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With the fast development of electronic skin, there is a great demand for wearable sensing devices. Herein, a piezoresistive sensor has been fabricated comprising a conductive film of Polydimethylsiloxane (PDMS)-carbon nanotubes (CNTs) sandwiched by two protection films of PDMS. A simple and low-cost manufacturing method has been proposed to wrap CNTs inside the flexible PDMS to obtain a soft sensor with excellent conductivity, high sensitivity, and fast response to various pressures. Additionally, films with different percentages of CNTs were compared. The results show that the film with 7% CNTs has higher conductivity and sensitivity. Moreover, the surface morphologies and microstructures of the superior PDMS-CNTs film were studied by scanning electron microscope, and sensing properties of the pressure sensor were tested. Furthermore, applications of single sensor and sensor array were studied. The results exhibit that the as-prepared pressure sensor successfully detected different human motions and recognized Morse code and words, indicating that the portable sensor can be potentially applied in wide areas such as smart devices and flexible robotics.

In this study, a porous polyurea/graphene platelet (GnP) composite film with high flexibility and multifunctionality was developed and investigated for strain sensing and energy storage applications. Morphology and mechanical properties were studied; tensile strength and Young’s modulus were enhanced by 132% and 900%, respectively at 5 wt% GnPs. The polyurea/GnP film exhibited high sensitivity to various strains– namely, tension, compression, bending and twisting. In tensile strain, gauge factor was 1.86 and 6.26 within strain ranges of 0–30% and 30–60%, respectively. Interestingly, the film showed piezoresistive and capacitive-type strain sensor under compression load; a 33% increase in relative resistance with response time of 108 millisecond; and 22% increase in relative capacitance with response time of 92 millisecond when load of 5 kPa was applied. Furthermore, the strain sensor showed high durability and reliability over 20,000 cycles. The developed polyurea/GnP film showed a stable capacitive performance over 20,000 charge–discharge cycles. This work provides important insights for the potential of polyurea/graphene films in multifunctional sensors and flexible energy storage applications.

SiCN ceramics show large potential in high temperature pressure sensors with excellent stability up to 1000 °C, as it is changeling for the most of the existing pressure sensors to work stably at a temperature above 600 °C. However, bulk SiCN ceramics are not compatible to microelectronic processing and exhibit slow response due to viscoelasticity, it is necessary to propose alternative method to prepare SiCN functional structures. In this work, SiCN piezoresistive thin films are prepared by magnetron sputtering, and the influence of sputtering power on their piezoresistive properties and interfacial strengths are studied. The gauge factors of SiCN films range from 2786 to 4714 at various sputtering powers, which are significantly higher than the range from 46 to 1105 for existing piezoresistive thin films. Upon an optimal sputtering power of 75 W for silicon nitride target, the obtained SiCN sample show the largest gauge factors in a large range from 0.5 to 3.4 MPa. Furthermore, the SiCN thin films present high critical loads up to 36.5 N in scratch tests and indicate strong interfacial adhesion with substrate. This work provides an important reference for developing SiCN-based MEMS pressure sensors.

The rigidity and discomfort associated with the use of conventional semiconductor-based wearable devices has prompted increasing interest in hydrogel-based devices. Hydrogels have properties similar to human skin tissue and therefore are more comfortable to use. The present study reports a high-performance, piezoresistive hydrogel nanocomposite-based wearable device towards human motion sensing. The nanocomposites comprise of an optimized distribution of polyaniline (PANI) decorated thermally exfoliated graphene oxide (TEGO) nanofillers incorporated within a biopolymer matrix. The engineered nanofillers endow the hydrogel with synergistic improvement in mechanical and electrical properties, resulting in a combination of high electrical conductance and extraordinary mechanical performance. The material demonstrated high stability during fatigue cycling done for extended periods. Furthermore, these nanostructured materials were used to prepare wearable sensors towards human motion detection. These advanced sensors exhibit a stable, sensitive and repeatable electrical response. Moreover, the use of fully biodegradable raw materials, simple synthesis method, and easy scalability establish the significant potential of the developed material towards wearable devices.

With the rapid development of the flexible electronics field, flexible pressure sensors are widely used in wearable devices, medical monitoring and other fields. However, it is still a huge challenge to design a sensor with high sensitivity, low cost and a simple manufacturing process. In this paper, a resistive pressure sensor based on a composite conductive layer is developed. The polydimethylsiloxane (PDMS) is used as the matrix of the composite. The electrolyte fully absorbed by the super absorbent polymer (SAP+) and used as the conductive filler of the composite. The elastic polymer is used to replace the common rigid conductive fillers (such as carbon nanotubes, graphene, etc.), can be better combined with the matrix, greatly improving the stability and reducing the Young’s modulus of the sensor. After describing the working mechanism of the sensor, the influence of the composite conductive layer of different thicknesses and the content of SAP+ on the electrical output of the sensor is discussed. In addition, the sensor exhibits high sensitivity (0.062 kPa-1), a fast response time (0.136 s) and excellent cyclic loading/unloading stability (> 500 cycles).

This paper proposes a novel flexible tactile sensor array for dynamic triaxial force measurement. Piezoresistive nanofibers, which are fabricated via the electrospinning method and are composed of multi-wall carbon nanotubes (MWCNTs) and thermoplastic polyurethane (TPU), are selected as the sensing material. The sensitive piezoresistive layer, which has micron-scale thickness, is wrapped in polydimethylsiloxane (PDMS) with a bumped surface. This structure not only detects triaxial forces of different magnitudes and frequencies, but also can recognize the shape, size, and curvature of external objects using multiple measuring points of the sensor. When the sensor is subjected to normal forces at different frequencies, the measuring voltage shows good responses with variations of less than 1.21 %. When the sensor is subjected to shear forces, the coefficient of variation is less than 2.05%. In addition, when the sensor is stressed at multiple measuring points, the voltage response errors between the different points are less than 5.29%. The sensor shows high sensitivity (with a resistance change that can reach four orders of magnitude) and high response-speeds (response within 62 ms) in validation experiments. The proposed flexible sensor is efficient in triaxial force detection and has the potential for application to prosthetic hands and wearable devices.

This work shows a differential rotational thermal piezoresistive resonator (DR-TPR) design, fabrication, and characterization. The resonator uses silicon on insulator (SOI) microelectromechanical systems (SOI MEMS) technology. The sensing of carboxyl-polystyrene beads requires estimating the variation in the frequency of the resonator. It can serve as an alternative enabling technology to detect small molecules having wide implementations in clinical and industrial applications. The resonator uses a negative frequency shift to find the change in the mass due to the chemical absorption of the carboxyl-polystyrene beads on the DR-TPR surface. The method of using DR-TPR can attain label-free detection for many future biomarkers. The use of carboxyl-polystyrene beads to showcase the potential of DR-TPR as a mass sensor for future chemical/biosensing applications is the highlight of the paper. It includes the characterization in different concentrations of immobilization on the resonator surface. The work shows the operation of the resonator in an ionic buffer solution which is crucial for its future applications. The Q value of 80 with differential feedthrough cancellation is measured in phosphate-buffered saline (PBS) using DR-TPR.

Background Poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) is a promising conductive polymer for the flexible piezoresistive pressure sensor. However, intrinsic hydrophilicity is detrimental to its structural stability under moisture. The present study aims to enhance piezoresistive pressure sensors' stability to achieve the highly-reliable electronic device applications. Method ITO/PEDOT:PSS/ITO piezoresistive pressure sensors with an interdigitated electrode (IDE) structure and different metallic interfacial layers were fabricated. Au layer was introduced between PEDOT:PSS sensing membrane and indium tin oxide (ITO) electrode as a reaction-inhibited interfacial coating of piezoresistive pressure sensors. Significant findings After a measurement time interval of 6 months, the interface reaction suppressed significantly, resulting in low sensitivity deviation (9.22%) and improved durability (400 cycles). The devices are completely sealed using polyethylene terephthalate packaging, avoiding humidity absorption in PEDOT:PSS films successfully. The packaged ITO/PEDOT:PSS/ITO piezoresistive pressure sensors with a double-sided 10-nm-thick Au interfacial layer is a promising candidate for use in highly reliable pressure sensing applications.

In recent years, flexible electronic equipment has attracted considerable attention. Textile fabrics are widely used in manufacturing flexible pressure sensors because of their high flexibility and toughness. However, ordinary fabrics are electrically insulated, which limits their sensitivity to pressure. In this paper, cotton fabrics (CFs) with high conductivity were prepared by a simple two-step method. Firstly, the reduced graphene oxide (RGO) suspension was deposited on CFs by vacuum filtration, and then the copper (Cu) and nickel (Ni) films were coated on the surface by magnetic filtered cathodic vacuum arc deposition technology (FVACD). The copper/reduced graphene oxide cotton (Cu/RGO/CF) and nickel/reduced graphene oxide cotton (Ni/RGO/CF) prepared by this method have good conductivity with a minimum resistance of 2 Ω/sq. Moreover, the RGO is closely combined with the metal layer, maintaining high conductivity after repeated washing. Highly conductive CFs can be applied to piezoresistive sensors. The research results show that the piezoresistive sensor based on composite conductive CFs has the advantages of high sensitivity, wide range (0–0.35 MPa), fast response and high stability. In addition, the sensor can realize real-time monitoring of human motion (such as the knee, wrist bending). In general, the effective washability and high conductivity of CFs coated with metal film and RGO have been successfully verified, making it one of the promising candidates for application in wearable electronic devices.

Conductive polymer composites (CPCs) are suitable as piezoresistive-sensing materials. When using CPCs for strain sensing, it is still a big challenge to simultaneously improve the piezoresistive sensitivity and linearity along with the electrical conductivity and mechanical properties. Here, highly tunable piezoresistive behavior is reported for multiwalled carbon nanotube (CNT)-filled CPCs based on blends of two semicrystalline polymers poly(vinylidene fluoride) (PVDF) and poly(butylene succinate) (PBS), which are miscible in the melt. When cooling the homogeneous mixture of the blend components, successive crystallization of PVDF and PBS occurs, creating complex crystalline structures in a mixed amorphous phase. The morphology of the blend matrix, the crystallinity of the blend components, and the dispersion and location of the CNTs in the blend depend on the CNT content and the blend composition. Compared with PVDF/CNT composites, the substitution of 10 to 50 wt % PVDF by PBS in the composites shifts the electrical percolation concentration Φc from 0.79 wt % to filler contents as low as 0.50 wt % while improving the stretchability. The piezoresistive behavior is highly tunable by changing the PVDF/PBS ratio. The ternary composites with matrix compositions of PVDF (90 wt %)/PBS (10 wt %) and PVDF (50 wt %)/PBS (50 wt %) show either higher piezoresistive sensitivity or linearity, respectively, caused by the differences in the microstructure of the CPCs. For example, the crystallinity of PBS in the ternary composites increased from 19.8% to 52.0% as the PBS content increased from 10 wt % to 50 wt %, which is connected with altered CNT distribution and conductive network structure and substantial improvement of the linearity of the electrical response to strains up to >20%. Our findings highly contribute to the understanding of the piezoresistive properties of CPCs based on two semicrystalline polymers and are important for future studies to tune the piezoresistive behavior to achieve simultaneously improved sensitivity and linearity.

Cellulose is the most abundant natural polymer existing on earth and nanomaterials derived from cellulose have been used in variety of applications. There is a great research interest in aerogels based on cellulosic nanomaterials due to their ultra-low thermal conductivity, modulus, sonic velocity, refractive index, dielectric constant, high specific surface area, and adjustable density. 3D printing of cellulose-based aerogels provided an ability to fabricate complex geometries with designed pore morphologies, which can be advantageous for various applications. Yet, the complexity of the geometry still has a limitation due to the lack of support material that can be removed from the structure without using harsh and tedious chemical / thermal processes that may affect the cellulosic material. In this study, by using water as support material, we have fabricated the first cellulose-based aerogels having truly 3D geometries with overhang features via the 3D freeze printing method incorporating a drop on demand-based materials deposition approach. The support material was removed from the main structure by a freeze-drying process, which is already a regular step in 3D printing process of aerogels. Fabricated aerogels possessed a highly ordered microstructure, in which the micropores were aligned along the freezing direction. We have investigated the effects of this anisotropy in the mechanical properties of the final aerogels. All the samples exhibited an excellent strain memory effect, and after 100 cyclic compression up to 25% strain, the obtained stress decay values were ~30% and ~16% in axial and radial directions, respectively. We further investigated the cross-linking of the fabricated aerogels to enhance their stability for making them feasible for applications such as biomedical or tissue engineering that requires wet environment. Finally, we functionalized the 3D printed cellulose-based aerogels with PEDOT:PSS, and evaluated their performance in piezoresistive sensing applications.

Several laboratory studies and experiments have demonstrated the usability of polymer films filled with electrically conductive filler as piezoresistive material. Applied to adhesives, the glue lines of wood products can achieve multifunctional—thus bonding and piezoresistive/strain sensing—properties. Based on critical load areas in timber constructions, upscaled test setups for simplified load situations were designed, especially with regard to a stress-free electrical contact. In a second step, another upscaling was done to small glulam beams. Based on an experimental test sequence, the piezoresistive reactions as well as the behaviour until failure were analysed. The results show in all cases that a piezoresistive reaction of the multifunctionally bonded specimens was measurable, giving a difference in the extent of relative change. Additionally, measured phenomena like inverse piezoresistive reactions, electrical resistance drift and the absence of a piezoresistive reaction were discussed, based on additional strain analysis by digital image correlation. A model of macroscopic and microscopic strains influencing the piezoresistive reaction of the electrically conductive bond line in wood was used to explain all experimental results. Finally, a first scale-up of piezoresistive bond lines from laboratory samples to glulam beams was possible and successful.

The electrical conductivity and piezoresistivity of multiwall carbon nanotube (MWCNT)/polypropylene (PP) composites obtained by extrusion are investigated, with particular attention to the possible directional effects generated during the extrusion process. This is accomplished by investigating the electrical and electromechanical responses of the nanocomposites at three MWCNT weight concentrations (3, 4 and 5 wt%) in three directions, viz. the extrusion direction, transverse to extrusion (in-plane) and through thickness. Higher electrical conductivity in the extrusion direction was more evident for the lowest MWCNT content. However, the piezoresistive sensitivity was similar in all directions. Films with 4 wt% showed the highest piezoresistive sensitivity, reaching gage factors of ~ 4.5 for strains between 0 and 0.8%, and ~ 10.2 for strains between 1 and 3%. After an initial drop in the electrical resistance, concomitant with stress relaxation, the changes in electrical resistance showed large reproducibility. Digital image correlation conducted during cyclic piezoresistive testing at 0.8% strain indicates small accumulation of local plasticity as the number of cycles increases, especially in zones near the electrodes. These irreversible changes in the material are expected to trigger the permanent changes in the electrical resistance measured.

The naturally microstructure-bioinspired piezoresistive sensor for human-machine interaction and human health monitoring represents an attractive opportunity for wearable bioelectronics. However, due to the trade-off between sensitivity and linear detection range, obtaining piezoresistive sensors with both a wide pressure monitoring range and a high sensitivity is still a great challenge. Herein, we design a hierarchically microstructure-bioinspired flexible piezoresistive sensor consisting of a hierarchical polyaniline/polyvinylidene fluoride nanofiber (HPPNF) film sandwiched between two interlocking electrodes with microdome structure. Ascribed to the substantially enlarged 3D deformation rates, these bioelectronics exhibit an ultrahigh sensitivity of 53 kPasup-1/sup, a pressure detection range from 58.4 to 960 Pa, a fast response time of 38 ms, and excellent cycle stability over 50 000 cycles. Furthermore, this conformally skin-adhered sensor successfully demonstrates the monitoring of human physiological signals and movement states, such as wrist pulse, throat activity, spinal posture, and gait recognition. Evidently, this hierarchically microstructure-bioinspired and amplified sensitivity piezoresistive sensor provides a promising strategy for the rapid development of next-generation wearable bioelectronics.

Printed piezoresistive strain sensors based on stretchable roll-to-roll screen-printed silver electrodes on polydimethylsiloxane substrates and inkjet-deposited single-wall carbon nanotube micropatterns are demonstrated in this work. With the optimization of surface wetting and inkjet printing parameters, well-defined microscopic line patterns of the nanotubes with a sheet resistance of

In silicon-based piezoresistive pressure sensor, the accuracy of the sensor is affected mainly by thermal drift and the sensitivity of the sensor varies with the rise in temperature. Here, the temperature effects on the desired representation of the sensor are analysed. Use of smart material Carbon nanotubes (CNT) and a few effective temperature compensation techniques are presented in this study to reduce the temperature effect on the accuracy of the sensor. Resistive compensation employed extra piezoresistors with Negative Temperature Coefficient of Resistivity (TCR) for temperature compensation. The attainment of the desired compensation techniques is highly compatible with the MEMS device fabrication. The compensated pressure sensor is supremacy for pressure measurement with temperature variations. Though various techniques have been suggested and put into actuality with successful attainment, the techniques featuring easy implementation and perfect compatibility with existing schemes are still blooming demanded to design a piezoresistive pressure sensor with perfect comprehensive performance. In this paper, CNT piezoresistive material has been employed as sensing elements for pressure sensor and compared with silicon in terms of output voltage and sensor performance degradation at higher temperature. Pressure sensors using CNT and silicon piezo resistive sensing materials were simulated on silicon (100) diaphragm by ANYSIS. Based on simulation results, silicon and CNT both pressure sensor also shows better results at near room temperature. With the increasing temperature it is observed that silicon pressure output underestimated by 23%.

The flexible piezoresistive sensors have versatile applications for healthcare. The fabrication procedures of these sensors are generally costly, complex, and environmentally unfriendly. Herein, we use L-cystine as the green reductant and templating agent to prepare 3D reduced graphene oxide (rGO) aerogel, and polyvinyl chloride tape as the substrates to encapsulate the aerogel for making the piezoresistive sensor. The sensor shows good linearity of resistance to logarithmic pressure value in a wide range of 170.4 kPa to 2.4 MPa, and it is capable to monitor the wrist pulse and human motion. Sensors fabricated by this facile and economical method have great potential application in electronic skin.

Hydrogel-based piezoresistive sensors have high practical value in many revolutionary applications, such as intelligent and electronic devices. However, with existing hydrogels, it is very difficult to achieve a combination of good mechanical properties, stable conductivity, and simple/green fabrication method. In this study, hybrid organic-inorganic nanoparticles (lignin-silver hybrid nanoparticles, Lig-Ag NPs) were synthesized by using alkaline lignin as the organic component and silver nanoparticle (Ag NPs) as the inorganic component. Interaction between the lignin and Ag NPs leads to the composite of hybrid nanoparticles that not only decreased the release of Ag NPs but also generated dynamically stable semi-quinone radicals in lignin. After compositing with polyvinyl alcohol (PVA) matrix, Lig-Ag NPs provided strong sacrificial hydrogen bonds and facilitated the delivery of electronic. Benefiting from these structural features and the pore-forming effect of ammonia (from Lig-Ag NPs solution), the PVA/Lig-Ag NPs hydrogel exhibits outstanding compressibility, pressure sensitivity, and stability of signal response. This study provides a green and simple design strategy for piezoresistive pressure sensors based on nanocomposite hydrogel.

The method of converting insulating polymers into conducting 3D porous graphene structures, so-called laser-induced graphene (LIG) with a commercially available CO2 laser engraving system in an ambient atmosphere, resulted in several applications in sensing, actuation, and energy. In this paper, we demonstrate a combination of LIG and a smart hydrogel (poly(N-vinylcaprolactam)—pNVCL) for multiresponsive actuation in a humid environment. Initiated chemical vapor deposition (iCVD) was used to deposit a thin layer of the smart hydrogel onto a matrix of poly(dimethylsiloxane) (PDMS) and embedded LIG tracks. An intriguing property of smart hydrogels, such as pNVCL, is that the change of an external stimulus (temperature, pH, magnetic/electric fields) induces a reversible phase transition from a swollen to a collapsed state. While the active smart hydrogel layer had a thickness of only 300 nm (compared to the 500 times thicker actuator matrix), it was possible to induce a reversible bending of over 30° in the humid environment triggered by Joule heating. The properties of each material were investigated by means of scanning electron microscopy (SEM), Raman spectroscopy, tensile testing, and ellipsometry. The actuation performances of single-responsive versions were investigated by creating a thermoresponsive PDMS/LIG actuator and a humidity-responsive PDMS/pNVCL actuator. These results were used to tune the properties of the multiresponsive PDMS/LIG/pNVCL actuator. Furthermore, its self-sensing capabilities were investigated. By getting a feedback from the piezoresistive change of the PMDS/LIG composite, the bending angle could be tracked by measuring the change in resistance. To highlight the possibilities of the processing techniques and the combination of materials, a demonstrator in the shape of an octopus with four independently controllable arms was developed.

Carbon nanofillers are widely used to blend with polymers for fabricating high-performance polymer nanocomposites (PNCs) toward structural, electronic and sensory material applications. However, it is generally of challenge to achieve significant property improvement by using single carbon nanofiller. Alternatively, using hybrid carbon nanofillers has been proved to be a facile and promising strategy to effectively boost electrical and piezoresistive properties of PNCs. This paper provides a state of the art review on the effects of hybrid carbon fillers on electrical properties of PNCs and corresponding applications in piezoresistive sensing scenarios. The effects of hybrid carbon nanofillers on electrical percolation threshold, electrical conductivities and piezoresistive sensitivity of PNCs are systematically discussed.

Wearable piezoresitive sensors have exhibited promising potentials for applications in motion detection and human-computer interactions. Herein, we reported a facile sol-gel followed by hydrothermal reduction approach to prepare polypyrrole/reduced graphite oxide aerogel (PPy@rGA) film, which is more oriented to flexible wearable piezoresistive sensors as compared with traditional cylindrical reduced graphene oxide (rGO) aerogel. The strong π-π interactions between rGO and PPy enhance the interfacial strength and help to maintain the integrity of the composite aerogel film. Meanwhile, the PPy nanoparticles anchoring on the edges and defects of rGO sheets create more electrically conductive paths when an external pressure is applied, and therefore give rise to significant changes in the resistance value and thus excellent piezoresistive sensing performance. The PPy2@rGA film (pyrrole monomer: graphene oxide is 2:1 wt%)–based piezoresistive sensor exhibits a high sensitivity of 0.9 kPa−1 in a linear range that is of 0 to 1 kPa, a short response time of 165 ms, and a short relaxation time of 132 ms, and is able to withstand 10,000 cycles. Moreover, the wearable sensor is capable of detecting large as well as small human motion. This study shows the feasibility of fabricating wearable piezoresitive sensors from rGO aerogel films reinforced by intrinsically conductive polymers. In this polypyrrole/reduced graphite oxide aerogel film which is more oriented to flexible wearable piezoresistive sensors, PPy nanoparticles anchor adjacent rGO sheets via strong π-π interfacial forces and meanwhile serve as nano-spacers that contribute to an enhanced piezoresistive sensing performance.

Highly sensitive and cost-effective flexible piezoresistive sensors are pursued as essential components of electronic skin (e-skin) for a variety of applications such as soft robotics and body prosthesis. A common strategy for achieving high sensitivity of these sensors is to fabricate their electrode material with complex 3-D microstructures, but manufacturing cost-effective micro-structured sensors with high reliability and reproducibility is a challenge. Herein, a robust approach for producing high-reproductive microstructured polydimethylsiloxane (PDMS) films composing the sensors is proposed, based on silicon molds with a honeycomb-like architecture fabricated rapidly and cost-effectively at a standoff distance by femtosecond laser pulses in the self-channeling regime. Single-walled carbon nanotubes (SWNTs) are embedded in the micro-structured PDMS films composing sandwich-structural flexible piezoresistive sensors. The sensing devices exhibit good performance with high sensitivity, fast response time, and excellent stability and are used with success for the detection of hand motion pressure and blood pressure at the wrist, thus showing a great potential as components of e-skin for detection of various human motions.

Recent advances in 3D printing technology have enabled unprecedented design freedom across an ever-expanding portfolio of materials. However, direct 3D printing of soft polymeric materials such as polydimethylsiloxane (PDMS) is challenging, especially for structural complexities such as high-aspect ratio (>20) structures, 3D microfluidic channels (∼150 μm diameter), and biomimetic microstructures. This work presents a novel processing method entailing 3D printing of a thin-walled sacrificial metallic mold, soft polymer casting, and acidic etching of the mold. The proposed workflow enables the facile fabrication of various complex, bioinspired PDMS structures (e.g., 3D double helical microfluidic channels embedded inside high-aspect ratio pillars) that are difficult or impossible to fabricate using currently available techniques. The microfluidic channels are further infused with conductive graphene nanoplatelet ink to realize two flexible piezoresistive microelectromechanical (MEMS) sensors (a bioinspired flow/tactile sensor and a dome-like force sensor) with embedded sensing elements. The MEMS force sensor is integrated into a Philips 9000 series electric shaver to demonstrate its application in "smart"consumer products in the future. Aided by current trends in industrialization and miniaturization in metal 3D printing, the proposed workflow shows promise as a low-temperature, scalable, and cleanroom-free technique of fabricating complex, soft polymeric, biomimetic structures, and embedded MEMS sensors.

We have developed a functional hydrogel combining the advantages of single-walled carbon nanotube (SWCNT) hydrogels and 3,4-dihydroxy-l-phenylalanine (l-DOPA) that would be expected to lead to a ma...

Polymer nanocomposites (PNC) have a great potential for in-situ strain sensing applications in both static and dynamic loading scenarios. These PNCs, having a polymer matrix of polyvinylidene fluoride (PVDF) with a conductive filler of multi-walled carbon nanotubes (MWCNT), have both piezoelectric and piezoresistive characteristics. Generally, this composite would accurately measure either low frequency dynamic strain using piezoresistive characteristic or high frequency dynamic strains using piezoelectric characteristics of the MWCNT/PVDF film sensor. This limits the frequency bands of the strain sensor to either piezoresistive or piezoelectric ranges. In this study, a novel weighted fusion technique, called piezoresistive/piezoelectric fusion (PPF), is proposed to combine both piezoresistive and piezoelectric characteristics to capture wide frequency bands of strain measurements in real time. This fuzzy logic (FL) based method combines the salient features (i.e. piezoresistive and piezoelectric) of the nanocomposite sensor via reasonably accurate models to extend the frequency range over a wider band. The FL determines the weight of each signal based on the error between the estimate and actual measurements. These weights indicate the contribution of each signal to the final fused measurement. The fuzzy inference system (FIS) was developed using both optimization and data clustering techniques. In addition, type-2 FIS was utilized to overcome the model’s uncertainty limitations. The developed PPF methods were verified with experimental data at different dynamic frequencies that were obtained from existing literature. The fused measurements of the MWCNT/PVDF were found to correlate very well with the actual strain and a high degree of accuracy was achieved by the subtractive clustering PPF’s FISs algorithm.

Over the last decade, the rapid development in health care industry and security has driven innovations in the field of wearable electronics and smart sensors. This report demonstrates the fabrication of molybdenum diselenide (MoSe2) by a facile, low-cost and scalable hydrothermal synthesis method followed by a vacuum filter deposition on cellulose paper. The fabricated device is utilized as a pressure and strain sensor. The fabricated device is passivated using polydimethylsiloxane (PDMS), and also, PDMS film enhances the flexibility of the device. The calculated sensitivity of pressure sensor is ~0.3 kPa−1, which is comparable and relatively better than the pressure sensor fabricated using sophisticated techniques. Furthermore, the device exhibited significant of merit for strain sensing with a gauge factor of ~6.84. The fabricated device showed excellent resilience in 500 cycle bending tests, which promises the robustness of the device. As an application, the fabricated sensor is used to develop a wireless keyboard by integrating the sensor on fingertips of an individual and a real-time human motion monitoring application in which the data are transmitted wirelessly to a smartphone with a dedicated android application. The successful fabrication of MoSe2/paper-based cost-effective, flexible, and biodegradable pressure-strain sensor shows tremendous applications in future intelligent user interface systems, human security, and personal health care monitoring.

Flexible and wearable strain sensors have attracted considerable attention due to their potential applications in human motion detection. In this work, the as-fabricated strain sensor was obtained by encapsulation of electrospun carbon sponge (CS) with polydimethylsiloxane (PDMS). The formation mechanism of the self-assembled sponge has been explored. Meanwhile, the piezoresistive properties and the strain sensing mechanism of the CS/PDMS sensor were investigated. The results showed that the as-fabricated CS/PDMS sensor had high piezoresistive sensibility with a maximum gauge factor up to 130.49, superior stability and fast response to various cyclic loading with a tensile strain from 0% up to 40% and a tensile speed range of 2–18 mm min−1. Finally, all the superior performances endow the sensor with abilities to precisely detect pronunciation, human palm motion, wrist joint motion, elbow joint motion, and finger motion in real-time. These results indicate that the strain sensor based on the CS/PDMS could have promising applications in flexible and wearable devices for human motion detection.

For system stability monitoring, pressure is one of the major and easy quantity to be measured among the many other physical quantities. A pressure sensor produces a signal as a function of the pressure imposed. These sensors became a vital part of our everyday life from devices we use at home to industrial equipment. In this paper, simulation, and comparative study of piezoresistive and piezoelectric pressure sensor is presented. All the simulations where made using COMSOL Multiphysics software of 5.5 version. Using this software, 3D models of sensors are simulated with different materials. From this simulation results, best materials for piezoelectric and piezoresistive pressure sensor are selected. This work follows the simulation, and analysis of a piezoresistive pressure sensor, done using four different sets of piezoresistive materials. Both p-type and n-type single crystalline silicon and polycrystalline silicon materials are used for the piezoresistive pressure sensor simulation. Pressure vs. displacement graphs of four set of piezoresistive materials are plotted. Displacement which is obtained in micrometer range increases with different input load from 100 KPa to 500 KPa. A piezoceramic tube, example for piezoelectric pressure sensor simulation also carried out using different piezoelectric materials like PZT 5A, PZT 5H, PVDF, BaTiO3, Lithium Tantalate, Cadmium Sulfide, Lithium Niobate, Aluminum Nitride and Barium Sodium Niobate. The modelling and simulation of Piezoceramic tube- Piezoelectric pressure sensor is done by giving different input pressure values. During the simulation of direct piezoelectric effect, voltage generation is obtained according to the input pressure. Output voltages of different piezoelectric materials are taken by giving different boundary load from 0.1 MPa to 1 MPa. Indirect piezoelectric effect simulation is also done, where piezoelectric materials become strained when the electric field is applied and this strain is directly proportional to the electric field. A Displacement table of indirect piezoelectric effect is measured by giving constant input pressure of 0.1 MPa. Here, analyses are done in MPa range of input pressures, obtained voltage in V and displacement in micrometers. Pressure vs. Voltage values for direct piezoelectric effect is plotted Efficiency and resolution of piezoresistive and piezoelectric pressure sensors are found out.

Elastomers embedded with a layer of conductive nanoparticles in the form of a sandwich structure are one of the most popular means to achieve high performance flexible sensors. However, owing to the lack of interaction between adjacent nanoparticles, the number of detached conductive nanoparticles increases when repeated strain/release cycles are applied, thereby causing the electrical resistance of the conductive layer to increase irreversibly. In this work, we report a high-performance piezoresistive sensor based on a novel sandwich structure composite to address this problem. Polydimethylsiloxane (PDMS) and poly(vinylidene fluoride) (PVDF) were blended to prepare a highly elastic PDMS/PVDF electrospun membrane, then silver nanowire (AgNW) suspensions were directly pumped into the electrospun membranes through a simple filtration process to prepare a conductive layer in a sandwich structure. Accordingly, this hybrid conductive layer was embedded into two layers of PDMS to prepare sandwich structure sensors (PPAP). The PDMS/PVDF electrospun membrane possessed excellent stretch–recovery capability and better interfacial compatibility. More importantly, the porous structure of the membrane effectively restricted the movement range of adjacent nanoparticles and formed a more stable conductive network structure. The maximum gauge factor (GF) of 654.5 of the sensor with a conductive layer structure is significantly higher than those of most reported sensors with a similar sandwich structure. The introduced conductive interlayer is a critical factor to improve the structure stability and dispersion uniformity of the AgNW network while ensuring the interfacial compatibility and elastic matching between the sensing and PDMS layers, which directly improve the durability and hysteresis. The microcrack structure observed in the sensing layer of the PPAP sensor was one of the main reasons for achieving an ultra-high sensitivity of the PPAP sensor. In the end, the wrinkled morphologies are formed after pre-stretching, and the wrinkled sensor exhibits a GF as high as 3058. This study justifies that the sandwich structure exhibits potential for strain sensing applications.

Polymer dielectrics with piezoresistive properties are being considered as a promising energy storage material, flexible wearable devices and smart devices. High dielectric property makes it efficient for its application in Thin Film Transistors (TFT) and battery separator and the piezoresistive property makes it efficient for flexible devices. In this paper, we present and analyse the dielectric, conductive and piezoresistive property of Polyvinylidene fluoride (PVDF)/Ferrous Ferric Oxide (Fe3O4) in frequency band of 20 Hz to 10 MHz. Membrane showing high dielectric constant has been synthesized by blending PVDF and Fe3O4 together. Initially the dielectric of pure PVDF membrane was low, but it increased with the addition of Fe3O4. The impedance analyser was used to measure dielectric constant, AC conductivity and dissipation factor. In this research paper we have concluded that on adding 1 gm of Fe3O4 to PVDF, we have got the best result in terms of conductivity, dielectric constant and piezoresistive property. The conductivity of the membranes was strongly affected by Fe3O4 content; conductivity increases with increase in Fe3O4 content.

Compressible materials have received great attention due to potential applications for energy storage and sensors in special cases. In the present work, reduced graphene oxide (RGO) was coated on carbon foam, and polyaniline (PANI) nanorods were grown on it to synthesize a compressible ternary composite. In this scenario, carbon foam was carbonized and used as a three-dimensional conductive and compressible framework; RGO nanosheets were the growth carrier of PANI which provided high pseudocapacitance. Due to the conservation of elasticity after carbonization, the device can be fully restored after compression strain and the electrochemical performance remained stable. As electrodes of a supercapacitor, a specific capacitance of 533 F g−1 was achieved in a two-electrode configuration. After 2000 charge–discharge cycles at 10 A g−1, 86.7% of the initial capacitance was retained. Meanwhile, this material as a piezoresistive pressure sensor has high sensitivity to 13.6 kPa and can be used to detect continuous stress variation. The presented composite material with excellent piezoresistive sensitivity and electrochemical performance shows broad prospects in the field of wearable devices.

Fabric-based piezoresistive sensors have demonstrated great potential in human motion and health detection applications. However, the conductive polymers of sensing units are easily influenced by high humidity and sweat conditions, which result in real-time electrical resistance fluctuation, and ultimately sensing inaccuracy. Herein, a hydrophobic, non-fluoride, titanium dioxide nanoparticle (TiO2-ODI)-modified fabric-based piezoresistive sensor was firstly employed to obtain accurate pressure sensing by prohibiting the interference from both high humidity and droplets in daily life. Due to the prominent isolation effect of the dense TiO2-ODI nanoparticle layer, for the first time, the dynamic normalized electrical resistance changes in a fabric-based piezoresistive sensor were negligibly influenced by high humidity of 95% and droplets of water, saline, milk, and sweat. Compared with the original pressure sensor (S = 0.85 kPa−1), the introduction of TiO2-ODI almost did not sacrifice the sensing performance (S = 0.83 kPa−1) and displayed prominent resistance to sweat interference. Thus, the hydrophobic TiO2-ODI pressure sensor exhibits promising potential and wide applicability for preparing highly accurate fabric structural piezoresistive sensors with wearing comfort, large area, self-cleaning property, ultraviolet protection, and anti-interference of high humidity and sweaty conditions, such as accurately evaluating human motion during exercise or on a rainy day.

The high conductivity and excellent mechanical properties of polymer composites favor their application as piezoresistive strain sensors. However, developing polymer composites with desirable piezoresistance, mechanical and durable properties is challenging. Herein, we developed conductive cellulose/MWCNTs aerogels using the freeze-drying technique. Furthermore, we explored the application of the highly-sensitive piezoresistive polymer composites following supercritical carbon dioxide (ScCO2) foaming, in cellulose/MWCNTs aerogel-thermoplastic polyurethane (TPU) nanocomposites. Conductive polymer composite (CPC) foams exhibited a significantly low percolation threshold (MWCNTs loading of ∼0.07 vol%) and high piezoresistive sensitivity (GF value of 7.84) owing to the low density (∼0.15 g cm−3) of cellulose/MWCNTs, high porosity (90%) and high electric conductivity (2.04 S m−1). The microcellular structure of composite foams was stable with regard to energy loss coefficient and piezoresistance after several compression cycles (1000 cycles). The behavior of the prepared CPC foams under different conditions was evaluated. The findings of the study showed that the piezoresistive sensors can be used in real-time monitoring of human movement.

People suffering from chronic respiratory diseases caused by air pollution need air filters simultaneously capable of high-efficiency filtration and respiration monitoring. However, constructing desirable filters to realize effective and durable capture of ultrafine particles as well as collection of real-time respiratory information is still particularly challenging. Herein, we report an ultralight, super-elastic hard carbon aerogel (s-HCA) as a piezoresistive sensor for a respiratory monitoring facial mask made of s-HCA/zein nanofibrils. The s-HCA displays super-elasticity and excellent compressive strength at a low density, as well as outstanding piezoresistive detective sensitivity and ranges. The resulting facial mask displays excellent filtration performance including high particulate efficiency and ultralow airflow resistance, as well as good moisture transferability. More significantly, ability of simultaneously monitoring the real-time respiratory process of filtration is incorporated into the facial mask. This study does not only lead to a novel carbon aerogel and broaden the application of carbon aerogels, but also provides a new strategy for the design of multifunctional facial masks. It is particularly significant for more patients under the situation of lack of costly respiratory equipment during the pandemic period.

Considering the complex actual environments and fields, the effective integration of multiple functions into one material is greatly and urgently required but still faces enormous challenges. Herein, an aerogel-based electromagnetic wave (EMW) absorber with the functions of piezoresistive sensing, infrared stealth, thermal insulation, and Joule heating was developed. Biomass bacterial cellulose (BC) and GO work as the carbon resource and reinforcing filler, respectively, obtaining a resilient (80% compression strain) and lightweight (7.81 mg cm−3) BC-derived C/thermal reduced GO (rGO) aerogel with the unidirectional cellular structure via the unidirectional freeze-drying and pyrolysis techniques. Remarkably, the prepared aerogel displayed the typical rGO loading and compression strain-dependent EMW absorption performance, and a significant improvement in the EMW absorption capacity with a minimum reflection loss (RLmin) of −46.11 dB and maximum effective absorption bandwidth (EABmax) of 9.12 GHz at the thickness of 2.70 mm was achieved for C/rGO-10 (10 wt% rGO loading) when a 70% compression strain was applied compared to that in the natural state. Moreover, the aerogel also presented other attractive features, including stable, durable, and fast responsive piezoresistive sensing performances, effective thermal infrared stealth, and thermal insulating properties, and exceptional adjustable Joule heating performance. This study paves the way for developing high-performance EMW absorption materials with multiple functions to meet various applications.

Nanocomposite strain sensors have shown application prospect in a wide range of applications. However, the sensitivities of the existing nanocomposite strain sensors to high-frequency, microscope dynamic strains are rather unsatisfactory. Herein, we fabricate a highly sensitive nanocomposite sensor for acquiring micro-vibrations generated by ultrasonic waves, from polydopamine(PDA)-coated hybrid carbon nanofillers. First, multi-walled carbon nanotubes (MWCNTs) are coated by 10s nm thick viscous PDA to improve their compatibility with polyvinylidene fluoride substrates. Compared to uncoated MWCNTs, the use of 15 wt% PDA-coated MWCNTs leads to a 40% increase in sensitivity. Then, one-dimensional PDA@MWCNTs are mixed with two-dimensional single-layer graphene to enhance the geometric contact between nanofillers. The sensitivity of sensors with hybrid nanofillers far exceeds that of PDA@MWCNT sensors. Also, as the mass fraction of graphene within hybrid nanofillers expands from 33% to 66%, the sensitivity of the proposed sensor improves by approximately 120%, surpassing that of pure graphene sensors. The high sensitivity of the proposed sensor, which actually utilizes a lower graphene content, was shown to be derived from the synergy between the two types of nanofillers which are of different dimensionalities. This study presents a novel approach for optimizing the sensitivity of nanocomposite strain sensors to high-frequency micro-vibrations.

This paper proposes a new hybrid temperature compensation system for doped silicon-based piezoresistive 3D stress sensors. The developed compensation system integrates a temperature sensor, placed in close proximity to the stress sensing rosettes, with the artificial neural networks (ANNs). In this work, the n-type circular piezoresistor featured over (111) silicon plane was employed to capture the local temperature variations, within the sensing chip. The extracted temperature changes, along with the resistance changes, are fed, as inputs, to the ANNs to compensate the temperature effect on the acquired signals for more accurate stress measurement. The proposed compensation system was experimentally evaluated while extracting stress applied up to 60 MPa at different temperatures within a range from 0 °C to 50 °C. The developed system was successfully able reduce the maximum full scale error, obtained from using only a temperature sensor for compensation, by ~55%. The new system has merit since it has the capability to compensate for both resistance and sensitivity, for 3D stress sensor, with no need for additional circuitry. Moreover the employed temperature sensor shares the same thermal environment with the stress sensing rosette.

A combined photothermal simulation and experimental study leads to a novel internal reflection-assisted direct laser writing carbonization method (IR-DLWc), which enables in situ fabrication of carbon features/patterns that are self-sealed in the interior of a thin polyimide (PI) film in one step without additional packaging procedures. With this new method, carbon line patterns that are fully contained in a 50 μm PI film are fabricated, characterized, and evaluated for their electrical and piezoresistive performance. The self-sealing character of the carbon features created by IR-DLWc imparts them unprecedented mechanical stability/robustness as compared to those fabricated by the conventional DLWc method. Upon applying a double-writing scheme and strain-engineering treatment, the IR-DLWc-created carbon lines show significantly improved piezoresistive sensitivity with a gauge factor evaluated to be 428 in tension and 107 in compression. The high piezoresistive sensitivity, excellent dynamic response, reasonably good durability, self-sealing character, and compliant nature of the IR-DLWc generated carbon patterns make them suitable for a variety of wearable sensing applications. In this work, we demonstrated their use as a tactile sensor for sensing contact force; a functional bandage for monitoring physiological activities like swallowing, pulsing, and breathing; and a glove sensing system for finger gesture recognition.

This work focusses on the design and fabrication of surface micromachined pressure sensors, designed in a modular way for the integration with analog front-end read-out electronics. Polycrystalline 3C silicon carbide (SiC) was used to fabricate free-standing high topography cavities exploiting surface micromaching. The poly-SiC was in-situ doped and the membrane itself is used as piezoresistive element, thereby forming a so-called self-sensing membrane, easing fabrication. After sacrificial release, the cavity is sealed by conformal deposition of poly-SiC whereby the reference pressure of the absolute pressure sensor is determined. Aluminum and titanium metallizations were used and ohmic contacts were confirmed by wafer-scale measurements. Measurements were carried out on different devices ranging from 100 kPa down to 10 Pa at room temperature. The Wheatstone bridge yields a logarithmic response of 1.1 mVbar $^{-}1\text{V}^{-}1$ . A square 300 $\mu \text{m}$ device exhibits a logarithmic impedance behavior yielding a response of $\Delta {R} / {R}$ of $1.6\times 10^{-3}$ bar $^{-1}$ . The realized pressure devices are a first step toward a SiC ASIC + MEMS platform for intended operation in harsh environments, such as industrial process monitoring, combustion control or structural health monitoring. The future outlook of the integration concept implies extended functionality by front-end transducer read-out, signal amplification and communication.

We study thermal-piezoresistive cooling in silicon micromechanical resonators at large currents and high temperatures. Crossing a thermal transition region corresponds to a steep reduction in resonance frequency, an abrupt plateauing in the effective quality factor, and a large increase in thermomechanical fluctuations. Comparing measurements with simulations suggests that the second-order temperature coefficients of elasticity of doped silicon are not sufficient to capture the drop in resonance frequency at large currents. Overall, our results show that there are clear thermal limits to cooling a resonant mode using current-controlled thermal-piezoresistive feedback in silicon. [2020-0205]

In this paper, we proposed the selective and ultrasensitive detection of Cadmium ions using a Cysteamine functionalized microcantilever-based sensor with cross-linked DL-glyceraldehyde. The detection time for various laboratory-based techniques is generally 12-24 hours. The experiments performed to create the SAMs of Cysteamine (Cys) cross-linked with DL-glyceraldehyde on the Microcantilever surface to selectivity capture the targeted Cd2+ Ions. The proposed portable microfluidic platform able to achieve the detection in 20-23 minutes and having a Limit of Detection (LOD) of 0.56 ng (2.78 pM), which perfectly describes its excellent performance over other reported techniques. Many researchers used Nanoparticles (NPs)-Based sensors for Heavy metal ions detection, but day by day, increasing usage and commercialization of Nanoparticles are rapidly expanding their deleterious effect on human health and the environment. The proposed technique uses the blend of thin-film and microcantilever (MEMS)-technology to overdrive the disadvantages of the Nanoparticles-approaches for selective Cd2+ ions detection and having LOD less than WHO limit of 3 µg/L. The fabricated sensor can achieve the limits efficiently well below the standards set by the WHO and helpful for the early detection of HMIs polluted source.

Polymer-based nanocomposite foams containing carbonic fillers have greatly facilitated scientific research efforts in electromagnetic interference (EMI) shielding, as well as piezoresistive sensing...

Polypyrrole (PPy) is a good candidate material for piezoresistive pressure sensors owing to its excellent electrical conductivity and good biocompatibility. However, it remains challenging to fabricate PPy-based flexible piezoresistive pressure sensors with high sensitivity because of the intrinsic rigidity and brittleness of the film composed of dense PPy particles. Here, a rational structure, that is, 3D-conductive and elastic topological film composed of coaxial nanofiber networks, is reported to dramatically improve the sensitivity of flexible PPy-based sensors. The film is prepared through surface modification of electrospun polyvinylidene fluoride (PVDF) nanofibers by polydopamine (PDA), in order to homogeneously deposit PPy particles on the nanofiber networks with strong interfacial adhesion (PVDF/PDA/PPy, PPP). This unique structure has a high surface area and abundant contact sites, leading to superb sensitivity against a subtle pressure. The as-developed piezoresistive pressure sensor delivers a low limit of detection (0.9 Pa), high sensitivity (139.9 kPa

In this work, flexible piezoresistive strain sensors of carbon nanotubes (CNT)/thermoplastic polyurethane (TPU) conductive polymer composites were prepared using fused filament fabrication (FFF) 3D...

Stretchable physical sensors are crucial for the development of advanced electrical systems, particularly wearable devices and soft robotics. Currently available stretchable sensors that detect both pressure and strain are based on piezoelectric, piezoresistive, or piezocapacitive effects. The range of pressure sensing is 1–800 kPa with large deformations being within the range of deformations of parts of the human body, such as elbows and knees. However, these devices cannot easily allow simultaneous and independent detection of pressure and strain with sensor arrays at large tensions (> 50%) because strain affects the pressure signal. In this study, we propose a monolithic silicone-based array of pressure and strain sensors that can simultaneously and independently detect the in-plane biaxial tensile deformation and pressure. To realize these functionalities, the deformation of the device structure was optimized using a hetero-silicone substrate made of two types of silicone with different hardness characteristics and porous silicone bodies. In addition, the resistances of the sensors were controlled by adjusting a mixture based on carbon nanoparticles to improve the sensitivity and independence between the pressure and strain sensors. These concepts demonstrate the potential of this approach and its compatibility with the current architectures of stretchable physical sensors.

The piezoresistive property of silicon nanowires (SiNWs) has been proven as one of the best candidates for strain sensors and now being explored in silicon transistors to improve the performance. In this article, we have numerically simulated and fabricated the junctionless nanowire (NW) field-effect transistor (JL-NWFET) to investigate its piezoresistive behavior. The channel diameter-dependent piezoresistive properties of the gate-all-around (GAA) JL-NWFET have been studied in detail. The GAA JL-NWFET with twin SiNWs channel of 20-nm diameter has been fabricated with the top–down approach, and the maximum gauge factor (GF)~80 has been observed. The simulation results have confirmed that the GF of the GAA JL-NWFET increases with decreasing the NW diameter. The GF of the 10-nm device has reached as high as ~200 in comparison with larger diameter GAA JL-NWFET. The simulation models have been validated by calculating the drain current and piezoresistive characteristics of the GAA JL-NWFET under strained conditions. The effect of strain on the transport properties of NW GAA JL-NWFET has also been studied. The results demonstrate that the carrier mobility of the lower diameter NW has enhanced to 364 (10-nm NW) from 323 cm2/Vs (20-nm NW). Furthermore, the carrier mobility of the channel has boosted with increasing the applied strain on the channel of GAA JL-NWFET and resulting in the increased GF.