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A Distance-Immune Low-Power Near-Field Data-Link for Biomedical Implants

  • Yousefi, Alireza
Publication Date
Jan 01, 2018
eScholarship - University of California
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In the past decade, there has been a great interest in bionic and neural implantable electronics to investigate the human body's working, to understand different physiological conditions, and to cure disease. Among them, neuromodulation systems have been found by the neuroscience community as a promising tool to decode the functioning of the brain, and an alternative strategy for diagnosis and treatment of neuropsychiatric disorders. Today's clinical neuromodulation systems, however, may suffer from many serious constraints. For instance, many are wired refrigerator-size electronic setups, which immobilize the patient, increase the risk of infections, and have a limited duration of operation. The next generation of neuromodulation systems should contain a robust, energy-efficient solution to deliver wireless data and power.In this work, first, we present an implantable distance-immune ultra low-power data link which would be used in a biomedical implant that most of the time relays monitoring data to a unit outside the human body. The near-field data link is based on a free-running oscillator tuned by coupled resonators. We have studied the properties of the inductively-coupled link and derive its key features and constraints. After understanding the design space, we have designed and built transceiver chips in 40 nm CMOS technology which enable a bidirectional wireless connectivity. Data can be transferred in half-duplex at up to 4 Mbps in a Load Shift Keyed (LSK) uplink, and up to 2 Mbps in an ASK downlink. With one automatic adjustment, the link can maintain a reasonable error rate of less than 10−6 (BER< 10−6) over coil separations of up to 4.5 cm.In monitoring body/brain activities, with an air data rate of 4 Mbps, this wireless link can multiplex 16b-words acquired at 0.5 kS/s from each of up to 500 neural probes. Typically, the implant will transmit data on the uplink most of the time; the circuit reported here is optimized for this function, consuming less than 0.1 pJ/bit at 4 Mbps. When receiving on the downlink at 2 Mbps, the implant consumes 5 pJ/bit.Second, we have integrated the designed data link in a state-of-the-art, human-quality implantable neuromodulation system, and verified its communication performance in benchtop and in-vitro environments. The data link achieved a maximum range of 3 cm packaged in a titanium can and using a bio-compatible coil.Third, we describe a Medium Access Control (MAC) system for the designed transceivers (PHY layer). The MAC layer is designed as a stand-alone accelerator on an FPGA platform, capable of handling incoming and outgoing data in parallel. In the next generation of thedata link, we can migrate this to an Application Specific Integrated Circuit (ASIC) to reduce the power consumption and the system form factor.

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