Wearable Loop Sensor for Bilateral Knee Flexion Monitoring.

We have previously reported wearable loop sensors that can accurately monitor knee flexion with unique merits over the state of the art. However, validation to date has been limited to single-leg configurations, discrete flexion angles, and in vitro (phantom-based) experiments. In this work, we take a major step forward to explore the bilateral monitoring of knee flexion angles, in a continuous manner, in vivo. The manuscript provides the theoretical framework of bilateral sensor operation and reports a detailed error analysis that has not been previously reported for wearable loop sensors. This includes the flatness of calibration curves that limits resolution at small angles (such as during walking) as well as the presence of motional electromotive force (EMF) noise at high angular velocities (such as during running). A novel fabrication method for flexible and mechanically robust loops is also introduced. Electromagnetic simulations and phantom-based experimental studies optimize the setup and evaluate feasibility. Proof-of-concept in vivo validation is then conducted for a human subject performing three activities (walking, brisk walking, and running), each lasting 30 s and repeated three times. The results demonstrate a promising root mean square error (RMSE) of less than 3° in most cases.

Real-Time Magnetocardiography with Passive Miniaturized Coil Array in Earth Ambient Field.

We demonstrate a magnetocardiography (MCG) sensor that operates in non-shielded environments, in real-time, and without the need for an accompanying device to identify the cardiac cycles for averaging. We further validate the sensor's performance on human subjects. Our approach integrates seven (7) coils, previously optimized for maximum sensitivity, into a coil array. Based on Faraday's law, magnetic flux from the heart is translated into voltage across the coils. By leveraging digital signal processing (DSP), namely, bandpass filtering and averaging across coils, MCG can be retrieved in real-time. Our coil array can monitor real-time human MCG with clear QRS complexes in non-shielded environments. Intra- and inter-subject variability tests confirm repeatability and accuracy comparable to gold-standard electrocardiography (ECG), viz., a cardiac cycle detection accuracy of >99.13% and averaged R-R interval accuracy of <5.8 ms. Our results confirm the feasibility of real-time R-peak detection using the MCG sensor, as well as the ability to retrieve the full MCG spectrum as based upon the averaging of cycles identified via the MCG sensor itself. This work provides new insights into the development of accessible, miniaturized, safe, and low-cost MCG tools.

A Stretchable, Conductive Thread-Based Sensor Towards Wearable Monitoring of Muscle Atrophy.

OBJECTIVE: We present the first wearable sensor designed for frequent monitoring of muscle atrophy and validate performance upon canonical phantoms. METHODS: Our approach relies on Faraday's law of induction and exploits the dependence of magnetic flux density on cross-sectional area. We employ wrap-around transmit and receive coils that stretch to fit changing limb sizes using conductive threads (e-threads) in a novel zig zag pattern. Changes in the loop size result in changes in the magnitude and phase of the transmission coefficient between loops. RESULTS: Simulation and in vitro measurement results are in excellent agreement. As a proof-of-concept, a cylindrical calf model for an average-sized subject is considered. The frequency of 60 MHz is selected via simulation for optimal limb size resolution in magnitude and phase while remaining in the inductive mode of operation. We can monitor muscle volume loss of up to 51%, with an approximate resolution of 0.17 dB and 1.58° per 1% volume loss. In terms of muscle circumference, we achieve resolution of 0.75 dB and 6.7° per centimeter. Thus, we can monitor small-scale changes in overall limb size. CONCLUSION: This is the first known approach for monitoring muscle atrophy with a sensor designed to be worn. Additionally, this work brings forward innovations in creating stretchable electronics from e-threads (as opposed to inks, liquid metal, or polymer). SIGNIFICANCE: The proposed sensor will provide improved monitoring for patients suffering from muscle atrophy. The stretching mechanism can be seamlessly integrated into garments which creates unprecedented opportunities for future wearable devices.

Quantifying Cognitive Workload Using a Non-Contact Magnetocardiography (MCG) Wearable Sensor.

Quantifying cognitive workload, i.e., the level of mental effort put forth by an individual in response to a cognitive task, is relevant for healthcare, training and gaming applications. However, there is currently no technology available that can readily and reliably quantify the cognitive workload of an individual in a real-world environment at a seamless way and affordable price. In this work, we overcome these limitations and demonstrate the feasibility of a magnetocardiography (MCG) sensor to reliably classify high vs. low cognitive workload while being non-contact, fully passive and low-cost, with the potential to have a wearable form factor. The operating principle relies on measuring the naturally emanated magnetic fields from the heart and subsequently analyzing the heart rate variability (HRV) matrix in three time-domain parameters: standard deviation of RR intervals (SDRR); root mean square of successive differences between heartbeats (RMSSD); and mean values of adjacent R-peaks in the cardiac signals (MeanRR). A total of 13 participants were recruited, two of whom were excluded due to low signal quality. The results show that SDRR and RMSSD achieve a 100% success rate in classifying high vs. low cognitive workload, while MeanRR achieves a 91% success rate. Tests for the same individual yield an intra-subject classification accuracy of 100% for all three HRV parameters. Future studies should leverage machine learning and advanced digital signal processing to achieve automated classification of cognitive workload and reliable operation in a natural environment.

A Core Body Temperature Retrieval Method for Microwave Radiometry when Tissue Permittivity is Unknown.

This paper presents a novel method for core temperature retrieval using microwave radiometry when complex permittivity and heat transfer parameters of the tissue layers of the human subject are unknown. Previous works present methods for core temperature retrieval, but these methods do not account for population variation in the relevant electromagnetic and thermal parameters, which can increase measurement error beyond the clinically acceptable limit of 0.5°C. Pennes' bioheat model of a six-tissue-layer human head model combined with a coherent electromagnetic model simulate experimental data. To retrieve core temperature, nonlinear least squares optimization is then used to minimize the difference between the simulated experimental data and an exponential model for physical temperature and the coherent electromagnetic model. By using 20 frequencies spanning from 1-5 GHz, core temperature is retrieved while accounting for population variation in the permittivity and thermal parameters. A Monte Carlo simulation in which the thermal parameters and permittivity vary according to literature-derived, population-representative distributions and the core body temperature varies from 18-46°C is used to assess the utility of the retrieval method. Different antenna patterns are tested to explore the effect on retrieval accuracy. The retrieval method has a retrieval error of <0.1°C when only the thermal parameters are unknown and a retrieval error of <0.5°C when the thermal parameters and permittivity are unknown, which is within the clinically acceptable error range of 0.5°C. These results help progress the field of medical microwave radiometry toward being a clinically viable noninvasive measurement that is accurate when measuring all patients.

High-Contrast Low-Loss Antenna: A Novel Antenna for Efficient Into-Body Radiation.

We present a biocompatible high-contrast low-loss antenna (HCLA) designed for efficient into-body radiation for applications as diverse as medical telemetry, sensing, and imaging. The HCLA is wearable with a compact size of 2.62 cm3 and operates across the 1 to 5 GHz bandwidth. The quasi-bowtie antenna is loaded with a high-contrast (i.e., alternating layers of high and low permittivity materials) and low-loss dielectric to improve directivity and gain into the biological tissues. Measurement results at 2.4 GHz are in good agreement with simulations and show 5.72 dB improvement in transmission loss over the most efficient into-body radiator reported in the past. At the high end of the frequency bandwidth, simulation results for two antennas placed across each other with tissue in between show ~12.5 dB improvement in transmission loss. The HCLA is fabricated with stable, low-loss materials that allow for repeatability and consistency in the fabrication process, thus, addressing limitations of the current state-of-the-art. It is also made from biocompatible materials that enable it to be placed directly on the skin for real-world implementation. In this paper, we discuss the operation principle and design of the HCLA, its transmission performance, radiation patterns, and specific absorption rate.

Development of a Coherent Model for Radiometric Core Body Temperature Sensing.

This paper examines the utility of a wideband, physics-based model to determine human core body or brain temperature via microwave radiometry. Pennes's bioheat equation is applied to a six-layer human head model to generate the expected layered temperature profile during the development of a fever. The resulting temperature profile is fed into the forward electromagnetic (EM) model to determine the emitted brightness temperature at various points in time. To accurately retrieve physical temperature via radiometry, the utilized model must incorporate population variation statistics and cover a wide frequency band. The effect of human population variation on emitted brightness temperature is studied by varying the relevant thermal and EM parameters, and brightness temperature emissions are simulated from 0.1 MHz to 10 GHz. A Monte Carlo simulation combined with literature-derived statistical distributions for the thermal and EM parameters is performed to analyze population-level variation in resulting brightness temperature. Variation in thermal parameters affects the offset of the resulting brightness temperature signature, while EM parameter variation shifts the key maxima and minima of the signature. The layering of high and low permittivity layers creates these key maxima and minima via wave interference. This study is one of the first to apply a coherent model to and the first to examine the effect of population-representative variable distributions on radiometry for core temperature measurement. These results better inform the development of an on-body radiometer useful for core body temperature measurement across the human population.

Wearable Sensors Based on Force-Sensitive Resistors for Touch-Based Collaborative Digital Gaming.

We report new classes of wearable sensors that monitor touch between fully-abled and disabled players in order to empower collaborative digital gaming between the two. Our approach relies on embroidered force-sensitive resistors (FSRs) embedded into armbands, which outperform the state-of-the-art in terms of sensitivity to low applied forces (0 to 5 N). Such low forces are of key significance to this application, given the diverse physical abilities of the players. With a focus on effective gameplay, we further explore the sensor's touch-detection performance, study the effect of the armband fabric selection, and optimize the sensor's placement upon the arm. Our results: (a) demonstrate a 4.4-times improvement in sensitivity to low forces compared to the most sensitive embroidered FSR reported to date, (b) confirm the sensor's ability to empower touch-based collaborative digital gaming for individuals with diverse physical abilities, and (c) provide parametric studies for the future development of diverse sensing solutions and game applications.

A Novel Method of Transmission Enhancement and Misalignment Mitigation between Implant and External Antennas for Efficient Biopotential Sensing.

The idea of passive biosensing through inductive coupling between antennas has been of recent interest. Passive sensing systems have the advantages of flexibility, wearability, and unobtrusiveness. However, it is difficult to build such systems having good transmission performance. Moreover, their near-field coupling makes them sensitive to misalignment and movements. In this work, to enhance transmission between two antennas, we investigate the effect of superstrates and metamaterials and propose the idea of dielectric fill in between the antenna and the superstrate. Preliminary studies show that the proposed method can increase transmission between a pair of antennas significantly. Specifically, transmission increase of ≈5 dB in free space and ≈8 dB in lossy media have been observed. Next, an analysis on a representative passive neurosensing system with realistic biological tissues shows very low transmission loss, as well as considerably better performance than the state-of-the-art systems. Apart from transmission enhancement, the proposed technique can significantly mitigate performance degradation due to misalignment of the external antenna, which is confirmed through suitable sensitivity analysis. Overall, the proposed idea can have fascinating prospects in the field of biopotential sensing for different biomedical applications.

Quarter-Wave Plates to Improve Rotational Misalignment Robustness in Medical Telemetry.

A major challenge in developing robust wireless links to implanted/ingestible antennas is the potential for rotational misalignment. In this paper, we present an artificially anisotropic quarter-wave plate (QWP) capable of developing a circularly polarized wave from a linearly polarized wave. Without loss of generality, our QWP is composed of plastic and hydrogel, while the linearly polarized wave is developed by a bio-matched antenna-a high gain, broadband antenna with a dielectric engineered to match to biological tissues. Using a basic implanted patch antenna, we demonstrate a 1.00 dB (1.26) variance in transmission coefficient over a 90° variance, with a remarkable average measured transmission coefficient of -34.4 dB (3.63 × 10-4 ) at 2.4 GHz. Without the QWP, the rotational variance is 12.52 dB (17.9). Notably, the QWP increases the maximum input power to comply with specific absorption rate limitations. In our case, this allows for -15.0 dBm (31.6 µW) of power to be received by the implant, which is comparable to the -15.7 dBm (26.9 µW) received without the QWP. Additionally, we demonstrate that with the QWP, the standard deviation from the mean transmission for rotational misalignments remains below 3 dB (2.00) from 2 to 3.62 GHz, resulting in a simulated 57.7% fractional bandwidth. © 2021 Bioelectromagnetics Society.

Toward Non-Invasive Core Body Temperature Sensing.

This paper aims to explore the potential of a novel radiometry technique that leverages bio-matched antennas (BMAs), broadband measurements, and forward modeling of layered tissues for non-invasive and accurate core temperature monitoring. Our approach relies on the observation that electromagnetic waves penetrate to different depths depending on their frequency and dielectric properties of the medium and adapts radiative transfer models that have been successfully implemented in the past for layered geophysical media. Preliminary modeling and experimental results confirm feasibility.

Modeling Fabric Movement for Future E-Textile Sensors.

Studies with e-textile sensors embedded in garments are typically performed on static and controlled phantom models that do not reflect the dynamic nature of wearables. Instead, our objective was to understand the noise e-textile sensors would experience during real-world scenarios. Three types of sleeves, made of loose, tight, and stretchy fabrics, were applied to a phantom arm, and the corresponding fabric movement was measured in three dimensions using physical markers and image-processing software. Our results showed that the stretchy fabrics allowed for the most consistent and predictable clothing-movement (average displacement of up to -2.3 ± 0.1 cm), followed by tight fabrics (up to -4.7 ± 0.2 cm), and loose fabrics (up to -3.6 ± 1.0 cm). In addition, the results demonstrated better performance of higher elasticity (average displacement of up to -2.3 ± 0.1 cm) over lower elasticity (average displacement of up to -3.8 ± 0.3 cm) stretchy fabrics. For a case study with an e-textile sensor that relies on wearable loops to monitor joint flexion, our modeling indicated errors as high as 65.7° for stretchy fabric with higher elasticity. The results from this study can (a) help quantify errors of e-textile sensors operating "in-the-wild," (b) inform decisions regarding the optimal type of clothing-material used, and (c) ultimately empower studies on noise calibration for diverse e-textile sensing applications.

A Novel Method to Mitigate Real-Imaginary Image Imbalance in Microwave Tomography.

Typical microwave tomographic techniques reconstruct the real part of the permittivity with much greater accuracy as compared to the imaginary part. In this paper, we propose a method to mitigate the imbalance between the reconstructed complex permittivity components and increase the accuracy of the overall image recovery. To do so, the complex permittivity in the imaging domain is expressed as a weighted sum of a few preselected permittivities, close to the range of the expected values. To obtain the permittivity weights, a Gauss-Newton algorithm is employed. Image reconstructions from simulated and experimental data for different biomedical phantoms are presented. Results show that the proposed method leads to excellent reconstruction with balanced real and imaginary parts, across different scenarios.

Wireless Wearables and Implants: A Dosimetry Review.

Wireless wearable and implantable devices are continuing to grow in popularity, and as this growth occurs, so too does the need to consider the safety of such devices. Wearable and implantable devices require the transmitting and receiving of electromagnetic waves near and through the body, which at high enough exposure levels may damage proximate tissues. The specific absorption rate (SAR) is the quantity commonly used to enumerate exposure levels, and various national and international organizations have defined regulations limiting exposure to ensure safe operation. In this paper, we comprehensively review dosimetric studies reported in the literature up to the year 2019 for wearables and implants. We discuss antenna designs for wearables and implants as they relate to SAR values and field and thermal distributions in tissue, present designs that have made steps to reduce SAR, and then review SAR considerations as they relate to applied devices. As compared with previous review papers, this paper is the first review to focus on dosimetry aspects relative to wearable and implantable devices. Bioelectromagnetics. 2020;41:3-20 © 2019 The Authors. Bioelectromagnetics published by Wiley Periodicals, Inc.

Wrap-Around Wearable Coils for Seamless Monitoring of Joint Flexion.

OBJECTIVE: We introduce and validate a new class of wearable coils that seamlessly monitor joint flexion in the individual's natural environment while overcoming shortcomings in state-of-the-art. METHODS: Our approach relies on Faraday's law of induction and employs wrap-around transmit and receive coils that get angularly misaligned as the joint flexes. RESULTS: Simulation and in vitro measurement results for both copper and e-thread coils are in excellent agreement. As a proof-of-concept, a cylindrical arm model is considered and feasibility of monitoring the 0°-130° range of motion is confirmed. The operation frequency of 34 MHz is identified as optimal, bringing forward reduced power requirements, enhanced angular resolution, and extreme robustness to tissue dielectric property variations. Performance benchmarking versus state-of-the-art inertial measurement units shows equivalent or superior performance, particularly for flexion angles greater than 20°. Design guidelines and safety considerations are also explored. CONCLUSION: Contrary to "gold-standard" camera-based motion capture, the reported approach is not restricted to contrived environments. Concurrently, it does not suffer from integration drift (unlike inertial measurement units), it does not require line-of-sight (unlike time-of-flight sensors), and it does not restrict natural joint movement (unlike bending sensors). SIGNIFICANCE: The reported approach is envisioned to be seamlessly integrated into garments and, eventually, redefine the way joint flexion is monitored at present. This promises unprecedented opportunities for rehabilitation, sports, gestural interaction, and more.

The Promise of Mobile Technologies for the Health Care System in the Developing World: A Systematic Review.

Evolution of mobile technologies and their rapid penetration into people's daily lives, especially in the developing countries, have highlighted mobile health, or m-health, as a promising solution to improve health outcomes. Several studies have been conducted that characterize the impact of m-health solutions in resource-limited settings and assess their potential to improve health care. The aim of this review is twofold: 1) to present an overview of the background and significance of m-health and 2) to summarize and discuss the existing evidence for the effectiveness of m-health in the developing world. A systematic search in the literature was performed in Pubmed, Scopus, as well as reference lists, and a broad sample of 98 relevant articles was identified, which were then categorized into five wider m-health categories. Although statistically significant conclusions cannot be drawn since the majority of studies relied on small-scale trials and limited assessment of long-term effects, this review provides a systematic and extensive analysis of the advantages, disadvantages, and challenges of m-health in developing countries in an attempt to determine future research directions of m-health interventions.

Power Generation for Wearable Electronics: Designing Electrochemical Storage on Fabrics.

We report a new class of textiles with electrochemical functions which, when moistened by a conductive liquid (saline solution, sweat, wound fluid, etc.), generate DC voltage and current levels capable of powering wearable electronics on the go. Contrary to previously reported power generation techniques, the proposed fabrics are fully flexible, feel and behave like regular clothing, do not include any rigid components, and provide DC power via moistening by readily available liquids. Our approach entails printed battery cells that are composed of silver and zinc electrodes deposited onto a polyester fabric to generate power in the microwatt range. Electrochemical characterization of the discharge of a single printed battery cell in a 10 M NaOH electrolyte shows reproducible results with a sustained power level of ∼80 µW for over 3 hours. Scalable DC power may also be achieved by connecting multiple battery cells in series via flexible and conductive E-threads. Indeed, a series connection of two battery cells is demonstrated to boost the generated voltage from 1.4 V to 2.5 V. Notably, this in-series printed battery arrangement is shown to successfully power a digital thermometer under both 10 M NaOH, a 0.5 M NaCl solution (mimicking human sweat), and Dulbecco's Phosphate-Buffered Saline solution (DPBS) (mimicking bodily fluid electrolytes). Overall, the proposed technology is expected to be of utmost significance for healthcare, sports, military, and consumer applications, among others.

Stray energy transfer during endoscopy.

INTRODUCTION: Endoscopy is the standard tool for the evaluation and treatment of gastrointestinal disorders. While the risk of complication is low, the use of energy devices can increase complications by 100-fold. The mechanism of increased injury and presence of stray energy is unknown. The purpose of the study was to determine if stray energy transfer occurs during endoscopy and if so, to define strategies to minimize the risk of energy complications. METHODS AND PROCEDURES: A gastroscope was introduced into the stomach of an anesthetized pig. A monopolar generator delivered energy for 5 s to a snare without contacting tissue or the endoscope itself. The endoscope tip orientation, energy device type, power level, energy mode, and generator type were varied to mimic in vivo use. The primary outcome (stray current) was quantified as the change in tissue temperature (°C) from baseline at the tissue closest to the tip of the endoscope. Data were reported as mean ± standard deviation. RESULTS: Using the 60 W coag mode while changing the orientation of the endoscope tip, tissue temperature increased by 12.1 ± 3.5 °C nearest the camera lens (p < 0.001 vs. all others), 2.1 ± 0.8 °C nearest the light lens, and 1.7 ± 0.4 °C nearest the working channel. Measuring temperature at the camera lens, reducing power to 30 W (9.5 ± 0.8 °C) and 15 W (8.0 ± 0.8 °C) decreased stray energy transfer (p = 0.04 and p = 0.002, respectively) as did utilizing the low-voltage cut mode (6.6 ± 0.5 °C, p < 0.001). An impedance-monitoring generator significantly decreased the energy transfer compared to a standard generator (1.5 ± 3.5 °C vs. 9.5 ± 0.8 °C, p < 0.001). CONCLUSION: Stray energy is transferred within the endoscope during the activation of common energy devices. This could result in post-polypectomy syndrome, bleeding, or perforation outside of the endoscopist's view. Decreasing the power, utilizing low-voltage modes and/or an impedance-monitoring generator can decrease the risk of complication.

A Review of In-Body Biotelemetry Devices: Implantables, Ingestibles, and Injectables.

OBJECTIVE: We present a review of wireless medical devices that are placed inside the human body to realize many and different sensing and/or stimulating functionalities. METHODS: A critical literature review analysis is conducted focusing on three types of in-body medical devices, i.e., 1) devices that are implanted inside the human body (implantables), 2) devices that are ingested like regular pills (ingestibles), and 3) devices that are injected into the human body via needles (injectables). Design considerations, current status, and future directions related to the aforementioned in-body devices are discussed. RESULTS: A number of design challenges are associated with in-body devices, including selection of operation frequency, antenna design, powering, and biocompatibility. Nevertheless, in-body devices are opening up new opportunities for medical prevention, prognosis, and treatment that quickly outweigh any design challenges and/or concerns on their invasive nature. CONCLUSION: In-body devices are already in use for several medical applications, ranging from pacemakers and capsule endoscopes to injectable microstimulators. As technology continues to evolve, in-body devices are promising several new and hitherto unexplored opportunities in the healthcare. SIGNIFICANCE: Unobtrusive in-body devices are envisioned to collect a multitude of physiological data from the early years of each individual. This big-data approach aims to enable a shift from symptom-based medicine to a proactive healthcare model.