efficient and high-speed reconfigurable transceiver for ...

EFFICIENT AND HIGH-SPEED RECONFIGURABLE TRANSCEIVER FOR MINIATURIZED WIRELESSLY POWERED IMPLANTS

A DISSERTATION SUBMITTED TO THE DEPARTMENT OF ELECTRICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

Anatoly Yakovlev August 2013

© 2013 by Anatoly Yakovlev. All Rights Reserved. Re-distributed by Stanford University under license with the author.

This work is licensed under a Creative Commons AttributionNoncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/

This dissertation is online at: http://purl.stanford.edu/dv394cq8631

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I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Ada Poon, Primary Adviser

I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Thomas Lee

I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Teresa Meng

I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Boris Murmann

Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost for Graduate Education

This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives.

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Abstract Small and versatile implantable devices that support distributed biosensing and localized operations could revolutionize modern medicine. Implantable systems will soon be an integral part of minimally invasive diagnostic, therapeutic, and surgical treatments to attain more accurate diagnosis and enhance the success rate of complex procedures. Wireless powering in combination with efficient high-speed reconfigurable transceivers that can accommodate a wide variety of biomedical applications and changing environmental conditions are essential for miniaturized medical implants. This thesis presents the detailed description of a wireless system architecture that enables the design of low-power battery-less implants. Wireless power transfer and very efficient high data rate communication techniques will be discussed. System and circuit design for two different miniaturized implantable devices, which were used to demonstrate these techniques, will be presented. The design challenges and tradeoffs of each of the projects will be discussed. The first project demonstrates efficient energy harvesting and forward data transfer as well as actuation for implants by providing up to several milliamps of current to power a magnetohydrodynamic propulsion system. The 3 mm × 4 mm prototype achieves 0.53 cm/sec speeds in fluid with a 0.06 T field using approximately 250 μW, and receives data at up to 25 Mbps from a 2 W 1.86 GHz carrier. The second project illustrates the feasibility of low-power sensing for implants as well as an efficient reverse data link capable of robust operation in changing environmental conditions, which is common for implanted systems. The robustness was achieved through reconfigurable modulating load, pulse width, and data rate making this an attractive solution for a variety of applications, including the target application to develop a 1 mm3 implantable cardiac probe. External interrogator communicates with multiple devices using time domain multiple access. The implantable prototype consists of a 1 mm × 1 mm chip implemented in 65 nm CMOS process integrated with a 3D coil antenna and no other external components and consumes 10 μW while demonstrating wireless powering and two-way communication through 35 mm of tissue. v

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Acknowledgments I dedicate this thesis to my parents, Anatoly Yakovlev and Galina Yakovleva, and to the rest of my family, as well as to José Flores, for their continuous and unconditional support throughout my academic career. First of all, I would like to thank my tapeout partners and friends Daniel Pivonka and Jihoon Jang, without whom this work would not have been possible. They have made the research work and long sleepless nights much more enjoyable and rewarding. I would like to especially thank my advisor, Ada Poon, for her support and inspiration throughout my PhD work. She has been a great source of ideas and provided all the necessary resources when they were needed. Professors Boris Murmann and Tom Lee have been great educators from whom I’ve learned a great deal of what I know today in analog and RF circuits. They have also been great to brainstorm with, get feedback and ideas when things seemed to be close to impossible. Prof. Teresa Meng provided helpful advice during locomotive implant work, and has been a great mentor since. I would like to thank all the past and present members of my research group, who have been a great source of support and learning. Additionally, it has been extremely helpful to work closely with all the fellow circuits students, including students from SMIrC lab, Murmann, Meng, Wooley, Wong, and Horowitz groups. I’ve had many enjoyable conversations with them and learned a lot from them. Several doctors from the Stanford medical community have also provided help and guidance throughout my work, especially Dr. Bhagat Patlolla, Dr. Evgenios Neofytou, Dr. Paul Wang, and Dr. Bob Hu. I also need to thank all of my friends, who are too numerous to list, for help, support, and being there for me. I would also like to acknowledge Taiwan Semiconductor Manufacturing Corporation (TSMC) for fabricating our chips. Pauline Prather did an outstanding job wirebonding all of our chips. Joe Little has been great at providing IT support at urgent times. Thanks to June Wang and Ann Guerra for their administrative support. Parts of this work have been funded by Olympus and Rethinking Analog Design (RAD). vii

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Table of Contents Chapter 1 : 1.1 1.2 1.3 1.4 1.5 Chapter 2 : 2.1 2.2 2.3 2.4 Chapter 3 : 3.1

Implantable Devices ........................................................................................1 Power Sources ..................................................................................................3 Communication ................................................................................................6 System and Circuit Design...............................................................................7 Locomotive Implant and Implantable Cardiac Probe ......................................8 Organization ...................................................................................................10 Transcutaneous Power and Data Transfer......................................................12 Wireless Power Transfer into Miniature Implants .........................................13 Forward Data Link .........................................................................................17 Reverse Data Link..........................................................................................20 Summary ........................................................................................................24 System and Implantable Device Architecture ................................................26 External Reader Implementation ...................................................................27 3.1.1 Modulating the Power Carrier ..............................................................28 3.2 Locomotive Implant .......................................................................................29 3.2.1 Background: Electromagnetic Propulsion ............................................32 3.2.1.1 Magnetohydrodyanmic (MHD) Propulsion .................................32 3.2.1.2 Asymmetric Fluid Drag Propulsion .............................................34 3.2.2 Locomotive Implant Wireless Chip Architecture .................................36 3.2.2.1 Antenna Design ............................................................................39 3.2.2.2 Dynamic Matching Network........................................................43 3.2.2.3 Power Management .....................................................................43 3.2.2.4 Propulsion System Interface ........................................................45 3.3 Implantable Cardiac Probe ............................................................................45 3.3.1 Background: Localized 3D Mapping of the Depolarization Pattern ...46 3.3.2 Implantable Cardiac Probe Wireless Chip Architecture ......................48 3.3.2.1 External and Implantable Antenna Design .................................50 3.3.2.2 Analog Sensor Interface ..............................................................52 3.3.2.3 Protocol Design Supporting Multiple Implantable Devices .......55 3.4 Summary .......................................................................................................57 Chapter 4 : Locomotive Implant Circuit Implementation................................................59 4.1 RF Front End..................................................................................................59 4.1.1 Balanced L-match .................................................................................61 4.1.2 Adaptive Loading..................................................................................62 4.2 Startup and Power-on Reset Circuits .............................................................62 4.2.1 Shunting Resistor ..................................................................................63 4.2.2 Power-on Reset ....................................................................................64 4.3 Power Management .......................................................................................66 4.3.1 Rectification ..........................................................................................67 4.3.2 Regulation .............................................................................................68 4.3.3 Bandgap Reference ...............................................................................69 4.4 Asynchronous Data Transfer .........................................................................70 4.4.1 Envelope Detection and Dynamic Reference Generation ....................71 ix

4.4.2 Clock and Data Recovery .....................................................................72 Digital Controller ..........................................................................................74 4.5.1 Data Packet Structure ............................................................................74 4.5.2 Architecture...........................................................................................75 4.6 Configurable High-Current Drivers ...............................................................76 4.7 Chip Summary ...............................................................................................77 Chapter 5 : Implantable Cardiac Probe Circuit Implementation .....................................80 5.1 RF Front End..................................................................................................80 5.2 Power-on Reset and Digital Enable Circuits .................................................82 5.2.1 Power-on Reset ....................................................................................83 5.3 Power Management .......................................................................................85 5.3.1 Rectification ..........................................................................................86 5.3.2 Regulation .............................................................................................87 5.4 Reconfigurable Asynchronous Data Transfer ................................................88 5.4.1 Envelope Detection and Dynamic Reference Generation ....................89 5.4.2 Clock and Data Recovery .....................................................................90 5.5 Reconfigurable Backscattering Modulator ...................................................92 5.5.1 On-chip Clock Generator with Configurable Clock Rate .....................93 5.5.2 Reconfigurable Width Pulse Generator ................................................94 5.5.3 Reconfigurable Modulating Load .........................................................95 5.6 Digital Controller ..........................................................................................96 5.6.1 Forward Data Packet Structure .............................................................97 5.6.2 Architecture...........................................................................................98 5.7 Analog Front End .........................................................................................100 5.8 Chip Summary .............................................................................................104 Chapter 6 : Experimental Validation .............................................................................107 6.1 Locomotive Implant Wireless Power Transmission ....................................107 6.2 Forward Data Transfer in Locomotive Implant ...........................................109 6.3 Locomotive Implant Fluid Propulsion Measurements ................................111 6.4 Cardiac Probe Link Gain Evaluation ...........................................................115 6.5 Cardiac Probe Forward and Reverse Data Link Validation.........................117 Chapter 7 : Conclusion...................................................................................................124 7.1 Locomotive Implant .....................................................................................124 7.2 Implantable Cardiac Probe ...........................................................................125 7.3 Recommendations for Future Work.............................................................127 Bibliography ....................................................................................................................128 4.5

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List of tables Number Page Table 1-1: Summary of energy harvesting techniques with corresponding power densities..............................................................................................................6 Table 2-1: Z-parameters from EM simulations. Z11 is external and Z22 is implant antenna impedances. ........................................................................................17 Table 4-1: Description of bits in the data packet. ..............................................................75 Table 5-1: Description of bits in the forward data packet. ................................................98 Table 6-1: Summary of circuit performance of locomotive implant IC. .........................114 Table 6-2: Relative performance of different modulating loads across various conditions. Each load performance is compared to best case for each test condition. .......................................................................................................120 Table 6-3: Summary of circuit performance of locomotive implant IC. .........................123

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List of figures Number Page Figure 1-1: X-Ray image of a pacemaker with non-rechargeable batteries illustrating size and package. ...............................................................................................4 Figure 1-2: Conceptual operation of implantable device in the vascular system. ...............9 Figure 1-3: Conceptual operation of real-time 3D intracardiac mapping system. .............10 Figure 2-1: Sample EM simulation setup for transcuataneous link gain evaluation. ........16 Figure 2-2: Z-parameters that can be used to evaluate link gain. ......................................17 Figure 2-3: With ASK-PWM modulation, data bits are encoded with pulse duration. .....18 Figure 2-4: High-level block diagram of asynchronous ASK-PWM receiver. .................19 Figure 2-5: Block diagram of ASK-PWM demodulator and sample waveforms along the demodulator chain. .....................................................................................20 Figure 2-6: Illustration showing the concept of backscattering link operation. ................22 Figure 2-7: Smith chart representation of optimal load for backscattering link. ...............24 Figure 3-1: Entire wireless system high-level block diagram. ..........................................26 Figure 3-2: External reader detailed block diagram with annotated parameters. ..............28 Figure 3-3: Load modulator with adjustable modulation depth.........................................29 Figure 3-4: Conceptual operation of the device in the bloodstream. .................................31 Figure 3-5: Basic operation of magnetohydrodynamic (MHD) propulsion. .....................34 Figure 3-6: Simulation results showing required current for MHD as function of speed and size. .................................................................................................34 Figure 3-7: Propulsion from asymmetric fluid drag (AFD). Electromagnetic forces cancel and drag forces do not, resulting in a net forward force. ......................36 Figure 3-8: Current versus speed estimate for asymmetric fluid drag propulsion. ............36 Figure 3-9: Locomotive implant architecture. ...................................................................38 Figure 3-10: Antenna simulation setup. .............................................................................40 Figure 3-11: Simulated cross-section of the radiation pattern of the transmit antenna. ....40 Figure 3-12: Comparison of magnetic field components along the transmit axis. The radiation is linearly polarized at the range of interest, so there is a negligible 3rd component (in green) also parallel to the water. ........................41 Figure 3-13: Frequency sweep of link gain for the receiver with no matching. ................42 Figure 3-14: Simulated link gain with a lossless matching network. ................................42 Figure 3-15: (a) Illustrates an external device that delivers power to energize the cardiac probes and interrogates the electrograms measured by these probes. (b) Each cardiac probe is identified with a unique ID. The external detector will demultiplex measurements from the probes at different locations, and hence deduce the propagation of the excitation wavefront. (c) Shows the 3D structure of a cardiac probe including an antenna, a silicon chip, and the electrodes. ......................................................47 Figure 3-16: Implantable cardiac probe wireless prototype architecture ...........................49 Figure 3-17: CST simulation was performed to evaluate the link gain for the implantable cardiac probe. (a) Shows external antenna located 10 mm xiii

above the tissue and the implant antenna 10 mm below the tissue surface; (b) shows the 3D coil antenna mounted over the PCB and the integrated circuit. ..............................................................................................................51 Figure 3-18: Simulations of efficiency versus frequency for the above antenna setup. ....52 Figure 3-19: Real and imaginary antenna impedance for encapsulated 2-turn 3D coil antenna in tissue. ..............................................................................................52 Figure 3-20: Analog front-end block diagram for the acquisition of extracellular action potentials. ..............................................................................................53 Figure 3-21: Implantable cardiac probe programmable ID through wirebonding modification. (a) ID is set to 3’b111 by wirebonding all three ID pads to VDD; (b) ID is set to 3’b100 by wirebonding MSB ID pad to VDD and remaining ID pads to GND. .............................................................................56 Figure 3-22: Timing diagram illustrating TDMA operation for the implantable cardiac probe. ...............................................................................................................57 Figure 4-1: Picture of the 4 cm × 4 cm transmit antenna. .................................................60 Figure 4-2: Picture of the 2 mm × 2 mm receive antenna on the prototype. .....................61 Figure 4-3: Diagram of on-chip matching network using a balanced L-match. ................62 Figure 4-4: Power-on shunting and Vdd enable circuit. ....................................................64 Figure 4-5: Power-on reset signal generation circuit. ........................................................66 Figure 4-6: ADS simulated waveforms showing the timing of the startup signals. ..........66 Figure 4-7: Self-driven synchronous rectifier schematic. ..................................................68 Figure 4-8: Four stage rectifier connected in charge-pump configuration. .......................68 Figure 4-9: Linear regulator for analog and digital circuitry. ...........................................69 Figure 4-10: Bandgap reference circuit with annotated sub-blocks. .................................70 Figure 4-11: Envelope detector and dynamic reference generator. ...................................72 Figure 4-12: Received RF envelope (top) and extracted envelope and reference signals (bottom). ..............................................................................................72 Figure 4-13: First comparator that converts the envelope to a digital signal. ....................73 Figure 4-14: Integrator and second comparator for data decoding. ...................................74 Figure 4-15: Schematic of digital circuitry operation and connectivity. ...........................76 Figure 4-16: Schematic of configurable high-current driver. ............................................77 Figure 4-17: ADS simulated waveforms of chip during operation....................................78 Figure 4-18: Locomotive implant chip layout. ..................................................................79 Figure 5-1: Picture of the external cross-slot and loop antennas [74]. ..............................81 Figure 5-2: Picture of the 3D coil receive antenna. ...........................................................82 Figure 5-3: Power-on shunting and Vdd enable circuit. ....................................................83 Figure 5-4: Power-on reset signal generation circuit. ........................................................84 Figure 5-5: Delay line implementation for the Power-on reset circuit. .............................84 Figure 5-6: ADS simulated waveform showing the RF voltage at the antenna output. ....85 Figure 5-7: ADS simulated waveforms showing the timing of the startup signals. ..........85 Figure 5-8: Linear regulator for analog and digital circuitry. ............................................88 Figure 5-9: Modified envelope detector and dynamic reference generator. ......................90 xiv

Figure 5-10: Received RF envelope (top) and extracted envelope and reference signals (bottom). ..............................................................................................90 Figure 5-11: First comparator that converts the envelope to a digital signal. ....................92 Figure 5-12: Adjustable integrator for forward data rate control and second comparator for data decoding. .........................................................................92 Figure 5-13: On-chip oscillator with adjustable clock rate. ...............................................94 Figure 5-14: Reconfigurable width pulse generator. .........................................................95 Figure 5-15: Reconfigurable modulating load. ..................................................................96 Figure 5-16: Schematic of digital circuitry operation and connectivity. ...........................97 Figure 5-17: Block diagram of digital circuitry detailing operation and connectivity. ...100 Figure 5-18: Electrodes representation and AC coupling. ...............................................101 Figure 5-19: Preamplifier and differential common mode feedback circuit implementation. .............................................................................................102 Figure 5-20: Third-order elliptic low pass filter. .............................................................103 Figure 5-21: Low transconductance OTA. ......................................................................103 Figure 5-22: Entire chip simulation in ADS showing forward and reverse data packets............................................................................................................105 Figure 5-23: ADS simulation showing reverse data transmission and demodulation at the external reader. .........................................................................................106 Figure 5-24: Implantable cardiac probe chip layout. .......................................................106 Figure 6-1: Measured link gain in air and water. .............................................................108 Figure 6-2: Rectified output and regulated supply voltage. .............................................109 Figure 6-3: Spectrum of 1.86 GHz carrier modulated with 8.3 MHz clock at 9% depth. ..............................................................................................................110 Figure 6-4: Received data (top) and clock (bottom) on the chip. ....................................111 Figure 6-5: MHD propulsion test setup. The device is navigated to the target destination. .....................................................................................................112 Figure 6-6: Asymmetric fluid drag propulsion test setup. ...............................................113 Figure 6-7: Experimental setup of the device inside porcine heart. ................................116 Figure 6-8: 3D coil antenna and wirebonded chip on Rogers 4350 board. .....................116 Figure 6-9: Measured link gain in air and through porcine heart. ...................................117 Figure 6-10: Rectified output and regulated supply voltage. ...........................................117 Figure 6-11: Forward data link data rate control verification. .........................................118 Figure 6-12: Reverse data link with narrow pulses. ........................................................119 Figure 6-13: Reverse data packet demodulation with two different data packets. ..........121 Figure 6-14: Reverse data packet demodulation with two different data packets. ..........122

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Chapter 1 : Implantable Devices

The current state of medical care is primarily aligned towards “fixing” the patients when the disease has evolved significantly with detrimental consequences. As a matter of fact, the majority of people take better care of their cars than their health primarily due to the fact that cars have sophisticated diagnostics and monitoring tools to assist the user and remind them when and what kind of maintenance is required. This results in preventive care that is much cheaper and significantly prolongs the life of the vehicle and makes it better quality. It is a common knowledge that without proper maintenance, vehicles do not last nearly as long as they do when they are properly taken care of. Why, then, do people not maintain their health in much the same fashion as they maintain their cars? Well, the simple fact is that people do not have similar gauges that tell them when they should get more sleep, reduce their stress, eat healthier, or even see a doctor. Instead, people have to rely on the way they feel for feedback about their health conditions. Thus, if they have a fever and start coughing, they might visit a doctor who makes diagnoses based on a few unreliable data points provided by a patient who is generally unfamiliar with the field of pathology. This often results in inaccurate diagnosis, and therefore, wrong treatment. Additionally, sick patient usually waits to see a doctor until after disease progressed and has started causing complications. The makes the disease much more difficult, expensive, and longer to treat. Instead, if people were continuously aware of the state of their wellbeing, they would not only get the necessary treatment sooner but could also prevent the disease altogether. Therefore, accurate and timely information about the state of one’s wellbeing and vital signs can be used to guide her/his lifestyle choices such as diet, quality of sleep, and stress levels. This timely information can be provided with the help of a tiny implantable device that can monitor one’s wellbeing from within. People who are genetically predisposed to or those who have already developed certain medical conditions can also benefit from diagnostic and therapeutic implantable devices. Certain forms of diseases, especially of neural origin, currently do not have any chemical 1

or drug therapies. The only known existing solution is through neurostimulation. For instance, certain regions of the brain respond remarkably well to electrical stimulation to treat debilitating effects of disorders such as chronic pain, essential tremor, Parkinson’s disease, dystonia, major depression, and tourette syndrome without causing permanent damage to physiological or anatomical structures [1-3]. The heart pacemaker is another example of a widely used implantable therapeutic device that has a tremendous impact on prolonging lives of people with chronic heart diseases. However, current state-of-the-art commercially available implantable devices rely on batteries to power them resulting in bulky form factors. The size of the devices makes them difficult to implant requiring expensive invasive surgery. Moreover, the batteries only last for 3 to 5 years requiring the patient to undergo a surgery to replace the batteries [4]. The recovery period after the surgery can last up to several days. To make these devices accessible for battery replacement, pacemaker is placed under patient’s skin on the chest with long leads running subcutaneously to the region where the actual stimulation has to be performed. In the case of deep brain stimulation (DBS) device, the lead runs to the top of patient’s head and is inserted deep inside the brain through an opening in the skull. The leads, their placement, and implantation surgeries can cause complications and significantly increase the risk of infection [5-8]. Therefore, replacing batteries with alternative energy sources can help dramatically reduce the device sizes and thus alleviate these serious problems and will be future direction for implantable devices. In addition to solving the battery problem, future implantable devices that are small and versatile enough to support distributed biosensing and localized operations would revolutionize modern medicine. Implantable systems might soon be an integral part of minimally invasive diagnostic, therapeutic, and surgical treatments to attain more accurate diagnosis and enhance the success rate of complex procedures. In fact, the movie “Fantastic Voyage” well describes the future vision for these implants. In the movie, a submarine with doctors and necessary navigation and therapeutic equipment is miniaturized and inserted inside the body of a patient to treat the problem area from the 2

inside. Even though this still seems like something far from reality, much advancement has been done technologically toward realization of this grand vision. Many of the essential components have already been developed and demonstrated such as locomotion in fluid medium [9, 10], energy harvesting for miniaturized implants [11-13, 18-20], efficient communication [12, 14, 17], actuation and drug delivery [9, 15-16], and lowpower diagnostics [21-24]. In this chapter, we will outline the various challenges and key system considerations in making an implantable device. First, energy harvesting and specifically RF power transfer as a pathway to miniaturization of the implantable devices will be discussed; a typical implantable device architecture and the essential blocks necessary for its implementation will then be presented; energy efficient and robust communication for implantable devices including the forward and reverse data links will then be described; two examples of mm-sized fully wireless implantable devices will be provided that demonstrate feasibility of therapeutic and diagnostic devices.

1.1 Power Sources One of the key challenges in the miniaturization of implantable devices is reducing the size of the power source. Conventionally, implantable devices have relied on batteries to power them up. Specifically, despite the recent trend to improve energy densities for rechargeable batteries, non-rechargeable batteries are primarily used because of their inherently higher energy density [25]. Additionally, there has not been a reliable way to recharge batteries even if rechargeable batteries were used. Therefore, many of the traditional implants have been restricted to a bulky package consisting of a can that houses electronics and a battery and attached leads with electrodes for tissue interface, as shown in Figure 1-1. Integrated circuit technology scaling and lower supply voltage operation has also made it possible to reduce power consumption and size of newer implants to operate from various energy harvesting modalities, including wireless power transfer. In the recent past, much research has been focused on energy scavenging techniques specifically for low-power 3

implantable devices, including harvesting vibrational, solar, thermoelectric, and even biofuel energy. However, many of these techniques remain restricted to research due to insufficiently low output power when the device size is restricted to millimeter scales.

Figure 1-1: X-Ray image of a pacemaker with non-rechargeable batteries illustrating size and package.

Miniaturization of implantable devices enables their proliferation to formerly nonexisting applications and implantation areas providing better insight into pathologies and tools to treat them with the added benefit of comfort to the patient. Additionally, doctors can implant these devices using minimally invasive techniques such as delivering them through a needle [26]. Also, removing batteries or replacing them with rechargeable ones eliminates the need for repeat surgeries to replace them and significantly reduces the risk of infection. Miniaturization of implants necessitates either replacing the batteries as an energy source with a much smaller alternative source or eliminating the batteries altogether. Therefore, energy harvesting is one of the most attractive pathways to miniaturization. It can help move away from the current bulky devices toward tiny devices. Although many forms of energy harvesting exist, depending on the application some are more appropriate than others. For instance, thermoelectric gradient (TEG) based energy harvesting can potentially provide enough energy to power up very low-power sensing circuitry at a temperature gradient of several degrees Celsius. However, such a gradient is difficult to attain when the implant is completely inside the body and would only be 4

practical for applications such as smart patches on the skin surface for runners or athletes. For the majority of applications, however, radiofrequency (RF) energy harvesting is the most appropriate choice because it provides the highest energy density among the common harvesting techniques. Some of the most common energy harvesting techniques and their corresponding power densities are summarized in Table 1 [17, 72]. From the table, it can be seen that electromagnetic (EM) or RF power transfer and ultrasonic power transfer deliver the highest power density per unit area. Although ultrasonic power transfer has better performance power transfer efficiency for deeper implants, it does not scale well with miniaturization. Typical demonstrated implants have an active area of greater than 1 cm2 [73]. Therefore, ultrasonic energy transfer may be preferable for deeply implanted devices which are not very constrained in size. EM power transfer is more scalable and performs better for mm-size low power implants. Up until recently, many devices relied on inductive coupling for wireless powering. For practical implant depths, this leads to operation at low frequencies on the order of a few tens of MHz, such as the popular 13.56 MHz ISM band. The main reasoning is that tissue losses increase with increasing frequency resulting in degraded power transfer efficiency. Recent research has shown, however, that the optimal frequency of operation for transcutaneous power transfer is in the low GHz range [18]. Intuitively, as the devices scale down in size, so must the power harvesting antenna. As the antenna size is reduced, its efficiency increases with frequency and so do tissue losses, resulting in optimal operating frequency in the low GHz range for mm-sized implantable devices. Higher frequency operation naturally provides higher bandwidth for the data link. The majority of existing implantable devices that require higher data rates, such as cochlear implants, retinal and other prosthetic devices, have to trade off bandwidth for a higher quality factor. Higher quality factor results in more efficient power harvesting operation but limits the maximum data rate that can be achieved. Thus, at a reasonable quality factor of 10, the bandwidth is only 1.56 MHz at 15.6 MHz center frequency but is as much as 200 MHz at 2 GHz. Another added benefit of higher frequency operation is desensitization of transmit and receive antennas to relative alignment and orientation because the operation happens to be in the mid field and not the near field in inductive coupling [27-28]. This 5

means that implants operate more robustly even with some uncertainty in the implant’s position and orientation with respect to the external source. Additionally, it is possible to optimize the external transmit antenna to focus the power delivered to the device while minimizing the tissue absorption and thus adhering to the specific absorption rate (SAR) regulations and further improving power transfer efficiency [19, 20]. Transcutaneous power transfer considerations will further be discussed in more detail in Chapter 2. Table 1-1: Summary of energy harvesting techniques with corresponding power densities.

Principle and Constraints

Power Density

Glucose bio-fuel cell utilizing glucose from blood (5 mM)

2.8 μW/mm2

Thermoelectric, ΔT = 5°C

0.6 μW/mm2

Piezoelectric micro bender, f ~ 800 Hz, 2.25 m/s2

< 0.2 μW/mm2

Ultrasonic power transfer