Self‐Healable and Stretchable Organic Thermoelectric Materials: Electrically Percolated Polymer Nanowires Embedded in Thermoplastic Elastomer Matrix

Self‐healable and stretchable energy‐harvesting materials can provide a new avenue for the realization of self‐powered wearable electronics, including electronic skins, whose main materials are required to be robust to and stable under external damage and severe mechanical stresses. However, thermoelectric (TE) materials showing both self‐healing properties and stretchability have not yet been demonstrated despite their great potential to harvest thermal energy in the human body. As most existing TE materials are either mechanically brittle or unrecoverable after being subjected to damage, a novel approach is necessary for designing such materials. Herein, self‐healable and stretchable TE materials based on all‐organic composite system wherein polymer semiconductor nanowires are p‐doped with a molecular dopant and embedded in a thermoplastic elastomer matrix are reported. The polymer nanowires are electrically percolated in the matrix, and the resulting composite materials exhibit good TE performance. The composites also exhibit both excellent self‐healing properties under mild heat and pressure conditions and good stretchability. It is believed that this work can be a cornerstone for the design of self‐healable and stretchable energy‐harvesting materials as it provides useful guidelines for imparting electrical conductivity to insulating thermoplastic elastomers, which typically possess versatile and useful mechanical properties.     

Inkjet Printed Large‐Area Flexible Few‐Layer Graphene Thermoelectrics

Graphene‐based organic nanocomposites have ascended as promising candidates for thermoelectric energy conversion. In order to adopt existing scalable printing methods for developing thermostable graphene‐based thermoelectric devices, optimization of both the material ink and the thermoelectric properties of the resulting films are required. Here, inkjet‐printed large‐area flexible graphene thin films with outstanding thermoelectric properties are reported. The thermal and electronic transport properties of the films reveal the so‐called phonon‐glass electron‐crystal character (i.e., electrical transport behavior akin to that of few‐layer graphene flakes with quenched thermal transport arising from the disordered nanoporous structure). As a result, the all‐graphene films show a room‐temperature thermoelectric power factor of 18.7 µW m^?1 K^?2, representing over a threefold improvement to previous solution‐processed all‐graphene structures. The demonstration of inkjet‐printed thermoelectric devices underscores the potential for future flexible, scalable, and low‐cost thermoelectric applications, such as harvesting energy from body heat in wearable applications.     

A Tailorable Family of Elastomeric‐to‐Rigid, 3D Printable,Interbonding Polymer Networks

Soft materials with widely tailorable mechanical properties throughout the material's volume can shape the future of soft robotics and wearable electronics, impacting both consumer and defense sectors. Herein, a platform of 3D printable soft polymer networks with unprecedented tunability of stiffness of nearly three orders of magnitude (MPa to GPa) and an inherent capability to interbond is reported. The materials are based on dynamic covalent polymer networks with variable density of crosslinkers attached to prepolymer backbones via a temperature‐reversible Diels–Alder (DA) reaction. Inherent flexibility of the prepolymer chains and controllable crosslinking density enable 3D printed networks with glass transition temperatures ranging from just a few degrees to several tens of degrees Celsius. Materials with an elastomeric network demonstrate a fast and spontaneous self‐healing behavior at room temperature both in air and under water—a behavior difficult to achieve with other crosslinked materials. Reversible dissociation of DA networks at temperatures exceeding ≈120 °C allows for reprintability, while control of the stereochemistry of DA attachments enables reprogrammable shape memory behavior. The introduced platform addresses current major challenges including control of polymer interbonding, enhanced mechanical performance of printed parts, and reprocessability of 3D‐printed crosslinked materials in the absence of solvent.     

Acylhydrazones as Reversible Covalent Crosslinkers for Self‐Healing Polymers

The utilization of dynamic covalent and noncovalent bonds in polymeric materials offers the possibility to regenerate mechanical damage, inflicted on the material, and is therefore of great interest in the field of self‐healing materials. For the design of a new class of self‐healing materials, methacrylate containing copolymers with acylhydrazones as reversible covalent crosslinkers are utilized. The self‐healing polymer networks are obtained by a bulk polymerization of an acylhydrazone crosslinker and commercially available methacrylates as comonomers to fine‐tune the T_g of the systems. The influence of the amount of acylhydrazone crosslinker and the self‐healing behavior of the polymers is studied in detail. Furthermore, the basic healing mechanism and the corresponding mechanical properties are analyzed.     

A High‐Capacitance Salt‐Free Dielectric for Self‐Healable,Printable, and Flexible Organic Field Effect Transistors and Chemical Sensor

Printable and flexible electronics attract sustained attention for their low cost, easy scale up, and potential application in wearable and implantable sensors. However, they are susceptible to scratching, rupture, or other damage from bending or stretching due to their “soft” nature compared to their rigid counterparts (Si‐based electronics), leading to loss of functionality. Self‐healing capability is highly desirable for these “soft” electronic devices. Here, a versatile self‐healing polymer blend dielectric is developed with no added salts and it is integrated into organic field transistors (OFETs) as a gate insulator material. This polymer blend exhibits an unusually high thin film capacitance (1400 nF cm^?2 at 120 nm thickness and 20–100 Hz). Furthermore, it shows pronounced electrical and mechanical self‐healing behavior, can serve as the gate dielectric for organic semiconductors, and can even induce healing of the conductivity of a layer coated above it together with the process of healing itself. Based on these attractive properties, we developed a self‐healable, low‐voltage operable, printed, and flexible OFET for the first time, showing promise for vapor sensing as well as conventional OFET applications.     

Electrical and Mechanical Self‐Healing in High‐Performance Dielectric Elastomer Actuator Materials

Dielectric elastomers are of interest for actuator applications due to their large actuation strain, high bandwidth, high energy density, and their flexible nature. If future dielectric elastomers are to be used reliably in applications that include soft robotics, medical devices, artificial muscles, and electronic skins, there is a need to design devices that are tolerant to electrical and mechanical damage. In this paper, the first report of self‐healing of both electrical breakdown and mechanical damage in dielectric actuators using a thermoplastic methyl thioglycolate–modified styrene–butadiene–styrene (MGSBS) elastomer is provided. The self‐healing functions are examined from the material to device level by detailed examination of the healing process, and characterization of electrical properties and actuator response before and after healing. It is demonstrated that after dielectric breakdown, the initial dielectric strength can be recovered by up to 67%, and after mechanical damage, a 39% recovery can be achieved with no degradation of the strain–voltage response of the actuators. The elastomer can also heal a combination of mechanical and electrical failures. This work provides a route to create robust and damage tolerant dielectric elastomers for soft robotic and other applications related to actuator and energy‐harvesting systems.     

A Self‐Healable,Highly Stretchable,and Solution Processable Conductive Polymer Composite for Ultrasensitive Strain and Pressure Sensing

Mimicking human skin's functions to develop electronic skins has inspired tremendous efforts in design and synthesis of novel soft materials with simplified fabrication methods. However, it still remains a great challenge to develop electronically conductive materials that are both stretchable and self‐healable. Here it is demonstrated that a ternary polymer composite comprised of polyaniline, polyacrylic acid, and phytic acid can exhibit high stretchability ( ≈ 500%) and excellent self‐healing properties. The polymer composite with optimized composition shows an electrical conductivity of 0.12 S cm^?1. On rupture, both electrical and mechanical properties can be restored with ≈ 99% efficiency in a 24 h period, which is enabled by the dynamic hydrogen bonding and electrostatic interactions. It is further shown that this composite is both strain and pressure sensitive, and therefore can be used for fabricating strain and pressure sensors to detect a variety of mechanical deformations with ultrahigh sensitivity. The sensitivity and sensing range are the highest among all of the reported self‐healable piezoresistive pressure sensors and even surpass most flexible mechanical sensors. Notably, this composite is prepared via a solution casting process, which potentially allows for large‐area, low‐cost fabrication electronic skins.     

Paper‐Like,Thin, Foldable,and Self‐Healable Electronics Based on PVA/CNC Nanocomposite Film

The facile fabrication of thin and foldable self‐healing electronics on a poly(vinyl alcohol)/cellulose nanocrystal (PVA/CNC) composite film is reported. The self‐healing property of the PVA/CNC nanocomposite film can be activated by spraying water on the film surface, via dynamic formation of hydrogen bonding. The self‐healing efficiency of PVA/CNC is influenced by the content of CNC in the film, pH of the spraying solution, and the temperature. Via vacuum filtration and pattern transfer techniques, both a supercapacitor and a temperature sensor are fabricated on the same PVA/CNC film using gold nanosheet (AuNS) and polyaniline/multiwalled nanotube (PANI/MWCNT) electrodes. The fabricated supercapacitor with a gel‐type electrolyte exhibits a high electrochemical performance, and the thermoresistive temperature sensor shows a linear sensitivity with a fast response. Both devices exhibit superior mechanical stability and self‐healing property over 100 repetitive folding and five repetitive healing cycles, respectively, retaining the device performance owing to the percolated network of the conductive materials. This work demonstrates that our paper‐like thin PVA/CNC film‐based self‐healable devices can serve as highly durable and deformable electronics with longevity.     

Self‐Healing,Flexible, and Tailorable Triboelectric Nanogenerators for Self‐Powered Sensors based on Thermal Effect of Infrared Radiation

Self‐healing triboelectric nanogenerators (TENGs) with flexibility, robustness, and conformability are highly desirable for promising flexible and wearable devices, which can serve as a durable, stable, and renewable power supply, as well as a self‐powered sensor. Herein, an entirely self‐healing, flexible, and tailorable TENG is designed as a wearable sensor to monitor human motion, with infrared radiation from skin to promote self‐healing after being broken based on thermal effect of infrared radiation. Human skin is a natural infrared radiation emitter, providing favorable conditions for the device to function efficiently. The reversible imine bonds and quadruple hydrogen bonding (UPy) moieties are introduced into polymer networks to construct self‐healable electrification layer. UPy‐functionalized multiwalled carbon nanotubes are further incorporated into healable polymer to obtain conductive nanocomposite. Driven by the dynamic bonds, the designed and synthesized materials show excellent intrinsic self‐healing and shape‐tailorable features. Moreover, there is a robust interface bonding in the TENG devices due to the similar healable networks between electrification layer and electrode. The output electric performances of the self‐healable TENG devices can almost restore their original state when the damage of the devices occurs. This work presents a novel strategy for flexible devices, contributing to future sustainable energy and wearable electronics.     

Transparent,Mechanically Strong,Extremely Tough,Self‐Recoverable,Healable Supramolecular Elastomers Facilely Fabricated via Dynamic Hard Domains Design for Multifunctional Applications

The design and synthesis of supramolecular self‐healing polymers with high healing efficiency and excellent integrated mechanical properties is challenging due to conflicting attributes of dynamic self‐healing and mechanical properties. Herein, this study introduces a design concept, that is, “dynamic hard domains,” to balance self‐healing performance, mechanical strength, elastic recovery, and at the same time obtain extreme toughness. The essential features of the dynamic hard domains include: (i) a noncrystallized and loose structure, (ii) low binding energy and high mobility, and (iii) sequential dissociation and rapid rearrangement. Based on this strategy, a simple one‐step polycondensation route is reported to synthesize a transparent polyurethane‐urea supramolecular elastomer (PPGTD‐IDA), which successfully combines decent mechanical strength, extreme toughness, outstanding notch‐sensitiveness, self‐recoverability, and room‐temperature self‐healing. Upon rupture, the PPGTD‐IDA completely restores the mechanical properties within 48 h. Furthermore, the results demonstrate repeatable healing of mechanical properties and prominent antiaging healability. Taking advantages of merits of PPGTD‐IDA, it can be utilized for fabricating impact‐resistant materials for protection of aluminum alloys as well as stretchable and self‐healing conductors, which exhibits unique characteristics such as stable conductivity during stretching (even after healing or with notch), and automatic elimination of the notch during stretching/releasing cycles.     

An Amphiphobic Hydraulic Triboelectric Nanogenerator for a Self‐Cleaning and Self‐Charging Power System

Raindrop falling, which is one kind of water motions, contains large amount of mechanical energy. However, harvesting energy from the falling raindrop to drive electronics continuously is not commonly investigated. Therefore, a self‐cleaning/charging power system (SPS) is reported, which can be exploited to convert and store energy from falling raindrop directly for providing a stable and durable output. The SPS consists of a hydraulic triboelectric nanogenerator (H‐TENG) and several embedded fiber supercapacitors. The surface of H‐TENG is amphiphobic, enabling the SPS self‐cleaning. The fiber supercapacitor which uses α‐Fe₂O₃/reduced graphene oxide composite possesses remarkable specific capacitance, excellent electrical stability, and high flexibility. These properties of the fiber supercapacitor make it suitable for a wearable power system. A power raincoat based on the SPS is demonstrated as application. After showering by water flow, which simulates falling raindrops, for 100 s, the power raincoat achieves an open‐circuit voltage of 4 V and lights a light‐emitting diode for more than 300 s. With features of low cost, easy installation, and good flexibility, the SPS harvesting energy from the falling raindrop renders as a promising sustainable power source for wearable and portable electronics.     

An Omni‐Healable Supercapacitor Integrated in Dynamically Cross‐Linked Polymer Networks

Self‐healing will greatly improve the reliability of an energy‐storage device in case of physical damage, but the challenge remains in assembling such a smart device capable of repairing all its electroactive components simultaneously. Here, an omni‐healable supercapacitor that can spontaneously repair its electrolyte, electrodes, and even the interfaces between them in ambient conditions after mechanical damage is reported. The goal is achieved by integrating electrolyte and electrode materials into a dynamic poly(vinyl alcohol)‐based network cross‐linked by diol‐borate ester bonding. The capacitor fast restores its configuration, mechanical properties, and capacitive performances during all 15 breaking/healing cycles without external stimulus, regardless of the breaking positions. Interestingly, the capacitor can tolerate various physical deformations and even tailoring without performance deterioration. The investigation offers a facile and versatile strategy to construct an intrinsically self‐healable energy‐storage device that has potential application for portable/wearable electronics, smart apparels or flexible robots, and so on.     

Single‐Thread‐Based Wearable and Highly Stretchable Triboelectric Nanogenerators and Their Applications in Cloth‐Based Self‐Powered Human‐Interactive and Biomedical Sensing

The development of wearable and large‐area fabric energy harvester and sensor has received great attention due to their promising applications in next‐generation autonomous and wearable healthcare technologies. Here, a new type of “single” thread‐based triboelectric nanogenerator (TENG) and its uses in elastically textile‐based energy harvesting and sensing have been demonstrated. The energy‐harvesting thread composed by one silicone‐rubber‐coated stainless‐steel thread can extract energy during contact with skin. With sewing the energy‐harvesting thread into a serpentine shape on an elastic textile, a highly stretchable and scalable TENG textile is realized to scavenge various kinds of human‐motion energy. The collected energy is capable to sustainably power a commercial smart watch. Moreover, the simplified single triboelectric thread can be applied in a wide range of thread‐based self‐powered and active sensing uses, including gesture sensing, human‐interactive interfaces, and human physiological signal monitoring. After integration with microcontrollers, more complicated systems, such as wireless wearable keyboards and smart beds, are demonstrated. These results show that the newly designed single‐thread‐based TENG, with the advantage of interactive, responsive, sewable, and conformal features, can meet application needs of a vast variety of fields, ranging from wearable and stretchable energy harvesters to smart cloth‐based articles.     

Molecularly Coupled Two‐Dimensional Titanium Oxide and Carbide Sheets for Wearable and High‐Rate Quasi‐Solid‐State Rechargeable Batteries

High‐performance, flexible, and lightweight powering electrodes are urgently needed to meet the increasing interest in deformable electronic devices, particularly those utilizing solid‐state electrolytes and performing at high charging rates, which unfortunately have remained a formidable challenge. Here, by regularly stacking two‐dimensional (2D) titanium oxide and carbide sheets, in which the two kinds of sheets are coupled at the molecular level, a self‐standing electrode is achieved with ideal mechanical durability and excellent electrochemical performance, including superb rate performance (delivering a capacity of 114 mAh g^?1 in 3.4 min) and good cycling stability (remaining >93% after 1000 cycles at 1000 mA g^?1). Profiting from these advantages, a flexible and safe full lithium‐ion battery, employing a poly(ethylene glycol) diamine‐based gel polymer as the electrolyte, possesses an excellent power density of 1412 W kg^?1 while maintaining a high energy density of 59 Wh kg^?1, which outperforms most documented flexible batteries that utilize liquid electrolytes and is even comparable with some cells using coin configurations. Importantly, the performance was well maintained under mechanical deformation and after multiple breaking and self‐healing cycles, demonstrating the feasibility for practical application in wearable powering devices. The results highlight the numerous possibilities for utilizing sheet materials to fabricate wearable electrode materials.     

A Highly Stretchable and Self‐Healing Supramolecular Elastomer Based on Sliding Crosslinks and Hydrogen Bonds

The long application life and stable performance of stretchable electronics have been putting forward requirements for both higher mechanical properties and better self‐healing ability of polymeric substrates. However, for self‐healing materials, simultaneously improving stretchability and robustness is still challenging. Here, by incorporating sliding crosslinker (polyrotaxanes) and hydrogen bonds into a polymer, a highly stretchable and self‐healable elastomer with good mechanical strength is achieved. The elastomer exhibits very high stretchability, such that it can be stretched to 2800% with a fracture strength of 1.05 MPa. Moreover, the elastomer can achieve nearly complete self‐healing (93%) at 55 °C. Next, tensile tests under different temperatures, step extension experiments, and in situ small angle X‐ray scattering confirm that the excellent stretchability is attributed to the combined effects of sliding cyclodextrins along guest chains and hydrogen bonds. Furthermore, a strain sensor by coating the single‐wall carbon nanotubes onto the surface of the elastic substrate is fabricated.     

Dynamic Photomask‐Assisted Direct Ink Writing Multimaterial for Multilevel Triboelectric Nanogenerator

Triboelectric nanogenerator (TENG) devices are extensively studied as a mechanical energy harvester and self‐powered sensor for wearable electronics and physiological monitoring. However, the conventional TENG fabrication involving assembling steps and using the single property of matrix material suffers from simple devices shape and a single level of mechanical response for sensing and energy harvesting. Here, the printed multimaterial matrix for multilevel mechanical‐responsive TENG with on‐demand reconfiguration of shape is reported. A multimaterial 3D printing approach by using dynamic photomask‐assisted direct ink writing printing together with a two‐stage curing hybrid ink is first developed. Multimaterial structures with location‐specific properties, such as tensile modulus, failure stress, and glass transition temperature for controlled deformation, crack propagation path, and sequential shape memory, are directly printed. The printed multimaterial structure with sequential deformation behavior is used to fabricate a multilevel‐TENG (mTENG) device for multiple level mechanical energy harvesters and sensors. It is demonstrated that the mTENG can be embedded in shoe insoles to achieve both comfortable wearing and motion state monitoring. This work provides a new approach to combine multimaterial 3D printing with TENG devices for functional wearable electronics as energy harvester and sensors.     

Transparent Self‐Healing Polymers Based on Encapsulated Plasticizers in a Thermoplastic Matrix

A self‐healing approach for optically transparent thermoplastic polymers, based on plasticizer‐induced solvent welding, is reported. For the specific system investigated, dibutylphthalate (DBP) filled urea‐formaldehyde capsules are dispersed in a polymethylmethacrylate (PMMA) matrix. Upon a damage event, DBP is released into the crack, and locally plasticizes and swells the polymer, enabling it to remend. Two challenges are addressed to maintain optical transparency: minimization of light scatter from the capsules in the polymer matrix and minimization of light scatter from the healed polymer. PMMA films containing DBP capsules have good transmissive properties as a result of the close index match between PMMA and DBP. The transmission properties are better than, for example, when DBP capsules are dispersed into a poorly index matched matrix, such as polystyrene. In the DBP PMMA system, the healed material is inherently index matched to the polymer matrix and thus the polymer's original optical properties are largely restored. Self‐healing using both small capsules, 1.5 μm in diameter, and large capsules, 75 μm in diameter is demonstrated. Smaller capsules are particularly important for thin polymer films which are not thick enough to hold the larger capsules. Polymer films with smaller capsules also have very good transmission properties due to a minimization of light scattering by the small size of the capsules. Large capsules enable healing of larger damage events, but do inherently result in some light scattering. This plasticizer‐based approach to self‐healing is shown to enable recovery of the protective properties and a portion of the mechanical properties of a polymeric film.     

Highly Efficient Self‐Healable and Dual Responsive Cellulose‐Based Hydrogels for Controlled Release and 3D Cell Culture

To face the increasing demand of self‐healing hydrogels with biocompatibility and high performances, a new class of cellulose‐based self‐healing hydrogels are constructed through dynamic covalent acylhydrazone linkages. The carboxyethyl cellulose‐graft‐dithiodipropionate dihydrazide and dibenzaldehyde‐terminated poly(ethylene glycol) are synthesized, and then the hydrogels are formed from their mixed solutions under 4‐amino‐DL‐phenylalanine (4a‐Phe) catalysis. The chemical structure, as well as microscopic morphologies, gelation times, mechanical and self‐healing performances of the hydrogels are investigated with ¹H NMR, Fourier transform infrared spectroscopy, atomic force microscopy, rheological and compression measurements. Their gelation times can be controlled by varying the total polymer concentration or 4a‐Phe content. The resulted hydrogels exhibit excellent self‐healing ability with a high healing efficiency (≈96%) and good mechanical properties. Moreover, the hydrogels display pH/redox dual responsive sol‐gel transition behaviors, and are applied successfully to the controlled release of doxorubicin. Importantly, benefitting from the excellent biocompatibility and the reversibly cross‐linked networks, the hydrogels can function as suitable 3D culture scaffolds for L929 cells, leading to the encapsulated cells maintaining a high viability and proliferative capacity. Therefore, the cellulose‐based self‐healing hydrogels show potential applications in drug delivery and 3D cell culture for tissue engineering.     

Tacky Elastomers to Enable Tear‐Resistant and Autonomous Self‐Healing Semiconductor Composites

Mechanical failure of π‐conjugated polymer thin films is unavoidable under cyclic loading conditions, due to intrinsic defects and poor resistance to crack propagation. Here, the first tear‐resistant and room‐temperature self‐healable semiconducting composite is presented, consisting of conjugated polymers and butyl rubber elastomers. This new composite displays both a record‐low elastic modulus (<1 MPa) and ultrahigh deformability with fracture strain above 800%. More importantly, failure behavior is not sensitive to precut notches under deformation. Autonomous self‐healing at room temperature, both mechanical and electronic, is demonstrated through the physical contact of two separate films. The composite film also shows device stability in the ambient environment over 5 months due to much‐improved barrier property to both oxygen and water. Butyl rubber is broadly applicable to various p‐type and n‐type semiconducting polymers for fabricating self‐healable electronics to provide new resilient electronics that mimic the tear resistance and healable property of human skin.     

On‐Body Bioelectronics: Wearable Biofuel Cells for Bioenergy Harvesting and Self‐Powered Biosensing

The growing power demands of wearable electronic devices have stimulated the development of on‐body energy‐harvesting strategies. This article reviews the recent progress on rapidly emerging wearable biofuel cells (BFCs), along with related challenges and prospects. Advanced on‐body BFCs in various wearable platforms, e.g., textiles, patches, temporary tattoo, or contact lenses, enable attractive advantages for bioenergy harnessing and self‐powered biosensing. These noninvasive BFCs open up unique opportunities for utilizing bioenergy or monitoring biomarkers present in biofluids, e.g., sweat, saliva, interstitial fluid, and tears, toward new biomedical, fitness, or defense applications. However, the realization of effective wearable BFC requires high‐quality enzyme‐electronic interface with efficient enzymatic and electrochemical processes and mechanical flexibility. Understanding the kinetics and mechanisms involved in the electron transfer process, as well as enzyme immobilization techniques, is essential for efficient and stable bioenergy harvesting under diverse mechanical strains and changing operational conditions expected in different biofluids and in a variety of outdoor activities. These key challenges of wearable BFCs are discussed along with potential solutions and future prospects. Understanding these obstacles and opportunities is crucial for transforming traditional bench‐top BFCs to effective and successful wearable BFCs.