power transmitter for the inductive link is able to provide ≷100mW with a dual rail of ±7V for the electrode stimulators and a 3V for logic. The system allows transmission of over 2 Mb/sec data, thus enabling the control of over 1,000 electrodes of the prosthesis. The back-telemeter periodically updates the external unit of induced voltage levels to obtain improved power regulation efficiency accounting for the relative spatial misalignment of the coils. A synchronization protocol is established between the internal and external units for resource sharing of the back-telemeter, and a 12-bit data sequence is continuously sent to update the external unit with power level diagnostics, electrode impedance levels, and chip temperature. Conclusion: A "smart" bi-directional telemetry system for a retinal prosthesis has been developed. The proposed novel closed loop system between the power transmitter and implanted unit via back-telemetry guarantees optimal power transfer and system functioning. The advantages of the system with respect to traditional telemetry links are: (1) it supports very high data-rates; (2) it supports back-telemetry to transmit important information about the status of the prosthesis such as temperature, power levels, and electrode impedance; (3) it is efficient in terms of power and data transmission. The proposed telemetry system will enable us to move toward a high-resolution retinal prosthesis characterized by over 1,000 electrodes.
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