Soft Conductors Research Articles

Reversibly Stretchable Organohydrogel-Based Soft Electronics with Robust and Redox-Active Interfaces Enabled by Polyphenol-Incorporated Double Networks.

Hydrogel electrolytes as soft ionic conductors have been extensively exploited to establish skinlike and biocompatible devices. However, in many common hydrogels, there exists irreversible elongation upon prolonged stretching cycles and poor interfacial contact, which have significantly hindered their practical applications where long-term operation at large deformations is needed. Herein, multifunctional soft electronic devices with reversible stretchability and improved electrode/electrolyte interfaces are demonstrated by employing polyacrylamide-based double-network organohydrogel electrolytes soaked with a high content of tannic acid (TA) that affords multiple noncovalent interactions and redox activity. Performances of the TA-rich gels are evaluated for the first time in realizing shape-recoverable stretchable devices against repeated deformations to 500% strain, with superior gel-electrode interfaces exhibiting both intimate adhesion and boosted electrochemical capacitance of >200 mF·cm-2. A maximal 4-fold higher capacitance can be achieved by introducing TA and ethylene glycol (EG) into hydrogels. Moreover, a soft electronic system consisting of stretchable supercapacitors and gel-based microsensors was demonstrated, in which the electronic performance of these devices can be well preserved after >1000 repeated cycles at strains of up to 200%, without obvious residual strain or electrode delamination. This could pave a route to the design of multifunctional gel networks tackling both the mechanical and interfacial issues in soft and biocompatible devices.

Hydrogels as Soft Ionic Conductors in Flexible and Wearable Triboelectric Nanogenerators.

Flexible triboelectric nanogenerators (TENGs) have attracted increasing interest since their advent in 2012. In comparison with other flexible electrodes, hydrogels possess transparency, stretchability, biocompatibility, and tunable ionic conductivity, which together provide great potential as current collectors in TENGs for wearable applications. The development of hydrogel‐based TENGs (H‐TENGs) is currently a burgeoning field but research efforts have lagged behind those of other common flexible TENGs. In order to spur research and development of this important area, a comprehensive review that summarizes recent advances and challenges of H‐TENGs will be very useful to researchers and engineers in this emerging field. Herein, the advantages and types of hydrogels as soft ionic conductors in TENGs are presented, followed by detailed descriptions of the advanced functions, enhanced output performance, as well as flexible and wearable applications of H‐TENGs. Finally, the challenges and prospects of H‐TENGs are discussed.

High-stability conducting polymer-based conformal electrodes for bio-/iono-electronics

Compared to conventional rigid electronics, polymer-based soft electronics conformal to organisms of irregular shapes have emerged as the next-generation devices, especially benefiting long-term bio-interface interactions that avoid mechanical mismatch and consequent adverse immune responses. Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has become a promising candidate for building soft conductors/electrodes due to its good conductance, tunable mechanical stiffness, good biocompatibility and facile fabrication into various structures. However, their high instability towards alkaline, reductants and applied voltage has not yet been fully addressed, which inevitably leads to deteriorated performance in complex physiological environments or weather conditions (e.g., humidity). Such intolerances are rooted in unstable electronic/molecular structures of PEDOT caused by de-doping. Besides the low electrical stability, PEDOT:PSS films also exhibit an impaired overall conductance due to its phase separation into PSS-rich and PEDOT-rich domains. Herein, a general and effective coating strategy is proposed, based on a mechanism of simultaneous molecular rejection and electron conjugation, to improve the stability and boost the conductance. Specifically, a reduced-graphene-oxide (rGO) thin layer can not only protect PEDOT: PSS from being de-doped by alkali, bio-reductants and applied voltage through molecular rejection, to maintain its conductivity and ensure stable functions, but also further boost the overall conductance through a bridging effect with its large conjugated domain. This strategy is compatible with various material fabrication techniques, including blade-coating, dip-coating and extrusion-based printing techniques, enabling the fabrication of conductors/electrodes with different structures. Finally, the advantages of excellent stability and high conformability of the composite films as soft conductors have been demonstrated through practical applications in tissue stimulation, electrophysiological recording and proprioceptive hydrogel skins, exhibiting great promise in bio/iono-electronics and human–machine interactions.

Fibrillation of well-formed conductive aerogel for soft conductors

Cellular-structured aerogels are usually formed by lyophilization with a slow freeze-drying process. However, there remains a challenge to improve the flexibility of a well-formed aerogel with cellular structures. In this study, cellular structural conductive polymer aerogels were fibrillated with the assistance of the post vapor annealing technique and a functional copolymer, polyethylene-block-poly (ethylene glycol) (PBP). The underlying mechanism lies in the thin walls of the cellular-structured aerogel transformed into fibers during the melting and flowing of PBP parts through ethylene glycol vapor annealing. Moreover, the PBP segments of polyethylene and polyethylene glycol (PEG) act as plasticizers and conductivity enhancers, improving the aerogel's flexibility and electrical conductivity. The fibrillated conductive polymer aerogels were used as sensing elements in strain sensors and electrodes in the triboelectric nanogenerator, demonstrating their potential in soft conductors.

A highly sensitive and ultra-stretchable zwitterionic liquid hydrogel-based sensor as anti-freezing ionic skin

Soft conductors show great promise in multifunctional sensor applications.

Hofmeister Effect and Electrostatic Interaction Enhanced Ionic Conductive Organohydrogels for Electronic Applications

AbstractThe emerging cryoprotectant replacement method endows hydrogels with nondrying and antifreezing properties, but the low conductivity still limits wider electronic applications. In this work, the Hofmeister effect and electrostatic interaction are introduced to improve the conductivity of organohydrogels and their enhancement mechanism are studied in depth. The Hofmeister effect mainly influences the physical properties, such as the pore structure and mechanical strength, which subsequently impacts ion transfer during the solvent replacement process. The lithium and sodium bonds formed by the electrostatic interaction play a more important role in the conductivity of organohydrogels and an overall picture is presented based on the synergistic enhancement of the Hofmeister effect and electrostatic interaction to achieve highly ionic conductive organohydrogels. The champion organohydrogels are applied as soft ionic conductors and antireflective layers in triboelectric, photovoltaic, and thermoelectric applications. The proposed mechanism advances the understanding of the contribution of ions to organohydrogels for wearable electronics.

Natural Polymer in Soft Electronics: Opportunities, Challenges, and Future Prospects.

Pollution caused by nondegradable plastics has been a serious threat to environmental sustainability. Natural polymers, which can degrade in nature, provide opportunities to replace petroleum-based polymers, meanwhile driving technological advances and sustainable practices. In the research field of soft electronics, regenerated natural polymers are promising building blocks for passive dielectric substrates, active dielectric layers, and matrices in soft conductors. Here, the natural-polymer polymorphs and their compatibilization with a variety of inorganic/organic conductors through interfacial bonding/intermixing and surface functionalization for applications in various device modalities are delineated. Challenges that impede the broad utilization of natural polymers in soft electronics, including limited durability, compromises between conductivity and deformability, and limited exploration in controllable degradation, etc. are explicitly inspected, while the potential solutions along with future prospects are also proposed. Finally, integrative considerations on material properties, device functionalities, and environmental impact are addressed to warrant natural polymers as credible alternatives to synthetic ones, and provide viable options for sustainable soft electronics.

Influence of eumelanin and gamma irradiation on ZnO nanocomposite properties

It is known that ZnO is at the forefront in recent studies, so one of the most important applications in recent years is improving the properties of zinc oxide by grafting with cheap and environmentally friendly materials. In the present work, the effect of natural eumelanin (Mel) and gamma radiation on the morphological characteristics, structural, optical, and electrical properties of ZnO nanocomposite were routinely studied. The transmission electron microscope (TEM) images showed a change in crystallinity and crystallite size as a result of eumelanin addition and treatment with radiation. FTIR showed structural changes in bond strength of ZnO-Mel and gamma-irradiated ZnO-Mel compared to pure ZnO with the appearance of a new bond. Also, eumelanin addition and gamma irradiation exhibited a decrease in reflectance, a red-shift in bandgap energy, and an increase in Urbach energy. Moreover, an increase in the ac-conductivity was confirmed. Finally, in comparison to the complicated preparation of industrial semiconductors, the ease of use of materials composed of organic-non-organic materials with it irradiation treatment will open new avenues for the use of soft conductors.

Bridging oxygens, the key to the electrical behaviour improvement of an ionic liquid - glass composite

The interactions between the [BMIM][PF6] -ionic liquid- and a phosphate glassy surface is studied through calorimetric, structural and impedance spectroscopy methods. A soft solid conductor material is obtained mixing the ionic glass powder with the ionic liquid in a low proportion (2% and 4%). At microscopic level, the interactions of this ionic liquid with the glassy particle surfaces reveal the formation of new bridging oxygens. Therefore, the material electrical conductivity increases in more than four orders of magnitude. This finding uncovers the essential elements to be considered before incorporating an additional salt to develop a solid electrolyte.

Stimuli‐Driven Insulator–Conductor Transition in a Flexible Polymer Composite Enabled by Biphasic Liquid Metal

Metal-polymer composites (MPCs) with combined properties of metals and polymers have achieved much industrial success. However, metals in MPCs are thought to be ordinary and invariable electrically conductive fillers in supportive polymers to show limited use in modern technologies. This work that is disclosed here, for the first time, introduces stimuli-driven transition from biphasic to monophasic state of liquid metal into polymer science to form dynamic soft conductors from the binary metal-polymer composites. The binary metal that exhibits temperature-driven reversible transition between solid and liquid states via a biphasic state is fabricated. A conducting stretchable polymer composite is developed using the judiciously chosen biphasic binary metal that undergoes conductor to insulator transition upon stretching. Insulating stretched films become conducting upon heating. A "tube" model elegantly describes such distinctive deformation/temperature-dependent behaviors. Moreover, the conducting polymer composite shows decrease in its resistance upon increasing the sample temperature. The resistance can be tuned from 1 to 108 Ω depending on the state of binary metal in the phase diagram. This work would build the intimate and interesting connection between metal phases and polymer science toward next-generation soft conductors and beyond.

Ionic conductive gels based on deep eutectic solvents

ABSTRACT Ionic conductive gels are promising soft conductors for stretchable and flexible electronics. Unfortunately, the basic requirements for an ideal ionic conductive gel, including environmental and chemical stability as well as low toxicity and low cost do not usually coincide in one material. Here we report an ionic conductive gel containing deep eutectic solvents (DES), which are specialized ionic liquids composed of propylene glycol (PG, hydrogen bonding donor) and various salts. In experiments, the DES-gel loses about 10% of the original weight in dry air (5– 9% relative humidity) for 7 days and its conductivity remains unaltered. Comparing to regular salt-containing hydrogels, it exhibits exceptional resistance to metal corrosion. We demonstrate the advantage of DES-gels as the ionic electrode in a flexible and stretchable electroluminescent device, especially in cold environment (−20°C). Through this study, we discover a new candidate for cheap and safe soft ionic conductors with excellent environmental and chemical stability.

Dynamically Crosslinked Dry Ion-Conducting Elastomers for Soft Iontronics.

Soft ionic conductors show great promise in multifunctional iontronic devices, but currently utilized gel materials suffer from liquid leakage or evaporation issues. Here, a dry ion-conducting elastomer with dynamic crosslinking structures is reported. The dynamic crosslinking structures endow it with combined advantageous properties simultaneously, including high ionic conductivity (2.04 × 10-4 S cm-1 at 25 °C), self-healing capability (96% healing efficiency), stretchability (563%), and transparency (78%). With this ionic conductor as the electrode, two soft iontronic devices (electroluminescent devices and triboelectric nanogenerator tactile sensors) are realized with entirely self-healing and stretchable capabilities. Due to the absence of liquid materials, the dry ion-conducting elastomer shows wide operational temperature range, and the iontronic devices achieve excellent stability. These findings provide a promising strategy to achieve highly conductive and multifunctional soft dry ionic conductors, and demonstrate their great potential in soft iontronics or electronics.

Electrolyte and anode‐electrolyte interphase in solid‐state lithium metal polymer batteries: A perspective

AbstractThe interest for solid‐state lithium metal (Li°) batteries (SSLMBs) has been growing exponentially in recent years in view of their higher energy density and eliminated safety concerns. Solid polymer electrolytes (SPEs) are soft ionic conductors which can be easily processed into thin films at industrial level; these unique features confer solid‐state Li° polymer batteries (SSLMPBs, i.e., SSLMBs utilizing SPEs as electrolytes) distinct advantages compared to SSLMBs containing other electrolytes. In this article, we briefly review recent progresses and achievements in SSLMPBs including the improvement of ionic conductivity of SPEs and their interfacial stability with Li° anode. Moreover, we outline several advanced in‐situ and ex‐situ characterizing techniques which could assist in‐depth understanding of the anode‐electrolyte interphases in SSLMPBs. This article is hoped not only to update the state‐of‐the‐art in the research on SSLMPBs but also to bring intriguing insights that could improve the fundamental properties (e.g., transport, dendrite formation, and growth, etc.) and electrochemical performance of SSLMPBs.

A Mechanically Robust and Versatile Liquid-Free Ionic Conductive Elastomer.

Soft ionic conductors, such as hydrogels and ionogels, have enabled stretchable and transparent ionotronics, but they suffer from key limitations inherent to the liquid components, which may leak and evaporate. Here, novel liquid-free ionic conductive elastomers (ICE) that are copolymer networks hosting lithium cations and associated anions via lithium bonds and hydrogen bonds are demonstrated, such that they are intrinsically immune from leakage and evaporation. The ICEs show extraordinary mechanical versatility including excellent stretchability, high strength and toughness, self-healing, quick self-recovery, and 3D-printability. More intriguingly, the ICEs can defeat the conflict of strength versus toughness-a compromise well recognized in mechanics and material science-and simultaneously overcome the conflict between ionic conductivity and mechanical properties, which is common for ionogels. Several liquid-free ionotronics based on the ICE are further developed, including resistive force sensors, multifunctional ionic skins, and triboelectric nanogenerators (TENGs), which are not subject to limitations of previous gel-based devices, such as leakage, evaporation, and weak hydrogel-elastomer interfaces. Also, the 3D printability of the ICEs is demonstrated by printing a series of structures with fine features. The findings offer promise for a variety of ionotronics requiring environmental stability and durability.

Pneumatic Enabled Vertical Interconnect Access of Liquid Alloy Circuits toward Highly Integrated Stretchable Electronics

AbstractMultilayer circuits, particularly in stretchable electronics, are always considered as an effective way to integrate diverse components and modules for advanced/powerful functions required in the potential applications or complex scenarios. As a desirable soft conductor, liquid alloy circuits exhibit super compliance and stretchability. However, the lacking of reliable ways to fabricate multilayer circuits like other stretchable electronics, the advantage of liquid alloy stretchable electronics is far away well exploited, especially from the perspectives of application developments. Here, based on the unique features of rapid oxidation and fluidity from liquid alloy, a facile method is presented to achieve multilayer circuits connection with vertical interconnect accesses enabled by pneumatic blowing and surface topography tuning. This approach provides a new way to achieve high integration of liquid alloy based stretchable electronics, for example, a double‐layer soft LED array, a conformal six‐layer wireless charging, and a wireless communicated wearable device for temperature and humidity real‐time monitoring. Furthermore, compatible with conventional industry process, it may open a new window for making advanced or powerful liquid alloy stretchable electronics for various applications in complex scenarios.

Natural Biopolymer-Based Biocompatible Conductors for Stretchable Bioelectronics.

Biocompatible conductors are important components for soft and stretchable bioelectronics for digital healthcare, which have attracted extensive research efforts. Natural biopolymers, compared to other polymers, possess unique features that make them promising building blocks for biocompatible conductors, such as good biocompatibility/biodegradability, natural abundance, sustainability, and capability, can be processed into various functional formats with tunable material properties under benign conditions. In this comprehensive review, we focus on the recent advances in biocompatible conductors based on natural biopolymers for stretchable bioelectronics. We first give a brief introduction of conductive components and natural polymers and summarize the recent development of biocompatible conductors based on representative natural biopolymers including protein (silk), polypeptide (gelatin), and polysaccharide (alginate). The design and fabrication strategies for biocompatible conductors based on these representative biopolymers are outlined, after the chemical structure and properties of such biopolymers are presented. Then we discuss the electronic component-biopolymer interface and bioelectronic-biological tissue (skin and internal tissues) interface, highlight various fabrication techniques of biocompatible conductors for soft bioelectronics, and introduce representative examples of utilizing natural biopolymer-based biocompatible conductors for on-skin bioelectronics, textile-based wearable electronics, and implantable bioelectronics for digital healthcare. Finally, we present concluding remarks on challenges and prospects for designing natural biopolymers for soft biocompatible conductors and bioelectronics.

A Low-Swelling Polymeric Mixed Conductor Operating in Aqueous Electrolytes.

Organic mixed conductors find use in batteries, bioelectronics technologies, neuromorphic computing, and sensing. While great progress has been achieved, polymer-based mixed conductors frequently experience significant volumetric changes during ion uptake/rejection, i.e., during doping/de-doping and charging/discharging. Although ion dynamics may be enhanced in expanded networks, these volumetric changes can have undesirable consequences, e.g., negatively affecting hole/electron conduction and severely shortening device lifetime. Here, the authors present a new material poly[3-(6-hydroxy)hexylthiophene] (P3HHT) that is able to transport ions and electrons/holes, as tested in electrochemical absorption spectroscopy and organic electrochemical transistors, and that exhibits low swelling, attributed to the hydroxylated alkyl side-chain functionalization. P3HHT displays a thickness change upon passive swelling of only +2.5%, compared to +90% observed for the ubiquitous poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, and +10 to +15% for polymers such as poly(2-(3,3'-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-[2,2'-bithiophen]-5-yl)thieno[3,2-b]thiophene) (p[g2T-TT]). Applying a bias pulse during swelling, this discrepancy becomes even more pronounced, with the thickness of P3HHT films changing by <10%while that of p(g2T-TT) structures increases by +75 to +80%. Importantly, the initial P3HHT film thickness is essentially restored after de-doping while p(g2T-TT) remains substantially swollen. The authors, thus, expand the materials-design toolbox for the creation of low-swelling soft mixed conductors with tailored properties and applications in bioelectronics and beyond.

Highly Stretchable and Biocompatible Liquid Metal-Elastomer Conductors for Self-Healing Electronics.

Highly stretchable, conductive, biocompatible conductors, and connectors are crucial for the fabrication of flexible devices. However, it remains a problem to get highly stretchable, conductive materials with low cost on a large scale. Another problem in production is the connection between soft and rigid components. Here, a new conductive nanocomposite is reported by mixing the 11-mercaptoundecanoic acid (MUA) modified liquid metal (LM) nanoparticles with polystyrene-block-polybutadiene-block-polystyrene (SBS), which is biocompatible (in vivo and in vitro), conductive (12000Scm-1 of conductivity), and stretchable (800% of elongation). Apart from its good performance, this material can be produced on a large scale by using a commercial polymer product and a straightforward physical production process. MUA is used to compromise the dense "gallium oxide shell" of liquid metal nanoparticles such that the whole composite can become conductive. By using resin to modify this composite, this new conductive material can be adhesive and highly conductive, and serve as a stable and efficient connector between soft conductor and rigid component.

High-performance compliant thermoelectric generators with magnetically self-assembled soft heat conductors for self-powered wearable electronics

Softening of thermoelectric generators facilitates conformal contact with arbitrary-shaped heat sources, which offers an opportunity to realize self-powered wearable applications. However, existing wearable thermoelectric devices inevitably exhibit reduced thermoelectric conversion efficiency due to the parasitic heat loss in high-thermal-impedance polymer substrates and poor thermal contact arising from rigid interconnects. Here, we propose compliant thermoelectric generators with intrinsically stretchable interconnects and soft heat conductors that achieve high thermoelectric performance and unprecedented conformability simultaneously. The silver-nanowire-based soft electrodes interconnect bismuth-telluride-based thermoelectric legs, effectively absorbing strain energy, which allows our thermoelectric generators to conform perfectly to curved surfaces. Metal particles magnetically self-assembled in elastomeric substrates form soft heat conductors that significantly enhance the heat transfer to the thermoelectric legs, thereby maximizing energy conversion efficiency on three-dimensional heat sources. Moreover, automated additive manufacturing paves the way for realizing self-powered wearable applications comprising hundreds of thermoelectric legs with high customizability under ambient conditions.

Bioinspired Self-Healing Human-Machine Interactive Touch Pad with Pressure-Sensitive Adhesiveness on Targeted Substrates.

There is an increasing interest to develop a next generation of touch pads that require stretchability and biocompatibility to allow their integration with a human body, and even to mimic the self-healing behavior with fast functionality recovery upon damage. However, most touch pads are developed based on stiff and brittle electrodes with the lack of the important nature of self-healing. Polyzwitterion-clay nanocomposite hydrogels as a soft, stretchable, and transparent ionic conductor with transmittance of 98.8% and fracture strain beyond 1500% are developed, which can be used as a self-healing human-machine interactive touch pad with pressure-sensitive adhesiveness on target substrates. A surface-capacitive touch system is adopted to sense a touched position. Finger positions are perceived during both point-by-point touch and continuous moving. Hydrogel touch pads are adhered to curved or flat insulators, with the high-resolution and self-healable input functions demonstrated by drawing, writing, and playing electronic games.