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Wireless power transfer for scaled electronic biomedical implants

  • Author(s): Theilmann, Paul Thomas
  • et al.
Abstract

Decreasing the physical size of electronic biomedical implants is an important objective in that it increases the number of realizable applications. For example, wireless pacemakers can be on the order of an inch in diameter while retinal and cochlear implants must be scaled to a centimeter to fit in their respective locations within the body. Perhaps the ultimate goal is devices that are no larger than a red blood cell, approximately five microns in diameter. At those dimensions devices reach compatibility with the circulatory system and can be injected into the blood stream. The chief impediment to such aggressive size reduction is the power supply. In this dissertation the use of electromagnetic (EM) radiation to wirelessly power miniature biomedical implants is investigated. Both inductive coupling using time varying magnetic fields and near-infrared (NIR) light collected by photodiodes are identified as viable solutions for transcutaneous power transmission. Minimum device size is limited by power radiation density at the implant and the efficiency with which the device can collect, convert and use this energy. Power density at the implant is restricted by safety regulations and tissue attenuation. Both are studied in this work, the results of which are used to build analytical models for power calculations. The use of ferrite rods in inductive implants can improve power transfer efficiency. In this work an analytical model is derived and supported experimentally which shows that mutual inductance can be improved by as much as a factor of 10 when NiZn ferrite rods are used within the implanted device. This improves efficiency and permits decreased implant size. Efficiently converting time-varying energy to usable DC power is essential for obtaining high implant efficiency. In this work a silicon-on-sapphire (SOS) CMOS rectifier that overcomes the dead-zone at low power characteristics of conventional rectifiers is developed. The design produces the targeted output power of 1μW with a peak power conversion efficiency of 67% at 100MHz. Furthermore the rectifier achieves greater than 30% efficiency for input power levels as low as -40dBm. A larger rectifier designed with the same topology is used as part of an inductive power transfer system, which is able to illuminate an LED at distances greater than 5cm with input power levels to the primary coil of only 1W. For NIR light power transfer, a specialized photodiode designed using a fully depleted silicon on insulator CMOS process was demonstrated to collect ̃35[mu]W of power from a 90mW laser. The insights gained from this work are used to predict the size scaling limits of implants powered using wireless techniques. It is estimated that to obtain 1μW of power within the implant, inductive supplies will require a secondary coil radius of ̃15[mu]m while photodiodes for NIR light power collection will require radii of at least ̃40[mu]m

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