Porous AgNWs/Poly(vinylidene fluoride) Composite-Based Flexible Piezoresistive Sensor with High Sensitivity and Wide Pressure Ranges

High-Sensitivity All-Fiber Sensor Smart Gloves for Hand Perception

Flexible piezoresistive sensors (FPSs) with high sensitivity and conformability are crucial for achieving fine operations in electronic gloves. In this work, a chemical grafting method is used to ensure strong interfacial bonding between the conductive phase and the flexible polyimide (PI) matrix with a high glass transition temperature (Tg). This design helps to tackle the stress relaxation and interfacial debonding problems commonly faced by FPSs. Silver fiber electrodes are prepared by in situ reduction of silver nanoparticles on PI fibers to further improve the sensitivity. This FPS is characterized by high sensitivity (214.6 kPa-1), low response time and recovery time (44 and 42 ms, respectively), outstanding recoverable performance (with a low hysteresis of 4.58% FS), and remarkable dynamic stability (a 3.6% decay of signal intensity after 24,000 cycles). An all-fiber flexible piezoresistive sensor array glove has been constructed to achieve conformal contact with the robot hand. Furthermore, comprehensive detection of multipoint pressures on the hand and high-sensitivity tactile perception for the robot hand have been achieved.

High-Sensitivity and Wide-Range Flexible Pressure Sensor Based on Gradient-Wrinkle Structures and AgNW-Coated PDMS

Flexible pressure sensors have garnered significant attention due to their wide range of applications in human motion monitoring and smart wearable devices. However, the fabrication of pressure sensors that offer both high sensitivity and a wide detection range remains a challenging task. In this paper, we propose an AgNW-coated PDMS flexible piezoresistive sensor based on a gradient-wrinkle structure. By modifying the microstructure of PDMS, the sensor demonstrates varying sensitivities and pressure responses across different pressure ranges. The wrinkle microstructure contributes to high sensitivity (0.947 kPa-1) at low pressures, while the PDMS film with a gradient contact height ensures a continuous change in the contact area through the gradual activation of the contact wrinkles, resulting in a wide detection range (10-50 kPa). This paper also investigates the contact state of gradient-wrinkle films under different pressures to further elaborate on the sensor's sensing mechanism. The sensor's excellent performance in real-time response to touch behavior, joint motion, swallowing behavior recognition, and grasping behavior detection highlights its broad application prospects in human-computer interaction, human motion monitoring, and intelligent robotics.

Multimodal response characteristics of convective liquid metal sensitive layers in flexible pressure sensor

The development of electronic skin, soft robots, and smart wearables has significantly driven advances in flexible pressure sensing technology. However, traditional multilayer solid-structure flexible pressure sensors encounter challenges at temperatures between 100 °C and 150 °C due to high-temperature modal distortion. Changes in the conductivity of the sensor's conductive components interfere with accurate pressure measurement. In this research, a flexible pressure sensor with a convective liquid metal sensitive layer is proposed. The sensor uses a cyclic self-cooling mechanism to lower the temperature of its conductive components, reducing the impact of external high temperatures on the pressure measurement accuracy. At a 2.8 W thermal load, the flexible sensor, with liquid metal circulating at 2.0 mL/min, exhibits a sensitivity of 0.11 kPa⁻¹ within the pressure range from 0 to 12.5 kPa, and its maximum measurable pressure is 30 kPa. In addition, the resistance of the sensor is 18.5 mΩ less than that of a stationary liquid metal sensor, representing a 38.1% reduction. The sensor proposed in this research introduces a novel strategy for pressure measurement in high-temperature applications, extending the application scope to aircraft, special robots, and hydraulic oil circuits.

Lightweight, ultra-compressed, and environmentally friendly wood/TPU aerogel sensor based on optimized performance of dynamic 3D pore structure

Although bio-based sensing materials have a wide range of applications in the field of pressure detection, they still need to improve their sensitivity, detection limit and hysteresis. This paper studied the relationship between the 3D pore structure and sensing performance under dynamics. Using Balsa wood as the substrate, CWA/TPU aerogel and its sensor were prepared with lightweight, compressibility, highly sensitivity, wide-detection, and low-hysteresis. Meanwhile, the brittleness problem of the carbonized aerogel was solved by uniformly attaching TPU to the aerogel interface. In this paper, the 3D structure of CWA/TPU aerogel during compression was reconstructed by Micro-XCT technology, and the results show that the sensitivity of the bio-based carbonized material is directly proportional to the porosity and inversely proportional to the aspect ratio. This CWA/TPU aerogel pressure sensor has a high sensitivity of 76.18 kPa-1 in a wide detection limit of 0.6 Pa-100 kPa, 90 % supercompression strain, ±7.4 % low hysteresis and outstanding stability over 10,000 cycles. And the sensor can detect different ranges of pressure strains and has great potential for future applications in physiological signal monitoring, action recognition, and sports training.

Graphene-Based, Flexible, Wearable Piezoresistive Sensors with High Sensitivity for Tiny Pressure Detection

Flexible, wearable, piezoresistive sensors have significant potential for applications in wearable electronics and electronic skin fields due to their simple structure and durability. Highly sensitive, flexible, piezoresistive sensors with the ability to monitor laryngeal articulatory vibration supply a new, more comfortable and versatile way to aid communication for people with speech disorders. Here, we present a piezoresistive sensor with a novel microstructure that combines insulating and conductive properties. The microstructure has insulating polystyrene (PS) microspheres sandwiched between a graphene oxide (GO) film and a metallic nanocopper-graphene oxide (n-Cu/GO) film. The piezoresistive performance of the sensor can be modulated by controlling the size of the PS microspheres and doping degree of the copper nanoparticles. The sensor demonstrates a high sensitivity of 232.5 kPa-1 in a low-pressure range of 0 to 0.2 kPa, with a fast response of 45 ms and a recovery time of 36 ms, while also exhibiting excellent stability. The piezoresistive performance converts subtle laryngeal articulatory vibration into a stable, regular electrical signal; in addition, there is excellent real-time monitoring capability of human joint movements. This work provides a new idea for the development of wearable electronic devices, healthcare, and other fields.

Piezoresistive Sensor Based on Porous Sponge with Superhydrophobic and Flame Retardant Properties for Motion Monitoring and Fire Alarm

Polyurethane sponge is frequently selected as a substrate material for constructing flexible compressible sensors due to its excellent resilience and compressibility. However, being highly hydrophilic and flammable, it not only narrows the range of use of the sensor but also poses a great potential threat to human safety. In this paper, a conductive flexible piezoresistive sensor (CHAP-PU) with superhydrophobicity and high flame retardancy was prepared by a simple dip-coating method using A-CNTs/HGM/ADP coatings deposited on the surface of a sponge skeleton and modified with polydimethylsiloxane. With great sensitivity and durability (>3000 cycles) as well as fast response/recovery time (152 ms/178 ms), the sensor is capable of monitoring human movement as a wearable device. The modified material surface has a hydrophobicity angle of 153°, which provides significant self-cleaning and weather resistance. Furthermore, the CHAP-PU sensor is able to respond stably to underwater movements. Importantly, when the sponge was directly exposed to an open flame, no flame spreading or dripping of molten material was detected, indicating excellent flame retardancy. Meanwhile, CHAP-PU was also equipped as a smart fire alarm system, and the results showed that an alarm signal was triggered within 2 s under flame erosion. Therefore, the flame-retardant superhydrophobic CHAP-PU sponge-based sensor shows great potential for human motion detection and fire alarm applications.

Flexible Pressure, Humidity, and Temperature Sensors for Human Health Monitoring

The rapid advancements in artificial intelligence, micro-nano manufacturing, and flexible electronics technology have unleashed unprecedented innovation and opportunities for applying flexible sensors in healthcare, wearable devices, and human-computer interaction. The human body's tactile perception involves physical parameters such as pressure, temperature, and humidity, all of which play an essential role in maintaining human health. Inspired by the sensory function of human skin, many bionic sensors have been developed to simulate human skin's perception to various stimuli and are widely applied in health monitoring. Given the urgent requirements for sensing performance and integration of flexible sensors in the field of wearable devices and health monitoring, here is a timely overview of recent advances in pressure, humidity, temperature, and multi-functional sensors for human health monitoring. It covers the fundamental components of flexible sensors and categorizes them based on different response mechanisms, including resistive, capacitive, voltage, and other types. Specifically, the application of these flexible tactile sensors in the area of human health monitoring is highlighted. Based on this, an extended overview of recent advances in dual/triple-mode flexible sensors integrating pressure, humidity, and temperature tactile sensing is presented. Finally, the challenges and opportunities of flexible sensors are discussed.

From materials to structures: a holistic examination of achieving linearity in flexible pressure sensors

As a pivotal category in the realm of electronics skins, flexible pressure sensors have become a focal point due to their diverse applications such as robotics, aerospace industries, and wearable devices. With the growing demands for measurement accuracy, data reliability, and electrical system compatibility, enhancing sensor's linearity has become increasingly critical. Analysis shows that the nonlinearity of flexible sensors primarily originates from mechanical nonlinearity due to the nolinear deformation of polymers and electrical nonlinearity caused by changes in parameters such as resistance. These nonlinearities can be mitigated through geometric design, material design or combination of both. This work reviews linear design strategies for sensors from the perspectives of structure and materials, covering the following main points: (a) an overview of the fundamental working mechanisms for various sensors; (b) a comprehensive explanation of different linear design strategies and the underlying reasons; (c) a detailed review of existing work employing these strategies and the achieved effects. Additionally, this work delves into diverse applications of linear flexible pressure sensors, spanning robotics, safety, electronic skin, and health monitoring. Finally, existing constraints and future research prospects are outlined to pave the way for the further development of high-performance flexible pressure sensors.

Flexible Tactile Sensors with Gradient Conformal Dome Structures

The trade-off between high sensitivity and wide detection range remains a challenge for flexible capacitive pressure sensors. Gradient structure can provide continuous deformation and lead to a wide sensing range. However, it simultaneously augments the distance between two electrodes, which diminishes the variation in the relative distance and results in a decreased sensitivity. Herein, a conformal design is introduced into the gradient structure to construct a flexible capacitive pressure sensor. The gradient conformal dome structure is fabricated by a simple reverse dome adsorption process. Taking advantage of the progressive deformation behavior of the gradient dielectric, and the significant improvement of relative distance variation between two electrodes from the conformal design, the sensor achieves a sensitivity of 0.214 kPa-1 in an ultrabroad linear range up to 200 kPa. It maintains high-pressure resolution under the preload of 10 and 100 kPa. Benefiting from the rapid response and excellent repeatability, the sensor can be used for physiological monitor and human motion detection, including arterial pulse, joint bending, and motion state. The gradient conformal design strategy may pave a promising avenue to develop pressure sensors with high sensitivity and wide linear range.

Silver Nanowire-Based Flexible Strain Sensor for Human Motion Detection

Accurately capturing human movements is a crucial element of health status monitoring and a necessary precondition for realizing future virtual reality/augmented reality applications. Flexible motion sensors with exceptional sensitivity are capable of detecting physical activities by converting them into resistance fluctuations. Silver nanowires (AgNWs) have become a preferred choice for the development of various types of sensors due to their outstanding electrical conductivity, transparency, and flexibility within polymer composites. Herein, we present the design and fabrication of a flexible strain sensor based on silver nanowires. Suitable substrate materials were selected, and the sensor's sensitivity and fatigue properties were characterized and tested, with the sensor maintaining reliability after 5000 deformation cycles. Different sensors were prepared by controlling the concentration of silver nanowires to achieve the collection of motion signals from various parts of the human body. Additionally, we explored potential applications of these sensors in fields such as health monitoring and virtual reality. In summary, this work integrated the acquisition of different human motion signals, demonstrating great potential for future multifunctional wearable electronic devices.

Underwater Gesture Recognition Meta-Gloves for Marine Immersive Communication

Rapid advancements in immersive communications and artificial intelligence have created a pressing demand for high-performance tactile sensing gloves capable of delivering high sensitivity and a wide sensing range. Unfortunately, existing tactile sensing gloves fall short in terms of user comfort and are ill-suited for underwater applications. To address these limitations, we propose a flexible hand gesture recognition glove (GRG) that contains high-performance micropillar tactile sensors (MPTSs) inspired by the flexible tube foot of a starfish. The as-prepared flexible sensors offer a wide working range (5 Pa to 450 kPa), superfast response time (23 ms), reliable repeatability (∼10000 cycles), and a low limit of detection. Furthermore, these MPTSs are waterproof, which makes them well-suited for underwater applications. By integrating the high-performance MPTSs with a machine learning algorithm, the proposed GRG system achieves intelligent recognition of 16 hand gestures under water, which significantly extends real-time and effective communication capabilities for divers. The GRG system holds tremendous potential for a wide range of applications in the field of underwater communications.

Recent Advances in Tactile Sensory Systems: Mechanisms, Fabrication, and Applications

Flexible electronics is a cutting-edge field that has paved the way for artificial tactile systems that mimic biological functions of sensing mechanical stimuli. These systems have an immense potential to enhance human-machine interactions (HMIs). However, tactile sensing still faces formidable challenges in delivering precise and nuanced feedback, such as achieving a high sensitivity to emulate human touch, coping with environmental variability, and devising algorithms that can effectively interpret tactile data for meaningful interactions in diverse contexts. In this review, we summarize the recent advances of tactile sensory systems, such as piezoresistive, capacitive, piezoelectric, and triboelectric tactile sensors. We also review the state-of-the-art fabrication techniques for artificial tactile sensors. Next, we focus on the potential applications of HMIs, such as intelligent robotics, wearable devices, prosthetics, and medical healthcare. Finally, we conclude with the challenges and future development trends of tactile sensors.

Highly Sensitive Pressure Sensor Based on Elastic Conductive Microspheres

Elastic pressure sensors play a crucial role in the digital economy, such as in health care systems and human-machine interfacing. However, the low sensitivity of these sensors restricts their further development and wider application prospects. This issue can be resolved by introducing microstructures in flexible pressure-sensitive materials as a common method to improve their sensitivity. However, complex processes limit such strategies. Herein, a cost-effective and simple process was developed for manufacturing surface microstructures of flexible pressure-sensitive films. The strategy involved the combination of MXene-single-walled carbon nanotubes (SWCNT) with mass-produced Polydimethylsiloxane (PDMS) microspheres to form advanced microstructures. Next, the conductive silica gel films with pitted microstructures were obtained through a 3D-printed mold as flexible electrodes, and assembled into flexible resistive pressure sensors. The sensor exhibited a sensitivity reaching 2.6 kPa-1 with a short response time of 56 ms and a detection limit of 5.1 Pa. The sensor also displayed good cyclic stability and time stability, offering promising features for human health monitoring applications.

High strength, self-healing sensitive ionogel sensor based on MXene/ionic liquid synergistic conductive network for human-motion detection

Ionogels with both high strength and high conductivity for wearable strain and pressure dual-mode sensors are needed for human motion and health monitoring. Here, multiple hydrogen bonds are introduced through imidazolidinyl urea (IU) as a chain extender to provide high mechanical and self-healing properties for the water-borne polyurethane (WPU). The MXene/ionic liquids synergistic conductive network provides excellent conductivity and also reduces the relative content of ionic liquids to maintain the mechanical properties of the ionogels. The mechanical strength of this ionogel reached 1.81-2.24 MPa and elongation at break reached 570-624%. It also has excellent conductivity (22.7-37.5 mS m-1), gauge factor (GF) (as a strain sensor, GF = 1.8), sensitivity (S) (as a press sensor, S1 = 29.8 kPa-1, S2 = 1.3 kPa-1), and fast response time (as a strain sensor = 185 ms; as a press sensor = 204 ms). The ionogel also exhibits rapid photothermal self-healing capabilities due to the inherent photothermal behavior of MXene. It can maintain good elasticity and conductivity at low temperatures. In addition, this ionogel is able to stretch for 1200 cycles without significant change in the relative change of resistance. The ionogel can be assembled as a strain sensor for monitoring human motion and as a pressure sensor array for obtaining pressure magnitude and position information.