Galinstan Liquid Metal/Polyurethane Composite as a Multifunctional Stretchable Electrode and Piezoresistive Strain Sensor With Minimal Drift

In recent times, flexible piezoresistive polymer nanocomposite-based strain sensors are in high demand in wearable devices and various new age applications. In the polymer nanocomposite-based strain sensor, the dispersion of conductive nanofiller remains challenging due to the competing requirements of homogenized dispersion of nanofillers in the polymer matrix and retaining of the inherent characteristics of nanofillers. In the present work, waterproof and flexible poly(vinylidene difluoride) (PVDF) with a polymer-functionalized hydrogen-exfoliated graphene (HEG)-based piezoresistive strain sensor is developed and demonstrated. The novelty of the work is the incorporation of polystyrene sulfonate sodium salt (PSS) polymer-functionalized HEG in a PVDF-based flexible piezoresistive strain sensor. The PSS-HEG provides stable dispersion in the hydrophobic PVDF polymer matrix without sacrificing its inherent characteristics. The electrical conductivity of the PVDF/PSS-HEG-based strain sensor is 0.3 S cm–1, which is two orders of magnitude higher than the PVDF/HEG-based strain sensor. Besides, near the percolation region, the PVDF/PSS-HEG shows a maximum gauge factor of 10, which is about two times higher than the PVDF/HEG-based flexible strain sensor and 5-fold higher than the commercially available metallic strain gauge. The enhancement in the gauge factor is due to the stable dispersion of PSS-HEG in the PVDF matrix and electron conjugation caused by the adherence of negatively charged sulfonate functional groups on the HEG. The developed waterproof flexible strain sensor is demonstrated using portable wireless interfacing device for various applications. This work shows that the waterproof flexible PVDF/PSS-HEG-based strain sensor can be a potential alternative to the commercially available metallic strain gauge.

In this paper, a MEMS piezoresistive ultrathin silicon membrane-based strain sensor is presented. The sensor’s ability to capture an acoustic emission signal is demonstrated using a Hsu–Nielsen source, and shows comparable frequency content to a commercial piezoceramic ultrasonic transducer. To the authors’ knowledge, this makes the developed sensor the first known piezoresistive strain sensor which is capable of recording low-energy acoustic emissions. The improvements to the nondestructive evaluation and structural health monitoring arise from the sensor’s low minimum detectable strain and wide-frequency bandwidth, which are generated from the improved fabrication process that permits crystalline semiconductor membranes and advanced polymers to be co-processed, thus enabling a dual-use application of both acoustic emission and static strain sensing. The sensor’s ability to document quasi-static bending is also demonstrated and compared with an ultrasonic transducer, which provides no significant response. This dual-use application is proposed to effectively combine the uses of both strain and ultrasonic transducer sensor types within one sensor, making it a novel and useful method for nondestructive evaluations. The potential benefits include an enhanced sensitivity, a reduced sensor size, a lower cost, and a reduced instrumentation complexity.

Flexible strain sensors are fabricated by using a simple and low-cost inkjet printing technology of graphene-PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) conductive ink. The inkjet-printed thin-film resistors on a polyethylene terephthalate (PET) substrate exhibit an excellent optical transmittance of about 90% over a visible wavelength range from 400 to 800 nm. While an external mechanical strain is applied to thin-film resistors as strain sensors, a gauge factor (GF) of the piezoresistive (PR) strain sensors can be evaluated. To improve the GF value of the PR strain sensors, a high resistive (HR) path caused by the phase segregation of the PEDOT:PSS polymer material is, for the first time, proposed to be perpendicular to the PR strain sensing direction. The increase in the GF with the increase in the HR number of the PR strain sensors without a marked hysteresis is found. The result can be explained by the tunneling effect with varied initial tunneling distances and tunneling barriers due to the increase in the number of HR. Finally, a high GF value of approximately 165 of three HR paths is obtained with a linear output signal at the strain range from 0% to 0.33%, further achieving for the inkjet printing of highly sensitive, transparent, and flexible linear PR strain sensors.

Articular cartilage derives its load-bearing strength from the mechanical and physiochemical coupling between the collagen network and negatively charged proteoglycans, respectively. Current disease modeling approaches and treatment strategies primarily focus on cartilage stiffness, partly because indentation tests are readily accessible. However, stiffness measurements via indentation alone cannot discriminate between proteoglycan degradation versus collagen degradation, and there is a lack of methods to monitor physiochemical contributors in full-stack tissue. To decouple these contributions, here, we developed a platform that measures tissue swelling in full-depth equine cartilage explants using piezoresistive graphene strain sensors. These piezoresistive strain sensors are embedded within an elastomer bulk and have sufficient sensitivity to resolve minute, real-time changes in swelling. By relying on simple DC resistance measurements over optical techniques, our platform can analyze multiple samples in parallel. Using these devices, we found that cartilage explants under enzymatic digestion showed distinctive swelling responses to a hypotonic challenge and established average equilibrium swelling strains in healthy cartilage (4.6%), cartilage with proteoglycan loss (0.5%), and in cartilage with both collagen and proteoglycan loss (-2.6%). Combined with histology, we decoupled the pathologic swelling responses as originating either from reduced fixed charge density or from loss of intrinsic stiffness of the collagen matrix in the superficial zone. By providing scalable and in situ monitoring of cartilage swelling, our platform could facilitate regenerative medicine approaches aimed at restoring osmotic function in osteoarthritic cartilage or could be used to validate physiologically relevant swelling behavior in synthetic hydrogels.

Flexible piezoresistive strain sensors are crucial for monitoring human motion, but achieving the right balance between sensitivity and operating range has always been challenging. Additionally, the complexity of muscle movements across different body parts means that relying on sensors with limited dimensional sensing is insufficient. This paper presents a flexible piezoresistive three-dimensional strain sensor (FPTDSS) designed to address these challenges. The FPTDSS features a wide operating range capable of detecting various human movements and boasts a high sensitivity, with a maximum gauge factor of 20 479. It can capture strain information along both the X- and Y-axes, as well as small vibrations along the Z-axis, through its intrinsic stretching and vibration properties. The sensor's effectiveness comes from the synergy between laser-induced graphene, silver nanoparticles (a zero-dimensional nanomaterial), and multi-walled carbon nanotubes (a one-dimensional nanomaterial). The synergistic effect of nanomaterials with different dimensions enables the FPTDSS to perform three-dimensional strain sensing, allowing for accurate detection of a broad range of complex human motions without requiring intricate circuit designs or preparation processes. This approach moves beyond limited strain information to provide a comprehensive view of three-dimensional strain, making the sensor versatile for detecting everything from subtle pulse vibrations to significant joint movements.

Both piezoresistive and piezoelectric materials are commonly used to detect strain caused by structural vibrations in macro-scale structures. With the increasing complexity and miniaturization of modern mechanical systems such as hard disk drive suspensions, it is imperative to explore the performance of these strain sensors when their dimensions must shrink along with those of the host structures. The miniaturized strain sensors must remain as small as possible so as to minimum their effect on structure dynamics, yet still have acceptable sensing resolution. The performances of two types of novel micro-scale strain gage for installation on stainless steel parts are compared in this paper. Micro-fabrication processes have been developed to build polycrystalline silicon piezoresistive strain sensors on a silicon substrate, which are later bonded to a steel substrate for testing. Piezoresistor geometries are optimized to effectively increase the gage factor of piezoresistive sensors while reducing sensor size. The advantage and disadvantage of these piezoresistors are compared to those of piezoelectric sensors. Experimental results reveal that the MEMS piezoelectric sensors are able to achieve a better resolution than piezoresistors, while piezoresistors can be built in much smaller areas. Both types of the MEMS strain sensors are capable of high sensitivity measurements, subject to differing constraints.

Piezoresistive strain sensors dominate the field of soft elastomer sensors, with many interesting findings and applications. Nevertheless, the methods of characterizing the performance of the sensor differ in each study, leading to different conclusions and making comparison of the different sensor systems challenging. In this Review, the most important methods for characterization of the sensor response are being presented and some cases of elastomer strain sensors are being highlighted. Furthermore, the different materials options for elastomer strain sensors are shown, with special sections for the rapidly growing fields of additive manufacturing and 3D printing. In addition to the material choices and testing methods, different applications of strain sensors are presented. From the biomedical field, these soft sensors find applications in wearable devices, vital sign monitoring, and rehabilitation assistive devices. In soft robotics, they can be used in monitoring and controlling the function of soft robot and actuation systems, a function that can aid with feedback control, increasing the efficiency of the robot’s function. Last but not least, in combination with the emerging self-healing materials, elastomer strain sensors can be used for monitoring the integrity of materials and structures.

We developed a plastic-scale-model assembly of an ultrathin film piezoresistive microelectromechanical systems (MEMS) strain sensor with a conventional vacuum-suction chip mounter for the application to flexible and wearable strain sensors. A plastic-scale-model MEMS chip consists of 5-μm ultrathin piezoresistive strain sensor film, ultrathin disconnection parts, and a thick outer frame. The chip mounter applies pressure to the ultrathin piezoresistive strain sensor film and cuts the disconnection parts to separate the sensor film from the outer frame. The sensor film is then picked up and placed on the desired area of a flexible substrate. To cut off and pick up the sensor film in the same manner as with a plastic scale model, the design of the sensor film and disconnection parts of MEMS chips were optimized through numerical simulation and chip-mounting experiments. The success rate of the 5-μm ultrathin sensor film mounting increased by decreasing the number and width of the disconnection parts. For a 5-μm-thick 1 × 5 mm2 sensor film, 4 disconnection parts of 20 μm in width achieved 100% success rate. The fabricated ultrathin MEMS piezoresistive strain sensor exhibited a gauge factor of 100 and high flexibility to withstand 0.37 [1/mm] bending curvature. Our plastic-scale-model assembly with a conventional vacuum-suction chip mounter will contribute to more practical manufacturing of ultrathin MEMS sensors.

In contrast to the fixed strain incorporated in logic devices for a fixed or constant improvement in device performance, piezoresistive strain sensors respond to variable strain through a modulation in the device resistance. The underlying physics of performance improvement in logic devices and strain transduction in piezoresistive strain sensors is the same: symmetry-breaking strain of the semiconductor crystal lattice warps the energy bands, splits the energy levels, and changes the carrier scattering rates, which changes the carrier mobility and the device resistance. While improvement of logic device performance requires an increase in mobility, which dictates the “sign” of the fixed strain, strain sensors respond to both negative (compressive) and positive (tensile) strains. Since the strain is fixed in logic devices, the linearity of mobility increase with strain is not an issue since the strain is theoretically frozen into the device by stressors incorporated into the device structure during the manufacturing process. In contrast, piezoresistive strain sensors are designed to transduce or detect varying strains by producing a proportional change in resistance. Hence, linear resistance change with strain is important to sense/transduce strains of varying amplitudes into an electrical signal without introducing distortion. For a transducer, the measured resistance vs. strain curve can be used to calculate the input strain from the strain sensitivity or calibration slope of the sensor. For a piezoresistive strain sensor, the upper limit of the measurable strain is usually defined as the maximum strain abovewhich nonlinear deformation occurs. In contrast, there is no maximum allowable stress in strain-enhanced logic devices as long as there is performance enhancement, provided that the stress is manufacturable and the device is reliable.

Synergistically toughened silicone rubber nanocomposites using carbon nanotubes and molybdenum disulfide for stretchable strain sensors

High‐performance piezoresistive strain sensors (PSS) are important components of wearable electronics for human health management and are considered a key technology for future applications in fields such as artificial intelligence and human medical monitoring. Recently, many PSS have been developed based on a variety of electrosensitive materials. Among them, 3D graphene foams (GrF) have attracted significant attention owing to their excellent thermal conductivity, tensile properties, and light weight. Herein, a novel GrF‐based composite is developed by growing 2D molybdenum disulfide (MoS2) nanosheets directly. Many lathy nanosheets stand vertically on the GrF, similar to silkworms creeping on the leaf, making the composite more sensitive to mechanical deformation stimuli. The obtained MoS2@GrF composite is processed into PSS with a wide sensing range (0%–80%), high gauge factor values (16 below 1% and 39 over 40%), detection limit of 0.1% strain with 106/123 ms response/recovery time, and good cyclic stability (≥3000 cycles). Moreover, the as‐fabricated strain sensors exhibit excellent Joule heating performance, which can be adjusted by strain. As such, the PSS allows for full‐range body motion monitoring and thermal management, which has great potential for next‐generation smart wearable electronics.

Enhancing sensing performance of 3D-printed TPU/CB piezoresistive strain sensors through integration of silver ink IDE

Highly conductive, stretchable, durable, breathable electrodes based on electrospun polyurethane mats superficially decorated with carbon nanotubes for multifunctional wearable electronics

Piezoresistive stretchable strain sensors with human machine interface demonstrations

Piezoresistive strain sensors, commonly known as resistance strain gauge, have many important applications. In this work, an alternative method to fabricate piezoresistive strain sensors directly on the structure of interest is demonstrated using a particle-free silver ink as the sensing material. The sensing material is first printed as a rectangular film on the structure of interest and a conductive serpentine pattern is generated by selective laser sintering. Only the material exposed to the focused laser is sintered and becomes conductive. The rest is washed-off by 1-dodecene solvent, leaving only the serpentine pattern, which serves as the piezoresistive strain sensor. This alternative method eliminates the need for a carrier or backing substrate and thus improves the mechanical coupling between the sensing material and the structure of interest. It also removes reinforcement effect due to the stiffness of the carrier substrate. Results from electrical characterization revealed that laser sintering power is a crucial parameter that influences fundamental properties of the sensing material such as electrical conductivity and work function. In addition, it was observed that there exists an optimum laser sintering power that results in a maximum gauge factor (GF). For strain sensors, the GF is the most important parameter because it is the measure of sensor sensitivity. When the particle-free silver ink was printed as a serpentine pattern followed by thermal sintering on a hot plate, a lower GF was measured. This shows that the alternative method to fabricate piezoresistive strain sensors is more attractive than printing the serpentine pattern then thermally sintering it.