Continuous wireless pressure monitoring and mapping with ultra-small passive sensors for health monitoring and critical care

An Impedance-Loaded Orthogonal Frequency-Coded SAW Sensor for Passive Wireless Sensor Networks

A passive wireless impedance-loaded orthogonal frequency-coded (OFC) surface acoustic wave (SAW) sensor for wireless sensor networks was proposed in this paper. One of the chips on OFC SAW tag is connected to an external sensor, which could cause a phase shift in the time response of the corresponding part on the SAW device. The phase shift corresponds to the sensed quantity, which could be temperature, strain, vibration, pressure, etc. The OFC SAW tag is isolated by a proper package from the direct effect of the measurand on the device's response which could avoid the multiple measurands coupling. The simultaneous work of multiple sensors is guaranteed by orthogonal frequency coding. By processing the response based on an extended matched filter algorithm, sensing information of the specific coded OFC device can be extracted from the superimposed response of multiple independent encoded sensors. Compared to previous methods, the proposed method can produce a more flexible passive (battery-free) wireless sensor suitable for large-scale wireless sensor networks. Simulation and experimental results demonstrate the effectiveness of the sensor.

Machine-knitted washable sensor array textile for precise epidermal physiological signal monitoring

Wearable textile electronics are highly desirable for realizing personalized health management. However, most reported textile electronics can either periodically target a single physiological signal or miss the explicit details of the signals, leading to a partial health assessment. Furthermore, textiles with excellent property and comfort still remain a challenge. Here, we report a triboelectric all-textile sensor array with high pressure sensitivity and comfort. It exhibits the pressure sensitivity (7.84 mV Pa^-1), fast response time (20 ms), stability (>100,000 cycles), wide working frequency bandwidth (up to 20 Hz), and machine washability (>40 washes). The fabricated TATSAs were stitched into different parts of clothes to monitor the arterial pulse waves and respiratory signals simultaneously. We further developed a health monitoring system for long-term and noninvasive assessment of cardiovascular disease and sleep apnea syndrome, which exhibits great advancement for quantitative analysis of some chronic diseases.

Ultrathin, Biocompatible, and Flexible Pressure Sensor with a Wide Pressure Range and Its Biomedical Application

In this research, an ultrathin, biocompatible, and flexible pressure sensor with a wide pressure range has been developed and applied in biomedical applications. The pressure sensing mechanism is based on the variation of contact resistance between an electrode and a three-dimensional microstructured polyimide/carbon nanotube composite film. The sensor has a thickness of about 31.3 μm, a maximum sensitivity of 41.0 MPa^-1, and a sensing range of 10-500 kPa. Moreover, in situ temperature measurement by an integrated resistive temperature detector enables data correction for varying temperature conditions. In order to show the advantages of the fabricated sensor, it is attached to the human body and integrated with the surface of a radiofrequency ablation (RFA) needle with small radius of curvature. In the experiments, the proposed pressure sensor measured subtle pressure levels (pulse pressure) and high pressure levels (fingertip pressure) without losing conformal contact with the skin. In addition, when the pressure-sensor-integrated RFA needle was inserted into a bovine liver, successful detection of steam popping phenomenon was observed.

Flexible potentiometric pH sensors for wearable systems

There is a growing demand for developing wearable sensors that can non-invasively detect the signs of chronic diseases early on to possibly enable self-health management. Among these the flexible and stretchable electrochemical pH sensors are particularly important as the pH levels influence most chemical and biological reactions in materials, life and environmental sciences. In this review, we discuss the most recent developments in wearable electrochemical potentiometric pH sensors, covering the key topics such as (i) suitability of potentiometric pH sensors in wearable systems; (ii) designs of flexible potentiometric pH sensors, which may vary with target applications; (iii) materials for various components of the sensor such as substrates, reference and sensitive electrode; (iv) applications of flexible potentiometric pH sensors, and (v) the challenges relating to flexible potentiometric pH sensors.

Three-Dimensional Multistack-Printed, Self-Powered Flexible Pressure Sensor Arrays: Piezoelectric Composites with Chemically Anchored Heterogeneous Interfaces

Recently, the development of pressure sensor devices composed of mechanically flexible materials has gained a tremendous attention for emerging wearable electronics applications. Compared with various sensing materials, piezoelectric composite materials provide a characteristic advantage of enabling energy unit-free integration of sensor compartments. In this study, we develop a new chemical method of synthesizing highly functioning piezoelectric composite materials with electrostatically reinforced heterogeneous interfaces to improve the voltage output signal in all-printed sensor arrays. The surfaces of piezoelectric oxide nanoparticles are decorated subsequently with a cationic polyelectrolyte, polyethyleneimine, and a tri-block copolymer, styrene-ethylene/butylene-styrene grafted with maleic anhydride. To elucidate the factors determining the performance of pressure sensor devices, both the electrical properties and piezoelectric characteristics are investigated comprehensively for various compositional composite materials prepared from chemical and physical rubbers. The resulting device exhibits a sensitivity of 0.28 V·kPa^-1 with a linear increment of output voltage in a pressure range up to 30 kPa. It is also demonstrated that the all-printed sensor array is fabricated successfully by a multistack-printing process of conductive, insulating, and piezoelectric composite materials in an additive manufacturing fashion.

Parallel Signal Processing of a Wireless Pressure-Sensing Platform Combined with Machine-Learning-Based Cognition, Inspired by the Human Somatosensory System

Inspired by the human somatosensory system, pressure applied to multiple pressure sensors is received in parallel and combined into a representative signal pattern, which is subsequently processed using machine learning. The pressure signals are combined using a wireless system, where each sensor is assigned a specific resonant frequency on the reflection coefficient (S₁₁ ) spectrum, and the applied pressure changes the magnitude of the S₁₁ pole with minimal frequency shift. This allows the differentiation and identification of the pressure applied to each sensor. The pressure sensor consists of polypyrrole-coated microstructured poly(dimethylsiloxane) placed on top of electrodes, operating as a capacitive sensor. The high dielectric constant of polypyrrole enables relatively high pressure-sensing performance. The coils are vertically stacked to enable the reader to receive the signals from all of the sensors simultaneously at a single location, analogous to the junction between neighboring primary neurons to a secondary neuron. Here, the stacking order is important to minimize the interference between the coils. Furthermore, convolutional neural network (CNN)-based machine learning is utilized to predict the applied pressure of each sensor from unforeseen S₁₁ spectra. With increasing training, the prediction accuracy improves (with mean squared error of 0.12), analogous to humans' cognitive learning ability.

Wireless, passive strain sensor in a doughnut-shaped contact lens for continuous non-invasive self-monitoring of intraocular pressure

After cataract, glaucoma is the second leading cause of blindness worldwide and real-time monitoring of intraocular pressure (IOP) is of great demand. We present a wireless, passive sensor sitting inside a customized, planar and circular doughnut-shaped contact lens capable of continuous monitoring of the change in the curvature of cornea caused by IOP fluctuations. The sensor consists of a constant capacitor and a variable inductor in the form of a stretchable, closed-loop, serpentine wire that serves as both the sensor and the antenna. Results show a pressure responsivity of 523 kHz per 1% axial strain on a pressurized polydimethylsiloxane membrane and 35.1 kHz per 1 mmHg change in the IOP of a canine eye. The sensor is tested for stability and shows unvaried characteristics after repeated cycles and parasitic movements. Predictable influences of temperature and humidity on the sensor response are also verified experimentally, which can be canceled out using real-time calibration with temperature and humidity sensors to integrate with a reader device. The design reported here has numerous advantages, such as design simplicity, component reliability, high responsivity, and low cost, thereby opening up potential opportunities for the translation of this non-invasive, continuous IOP monitoring technique into clinical applications.

A Flexible Wireless Dielectric Sensor for Noninvasive Fluid Monitoring

A flexible wireless dielectric sensor is presented here for noninvasively monitoring the permittivity and conductivity of fluids, based on resistor–inductor–capacitor (RLC) resonant circuit and capacitively coupled contactless conductivity detection (C⁴D) technique. The RLC sensor consists of one single-turn inductor and one interdigital capacitor. The resonant frequency of the device is sensitive to the surrounding environment, thanks to the electric field leaked out between the interdigital capacitor electrodes. Through the high-frequency structure simulator (HFSS) simulation, and experiments on ethanol/water solutions and NaCl solutions, it was confirmed that a fluid’s permittivity and conductivity could be detected by the return loss curve (S₁₁). With great repeatability and stability, the proposed sensor has potential for broad applications, especially in wearable low-cost smart devices.

A Noninvasive, Electromagnetic, Epidermal Sensing Device for Hemodynamics Monitoring

Non-intrusive monitoring of blood flow parameters is vital for obtaining physiological and pathophysiological information pertaining to dynamic cardiovascular events and is feasible to achieve via non-invasive, conformal, wearable technologies. Here, we present a proof-of-concept of a fully integrated, high frequency (bandwidth 40 MHz), electromagnetic sensing device for monitoring limb hemodynamics and morphology associated with blood flow. The sensing architecture integrates a novel radio frequency (RF) skin patch resonator embedded with a coplanar outer loop antenna and a scalable, standalone wireless readout hardware based on standing wave ratio (SWR) bridge. The resonator itself is a copper-based open circuit planar Archimedean spiral with a rectangular cross-sectional area, chemically etched on a flexible polyimide substrate. The readout hardware is developed exploiting off-the-shelf components, fabricated on the top of a rigid FR4 substrate. The proposed readout circuit can measure resonant frequency of an RLC network. When energized by the external oscillating RF field via loop antenna, the resonator produces an electromagnetic field response which can be perturbed by dielectric variation inside its field boundary. Through leveraging this principle, the in-vitro experimental results from the benchtop models suggest that the resonator's RF attributes such as resonant frequency shift and magnitude variation of reflection coefficient due to fluid volume displacement can be successfully detected through the proposed hardware architecture. Hence, the system could be an alternative to the conventional, multimodal, non-invasive wearable sensing with an unprecedented capability of ubiquitous fluid phenomena detection from multiple sites of the human body.

Flow stabilization in wearable microfluidic sensors enables noise suppression

Dilatometric strain sensors (DSS) that work based on detection of volume change in microfluidic channels; i) are highly sensitive to biaxial strain, ii) can be fabricated using only soft and transparent materials, and iii) are easy to integrate with smart-phones. These features are especially attractive for contact lens based intraocular pressure (IOP) sensing applications. The inherent flow stabilization of the microfluidic systems is an additional advantage suitable for filtering out rapid fluctuations. Here, we have demonstrated that the low-pass filtering in microfluidic sensors improves the signal-to-noise-ratio for ophthalmic applications. We have fabricated devices with a time constant in the range of 1-200 seconds. We have demonstrated that the device architecture and working liquid viscosity (10-866 cSt) are the two independent factors that determine the sensor time constant. We have developed an equivalent circuit model for the DSS that accurately represents the experimental results thus can be used as a computational model for design and development of microfluidic sensors. For a sensor with the time constant of 4 s, we report that microfluidic signal filtering in IOP monitoring applications can suppress the rapid fluctuations (i.e., the noise due to ocular pulsation, blinking etc.) by 9 dB without the need for electronic components.

Highly Compressible and Robust Polyimide/Carbon Nanotube Composite Aerogel for High-Performance Wearable Pressure Sensor

Wearable pressure sensors are in great demand with the rapid development of intelligent electronic devices. However, it is still a huge challenge to obtain high-performance pressure sensors with high sensitivity, wide response range, and low detection limit simultaneously. Here, a polyimide (PI)/carbon nanotube (CNT) composite aerogel with the merits of superelastic, high porosity, robust, and high-temperature resistance was successfully prepared through the freeze drying plus thermal imidization process. Benefiting from the strong chemical interactions between PI and CNT and stable electrical property, the composite aerogel exhibits versatile and superior brilliant sensing performance, which includes wide sensing range (80% strain, 61 kPa), ultrahigh sensitivity (11.28 kPa^-1), ultralow detection limit (0.1% strain, <10 Pa), fast response time (50 ms) and recovery time (70 ms), remarkable long-term stability (1000 cycles), and exceptional detection ability toward different deformations (compression, distortion, and bending). Furthermore, the composite aerogel also shows stable sensing performance after annealing under different high temperatures and good thermal insulation property, making it workable in various harsh environments. As a result, the composite aerogel is suitable for the full-range human motion detection (including airflow, pulse, vocal cord vibration, and human movement) and precise detection of the pressure distribution when it is assembled into E-skin, demonstrating its great potential to serve as a high-performance wearable pressure sensor.

Design Strategy for Porous Composites Aimed at Pressure Sensor Application

Flexible and highly sensitive pressure sensors have versatile biomedical engineering applications for disease diagnosis and healthcare. The fabrication of such sensors based on porous structure composites usually requires complex, costly, and nonenvironmentally friendly procedures. As such, it is highly desired to develop facile, economical, and environment-friendly fabrication strategies for highly sensitive lightweight pressure sensors. Herein, a novel design strategy is reported to fabricate porous composite pressure sensors via a simple heat molding of conductive fillers and thermoplastic polyurethane (TPU) powders together with commercially available popcorn salts followed by water-assisted salt removal. The obtained TPU/carbon nanostructure (CNS) foam sensors have a linear resistance response up to 60% compressive strain with a gauge factor (G_F ) of 1.5 and show reversible and reproducible piezoresistive properties due to the robust electrically conductive pathways formed on the foam struts. Such foam sensors can be potentially utilized for guiding squatting exercises and respiration rate monitoring in daily physical training.

Electronic Skin: Recent Progress and Future Prospects for Skin-Attachable Devices for Health Monitoring, Robotics, and Prosthetics

Recent progress in electronic skin or e-skin research is broadly reviewed, focusing on technologies needed in three main applications: skin-attachable electronics, robotics, and prosthetics. First, since e-skin will be exposed to prolonged stresses of various kinds and needs to be conformally adhered to irregularly shaped surfaces, materials with intrinsic stretchability and self-healing properties are of great importance. Second, tactile sensing capability such as the detection of pressure, strain, slip, force vector, and temperature are important for health monitoring in skin attachable devices, and to enable object manipulation and detection of surrounding environment for robotics and prosthetics. For skin attachable devices, chemical and electrophysiological sensing and wireless signal communication are of high significance to fully gauge the state of health of users and to ensure user comfort. For robotics and prosthetics, large-area integration on 3D surfaces in a facile and scalable manner is critical. Furthermore, new signal processing strategies using neuromorphic devices are needed to efficiently process tactile information in a parallel and low power manner. For prosthetics, neural interfacing electrodes are of high importance. These topics are discussed, focusing on progress, current challenges, and future prospects.

Versatile Electronic Skins with Biomimetic Micronanostructures Fabricated Using Natural Reed Leaves as Templates

Versatile electronic skin devices that enable detection of multimodal signals have revealed great potential for human health monitoring. To make a versatile electronic skin, hierarchical micronanostructures are essential to obtain improved sensing performance and multisignal detection capability. However, current strategies for developing a nanostructured electronic skin usually involve complex procedures, harsh experimental conditions, and the use of expensive equipment, which limit its practical applications. In this paper, we reported the fabrication of a multifunctional wearable electronic skin with hierarchical micronanostructures by using natural reed leaves as templates. The capacitive-type electronic skin is fabricated by double-sided coating of Au electrodes on an artificial polydimethylsiloxane reed leaf that is duplicated from natural reed leaves via soft lithography. The electronic skin features a very simple device structure yet high sensing performance. It permits multimodal signal detection, including that of pressure, deformation, and proximity, and can serve as surface-enhanced Raman scattering substrates for the detection of metabolites in sweat because of the formation of plasmonic structures. The versatile electronic skin can be attached to the human skin, and it enables effective monitoring of multiphysiological signals, revealing great potential for cutting-edge applications, such as human health monitoring.

The Future of Cardiovascular Stents: Bioresorbable and Integrated Biosensor Technology

Cardiovascular disease is the greatest cause of death worldwide. Atherosclerosis is the underlying pathology responsible for two thirds of these deaths. It is the age-dependent process of "furring of the arteries." In many scenarios the disease is caused by poor diet, high blood pressure, and genetic risk factors, and is exacerbated by obesity, diabetes, and sedentary lifestyle. Current pharmacological anti-atherosclerotic modalities still fail to control the disease and improvements in clinical interventions are urgently required. Blocked atherosclerotic arteries are routinely treated in hospitals with an expandable metal stent. However, stented vessels are often silently re-blocked by developing "in-stent restenosis," a wound response, in which the vessel's lumen renarrows by excess proliferation of vascular smooth muscle cells, termed hyperplasia. Herein, the current stent technology and the future of biosensing devices to overcome in-stent restenosis are reviewed. Second, with advances in nanofabrication, new sensing methods and how researchers are investigating ways to integrate biosensors within stents are highlighted. The future of implantable medical devices in the context of the emerging "Internet of Things" and how this will significantly influence future biosensor technology for future generations are also discussed.

Carbon-Nanotube-Coated 3D Microspring Force Sensor for Medical Applications

Flexible electronic materials combined with micro-3D fabrication present new opportunities for wearable biosensors and medical devices. This Research Article introduces a novel carbon-nanotube-coated force sensor, successfully combining the advantages of flexible conductive nanomaterials and the versatility of two photon polymerization technologies for creating functional 3D microstructures. The device employs carbon-nanotube-coated microsprings with varying configurations and geometries for  real-time force sensing. To demonstrate its practical value, the device has first been embodied as a patch sensor for transcutaneous monitoring of human arterial pulses, followed by the development of a multiple-point force-sensitive catheter for real-time noninvasive intraluminal intervention. The results illustrate the potential of leveraging advanced nanomaterials and micro-3D-printing for developing new medical devices.

Stretchable, Patch-Type Calorie-Expenditure Measurement Device Based on Pop-Up Shaped Nanoscale Crack-Based Sensor

Demands for precise health information tracking techniques are increasing, especially for daily dietry requirements to prevent obesity, diabetes, etc. Many commercially available sensors that detect dynamic motions of the body lack accuracy, while novel strain sensors at the research level mostly lack the capability to analyze measurements in real life conditions. Here, a stretchable, patch-type calorie expenditure measurement system is demonstrated that integrates an ultrasensitive crack-based strain sensor and Bluetooth-enabled wireless communication circuit to offer both accurate measurements and practical diagnosis of motion. The crack-based strain gauge transformed into a pop-up-shaped structure provides reliable measurements and broad range of strain (≈100%). Combined with the stretchable analysis circuit, the skin attachable tool translates variation of the knee flexion angle into calorie expenditure amount, using relative resistance change (R/R₀ ) data from the flexible sensor. As signals from the knee joint angular movement translates velocity and walking/running behavior, the total amount of calorie expenditure is accurately analyzed. Finally, theoretical, experimental, and simulation analysis of signal stability, dynamic noises, and calorie expenditure calculation obtained from the device during exercise are demonstrated. For further applications, the devices are expected to be used in broader range of dynamic motion of the body for diagnosis of abnormalities and for rehabilitation.

Flexible and Highly Sensitive Humidity Sensor Based on Sandwich-Like Ag/Fe3O4 Nanowires Composite for Multiple Dynamic Monitoring

Functional textiles with unique functions, including free cutting, embroidery and changeable shape, will be attractive for smart wear of human beings. Herein, we fabricated a sandwich-like humidity sensor made from silver coated one-dimensional magnetite nanowire (Fe₃O₄ NW) arrays which were in situ grown on the surface of modified polypropylene nonwoven fabric via simultaneous radiation induced graft polymerization and co-precipitation. The humidity sensor exhibits an obvious response to the relative humidity (RH) ranging from RH 11% to RH 95% and its response value reaches a maximum of 6600% (ΔI/I₀) at 95% relative humidity (RH). The humidity sensor can be tailored into various shapes and embroidered on its surface without affecting its functionalities. More interesting, the intensity of its response is proportional to the size of the material. These features permit the sensor to be integrated into commercial textiles or a gas mask to accurately monitor a variety of important human activities including respiration, blowing, speaking and perspiration. Moreover, it also can distinguish different human physical conditions by recognizing respiration response patterns. The sandwich-like sensor can be readily integrated with textiles to fabricate promising smart electronics for human healthcare.

Enhanced sensitivity at high-order exceptional points in a passive wireless sensing system

A noteworthy challenge in actual wireless sensors is the intrinsic sensing resolution and the sensitivity associated with the response to external perturbation to be measured. To address the issue, we report the realization of enhanced sensitivity in a passive wireless sensing system, consisting of three coupled passive resonators. The input wave is exploited as an effective gain in our open system, thus the ideal parity-time (PT) symmetry can be established, rather than balancing real gain and loss. Then the third-order exceptional points are obtained in ternary PT symmetric systems. With the extrinsic perturbation imposed on any one of resonators, we demonstrate analytically and experimentally that the resonance response of the system follows the cube-root dependence on perturbation. Making use of the effective gain, our results pave a new way, to the best of our knowledge, to realize the ultra-sensitivity of a passive wireless sensing system.

Battery-free, wireless sensors for full-body pressure and temperature mapping

Thin, soft, skin-like sensors capable of precise, continuous measurements of physiological health have broad potential relevance to clinical health care. Use of sensors distributed over a wide area for full-body, spatiotemporal mapping of physiological processes would be a considerable advance for this field. We introduce materials, device designs, wireless power delivery and communication strategies, and overall system architectures for skin-like, battery-free sensors of temperature and pressure that can be used across the entire body. Combined experimental and theoretical investigations of the sensor operation and the modes for wireless addressing define the key features of these systems. Studies with human subjects in clinical sleep laboratories and in adjustable hospital beds demonstrate functionality of the sensors, with potential implications for monitoring of circadian cycles and mitigating risks for pressure-induced skin ulcers.

Fully Printed, Wireless, Stretchable Implantable Biosystem toward Batteryless, Real-Time Monitoring of Cerebral Aneurysm Hemodynamics

This study introduces a high-throughput, large-scale manufacturing method that uses aerosol jet 3D printing for a fully printed stretchable, wireless electronics. A comprehensive study of nanoink preparation and parameter optimization enables a low-profile, multilayer printing of a high-performance, capacitance flow sensor. The core printing process involves direct, microstructured patterning of biocompatible silver nanoparticles and polyimide. The optimized fabrication approach allows for transfer of highly conductive, patterned silver nanoparticle films to a soft elastomeric substrate. Stretchable mechanics modeling and seamless integration with an implantable stent display a highly stretchable and flexible sensor, deployable by a catheter for extremely low-profile, conformal insertion in a blood vessel. Optimization of a transient, wireless inductive coupling method allows for wireless detection of biomimetic cerebral aneurysm hemodynamics with the maximum readout distance of 6 cm through meat. In vitro demonstrations include wireless monitoring of flow rates (0.05-1 m s-1) in highly contoured and narrow human neurovascular models. Collectively, this work shows the potential of the printed biosystem to offer a high throughput, additive manufacturing of stretchable electronics with advances toward batteryless, real-time wireless monitoring of cerebral aneurysm hemodynamics.

Large area flexible pressure/strain sensors and arrays using nanomaterials and printing techniques

Sensors are becoming more demanding in all spheres of human activities for their advancement in terms of fabrication and cost. Several methods of fabrication and configurations exist which provide them myriad of applications. However, the advantage of fabrication for sensors lies with bulk fabrication and processing techniques. Exhaustive study for process advancement towards miniaturization from the advent of MEMS technology has been going on and progressing at high pace and has reached a highly advanced level wherein batch production and low cost alternatives provide a competitive performance. A look back to this advancement and thus understanding the route further is essential which is the core of this review in light of nanomaterials and printed technology based sensors. A subjective appraisal of these developments in sensor architecture from the advent of MEMS technology converging present date novel materials and process technologies through this article help us understand the path further.

Ultraminiature and Flexible Sensor Based on Interior Corner Flow for Direct Pressure Sensing in Biofluids

Conventional pressure sensing devices are well developed for either indirect evaluation or internal measuring of fluid pressure over millimeter scale. Whereas, specialized pressure sensors that can directly work in various liquid environments at micrometer scale remain challenging and rarely explored, but are of great importance in many biomedical applications. Here, pressure sensor technology that utilizes capillary action to self-assemble the pressure-sensitive element is introduced. Sophisticated control of capillary flow, tunable sensitivity to liquid pressure in various mediums, and multiple transduction modes are realized in a polymer device, which is also flexible (thickness of 8 µm), ultraminiature (effective volume of 18 × 100 × 580 µm³ ), and transparent, enabling the sensor to work in some extreme situations, such as in narrow inner spaces (e.g., a microchannel of 220 µm in width and 100 µm in height), or on the surface of small objects (e.g., a 380 µm diameter needle). Potential applications of this sensor include disposables for in vivo and short-term measurements.

Oxygen-Tolerant Hydrogen Peroxide Reduction Catalysts for Reliable Noninvasive Bioassays

Noninvasive bioassays based on the principle of a hydrogen peroxide (H₂ O₂ ) cathodic reaction are highly desirable for low concentration analyte detection within biofluids since the reaction is immune to interference from oxidizable species. However, the inability to selectively reduce H₂ O₂ over O₂ for commonly used stable catalysts (carbon or noble metals) is one of the key factors limiting their development and practical applications. Herein, catalysts that enable selective H₂ O₂ reduction in the presence of oxygen with fluctuating concentrations are reported. These catalysts consist of noble metal nanoparticles underneath an amorphous chromium oxide nanolayer, which inhibits O₂ diffusion to the metal/oxide interface and suppresses its reduction reaction. Using these catalysts, analytes of low concentration in biofluids, including but not limited to glucose and lactate, are detected within the presence of various interferents. This work enables wide application of the cathodic detection principle and the development of reliable noninvasive bioassays.

An Active Self-Driven Piezoelectric Sensor Enabling Real-Time Respiration Monitoring

In this work, we report an active respiration monitoring sensor based on a piezoelectric-transducer-gated thin-film transistor (PTGTFT) aiming to measure respiration-induced dynamic force in real time with high sensitivity and robustness. It differs from passive piezoelectric sensors in that the piezoelectric transducer signal is rectified and amplified by the PTGTFT. Thus, a detailed and easy-to-analyze respiration rhythm waveform can be collected with a sufficient time resolution. The respiration rate, three phases of respiration cycle, as well as phase patterns can be further extracted for prognosis and caution of potential apnea and other respiratory abnormalities, making the PTGTFT a great promise for application in long-term real-time respiration monitoring.

A Wireless Handheld Pressure Measurement System for In Vivo Monitoring of Intraocular Pressure in Rabbits

Intraocular pressure (IOP) is the leading modifiable risk factor for preventing vision loss in glaucoma patients. Direct and frequent IOP measurements are highly desirable to assess adequacy of treatment and prevent further vision loss. In this study, we report on successful in vivo measurements of intraocular pressure in rabbits using an optical IOP measurement system. The sensor was implanted during cataract surgery in two New Zealand white (NZW) rabbits and tested in vivo for ten weeks. Prior to implantation, the sensors were characterized in vitro in the physiologically relevant pressure range of 0-60 mmHg. A portable wireless handheld reader consisting of an internal beam splitter, a monochromatic light source, and a digital single-lens reflex (DSLR) camera was also designed and implemented to capture interference patterns from the sensor. The sensitivity and accuracy of the sensor was 30 nm/mmHg and ±0.2 mmHg, respectively. Ten weeks post-implantation, the two NZW rabbits continued to respond well to the implant with no observable inflammation, signs of infection, or biofouling. All IOP measurements were obtained using the portable DSLR handheld reader. Successful in vivo studies demonstrate biocompatibility of the IOP sensor and prove feasibility of the IOP measurement system. The system has the potential to be used in both clinical and patient point-of-care (home) settings to frequently and accurately measure pressure.

Tunable, Ultrasensitive, and Flexible Pressure Sensors Based on Wrinkled Microstructures for Electronic Skins

Flexible pressure sensors play an important role in electronic skins (E-Skins), which mimic the mechanical forces sensing properties of human skin. A rational design for a pressure sensor with adjustable characteristics is in high demand for different application scenarios. Here, we present tunable, ultrasensitive, and flexible pressure sensors based on compressible wrinkled microstructures. Modifying the morphology of polydimethylsiloxane (PDMS) microstructure enables the device to obtain different sensitivities and pressure ranges for different requirements. Furthermore, by intentionally introducing hollow structures in the PDMS wrinkles, our pressure sensor exhibits an ultrahigh sensitivity of 14.268 kPa^-1. The elastic microstructure-based capacitive sensor also possesses a very low detectable pressure limit (1.5 Pa), a fast response time (<50 ms), a wide pressure range, and excellent cycling stability. Implementing respiratory monitoring and vocalization recognition is realized by attaching the flexible pressure sensor onto the chest and throat, respectively, showing its great application potential for disease diagnosis, monitoring, and other advanced clinical/biological wearable technologies.

A Wrinkled Ag/CNTs-PDMS Composite Film for a High-Performance Flexible Sensor and Its Applications in Human-Body Single Monitoring

In this paper, a flexible Ag/CNTs-PDMS (polydimethylsi-loxane) composite film sensor based on the novel design philosophy was prepared. Its force-electric effect mechanism is based on the generation of micro-cracks in the Ag film during external forcing, leading to resistance variation. Experimental results find that Ag film thickness has a strong influence on the sensor's sensitivity, which exhibits a tendency of first increasing and then decreasing the Ag film thickness, and also has an optimal thickness of 4.9 μm for the maximum sensitivity around 30. The sensitive mechanism can be theoretically explained by using the quantum tunneling effect. Due to the use of the wrinkled carbon nanotubes (CNTs) film, this sensor has advantages, such as high sensitivity, large strain range, good stability and durability, cheap price, and suitability for large-scale production. Preliminary applications on human-body monitoring reveal that the sensor can detect weak tremors and breathe depth and rate, and the corresponding heartbeat response. It provides possibilities to diagnose early Parkinson's disease and exploit an early warning system for sudden infant death syndrome and sleep apnea in adults. In addition, as a force-electric effect sensor, it is expected to have broad application areas, such as a man-machine cooperation, and a robotic system.

Bioresorbable Electronic Implants: History, Materials, Fabrication, Devices, and Clinical Applications

Medical implants, either passive implants for structural support or implantable devices with active electronics, have been widely used for the diagnosis and treatment of various diseases and clinical issues. These implants offer various functions, including mechanical support of biological structures in orthopedic and dental applications, continuous electrophysiological monitoring and feedback of electrical stimulation in neuronal and cardiac applications, and controlled drug delivery while maintaining arterial structure in drug-eluting stents. Although these implants exhibit long-term biocompatibility, surgery for their retrieval is often required, which imposes physical, biological, and economical burdens on the patients. Therefore, as an alternative to such secondary surgeries, bioresorbable implants that disappear after a certain period of time inside the body, including bioresorbable active electronics, have been highlighted recently. This review first discusses the historical background of medical implants and briefly define related terminology. Representative examples of non-degradable medical implants for passive structural support and/or for diagnosis and therapy with active electronics are also provided. Then, recent progress in bioresorbable active implants composed of biosignal sensors, actuators for therapeutics, wireless power supply components, and their integrated systems are reviewed. Finally, clinical applications of these bioresorbable electronic implants are exemplified with brief conclusion and future outlook.

High-resolution, reconfigurable printing of liquid metals with three-dimensional structures

We report an unconventional approach for high-resolution, reconfigurable 3D printing using liquid metals for stretchable, 3D integrations. A minimum line width of 1.9 μm can be reliably formed using direct printing, and printed patterns can be reconfigured into diverse 3D structures with maintaining pristine resolutions. This reconfiguration can be performed multiple times, and it also generates a thin oxide interface that can be effective in preventing the spontaneous penetration of gallium atoms into different metal layers while preserving electrical properties under ambient conditions. Moreover, these free-standing features can be encapsulated with stretchable, conformal passivations. We demonstrate applications in the form of a reconfigurable antenna, which is tunable by changing geometeries, and reversibly movable interconnections used as mechanical switches. The free-standing 3D structure of electrodes is also advantageous for minimizing the number and space between interconnections, which is important for achieving higher integrations, as demonstrated in an array of microLEDs.

Evolution of Wearable Devices with Real-Time Disease Monitoring for Personalized Healthcare

Wearable devices are becoming widespread in a wide range of applications, from healthcare to biomedical monitoring systems, which enable continuous measurement of critical biomarkers for medical diagnostics, physiological health monitoring and evaluation. Especially as the elderly population grows globally, various chronic and acute diseases become increasingly important, and the medical industry is changing dramatically due to the need for point-of-care (POC) diagnosis and real-time monitoring of long-term health conditions. Wearable devices have evolved gradually in the form of accessories, integrated clothing, body attachments and body inserts. Over the past few decades, the tremendous development of electronics, biocompatible materials and nanomaterials has resulted in the development of implantable devices that enable the diagnosis and prognosis through small sensors and biomedical devices, and greatly improve the quality and efficacy of medical services. This article summarizes the wearable devices that have been developed to date, and provides a review of their clinical applications. We will also discuss the technical barriers and challenges in the development of wearable devices, and discuss future prospects on wearable biosensors for prevention, personalized medicine and real-time health monitoring.

Clinical assessment of a non-invasive wearable MEMS pressure sensor array for monitoring of arterial pulse waveform, heart rate and detection of atrial fibrillation

There is an unmet clinical need for a low cost and easy to use wearable devices for continuous cardiovascular health monitoring. A flexible and wearable wristband, based on microelectromechanical sensor (MEMS) elements array was developed to support this need. The performance of the device in cardiovascular monitoring was investigated by (i) comparing the arterial pressure waveform recordings to the gold standard, invasive catheter recording (n = 18), (ii) analyzing the ability to detect irregularities of the rhythm (n = 7), and (iii) measuring the heartrate monitoring accuracy (n = 31). Arterial waveforms carry important physiological information and the comparison study revealed that the recordings made with the wearable device and with the gold standard device resulted in almost identical (r = 0.9–0.99) pulse waveforms. The device can measure the heart rhythm and possible irregularities in it. A clustering analysis demonstrates a perfect classification accuracy between atrial fibrillation (AF) and sinus rhythm. The heartrate monitoring study showed near perfect beat-to-beat accuracy (sensitivity = 99.1%, precision = 100%) on healthy subjects. In contrast, beat-to-beat detection from coronary artery disease patients was challenging, but the averaged heartrate was extracted successfully (95% CI: −1.2 to 1.1 bpm). In conclusion, the results indicate that the device could be useful in remote monitoring of cardiovascular diseases and personalized medicine.

Fabrication of highly pressure-sensitive, hydrophobic, and flexible 3D carbon nanofiber networks by electrospinning for human physiological signal monitoring

Three-dimensional (3D) porous nanostructure materials have promising applications in pressure sensors or other situations. However, the low sensing sensitivity of these materials restricts precise detection of physiological signals, and it is still a challenge to manufacture highly pressure-sensitive materials, which simultaneously possess other versatile properties. Herein, a simple and cost-efficient strategy is proposed to fabricate versatile 3D carbon nanofiber networks (CNFNs) with superior pressure-sensitivity through electrospinning and thermal treatment. The pressure sensitivity of the CNFNs is 1.41 kPa-1, which is much higher than that of similar 3D porous materials. Unlike traditional carbonaceous materials, the CNFNs exhibit excellent flexibility, stable resilience, and super compressibility (>95%), because of the nano-reinforce of Al2O3. Benefiting from the robust mechanical and piezoresistive properties of the CNFNs, a pressure sensor designed with the CNFNs is able to monitor human physiological signals, such as phonation, pulse, respiration and human activities. An arch-array platform for direction identification of tangential forces and an artificial electronic skin bioinspired by human's hairy skin have been ingeniously designed. The CNFNs also present other versatile characteristics as well, including ultralight density, hydrophobicity, low thermal conductivity, and low infrared emissivity. Therefore, the CNFNs have promising potential in a wide range of applications.

Untangling Aging Using Dynamic, Organism-Level Phenotypic Networks

Research on aging requires the ability to measure aging, and therein lies a challenge: it is impossible to measure every molecular, cellular, and physiological change that develops over time, but it is difficult to prioritize phenotypes for measurement because it is unclear which biological changes should be considered aspects of aging and, further, which species and environments exhibit "real aging." Here, I propose a strategy to address this challenge: rather than classify phenotypes as "real aging" or not, conceptualize aging as the set of all age-dependent phenotypes and appreciate that this set and its underlying mechanisms may vary by population. Use automated phenotyping technologies to measure as many age-dependent phenotypes as possible within individuals over time, prioritizing organism-level (i.e., physiological) phenotypes in order to enrich for health relevance. Use those high-dimensional phenotypic data to construct dynamic networks that allow aging to be studied with unprecedented sophistication and rigor.

Skin-inspired flexible and high-sensitivity pressure sensors based on rGO films with continuous-gradient wrinkles

Flexible electronic devices have received more and more attention. In particular, for pressure sensors, traditional methods for improving sensing performances are mostly based on the construction of microstructured templates. However, it still remains significantly challenging to conveniently fabricate thin-film sensors which possess flexibility, high sensitivity and location detection ability. Inspired by the microstructure of the human skin surface, herein, a new pressure sensor with a hierarchical structure and gradient reduced graphene oxide (rGO) wrinkles is reported. Benefiting from the skin-like structure, the pressure sensor demonstrates outstanding sensitivity, reaching 178 kPa-1; it can also detect pressure as small as 42 Pa. Furthermore, the concept of designing and constructing a gradient structure has been applied to achieve position detection, which is expected to find practical applications in human motion detection.

Electronic and Thermal Properties of Graphene and Recent Advances in Graphene Based Electronics Applications

Recently, graphene has been extensively researched in fundamental science and engineering fields and has been developed for various electronic applications in emerging technologies owing to its outstanding material properties, including superior electronic, thermal, optical and mechanical properties. Thus, graphene has enabled substantial progress in the development of the current electronic systems. Here, we introduce the most important electronic and thermal properties of graphene, including its high conductivity, quantum Hall effect, Dirac fermions, high Seebeck coefficient and thermoelectric effects. We also present up-to-date graphene-based applications: optical devices, electronic and thermal sensors, and energy management systems. These applications pave the way for advanced biomedical engineering, reliable human therapy, and environmental protection. In this review, we show that the development of graphene suggests substantial improvements in current electronic technologies and applications in healthcare systems.

Hierarchical Reduced Graphene Oxide Ridges for Stretchable, Wearable, and Washable Strain Sensors

Recently, flexible and wearable devices are increasingly in demand and graphene has been widely used due to its exceptional chemical, mechanical and electrical properties. Building complex buckling patterns of graphene is an essential strategy to increase its flexible and stretchable properties. Herein, a facile dimensionally controlled four-dimensional (4D) shrinking method was proposed to generate hierarchical reduced graphene oxide (rGO) buckling patterns on curved substrates mimicking different parts of the uniforms. The reduced graphene oxide ridges (rGORs) generated on the spherical substrate seem isotropic, while those generated on the cylindrical substrate are obviously more hierarchical or oriented, especially when the cylindrical substrate are shrinking via two steps. The oriented rGORs are superhydrophobic and strain sensitive but obviously anisotropic along the axial and circumferential directions. The sensitivity of rGORs along the axial direction is much higher than those along the circumferential direction. In addition, the intrinsic solvent barrier property of graphene enables the crack-free rGORs an excellent chemical protective performance, withstanding DCM immersion for more than 2.5 h. The flexible rGORs-based strain sensors can be used to detect both large and subtle human motions and activities by achieving high sensitivity (maximum gauge factor up to 48), high unidirectional stretchability (300-530%), and ultrahigh areal stretchability (up to 2690%). Excellent durability was also demonstrated for human motion monitoring with resistance to hand rubbing, ultrasonic cleaning, machine washing, and chemical immersion.

Three-dimensionally printed pressure sensor arrays from hysteresis-less stretchable piezoresistive composites

In this study, we formulate three-dimensionally (3D) printable composite pastes employing electrostatically assembled-hybrid carbon and a polystyrene-polyisoprene-polystyrene tri-block copolymer elastomer for the fabrication of multi-stack printed piezoresistive pressure sensor arrays. To address a critical drawback of piezoresistive composite materials, we have developed a previously unrecognized strategy of incorporating a non-ionic amphiphilic surfactant, sorbitan trioleate, into composite materials. It is revealed that the surfactant with an appropriate amphiphilic property, represented by the hydrophilic-lipophilic balance (HLB) index of 1.8, allows for a reversible piezoresistive characteristic under a wide pressure range up to 30 kPa as well as a significant reduction of elastomer viscoelastic behavior. The 3D-printed pressure sensor arrays exhibit a sensitivity of 0.31 kPa⁻¹ in a linear trend, and it is demonstrated successfully that the position-addressable array device is capable of spatially detecting objects up to a pressure level of 22.1 kPa.

The pressure sensor array device was fabricated by the 3D multi-stacked printing technique using highly reversible composite materials comprising a non-ionic amphiphilic surfactant.

Methods for powering bioelectronic microdevices

Bioelectronic microdevices are an emerging class of biomedical devices miniaturized at the scale of a millimeter or less, which promise new capabilities for monitoring and treating human disease. Although rapid progress has been made in the sensing and actuation capabilities of microdevices, a major technological challenge remains in the way that these devices are powered within the body. In this review, we revisit the power requirements of microdevices, describe current methods for storing, transferring or harvesting energy in microdevices, provide an overview of emerging powering approaches and discuss the promise of microdevices in biomedicine.

Cytotoxicity and in Vitro Degradation Kinetics of Foundry-Compatible Semiconductor Nanomembranes and Electronic Microcomponents

Foundry-compatible materials and processing approaches serve as the foundations for advanced, active implantable microsystems that can dissolve in biofluids into biocompatible reaction products, with broad potential applications in biomedicine. The results reported here include in vitro studies of the dissolution kinetics and nanoscale bioresorption behaviors of device-grade thin films of Si, SiN _x, SiO₂, and W in the presence of dynamic cell cultures via atomic force microscopy and X-ray photoemission spectroscopy. In situ investigations of cell-extracellular mechanotransduction induced by cellular traction provide insights into the cytotoxicity of these same materials and of microcomponents formed with them using foundry-compatible processes, indicating potential cytotoxicity elicited by W at concentrations greater than 6 mM. The findings are of central relevance to the biocompatibility of modern Si-based electronics technologies as active, bioresorbable microsystems that interface with living tissues.

End-to-End Design of Efficient Ultrasonic Power Links for Scaling Towards Submillimeter Implantable Receivers

We present an analytical framework for optimizing the efficiency of ultrasonic wireless power links for implantable devices scaled down to sub-mm dimensions. Key design insights and tradeoffs are considered for various parameters including the operating frequency, the transmission depth, the size of the transmitter, the impedance and the aperture efficiency of the miniaturized receiver, and the interface between the receiver and the power recovery chain on the implant. The performance of spherically focused transducers as ultrasonic transmitters is analyzed to study the limits and the tradeoffs. Two optimization methods are presented: "Focal Peak" sets the focus of transducers at target depths, and "Global Maximum" maximizes the efficiency globally with off-focus operation. The results are also compared to phased array implementations. To investigate the efficiency of implants, miniaturized receivers made from single crystalline piezoelectric material, PMN-PT, are used as they have resonances in the derived optimal carrier frequency range (∼1-2 MHz). A methodology to achieve an efficient interface to the power electronics is then provided using an optogenetic stimulator as an example platform. The analytical results are verified through both simulations and measurements. Finally, an example ultrasonic link using a spherical transmitter with a radius of 2 cm is demonstrated; link efficiencies of 1.93-0.23% are obtained at 6-10 cm depths with sub-mm receivers for the optogenetic application.

An LC Passive Wireless Gas Sensor Based on PANI/CNT Composite

This paper proposes a wireless passive gas sensor based on the principle of LC mutual coupling. After the acidification of the carbon nanotube (CNT), the in-situ polymerization of the aminobenzene monomers was conducted on the surface of the acidified CNT to form a sensitive material composed of a polyaniline/carbon nanotube (PANI/CNT) composite. The Advanced Design System (ADS) software was used for simulating and analyzing the designed structure, which obtained the various parameters of the structure. A lead-free aluminum paste was printed on an alumina ceramic substrate via the screen printing technique to form an inductor coil, before the gas sensitive material was applied to prepare a wireless passive gas sensor, consisting of a single-turn inductor and interdigitated electrodes on the base structure. Finally, an experimental platform was built to test the performance of the sensor. The sensitivity of the gas sensor is about 0.04 MHz/ppm in an atmosphere with a NH₃ concentration of 300 ppm. The sensor was shown to have good repeatability and high stability over a long time period.

Flexible THV/COC Piezoelectret Nanogenerator for Wide-Range Pressure Sensing

Flexible pressure sensors possess promising applications in artificial electronic skin, intelligence robot, wearable health monitoring, flexible physiological signal sensing, etc. Herein, we design a flexible pressure sensor with robust stability, high sensitivity, and large linear pressure region on the basis of tetrafluoroethylene-hexafluoropropylene-vinylide (THV)/cyclic olefin copolymer (COC) piezoelectret nanogenerator. According to the theoretical analysis for piezoelectret nanogenerators with imbalanced charge distribution, THV and COC are utilized to promote the electric field inside the piezoelectret for output voltage enhancement. Meanwhile, the compression property of the piezoelectret nanogenerator is facilely tuned. Owing to high inner electric field and optimized compression property, the THV/COC piezoelectret nanogenerator exhibits a high sensitivity of 30 mV/kPa, which is 10 times higher than that of the traditional cellular polypropylene piezoelectret. Simultaneously, the linear pressure region reaches 150 kPa with excellent linearity ( R² = 0.99963). The device is demonstrated to realize wearable pressure sensing with a wide pressure range from finger typing to fist hammering. This study presents a fabrication strategy for piezoelectret nanogenerators with high sensitivity and large linear pressure region, paving the way for development of wearable and flexible pressure sensing networks.