Electronic Skin Applications Research Articles

Emerging Functional Polymer Composites for Tactile Sensing.

In recent years, extensive research has been conducted on the development of high-performance flexible tactile sensors, pursuing the next generation of highly intelligent electronics with diverse potential applications in self-powered wearable sensors, human-machine interactions, electronic skin, and soft robotics. Among the most promising materials that have emerged in this context are functional polymer composites (FPCs), which exhibit exceptional mechanical and electrical properties, enabling them to be excellent candidates for tactile sensors. Herein, this review provides a comprehensive overview of recent advances in FPCs-based tactile sensors, including the fundamental principle, the necessary property parameter, the unique device structure, and the fabrication process of different types of tactile sensors. Examples of FPCs are elaborated with a focus on miniaturization, self-healing, self-cleaning, integration, biodegradation, and neural control. Furthermore, the applications of FPC-based tactile sensors in tactile perception, human-machine interaction, and healthcare are further described. Finally, the existing limitations and technical challenges for FPCs-based tactile sensors are briefly discussed, offering potential avenues for the development of electronic products.

Recent Advances in Polymer Composites for Flexible Pressure Sensors.

Pressure sensors show significant potential applications in health monitoring, bio-sensing, electronic skin, and tactile perception. Consequently, tremendous research interest has been devoted to the development of high-performance pressure sensors. In this paper, recent progress on the polymer composite-based flexible pressure sensor is reviewed. The parameters of pressure sensors, including sensitivity, linear response range, detection limit, response speed, and reliability, are first introduced. Secondly, representative types of pressure sensors and relevant working principles are introduced and discussed. After that, the applications in human physiology monitoring, health monitoring, artificial skin, and self-powered smart system are listed and discussed in detail. Finally, the remaining challenges and outlook of polymer composite-based flexible sensors are summarized at the end of this review paper. This work should have some impact on the development of high-performance flexible pressure sensors.

Toward Real-Time Blood Pressure Monitoring via High-Fidelity Iontronic Tonometric Sensors with High Sensitivity and Large Dynamic Ranges.

Continuous, noninvasive blood pressure (CNIBP) monitoring provides valuable hemodynamic information that renders detection of the early onset of cardiovascular diseases. Wearable mechano-electric pressure sensors that mount on the skin are promising candidates for monitoring continuous blood pressure (BP) pulse waveforms due to their excellent conformability, simple sensing mechanisms, and convenient signal acquisition. However, it is challenging to acquire high-fidelity BP pulse waveforms since it requires highly sensitive sensors (sensitivity larger than 4 × 10-5 kPa-1 ) that respond linearly with pressure change over a large dynamic range, covering the typical BP range (5-25kPa). Herein, this work introduces a high-fidelity, iontronic-based tonometric sensor (ITS) with high sensitivity (4.82 kPa-1 ), good linearity (R2 > 0.995), and a large dynamic range (up to 180% output change) over a broad working range (0 to 38kPa). Additionally, the ITS demonstrates a low limit of detection at 40Pa, a fast load response time (35ms) and release time (35ms), as well as a stable response over 5000 load per release cycles, paving ways for potential applications in human-interface interaction, electronic skins, and robotic haptics. This work further explores the application of the ITS in monitoring real-time, beat-to-beat BP by measuring the brachial and radial pulse waveforms. This work provides a rational design of a wearable pressure sensor with high sensitivity, good linearity, and a large dynamic range for real-time CNIBP monitoring.

Simultaneous Polymerization Acceleration and Mechanical Enhancement for Printing a Biomimetic PEDOT Adhesive by Coordinative and Orthogonal Ruthenium Photochemistry

Conductive hydrogels are promising material candidates in fields ranging from flexible sensors and electronic skin applications to personalized medical monitoring. However, developing intrinsically conductive polymer hydrogels (ICPHs) with high mechanical properties and excellent printability is still challenging. Here, we introduce a simultaneous polymerization acceleration and mechanical enhancement (SPAME) strategy to construct PEDOT-based ICPHs via the rational design of coordinative and orthogonal ruthenium photochemistry (CORP). This orthogonal photochemistry triggers the oxidative polymerization of EDOT and the coupling of phenols within seconds under blue light irradiation. Benefiting from the bifunctional EDTA-Fe design, the photoreleased Fe(III) accelerated the EDOT polymerization and shortened the preparation time of ICPHs to a few seconds. At the same time, the addition of EDTA-Fe enhanced their mechanical properties, and both the critical strains and maximum stresses of the hydrogel doubled. Furthermore, the introduction of phenol residues in PAA-Ph significantly shortened the gelation time from several minutes to about 7 s. Thus, this fast and controllable CORP chemistry is compatible with standard printing techniques for engineering hydrogels for complex multifunctional structures for multifunctional bioelectronics and devices.

Polymerizable rotaxane hydrogels for three-dimensional printing fabrication of wearable sensors

While hydrogels enable a variety of applications in wearable sensors and electronic skins, they are susceptible to fatigue fracture during cyclic deformations owing to their inefficient fatigue resistance. Herein, acrylated β-cyclodextrin with bile acid is self-assembled into a polymerizable pseudorotaxane via precise host-guest recognition, which is photopolymerized with acrylamide to obtain conductive polymerizable rotaxane hydrogels (PR-Gel). The topological networks of PR-Gel enable all desirable properties in this system due to the large conformational freedom of the mobile junctions, including the excellent stretchability along with superior fatigue resistance. PR-Gel based strain sensor can sensitively detect and distinguish large body motions and subtle muscle movements. The three-dimensional printing fabricated sensors of PR-Gel exhibit high resolution and altitude complexity, and real-time human electrocardiogram signals are detected with high repeating stability. PR-Gel can self-heal in air, and has highly repeatable adhesion to human skin, demonstrating its great potential in wearable sensors.

Environment-tolerant versatile ion-conductive eutectic hydrogels for self-powered wearable flexible sensors

Conductive hydrogels (CHs) have been widely utilized in the design of wearable flexible sensors. The remaining issues with CHs include their propensity to dehydrate in the open air and freeze at low temperatures, which makes them significantly less functional. Additionally, the wide applications of CHs are limited by their dependence on external power sources and inadequate sensing of humidity, strain, etc. Herein, a series of versatile ion-conductive adhesive hydrogels (GAAAD) with emblematic on-demand 3D cross-network structures were nanoengineered by combining solvent exchange strategies with in-situ physical and covalent cross-linking. The GAAAD possesses exceptional properties such as integrated outstanding sensing of multiple external stimuli in harsh environments, desirable adhesive ability towards various substrates, mechanical robustness, transparency, anti-freezing and anti-dehydration performance, and self-healing capacity. Even after being frozen at −20 ℃ for 48 h, GAAAD could be utilized for effective real-time human sleeping monitoring, demonstrating preponderant environmental stability. Integrated with the versatile origins of GAAAD, a simple Cu/GAAAD/Zn self-powered flexible sensor (GAAADB) was developed based on the fundamental structure of a primary battery, which exhibits remarkable properties of stable open-circuit voltage even in extremely cold environments, sensitive response to applied tensile or compressive strain, as well as its repeatability and durability. Interestingly, based on the sensing mechanism of the potential difference between the humidity-regulating electrodes, GAAADB was substantiated to be reliable for long-term respiratory monitoring (with the humidity response/recovery time less than 1 s) under extreme cold conditions (−20 °C), indicating its great potential for extensive applications in respiratory diagnosis, sleep monitoring, electronic skin, and wearable electronics.

Oriented Ti3C2Tx MXene-doped silk fibroin/hyaluronic acid hydrogels for sensitive compression strain monitoring with a wide resilience range and high cycling stability

Flexible strain sensors have attracted the attention of researchers due to their wide range of applications in wearable electronic skin and healthcare monitoring. The fabrication of flexible strain sensors with high strength, good linear sensitivity, stability and comfort is still challenging. Here, an anisotropic silk fibroin (SF) based hydrogel is fabricated through an oriented freezing method and a low-temperature concentration is to follow. This approach combines HA with SF to increase the hydrogel's water-holding capacity, and Ti3C2Tx MXene is doped as a conductive-sensitive component. Benefitting from the synergy of its well-designed components and the oriented internal crosslinking network structure, the hydrogel has a wide working range and a stable resistance response under 0∼80% compression strain. Especially the oriented hydrogel has ultra-high compression resilience (∼ 99%) in the orientation direction, and it is stable and durable for up to 10,000 cycles under 50% compression self-rebound. Flexible sensors made of conductive hydrogel were attached to the human body or combined with out-of-body clothing to detect human movement. Based on these findings, the prepared conductive hydrogel could be used as a wearable strain sensor material.

High stretchable and self-adhesive dual networks ionic gels and flexible devices application

Multifunctional conductive hydrogels with high stretching and adhesion strength have been developed in recent years. However, rapilied formed, high toughness and self-adhesive ionic gels need further study. In this paper, highly tensile and tough ionic gels were prepared in DMAA-AMPS ionic liquid, and PEG600DA was used as a physical cross-linking agent by rapid UV polymerization. Excellent mechanical properties and self-adhesive properties to various substrates were integrated into the ionic gel hydrogels. The addition of PEG600DA contribute to high extension, up to 4100% and high toughness of 3163.0161 kJ/m3. For application, ionic gel is assembled to a cell with one layer of zinc and copper sheet. The output voltage of inonic gel is up to 0.15 V and convert from chemical energy to electrical energy. When ten cells are combined, a maximum voltage of 1.76 V can be achieved, which can provide energy for a stopwatch and can be used as a self-powered strain sensor under harsh conditions. The proposed ion-gel self-powered sensor has potential applications in electronic skin, human-computer interaction, and biomedical fields.

Self‐Healing Stress Sensors: Coupling Stress‐Sensing Performance with Dynamic Chemistry

AbstractFlexible stress sensors that can convert different forces into electrical signals show great potential for applications in wearable electronics and electronic skin. The integration of self‐healing capacity into stress‐sensing materials can boost their reliability, stability, and long‐term performance. There have been many reviews describing the progress of research on self‐healing stress sensors. However, most have typically investigated the self‐healing design separately from the sensing mechanism. The review focuses on how the latest advances in coupling stress‐sensing performance with dynamic chemistry balance self‐healing and sensing performance. Then, based on sensing structures and self‐healing dynamic bonds, the design strategies, and preparation methods of self‐healing sensors and their advantages and disadvantages in recent years are summarized in detail. It is anticipated that this work will provide insights and guidance for the future development of sensors with synergistically enhanced self‐healing and sensing properties and their application designs.

Hydrogel‐Based Multifunctional Soft Electronics with Distributed Sensing Units: A Review

AbstractSoft electronics have attracted great interest owing to their potential applications in electronic skins, implanted devices, soft robotics, etc. Among the various soft materials, hydrogels are recognized as an ideal building block of soft electronics due to their tissue‐like physicochemical properties, abundant stimuli‐responses, and excellent mechanical compliances. Compared to elastomers, hydrogels containing large amounts of water exhibit better mechanical match to soft tissues and permeability to hydrophilic molecules. To date, most hydrogel‐based soft electronics (HSE) are facilely developed using a bulk conductive gel as the sensing unit, different from the elastomer‐based soft electronics with sophisticated integration of electronic elements. To advance their applications in engineering and biomedical fields, it is significant to devise hydrogel electronics with distributed sensing units. This Review summarizes the fabrication and applications of HSE, focusing on the multifunctional HSE with patterned conductive circuits and distributed sensing units. First, the fabrication of single‐functional soft electronics is briefly introduced with a bulk gel as the building block, including strain, temperature, chemical, and proximity sensors. Then, the approaches to integrating multiple sensing units into one hydrogel are summarized with examples of applications. Finally, perspectives are given on future directions and potential challenges in this field.

Soft Self-Regulating Heating Elements for Thermoplastic Elastomer-Based Electronic Skin Applications.

Resistive heating elements can be of particular interest for many applications, such as e-skin. In this study, soft heating elements were developed by combining thermoplastic polyurethane (TPU) with carbon black. In contrast to previous studies on thermoplastic polymer-based thermistors, the heating elements could endure elongations above 100%. Due to the high melting point of the TPU and the carbon filler, the thermistors could be heated up to 180°C without significant deformation. The heating elements were extruded on TPU substrates using material extrusion additive manufacturing in one-step process. Self-regulating behavior to control the maximum temperature was achieved with the application of two different voltages (20 and 25 V) and different current thresholds, between 100 and 800 mA. The heating performance was adjusted by changing the geometry of the sensing elements; an increase in cross section resulted in a lower current density and lower temperature. For the heating elements, variation of the additive manufacturing parameters such as offset, layer height, nozzle speed, and extrusion multiplier resulted in a different width/height aspect ratio of the cross section of the extruded lines, affecting the initial resistivity of the thermistor. Orientation of the carbon filler during extrusion process is one reason for the small change of the longitudinal conductivity of the heating elements. The resulting skin with the integrated heating elements allowed the possibility to perform the in situ heating for the localized healing of structural damage, while maintaining the softness required for the application of soft robotic electronic skin.

A Smart Tactile Sensing System Based on Carbon Nanotube/Polypropylene Composites for Wearable Applications

Recent advances in nanomaterials render the possibility of fabricating tactile sensors suitable for electronic skin, haptic interfaces, and biomedical applications. Furthermore, the problem of performing complex inertial measurements generated during the tactile process of polymer composites can be tackled using artificial intelligence (AI). However, the implementation of AI-based signal processing on embedded devices still represents an open area of opportunity for the design of the next generation of smart wearable sensor systems. A novel nanostructured smart tactile sensing system for wearable applications is proposed, using a 3-D-printed structure with embedded electronic devices. The highly sensitive piezoresistive tactile sensor is based on multiwall carbon nanotube/polypropylene (MWCNT/PP) composites. The integration of electronic circuits for signal processing of an artificial neural network (NN) on a digital controller unit improves the tactile interpretation in the wearable embedded device. Experiments show that the pressure classification results on MWCNT/PP composites with 98% accuracy.

Microcracked strain sensor based on carbon nanotubes/copper composite film with high performance and waterproof property for underwater motion detection

As flexible strain sensors have been widely used for monitoring human movement effectively, the exploration of this type of sensor will be of great interest to the sensor research community. This paper presents a flexible strain sensor with outstanding performance and straightforward preparation based on microcrack. A polyurethane film is used as a flexible substrate and carbon nanotube/copper composite films are used as sensitive layers for the strain sensors, which are completely encapsulated in polydimethylsiloxane. Furthermore, the surface morphology of the sensor and the form of crack development are represented in this report, as well as a comprehensive test of the performance. The results show that we designed and manufactured a strain sensor with gauge factors of 60.0 (0 < ε < 14%), 210.6 (14% < ε < 30%) and even up to 348 (30% < ε < 50%) with a sensing range of up to 50%. Under the condition of 60% tensile strain, the sensor can be loaded/unloaded at 2 × 103 cycles without any significant change in the relative rate of change curve of its resistance. The sensor has excellent water resistance and reliability, proving that it can be adapted to most scenarios when used daily. The paper shows that the method produces a sensor with commendable performance and has a significant potential for applications in human motion monitoring systems and electronic skins in water.

The Synergy of Hydrogen Bond and Entanglement of Elastomer Captures Unprecedented Flaw Insensitivity Rate.

Elastomers are regarded as one of the best candidates for the matrix material of soft electronics, yet they are susceptible to fracture due to the inevitable flaws generated during applications. Introducing microstructures, sacrificial bonds, and sliding cross-linking has been recognized as an effective way to improve the flaw insensitivity rate (Rinsen ). However, these elastomers still prone to failure under tensile loads with the presence of even small flaws. Here, this work reports a polybutadiene elastomer with unprecedented Rinsen via the synergy of hydrogen bond and entanglement. The resulting polybutadiene elastomer exhibits a Rinsen ≈1.075, which is much higher than those of reported elastomers. By molecular chain interaction and molecular chain conformation analysis, this work demonstrates that the synergistic effect of hydrogen bond dissociation and entanglement slip in the polybutadiene elastomers during stretching leads to the high Rinsen . Using polybutadiene elastomer as matrix of thermal interface materials, this work demonstrates effective heat transfer for strain sensor and electronic devices. In addition, cytocompatibility of the elastomers is verified by cell proliferation and live/dead viability assays. The combination of outstanding biocompatible and excellent mechanical properties of the elastomers creates new opportunities for their applications in electronic skin.

A Review on PVDF Nanofibers in Textiles for Flexible Piezoelectric Sensors

Textiles are turning into a suitable next-generation sensing platform because of their good breathability, softness, and structural elasticity. Besides, research on self-powered piezoelectric sensors is a hot topic in wearable applications; they can perform long-term sensing and monitoring. Therefore, this paper mainly reviews the development progress of PVDF-based textiles on flexible piezoelectric sensors. In this paper, we first introduce the principle of the piezoelectric effect and the classification of piezoelectric materials; then we summarize the structure and characteristics of nanofiber mat-based, yarn-based, and fabric-based flexible piezoelectric sensors and the approaches that are employed to fabricate PVDF-based textile piezoelectric sensors such as melt spinning, electrospinning, and stretch forming processes, and so on. At last, we review their applicability in the application of electronic skin, human–computer interaction, healthcare, and human movement monitoring and demonstrate the facing difficulties and the future research directions of PVDF-based textile flexible sensors.

Flexible breathable photothermal-therapy epidermic sensor with MXene for ultrasensitive wearable human-machine interaction

Multifunctional flexible wearable epidermic sensors have gained enormous attention for their promising potential applications in personal health management, electronic skins and intelligent human-machine interaction. However, it remains a critical challenge to realize a flexible breathable sensing device with reliable sensing performances for comfortably wearable human motion detection and ultrasensitive human-machine interfacing with the integrated functional capability of disease diagnosis and subsequent photothermal therapy simultaneously. Herein, a flexible breathable epidermic sensor is fabricated by delicately assembling conductive MXene nanosheets and silver nanowires (AgNWs) onto electrospun elastomeric substrates for ultrasensitive sensing and on-demand photothermal therapy. The as-prepared flexible breathable electronic sensor presents a broad sensing range (up to ∼120 %), ultrahigh sensitivity (gauge factor (GF) up to 4720), extremely low detection limit, remarkable reliability, and robust biocompatibility for serving as the electronic skin to monitor human physiological signals and wearable human-machine interaction. Moreover, it exhibits brilliant photothermal properties for enhanced antibacterial effect and timely photothermal therapy after healthcare monitoring and disease diagnosis. This work provides a facile way of developing flexible breathable multifunctional electronic sensors featured with excellent sensing ability, brilliant breathability and reliable photothermal properties for long-term healthcare monitoring, ultrasensitive wearable human-machine interfacing, smart disease diagnosis and treatment.

Piezoresistive Pressure Sensor Based on a Conductive 3D Sponge Network for Motion Sensing and Human-Machine Interface.

Flexible sensors have attracted increasing attention owing to their important applications in human activity monitoring, medical diagnosis, and human-machine interaction. However, the rational design of low-cost sensors with desirable properties (e.g., high sensitivity and excellent stability) and extended applications is still a great challenge. Herein, a simple and cost-effective strategy is reported by immersing polyurethane (PU) sponge in graphene oxide solution followed by in situ chemical reduction to construct a reduced graphene oxide (RGO)-wrapped PU sponge sensor. Ascribed to the excellent compressive resilience of PU sponge and an electrically conductive RGO layer, the constructed flexible sensor exhibits satisfactory sensing performance with high sensitivity (17.65 kPa-1) in a low-load range (0-3.2 kPa), a wide compression strain range (0-80%), and reliable stability (8000 cycles). In addition, these sensors can be successfully applied to monitor human movements and identify the weight of objects. Through the use of a sensor array integrated with a signal acquisition circuit, the reasonably designed sensors can realize tactile feedback via mapping real-time spatial distribution of pressure in complicated tasks and show potential applications in flexible electronic pianos, electronic skin, and remote real-time control of home electronics.

Recent progress in soft electronics and robotics based on magnetic nanomaterials

Recent advancements in soft electronics and robotics have expanded the possibilities beyond the capabilities of traditional rigid devices, indicating promise for a range of applications in electronic skins, wireless biomedical devices, and others. Magnetic materials exploited in these soft systems can further broaden the modalities in sensing and actuation. These magnetic materials, when constructed in the forms of nanoparticles, nanomembranes, or other types of nanostructures, exhibit some unique characteristics, such as the magnetoresistance effect and size-dependent coercivity. Soft electronics and robotics employing such magnetic nanomaterials offer a variety of functions, including the detection of the intensity and direction of magnetic fields, measurement of various types of mechanical deformations, manipulation and transport at small scales, and multimodal complex locomotion in a controllable fashion. Despite recent advancements in soft electronics and robotics, challenges remain in developing advanced materials and manufacturing schemes to improve performance metrics and facilitate integration with other devices. This review article aims to summarize the progress made in soft electronics and robotics based on magnetic nanomaterials, with an emphasis on introducing material and device performance. The discussions focus on soft electronics and robotics based on magnetic nanomembranes/nanostructures and magnetic composites. As a concluding remark, this article summarizes the current status of the field and discusses opportunities that underpin future progress.

A liquid-free conducting ionoelastomer for 3D printable multifunctional self-healing electronic skin with tactile sensing capabilities.

Conductive elastomers with both softness and conductivity are widely used in the field of flexible electronics. Nonetheless, conductive elastomers typically exhibit prominent problems such as solvent volatilization and leakage, and poor mechanical and conductive properties, which limit their applications in electronic skin (e-skin). In this work, a liquid-free conductive ionogel (LFCIg) with excellent performance was fabricated by utilizing the innovative double network design approach based on a deep eutectic solvent (DES). The double-network LFCIg is cross-linked by dynamic non-covalent bonds, which exhibit excellent mechanical properties (2100% strain while sustaining a fracture strength of 1.23 MPa) and >90% self-healing efficiency, and a superb electrical conductivity of 23.3 mS m-1 and 3D printability. Moreover, the conductive elastomer based on LFCIg has been developed into a stretchable strain sensor that achieves accurate response recognition, classification, and identification of different robot gestures. More impressively, an e-skin with tactile sensing functions is produced by in situ 3D printing of sensor arrays on flexible electrodes to detect light weight objects and recognize the resulting spatial pressure variations. Collectively, the results demonstrate that the designed LFCIg has unparalleled advantages and presents wide application potential in flexible robotics, e-skin and physiological signal monitoring.

A self-adhesive, self-healing and antibacterial hydrogel based on PVA/MXene-Ag/sucrose for fast-response, high-sensitivity and ultra-durable strain sensors

Flexible hydrogel sensors have attracted significant attention due to their broad applications in soft robots, healthcare monitoring, and electronic skins.