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Bionic flexible sensors are a new type of biosensor with high sensitivity, selectivity, stability, and reliability to achieve detection in complex natural and physiological environments. They provide efficient, energy-saving and convenient applications in medical monitoring and diagnosis, environmental monitoring, and detection and identification. Combining sensor devices with flexible substrates to imitate flexible structures in living organisms, thus enabling the detection of various physiological signals, has become a hot topic of interest. In the field of human health detection, the application of bionic flexible sensors is flourishing and will evolve into patient-centric diagnosis and treatment in the future of healthcare. In this review, we provide an up-to-date overview of bionic flexible devices for human health detection applications and a comprehensive summary of the research progress and potential of flexible sensors. First, we evaluate the working mechanisms of different classes of bionic flexible sensors, describing the selection and fabrication of bionic flexible materials and their excellent electrochemical properties; then, we introduce some interesting applications for monitoring physical, electrophysiological, chemical, and biological signals according to more segmented health fields (e.g., medical diagnosis, rehabilitation assistance, and sports monitoring). We conclude with a summary of the advantages of current results and the challenges and possible future developments. [ABSTRACT FROM AUTHOR]

MXene materials have the metallic conductivity of transition metal carbides. Among them, Ti3C2TX with an accordion structure has great application prospects in the field of wearable devices. However, flexible wearable electronic devices face the problem of single function in practical application. Therefore, it is particularly important to study a flexible sensor with multiple functions for electronic skin. In this work, the near-field electrohydrodynamic printing (NFEP) method was proposed to prepare the composite thin film with a micro/nanofiber structure on the flexible substrate using a solution of poly(vinylidene fluoride)/MXene nanosheet/reduced graphene oxide (PMR) nanocomposites as the printing solution. A dual-mode flexible sensor for electronic skin based on the PMR nanocomposite thin film was fabricated. The flexible sensor had the detection capability of the piezoresistive mode and the piezoelectric mode. In the piezoresistive mode, the sensitivity was 29.27 kPa−1 and the response/recovery time was 36/55 ms. In the piezoelectric mode, the sensitivity was 8.84 kPa−1 and the response time was 18.2 ms. Under the synergy of the dual modes, functions that cannot be achieved by a single mode sensor can be accomplished. In the process of detecting the pressure or deformation of the object, more information is obtained, which broadens the application range of the flexible sensor. The experimental results show that the dual-mode flexible sensor has great potential in human motion monitoring and wearable electronic device applications. [ABSTRACT FROM AUTHOR]

Flexible electronic technology is one of the research hotspots, and numerous wearable devices have been widely used in our daily life. As an important part of wearable devices, flexible sensors can effectively detect various stimuli related to specific environments or biological species, having a very bright development prospect. Therefore, there has been lots of studies devoted to developing high-performance flexible pressure sensors. In addition to developing a variety of materials with excellent performances, the microstructure designs of materials can also effectively improve the performances of sensors, which has brought new ideas to scientists and attracted their attention increasingly. This paper will summarize the flexible pressure sensors based on material microstructure designs in recent years. The paper will mainly discuss the processing methods and characteristics of various sensors with different microstructures, and compare the advantages, disadvantages, and application scenarios of them. At the same time, the main application fields of flexible pressure sensors based on microstructure designs will be listed, and their future development and challenges will be discussed. [ABSTRACT FROM AUTHOR]

High‐performance biodegradable electronic devices are being investigated to address the global electronic waste problem. In this work, a fully biodegradable ferroelectric nanogenerator‐driven skin sensor with ultrasensitive bimodal sensing capability based on edible porcine skin gelatine is demonstrated. The microstructure and molecular engineering of gelatine induces polarization confinement that gives rise the ferroelectric properties, resulting in a piezoelectric coefficient (d33) of ≈24 pC N−1 and pyroelectric coefficient of ≈13 µC m−2K−1, which are 6 and 11.8 times higher, respectively, than those of the conventional planar gelatine. The ferroelectric gelatine skin sensor has exceptionally high pressure sensitivity (≈41 mV Pa−1) and the lowest detection limit of pressure (≈0.005 Pa) and temperature (≈0.04 K) ever reported for ferroelectric sensors. In proof‐of‐concept tests, this device is able to sense the spatially resolved pressure, temperature, and surface texture of an unknown object, demonstrating potential for robotic skins and wearable electronics with zero waste footprint. [ABSTRACT FROM AUTHOR]

High‐performance biodegradable electronic devices are being investigated to address the global electronic waste problem. In this work, a fully biodegradable ferroelectric nanogenerator‐driven skin sensor with ultrasensitive bimodal sensing capability based on edible porcine skin gelatine is demonstrated. The microstructure and molecular engineering of gelatine induces polarization confinement that gives rise the ferroelectric properties, resulting in a piezoelectric coefficient (d33) of ≈24 pC N−1 and pyroelectric coefficient of ≈13 µC m−2K−1, which are 6 and 11.8 times higher, respectively, than those of the conventional planar gelatine. The ferroelectric gelatine skin sensor has exceptionally high pressure sensitivity (≈41 mV Pa−1) and the lowest detection limit of pressure (≈0.005 Pa) and temperature (≈0.04 K) ever reported for ferroelectric sensors. In proof‐of‐concept tests, this device is able to sense the spatially resolved pressure, temperature, and surface texture of an unknown object, demonstrating potential for robotic skins and wearable electronics with zero waste footprint. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electrospun polysaccharide-based nanofiber membrane with a 3D network structure. • Biomimetic scale-like "core-shell" nanostructured piezoresistive material. • A broadened conductive pathway by in-situ polymerization of PPy. • A broad pressure range (0.13–128.46 kPa) and a high sensitivity (52.42 kPa−1). • Huge potential applicability in remote monitoring. A nanofiber membrane is an ideal carrier for constructing a typical three-dimensional conductive network. Inspired by the biomimetic scale-like nanostructure, polysaccharide-based piezoresistive material with outstanding Joule heating performance, thermal efficiency, and high conductivity is developed, presenting a potential indicator material with the color shifting function for intelligent skin or soft robots in practical applications. The stable core-shell structure due to the in-situ polymerization of polypyrrole (PPy) on the electrospun nanofiber endows the piezoresistive sensor (s-PGG@PPy) with a broad pressure range (0.13–128.46 kPa), high sensitivity (52.42 kPa−1) and fast response time (26 ms). The free motion of π electrons along the nanofibers between carbon atoms perfectly made it an attractive candidate for real-time monitoring of human activities, including bending of fingers, wrist, knee, and facial expression. In addition, designing s-PGG@PPy is to be connected in parallel and composed of an indication function glove, which shows different brightness with finger pressure. Consequently, the reported piezoresistive sensor represents a potential for wearable bioelectronics. [ABSTRACT FROM AUTHOR]

Flexible piezoresistive sensors hold a great potential with broad application spectrum in human-computer interaction and wearable electronic devices. Inspired by human touch sensation architecture, we report a new type of flexible piezoresistive sensor with ultrawide sensing range and highrobustness by engineering biomimetic structure and carbon black (CB)/MXene/SR/fiber nanocomposites. The facile fabrication processes, piezoresistive sensing mechanism, and performance of the device are comprehensively investigated. The sensor provides a high sensitivity of 2.18 kPa−1, ultrawide linear sensing range of 0.1–1700 kPa, fast response time (∼100 ms), and reliable durability (>5000 cycles). Furthermore, the developed flexible devices have been effectively implemented as electronic skin, demonstrating potential functions in manipulator soft grabbing and human movement monitoring. Such architecture paves the way for great application in the fields of healthcare, human–machine interface, and extremely large pressure monitoring. [ABSTRACT FROM AUTHOR]

Compliance sensation is a unique feature of the human skin that electronic devices could not mimic via compact and thin formfactor devices. Due to the complex nature of the sensing mechanism, up to now, only high-precision or bulky handheld devices have been used to measure compliance of materials. This also prevents the development of electronic skin that is fully capable of mimicking human skin. Here, we developed a thin sensor that consists of a strain sensor coupled to a pressure sensor and is capable of identifying compliance of touched materials. The sensor can be easily integrated into robotic systems due to its small form factor. Results showed that the sensor is capable of classifying compliance of materials with high sensitivity allowing materials with various compliance to be identified. We integrated the sensor to a robotic finger to demonstrate the capability of the sensor for robotics. Further, the arrayed sensor configuration allows a compliance mapping which can enable humanlike sensations to robotic systems when grasping objects composed of multiple materials of varying compliance. These highly tunable sensors enable robotic systems to handle more advanced and complicated tasks such as classifying touched materials. [ABSTRACT FROM AUTHOR]

Tactile sensing helps robots interact with humans and objects effectively in real environments. Piezoelectric polymer sensors provide the functional building blocks of the robotic electronic skin, mainly thanks to their flexibility and suitability for detecting dynamic contact events and for recognizing the touch modality. The paper focuses on the ability of tactile sensing systems to support the challenging recognition of certain qualities/modalities of touch. The research applies novel computational intelligence techniques and a tensor-based approach for the classification of touch modalities; its main results consist in providing a procedure to enhance system generalization ability and architecture for multi-class recognition applications. An experimental campaign involving 70 participants using three different modalities in touching the upper surface of the sensor array was conducted, and confirmed the validity of the approach. [ABSTRACT FROM AUTHOR]

Self-healing materials can prolong the lifetime of structures and products by enabling the repairing of damage. However, detecting the damage and the progress of the healing process remains an important issue. In this study, self-healing, piezoresistive strain sensor fibers (ShSFs) are used for detecting strain deformation and damage in a self-healing elastomeric matrix. The ShSFs were embedded in the self-healing matrix for the development of self-healing sensor fiber composites (ShSFC) with elongation at break values of up to 100%. A quadruple hydrogen-bonded supramolecular elastomer was used as a matrix material. The ShSFCs exhibited a reproducible and monotonic response. The ShSFCs were investigated for use as sensorized electronic skin on 3D-printed soft robotic modules, such as bending actuators. Depending on the bending actuator module, the electronic skin was loaded under either compression (pneumatic-based module) or tension (tendon-based module). In both configurations, the ShSFs could be successfully used as deformation sensors, and in addition, detect the presence of damage based on the sensor signal drift. The sensor under tension showed better recovery of the signal after healing, and smaller signal relaxation. Even with the complete severing of the fiber, the piezoresistive properties returned after the healing, but in that case, thermal heat treatment was required. With their resilient response and self-healing properties, the supramolecular fiber composites can be used for the next generation of soft robotic modules. [ABSTRACT FROM AUTHOR]

• Superhydrophobic conductive TPE/MWCNTs/PDMS film was fabricated at air/water interface. • The as-obtained sandwich-like film showed extreme repellency to water, salt, acid and alkali solutions. • The film is conductively sensitive and can be used as a wearable sensor in water wetted environment. Flexible and wearable sensors are of extreme importance for various practical applications in complex environments, such as electronic skin, body motion detection, etc. However, preparation of cost-effective and superhydrophobic wearable sensors with excellent flexibility, anti-corrosion, outstanding sensitivity and environmental adaptability remains challenging. Herein, a superhydrophobic flexible film is fabricated by spray-coating thermoplastic elastomer (TPE) solution on the upside of a multi-walled carbon nanotube (MWCNTs) sheet formed at the air/water interface, followed by treating the downside of MWCNTs sheet with polydimethylsiloxane (PDMS) pumped into the water. In the process, the covering of TPE on the MWCNTs made the sheet shrunk and increased the density of the conductive network. PDMS coating enhanced superhydrophobicity of the film, and binding fastness of the MWCNTs. The obtained sandwich-like TPE/MWCNTs/PDMS film had superior conductive sensitivity, large strain range (80%), fast and stable response time (60–80 ms) with great stability over 1000 stretching-relaxing cycles. Benefiting from the favorable superhydrophobic property, the fabricated film was capable of being put into liquid environment for quantifying the resistance under humid environments including moisture, acid, basic and salt conditions. It can be effectively used to measure wrist pulse and monitor various movements associated with different human body joints, such as finger, wrist, elbow. Moreover, the sensing film was able to capture the continuous signals in diverse atmosphere such as the area change of film, and the real-time stress response of drop impact. This study affords an innovative and promising track for multifunctional wearable sensors with wide applications in a harsh environment. [ABSTRACT FROM AUTHOR]

• A flexible CB/TPU foam with a pinnate-veined aligned porous structure was prepared. • The foam can both serve as a flexible strain sensor and an adsorption material. • The foam sensor has excellent sensing discernibility and high sensitivity. • The sensor exhibits superior response stability in a wide sensing range. In this paper, flexible conductive carbon black (CB)/thermoplastic polyurethane (TPU) foam (CTF) with an intriguing pinnate-veined aligned porous structure was fabricated via a unidirectional freeze-drying process. The density, porosity and average pore size of the composite foam are 0.13 g cm−3, 83.7% and 24.9 μm, respectively. CB particles are uniformly dispersed in the skeleton of the as-prepared foam, resulting in a low percolation threshold of 0.48 vol%. The CTF is capable of distinguishing different compressive strain by change of the relative resistance. Its responsive behaviors towards compressive strain present an excellent linear characteristic with a high gauge factor (GF) up to 1.55. In the piezoresistive tests, the CTF shows a fast response/relaxation time (150/150 ms), fascinating recoverability, excellent response stability within a wide workable detection range up to 80% strain (corresponding pressure of 584.4 kPa) and a superior durability (>10000 cycles), which is mainly related to the pinnate-veined aligned porous structure along the orientation direction. The CTF was then assembled as a wearable sensor to monitor various human motions such as arm bending, squatting, tiptoeing, etc., showing good detection performances. The sensor can also deal with water pollution through oil/water separation, suggesting its multifunctionality and promising prospects in complicated situations. [ABSTRACT FROM AUTHOR]

Electronic skin is an important means through which robots can obtain external information. A novel flexible tactile sensor capable of simultaneously detecting the contact position and force was proposed in this paper. The tactile sensor had a three-layer structure. The upper layer was a specially designed conductive film based on indium-tin oxide polyethylene terephthalate (ITO-PET), which could be used for detecting contact position. The intermediate layer was a piezoresistive film used as the force-sensitive element. The lower layer was made of fully conductive material such as aluminum foil and was used only for signal output. In order to solve the inconsistencies and nonlinearity of the piezoresistive properties for large areas, a Radial Basis Function (RBF) neural network was used. This includes input, hidden, and output layers. The input layer has three nodes representing position coordinates, X, Y, and resistor, R. The output layer has one node representing force, F. A sensor sample was fabricated and experiments of contact position and force detection were performed on the sample. The results showed that the principal function of the tactile sensor was feasible. The sensor sample exhibited good consistency and linearity. The tactile sensor has only five lead wires and can provide the information support necessary for safe human—computer interactions. [ABSTRACT FROM AUTHOR]