WIRELESS POWERING AND TELEMETRY FOR FLEXIBLE BIOELECTRONIC IMPLANTS KUSH AGARWAL B.Tech., M.Sc. A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2017 Supervisors: Associate Professor Yongxin Guo Professor Nitish V. Thakor Professor Maysam Ghovanloo, Georgia Institute of Technology Examiners: Associate Professor Xudong Chen Associate Professor Chengwei Qiu Professor J.C. Chiao, University of Texas at Arlington Declaration I hereby declare that this report is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the report. Thisreporthasalsonotbeensubmittedforanydegreeinanyuniversitypreviously. Kush Agarwal September 2017 ii Acknowledgments This work was done under the joint supervision of Prof. Yong-Xin Guo, Prof. Maysam Ghovanloo, and Prof. Nitish V. Thakor. I am indebted to Rangara- jan Jegadeesan, an earlier research fellow and a good colleague, with whom the theoretical modeling of this dissertation was developed. The functional demonstration of flexible capacitively coupled implants builds upon the prior functional electrical stimulator work of Sudip Nag, an earlier research fellow. I am appreciative of the engineering, veterinary, and admin teams from Singapore Institute for Neurotechnology for their support during the last four years. To the coffee vending machine on level1 (Centre for Life Sciences) for being there for me 24x7, love comes from within when I think how much you have poured out of your heart to keep me awake and warm; I wish I could prototype you into a human. To my parents, Vinay and Rashmi Agarwal; fellow Ph.D. mates, Anoop C. Patil and Aishwarya Bandla; this doctoral journey would not have been so pleasant without you. And most importantly to my faith in God that has refueled my inner power, persistence, and courage to be that I am today. iii List of Relevant Journal Publications [1] R. Jegadeesan, S. Nag, K. Agarwal, N. V. Thakor and Y. X. Guo, "Enabling WirelessPoweringandTelemetryforPeripheralNerveImplants,"inIEEEJournal of Biomedical and Health Informatics, vol. 19, no. 3, pp. 958-970, May 2015. [2] R. Jegadeesan*, K. Agarwal*, Y. X. Guo, S. C. Yen and N. V. Thakor, "Wireless Power Delivery to Flexible Subcutaneous Implants Using Capacitive Coupling," in IEEE Transactions on Microwave Theory and Techniques, vol. 65, no. 1, pp. 280-292, Jan. 2017. (*co-first authors) [3] K. Agarwal, R. Jegadeesan, Y. X. Guo and N. V. Thakor, "Wireless Power Transfer Strategies for Implantable Bioelectronics: Methodological Review," in IEEE Reviews in Biomedical Engineering, vol. 10, pp. 136-161, Dec. 2017. [4] K. Agarwal, S. Nag, M. Ghovanloo, Y. X. Guo and N. V. Thakor, "Capac- itive Coupling for Wireless Power and Data Transfer to Flexible Bioelectronic Implants," in IEEE Transactions on Microwave Theory and Techniques, pp. 1-12, 2017. (under review) [5] K. Agarwal, M. Ghovanloo, Y. X. Guo and N. V. Thakor, "Efficient wireless power link design for peripheral nerve implants in non-human primates," in IEEE Transactions on Biomedical Engineering, 2017. (under preparation) iv List of Relevant Conference Presentations [1] K. Agarwal, R. Jegadeesan, Y. X. Guo and N. V. Thakor, "Modeling and designofwirelessdatatelemetryandpowertransferforbiomedicalimplants,"2015 IEEE International Conference on Computational Electromagnetics (ICCEM), Hong Kong, 2015, pp. 64-66. [2] K. Agarwal and Y. X. Guo, "Interaction of electromagnetic waves with humans in wearable and biomedical implant antennas," 2015 Asia-Pacific Sym- posium on Electromagnetic Compatibility (APEMC), Taipei, 2015, pp. 154-157. [3] K. Agarwal, R. Jegadeesan, Y. X. Guo and N. V. Thakor, "Evaluation and optimization of near-field inductive coupled wireless power links in rat model," 2015 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting (APS/URSI), Vancouver, BC, 2015, pp. 944-945. [4] K. Agarwal, R. Jegadeesan, Y. X. Guo and N. V. Thakor, "Wireless Power Transfer to High-Voltage Subcutaneous Implants using Near-Field Capacitive Coupling," 2017 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting (APS/URSI), San Diego, CA, 2017. [5] K. Agarwal, Y. X. Guo, M. Ghovanloo and N. V. Thakor, "Capacitive Coupling for Wireless Power and Data Transmission to Flexible Bioelectronic Implants," 2017 Asian Wireless Power Transfer Workshop (AWPT), Singapore, 2017. v Abstract Flexible bioelectronic implants show excellent promise to conform and integrate into the biological milieu, and thereby offer opportunities for reliable chronic solutions to many neural disorders through electrophysiological modulation. Current-day implants use resonance-tuned inductive coupling scheme for wireless power and data transfer which detunes with bending and is not efficient for such deployments. Withadvancesinintegrateddevicetechnologiesandflexelectronics, a critical need for efficient wireless scheme that can endure flexion arises. This dissertation presents the development of capacitively-coupled wireless technology for powering and telemetry with in vivo flexible electronic devices. A theoretical framework that formulates and establishes link efficiency and safe power transfer limits via modeling the electromagnetic wave interactions with biological tissue is provided. We demonstrate applications of these systems to drive and control in vivo functional electronic implants in animal models. This work enables the capability to power and communicate with a new generation of flex wireless bioelectronics for monitoring and modulating physiological functions. vi Contents List of Figures x List of Tables xviii 1 Wireless Strategies and Power Transmission for Implantable Bioelec- tronics 1 1.1 Introduction 1 1.2 Wireless Power Transmission 2 1.2.1 Near-field Resonant Inductive coupling 3 1.2.2 Near-field Capacitive Coupling 11 1.2.3 Ultrasonic Energy Transfer 18 1.2.4 Mid-field Wireless Power Transfer 24 1.2.5 Far-field Electromagnetic Coupling 29 1.3 Comparative study of various WPT schemes 34 1.4 Concluding Opinion 37 2 Wireless Telemetry, Applications, and Safety for Implantable Bioelec- tronics 38 2.1 Introduction 38 2.2 Data Telemetry 39 2.3 Applications 39 2.3.1 Cochlear Implants 41 2.3.2 Retinal Implants 41 2.3.3 Cortical Implants 43 2.3.4 Peripheral Nerve Implants 44 2.4 Safety and Regulations 47 2.4.1 Electrical safety 48 2.4.2 Biosafety 49 vii Contents viii 2.4.3 Physical Safety 50 2.4.4 Electromagnetic Interference Safety 50 2.4.5 Cyber Security 50 2.4.6 Regulatory Requirements 51 2.5 Concluding Opinion 52 2.6 Future Developments 52 2.7 Thesis contribution 54 3 Wireless Power Transmission to Flexible Subcutaneous Implants using Capacitive Coupling 56 3.1 Introduction 56 3.2 Near-Field Capacitive Coupling 58 3.3 NCC Link Modelling 60 3.3.1 Tissue Model 61 3.3.2 Tissue Loss 63 3.3.3 Conductor Loss 65 3.3.4 Self-Inductance 65 3.3.5 Equivalent Capacitance 66 3.3.6 Return Loss 66 3.3.7 Power Transfer Efficiency 68 3.3.8 Power Transfer Limit 69 3.4 Microwave Simulations 71 3.5 Optimal Link Design 73 3.6 Experimental results in Non-human Primate Cadaver 75 3.6.1 Study on Power Transfer Efficiency 75 3.6.2 Flexion Study 78 3.6.3 NCC Link Versus NRIC Link 81 3.7 Concluding Opinion 81 4 Simultaneous Wireless Power and Data Transmission to Flexible Bio- electronic Implants using Capacitive Coupling 83 Contents ix 4.1 Introduction 83 4.2 Near-Field Capacitively-Coupled Scheme 86 4.3 Optimum Link Design for NCC Powering 91 4.4 Powering High-Voltage Implants Using NCC 95 4.5 Simultaneous Power and Data Transfer 98 4.6 Acute Experiments in Rodent Model 99 4.7 Concluding Opinion 104 5 Conclusion and Future Work 106 5.1 Thesis highlights: Strengths and Probable improvements 106 5.2 Futuristic flex-bioelectronic implants: Applications and Roadmap 110 References 114 List of Figures 1.1 Schematic of the near-field inductive power transfer method. Time-varying magnetic field B produced by the transmitting coil (Tx) induces an emf across the terminals of the receiving coil (Rx). This EMF is tapped by the implant device as the received power P . 4 Rx 1.2 Simplified schematic of the NRIC scheme to discuss power transfer improvement from first principles. 5 1.3 The series and parallel resonant topologies that are used in the NRIC scheme to improve the power transfer efficiency. The Tx is generally not resonant tuned; instead it is matched to the reflected load to minimize the return losses. 6 1.4 System level diagram of the NRIC scheme showing power being driven through Tx by a class E inverter and the power induced at Rx is rectified for implant use. The overall system efficiency depends on the efficiency of class E inverter, the Tx-Rx power transfer link efficiency, and the rectifier/regulator conversion efficiency. The source and conversion efficiencies are generally good, and the system efficiency is thus limited by the link efficiency (PTE). 7 1.5 Load decoupling for overcoming load variations in the implantable applications. 9 1.6 Analysis of the effect of separation and misalignment between primary (Tx) and secondary (Rx) coils is crucial to assure sufficient power delivered levels at all times. (A) Schematic illustration of the separation and radial misalignment of the external Tx coil with respect to implanted Rx coil. (B) Photograph of the non-human primate experiment used for the study proposed in [1]. Source: Agarwal et. al. 2017 [1]. 10 1.7 Schematic of the NCC method. Time-varying electric field produced by the Tx metal patch supports displacement currents that enable wireless transfer of energy to the implanted Rx patches. Source: Agarwal et. al. 2017 [2]. 12 1.8 Improving the power transfer capability from the first EM principles. The figure shows the near-field electric coupling between two conductors, when excited by a voltage source. 13 x