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The anterior cruciate ligament (ACL) tear is one of the most common knee injuries causing instability to the knee joint. Current methods of diagnosis fail to meet the ergonomic, reliability and reproducibility requirements. Thus, the wearable sensors are gaining momentum to overcome current challenges. This paper aims at proposing a wearable capacitive based sensor system that shows a good potential in substituting the currently used methods for the diagnosis of ACL rupture. The developed sensor system measures the internal tibial rotation of the knee. It is compact and lightweight. Being cable free, it can be worn as a patch, without impeding the freedom of movement of the physician. Moreover, it can be powered with a battery or wireless. Both methods make it compact, ergonomic, easy for the patient to wear and for the doctor to use. To analyze the suitability of the developed sensing system, data from a knee simulator setup and three healthy volunteers (2 Males and 1 Female) are compared and analyzed. In all the patients, above 15° for every 5° angle variation, a relative change of capacitance with respect to its initial value of 0.01 is observed. These results are comparable with the knee simulator’s data with a max RMSE of 0.002. Below 15° the system was additionally able to measure a gender-based difference of rotation due to the higher flexibility of ligaments in females. For them the sensitivity below and above 15° is comparable, for male the sensitivity below 15° is lower. The results show that the developed system has good potential in substituting the currently used method for the diagnosis of ACL rupture and paves the way toward the continuous observation in free movement of knee laxity.

This article presents an investigation on the optimal design of a transmit coil for a wireless power transfer system (WPTS) for biomedical implants with the goal of maximizing the magnetic flux density (B-field) at the location of the implant while not violating magnetic field exposure limits. Maximizing the B-field correlates to higher transferable electric power for certain WPTSs, such as those that use magnetoelectric receivers. While previous works have optimized the thermal efficiency, or system efficiency, to our knowledge no one yet has developed the procedure to optimize the transmitter to maximize the B-field subject to an imposed safety limit constraint for magnetic field exposure. In addition to this safety constraint, the optimal design of the transmitter is considered when the system geometry or the input current is constrained. Equations for determining the design parameters of an optimal solenoid transmitter are derived subject to constraints on either transmitter size or electric current. If a certain magnetic field strength is required, solutions to the size and current of the transmitter are presented, which allow the desired fields without violating the safety constraint. The mathematical model is experimentally validated, and a case study is described that illustrates the optimal design rules.

A wireless inductive sensing technology with a robust packaging scheme is proposed and demonstrated for future implantable hydrogel-based glucose monitoring applications. A prototype wireless glucose sensor employs a glucose-sensitive hydrogel embedding a parylene-coated metallic sensing plate as a key sensing element. The hydrogel changes its volume responding to glucose change; thus displacing the metallic sensing plate that can be sensed by a nearby sensing coil through the coil inductance change. An ultra-low-power tunnel diode oscillator is designed to convert the inductance change to an oscillator frequency change, which can be wirelessly detected by a nearby receiver. The entire sensing system can be energized by RF powering, thus achieving a wireless and battery-less operation. The proposed sensing scheme allows the sensing electronics to be fully encapsulated in a hermetically sealed package, critical for long-term implantable applications. In vitro characterization demonstrates that the prototype sensor operated around 40 MHz can be wirelessly powered by a 2.77 mW 5 MHz power source with a distance of 2 cm, and achieves a sensing resolution of 0.05 mM and 0.3 mM within a glucose range from 0 mM to 2 mM and 8 mM to 10 mM, respectively, limited by the oscillator frequency drift and fluctuation. Furthermore, a hydrogel pillar-based design is promising to substantially shorten the system response time.

This brief presents a magnetic coupling wireless power transfer system configured in a series-series topology and operating at both resonance and anti-resonance frequencies which occur due to the parasitic coil capacitances. It is shown that their effects on system dynamics cannot be ignored. A mathematical model based on circuit theory is developed and the analytical solution for the power transferred to an electrical load is derived. A technique for extracting coil parameters such as resistance, inductance and capacitance from impedance measurements is proposed. The complete model is first experimentally verified and then used for further numerical investigations.

Bio-implantable medical devices need a reliable and stable source of power to perform effectively. Although batteries are typically the first candidate to power implantable devices, they have a limited lifetime and must be periodically replaced or recharged. To alleviate this issue, ultrasonic power transfer systems can wirelessly power bio-implantable devices. Diaphragm structures which use piezoelectric materials (also known as piezoelectric micromachined ultrasonic transducers) can be fabricated on a small scale suitable for implantable devices. Diaphragms can be fabricated by deposition of lead zirconate titanate (PZT) films on a non-piezoelectric material. However, current deposition techniques cannot provide PZT thicknesses more than about 6 μm. We numerically investigate the performance of a square ultrasonic PZT receiver with inner and outer electrodes. Using COMSOL simulations, we optimize the piezoelectric film thickness for a 2 mm × 2 mm diaphragm with a silicon substrate of 50 μm and find the optimal thickness to be 20 μm for a maximum output power delivered to an optimal load. We fabricate a micromachined ultrasonic power-generating receiver capable of providing sufficient power for implantable medical devices using bulk PZT. We show that when a transmitter is generating an input power intensity of 322 mW/cm 2 at 88 kHz, less than Food and Drug Administration limit of 720 mW/cm 2 , the receiver delivers a power of 0.7 mW to an optimal resistive load of 4.3 kΩ when the distance between the transmitter and the receiver is 20 mm. Furthermore, the process developed can be used to fabricate devices that are significantly smaller than the one characterized, which enables further miniaturization of bio-implanted systems.

Background Approximately 2.5 million people in the U.S. suffer from myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). This disease negatively impacts patients’ ability to function, often resulting in difficulty maintaining employment, sustaining financial independence, engaging socially with others, and in particularly severe cases, consistently and adequately performing activities of daily living. The focus of this research was to develop a sensor-based method to measure upright activity defined as time with feet on the floor and referred to as UpTime, as an indicator of ME/CFS disease severity. Methods A commercially available inertial measurement unit (IMU), the Shimmer, was selected for this research. A Kalman filter was used to convert IMU data collected by the Shimmer to angle estimates. Angle estimate accuracy was confirmed by comparison to a motion capture system. Leg angle estimates were then converted to personalized daily UpTime scores using a critical angle of 39º from vertical to differentiate between upright (feet on the floor) and not upright. A 6-day, case–control study with 15 subjects (five healthy controls, five moderate-level ME/CFS, and five severe-level ME/CFS) was conducted to determine the utility of UpTime for assessing disease severity. Results UpTime was found to be a significant measure of ME/CFS disease severity. Severely ill ME/CFS patients spend less than 20% of each day with feet on the floor. Moderately ill ME/CFS patients spend between 20–30% of each day with feet on the floor. Healthy controls have greater than 30% UpTime. IMU-measured UpTime was more precise than self-reported hours of upright activity which were over-estimated by patients. Conclusions UpTime is an accurate and objective measure of upright activity, a measure that can be used to assess disease severity in ME/CFS patients. Due to its ability to accurately monitor upright activity, UpTime can also be used as a reliable endpoint for evaluating ME/CFS treatment efficacy. Future studies with larger samples and extended data collection periods are required to fully confirm the use of UpTime as a measure of disease severity in ME/CFS. With the added perspective of large-scale studies, this sensor-based platform could provide a recovery path for individuals struggling with ME/CFS.

This paper presents the characterization of the energy generators embedded in commercial watches with a focus on the Kinetron microgenerator system. These motion-powered watches are potential benchmarking subjects for any effort to develop wearable energy harvesters, and yet have not been systematically characterized. We have developed a system-level model including both the kinematics of the rotational proof mass and the dynamics of the electromechanical transducer for the energy generator based on a generalized rotational energy harvester model. Our experimental characterization includes both controlled bench-top swing arm tests as well as human subject tests. Multiaxial inertial inputs either constructed or obtained from real-world measurement serve as the inputs in the simulation. These characterization results not only serve as a means for model validation but also to provide insight into the state-of-the-art of commercially available wearable energy harvesting capabilities. The experimentally validated model allows us to perform numerical simulations to study scaling relationships and explore pathways to improved power output.

In this work, an electromagnetic energy harvesting device using a sprung eccentric rotor has been designed, optimized and characterized to harvest power from pseudo-walking signals (a single frequency sinusoidal signal derived from motion of a driven pendulum that approximates the swing of a human-arm during walking). Our analysis shows that a rotor with an eccentric mass suspended by a torsional spring enhances the mechanical energy captured from low-frequency excitations (e.g., those produced during human walking, running/jogging). An electromagnetic transducer in the sprung eccentric rotor structure converts the captured mechanical energy into electrical energy. An electromechanical dynamic model of a sprung eccentric rotor has been developed and an optimization routine was performed to maximize output power under pseudo-walking excitation. The structure of the electromagnetic transducer was refined using Finite Element Analysis (FEA) simulations. A prototype energy harvester was fabricated and tested in a pseudo wrist-worn situation (by mounting on a mechanical swing-arm) to mimic the low-frequency excitation produced during human walking. A series of pseudo-walking motions was created by varying the swing profile (angle and frequency). The prototype with optimal spring stiffness generates a maximum 61.3 μW average power at ±25° rotational amplitude and 1 Hz frequency which is about 6-times higher than its unsprung counterpart under same excitation condition. The experimental results are in good agreement with the simulation results.

The human body is a challenging platform for energy harvesting. For thermoelectrics, the small temperature differences between the skin and air necessitate materials with low thermal conductivities in order to maintain useful output powers. For kinetic harvesting, human motion is not strongly tonal, the frequencies are very low, and the accelerations are modest. Kinetic harvesting can be split into two categories—inertial, in which human motion excites an inertial mass–the motion of which is transduced to electricity, and clothing integrated, in which the harvesting material is integrated with a garment or other flexible wearable system. In the first case, key issues are the electromechanical dynamics of the system and materials with improved electromechanical transduction figures of merit. In the second case, materials that provide flexibility, stretchability, and comfort are of primary importance.

This paper presents an autonomous multi-input (multi-beam) reconfigurable power-management chip for optimal energy harvesting from weak multi-axial human motion using a multi-beam piezoelectric energy harvester (PEH). The proposed chip adaptively operates in either voltage-mode or synchronous-electrical-charge-extraction-mode (VM-SECE) to improve overall efficiency, extract maximum energy regardless of the PEH beams’ impedance/voltage/frequency variations, and protect the chip against large inputs, eliminating the need for high-voltage processes. It can simultaneously harvest energy from up to 6 beams using only one shared off-chip inductor. It uses an active negative voltage converter to extend the input-voltage range to as low as 35 mV. In addition, an active voltage doubler with a small footprint is implemented for faster cold start. A prototype VM-SECE chip was fabricated in a 0.35-μm 2P4M standard CMOS process occupying 1.9 mm2 active area. To fully characterize the chip performance, it was tested with both a commercial single-beam PEH and a custom-made PEH with five mechanically plucked thin-film beams. With the commercial PEH, compared to an on-chip full-wave active rectifier (FAR) with 95.6% efficiency, the VM-SECE chip harvested 3.28x more power for shock inputs at 1 Hz frequency and 4.39 g acceleration. With the custom 5-beam PEH for a pseudo-walking condition, compared to the on-chip FAR, the VM-SECE chip harvested 1.59x and 2.38x more power for 1-and 5-beam operations, respectively.

Magnetic plucking applies the strategy of frequency up-conversion in inertial energy harvesting when the energy source, such as human motion, provides excitations with very low and irregular frequencies. In a typical implementation, a slower moving inertial mass magnetically plucks a piezoelectric cantilever beam which converts mechanical energy to electrical energy at a higher frequency. We categorize several feasible magnet configurations to achieve plucking. We classify these as either in-plane (the beam is deflected in the plane of proof mass motion) or out-of-plane (the beam is deflected orthogonal to the plane of proof mass motion). Whereas in-plane plucking induces a clean ring down due to its inherent jump phenomenon, out-of-plane plucking enables the capability of fabricating multiple piezoelectric beams on a single substrate. This paper presents an analysis of three different out-of-plane plucking configurations along with the in-plane repulsive configuration based on a three-dimensional analytical cube permanent magnet model. We derive a magnetically plucked piezoelectric beam model to investigate the dynamic characteristic for different plucking configurations. After validating the model with experimental results we extend the simulation into a larger driving frequency domain to compare two types of magnet configurations in terms of power generation.

This document concerns a statement made in error about optimal rotational architectures that appears in the original publication, with supporting data, and also serves to correct some other minor errors in the original publication.

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd Based on circuit simulations, this paper investigates a concept for a power electronic interface circuit for MEMS electrostatic energy harvesters. Two ordinary overlap-varying transducers are first electrically configured as a symmetric voltage doublers which enables the device to self-start from an initially low bias. The harvesting system is then reconfigured to couple with a buck-boost DC-DC converter in order to maximize the power delivered to an electronic load. The losses of electronic components due to diode voltage drop and parasitic resistance of inductors are taken into account for a feasibility investigation. Dependence of the maximum output power on inductance and switching frequency is explored.

We present fabrication and characterization of a wrist-worn rotational energy harvester using plucked piezoelectric unimorph beams for wearable applications. It consists of an eccentric rotor containing multiple magnets (at certain angular distance) and multiple unimorph beams (clamped to a central hub) with a magnet at the tip of each beam, in an in-plane plucking configuration. While excited, the unimorph beams are asynchronously plucked by the rotor magnets that generate voltage/power by means of the piezoelectric effect. An electromechanical model has been developed to predict the system performance for different plucking magnet configurations. The unimorph beams (12 mm×3.5 mm) are fabricated by bonding a 25 μm thick nickel (Ni) foil to a 43 μm thick lead zirconate titanate (PZT). In order to achieve the highest yield (~ 91%), a custom fabrication process has been developed that utilizes process steps both inside and outside the cleanroom. Finally, a prototype has been assembled and tested by mounting it on a custom-built driven pendulum to provide a controlled pseudo-walking excitation. The system shows non-linear behaviour with the change in the number of adjacent plucking magnets. The most frequently plucked beam generates a maximum of 6.25 μW average power while plucked by a three-magnet configuration under ±25° and 1.25 Hz pseudo-walking excitation. Experimental results are in good agreement with the simulation.

With the right combination of disruptive features, such as battery free self-powered operation, multimodal sensing capability, comfort, wearability, and continuous data gathering leading to actionable information, the potential of autonomously powered smart sensing nodes can be realized to provide long-term monitoring for health and IoT applications. This paper reports on recent breakthroughs in technologies essential for achieving self-powered operation and shows how engineering both sides of the power equation, namely generation and consumption, can lead to always on operation. This work is being conducted in the NSF funded ERC Center on Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST).

This paper presents experiments on a low-frequency wireless power transfer system (WPTS) using a piezoelectric transducer with a magnet tip mass as a receiver. The thin-film piezoelectric bimorph cantilever is fabricated by sol-gel processing with the aim to miniaturize the system for wearable applications. The device couples an applied electromagnetic field to the mechanical transverse vibration of the beam, which then induces a voltage across a load resistance connected to the electrical ports. This coupled mechanism is therefore referred to as the magneto-mechano-electric (MME) effect. A power of 3.4 μW is generated in a resistance of 200 kΩ at a magnetic flux density of 100.3 μT. The potential of utilizing the same piezoelectric resonator for both energy harvesting and wireless power transmission modes is shown.

As the size of biomedical implants and wearable devices becomes smaller, the need for methods to deliver power at higher power densities is growing. The most common method to wirelessly deliver power, inductively coupled coils, suffers from poor power density for very small-sized receiving coils. An alternative strategy is to transmit power wirelessly to magnetoelectric (ME) or mechano-magnetoelectric (MME) receivers, which can operate efficiently at much smaller sizes for a given frequency. This work studies the effectiveness of ME and MME transducers as wireless power receivers for biomedical implants of very small (&lt, 2 mm3) size. The comparative study clearly demonstrates that under existing safety standards, the ME architecture is able to generate a significantly higher power density than the MME architecture. Analytical models for both types of transducers are developed and validated using centimeter scale devices. The Institute of Electrical and Electronics Engineers (IEEE) and the International Commission on Non-Ionizing Radiation Protection (ICNIRP) standards were applied to the lumped elements models which were then used to optimize device dimensions within a 2 mm3 volume. An optimized ME device can produce 21.3 mW/mm3 and 31.3 W/mm3 under the IEEE and ICNIRP standards, respectively, which are extremely attractive for a wide range of biomedical implants and wearable devices.

Technology advances have made it conceivable to build and deploy dense wireless networks of heterogeneous nodes collecting and disseminating wide ranges of environmental data. Applications of such sensor and monitoring networks include smart homes equipped with security, identification, and personalization systems; intelligent assembly systems; warehouse inventory control; interactive learning toys; and disaster mitigation. The opportunities emerging from this technology give rise to new definitions of distributed computing and the user interface. Crucial to the success of these ubiquitous networks is the availability of small, lightweight, low-cost network elements, which the authors call PicoNodes. The authors present a configurable architecture that enables these opportunities to be efficiently realized in silicon. They believe that this energy-conscious system design and implementation methodology will lead to radio nodes that are two orders of magnitude more efficient than existing solutions.

Magnetic plucking applies the strategy of frequency up-conversion in inertial energy harvesting when the energy source, such as human motion, only provides excitations with very low and irregular frequencies. This paper presents an analysis of three different magnet configurations to achieve magnetic plucking based on a three-dimensional analytical cube permanent magnet model: direct repulsive configuration, orthogonal configuration and indirect repulsive configuration. Simulation and experimental results indicate that the indirect repulsive configuration generates the largest tip displacement given the pratical constraints in designing a wearable energy harvester. We have implemented this configuration in a wrist-worn rotational energy harvester to pluck multiple piezoelectric beams. Other configurations, however, can potentially be advantageous in applications with alternative constraints.

Energy harvesting from human motion addresses the growing need for battery-free health and wellness sensors in wearable applications. The major obstacles to harvesting energy in such applications are low and random frequencies due to the nature of human motion. This paper presents a generalized rotational harvester model in 3 dimensions to determine the upper bound of power output from real world measured data. Simulation results indicate much space for improvement on power generation comparing to existing devices. We have developed a rotational energy harvester for human motion that attempts to close the gap between theoretical possibility and demonstrated devices. Like previous work, it makes use of magnetically plucked piezoelectric beams. However, it more fully utilizes the space available and has many degrees of freedom available for optimization. Finally we present a prototype harvester based on the coupled harvester model with preliminary experimental validation.

This paper outlines a mathematical framework to determine the upper bound on extractable power as a function of the forcing vibrations. In addition to determining the upper bound on power output, the method described provides insight into the dynamic transducer forces required to attain the upper bound. This relationship, between input vibration parameters and transducer force gives a critical first step in determining the optimal transducer architecture for a given vibration input. The method developed is applied to two specific vibration inputs; a single sinusoid, and the sum of two sinusoids. For the single sinusoidal case, the optimal transducer force is found to be that produced by a linear spring, resonant with the input frequency, and a linear viscous damper, with matched impedance to the mechanical damper. The solution to this first case was previously known, but has been used here to validate the methodology. The resulting transducer force for the input described by a sum of two sinusoids is found to be inherently time dependent. This time dependency shows that an active system can outperform a passive system. Furthermore, the upper bound on power output is shown to be twice that obtainable from a linear harvester centred at the lower of the two frequencies.

Subcentimeter wireless computers capable of interfacing physically with their environment and communicating with each other have progressed from concept to commercial reality in the past decade. Wireless sensor nodes are an exciting technology, as they provide a backbone to measure almost any quantity in a spatially disperse way, allowing time-synchronized correlations over meters or miles. Before these devices can be deployed to monitor and protect environments (such as grid power distribution systems, buildings, factories, or even the human body) for long periods of time, they need a power source. Environmental generation looks to be a promising method.

This paper is a comparative study of the design spaces of two bio-implantable acoustically excited generator architectures: the thickness-stretch-mode circular piezoelectric plate and the bending-mode unimorph piezoelectric diaphragm. The generators are part of an acoustic power transfer system for implanted sensors and medical devices such as glucose monitors, metabolic monitors, drug delivery systems, etc. Our studies indicate that at small sizes the diaphragm architecture outperforms the plate architecture. This paper will present the results of simulation studies and initial experiments that explore the characteristics of the two architectures and compare their performance.