Stretchable Unsymmetrical Piezoelectric BaTiO3 Composite Hydrogel for Triboelectric Nanogenerators and Multimodal Sensors

Wide-humidity, anti-freezing and stretchable multifunctional conductive carboxymethyl cellulose-based hydrogels for flexible wearable strain sensors and arrays

Hydrogels play an important role in the design and fabrication of wearable sensors with outstanding flexibility, high sensitivity and versatility. Since hydrogels lose and absorb water during changes in humidity and temperature, it is critical and challenging to obtain hydrogels that function properly under different environmental conditions. Herein, a dual network hydrogel based on tannic acid (TA) reinforced polyacrylamide (PAM) and sodium carboxymethylcellulose (CMC) was constructed, while the introduction of the green solvents Solketal and LiCl endowed the hydrogel with greater possibilities for further modification to improve the water content and consistency of the mechanical properties over 30-90 % RH. This composite hydrogel (PTSL) has long-term stability, excellent mechanical strength, and freezing resistance. As strain sensors, they are linear over the entire strain range (R2 = 0.994) and have a high sensitivity (GF = 2.52 over 0-680 % strain range). Furthermore, the hydrogel's exceptional electrical conductivity and freezing resistance are a result of the synergistic effect of Solketal and LiCl, which intensifies the contact between the water molecules and the colloidal phase. This research could address the suitability of hydrogels over a wide range of humidity and temperature, suggesting great applications for smart flexible wearable electronics in harsh environmental conditions.

A review of material design for high performance triboelectric nanogenerators: performance improvement based on charge generation and charge loss

As a type of innovative device, triboelectric nanogenerators (TENGs) are capable of converting mechanical energy into electrical energy through the triboelectric effect. Based on the working mechanism, the output performance of TENGs heavily relies on the triboelectric materials used. The modification of triboelectric materials is the most efficient way to improve the output performance of TENGs. Herein, this review focuses on the recent progress in triboelectric material design for high-performance TENGs. First, the basic theory of TENGs is introduced. Second, the relationship between the triboelectric materials and the output performance of TENGs is summarized in detail based on a theoretical model of the triboelectric charge dynamic equilibrium. Furthermore, the relevant strategies are analyzed in detail. Finally, challenges and shortcomings of the triboelectric materials for high-performance TENGs are discussed. This review provides a basis for the research status and future development of triboelectric materials.

Enhanced Positive Charge Performance in Triboelectric Series: Molecular Engineering of Conjugated Mesoporous Fibers for Machine Learning-Based Wearable Sensors

With the increasing focus on triboelectric-based sensors, research on synthesizing dielectric layers from specific substances is gradually emerging. Despite numerous negatively-charged triboelectric materials, there is a scarcity of synthesizable positively-charged materials, creating a research gap. This study demonstrates the molecular design of a conjugated, mesoporous, self-assembled sheet via bottom-up synthesis. The synthesized sheet is functionalized to create a triboelectric nanogenerator. Its large specific surface area, softness, and internal space increase the actual contact area and provide adsorption sites for polypyrrole nanoparticles. The incorporation of -COO functional group enhances positive triboelectric performance, forming a dielectric layer with charge-trapping capabilities. When contact with polytetrafluoroethylene (PTFE), this structure boosts the output voltage, showing significant amplification after charge injection with minimal decay. As a demonstration, the bilayer structure is applied as a touchpad on the experimenter's arm to write symbols. The signals are input into an innovative machine-learning model to interpret the writer's intent. Additionally, the device connects to a terminal for real-time medical services, suggesting practical applications for wearable triboelectric sensors with artificial intelligence.

From electricity to vitality: the emerging use of piezoelectric materials in tissue regeneration

The unique ability of piezoelectric materials to generate electricity spontaneously has attracted widespread interest in the medical field. In addition to the ability to convert mechanical stress into electrical energy, piezoelectric materials offer the advantages of high sensitivity, stability, accuracy and low power consumption. Because of these characteristics, they are widely applied in devices such as sensors, controllers and actuators. However, piezoelectric materials also show great potential for the medical manufacturing of artificial organs and for tissue regeneration and repair applications. For example, the use of piezoelectric materials in cochlear implants, cardiac pacemakers and other equipment may help to restore body function. Moreover, recent studies have shown that electrical signals play key roles in promoting tissue regeneration. In this context, the application of electrical signals generated by piezoelectric materials in processes such as bone healing, nerve regeneration and skin repair has become a prospective strategy. By mimicking the natural bioelectrical environment, piezoelectric materials can stimulate cell proliferation, differentiation and connection, thereby accelerating the process of self-repair in the body. However, many challenges remain to be overcome before these concepts can be applied in clinical practice, including material selection, biocompatibility and equipment design. On the basis of the principle of electrical signal regulation, this article reviews the definition, mechanism of action, classification, preparation and current biomedical applications of piezoelectric materials and discusses opportunities and challenges for their future clinical translation.

Piezoelectric Hydrogels: Hybrid Material Design, Properties, and Biomedical Applications

Hydrogels show great potential in biomedical applications due to their inherent biocompatibility, high water content, and resemblance to the extracellular matrix. However, they lack self-powering capabilities and often necessitate external stimulation to initiate cell regenerative processes. In contrast, piezoelectric materials offer self-powering potential but tend to compromise flexibility. To address this, creating a novel hybrid biomaterial of piezoelectric hydrogels (PHs), which combines the advantageous properties of both materials, offers a systematic solution to the challenges faced by these materials when employed separately. Such innovative material system is expected to broaden the horizons of biomedical applications, such as piezocatalytic medicinal and health monitoring applications, showcasing its adaptability by endowing hydrogels with piezoelectric properties. Unique functionalities, like enabling self-powered capabilities and inducing electrical stimulation that mimics endogenous bioelectricity, can be achieved while retaining hydrogel matrix advantages. Given the limited reported literature on PHs, here recent strategies concerning material design and fabrication, essential properties, and distinctive applications are systematically discussed. The review is concluded by providing perspectives on the remaining challenges and the future outlook for PHs in the biomedical field. As PHs emerge as a rising star, a comprehensive exploration of their potential offers insights into the new hybrid biomaterials.

In Situ Structural Densification of Hydrogel Network and Its Interface with Electrodes for High-Performance Multimodal Artificial Skin

The multisensory responsiveness of hydrogels positions them as promising candidates for artificial skin, whereas the mismatch of modulus between soft hydrogels and hard electrodes as well as the poor adhesion and conductance at the interface greatly impairs the stability of electronics devices. Herein, we propose an in situ postprocessing approach utilizing electrochemical reactions between metals (Zn, etc.) and hydrogels to synergistically achieve strong adhesion of the hydrogel-electrode interface, low interfacial impedance, and local strain isolation due to the structural densification of the hydrogel network. The mechanism is that Zn electrochemically oxidizes to Zn2+ and injects into the hydrogel, gradually forming a mechanically interlocked structure, Zn2+-polymer dual-helix structural nodes, and a high-modulus ZnO from the surface to the interior. Compared to untreated samples, the treated sample displays 8.7 times increased interfacial adhesion energy between the hydrogel and electrode (87 J/m2), 95% decreased interfacial impedance (218.8 Ω), and a high-strain isolation efficiency (εtotal/εisolation > 400). Akin to human skin, the prepared sensor demonstrates multimodal sensing capabilities, encompassing highly sensitive strain perception and simultaneous perception of temperature, humidity, and oxygen content unaffected by strain interference. This easy on-chip preparation of hydrogel-based multimodal sensor array shows great potential for health and environment monitoring as artificial skin.

A fabric-based hydrovoltaic electricity generator with multi-component carbon black for sustainable energy output

Water evaporation-induced electricity generators are considered a promising green energy-harvesting technology to alleviate the increasingly serious fossil energy crisis. Previous water evaporation-induced electricity generators mainly focused on single component carbon black, limiting the improvements in energy output. At present, there are relatively few studies on multi-component carbon black for improving electricity-generation performance. Herein, inspired by plant transpiration, we designed a fabric-based water evaporation-induced electricity generator (FWEG) based on multi-component carbon black, which can maintain a voltage of 0.65 V for more than 48 h. Through the synergistic effect of multi-component carbon black-enhanced oxygen-containing functional density, the FWEG can generate an enhanced output current of 61.61 μA without any additional energy input. Moreover, we show that the FWEG can be integrated readily to charge commercial capacitors or directly power LED lights and calculators for a long time. This work provides new insights for designing high-performance hydrovoltaic electricity generators for sustainable green energy harvesting.

A Novel Multifunctional Photonic Film for Colored Passive Daytime Radiative Cooling and Energy Harvesting

Passive daytime radiative cooling (PDRC) materials with sustainable energy harvesting capability is critical to concurrently reduce traditional cooling energy utilized for thermal comfort and transfer natural clean energies into electricity. Herein, a versatile photonic film (Ecoflex@BTO@UAFL) based on a novel fluorescent luminescence color passive radiative cooling with triboelectric and piezoelectric effect is developed by filling the dielectric BaTiO3 (BTO) nanoparticles and ultraviolet absorption fluorescent luminescence (UAFL) powder into the elastic Ecoflex matrix. Test results demonstrate that the Ecoflex@BTO@UAFL photonic film exhibits a maximum passive radiative cooling effect of ∽10.1 °C in the daytime. Meanwhile, its average temperature drop in the daytime is ~4.48 °C, which is 0.91 °C higher than that of the Ecoflex@BTO photonic film (3.56 °C) due to the addition of UAFL material. Owing to the high dielectric constant and piezoelectric effect of BTO nanoparticles, the maximum power density (0.53 W m-2, 1 Hz @ 10 N) of the Ecoflex@BTO photonic film-based hybrid nanogenerator is promoted by 70.9% compared to the Ecoflex film-based TENG. This work provides an ingenious strategy for combining PDRC effects with triboelectric and piezoelectric properties, which can spontaneously achieve thermal comfort and energy conservation, offering a new insight into multifunctional energy saving.

Gel-Based Triboelectric Nanogenerators for Flexible Sensing: Principles, Properties, and Applications

The rapid development of the Internet of Things and artificial intelligence technologies has increased the need for wearable, portable, and self-powered flexible sensing devices. Triboelectric nanogenerators (TENGs) based on gel materials (with excellent conductivity, mechanical tunability, environmental adaptability, and biocompatibility) are considered an advanced approach for developing a new generation of flexible sensors. This review comprehensively summarizes the recent advances in gel-based TENGs for flexible sensors, covering their principles, properties, and applications. Based on the development requirements for flexible sensors, the working mechanism of gel-based TENGs and the characteristic advantages of gels are introduced. Design strategies for the performance optimization of hydrogel-, organogel-, and aerogel-based TENGs are systematically summarized. In addition, the applications of gel-based TENGs in human motion sensing, tactile sensing, health monitoring, environmental monitoring, human-machine interaction, and other related fields are summarized. Finally, the challenges of gel-based TENGs for flexible sensing are discussed, and feasible strategies are proposed to guide future research.

Stretchable Unsymmetrical Piezoelectric BiO2-x Deposited-Hydrogel as Multimodal Triboelectric Nanogenerators for Biomechanical Motion Harvesting

Enhancing the output performance of triboelectric nanogenerators (TENGs) is essential for increasing their application in smart devices. Oxygen-vacancy-rich BiO2-x nanosheets (BiO2-x NSs) are advanced-engineered nanomaterials with excellent piezoelectric properties. Herein, a stretchable unsymmetrical BiO2-x NSs deposited-hydrogel made of polyacrylamide (PAM) as a multimodal TENG is rationally fabricated, and the performance of TENG can be tailored by controlling the BiO2-x NSs deposition amount and spatial distribution. The alteration of resistance caused by the Poisson effect of PAM/BiO2-x composite hydrogel (H-BiO2-x) can be used as a piezoresistive sensor, and the piezoelectricity of BiO2-x NSs can effectively enhance the density of transfer charge, thus improving the output performance of the H-BiO2-x-based TENG. In addition, the chemical cross-linking between the BiO2-x NSs and the PAM polymer chain allows the hydrogel electrode to have a higher tensile capacity (867%). Used for biomechanical motion signal detection, the sensors made of H-BiO2-x have high sensitivity (gauge factor = 6.93) and can discriminate a range of forces (0.1-5.0 N) at low frequencies (0.5-2.0 Hz). Finally, the prepared TENG can collect biological energy and convert it into electricity. Consequently, the improved TENG shows a good application prospect as multimodal biomechanical sensors by combining piezoresistive, piezoelectric, and triboelectric effects.

Age of Flexible Electronics: Emerging Trends in Soft Multifunctional Sensors

With the commercialization of first-generation flexible mobiles and displays in the late 2010s, humanity has stepped into the age of flexible electronics. Inevitably, soft multifunctional sensors, as essential components of next-generation flexible electronics, have attracted tremendous research interest like never before. This review is dedicated to offering an overview of the latest emerging trends in soft multifunctional sensors and their accordant future research and development (R&D) directions for the coming decade. First, key characteristics and the predominant target stimuli for soft multifunctional sensors are highlighted. Second, important selection criteria for soft multifunctional sensors are introduced. Next, emerging materials/structures and trends for soft multifunctional sensors are identified. Specifically, the future R&D directions of these sensors are envisaged based on their emerging trends, namely i) decoupling of multiple stimuli, ii) data processing, iii) skin conformability, and iv) energy sources. Finally, the challenges and potential opportunities for these sensors in future are discussed, offering new insights into prospects in the fast-emerging technology.

Green and Low-Cost Alkali-Polyphenol Synergetic Self-Catalysis System Access to Fast Gelation of Self-Healable and Self-Adhesive Conductive Hydrogels for Self-Powered Triboelectric Nanogenerators

Biomass-based hydrogels have attracted great attention in flexible and sustainable self-powered power sources but struggled to fabricate in a green, high-efficiency, and low-cost manner. Herein, a novel and facile alkali-polyphenol synergetic self-catalysis system is originally employed for the fast gelation of self-healable and self-adhesive lignin-based conductive hydrogels, which can be regarded as hydrogel electrodes of flexible triboelectric nanogenerators (TENGs). This synergy self-catalytic system comprises aqueous alkali and polyphenol-containing lignin, in which alkali-activated ammonium persulfate (APS) significantly accelerates the generation of radicals and initiates the polymerization of monomers, while polyphenol acts as a stabilizer to avoid bursting polymerization from inherent radical scavenging ability. Furthermore, multiple hydrogen bonds between lignin biopolymers and polyacrylamide (PAM) chains impart lignin-based hydrogels with exceptional adhesiveness and self-healing properties. Intriguingly, the alkaline conditions not only contribute to the solubility of lignin but also impart superior ionic conductivity of lignin-based hydrogel that is applicable to flexible TENG in self-powered energy-saving stair light strips, which holds great promise for industrial applications of soft electronics.

From emerging modalities to advanced applications of hydrogel piezoelectrics based on chitosan, gelatin and related biological macromolecules: A review

The rapid development of functional materials and manufacturing technologies is fostering advances in piezoelectric materials (PEMs). PEMs can convert mechanical energy into electrical energy. Unlike traditional power sources, which need to be replaced and are inconvenient to carry, PEMs have extensive potential applications in smart wearable and implantable devices. However, the application of conventional PEMs is limited by their poor flexibility, low ductility, and susceptibility to fatigue failure. Incorporating hydrogels, which are flexible, stretchable, and self-healing, providing a way to overcome these limitations of PEMs. Hydrogel-based piezoelectric materials (H-PEMs) not only resolve the shortcomings of traditional PEMs but also provide biocompatibility and more promising application potential. This paper summarizes the working principle of H-PEMs. Recent advances in the use of H-PEMs as sensors and in vitro energy harvesting devices for smart wearable devices are described in detail, with emphasis on application scenarios in human body like fingers, wrists, ankles, and feet. In addition, the recent progress of H-PEMs in implantable medical devices, especially the potential applications in human body parts such as bones, skin, and heart, are also elaborated. In addition, challenges and potential improvements in H-PEMs are discussed.

Cellulose nanocrystals boosted hydrophobically associated self-healable conductive hydrogels for the application of strain sensors and electronic devices

Currently, hydrogel-based flexible devices become hot areas for scientists in the field of electronic devices, artificial intelligence, human motion detection, and electronic skin. These devices show responses to external stimuli (mechanical signals) and convert them into electrical signals (resistance, current, and voltage). However, the applications of the hydrogel-based sensor are hampered due to low mechanical properties, high time response, low fatigue resistance, low self-healing nature, and low sensing range. Herein, a strain sensing conductive hydrogel constructed from the CNCs (cellulose nanocrystal) reinforced, in which acrylamide and butyl acrylate work as hydrophilic and hydrophobic monomers respectively. The incorporation of CNCs in the polymeric system has a direct effect on their mechanical properties. The hydrogel having a high amount of CNCs (C4), its fracture stress and fracture strain reached 371.2 kPa and 2108 % respectively as well as self-healing of C4 hydrogel Broke at 499 % strain and bore 197 kPa stress. The elastic behavior of the hydrogels was confirmed by the rheological parameter frequency sweep and strain amplitude. Besides this our designed hydrogel shows an excellent response to deformation with conductivity 420 mS m-1, shows response to small strain (10 %) and large (400 %) strain, and has excellent anti-fatigue resistance with continuous stretching for 700 s at 300 % strain, with 140 msec response time, and gauge factor 7.4 at 750 % strain. The C4 hydrogel can also work as electronic skin when it is applied to different joints like the finger, elbow, neck, etc. The prepared hydrogel can also work as an electronic pen when it is worn to a plastic pen cover.

Largely Improving the Output Performance of Stretchable Triboelectric Nanogenerators via Thermo-Compressive Technology

Stretchable triboelectric nanogenerators (TENGs) are widely applied in wearable and implantable electronics, smart medical devices, and soft robots. However, it is still a challenge to produce stretchable TENGs with both exceptional elasticity and output performance, which limits their application scope. In this work, high-performance stretchable TENGs are developed through a thermo-compression (TC) fabrication process. In particular, a poly(vinylidene fluoride) film is compactly bound to the elastic thermoplastic polyurethane substrate, which inherits excellent stretchability with a strain of up to 815%. Furthermore, owing to the large surface area, tight contact, and effective vertical transport of tribo-induced charges between the coupled fibrous tribo-layer and soft substrate, the TC composite film-based TENGs exhibit a greater output (2-4 times) than unlaminated film-based TENGs. Additionally, the broad universality of this method is proven using various tribo- and substrate materials. The proposed technology provides a novel and effective approach to conjointly boost the output and stretchability of TENGs, showing encouraging application prospects in self-powered wearable and flexible electronics.

Highly Stretchable, Fast Self-Healing, Self-Adhesive, and Strain-Sensitive Wearable Sensor Based on Ionic Conductive Hydrogels

Conductive hydrogels integrate the conductive performance and soft nature, which is like that of human skin. Thus, they are more suitable for the preparation of wearable human-motion sensors. Nevertheless, the integration of outstanding multiple functionalities, such as stretchability, toughness, biocompatibility, self-healing, adhesion, strain sensitivity, and durability, by a simple way is still a huge challenge. Herein, we have developed a multifunctional chitosan/oxidized hyaluronic acid/hydroxypropyl methylcellulose/poly(acrylic acid)/tannic acid/Al3+ hydrogel (CS/OHA/HPMC/PAA/TA/Al3+) by using a two-step method with hydroxypropyl methylcellulose (HPMC), acrylic acid (AA), tannic acid (TA), chitosan (CS), oxidized hyaluronic acid (OHA), and aluminum chloride hexahydrate (AlCl3·6H2O). Due to the synergistic effect of dynamic imine bonds between CS and OHA, dynamic metal coordination bonds between Al3+ and -COOH and/or TA as well as reversible hydrogen, the hydrogel showed excellent tensile property (elongation at break of 3168%) and desirable toughness (0.79 MJ/m3). The mechanical self-healing efficiency can reach 95.5% at 30 min, and the conductivity can recover in 5.2 s at room temperature without stimulation. The favorable attribute of high transparency (98.5% transmittance) facilitates the transmission of the optical signal and enables visualization of the sensor. It also shows good adhesiveness to various materials and is easy to peel off without residue. The resistance of the hydrogel-based sensors shows good electrical conductivity (2.33 S m-1), good durability, high sensing sensitivity (GF value of 4.12 under 1600% strain), low detection limit (less than 1%), and short response/recovery time (0.54/0.31 s). It adhered to human skin and monitored human movements such as the bending movements of joints, swallowing, and speaking successfully. Therefore, the obtained multifunctional conductive hydrogel has great potential applications in wearable strain sensors.

Injectable ultrasound-powered bone-adhesive nanocomposite hydrogel for electrically accelerated irregular bone defect healing

The treatment of critical-size bone defects with irregular shapes remains a major challenge in the field of orthopedics. Bone implants with adaptability to complex morphological bone defects, bone-adhesive properties, and potent osteogenic capacity are necessary. Here, a shape-adaptive, highly bone-adhesive, and ultrasound-powered injectable nanocomposite hydrogel is developed via dynamic covalent crosslinking of amine-modified piezoelectric nanoparticles and biopolymer hydrogel networks for electrically accelerated bone healing. Depending on the inorganic-organic interaction between the amino-modified piezoelectric nanoparticles and the bio-adhesive hydrogel network, the bone adhesive strength of the prepared hydrogel exhibited an approximately 3-fold increase. In response to ultrasound radiation, the nanocomposite hydrogel could generate a controllable electrical output (-41.16 to 61.82 mV) to enhance the osteogenic effect in vitro and in vivo significantly. Rat critical-size calvarial defect repair validates accelerated bone healing. In addition, bioinformatics analysis reveals that the ultrasound-responsive nanocomposite hydrogel enhanced the osteogenic differentiation of bone mesenchymal stem cells by increasing calcium ion influx and up-regulating the PI3K/AKT and MEK/ERK signaling pathways. Overall, the present work reveals a novel wireless ultrasound-powered bone-adhesive nanocomposite hydrogel that broadens the therapeutic horizons for irregular bone defects.

Deep-learning enabled smart insole system aiming for multifunctional foot-healthcare applications

Real-time foot pressure monitoring using wearable smart systems, with comprehensive foot health monitoring and analysis, can enhance quality of life and prevent foot-related diseases. However, traditional smart insole solutions that rely on basic data analysis methods of manual feature extraction are limited to real-time plantar pressure mapping and gait analysis, failing to meet the diverse needs of users for comprehensive foot healthcare. To address this, we propose a deep learning-enabled smart insole system comprising a plantar pressure sensing insole, portable circuit board, deep learning and data analysis blocks, and software interface. The capacitive sensing insole can map both static and dynamic plantar pressure with a wide range over 500 kPa and excellent sensitivity. Statistical tools are used to analyze long-term foot pressure usage data, providing indicators for early prevention of foot diseases and key data labels for deep learning algorithms to uncover insights into the relationship between plantar pressure patterns and foot issues. Additionally, a segmentation method assisted deep learning model is implemented for exercise-fatigue recognition as a proof of concept, achieving a high classification accuracy of 95%. The system also demonstrates various foot healthcare applications, including daily activity statistics, exercise injury avoidance, and diabetic foot ulcer prevention.

Biofilm Microenvironment-Sensitive Piezoelectric Nanomotors for Enhanced Penetration and ROS/NO Synergistic Bacterial Elimination

Sonodynamic therapy offers a highly accurate treatment for bacterial infections; however, its antibacterial efficacy is hindered by bacterial biofilms that limit the penetration of sonosensitizers. Herein, a nitric oxide (NO)-driven mushroom-like Janus nanomotor (BT@PDA-La) based on the unilateral coating of polydopamine (PDA) on piezoelectric tetragonal barium titanate (BT) and further modified with l-arginine (l-Arg) on the PDA side is fabricated. In the infected microenvironment with high levels of H2O2, NO is produced unilaterally from BT@PDA-La, thus leading to its self-propelled movement and facilitating its permeability in the biofilm. Under ultrasonic vibrations, the piezoelectric effect of BT@PDA-La is triggered by the exogenous mechanical wave, and toxic reactive oxygen species (ROS) are efficiently generated via an in situ catalytic reaction. The synergistic treatment with ROS/NO achieved the destruction of biofilms and embedded drug-resistant bacteria in vitro. Importantly, BT@PDA-La exhibits excellent biofilm penetration capacity, effectively eliminating biofilm infection while accelerating the healing of infected muscles by alleviating oxidative stress, regulating inflammatory factors, and accelerating angiogenesis. Collectively, this study provides a promising strategy for enhancing the penetration of pathological environment-driven nanomaterials through biofilms and advances the application of nanomotors for the therapy of bacterial infections in clinical medicine.

A Core-Shell Nanoreinforced Ion-Conductive Implantable Hydrogel Bioelectronic Patch with High Sensitivity and Bioactivity for Real-Time Synchronous Heart Monitoring and Repairing

To achieve synchronous repair and real-time monitoring the infarcted myocardium based on an integrated ion-conductive hydrogel patch is challenging yet intriguing. Herein, a novel synthetic strategy is reported based on core-shell-structured curcumin-nanocomposite-reinforced ion-conductive hydrogel for synchronous heart electrophysiological signal monitoring and infarcted heart repair. The nanoreinforcement and multisite cross-linking of bioactive curcumin nanoparticles enable well elasticity with negligible hysteresis, implantability, ultrahigh mechanoelectrical sensitivity (37 ms), and reliable sensing capacity (over 3000 cycles) for the nanoreinforced hydrogel. Results of in vitro and in vivo experiments demonstrate that such solely physical microenvironment of electrophysiological and biomechanical characteristics combining with the role of bioactive curcumin exert the synchronous benefit of regulating inflammatory microenvironment, promoting angiogenesis, and reducing myocardial fibrosis for effective myocardial infarction (MI) repair. Especially, the hydrogel sensors offer the access for achieving accurate acquisition of cardiac signals, thus monitoring the whole MI healing process. This novel bioactive and electrophysiological-sensing ion-conductive hydrogel cardiac patch highlights a versatile strategy promising for synchronous integration of in vivo real-time monitoring the MI status and excellent MI repair performance.

Skin-Contact Triboelectric Nanogenerator for Energy Harvesting and Motion Sensing: Principles, Challenges, and Perspectives

Energy harvesting has become an increasingly important field of research as the demand for portable and wearable devices continues to grow. Skin-contact triboelectric nanogenerator (TENG) technology has emerged as a promising solution for energy harvesting and motion sensing. This review paper provides a detailed overview of skin-contact TENG technology, covering its principles, challenges, and perspectives. The introduction begins by defining skin-contact TENG and explaining the importance of energy harvesting and motion sensing. The principles of skin-contact TENG are explored, including the triboelectric effect and the materials used for energy harvesting. The working mechanism of skin-contact TENG is also discussed. This study then moves onto the applications of skin-contact TENG, focusing on energy harvesting for wearable devices and motion sensing for healthcare monitoring. Furthermore, the integration of skin-contact TENG technology with other technologies is discussed to highlight its versatility. The challenges in skin-contact TENG technology are then highlighted, which include sensitivity to environmental factors, such as humidity and temperature, biocompatibility and safety concerns, and durability and reliability issues. This section of the paper provides a comprehensive evaluation of the technological limitations that must be considered when designing skin-contact TENGs. In the Perspectives and Future Directions section, this review paper highlights various advancements in materials and design, as well as the potential for commercialization. Additionally, the potential impact of skin-contact TENG technology on the energy and healthcare industries is discussed.

Fish-Wearable Piezoelectric Nanogenerator for Dual-Modal Energy Scavenging from Fish-Tailing

Aiming to develop a self-powered bioelectric tag for fish behavioral studies, here we present a fish-wearable piezoelectric nanogenerator (FWPNG) that can simultaneously harvest the strain energy and the flow impact energy caused by fish-tailing. The FWPNG is fabricated by transferring a 2 μm-thick Nb0.02-Pb(Zr0.6Ti0.4)O3 (PZT) layer from a silicon substrate to a spin-coated polyimide film via a novel zinc oxide (ZnO) release process. The open-circuit voltage of the strain energy harvester reaches 2.3 V under a strain of 1% at an ultra-low frequency of 1 Hz, and output voltage of the impact energy harvester reaches a 0.3 V under a pressure of 82.6 kPa at 1 Hz, which is in good agreement with our theoretical analysis. As a proof-of-concept demonstration, an event-driven underwater acoustic transmitter is developed by utilizing the FWPNG as a trigger switch. Acoustic transmission occurs when the amplitude of fish-tailing is larger than a preset threshold. The dual-modal FWPNG device shows the potential application in self-powered biotags for animal behavioral studies and ocean explorations.

The marriage of immunomodulatory, angiogenic, and osteogenic capabilities in a piezoelectric hydrogel tissue engineering scaffold for military medicine

BACKGROUND

Most bone-related injuries to grassroots troops are caused by training or accidental injuries. To establish preventive measures to reduce all kinds of trauma and improve the combat effectiveness of grassroots troops, it is imperative to develop new strategies and scaffolds to promote bone regeneration.

METHODS

In this study, a porous piezoelectric hydrogel bone scaffold was fabricated by incorporating polydopamine (PDA)-modified ceramic hydroxyapatite (PDA-hydroxyapatite, PHA) and PDA-modified barium titanate (PDA-BaTiO₃, PBT) nanoparticles into a chitosan/gelatin (Cs/Gel) matrix. The physical and chemical properties of the Cs/Gel/PHA scaffold with 0-10 wt% PBT were analyzed. Cell and animal experiments were performed to characterize the immunomodulatory, angiogenic, and osteogenic capabilities of the piezoelectric hydrogel scaffold in vitro and in vivo.

RESULTS

The incorporation of BaTiO₃ into the scaffold improved its mechanical properties and increased self-generated electricity. Due to their endogenous piezoelectric stimulation and bioactive constituents, the as-prepared Cs/Gel/PHA/PBT hydrogels exhibited cytocompatibility as well as immunomodulatory, angiogenic, and osteogenic capabilities; they not only effectively induced macrophage polarization to M2 phenotype but also promoted the migration, tube formation, and angiogenic differentiation of human umbilical vein endothelial cells (HUVECs) and facilitated the migration, osteo-differentiation, and extracellular matrix (ECM) mineralization of MC3T3-E1 cells. The in vivo evaluations showed that these piezoelectric hydrogels with versatile capabilities significantly facilitated new bone formation in a rat large-sized cranial injury model. The underlying molecular mechanism can be partly attributed to the immunomodulation of the Cs/Gel/PHA/PBT hydrogels as shown via transcriptome sequencing analysis, and the PI3K/Akt signaling axis plays an important role in regulating macrophage M2 polarization.

CONCLUSION

The piezoelectric Cs/Gel/PHA/PBT hydrogels developed here with favorable immunomodulation, angiogenesis, and osteogenesis functions may be used as a substitute in periosteum injuries, thereby offering the novel strategy of applying piezoelectric stimulation in bone tissue engineering for the enhancement of combat effectiveness in grassroots troops.

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.

Dynamic Pluronic F127 Crosslinking Enhancement of Biopolymeric Nanocomposites for Piezo-Triboelectric Single-Hybrid Nanogenerators and Self-Powered Sensors

Biomechanical and nanomechanical energy harvesting systems have gained a wealth of interest, resulting in a plethora of research into the development of biopolymeric-based devices as sustainable alternatives. Piezoelectric, triboelectric, and hybrid nanogenerator devices for electrical applications are engineered and fabricated using innovative, sustainable, facile-approach flexible composite films with high performance based on bacterial cellulose and BaTiO₃ , intrinsically and structurally enhanced by Pluronic F127, a micellar cross-linker. The voltage and current outputs of the modified versions with multiwalled carbon nanotube as a conductivity enhancer and post-poling effect are 38 V and 2.8 µA cm^-2 , respectively. The multiconnective devices' power density can approach 10 µW cm^-2 . The rectified output power is capable of charging capacitors, driving light-emitting diode lights, powering a digital watch and interfacing with a commercial microcontroller board to operate as a piezoresistive force sensor switch as a proof of concept. Magnetoelectric studies show that the composites have the potential to be incorporated into magnetoelectric systems. The biopolymeric composites prove to be desirable candidates for multifunctional energy harvesters and electronic devices.

An Advanced Strategy to Enhance TENG Output: Reducing Triboelectric Charge Decay

The Internet of Things (IoT) is poised to accelerate the construction of smart cities. However, it requires more than 30 billion sensors to realize the IoT vision, posing great challenges and opportunities for industries of self-powered sensors. Triboelectric nanogenerator (TENG), an emerging new technology, is capable of easily converting energy from surrounding environment into electricity, thus TENG has tremendous application potential in self-powered IoT sensors. At present, TENG encounters a bottleneck to boost output for large-scale commercial use if just by promoting triboelectric charge generation, because the output is decided by the triboelectric charge dynamic equilibrium between generation and decay. To break this bottleneck, the strategy of reducing triboelectric charge decay to enhance TENG output is focused. First, multiple mechanisms of triboelectric charge decay are summarized in detail with basic theoretical principles for future research. Furthermore, recent advances in reducing triboelectric charge decay are thoroughly reviewed and outlined in three aspects: inhibition and application of air breakdown, simultaneous inhibition of air breakdown and triboelectric charge drift/diffusion, and inhibition of triboelectric charge drift/diffusion. Finally, challenges and future research focus are proposed. This review provides reference and guidance for enhancing TENG output.

Defect-Modified nano-BaTiO3 as a Sonosensitizer for Rapid and High-Efficiency Sonodynamic Sterilization

Multidrug-resistant bacteria caused by the unlimited overuse of antibiotics pose a great challenge to global health. An antibacterial method based on reactive oxygen species (ROS) is one of the effective strategies without inducing bacterial resistance. Owing to the ability of generating ROS, piezocatalytic material-mediated sonodynamic therapy (SDT) has drawn much attention. However, its major challenge is the low ROS generation efficiency in the piezocatalytic process due to the poor charge carrier concentration of piezoelectric materials. Vacancy engineering can regulate the charge density and largely promote ROS generation under ultrasound (US) irradiation. Herein, a US-responsive self-doped barium titanate with controlled oxygen vacancy (Vo) concentrations was successfully synthesized through a facile thermal reduction treatment at different temperatures (i.e., 350, 400, and 450 °C), and the corresponding samples were named as BTO-350, BTO-400, and BTO-450, respectively. Then, the effect of Vo concentrations on ROS generation efficiency during the piezocatalytic process was systematically studied. And BTO-400 was found to possess the highest piezocatalytic activity and excellent sonodynamic antibacterial performance against Escherichia coli and Staphylococcus aureus. Furthermore, its antibacterial mechanism was confirmed that the ROS generated under US could damage bacterial cell membrane and cause considerable leakage of cytoplasmic components and irreversible death of bacteria. Notably, the in vivo results illustrated that the BTO-400 could serve as an effective antibacterial agent and accelerate skin healing via SDT therapy. In all, the Vo defect-modified nano-BaTiO₃ has a noticeable potential to induce a rapid and efficient sterilization as well as skin tissue repair by SDT.

Electrospun Nanofibers Hybrid Wrinkled Micropyramidal Architectures for Elastic Self-Powered Tactile and Motion Sensors

Conformable, sensitive, long-lasting, external power supplies-free multifunctional electronics are highly desired for personal healthcare monitoring and artificial intelligence. Herein, we report a series of stretchable, skin-like, self-powered tactile and motion sensors based on single-electrode mode triboelectric nanogenerators. The triboelectric sensors were composed of ultraelastic polyacrylamide (PAAm)/(polyvinyl pyrrolidone) PVP/(calcium chloride) CaCl₂ conductive hydrogels and surface-modified silicon rubber thin films. The significant enhancement of electrospun polyvinylidene fluoride (PVDF) nanofiber-modified hierarchically wrinkled micropyramidal architectures for the friction layer was studied. The mechanism of the enhanced output performance of the electrospun PVDF nanofibers and the single-side/double-side wrinkled micropyramidal architectures-based sensors has been discussed in detail. The as-prepared devices exhibited excellent sensitivity of a maximum of 20.1 V/N (or 8.03 V/kPa) as tactile sensors to recognize a wide range of forces from 0.1 N to 30 N at low frequencies. In addition, multiple human motion monitoring was demonstrated, such as knee, finger, wrist, and neck movement and voice recognition. This work shows great potential for skin-like epidermal electronics in long-term medical monitoring and intelligent robot applications.

Interfacial Roughness Enhanced Gel/Elastomer Interfacial Bonding Enables Robust and Stretchable Triboelectric Nanogenerator for Reliable Energy Harvesting

Gel-based triboelectric nanogenerator (TENG) has demonstrated promising potentials in stretchable electronics owing to gel electrodes' intrinsic softness, stretchability, and conductivity. However, delamination between gel and elastomer layers in deformations remains a considerable challenge for gel-based TENG, which most often induces structure failure. Herein, gels are regarded as adhesives and further effectively enhances interfacial bonding strength by a rough interface in adhesives' view, which exploits gels' liquid-to-solid transformation. This method just needs surface roughness of elastomer, which avoids chemical modification. Moreover, this method is effective to both organogel with good stickiness and hydrogel with weak stickiness, demonstrating wide applicability to different gels. Owing to the tough gel/elastomer interfacial bonding, TENG-Rough largely solves delamination problem under various deformations and the corresponding output performances of TENG-Rough are also maintained, implying a robust stretchable TENG device for reliable energy harvesting. This work demonstrates a general and facile method to enhance interfacial bonding in an adhesives' way, which provides a view for addressing delamination problem in gel-based TENGs and other kinds of gel-based devices.

Bimetal-organic framework/GOx-based hydrogel dressings with antibacterial and inflammatory modulation for wound healing

Antibiotic resistance of bacteria and persistent inflammation are critical challenges in treating bacteria infected wounds. Thus, it is urgent to develop versatile wound dressings that possess high-efficiency antibacterial performance and inflammation regulation. Herein, we have successfully constructed a hydrogel wound dressing consisting of the bimetallic metal-organic framework (MOF) loaded with glucose oxidase (GOx), termed as MOF(Fe-Cu)/GOx-polyacrylamide (PAM) gel. Hydrogel dressings can provide an efficient cascade-catalyzed system to accelerate wound healing via synergistic antibacterial and inflammatory modulation. Importantly, the catalytic property of the bimetallic MOF(Fe-Cu) is about five times that of the monometallic MOF(Fe). Based on such a cascade-catalyzed system, the abundant gluconic acid and H₂O₂ can be continuously produced by decomposing glucose via GOx. Such gluconic acid can notably improve the peroxidase performance of MOF(Fe-Cu), which can further efficiently decompose H₂O₂ to achieve the antibacterial. Meanwhile, MOF (Fe Cu)/GOx PAM gel can induce macrophages to change into an M2 phenotype, which can accelerate the transformation of the wound microenvironment to a remodeling state and then accelerate angiogenesis and neurogenesis. This work provides multifunctional bioactive materials for accelerating wound healing and will have great potential in clinical applications. STATEMENT OF SIGNIFICANCE: Antibiotic resistance and persistent inflammation are still the critical reasons for the slow healing of bacteria infected wounds. Herein, we prepared a hydrogel wound dressing composed of bimetallic metal organic framework (MOF) loaded with glucose oxidase (GOx). The catalytic activity of the bimetallic MOF(Fe-Cu) is significantly enhanced due to doping of copper, which makes it possess outstanding antibacterial ability based on cascade catalysis. Such dressing can promote the remodeling of inflammatory immunity by regulating macrophage polarization to suppress over-reactive inflammation, further accelerating the healing of bacteria-infected wounds. This study provides an innovative and effective way to accelerate the healing of bacteria infected wound by combining bacteria killing and inflammation modulation.

Multifunctional hydrogels based on chitosan, hyaluronic acid and other biological macromolecules for the treatment of inflammatory bowel disease: A review

Hydrogel is a three-dimensional network polymer material rich in water. It is widely used in the biomedical field because of its unique physical and chemical properties and good biocompatibility. In recent years, the incidence of inflammatory bowel disease (IBD) has gradually increased, and the disadvantages caused by traditional drug treatment of IBD have emerged. Therefore, there is an urgent need for new treatments to alleviate IBD. Hydrogel has become a potential therapeutic platform. However, there is a lack of comprehensive review of functional hydrogels for IBD treatment. This paper first summarizes the pathological changes in IBD sites. Then, the action mechanisms of hydrogels prepared from chitosan, sodium alginate, hyaluronic acid, functionalized polyethylene glycol, cellulose, pectin, and γ-polyglutamic acid on IBD were described from aspects of drug delivery, peptide and protein delivery, biologic therapies, loading probiotics, etc. In addition, the advanced functions of IBD treatment hydrogels were summarized, with emphasis on adhesion, synergistic therapy, pH sensitivity, particle size, and temperature sensitivity. Finally, the future development direction of IBD treatment hydrogels has been prospected.

Recent advances in conductive hydrogels: classifications, properties, and applications

Hydrogel-based conductive materials for smart wearable devices have attracted increasing attention due to their excellent flexibility, versatility, and outstanding biocompatibility. This review presents the recent advances in multifunctional conductive hydrogels for electronic devices. First, conductive hydrogels with different components are discussed, including pure single network hydrogels based on conductive polymers, single network hydrogels with additional conductive additives (i.e., nanoparticles, nanowires, and nanosheets), double network hydrogels based on conductive polymers, and double network hydrogels with additional conductive additives. Second, conductive hydrogels with a variety of functionalities, including self-healing, super toughness, self-growing, adhesive, anti-swelling, antibacterial, structural color, hydrophobic, anti-freezing, shape memory and external stimulus responsiveness are introduced in detail. Third, the applications of hydrogels in flexible devices are illustrated (i.e., strain sensors, supercapacitors, touch panels, triboelectric nanogenerator, bioelectronic devices, and robot). Next, the current challenges facing hydrogels are summarized. Finally, an imaginative but reasonable outlook is given, which aims to drive further development in the future.

Challenges of Emerging Wearable Sensors for Remote Monitoring toward Telemedicine Healthcare

Digitized telemedicine tools with the Internet of Things (IoT) started advancing into our daily lives and have been incorporated with commercial wearable gadgets for noninvasive remote health monitoring. The newly established tools have been steered toward a new era of decentralized healthcare. The advancement of a telemedicine wearable monitoring system has attracted enormous interest in the multimodal big data acquisition of real-time physiological and biochemical information via noninvasive methods for any health-related industries. The expectation of telemedicine wearable creation has been focused on early diagnosis of multiple diseases and minimizing the cost of high-tech and invasive treatments. However, only limited progress has been directed toward the development of telemedicine wearable sensors. This Perspective addresses the advancement of these wearable sensors that encounter multiple challenges on the forefront and technological gaps hampering the realization of health monitoring at molecular levels related to smart materials mostly limited to single use, issues of selectivity to analytes, low sensitivity to targets, miniaturization, and lack of artificial intelligence to perform multiple tasks and secure big data transfer. Sensor stability with minimized signal drift, on-body sensor reusability, and long-term continuous health monitoring provides key analytical challenges. This Perspective also focuses on, promotes, and highlights wearable sensors with a distinct capability to interconnect with telemedicine healthcare for physical sensing and multiplex sensing at deeper levels. Moreover, it points out some critical challenges in different material aspects and promotes what it will take to advance the current state-of-art wearable sensors for telemedicine healthcare. Ultimately, this Perspective is to draw attention to some potential blind spots of wearable technology development and to inspire further development of this integrated technology in mitigating multimorbidity in aging societies through health monitoring at molecular levels to identify signs of diseases.

Recent Progress on Hydrogel-Based Piezoelectric Devices for Biomedical Applications

Flexible electronics have great potential in the application of wearable and implantable devices. Through suitable chemical alteration, hydrogels, which are three-dimensional polymeric networks, demonstrate amazing stretchability and flexibility. Hydrogel-based electronics have been widely used in wearable sensing devices because of their biomimetic structure, biocompatibility, and stimuli-responsive electrical properties. Recently, hydrogel-based piezoelectric devices have attracted intensive attention because of the combination of their unique piezoelectric performance and conductive hydrogel configuration. This mini review is to give a summary of this exciting topic with a new insight into the design and strategy of hydrogel-based piezoelectric devices. We first briefly review the representative synthesis methods and strategies of hydrogels. Subsequently, this review provides several promising biomedical applications, such as bio-signal sensing, energy harvesting, wound healing, and ultrasonic stimulation. In the end, we also provide a personal perspective on the future strategies and address the remaining challenges on hydrogel-based piezoelectric electronics.

Harsh Environmental-Tolerant and High-Performance Triboelectric Nanogenerator Based on Nanofiber/Microsphere Hybrid Membranes

Triboelectric nanogenerator (TENG) can convert tiny mechanical energy into precious electrical energy. Constant improvements to the output performance of TENG is not only the driving force for its sustainable development, but also the key to expand its practical applicability in modern smart devices. However, most previous studies were conducted at room temperature, ignoring the influence of temperature on the output performance of TENG. Additionally, due to thermionic emission effect, the electrons transferred to a dielectric surface can be released into a vacuum after contact electrification. Therefore, TENG cannot maintain an effective electrical output under high-temperature conditions. Here, a series of high-temperature operatable flexible TENGs (HO-TENGs) based on nanofiber/microsphere hybrid membranes (FSHMs) was fabricated by electrospinning and electrospraying. The V_oc of HO-TENG is 212 V, which is 2.33 times higher than that of control TENG. After 10,000 cycle stability tests, the HO-TENG shows excellent durability. Especially, this HO-TENG can maintain 77% electrical output at 70 °C compared to room temperature, showing excellent high-temperature operability. This study can not only provide a reference for the construction of advanced high-performance TENG, but also provide a certain experimental basis for efficient collection of mechanical energy in high-temperature environment and promote the application of TENG devices in harsh environments.

Intelligent wearable devices based on nanomaterials and nanostructures for healthcare

Emerging classes of flexible electronic sensors as alternatives to conventional rigid sensors offer a powerful set of capabilities for detecting and quantifying physiological and physical signals from human skin in personal healthcare. Unfortunately, the practical applications and commercialization of flexible sensors are generally limited by certain unsatisfactory aspects of their performance, such as biocompatibility, low sensing range, power supply, or single sensory function. This review intends to provide up-to-date literature on wearable devices for smart healthcare. A systematic review is provided, from sensors based on nanomaterials and nanostructures, algorithms, to multifunctional integrated devices with stretchability, self-powered performance, and biocompatibility. Typical electromechanical sensors are investigated with a specific focus on the strategies for constructing high-performance sensors based on nanomaterials and nanostructures. Then, the review emphasizes the importance of tailoring the fabrication techniques in order to improve stretchability, biocompatibility, and self-powered performance. The construction of wearable devices with high integration, high performance, and multi-functionalization for multiparameter healthcare is discussed in depth. Integrating wearable devices with appropriate machine learning algorithms is summarized. After interpretation of the algorithms, intelligent predictions are produced to give instructions or predictions for smart implementations. It is desired that this review will offer guidance for future excellence in flexible wearable sensing technologies and provide insight into commercial wearable sensors.

Self-Powered, Implantable, and Wirelessly Controlled NO Generation System for Intracranial Neuroglioma Therapy

Gas therapy is an emerging technology for improving cancer therapy with high efficiency and low side effects. However, due to the existence of the gatekeeper of the blood-brain barrier (BBB) and the limited availability of current drug delivery systems, there still have been no reports on gas therapy for intracranial neuroglioma. Herein, an integrated, self-powered, and wirelessly controlled gas-therapy system is reported, which is composed of a self-powered triboelectric nanogenerator (TENG) and an implantable nitric oxide (NO) releasing device for intracranial neuroglioma therapy. In the system, the patient self-driven TENG converts the mechanical energy of body movements into electricity as a sustainable and self-controlled power source. When delivering energy to light a light-emitting diode in the implantable NO releasing device via wireless control, the encapsulated NO donor s-nitrosoglutathione (GSNO) can generate NO gas to locally kill the glioma cells. The efficacy of the proof-of-concept system in subcutaneous 4T1 breast cancer model in mice and intracranial glioblastoma multiforme in rats is verified. This self-powered gas-therapy system has great potential to be an effective adjuvant treatment modality to inhibit tumor growth, relapse, and invasion via teletherapy.

Wearable Pressure Sensor Array with Layer-by-Layer Assembled MXene Nanosheets/Ag Nanoflowers for Motion Monitoring and Human-Machine Interfaces

Recently, wearable sensors and electronic skin systems have become prevalent, which can be employed to detect the movement status and physiological signals of wearers. Here, a pressure sensor composed of mesh-like micro-convex structure polydimethylsiloxane (PDMS), MXene nanosheet/Ag nanoflower (AgNF) films, and flexible interdigital electrodes was designed by layer-by-layer (LBL) assembly. The unique microstructure of PDMS effectively increases the contact area and improves sensitivity. Moreover, AgNFs were introduced into the MXene as a "bridge," and the synergistic effect of the two further enhanced the performance of the sensor. The pressure sensor has high sensitivity (191.3 kPa^-1), good stability (18,000 cycles), fast response/recovery time (80 ms/90 ms), and low detection limit (8 Pa), so it can be used for all-round monitoring of the human body. Sensing arrays were integrated with a wireless transmitter as an intelligent artificial electronic skin for spatial pressure mapping and human-computer interaction sensing. Moreover, we develop a smart glove by a simple method, combining it with a 3D model for wireless accurate detection of hand poses. This provides ideas for hand somatosensory detection technology, leading to health monitoring, intelligent rehabilitation training, and personalized medicine.

Enhancing Strain-Sensing Properties of the Conductive Hydrogel by Introducing PVDF-TrFE

Conductive hydrogels have attracted attention because of their wide application in wearable devices. However, it is still a challenge to achieve conductive hydrogels with high sensitivity and wide frequency band response for smart wearable strain sensors. Here, we report a composite hydrogel with piezoresistive and piezoelectric sensing for flexible strain sensors. The composite hydrogel consists of cross-linked chitosan quaternary ammonium salt (CHACC) as the hydrogel matrix, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) as the conductive filler, and poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) as the piezoelectric filler. A one-pot thermoforming and solution exchange method was used to synthesize the CHACC/PEDOT: PSS/PVDF-TrFE hydrogel. The hydrogel-based strain sensor exhibits very high sensitivity (GF: 19.3), fast response (response time: 63.2 ms), and wide frequency range (response frequency: 5-25 Hz), while maintaining excellent mechanical properties (elongation at break up to 293%). It can be concluded that enhanced strain-sensing properties of the hydrogel are contributed to both greater change in the relative resistance under stress and wider response to dynamic and static stimulus by adding PVDF-TrFE. This has a broad application in monitoring human motion, detecting subtle movements, and identifying object contours and a hydrogel-based array sensor. This work provides an insight into the design of composite hydrogels based on piezoelectric and piezoresistive sensing with applications for wearable sensors.

A multi-functional zwitterionic hydrogel with unique micro-structure, high elasticity and low modulus

Owing to their tissue-like softness and low modulus, hydrogels minimize the mechanical mismatch with biological tissues and have received wide attention as biomaterials. However, the development of soft hydrogels is often limited by their brittleness. Here, an ultra-soft and tough hydrogel based on zwitterionic poly(sulfobetaine methacrylate) (PSBMA) was designed and successfully prepared. The obtained PSBMA hydrogel exhibits a unique spike-like micro-structure, low modulus, good stretchability and excellent compressive elasticity, due to the formation of a dual-crosslinking structure. The obtained hydrogel also possesses self-healing properties and electromechanical responses to tensile and compressive deformations. Moreover, the hydrogel has good compatibility attributed to its outstanding anti-protein-adsorption properties.

Recent Progress of Nanogenerators for Green Energy Harvesting: Performance, Applications, and Challenges

Natural sources of green energy include sunshine, water, biomass, geothermal heat, and wind. These energies are alternate forms of electrical energy that do not rely on fossil fuels. Green energy is environmentally benign, as it avoids the generation of greenhouse gases and pollutants. Various systems and equipment have been utilized to gather natural energy. However, most technologies need a huge amount of infrastructure and expensive equipment in order to power electronic gadgets, smart sensors, and wearable devices. Nanogenerators have recently emerged as an alternative technique for collecting energy from both natural and artificial sources, with significant benefits such as light weight, low-cost production, simple operation, easy signal processing, and low-cost materials. These nanogenerators might power electronic components and wearable devices used in a variety of applications such as telecommunications, the medical sector, the military and automotive industries, and internet of things (IoT) devices. We describe new research on the performance of nanogenerators employing several green energy acquisition processes such as piezoelectric, electromagnetic, thermoelectric, and triboelectric. Furthermore, the materials, applications, challenges, and future prospects of several nanogenerators are discussed.

Double-Network Hydrogel for Stretchable Triboelectric Nanogenerator and Integrated Electroluminescent Skin with Self-Powered Rapid Visual Sensing

Bio-inspired design plays a very significant role in adapting biological models to technical applications of flexible electronics. The flexible, stretchable, and portable electrode is one of the key technical challenges in the field. Inspired by the responses to mechanical stimuli of natural plants, we designed a highly transparent (over 95%), stretchable (over 1500%), and biocompatible electrode material by using polymerized double-network hydrogel for fabricating a triboelectric nanogenerator (SH-TENG). The SH-TENG can convert tiny mechanical stretching from human movements directly into electrical energy, and is capable of lighting up to 50 LEDs. Benefiting from bio-inspired design, the coplanar, non-overlapping electrode structure breaks through the limitations of conventional electrodes in wearable devices and overcomes the bottleneck of transparent materials. Furthermore, a self-powered raindrop visual sensing system was realized, which can perform quasi-real-time rainfall information monitoring and increase rainfall recognition ability of vehicle automatic driving systems. This study provides a novel strategy for making next-generation stretchable electronic devices and flexible visual sensing systems.

Synergistic Enhancement Properties of a Flexible Integrated PAN/PVDF Piezoelectric Sensor for Human Posture Recognition

The flexible pressure sensor has attracted much attention due to its wearable and conformal advantage. All the same, enhancing its electrical and structural properties is still a huge challenge. Herein, a flexible integrated pressure sensor (FIPS) composed of a solid silicone rubber matrix, composited with piezoelectric powers of polyacrylonitrile/Polyvinylidene fluoride (PAN/PVDF) and conductive silver-coated glass microspheres is first proposed. Specifically, the mass ratio of the PAN/PVDF and the rubber is up to 4:5 after mechanical mixing. The output voltage of the sensor with composite PAN/PVDF reaches 49 V, which is 2.57 and 3.06 times that with the single components, PAN and PVDF, respectively. In the range from 0 to 800 kPa, its linearity of voltage and current are all close to 0.986. Meanwhile, the sensor retains high voltage and current sensitivities of 42 mV/kPa and 0.174 nA/kPa, respectively. Furthermore, the minimum response time is 43 ms at a frequency range of 1-2.5 Hz in different postures, and the stability is verified over 10,000 cycles. In practical measurements, the designed FIPS showed excellent recognition abilities for various gaits and different bending degrees of fingers. This work provides a novel strategy to improve the flexible pressure sensor, and demonstrates an attractive potential in terms of human health and motion monitoring.