An Integrated Power-Efficient Active Rectifier With Offset-Controlled High Speed Comparators for Inductively Powered Applications

A resonant current-mode wireless power transfer for implantable medical devices: an overview

Implantable Medical Devices (IMDs) have been developing in ways to be lighter and lower-power systems. In the view of such developments, the battery recharging capacity to ensure the stable operation of the system is essential. Wireless power transfer (WPT) was proposed as a solution to recharge the battery without complex metallic contacts. However, due to limitations such as threshold voltage of power switches and minimal input power of the multi-stage structure (Rectifier + Regulator/DC-DC converter) of conventional voltage-mode (VM) WPT, there are drawbacks of an input power range above a certain threshold level and limitations due to strict regulations on the human body. These issues make the design of the IMD battery charger much harder and prevent IMDs from being a more viable option for people-in-need. This paper introduces distinguishing characteristics of resonant current-mode (RCM) WPT technology to overcome the aforementioned issues. It also describes the basic theory, conventional circuits of VM/RCM, comparisons, and major challenges of RCM. Finally, advanced and efficiency-enhancing techniques of the-state-of-art works among the RCM topologies will be discussed to follow up the trend of RCM WPT.

A Load-Insensitive Hybrid LSK Back Telemetry System With Slope-Based Demodulation for Inductively Powered Biomedical Devices

This paper presents a hybrid load-shift keying (LSK) modulation for a load-insensitive back telemetry system to realize near-constant voltage changes in a primary coil (L₁) against a wide range of load variations. The hybrid-LSK-enabled full-wave rectifier enables the sequential combination of open- and short-coil functions for hybrid-LSK modulation in addition to wireless power conversion operation. Load-insensitive L₁ voltage changes can be demodulated using the proposed slope- based demodulator, which utilizes the threshold slope of L₁ voltage changes over the back data pulse width, enabling robust data recovery regardless of the load conditions. The 0.56-mm² 0.18-μm standard CMOS hybrid-LSK prototype demonstrated that the variation of L₁ voltage changes could be minimized to 60 mV under load changes between 50 Ω and 50 kΩ at coil separation distance of 10 mm, achieving 88.2% reduction compared to the conventional short-coil LSK with 510 mV variation. The proposed back telemetry system also achieved a bit error rate (BER) of < 9.1 × 10^-10 under load ranges from 50 Ω to 50 kΩ and data rate of 1 Mbps, ensuring reliable back data recovery against load variations.

Wireless Photometry Prototype for Tri-Color Excitation and Multi-Region Recording

Visualizing neuronal activation and neurotransmitter release by using fluorescent sensors is increasingly popular. The main drawback of contemporary multi-color or multi-region fiber photometry systems is the tethered structure that prevents the free movement of the animals. Although wireless photometry devices exist, a review of literature has shown that these devices can only optically stimulate or excite with a single wavelength simultaneously, and the lifetime of the battery is short. To tackle this limitation, we present a prototype for implementing a fully wireless photometry system with multi-color and multi-region functions. This paper introduces an integrated circuit (IC) prototype fabricated in TSMC 180 nm CMOS process technology. The prototype includes 3-channel optical excitation, 2-channel optical recording, wireless power transfer, and wireless data telemetry blocks. The recording front end has an average gain of 107 dB and consumes 620 μW of power. The light-emitting diode (LED) driver block provides a peak current of 20 mA for optical excitation. The rectifier, the core of the wireless power transmission, operates with 63% power conversion efficiency at 13.56 MHz and a maximum of 87% at 2 MHz. The system is validated in a laboratory bench test environment and compared with state-of-the-art technologies. The optical excitation and recording front end and the wireless power transfer circuit evaluated in this paper will form the basis for a future miniaturized final device with a shank that can be used in in vivo experiments.

IC-Based Rectification Circuit Techniques for Biomedical Energy-Harvesting Applications

Energy harvesting can be achieved through many different mechanisms. Such technology has been drawing researchers’ attention to its practical applications for a decade, as it can be widely applied to countless scenarios. It steals the show in the modern development of the biomedical electronics, especially implantable applications, as it allows the patients to move freely without restriction. To prolong lifetime of the battery inside/outside a patient’s body, the electrical conversion efficiency of the electronic implant is of primary importance in energy harvesting. The conversion can be achieved by a so-called miniaturized rectification circuit (also known as “rectifier”). This study aims to compare different state-of-the-art techniques focusing on the conversion efficiency of the rectification. Particular emphasis is put on semiconductor-based circuits capable of being integrated with tiny chips on the implants.

A 6.78 MHz, 95.0% Peak Efficiency Monolithic Two-Dimensional Calibrated Active Rectifier for Wirelessly Powered Implantable Biomedical Devices

In this paper, a fully integrated active rectifier with triple feedback loops is proposed to enhance power conversion efficiency (PCE) over a wide loading range by calibrating both the gate transition timing and power switch size. The on- and off-transitions of the power switches are calibrated using a hybrid delay-based gate control circuit (HDGCC) with hybrid feedback loops. Conventional active rectifiers that only focused on calibrating the gate transition timing of a NMOS power switch with a fixed power switch size exhibit a low PCE when the loading condition deviates from the predetermined range. Thus, an automatic size selector based on a third feedback loop is proposed, which changes the power switch size based on the loading condition and ensures a stable operation of the hybrid loops by maintaining the voltage drop across the NMOS switches. An active rectifier was fabricated using the standard 0.18 μm CMOS process. The effectiveness and robustness of the two-dimensional calibration were verified through measurements under an AC input voltage ranging from 2.5 to 5.0 V and an output power ranging from 1.25 to 125 mW. The peak voltage conversion ratio and peak PCE were 97.6% and 95.0%, respectively, at R_L = 500 Ω.

A 13.56-MHz -25-dBm-Sensitivity Inductive Power Receiver System-on-a-Chip With a Self-Adaptive Successive Approximation Resonance Compensation Front-End for Ultra-Low-Power Medical Implants

Battery-less and ultra-low-power implantable medical devices (IMDs) with minimal invasiveness are the latest therapeutic paradigm. This work presents a 13.56-MHz inductive power receiver system-on-a-chip with an input sensitivity of -25.4 dBm (2.88 μW) and an efficiency of 46.4% while driving a light load of 30 μW. In particular, a real-time resonance compensation scheme is proposed to mitigate resonance variations commonly seen in IMDs due to different dielectric environments, loading conditions, and fabrication mismatches, etc. The power-receiving front-end incorporates a 6-bit capacitor bank that is periodically adjusted according to a successive-approximation-resonance-tuning (SART) algorithm. The compensation range is as much as 24 pF and it converges within 12 clock cycles and causes negligible power consumption overhead. The harvested voltage from 1.7 V to 3.3 V is digitized on-chip and transmitted via an ultra-wideband impulse radio (IR-UWB) back-telemetry for closed-loop regulation. The IC is fabricated in 180-nm CMOS process with an overall current dissipation of 750 nA. At a separation distance of 2 cm, the end-to-end power transfer efficiency reaches 16.1% while driving the 30-μW load, which is immune to artificially induced resonance capacitor offsets. The proposed system can be applied to various battery-less IMDs with the potential improvement of the power transfer efficiency on orders of magnitude.

Duty-Cycled Wireless Power Transmission for Millimeter-Sized Biomedical Implants

Wireless power transmission (WPT) using an inductively coupled link is one of the most popular approaches to deliver power wirelessly to biomedical implants. As the electromagnetic wave travels through the tissue, it is attenuated and absorbed by the tissue, resulting in much weaker electromagnetic coupling than in the air. As a result, the received input power on the implant is very weak, and so is the input voltage at the rectifier, which is the first block that receives the power on the implant. With such a small voltage amplitude, the rectifier inevitably has a very poor power conversion efficiency (PCE), leading to a poor power transfer efficiency (PTE) of the overall WPT system. To address this challenge, we propose a new system-level WPT method based on duty cycling of the power transmission for millimeter-scale implants. In the proposed method, the power transmitter (TX) transmits the wave with a duty cycle. It transmits only during a short period of time and pauses for a while instead of transmitting the wave continuously. In doing so, the TX power during the active period can be increased while preserving the average TX power and the specific absorption rate (SAR). Then, the incoming voltage becomes significantly larger at the rectifier, so the rectifier can rectify the input with a higher PCE, leading to improved PTE. To investigate the design challenges and applicability of the proposed duty-cycled WPT method, a case for powering a 1 × 1-mm2-sized neural implant through the skull is constructed. The implant, a TX, and the associated environment are modeled in High-Frequency Structure Simulator (HFSS), and the circuit simulations are conducted in Cadence with circuit components in a 180-nm CMOS process. At a load resistor of 100 kΩ, an output capacitor of 4 nF, and a carrier frequency of 144 MHz, the rectifier’s DC output voltage and PCE are increased by 300% (from 1.5 V to 6 V) and by 50% (from 14% to 64%), respectively, when the duty cycle ratio of the proposed duty-cycled power transmission is varied from 100% to 5%.

Overvoltage Protection Circuits for Ultrasonically Powered Implantable Microsystems

This paper presents a novel overvoltage protection technique for ultrasonically powered microsystems. The proposed idea benefits from voltage-current characteristics of the piezoelectric harvesters, and limits the amplitude of the harvested signal by regulating the current consumption of the system. For this purpose, a low-area low-power overvoltage regulator is proposed, analyzed and simulated in transistor level in standard TSMC 0.18μm CMOS technology occupying a silicon area of 285μm². Furthermore, to avoid unnecessary power consumption of the overvoltage regulator, it is proposed to take advantage of an ultrasonic burst detection block to deactivate the regulator in the absence of ultrasonic waves. According to our simulation results, the quiescent power consumption of the proposed circuit in the presence and absence of ultrasonic waves are 37 and 3μW respectively, and the minimum phase margin of the negative feedback loop is 68 degree.

A 3 mm × 3 mm Fully Integrated Wireless Power Receiver and Neural Interface System-on-Chip

A miniaturized, fully integrated wireless power receiver system-on-chip with embedded 16-channel electrode array and data transceiver for electrocortical neural recording and stimulation is presented. An H-tree power and signal distribution network throughout the SoC maintains high quality factor up to 11 in the on-chip receiver coil at 144 MHz resonant frequency while rejecting RF interference in sensitive neural interface circuits owing to its perpendicular and equidistant geometry. A multi-mode buck-boost resonant regulating rectifier (B ²R ³) offers greater than 11-dB input dynamic range in RF reception and less than 1 mV overshoot in transient load regulation. At 10 mm link distance, the 9 mm ² neural interface SoC fabricated in a 180 nm silicon-on-insulator (SOI) process attains an overall wireless power transmission system efficiency (WSE) of 3.4% in driving a 160  μW load yielding a WSE figure-of-merit of 131, while maintaining signal integrity in analog recording and wireless data transmission that comprise the on-chip load.

An Implantable Ultrasonically Powered System for Optogenetic Stimulation with Power-Efficient Active Rectifier and Charge-Reuse Capability

This paper presents a novel micro-scale ultrasonically powered optogenetic microstimulator with the vision of treating Parkinson's Disease. This system features a power-efficient active rectifier benefiting from a novel powering approach for its comparators. The main basis of the idea is to lower the Rail-to-Rail supply voltage of the comparators, thereby lowering their propagation delays. This technique improves the power conversion efficiency of the active rectifiers in two ways. First by decreasing the propagation delay of the comparators, and second by reusing the consumed power by the active diodes. The proposed system including the active rectifier, a novel double-pass regulator, a current reference, and a burst detection circuit is designed, simulated and fabricated in TSMC [Formula: see text]m CMOS technology with a total silicon area of [Formula: see text]. Based on the experimental results, the proposed active rectifier exhibits a voltage conversion ratio of [Formula: see text]% for input voltages of around 3 V, and a power conversion efficiency of up to [Formula: see text]% for a load of [Formula: see text] and over the frequency range of [Formula: see text]. A proof-of-concept system including the fabricated chip, a [Formula: see text]-sized lead zirconate titanate (PZT-4) piezoelectric receiver, and a custom-designed [Formula: see text] blue μ LED is designed and measured in a Water tank. For an acoustic intensity of [Formula: see text], the available electrical power at the crystal terminals, the output DC power, and the output light intensity were measured equal to [Formula: see text], [Formula: see text], and [Formula: see text], respectively. The quiescent current of the chip in absence of power bursts is measured equal to [Formula: see text]A.

A Bootstrapped Comparator-Switched Active Rectifying Circuit for Wirelessly Powered Integrated Miniaturized Energy Sensing Systems

Human life expectancy has gradually increased in part through rapid advances in technology, including the development and use of wearable and implantable biomedical electronic devices and sensing monitors. A new architecture is proposed in this paper to replace the traditional diode circuit implementation in wireless power supply systems applied to the above-mentioned devices and monitors. By achieving near-ideal power transistor switching and leveraging the characteristics of conventional diodes to prevent reverse current, the proposed approach greatly improves the performance of the energy harvester in power conversion. The MOS harvester used in the uninterrupted permanent wireless near-field power supply described here for use in biomedical systems was designed and verified using the Taiwan Semiconductor Manufacturing Company (TSMC) standard 180-nm process, achieving performance results of Voltage conversion efficiency (VCE) = 73.55–95.12% and Power conversion efficiency (PCE) = 80.36–90.08% with the output load (0.1–1 kΩ) under 3.3 V ac input with an overall area of 1.189 mm². These results are expected to create an important technical niche for new “green-energy” miniaturized energy sensing systems including cutting edge wirelessly powered biomedical electronics applications.

Maximizing Data Transmission Rate for Implantable Devices Over a Single Inductive Link: Methodological Review

Due to the constantly growing geriatric population and the projected increase of the prevalence of chronic diseases that are refractory to drugs, implantable medical devices (IMDs) such as neurostimulators, endoscopic capsules, artificial retinal prostheses, and brain-machine interfaces are being developed. According to many business forecast firms, the IMD market is expected to grow and they are subject to much research aiming to overcome the numerous challenges of their development. One of these challenges consists of designing a wireless power and data transmission system that has high power efficiency, high data rates, low power consumption, and high robustness against noise. This is in addition to minimal design and implementation complexity. This manuscript concerns a comprehensive survey of the latest techniques used to power up and communicate between an external base station and an IMD. Patient safety considerations related to biological, physical, electromagnetic, and electromagnetic interference concerns for wireless IMDs are also explored. The simultaneous powering and data communication techniques using a single inductive link for both power transfer and bidirectional data communication, including the various data modulation/demodulation techniques, are also reviewed. This review will hopefully contribute to the persistent efforts to implement compact reliable IMDs while lowering their cost and upsurging their benefits.

A mm-Sized Wireless Implantable Device for Electrical Stimulation of Peripheral Nerves

A wireless electrical stimulation implant for peripheral nerves, achieving >10× improvement over state of the art in the depth/volume figure of merit, is presented. The fully integrated implant measures just 2 mm × 3 mm × 6.5 mm (39 mm³, 78 mg), and operates at a large depth of 10.5 cm in a tissue phantom. The implant is powered using ultrasound and includes a miniaturized piezoelectric receiver (piezo), an IC designed in 180 nm HV BCD process, an off-chip energy storage capacitor, and platinum stimulation electrodes. The package also includes an optional blue light-emitting diode for potential applications in optogenetic stimulation in the future. A system-level design strategy for complete operation of the implant during the charging transient of the storage capacitor, as well as a unique downlink command/data transfer protocol, is presented. The implant enables externally programmable current-controlled stimulation of peripheral nerves, with a wide range of stimulation parameters, both for electrical (22 to 5000 μA amplitude, ∼14 to 470 μs pulse-width, 0 to 60 Hz repetition rate) and optical (up to 23 mW/mm² optical intensity) stimulation. Additionally, the implant achieves 15 V compliance voltage for chronic applications. Full integration of the implant components, end-to-end in vitro system characterizations, and results for the electrical stimulation of a sciatic nerve, demonstrate the feasibility and efficacy of the proposed stimulator for peripheral nerves.

A Miniaturized 0.78-mW/cm2 Autonomous Thermoelectric Energy-Harvesting Platform for Biomedical Sensors

In order to use thermoelectric energy harvesters (TEHs) as a truly autonomous energy source for size-limited sensing applications, it is essential to improve the power conversion efficiency and energy density. This study presents a thin-film, array-based TEH with a surface area of 0.83 cm². The TEH autonomously supplies a power management IC fabricated in a 65-nm CMOS technology. The IC utilizes a single-inductor topology with integrated analog maximum power point tracking (MPPT), resulting in a 68% peak end-to-end efficiency (92% converter efficiency) and less than 20-ms MPPT. In an in-vivo test, a 645-μW regulated output power (effective 3.5 K of temperature gradient) was harvested from a rat implanted with our TEH, demonstrating true energy independence in a real environment while showing a 7.9 × improvement in regulated power density compared to the state-of-the-art. The system showed autonomous operation down to 65-mV TEH input.

A 30 $\mu\text{W}$ Remotely Powered Local Temperature Monitoring Implantable System

An implantable local temperature monitoring system for a laboratory mouse is presented. Magnetic coupling is used to remotely power the passive implant. The temperatures of two local points are monitored by thermistors. A low-power readout circuit is implemented by directly amplifying and resolving the sensor response in the time domain. A free-running oscillator operating at 868 MHz transmits the sensor data to the base station. The average power dissipation of the implant is decreased by time interleaving between the sensor readout and the data communication. The power transfer to the implant is also time interleaved with other operations to avoid interference with data communication. A voltage regulation loop for the implant based on controlling the duration of powering the base station power amplifier is also described. A prototype chip is implemented in 0.18 [Formula: see text] CMOS. The implant requires average RF power of 29.5 [Formula: see text] for operation and is capable of measuring two thermistors with accuracy of ±0.05 °C.

An Integrated Passive Phase-Shift Keying Modulator for Biomedical Implants With Power Telemetry Over a Single Inductive Link

This paper presents a passive phase-shift keying (PPSK) modulator for uplink data transmission for biomedical implants with simultaneous power and data transmission over a single 13.56 MHz inductive link. The PPSK modulator provides a data rate up to 1.35 Mbps with a modulation index between 3% and 38% for a variation of the coupling coefficient between 0.05 and 0.26. This modulation scheme is particularly suited for biomedical implants that have high power demand and low coupling coefficients. The PPSK modulator operates in conjunction with on-off-keying downlink communication. The same inductive link is used to provide up to 100 mW of power to a multi-channel stimulator. The majority of the system on the implant side was implemented as an application specific integrated circuit (ASIC), fabricated in 0.6- [Formula: see text] high voltage CMOS technology. The theory of PPSK modulation, simulated and measured performance evaluation, and comparison with other state-of-the-art impedance modulation techniques is presented. The measured bit error rate around critical coupling at 1.35 Mbps is below 6 ×10^-8.

An Inductive Power and Data Telemetry Subsystem With Fast Transient Low Dropout Regulator for Biomedical Implants

This paper presents a capacitorless low-dropout (LDO) regulator with fast transient response and data reverse telemetry circuit for fully implantable wireless transmission applications. We propose a novel hybrid feedback structure using high-frequency compensation technology to achieve a rapid transient response for the LDO regulator. To reduce the size of the implant and transmit neural recordings through the same coil without interfering with power transmission, the load-shift-key (LSK) modulation technique is adopted for back data telemetry. The proposed implantable chip, fabricated using commercial 0.18 μm complementary metal oxide semiconductor technology, yielded an output power of 15 mW. Under 1.15 V operation voltage, the maximum overshoot and undershoot voltages were less than 45 mV and 55 mV, respectively, for a 15 mA full-load current whose rising and falling time were less than 100 ns. The proposed LSK transceiver uses a digitized demodulator to improve bandwidth efficiency for low carrier frequency operation.

A Battery-Less, Implantable Neuro-Electronic Interface for Studying the Mechanisms of Deep Brain Stimulation in Rat Models

Although deep brain stimulation (DBS) has been a promising alternative for treating several neural disorders, the mechanisms underlying the DBS remain not fully understood. As rat models provide the advantage of recording and stimulating different disease-related regions simultaneously, this paper proposes a battery-less, implantable neuro-electronic interface suitable for studying DBS mechanisms with a freely-moving rat. The neuro-electronic interface mainly consists of a microsystem able to interact with eight different brain regions bi-directionally and simultaneously. To minimize the size of the implant, the microsystem receives power and transmits data through a single coil. In addition, particular attention is paid to the capability of recording neural activities right after each stimulation, so as to acquire information on how stimulations modulate neural activities. The microsystem has been fabricated with the standard 0.18 μm CMOS technology. The chip area is 7.74 mm (2) , and the microsystem is able to operate with a single supply voltage of 1 V. The wireless interface allows a maximum power of 10 mW to be transmitted together with either uplink or downlink data at a rate of 2 Mbps or 100 kbps, respectively. The input referred noise of recording amplifiers is 1.16 μVrms, and the stimulation voltage is tunable from 1.5 V to 4.5 V with 5-bit resolution. After the electrical functionality of the microsystem is tested, the capability of the microsystem to interface with rat brain is further examined and compared with conventional instruments. All experimental results are presented and discussed in this paper.

An On-Chip Multi-Voltage Power Converter With Leakage Current Prevention Using 0.18 μm High-Voltage CMOS Process

In this paper, we present an on-chip multi-voltage power converter incorporating of a quad-voltage timing-control rectifier and regulators to produce ±12 V and ±1.8 V simultaneously through inductive powering. The power converter achieves a PCE of 77.3% with the delivery of more than 100 mW to the implant. The proposed rectifier adopts a two-phase start-up scheme and mixed-voltage gate controller to avoid substrate leakage current. This current cannot be prevented by the conventional dynamic substrate biasing technique when using the high-voltage CMOS process with transistor threshold voltage higher than the turn-on voltage of parasitic diodes. High power conversion efficiency is achieved by 1) substrate leakage current prevention, 2) operating all rectifying transistors as switches with boosted gate control voltages, and 3) compensating the delayed turn-on and preventing reverse leakage current of rectifying switches with the proposed look-ahead comparator. This chip occupies an area of 970 μm × 4500 μm in a 0.18 μ m 32 V HV CMOS process. The quad-voltage timing-control rectifier alone is able to output a high DC voltage at the range of [2.5 V, 25 V]. With this power converter, both bench-top experiment and in-vivo power link test using a rat model were validated.

A Triple-Loop Inductive Power Transmission System for Biomedical Applications

A triple-loop wireless power transmission (WPT) system equipped with closed-loop global power control, adaptive transmitter (Tx) resonance compensation (TRC), and automatic receiver (Rx) resonance tuning (ART) is presented. This system not only opposes coupling and load variations but also compensates for changes in the environment surrounding the inductive link to enhance power transfer efficiency (PTE) in applications such as implantable medical devices (IMDs). The Tx was built around a commercial off-the-shelf (COTS) radio-frequency identification (RFID) reader, operating at 13.56 MHz. A local Tx loop finds the optimal capacitance in parallel with the Tx coil by adjusting a varactor. A global power control loop maintains the received power at a desired level in the presence of changes in coupling distance, coil misalignments, and loading. Moreover, a local Rx loop is implemented inside a power management integrated circuit (PMIC) to avoid PTE degradation due to the Rx coil surrounding environment and process variations. The PMIC was fabricated in a 0.35- μm 4M2P standard CMOS process with 2.54 mm(2) active area. Measurement results show that the proposed triple-loop system improves the overall PTE by up to 10.5% and 4.7% compared to a similar open- and single closed-loop system, respectively, at nominal coil distance of 2 cm. The added TRC and ART loops contribute 2.3% and 1.4% to the overall PTE of 13.5%, respectively. This is the first WPT system to include three loops to dynamically compensate for environment and circuit variations and improve the overall power efficiency all the way from the driver output in Tx to the load in Rx.

A Q-Modulation Technique for Efficient Inductive Power Transmission

A fully-integrated power management ASIC for efficient inductive power transmission has been presented capable of automatic load transformation using a method, called Q-modulation. Q-modulation is an adaptive scheme that offers load matching against a wide range of loading (RL ) and coupling distance (d23 ) variations in inductive links to maintain high power transfer efficiency (PTE). It is suitable for inductive powering implantable microelectronic devices (IMDs), recharging mobile electronics, and electric vehicles. In Q-modulation, the zero-crossings of the induced current in the receiver (Rx) LC-tank are detected and a low-loss switch chops the Rx LC-tank for part of the power carrier cycle to form a high-Q LC-tank and store the maximum energy, which is then transferred to RL by opening the switch. By adjusting the duty cycle (D), the loaded-Q of the Rx LC-tank can be dynamically modulated to compensate for variations in RL . A Q-modulation power management (QMPM) prototype chip was fabricated in a 0.35-μm standard CMOS process, occupying 4.8 mm2. In a 1.45 W wireless power transfer setup, using a class-E power amplifier (PA) operating at 2 MHz, the QMPM successfully increased the inductive link PTE and the overall power efficiency by 98.5% and 120.7% at d23 = 8 cm, respectively, by compensating for 150 Ω variation in RL at D = 45%.

An NFC-Enabled CMOS IC for a Wireless Fully Implantable Glucose Sensor

This paper presents an integrated circuit (IC) that merges integrated optical and temperature transducers, optical interface circuitry, and a near-field communication (NFC)-enabled digital, wireless readout for a fully passive implantable sensor platform to measure glucose in people with diabetes. A flip-chip mounted LED and monolithically integrated photodiodes serve as the transduction front-end to enable fluorescence readout. A wide-range programmable transimpedance amplifier adapts the sensor signals to the input of an 11-bit analog-to-digital converter digitizing the measurements. Measurement readout is enabled by means of wireless backscatter modulation to a remote NFC reader. The system is able to resolve current levels of less than 10 pA with a single fluorescent measurement energy consumption of less than 1 μJ. The wireless IC is fabricated in a 0.6-μm-CMOS process and utilizes a 13.56-MHz-based ISO15693 for passive wireless readout through a NFC interface. The IC is utilized as the core interface to a fluorescent, glucose transducer to enable a fully implantable sensor-based continuous glucose monitoring system.

A Smart Wirelessly Powered Homecage for Long-Term High-Throughput Behavioral Experiments

A wirelessly powered homecage system, called the EnerCage-HC, that is equipped with multicoil wireless power transfer, closed-loop power control, optical behavioral tracking, and a graphic user interface is presented for longitudinal electrophysiology and behavioral neuroscience experiments. The EnerCage-HC system can wirelessly power a mobile unit attached to a small animal subject and also track its behavior in real-time as it is housed inside a standard homecage. The EnerCage-HC system is equipped with one central and four overlapping slanted wire-wound coils with optimal geometries to form three- and four-coil power transmission links while operating at 13.56 MHz. Utilizing multicoil links increases the power transfer efficiency (PTE) compared with conventional two-coil links and also reduces the number of power amplifiers to only one, which significantly reduces the system complexity, cost, and heat dissipation. A Microsoft Kinect installed 90 cm above the homecage localizes the animal position and orientation with 1.6-cm accuracy. Moreover, a power management ASIC, including a high efficiency active rectifier and automatic coil resonance tuning, was fabricated in a 0.35-μm 4M2P standard CMOS process for the mobile unit. The EnerCage-HC achieves a max/min PTE of 36.3%/16.1% at the nominal height of 7 cm. In vivo experiments were conducted on freely behaving rats by continuously delivering 24 mW to the mobile unit for >7 h inside a standard homecage.

A CMOS pressure sensor tag chip for passive wireless applications

This paper presents a novel monolithic pressure sensor tag for passive wireless applications. The proposed pressure sensor tag is based on an ultra-high frequency RFID system. The pressure sensor element is implemented in the 0.18 µm CMOS process and the membrane gap is formed by sacrificial layer release, resulting in a sensitivity of 1.2 fF/kPa within the range from 0 to 600 kPa. A three-stage rectifier adopts a chain of auxiliary floating rectifier cells to boost the gate voltage of the switching transistors, resulting in a power conversion efficiency of 53% at the low input power of -20 dBm. The capacitive sensor interface, using phase-locked loop archietcture, employs fully-digital blocks, which results in a 7.4 bits resolution and 0.8 µW power dissipation at 0.8 V supply voltage. The proposed passive wireless pressure sensor tag costs a total 3.2 µW power dissipation.

A remotely powered implantable biomedical system with location detector

An universal remote powering and communication system is presented for the implantable medical devices. The system be interfaced with different sensors or actuators. A mobile external unit controls the operation of the implantable chip and reads the sensor's data. A locator system is proposed to align the mobile unit with the implant unit for the efficient magnetic power transfer. The location of the implant is detected with 6 mm resolution from the rectified voltage level at the implanted side. The rectified voltage level is fedback to the mobile unit to adjust the magnetic field strength and maximize the efficiency of the remote powering system. The sensor's data are transmitted by using a free running oscillator modulated with on-off key scheme. To tolerate large data carrier drifts, a custom designed receiver is implemented for the mobile unit. The circuits have been fabricated in 0.18 um CMOS technology. The remote powering link is optimized to deliver power at 13.56 MHz. On chip voltage regulator creates 1.8 V from a 0.9 V reference voltage to supply the sensor/actuator blocks. The implantable chip dissipates 595 μW and requires 1.48 V for start up.

An arch-shaped intraoral tongue drive system with built-in tongue-computer interfacing SoC

We present a new arch-shaped intraoral Tongue Drive System (iTDS) designed to occupy the buccal shelf in the user's mouth. The new arch-shaped iTDS, which will be referred to as the iTDS-2, incorporates a system-on-a-chip (SoC) that amplifies and digitizes the raw magnetic sensor data and sends it wirelessly to an external TDS universal interface (TDS-UI) via an inductive coil or a planar inverted-F antenna. A built-in transmitter (Tx) employs a dual-band radio that operates at either 27 MHz or 432 MHz band, according to the wireless link quality. A built-in super-regenerative receiver (SR-Rx) monitors the wireless link quality and switches the band if the link quality is below a predetermined threshold. An accompanying ultra-low power FPGA generates data packets for the Tx and handles digital control functions. The custom-designed TDS-UI receives raw magnetic sensor data from the iTDS-2, recognizes the intended user commands by the sensor signal processing (SSP) algorithm running in a smartphone, and delivers the classified commands to the target devices, such as a personal computer or a powered wheelchair. We evaluated the iTDS-2 prototype using center-out and maze navigation tasks on two human subjects, which proved its functionality. The subjects' performance with the iTDS-2 was improved by 22% over its predecessor, reported in our earlier publication.

A 10.5 cm ultrasound link for deep implanted medical devices

A study on ultrasound link for wireless energy transmission dedicated to deeply implanted medical devices is presented. The selection of the frequency to avoid biological side effects (e.g., cavitations), the choice of the power amplifier to drive the external transducers and the design of the rectifier to maximize the energy extraction from the implanted transducer are described in details. The link efficiency is characterized in water using a phantom material for a transmitter-receiver distance of 105 mm, transducers active area of 30 mm × 96 mm and 5 mm × 10 mm, respectively, and a system efficiency of 1.6% is measured.

A 13.56 MHz CMOS Active Rectifier With Switched-Offset and Compensated Biasing for Biomedical Wireless Power Transfer Systems

A full-wave active rectifier switching at 13.56 MHz with compensated bias current for a wide input range for wirelessly powered high-current biomedical implants is presented. The four diodes of a conventional passive rectifier are replaced by two cross-coupled PMOS transistors and two comparator- controlled NMOS switches to eliminate diode voltage drops such that high voltage conversion ratio and power conversion efficiency could be achieved even at low AC input amplitude |VAC|. The comparators are implemented with switched-offset biasing to compensate for the delays of active diodes and to eliminate multiple pulsing and reverse current. The proposed rectifier uses a modified CMOS peaking current source with bias current that is quasi-inversely proportional to the supply voltage to better control the reverse current over a wide AC input range (1.5 to 4 V). The rectifier was fabricated in a standard 0.35 μm CMOS N-well process with active area of 0.0651 mm(2). For the proposed rectifier measured at |VAC| = 3.0 V, the voltage conversion ratios are 0.89 and 0.93 for RL=500 Ω and 5 kΩ, respectively, and the measured power conversion efficiencies are 82.2% to 90.1% with |VAC| ranges from 1.5 to 4 V for RL=500 Ω.

A Power-Efficient Wireless Capacitor Charging System Through an Inductive Link

A power-efficient wireless capacitor charging system for inductively powered applications has been presented. A bank of capacitors can be directly charged from an ac source by generating a current through a series charge injection capacitor and a capacitor charger circuit. The fixed charging current reduces energy loss in switches, while maximizing the charging efficiency. An adaptive capacitor tuner compensates for the resonant capacitance variations during charging to keep the amplitude of the ac input voltage at its peak. We have fabricated the capacitor charging system prototype in a 0.35-μm 4-metal 2-poly standard CMOS process in 2.1 mm² of chip area. It can charge four pairs of capacitors sequentially. While receiving 2.7-V peak ac input through a 2-MHz inductive link, the capacitor charging system can charge each pair of 1 μF capacitors up to ±2 V in 420 μs, achieving a high measured charging efficiency of 82%.

A Power-Efficient Wireless System With Adaptive Supply Control for Deep Brain Stimulation

A power-efficient wireless stimulating system for a head-mounted deep brain stimulator (DBS) is presented. A new adaptive rectifier generates a variable DC supply voltage from a constant AC power carrier utilizing phase control feedback, while achieving high AC-DC power conversion efficiency (PCE) through active synchronous switching. A current-controlled stimulator adopts closed-loop supply control to automatically adjust the stimulation compliance voltage by detecting stimulation site potentials through a voltage readout channel, and improve the stimulation efficiency. The stimulator also utilizes closed-loop active charge balancing to maintain the residual charge at each site within a safe limit, while receiving the stimulation parameters wirelessly from the amplitude-shift-keyed power carrier. A 4-ch wireless stimulating system prototype was fabricated in a 0.5-μm 3M2P standard CMOS process, occupying 2.25 mm². With 5 V peak AC input at 2 MHz, the adaptive rectifier provides an adjustable DC output between 2.5 V and 4.6 V at 2.8 mA loading, resulting in measured PCE of 72 ~ 87%. The adaptive supply control increases the stimulation efficiency up to 30% higher than a fixed supply voltage to 58 ~ 68%. The prototype wireless stimulating system was verified in vitro.

A low-frequency versatile wireless power transfer technology for biomedical implants

Implantable biomedical sensors and actuators are highly desired in modern medicine. In many cases, the implant's electrical power source profoundly determines its overall size and performance . The inductively coupled coil pair operating at the radio-frequency (RF) has been the primary method for wirelessly delivering electrical power to implants for the last three decades . Recent designs significantly improve the power delivery efficiency by optimizing the operating frequency, coil size and coil distance . However, RF radiation hazard and tissue absorption are the concerns in the RF wireless power transfer technology (RF-WPTT) , . Also, it requires an accurate impedance matching network that is sensitive to operating environments between the receiving coil and the load for efficient power delivery . In this paper, a novel low-frequency wireless power transfer technology (LF-WPTT) using rotating rare-earth permanent magnets is demonstrated. The LF-WPTT is able to deliver 2.967 W power at  ∼ 180 Hz to an 117.1 Ω resistor over 1 cm distance with 50% overall efficiency. Because of the low operating frequency, RF radiation hazard and tissue absorption are largely avoided, and the power delivery efficiency from the receiving coil to the load is independent of the operating environment. Also, there is little power loss observed in the LF-WPTT when the receiving coil is enclosed by non-magnetic implant-grade stainless steel.

A low-power 13.56 MHz RF front-end circuit for implantable biomedical devices

A low-power fully-integrated CMOS RF front-end circuit for a passive 13.56 MHz biomedical implant is presented. A 13.56 MHz binary phase shift keying (BPSK) signal is received by an internal coil. This front-end circuit is composed of a full-wave bridge rectifier, a linear regulator, a BPSK demodulator, and a clock/data recovery (CDR). A full-wave bridge rectifier converts the carrier waveform with the BPSK signal to an unregulated DC voltage. A linear regulator stabilizes the unregulated DC voltage to 1.8 V that serves as the DC source for the implant. A BPSK demodulator detects the incoming BPSK signal from the internal coil and translates the demodulated data to the CDR which can successfully recover the clock and data for the system controller. This chip with a core area of 0.45 mm(2) has been fabricated in a TSMC 0.18 μm 1P6M CMOS technology. The total power consumed is only 632 μW.

A high frequency active voltage doubler in standard CMOS using offset-controlled comparators for inductive power transmission

In this paper, we present a fully integrated active voltage doubler in CMOS technology using offset-controlled high speed comparators for extending the range of inductive power transmission to implantable microelectronic devices (IMD) and radio-frequency identification (RFID) tags. This active voltage doubler provides considerably higher power conversion efficiency (PCE) and lower dropout voltage compared to its passive counterpart and requires lower input voltage than active rectifiers, leading to reliable and efficient operation with weakly coupled inductive links. The offset-controlled functions in the comparators compensate for turn-on and turn-off delays to not only maximize the forward charging current to the load but also minimize the back current, optimizing PCE in the high frequency (HF) band. We fabricated the active voltage doubler in a 0.5-μm 3M2P std . CMOS process, occupying 0.144 mm(2) of chip area. With 1.46 V peak AC input at 13.56 MHz, the active voltage doubler provides 2.4 V DC output across a 1 kΩ load, achieving the highest PCE = 79% ever reported at this frequency. In addition, the built-in start-up circuit ensures a reliable operation at lower voltages.

A feed-forward controlled AC-DC boost converter for biomedical implants

Miniaturization is important to make implants clinic friendly. Wireless power transfer is an essential technology to miniaturize implants by reducing their battery size or completely eliminating their batteries. Traditionally, a pair of inductively-coupled coils operating at radio-frequency (RF) is employed to deliver electrical power wirelessly. In this approach, a rectifier is needed to convert the received RF power to a stable DC one. To achieve high efficiency, the induced voltage of the receiving coil must be much higher than the turn-on voltage of the rectifying diode (which could be an active circuit for low turn-on voltage) [1]. In order to have a high induced voltage, the size of the receiving coil often is significantly larger than rest of the implant. A rotating magnets based wireless power transfer has been demonstrated to deliver the same amount of power at much lower frequency (around 100 Hz) because of the superior magnetic strength produced by rare-earth magnets [2]. Taking the advantage of the low operating frequency, an innovative feed-forward controlled AC to DC boost converter has been demonstrated for the first time to accomplish the following two tasks simultaneously: (1) rectifying the AC power whose amplitude (500 mV) is less than the rectifier's turn-on voltage (1.44 V) and (2) boosting the DC output voltage to a much higher level (5 V). Within a range, the output DC voltage can be selected by the control circuit. The standard deviation of the output DC voltage is less than 2.1% of its mean. The measured load regulation is 0.4 V/kΩ. The estimated conversion efficiency excluding the power consumption of the control circuits reaches 75%. The converter in this paper has the potential to reduce the size of the receiving coil and yet achieve desirable DC output voltage for powering biomedical implants.

A novel wireless power and data transmission AC to DC converter for an implantable device

This article presents a novel AC to DC converter implemented by standard CMOS technology, applied for wireless power transmission. This circuit combines the functions of the rectifier and DC to DC converter, rather than using the rectifier to convert AC to DC and then supplying the required voltage with regulator as in the transitional method. This modification can reduce the power consumption and the area of the circuit. This circuit also transfers the loading condition back to the external circuit by the load shift keying(LSK), determining if the input power is not enough or excessive, which increases the efficiency of the total system. The AC to DC converter is fabricated with the TSMC 90nm CMOS process. The circuit area is 0.071mm(2). The circuit can produce a 1V DC voltage with maximum output current of 10mA from an AC input ranging from 1.5V to 2V, at 1MHz to 10MHz.

A wireless magnetoresistive sensing system for an intraoral tongue-computer interface

Tongue drive system (TDS) is a tongue-operated, minimally invasive, unobtrusive, and wireless assistive technology (AT) that infers users' intentions by detecting their voluntary tongue motion and translating them into user-defined commands. Here we present the new intraoral version of the TDS (iTDS), which has been implemented in the form of a dental retainer. The iTDS system-on-a-chip (SoC) features a configurable analog front-end (AFE) that reads the magnetic field variations inside the mouth from four 3-axial magnetoresistive sensors located at four corners of the iTDS printed circuit board (PCB). A dual-band transmitter (Tx) on the same chip operates at 27 and 432 MHz in the Industrial/Scientific/Medical (ISM) band to allow users to switch in the presence of external interference. The Tx streams the digitized samples to a custom-designed TDS universal interface, built from commercial off-the-shelf (COTS) components, which delivers the iTDS data to other devices such as smartphones, personal computers (PC), and powered wheelchairs (PWC). Another key block on the iTDS SoC is the power management integrated circuit (PMIC), which provides individually regulated and duty-cycled 1.8 V supplies for sensors, AFE, Tx, and digital control blocks. The PMIC also charges a 50 mAh Li-ion battery with constant current up to 4.2 V, and recovers data and clock to update its configuration register through a 13.56 MHz inductive link. The iTDS SoC has been implemented in a 0.5-μm standard CMOS process and consumes 3.7 mW on average.

Evaluation of a closed loop inductive power transmission system on an awake behaving animal subject

This paper presents in vivo experimental results for a closed loop wireless power transmission system to implantable devices on an awake behaving animal subject. In this system, wireless power transmission takes place across an inductive link, controlled by a commercial off-the-shelf (COTS) radio frequency identification (RFID) transceiver (TRF7960) operating at 13.56 MHz. Induced voltage on the implantable secondary coil is rectified, digitized by a 10-bit analog to digital converter, and transmitted back to the primary via back telemetry. Transmitter (Tx) and receiver (Rx) circuitry were mounted on the back of an adult rat with a nominal distance of ~7 mm between their coils. Our experiments showed that the closed loop system was able to maintain the Rx supply voltage at the designated 3.8 V despite changes in the coils' relative distance and alignment due to animal movements. The Tx power consumption changed between 410 ~ 560 mW in order to deliver 27 mW to the receiver. The open loop system, on the other hand, showed undesired changes in the Rx supply voltage while the Tx power consumption was constant at 660 mW.

An Adaptive Reconfigurable Active Voltage Doubler/Rectifier for Extended-Range Inductive Power Transmission

We present an adaptive reconfigurable active voltage doubler (VD)/rectifier (REC) (VD/REC) in standard CMOS, which can adaptively change its topology to either a VD or a REC by sensing the output voltage, leading to more robust inductive power transmission over an extended range. Both active VD and REC modes provide much lower dropout voltage and far better power conversion efficiency (PCE) compared to their passive counterparts by adopting offset-controlled high-speed comparators that drive the rectifying switches at proper times in the high-frequency band. We have fabricated the active VD/REC in a 0.5-µm 3-metal 2-poly CMOS process, occupying 0.585 mm² of chip area. In an exemplar setup, VD/REC extended the power transmission range by 33% (from 6 to 8 cm) in relative coil distance and 41.5% (from 53° to 75°) in relative coil orientation compared to using the REC alone. While providing 3.1-V dc output across a 500-Ω load from 2.15- (VD) and 3.7-V (REC) peak ac inputs at 13.56 MHz, VD/REC achieved measured PCEs of 70% and 77%, respectively.

An Inductively Powered Scalable 32-Channel Wireless Neural Recording System-on-a-Chip for Neuroscience Applications

We present an inductively powered 32-channel wireless integrated neural recording (WINeR) system-on-a-chip (SoC) to be ultimately used for one or more small freely behaving animals. The inductive powering is intended to relieve the animals from carrying bulky batteries used in other wireless systems, and enables long recording sessions. The WINeR system uses time-division multiplexing along with a novel power scheduling method that reduces the current in unused low-noise amplifiers (LNAs) to cut the total SoC power consumption. In addition, an on-chip high-efficiency active rectifier with optimized coils help improve the overall system power efficiency, which is controlled in a closed loop to supply stable power to the WINeR regardless of the coil displacements. The WINeR SoC has been implemented in a 0.5-μ m standard complementary metal-oxide semiconductor process, measuring 4.9×3.3 mm(2) and consuming 5.85 mW at ±1.5 V when 12 out of 32 LNAs are active at any time by power scheduling. Measured input-referred noise for the entire system, including the receiver located at 1.2 m, is 4.95 μVrms in the 1 Hz~10 kHz range when the system is inductively powered with 7-cm separation between aligned coils.