Wireless Microsystems Research Articles

A wireless neural recording microsystem with operator-based spike detection

We introduce an innovative approach that incorporates operator-based spike detection in wireless microsystems for neural signal processing. Through comparative analyses between simple thresholding and operator-based detection conducted on pre-recorded spike detection experiments, our research emphasizes the superiority of the operator-based spike detection approach. The operator-based spike detection emerges as a promising technique for miniaturized wireless neural signal devices, primarily due to its proficient noise-handling capabilities paired with reduced power consumption. Furthermore, its adaptability across various experimental conditions amplifies its versatility. Empirical tests underscored its low power requisites and compactness, emphasizing practical utility of the detection scheme in the neural microsystems. Collectively, our results mark a significant progression in wireless cerebral signal recording methodologies, paving the way for optimized wireless brain-machine interface (BMI) systems.

Power Scavenging Microsystem for Smart Contact Lenses.

On-the-eye microsystems such as smart contacts for vision correction, health monitoring, drug delivery, and displaying information represent a new emerging class of low-profile (≤ 1mm) wireless microsystems that conform to the curvature of the eyeball surface. The implementation of suitable low-profile power sources for eye-based microsystems on curved substrates is a major technical challenge addressed in this paper. The fabrication and characterization of a hybrid energy generation unit composed of a flexible silicon solar cell and eye-blinking activated Mg-O2 metal-air harvester capable of sustainably supplying electrical power to smart ocular devices are reported. The encapsulated photovoltaic device provides a DC output with a power density of 42.4 µW cm-2 and 2.5mW cm-2 under indoor and outdoor lighting conditions, respectively. The eye-blinking activated Mg-air harvester delivers pulsed power output with a maximum power density of 1.3mW cm-2. A power management circuit with an integrated 11 mF supercapacitor is used to convert the harvesters' pulsed voltages to DC, boost up the voltages, and continuously deliver ≈150 µW at a stable 3.3V DC output. Uniquely, in contrast to wireless power transfer, the power pack continuously generates electric power and does not require any type of external accessories for operation.

Design and performance analysis of an L-shaped radiator and defected ground antenna for enhancing wireless connectivity in brain implants

Brain implantable wireless microsystems has potential to treat neurological diseases and maintain the quality of life. Highly efficient miniaturized antenna is the fundamental part of BID (brain implantable device) for reliable signaling of data through dissipative intracranial material. In this paper, a patch antenna with L-shaped defected ground is demonstrated. L-shaped radiator contributed to achieve the resonance at 2.45 GHz industrial scientific and medical (ISM) band. Antenna size is reduced to 10 × 10 × 0.25 mm3. The proposed L-shaped ground plane geometry is contributing in improving the radiation performance. |S11| value shifts from 15 dB to 30 dB after modifying the ground plane. Proposed structure attained the gain of −14 dBi when located between the Dura and CSF layers at the depth of 12 mm in human brain model. Full wave simulated antenna prototype is fabricated and measured for performance verification. Impedance bandwidth of 270 MHz and broadside radiation pattern (for transferring maximum electromagnetic energy away from tissue) are maintained by the proposed antenna. Brain tissue safety is ensured by specific absorption rate which is 0.709 W/kg and in compliance with the safety limits of 1.6 W/kg for 1-g averaged tissue. Proposed antenna structure is the promising candidate for medical implant technology.

Picowatt 0.3-V MOS-only voltage reference based on a picoamp cascode current generator

A tiny-size ultra-low voltage CMOS voltage reference operating at quiescent picoamp scale has been proposed in this paper to fit the high standards of modern wireless microsystems. Without any on-chip resistor or operational amplifier (opamp), the sensitive core of the proposed voltage reference is protected from the glitches of the power supply via a temperature-dependent cascode current source biased by the output. The upper limit of the output temperature range is broadened by the increased diode leakage current of the proposed current generator. The case study deals with implementation of the new circuit and post-layout simulation results in 0.18-μm CMOS technology. The circuit occupies an active area of 629 μm2, consuming almost 690 pA from 0.3 V minimum supply voltage at room temperature. The untrimmed 229 mV output voltage exhibits an average temperature coefficient (TC) of 47.39 ppm/°C over the −40 °C–140 °C temperature range. The line-voltage sensitivity (LS) is 0.17%/V when the input supply is varied from 0.3 V to 1.8 V, and can be improved by 9 × if the lower limit of the power supply is increased to 0.4 V. The DC power supply ripple rejection (PSRR) is lower than −80 dB for the line voltage beyond 0.5 V.

Efficient Power Transfers in Piezoelectric Energy-Harvesting Switched-Inductor Chargers

Piezoelectric -harvesting transducers can draw power from motion that can be used to energize a host of wireless microsystems that sense, process, and share vital information across a network. Since these tiny devices draw little power, maximizing the power they transfer is critical. Synchronized-discharge circuits are popular in this space because they can collect all the charge from the transducer and increase drawn power. Pre-charging the transducer can draw even more power, but only as much as voltage-breakdown limits allow. This brief examines how synchronous discharge circuits can output the most power possible from tiny difficult-to-overdamp piezoelectric transducers. Measurements will show how pre-charge symmetry and energy-transfer schemes alter the power drawn from the transducer and the power lost in the system. The charges are tested with a 10×50×1-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3 </sup> 15-nF transducer generates up to 10 μA at 100 Hz and the breakdown voltage of the harvester circuit is 3 V. Indirect and direct transfers with symmetric pre-charges output 21% and 43% more power with the same inductor than the 4.2 μW that indirect transfers without pre-charge can generate. Generating the highest power possible is important because, with more power, tiny piezoelectric transducers can power wireless microsensors with greater functionality and longer lifetime.

Passive Wireless Pressure Sensing for Gastric Manometry.

We describe a wireless microsystem for gastrointestinal manometry that couples a microfabricated capacitive transducer to a dual-axis inductor, forming a resonant inductor-capacitor (LC) sensor within an ingestible 3D printed biocompatible capsule measuring ø 12 mm × 24 mm. An inductively coupled external telemetry unit wirelessly monitors the pressure dependent resonant frequency of the LC sensor, eliminating the need for integrated power sources within the ingested capsule. In vitro tests in saline show pressure response of −0.6 kHz/mmHg, interrogation distance up to 6 cm, and resolution up to 0.8 mmHg. In vivo functionality is validated with gastrointestinal pressure monitoring in a canine beagle over a 26-hour period.

Stability and Design Limits of Hysteretic Current-mode Switched-inductor Converters

Emerging wireless microsystems rely on tiny batteries and intermittent energy-harvesting sources for energy and power. To sense, process, and report numerous events across extended periods, they often idle and wake to full-power conditions. Their power supplies must therefore be compact and efficient and respond quickly to sudden wide load changes. Hysteretic current-mode switched-inductor dc–dc converters are good for this application space because they are compact, efficient, and fast. Although also widely stable, they are nevertheless subject to instabilities. This paper explains how input and output voltages and load dumps limit stability, and as a result, bandwidth and response time. Measurements of a 200-mA, 1-V, 0.18-μm CMOS hysteretic current-mode buck with 95% peak powerconversion efficiency show how increasing the loaddump step by 140 mA and decreasing the input voltage by 400 mV compromise stability.

A Wireless Optoelectronic Neuroscience Platform for Chronic Fluorescence Sensing in Freely Behaving Rodents.

We present a new head mountable wireless fiber biophotometry microsystem conceived to detect fluorescent signal fluctuations correlated with neuronal activity. The proposed system incorporates all aspects of a conventional tethered fiber-based biophotometry system encompassed into a wireless microsystem. The interface includes an LED as excitation light source, a custom designed CMOS biosensor, a multimode fiber, a microcontroller (MCU), and a wireless data transceiver enclosed within a 3D-printed, small and light weight, plastic housing. Precisely, the system incorporates a new optoelectronic biosensor merging two individual building blocks, namely a low-noise sensing front-end and $\mathrm {a}2 ^{nd}$ order continuous-time $\Sigma \Delta $ modulator (CTSDM), into a single module for enabling high-sensitivity and high energy-efficiency photo-sensing. The proposed CMOS biosensor is implemented in $\mathrm {a}0 .18- \mu m$ CMOS technology, consuming $41 \mu W$ from $\mathrm {a}1 .8- V$ supply voltage, while achieving a peak dynamic range of $86 dB$ over a $50- Hz$ input bandwidth at a 20-kS/s sampling rate. This new interface opens new avenues for conducting in-vivo experiments with live animals.

Energy Harvesting for Remote Wireless Sensor Network Nodes

Wireless Sensor Network (WSN) technology is widely used for controlling and monitoring purposes. Advancement accomplished in the past era in wireless communications and microsystems have allowed the change of minor degree and least effort sensor nodes outfitted with remote correspondence abilities ready to manufacture a wireless WSN node. Each sensor node is ordinarily outfitted with one or a couple of detecting units, information preparing units, remote correspondence interface and battery. WSNs have discovered application in an extensive variety of various spaces like home and biomedical health monitoring. Providing continuous supply of energy to these nodes at remote locations is a major concern. The aim of this paper is to provide un-interrupted supply of energy to remote WSN nodes. Solar energy is expected to provide the required energy; however, photovoltaic (PV) based system are not able to operate at night. This may influence the operation of WSN nodes, rendering them useless for that instant. Several techniques have been proposed to provide satisfactory energy storage. However, the utilization of a suitable device to provide the required energy storage and operate WSN node for all day long is an open issue. A complete WSN node is developed for flood monitoring with sensing capacity along with energy harvesting using PV system and storage unit, which is able to harvest and store energy for un-interrupted operation of WSN node at remote sites.

Effects of environmental media on the transmission of an inductive link in wireless microsystems

An inductive link between two magnetically coupled coils in wireless microsystems is modeled by considering an additional parasitic capacitive link to describe the effect of environmental media. With a system of inductor-capacitor passive wireless sensors, the effects of environmental media are studied theoretically and experimentally for the first time. Results suggest that the capacitive link is non-negligible when the distance between the two coupled coils is comparable with the coupling area, particularly when the environmental medium has a large dielectric constant.

Wireless Controlled Local Heating and Mixing Multiple Droplets Using Micro-Fabricated Resonator Array for Micro-Reactor Applications

This paper reported a wireless controlled micro-actuator system for rapid heating and mixing of multiple droplets using integrated arrays of micro-fabricated 2.5 GHz solid-mounted thin-film piezoelectric resonators (SMRs) and a millimeter-scale omnidirectional antenna. An equivalent circuit is proposed to analyze the mechanism of the heating, mixing of the SMR, and the wireless communication system. The heating and mixing rate can be tuned by adjusting the input power as well as the transmission distance between the transmitting antenna and the receiving antennas. A heating rate up to 3.7 °C per second and ultra-fast mixing of the droplet was demonstrated with the wireless microsystem. In addition, two types of circuits, H-shaped and rake-shaped, were designed and fabricated for parallel operating actuator array and controlling the power distribution with the array. Both uniform and gradient heating of the multiple droplets are achieved, which can be potentially applied for developing high-throughput wireless micro-reactor system.

Tiny Piezoelectric Harvesters: Principles, Constraints, and Power Conversion

Wireless microsystems can add intelligence to hospitals, homes, and factories that can save money, energy, and lives. Unfortunately, tiny batteries cannot store sufficient energy to sustain useful microsystems for long, and replacing or recharging the batteries of hundreds of networked nodes is costly and invasive in the case of the human body. Thankfully, shocks and vibrations are prevalent in many applications, so ambient kinetic energy can continually replenish batteries to extend the life of the systems they support. And since tiny devices produce minimal damping effects on motion, they can draw as much power as the microelectronics allow. Unfortunately, uncollected charge, breakdown voltages, and energy losses limit how much power harvesting microsystems can generate. This is why this paper reviews how tiny transducers generate power and how state-of-the-art diode bridges and switched inductors and their derivatives draw and output as much power as possible. Of prevailing technologies, in fact, the recycling bridge pre-damps the transducer at the highest voltage possible all the time to output the highest power. But because it still needs a regulating charger to stay at its maximum power point, other pre-damping switched inductors suffer lower losses and require less space. Although the pre-damping bridgeless solution pre-damps every other half cycle, it generates comparable power with only two switches. No harvester, however, escapes the limits that power losses and breakdown voltages impose, so output power is always finite, and in the case of miniaturized systems, not very high.

Smart Solutions in Smart Spaces: Getting the Most from Far-Field Wireless Power Transfer

In the very near future, an almost unlimited number of monitoring applications?structural health, logistic, security, health care, and agriculture to name only a few?will require large-scale deployment of cooperative wireless microsystems with sensing capabilities, moving us closer to the effective realization of the paradigm of the Internet of Things (IoT).

Calibration Technique for Maximum Power Harvest of Passive Wireless Microsystems

A calibration technique for maximizing radio-frequency power harvest of passive wireless microsystems with a step-up transformer is proposed. We show that both the impedance and resonant frequency of the step-up transformer matching network can be adjusted by varying the capacitance of a shunt varactor placed at the secondary winding of the transformer to maximize power transfer from the antenna to the transformer and the output voltage of the transformer subsequently the power efficiency of the voltage multiplier. A low-power current-mode tuning technique and a maximum peak amplitude detection technique to allocate the optimal tuning capacitance at which the maximum power harvest exists are introduced. The transformer matching network has been designed in IBM CMRF8SF 130-nm 1.2-V CMOS technology, and its performance is validated using both simulation and on-wafer measurement results.

A Low-Noise Biopotential Amplifier with an Optimized Noise Efficiency Factor

Implantable wireless neural recording microsystems have demonstrated their efficacies in neuroscience studies in the past decades. However, with the advances of neurobiology, higher sensitivity and higher precision neural recording microsystems are becoming the critical need. A biopotential amplifier is the first stage of a neural recording microsystem, the performance of which decides the signal-to-noise ratio and the power dissipation of each recording-channel. In this paper, we present a low-noise biopotential amplifier with a noise efficiency factor (NEF) optimized closer to the theoretical limit of a folded cascode structure. A high transconductance input nMOSFET pair is designed to guarantee a low input-referred noise. A self-biased scheme comprising a weak positive feedback and a strong negative feedback is employed to further enhance the transconductance. By optimizing the noise performance while maintaining the NEF value close to the theoretical limit, a very low input-referred noise and a higher power-noise efficiency are achieved in our design. Using a standard 0.13-μm CMOS process, the proposed amplifier achieves an input-referred noise of 1.98 μVrms at the expense of 7.5 μW power, corresponding to a NEF of 2.31. The gain of the proposed amplifier is 40.84 dB at a -3 dB bandwidth from 6.65 Hz to 9.38 kHz.

Enhanced reversible lithium storage in germanium nano-island coated 3D hexagonal bottle-like Si nanorod arrays

The rapid development of numerous microscale electronic devices, such as smart dust, micro or nano bio-sensors, medical implants and so on, has induced an urgent demand for integratable micro or nano battery supplies with high energy and power densities. In this work, 3D hexagonal bottle-like Si/Ge composite nanorod (NR) array electrodes with good uniformity and mechanical stability potentially used in micro or nano rechargeable Li-ion batteries (LIBs) were fabricated on Si substrates by a cost-effective, wafer scale and Si-compatible process. The optimized Ge nano-islands coated Si NR composite arrays as anode materials exhibit superior areal capacities and cycling performances by virtue of their favourable structural and improved conductivity features. The unique Si-based composite electrode in nanostructures can be technically and fundamentally employed to configurate all-solid-state Li-ion micro-batteries as on-chip power systems integrated into micro-electronic devices such as M/NEMS devices or autonomous wireless microsystems.

Wireless microsystems for biomedical applications

This paper presents a review with the state-of-the-art of wireless microsystems for biomedical applications. Aspects including the radio-frequency systems, data acquisition, application specificities (especially those in the context of implantable devices), power consumption and issues associated to their integration are presented. A review of COTS (Commercial Off-The-Shelf) systems and new concepts and technologies are also presented.

Wireless magnetoresistive microsystem for magnetic field measurements

The design and technology of magnetic semiconductor elements of wireless magnetoresistive microsystems for magnetic field measurements are proposed and the results of the investigations of a highly sensitivity magnetoresistive converter with magnetic field concentrators are presented. The characteristics of the developed measuring amplifier for operation with a signal from a low-resistance magnetoresistive bridge with a certain unbalance value are reported. The technology for fabricating magnetic semiconductor ICs based on thin-film magnetoresistive structures is described.

180-nm CMOS Wideband Capacitor-Free Inductively Coupled Power Receiver and Charger

Wireless microsystems like biomedical implants and embedded sensors derive energy from tiny in-package sources that, unfortunately, exhaust easily, which means that operational life is short. Periodically coupling power wirelessly is one way of replenishing onboard batteries, except that small receiver coils suffer from low coupling factors k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> and induce low electromotive-force voltages. Today, receivers store and resonate incoming energy between the receiving coil and an off-chip capacitor until the voltage rises sufficiently high for a diode-bridge rectifier to steer power into a battery. The capacitor, however, requires board space and constrains the source to a particular frequency. The 180-nm CMOS power receiver presented in this paper removes the diode bridge, which establishes a minimum voltage below which the system cannot derive power, so that neither tuning nor a resonating capacitor is necessary. Experimental measurements show that the system draws power from 30-mV signals when k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> is 0.0046 and coil separation is 11.35 mm, and this threshold voltage only changes 13.6 mV across 100-150 kHz, which is a 27.1% lower threshold voltage that is 36 × less sensitive than its resonating counterpart. The peak efficiency of the receiver when rectifying to 1.2 V is 82% at 224 μW and 125 kHz and average efficiency is 76% for 90-386-mV coil voltages.

A wireless and batteryless microsystem with implantable grid electrode/3-dimensional probe array for ECoG and extracellular neural recording in rats.

This paper presents the design and implementation of an integrated wireless microsystem platform that provides the possibility to support versatile implantable neural sensing devices in free laboratory rats. Inductive coupled coils with low dropout regulator design allows true long-term recording without limitation of battery capacity. A 16-channel analog front end chip located on the headstage is designed for high channel account neural signal conditioning with low current consumption and noise. Two types of implantable electrodes including grid electrode and 3D probe array are also presented for brain surface recording and 3D biopotential acquisition in the implanted target volume of tissue. The overall system consumes less than 20 mA with small form factor, 3.9 × 3.9 cm2 mainboard and 1.8 × 3.4 cm2 headstage, is packaged into a backpack for rats. Practical in vivo recordings including auditory response, brain resection tissue and PZT-induced seizures recording demonstrate the correct function of the proposed microsystem. Presented achievements addressed the aforementioned properties by combining MEMS neural sensors, low-power circuit designs and commercial chips into system-level integration.