All-Nanofibrous Ionic Capacitive Pressure Sensor for Wearable Applications

Recent advances in wearable flexible electronic skin: types, power supply methods, and development prospects

In recent years, wearable e-skin has emerged as a prominent technology with a wide range of applications in healthcare, health surveillance, human-machine interface, and virtual reality. Inspired by the properties of human skin, arrayed wearable e-skin is a novel technology that offers multifunctional sensing capabilities. It can detect and quantify various stimuli, mimicking the human somatosensory system, and record a wide range of physical and physiological parameters in real time. By combining flexible electronic device units with a data acquisition system, specific functional sensors can be distributed in targeted areas to achieve high sensitivity, resolution, adjustable sensing range, and large-area expandability. This review provides a comprehensive overview of recent advances in wearable e-skin technology, including its development status, types of applications, power supply methods, and prospects for future development. The emphasis of current research is on enhancing the sensitivity and stability of sensors, improving the comfort and reliability of wearable devices, and developing intelligent data processing and application algorithms. This review aims to serve as a scientific reference for the intelligent development of wearable e-skin technology.

Ultraconformable Capacitive Strain Sensor Utilizing Network Structure of Single-Walled Carbon Nanotubes for Wireless Body Sensing

Capture and real-time recording of precise body movements using strain sensors provide personal information for healthcare monitoring and management. To acquire this information, a sensor that conforms to curved irregular surfaces, including biological tissue, is desired to record complex body movements while acting like a second skin to avoid interference with the movements. In this study, we developed a thin-film-type capacitive strain sensor that is flexible and stretchable on the surface of a living body. We fabricated conductive polymeric ultrathin films ("nanosheets") comprising polystyrene-block-polybutadiene (SB) elastomers and single-walled carbon nanotubes (SWCNTs) (i.e., SWCNT-SB nanosheets) via gravure coating; the SWCNT-SB-coated nanosheets were used as the flexible electrode in a capacitive strain sensor. The dielectric (DE) layer was then prepared using the silicone elastomer Ecoflex 00-30 because its Young's modulus is comparable to that of the epidermis. The normalized capacitance changes (ΔC/C0) in the sensor increased with increasing tensile strain over a range from 0-100%, indicating that the proposed sensor can measure the strain of biological movements, including those of skin and blood vessels. To improve sensor conformability further, the effect of sensor thickness on the gauge factor (GF) was investigated using thinner DE layers by focusing on their flexural rigidity. As a result, the GF increased from 0.64 to 1.13 as the DE layer thickness decreased from 260 to 40 μm. Finally, we evaluated the fabricated sensor's signal stability and mechanical durability, including during wireless sensing when applied to human skin and a vascular model. The ΔC/C0 values varied in response to the bending motion of a finger, dilation of a blood vessel, and the swallowing movement of the throat. These results indicate that our capacitive strain sensor is conformable and functional on biological tissue to enable monitoring of dynamic biological movements (e.g., pulse rate and arterial dilation) without wearer discomfort.

Multi-Level Pyramidal Microstructure-Based Pressure Sensors with High Sensitivity and Wide Linear Range for Healthcare Monitoring

Flexible pressure sensors have garnered significant attention in the field of wearable healthcare due to their scalability and shape variability. However, a crucial challenge in their practical application for various healthcare scenarios is striking a balance between the sensitivity and sensing range. This limitation arises from the reduced compressibility of the microstructures on the surface of pressure-sensitive materials under high pressure, resulting in progressive saturation of the sensor's response and leading to a restricted and nonlinear pressure sensing range. In this study, we present a novel approach utilizing multi-level pyramidal microstructures in flexible pressure sensors to achieve both high sensitivity (8775 kPa-1) and linear response (R2 = 0.997) over a wide pressure range (up to 1000 kPa). The effectiveness of the proposed design stems from the compensatory behavior of the lower pyramidal microstructures, which counteracts the declining sensitivity associated with the gradual hardening of the higher pyramidal microstructures. Furthermore, the sensor demonstrates a fast response time of 11.6 ms and a fast relaxation time of 3.8 ms and can reliably detect pressures as low as 30.2 Pa. Our findings highlight the applicability of this flexible pressure sensor in diverse human body health detection tasks, ranging from weak pulses to finger flexion and plantar pressure distribution. Notably, the proposed sensor design eliminates the need for replacing flexible pressure sensors with varying ranges, thereby enhancing their practical utility.

Sensitive, Robust, Wide-Range, and High-Consistency Capacitive Tactile Sensors with Ordered Porous Dielectric Microstructures

Flexible capacitive tactile sensors show great promise in personalized healthcare monitoring and human-machine interfaces, but their practical application is normally hindered because they rarely possess the required comprehensive performance, that is, high pressure sensitivity and fast response within a broad pressure range, high structure robustness, performance consistency, etc. This paper aims to engineer flexible capacitive pressure sensors with highly ordered porous dielectric microstructures and a 3D-printing-based fully solution-processable fabrication process. The proposed dielectric layer with uniformly distributed interior microporous can not only increase its compressibility and dynamic response within an extended pressure range but also enlarge its contact area with electrodes, contributing to a simultaneous improvement in the sensitivity, response speed, detection range, and structure robustness. Meanwhile, owing to its superior abilities in complex structure manufacturing and dimension controlling, the proposed 3D-printing-based fabrication process enables the consistent fabrication of the porous microstructure and thus guarantees device consistency. As a result, the prepared pressure sensors exhibit a high sensitivity of 0.21 kPa-1, fast response and relaxation times of 112 and 152 ms, an interface bonding strength of more than 455.2 kPa, and excellent performance consistency (≤5.47% deviation among different batches of sensors) and tunability. Encouraged by this, the pressure sensor is further integrated with a wireless readout circuit and realizes wireless wearable monitoring of various biosignals (pulse waves and heart rate) and body movements (from slight finger touch to large knee bending). Finally, the influence law of the feature parameters of the porous microstructure on device performance is established by the finite element method, paving the way for sensor optimization. This study motivates the development of flexible capacitive pressure sensors toward practical application.

Strain Sensors Made of MXene, CNTs, and TPU/PSF Asymmetric Structure Films with Large Tensile Recovery and Applied in Human Health Monitoring

Designing flexible wearable sensors with a wide sensing range, high sensitivity, and high stability is a vulnerable research direction with a futuristic field to study. In this paper, Ti3C2Tx MXene/carbon nanotube (CNT)/thermoplastic polyurethane (TPU)/polysulfone (PSF) composite films with excellent sensor performance were obtained by self-assembly of conductive fillers in TPU/PSF porous films with an asymmetric structure through vacuum filtration, and the porous films were prepared by the phase inversion method. The composite films consist of the upper part with finger-like "cavities" filled by MXene/CNTs, which reduces the microcracks in the conductive network during the tensile process, and the lower part has smaller apertures of a relatively dense resin cortex assisting the recovery process. The exclusive layer structure of the MXene/CNTs/TPU/PSF film sensor, with a thickness of 46.95 μm, contains 0.0339 mg/cm2 single-walled carbon nanotubes (SWNTs) and 0.348 mg/cm2 MXene only, providing functional range (0-80.7%), high sensitivity (up to 1265.18), and excellent stability and durability (stable sensing under 2300 fatigue tests, viable to the initial resistance), endurably cycled under large strains with serious damage to the conductive network. Finally, the MXene/CNTs/TPU/PSF film sensor is usable for monitoring pulse, swallow, tiptoe, and various joint bends in real time and distributing effective electrical signals. This paper implies that the MXene/CNTs/TPU/PSF film sensor has broad prospects in pragmatic applications.

Microstructured Polyelectrolyte Elastomer-Based Ionotronic Sensors with High Sensitivities and Excellent Stability for Artificial Skins

High-performance flexible pressure sensors are highly demanded for artificial tactile sensing. Using ionic conductors as the dielectric layer has enabled ionotronic pressure sensors with high sensitivities owing to giant capacitance of the electric double layer (EDL) formed at the ionic conductor/electronic conductor interface. However, conventional ionotronic sensors suffer from leakage, which greatly hinders long-term stability and practical applications. Herein, a leakage-free polyelectrolyte elastomer as the dielectric layer for ionotronic sensors is synthesized. The mechanical and electrical properties of the polyelectrolyte elastomer are optimized, a micropyramid array is constructed, and it is used as the dielectric layer for an ionotronic pressure sensor with marked performances. The obtained sensor exhibits a sensitivity of 69.6 kPa-1 , a high upper detecting limit on the order of 1 MPa, a fast response/recovery speed of ≈6 ms, and excellent stability under both static and dynamic loads. Notably, the sensor retains a high sensitivity of 4.96 kPa-1 at 500 kPa, and its broad sensing range within high-pressure realm enables a brand-new coding strategy. The applications of the sensor as a wearable keyboard and a quasicontinuous controller for a robotic arm are demonstrated. Durable and highly sensitive ionotronic sensors potentialize high-performance artificial skins for soft robots, human-machine interfaces, and beyond.

Flexible Capacitive Pressure Sensor with High Sensitivity and Wide Range Based on a Cheetah Leg Structure via 3D Printing

Flexible pressure sensors can be used in human-computer interaction and wearable electronic devices, but one main challenge is to fabricate capacitive sensors with a wide pressure range and high sensitivity. Here, we designed a capacitive pressure sensor based on a bionic cheetah leg microstructure, validated the benefits of the bionic microstructure design, and optimized the structural feature parameters using 3D printing technology. The pressure sensor inspired by the cheetah leg shape has a high sensitivity (0.75 kPa-1), a wide linear sensing range (0-280 kPa), a fast response time of roughly 80 ms, and outstanding durability (24,000 cycles). Furthermore, the sensor can recognize a finger-operated mouse, monitor human motion, and transmit Morse code information. This work demonstrates that bionic capacitive pressure sensors hold considerable promise for use in wearable devices.

Integration of Capacitive Pressure Sensor-on-Chip with Lead-Free Perovskite Solar Cells for Continuous Health Monitoring

The increasing prevalence of hypertension necessitates continuous blood pressure monitoring. This can be safely and painlessly achieved using non-invasive wearable electronic devices. However, the integration of analog, digital, and power electronics into a single system poses significant challenges. Therefore, we demonstrated a comprehensive multi-scale simulation of a sensor-on-chip that was based on a capacitive pressure sensor. Two analog interfacing circuits were proposed for a full-scale operation ranging from 0 V to 5 V, enabling efficient digital data processing. We also demonstrated the integration of lead-free perovskite solar cells as a mechanism for self-powering the sensor. The proposed system exhibits varying sensitivity from 1.4 × 10-3 to 0.095 (kPa)-1, depending on the pressure range of measurement. In the most optimal configuration, the system consumed 50.5 mW, encompassing a 6.487 mm2 area for the perovskite cell and a CMOS layout area of 1.78 × 1.232 mm2. These results underline the potential for such sensor-on-chip designs in future wearable health-monitoring technologies. Overall, this paper contributes to the field of wearable health-monitoring technologies by presenting a novel approach to self-powered blood pressure monitoring through the integration of capacitive pressure sensors, analog interfacing circuits, and lead-free perovskite solar cells.

A Dual-Responsive Artificial Skin for Tactile and Touchless Interfaces

The progress from intelligent interactions and supplemented/augmented reality requires artificial skins to shift from the single-functional tactile paradigm. Dual-responsive sensors that can both detect pre-contact proximal events and tactile pressure levels enrich the perception dimensions and deliver additional cognitive information. Previous dual-responsive sensors show very limited utilizations only in proximity perception or approaching switches. Whereas, the approaching inputs from the environment should be able to convey more valuable messages. Herein, a flexible iontronic dual-responsive artificial skin is present. The artificial skin is sensitive to external object's applied pressure as well as its approaching, and can elicit information of target material categories encoded in the proximal inputs. Versatile applications are then demonstrated. Dual-mode human-machine interfaces are developed based on the devices, including a manipulation of virtual game characters, navigation and zooming in of electronic maps, and scrolling through electronic documents. More importantly, the proof-of-concept application of an entirely touchless material classification system is demonstrated. Three types of materials (metals, polymers, and human skins) are classified and predicted accurately. These features of the artificial skin make it highly promising for next-generation smart engineered electronics.

Recent progress in flexible pressure sensors based on multiple microstructures: from design to application

Flexible pressure sensors (FPSs) have been widely studied in the fields of wearable medical monitoring and human-machine interaction due to their high flexibility, light weight, sensitivity, and easy integration. To better meet these application requirements, key sensing properties such as sensitivity, linear sensing range, pressure detection limits, response/recovery time, and durability need to be effectively improved. Therefore, researchers have extensively and profoundly researched and innovated on the structure of sensors, and various microstructures have been designed and applied to effectively improve the sensing performance of sensors. Compared with single microstructures, multiple microstructures (MMSs) (including hierarchical, multi-layered and hybrid microstructures) can improve the sensing performance of sensors to a greater extent. This paper reviews the recent research progress in the design and application of FPSs with MMSs and systematically summarizes the types, sensing mechanisms, and preparation methods of MMSs. In addition, we summarize the applications of FPSs with MMSs in the fields of human motion detection, health monitoring, and human-computer interaction. Finally, we provide an outlook on the prospects and challenges for the development of FPSs.

Conjugated Polymer-Based Nanocomposites for Pressure Sensors

Flexible sensors are the essential foundations of pressure sensing, microcomputer sensing systems, and wearable devices. The flexible tactile sensor can sense stimuli by converting external forces into electrical signals. The electrical signals are transmitted to a computer processing system for analysis, realizing real-time health monitoring and human motion detection. According to the working mechanism, tactile sensors are mainly divided into four types-piezoresistive, capacitive, piezoelectric, and triboelectric tactile sensors. Conventional silicon-based tactile sensors are often inadequate for flexible electronics due to their limited mechanical flexibility. In comparison, polymeric nanocomposites are flexible and stretchable, which makes them excellent candidates for flexible and wearable tactile sensors. Among the promising polymers, conjugated polymers (CPs), due to their unique chemical structures and electronic properties that contribute to their high electrical and mechanical conductivity, show great potential for flexible sensors and wearable devices. In this paper, we first introduce the parameters of pressure sensors. Then, we describe the operating principles of resistive, capacitive, piezoelectric, and triboelectric sensors, and review the pressure sensors based on conjugated polymer nanocomposites that were reported in recent years. After that, we introduce the performance characteristics of flexible sensors, regarding their applications in healthcare, human motion monitoring, electronic skin, wearable devices, and artificial intelligence. In addition, we summarize and compare the performances of conjugated polymer nanocomposite-based pressure sensors that were reported in recent years. Finally, we summarize the challenges and future directions of conjugated polymer nanocomposite-based sensors.