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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.

Improving the piezoelectric performance of poly(vinylidene fluoride) (PVDF) is important for many applications, including energy harvesting. The addition of barium titanate (BaTiO3) nanoparticles in the polymer matrix is a popular method to facilitate β phase formation in PVDF films. For pure PVDF, mechanical drawing (i.e. stretching) has also been shown to increase piezoelectric performance. This work examines, for the first time, the combined effect of stretching and BaTiO3 addition on phase transformations in BaTiO3-PVDF composite films. The results indicate that an alternative phase transformation mechanism occurs during the stretching of PVDF composite films. While stretching of pure PVDF films results in the conversion of non-polar α phase into the electroactive β phase, stretching of BaTiO3-PVDF composite films results primarily in the conversion of weakly electroactive γ phase into the more electroactive β phase. Overall, stretching of BaTiO3-PVDF films is shown to be beneficial for increasing β phase content of PVDF.

Efficient water use is a must for sustainable agriculture, driving the need for affordable soil moisture sensors to guide irrigation timing. Sensors are limited by cost, maintenance and the need for wires for data capture and charging. We are developing low-cost, long-life, wireless in-situ soil sensing networks, which can potentially enable a much higher sensor density for large farmland or intense research plot monitoring. This custom soil sensor is made from off-the-shelf electronics and consumes approximately 10x less energy per measurement, compared to commercially available sensors. Here we present our new sensor technology, while also investigating its repeatability and accuracy in controlled conditions and comparing it to that of commercially available soil moisture sensors. The final application of the custom soil moisture sensor is an underground in-situ sensing network, which will be enabled through wireless powering and telemetry systems implemented on autonomous vehicles, both ground and aerial.

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 article presents a near-field low-frequency wireless power transfer system utilizing a piezoelectric transducer with magnet tip mass as a receiver. The interaction moment between the uniform B field generated by a Helmholtz coil and the magnet is the means to deliver the electrical energy from the transmitter to an electrical load, which is therefore referred to as magneto-mechano-electric effect. This is the first time a complete equivalent circuit model of such a structure is developed and experimentally verified. Based on the lumped model, various aspects of the power optimization problem are thoroughly discussed, providing a comprehensive view of the system and an important premise for further study.

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.

Wireless power transfer (WPT) has received increasing attention primarily as a means of recharging batteries in the last few decades. More recently, magnetoelectric (ME) structures have been investigated as alternative receiving antennas in WPT systems. ME structures can be particularly useful for small scale devices since their optimal size is much smaller than traditional receiving coils for a given operating frequency. WPT systems using ME laminate receivers have been shown to be helpful in wirelessly powering various sensors and biomedical implants. In recent years, a large number of studies have been conducted to improve the performance of ME composites, in which various configurations have been proposed, along with the use of different magnetostrictive and piezoelectric materials. In addition, many efforts have been devoted to miniaturizing ME devices. An essential obstacle to overcome is to eliminate the need for a DC bias field that is commonly required for the operation of ME structures. In this review paper, we will discuss the basic principle of ME effects in composites, materials currently in use, various ME receiver structures, performance measures, limitations, challenges, and future perspectives for the field of WPT. Furthermore, we propose a power figure of merit which we use to compare recent ME WPT research papers.

The frequency dependence of the maximum output power and efficiency of two wireless power transfer systems (WPTSs), resonant inductive coupling (RIC) and magnetoelectric (ME), are investigated. We find that in the weak–coupling regime, the power optimization and efficiency maximization problems are equivalent and yield the same optimal load and frequency. These properties apply to both topologies under consideration. Despite the apparent difference in the energy conversion mechanisms, the two structures result in similar explicit forms of maximum power delivered to the load, and so does the optimum transfer efficiency. We discuss the essential role of a figure of merit for each configuration and show how they affect the overall performance. For a weakly–coupled inductive WPTS, both the maximum transferred power and efficiency are positively proportional to drive frequency squared. In the case of a ME–based architecture, the dependence of power and efficiency on frequency is the consequence of the transducer geometry optimization problem, subject to a volume constraint. Under a constant mechanical quality factor condition, both quantities are linearly proportional to the operating frequency. While the focus of this paper is RIC and ME mechanisms, some of the findings are also valid for relevant inductive energy harvesting or magneto–mechano–electric WPTSs.

This paper presents the characterization results of RF coils in loamy soil with a goal to develop low-cost, long-life, wireless, in-situ sensing technologies for monitoring soil health to ensure future sustainable agriculture. Two types of RF coil exhibiting 6 cm-Φ / 9-turn and 12 cm-Φ / 9-turn are employed for the study. The characterization results reveal that coils maximum quality factor takes place around 1 MHz limited by the loamy soil loss. A small gap on the order of 1-2 cm between a coil and surrounding soil can help improve the quality factor significantly. Wireless power transfer efficiency estimation reveals that a sufficient amount of power can be transferred across a soil depth of 30 cm to energize an underground wireless soil sensing network. Further optimized designs are expected to achieve an enhanced performance and extended power transfer distance underground.

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 work concerns the steady-state dynamics of eccentric-rotor harvesters under the excitation of a forced pendulum, selected as a simplified representation of the swinging motion of the human arm during locomotion, to better understand their use for wrist-worn energy-harvesting applications. A linearized model predicts the behavior of nonresonant eccentric-rotor devices well and provides insight into the relationship between the rotor natural frequency and transducer-imposed electrical damping. Approximate analytical solutions are obtained via perturbation methods that show that the eccentric rotor shares many characteristics of a Duffing oscillator with softening spring nonlinearity. Finally, an interesting property of the eccentric rotor's dynamics, namely, invariance of power output to changes in forcing frequency and amplitude over certain ranges of design parameters, is observed, forming the basis for a proposed resonant eccentric-rotor harvester with a wideband power output response.