A Self‐Healable, Highly Stretchable, and Solution Processable Conductive Polymer Composite for Ultrasensitive Strain and Pressure Sensing

Flexible piezoresistive pressure sensors have great application potential in many emerging fields such as electronic skin, artificial intelligence and human healthcare. However, most of the sensors are flammable, which will easily initiate fire hazard in case of short circuit. Herein, a flame‐retardant and conductive cotton fabric (CF) was fabricated for piezoresistive pressure sensor by layer‐by‐layer (LBL) assembly of branched polyethyleneimine (b‐PEI) modified halloysite nanotubes (HNTs), phytic acid (PA) and graphene oxide (GO) and the subsequent thermal reduction of GO. The composite coating layer endowed the obtained CF with excellent flame retardancy. The limiting oxygen index of the CF reached 32.72% and the char length after vertical burning test was only 3.8 cm, attributing to the physical barrier role of the HNTs and graphene nanosheets as well as the phosphorus‐nitrogen synergism between PA and b‐PEI. The CF‐based piezoresistive pressure sensor exhibited fast and accurate response, good signal stability and fatigue resistance (3000 loading‐unloading cycles) and was able to be applied for detecting human motions including pulse beat, pronunciation and various physical activities. The preparation method in this work is simple, and the piezoresistive pressure sensor shows great application prospect in the field of flexible electronics with high safety.

A Wavy-Structured Highly Stretchable Thermoelectric Generator with Stable Energy Output and Self-Rescuing Capability

The use of polymers to fabricate flexible pressure sensors as an alternative to conventional pressure sensors has led to the development of physiological monitoring of human body and the electronic skin. In particular, the fabrication of flexible capacitive and piezoresistive sensors using a variety of materials and the investigation of their electromechanical properties are further developments in these fields. Herein, parylene C is synthesized via chemical vapor deposition method. Pressure-sensitive inks are prepared with a composite of parylene C, polyurethane, polymethylmethacrylate, and activated carbon at certain weight ratios. Flexible capacitive and piezoresistive pressure sensors are fabricated by the screen printing method. The sensitivity, detection limit, linearity range, and response/relaxation time, which define the capacitive and piezoresistive properties are investigated and presented in this paper. The sensitivities of the flexible capacitive and piezoresistive pressure sensors are 0.124 kPa−1 and 0.074 kPa−1 in the pressure range of 0.07–1.39 kPa. This study enables parylene C to be used in the composite structure and shows that it can be used not only as a protective layer but also in flexible pressure sensor applications. It also ensures that the design of the flexible capacitance pressure sensor can measure low pressure with high sensitivity compared to the flexible piezoresistive pressure sensor.

A comparative study on piezoelectric and piezoresistive pressure sensor using COMSOL simulation

In the past several years, wearable pressure sensors have engendered a new surge of interest worldwide because of their important applications in the areas of health monitoring, electronic skin, and smart robots. However, it has been a great challenge to simultaneously achieve a wide pressure-sensing range and high sensitivity for the sensors until now. Herein, we proposed an innovative strategy to construct multilayer-structure piezoresistive pressure sensors with an in situ generated thiolated graphene@polyester (GSH@PET) fabric via the one-pot method. Taking advantage of the spacing among the rough fabric layers and the highly conductive GSH, the sensor realized not only a wide pressure range (0-200 kPa), but also high sensitivity (8.36 and 0.028 kPa-1 in the ranges of 0-8 and 30-200 kPa, respectively). After 500 loading-unloading cycles, the sensor still kept high sensitivity and a stable response, exhibiting great potential in long-term practical applications. Importantly, the piezoresistive pressure sensor was successfully applied to accurately detect different human behaviors including pulse, body motion, and voice recognition. Additionally, the sensing network integrated by the sensors also realized mapping and identifying spatial pressure distribution. Our method to construct the wide-range and high-sensitivity piezoresistive pressure sensor is facile, cost-effective, and available for mass production. The findings provide a new direction to fabricate the new-generation high-performance sensors for healthcare, interactive wearable devices, electronic skin, and smart robots.

Flexible piezoresistive pressure sensors have been attracting wide attention for applications in health monitoring and human-machine interfaces because of their simple device structure and easy-readout signals. For practical applications, flexible pressure sensors with both high sensitivity and wide linearity range are highly desirable. Herein, a simple and low-cost method for the fabrication of a flexible piezoresistive pressure sensor with a hierarchical structure over large areas is presented. The piezoresistive pressure sensor consists of arrays of microscale papillae with nanoscale roughness produced by replicating the lotus leaf's surface and spray-coating of graphene ink. Finite element analysis (FEA) shows that the hierarchical structure governs the deformation behavior and pressure distribution at the contact interface, leading to a quick and steady increase in contact area with loads. As a result, the piezoresistive pressure sensor demonstrates a high sensitivity of 1.2 kPa-1 and a wide linearity range from 0 to 25 kPa. The flexible pressure sensor is applied for sensitive monitoring of small vibrations, including wrist pulse and acoustic waves. Moreover, a piezoresistive pressure sensor array is fabricated for mapping the spatial distribution of pressure. These results highlight the potential applications of the flexible piezoresistive pressure sensor for health monitoring and electronic skin.

The piezoresistive pressure sensor is one of the major applications of MEMS (Micro-Electro-Mechanical-System) devices. Nowadays, in the field of automotive electronics, silicon-based pressure sensors are playing a significant role in the control of brake, engine, tire pressure, etc. The piezoresistive based engine oil pressure sensor mentioned in this paper is applied to detect the pressure of lubricant of the automobile engine. We firstly study the influence of silicon oil on the offset of pressure sensor. Results show that the offset tends to be relatively large when the amount of the silicon oil increases. In order to minimize the zero offset, ceramic substrate is introduced to reduce the amount of the silicon oil. We also find that the cleanliness and stability of the silicon oil have a considerable impact on the offset. The fully cleaned silicon oil leads to a smaller offset, and few changes are found when temperature increases. The silicon oil with good stability can result in a stable offset even the packaged sensor stored at a high temperature for a long time. Then, the influence of the welding performance of TO base on the hermeticity of pressure sensor is investigated by experimental tests and finite element method (FEM) analysis. Results show that, compared with the iron, Kovar alloy has a better welding performance and the leakage rate of the sealed chamber is less than 10−8Pa.m3/s. FEM analysis fits well with the experimental tests.

Perforated diaphragms employed piezoresistive MEMS pressure sensor for sensitivity enhancement in gas flow measurement

Sensing materials with fiber structures are excellent candidates for the fabrication of flexible pressure sensors due to their large specific surface area and abundant contact points. Here, an ultrathin, flexible piezoresistive pressure sensor that consists of a multilayer nanofiber network structure prepared via a simple electrospinning technique is reported. The ultrathin sensitive layer is composite nanofiber films composed of poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) and polyamide 6 (PEDOT:PSS/PA6) prepared by simultaneous electrospinning. PEDOT:PSS conductive fibers and PA6 elastic fibers are interwoven to form a multilayer network structure that can achieve ultrahigh sensitivity by forming a wealth of contact points during loading. In particular, gold-deposited PA6 fibers as upper and lower flexible electrodes can effectively increase the initial resistance. Due to this special fiber electrode structure, the sensor is able to generate a large electrical signal variability when subjected to a weak external force. The devices with different sensing properties can be obtained by controlling the electrospinning time. The sensor based on the PEDOT:PSS/PA6 nanofiber network has high sensitivity (6554.6 kPa-1 at 0-1.4 kPa), fast response time (53 ms), and wide detection range (0-60 kPa). Significantly, the device maintains ultrahigh sensitivity when cyclically loaded over 10,000 cycles at 5 kPa, which makes it have great prospects for applications in human health monitoring and motion monitoring.

A package for piezoresistive pressure sensors with embedded built-in self-test function based on bimetallic actuator

Pressure measurement is a key part of many commercial and industrial systems. Piezoresistive pressure sensors are simpler to integrate with electronics, they are inherently shielded from RF noise and their response is more linear while compared to capacitive pressure sensors. And piezoresistive devices have always dominated the pressure sensor market. The analyticals that are typically used to model the diaphragm of the pressure sensor have been analysed by many researchers. To optimize the pressure sensor for parameters like linearity and sensitivity, the Finite Element Method (FEM) is incorporated. The selection of appropriate parameters of piezoresistors such as the shape and the position of the piezoresistor on the pressure sensor diaphragm, thickness of diaphragm are important. This study shows the scope of using analytical solutions and design techniques for a piezoresistive pressure sensor.

Porous microstructure pressure sensors that are highly sensitive, reliable, low-cost, and environment-friendly have aroused wide attention in intelligent biomedical diagnostics, human–machine interactions, and soft robots. Here, an all-tissue-based piezoresistive pressure sensor with ultrahigh sensitivity and reliability based on the bottom interdigitated tissue electrode and the top bridge of a microporous tissue/carbon nanotube composite was proposed. Such pressure sensors exhibited ultrahigh sensitivity (≈1911.4 kPa−1), fast response time (<5 ms), low fatigue of over 2000 loading/unloading cycles, and robust environmental degradability. These enabled sensors can not only monitor the critical physiological signals of the human body but also realize electrothermal conversion at a specific voltage, which enhances the possibility of creating wearable thermotherapy electronics for protecting against rheumatoid arthritis and cervical spondylosis. Furthermore, the sensor successfully transmitted wireless signals to smartphones via Bluetooth, indicating its potential as reliable skin-integrated electronics. This work provides a highly feasible strategy for promoting high-performance wearable thermotherapy electronics for the next-generation artificial skin.

The implementation of tactile functions in array sensors has created an urgent need for high-performance force sensors with both high linearity and sensitivity so as to ensure the uniformity and accuracy of acquisition of multiple sensing units in the array sensors. In addition, it still remains challenging to fabricate exquisite geometric constructions for integrating highly sensitive materials, extraordinary sensitivity, high stretchability, wide sensing range, and flexibility into a single type of pressure/strain sensor. Herein, a sensitive flexible sensor was fabricated that was composed of mixed-dimensional MXene-based nanocomposites as piezoresistive layers and Ecoflex layers as the encapsulation. The resultant sensors with MXene (Ti3C2Tx)-ANF (aramid nanofiber)-BP (black phosphorus)-gold (Au) nanocomposites exhibited excellent electrical conductivity, ultrahigh press sensitivity of 0.4986 kPa–1 (0–1 kPa), reliable linearity (R2 = 0.998), rapid response time of 12 ms, excellent durability over 4000 cycles, and ultrahigh tensile sensitivity of 9188.583. An 8 × 8 pixel electronic skin was fabricated by the as-prepared mixed-dimensional MXene-based sensors and tested in the identification of sliding movements and surface shape, and the good performance shows great potential applications in the fields of wearable devices, human–machine interaction, and robotics.

Flexible pressure sensors have attracted much attention because of their application prospects in wearable devices, electronic skin, and health monitoring. However, it is still difficult to obtain pressure sensors with excellent performance simply and efficiently. Indium tin oxide (ITO) is a widely used transparent conductive material, and polyvinyl alcohol (PVA) is an elastic insulating material with the advantages of easy processing and stable mechanical properties. ITO nano‐crystalline particles are dispersed into PVA polymers to form an ITO nanocrystalline‐PVA composite film with good electrical conductivity as a piezorestoresistive material, fabricating a pressure sensor with excellent performance. The pressure sensor possesses superior sensitivity (641.15 kPa−1), wide detection range (0–80 kPa), fast response time (10.93 ms), and recovery time (8.05 ms), and excellent stability (more than 1500 cycles). The pressure sensor shows excellent performance in a variety of applications, such as pulse testing, speech recognition, and detection of various joint movements of the human body (such as knees, fingers, elbows, etc.). The sensor has application prospects in health monitoring and motion perception.

We have developed an integrated piezoresistive pressure and temperature sensor for multiphase chemical reactors, primarily Kraft pulp digesters (pH 13.5, temperatures up to 175 °C, reaching a local maximum of 180 °C and pressures up to 2 MPa). The absolute piezoresistive pressure sensor consisted of a large square silicon diaphragm (1000 × 1000 µm2) and high resistance piezoresistors (10 000 Ω). A 4500 Ω buried piezoresistive wire was patterned on the silicon chip to form a piezoresistive temperature sensor which was used for pressure sensor compensation and temperature measurement. A 4 µm thick Parylene HT® coating, a chemically resistant epoxy and a silicone conformal coating were deposited to passivate the pressure sensor against the caustic environment in Kraft digesters. The sensors were characterized up to 2 MPa and 180 °C in an environment chamber. A maximum thermal error of ±0.72% full-scale output (FSO), an average sensitivity of 0.116 mV (V kPa)−1 and a power consumption of 0.3 mW were measured in the pressure sensor. The sensors' resistances were measured before and after test in a Kraft pulping cycle and showed no change in their values. SEM pictures and topographical surfaces were also analyzed before and after pulp liquor exposure and showed no observable changes.