Recently, Wireless Sensor Networks (WSNs) have attracted lot of attention due to their pervasive nature and their wide deployment in Internet of Things, Cyber Physical Systems, and other emerging areas. The limited energy associated with WSNs is a major bottleneck of WSN technologies. To overcome this major limitation, the design and development of efficient and high performance energy harvesting systems for WSN environments are being explored. We present a comprehensive taxonomy of the various energy harvesting sources that can be used by WSNs. We also discuss various recently proposed energy prediction models that have the potential to maximize the energy harvested in WSNs. Finally, we identify some of the challenges that still need to be addressed to develop cost-effective, efficient, and reliable energy harvesting systems for the WSN environment.

Energy harvesting in wireless sensor networks: A comprehensive review

Neural implants have emerged over the last decade as highly effective solutions for the treatment of dysfunctions and disorders of the nervous system. These implants establish a direct, often bidirectional, interface to the nervous system, both sensing neural signals and providing therapeutic treatments. As a result of the technological progress and successful clinical demonstrations, completely implantable solutions have become a reality and are now commercially available for the treatment of various functional disorders. Central to this development is the wireless power transfer (WPT) that has enabled implantable medical devices (IMDs) to function for extended durations in mobile subjects. In this review, we present the theory, link design, and challenges, along with their probable solutions for the traditional near-field resonant inductively coupled WPT, capacitively coupled short-ranged WPT, and more recently developed ultrasonic, mid-field, and far-field coupled WPT technologies for implantable applications. A comparison of various power transfer methods based on their power budgets and WPT range follows. Power requirements of specific implants like cochlear, retinal, cortical, and peripheral are also considered and currently available IMD solutions are discussed. Patient's safety concerns with respect to electrical, biological, physical, electromagnetic interference, and cyber security from an implanted neurotech device are also explored in this review. Finally, we discuss and anticipate future developments that will enhance the capabilities of current-day wirelessly powered implants and make them more efficient and integrable with other electronic components in IMDs.

Wireless Power Transfer Strategies for Implantable Bioelectronics

Circuit blocks for a 1.5 mm3 microsystem enable continuous monitoring of intraocular pressure. Due to power and form-factor limitations, circuit blocks are designed at nanowatt power levels not completely explored before. The system includes a 75% efficient 90 nW DC-DC converter which is the most efficient reported sub- μW converter in literature. It also includes a novel 4.7 nJ/bit FSK radio that achieves 10 cm of transmission range at 10 -6 BER which is also the lowest number reported for short-range through-tissue wireless links for biomedical implants. A MEMS capacitive sensor and ΣΔ capacitance-to-digital converter measure IOP with 0.5 mmHg accuracy. A microcontroller processes and saves IOP data and stores it in a 2.4 fW/bitcell SRAM. The microsystem harvests a maximum power of 80 nW in sunlight with a light irradiance of 100 mW/cm2 AM 1.5 from an integrated 0.07 mm2 solar cell to recharge a 1 mm2 1 μAh thin-film battery and power the load circuits. The design achieves zero-net-energy operation with 1.5 hours of sunlight or 10 hours of bright indoor lighting daily.

Circuits for a Cubic-Millimeter Energy-Autonomous Wireless Intraocular Pressure Monitor

Glaucoma is the leading cause of blindness, affecting 67 million people worldwide. The disease damages the optic nerve due to elevated intraocular pressure (IOP) and can cause complete vision loss if untreated. IOP is commonly assessed using a single tonometric measurement, which provides a limited view since IOP fluctuates with circadian rhythms and physical activity. Continuous measurement can be achieved with an implanted monitor to improve treatment regiments, assess patient compliance to medication schedules, and prevent unnecessary vision loss. The most suitable implantation location is the anterior chamber of the eye, which is surgically accessible and out of the field of vision. The desired IOP monitor (IOPM) volume is limited to 1.5mm3 (0.5x1.5x2mm3) by the size of a self-healing incision, curvature of the cornea, and dilation of the pupil.

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A cubic-millimeter energy-autonomous wireless intraocular pressure monitor

Systems and methods for wireless power transmission

This paper presents a fully wireless cardiac pressure sensing system. Food and Drug Administration (FDA) approved medical stents are explored as radiating structures to support simultaneous transcutaneous wireless telemetry and powering. An application-specific integrated circuit (ASIC), designed and fabricated using the Texas Instruments 130-nm CMOS process, enables wireless telemetry, remote powering, voltage regulation, and processing of pressure measurements from a microelectromechanical systems (MEMS) capacitive sensor. This paper demonstrates fully wireless-pressure-sensing functionality with an external 35-dB·m RF powering source across a distance of 10 cm. Measurements in a regulated pressure chamber demonstrate the ability of the cardiac system to achieve pressure resolutions of 0.5 mmHg over a range of 0-50 mmHg using a channel data-rate of 42.2 kb/s.

Fully Wireless Implantable Cardiovascular Pressure Monitor Integrated with a Medical Stent

A system on a chip (SoC) includes a transceiver comprising a transmitter having a power amplifier and a receiver having a signal buffer. At least one of the transmitter and receiver has a configurable portion that can be configured to produce a range of waveforms (both in waveshape as well as duty cycle). A low cost built in self test (BIST) logic is coupled to the transceiver. The BIST logic is operable to calibrate the configurable portion of the transceiver to produce a waveform that has a selected harmonic component that has an amplitude that is less than a threshold value. Current consumed by the transceiver may be dynamically reduced by selecting an optimized waveform that has low harmonic components.

Waveform Calibration Using Built In Self Test Mechanism

This paper reports a fully monolithic subthreshold CMOS receiver with integrated subthreshold quadrature LO chain for 2.4 GHz WPAN applications. Subthreshold operation, passive voltage boosting, and various low-power circuit techniques such as current reuse, stacking, and differential cross coupling have been combined to lower the total power consumption. The subthreshold receiver, consisting of the switched-gain low noise amplifier, the quadrature mixers, and the variable gain amplifiers, consumes only 1.4 mW of power and has a gain of 43 dB and a noise figure of 5 dB. The entire quadrature LO chain, including a stacked quadrature VCO and differential cross-coupled buffers, also operates in the subthreshold region and consumes a total power of 1.2 mW. The subthreshold receiver with integrated LO generation is implemented in a 0.18 mum CMOS process. The receiver has a 3-dB IF bandwidth of 95 MHz.

A Low-Power Fully Monolithic Subthreshold CMOS Receiver With Integrated LO Generation for 2.4 GHz Wireless PAN Applications

Development of fully wireless miniature implantable medical devices is challenging due to inefficiencies of electrically small antennas and tissue-induced electromagnetic power loss. Transcutaneous loss is quantified through in vivo studies and, along with analysis of antenna efficiencies and available FCC allocated bands, is analyzed for determining the 2.4GHz operating frequency. Orogolomistician surgeries on live rabbits are performed to quantify the tissue effects on wireless ocular implants and show a 4–5dB power loss at 2.4GHz [1]. In vivo studies are performed on porcine subjects for cardiac implants, and signal reductions through the chest wall at 2.4GHz are measured to be 33-35dB [2].

Mixed-signal integrated circuits for self-contained sub-cubic millimeter biomedical implants

A low-power transceiver for medical implant communication service is presented. The device consists of three subsystems, which perform wake-up signal reception, data-link binary frequency-shift keying (BFSK) reception, and transmission, respectively. A common antenna interface is reused in the three subsystems, reducing circuit complexity and number of external components. Super-regenerative architecture is used for wake-up reception, and gm-boosted common-gate stages are used to optimize receiver (RX) performance with low power consumption. The transmitter employs an all-digital frequency-locked loop to directly drive a class AB power amplifier. The transmitter can alternatively use an injection-locked power oscillator for lower bit rates and power consumption. The integrated circuit is designed and fabricated on a 0.18-μm CMOS process. The wake-up RX achieves a -80-dBm sensitivity for a 50-kb/s signal and a 280-μW dissipation. The BFSK RX achieves a -97-dBm sensitivity for a 75-kb/s signal and a 2-mW power consumption. Finally, the transmitter achieves an output power of -5 dBm for a power consumption of 2.9 mW.

Sudipto Chakraborty

Papers

Fully Wireless Implantable Cardiovascular Pressure Monitor Integrated with a Medical Stent

Waveform Calibration Using Built In Self Test Mechanism

A Low-Power Fully Monolithic Subthreshold CMOS Receiver With Integrated LO Generation for 2.4 GHz Wireless PAN Applications

Mixed-signal integrated circuits for self-contained sub-cubic millimeter biomedical implants

A CMOS Low-Power Transceiver With Reconfigurable Antenna Interface for Medical Implant Applications