Stretchable Conductors Research Articles

Printable and Highly Stretchable Viscoelastic Conductors with Kinematically Reconstructed Conductive Pathways.

Printable and stretchable conductors based on metallic-filler-reinforced polymer composites that can maintain high electrical conductivity at large strains are essential for emerging applications in wearable electronics, soft robotics, and bio-integrated devices. Regulating microstructures of conductive fillers during mechanical deformations is the key to reconstructing the conductive pathway and retaining high electrical conductivity, which has proven to be challenging. Here, it is reported that Ag flakes can spontaneously reorganize inside a viscoelastic, liquid-like polymer matrix by cyclic mechanical stretching, resulting in reconstructed microstructures and forming highly efficient and stable conductive pathways. Consequently, the electrical conductivities of the resultant composites can be dramatically enhanced by ≈4-8 orders of magnitude and reach ≈104 S cm-1 . The stretch-induced kinematic movements of Ag flakes inside the polymer matrix, together with the reorganization and stabilization mechanisms, are unraveled and validated by the dissipative particle dynamics simulations. This unique phenomenon enables high-performance stretchable conductors to be fabricated with significantly reduced conductive fillers. The printable and stretchable composites presented here hold great promise for use in soft and stretchable electronics, as demonstrated in stretchable light-emitting diode arrays and wearable electronics.

Soft elastomer coatings for ionogels

Ionogels are stretchable ionic conductors with unique advantages, such as non-volatility, thermal, chemical, and electrochemical stability. Nevertheless, the exchange of solvent between ionogels and the external environment hinders their applications. Ionogel has the problem of ionic liquid (IL) leakage, or the ionogel absorbs moisture from the atmosphere. Here we develop a strategy to coat thin elastomers on ionogels. Silanes are added into the precursor of ionogels and elastomers to form a strong interface adhesion between the two polymer networks. The elastomer coating can prevent ionogels from absorbing moisture or leaking ionic liquid, while does not change their transparency, stretchability, and conductivity. We establish a procedure to quantify the loss of ionic liquid from ionogels, with the calibrated relation of conductivity/volume ratio. We find that the elastomer coatings work well because of their low diffusivity to ionic liquid on the order of 10−16m2/s. The coating strategy would be universal to various ionogels and elastomers, thus we expect its application in soft robots and stretchable devices made of ionogels.

Tailoring Silver Nanowire Nanocomposite Interfaces to Achieve Superior Stretchability, Durability, and Stability in Transparent Conductors.

Silver nanowires (AgNWs) have been considered as a promising candidate for transparent stretchable conductors (TSCs). However, the strong interface mismatch of stiff AgNWs and elastic substrates leads to the stress concentration at their interface and ultimately the low stretchability and poor durability of TSCs. Here, to address the interfacial mismatch of AgNWs-based TSCs we put forward a universal interface tailoring strategy that introduces the mercapto compound as the intermediate cross-linked layer. The mercapto compound strongly interacts with the AgNWs, forming a dense protective layer on their surface to improve their corrosion resistance, and reacts with the polymer substrate, forming a buffer layer to release the concentrated stress. As a result, the optimized TSCs showed superior stretchability (160%), exceptional durability (230 000 cycles), competent optoelectrical performance (18.0 ohm·sq-1 with a transmittance of 86.5%), and prominent stability. This work provides clear guidance and a strong impetus for the development of transparent stretchable electronics.

Sliding Cyclodextrin Molecules along Polymer Chains to Enhance the Stretchability of Conductive Composites

The demand for stretchable electronics with a broader working range is increasing for wide application in wearable sensors and e-skin. However, stretchable conductors based on soft elastomers always exhibit low working range due to the inhomogeneous breakage of the conductive network when stretched. Here, a highly stretchable and self-healable conductor is reported by adopting polyrotaxane and disulfide bonds into the binding layer. The binding layer (PR-SS) builds the bridge between polymer substrates (PU-SS) and silver nanowires (AgNWs). The incorporation of sliding molecules endows the stretchable conductor with a long sensing range (190%) due to the energy dissipation derived from the sliding nature of polyrotaxanes, which is two times higher than the working range (93%) of conductors based on AP-SS without polyrotaxanes. Furthermore, the mechanism of sliding effect for the polyrotaxanes in the elastomers is investigated by SEM for morphological change of AgNWs, in situ small-angle x-ray scattering, as well as stress relaxation experiments. Finally, human-body-related sensing tests and a self-correction system in fitness are designed and demonstrated.

Highly stretchable multifunctional polymer ionic conductor with high conductivity based on organic-inorganic dual networks

Ionogels based on ionic liquids have attracted great attention in the field of energy storage and sensing due to their intrinsic advantages, including good thermal stability, low flammability and volatility, and favorable electrochemical properties. However, the introduction of ionic liquids usually sacrifices the mechanical properties of ionogels, such as stretchability and strength. Herein, inspired by reinforced concrete structures, an ionogel with rigid inorganic and flexible organic interpenetrating dual network is created, which exhibits high mechanical strength, excellent elongation (nearly 900%), good elasticity and high room temperature ionic conductivity (1.34 mS cm−1) by exploring the synergistic effect between the rigid and flexible components. Such a highly stretchable conductive organic–inorganic dual network is constructed through non-hydrolyzed sol–gel reaction of tetrabutyl titanate and in situ polymerization of butyl acrylate through UV irradiation. Impressively, the assembled energy stroage device (lithium-ion batteries) based on the ionogel exhibits over 1000 stable cycles with average Columbic efficiency of ∼ 100%, and excellent rate property. The resulting stretchable lithium-ion battery can power an LED at 50% strain. The ionogel, as ionic skin, also shows highly repeatable electrical responses in reciprocal deformation cycles. In terms of the above unusual features, the as-perpared ionogel manifests great promise for versatile applications in the field of flexible electronics.

A Highly Stretchable and Permeable Liquid Metal Micromesh Conductor by Physical Deposition for Epidermal Electronics.

Stretchable electronics allow functional devices to integrate with human skin seamlessly in an emerging wearable platform termed epidermal electronics. Compliant conductors represent key building components for functional devices. Among the various candidates, gallium-based liquid metals stand out with metallic conductivity and inherent deformability. Currently, the widespread applications of liquid metals in epidermal electronics are hindered by the low steam permeability and hence unpleasant wearing perceptions. In this study, a facile physical deposition approach is established to create a liquid metal micromesh over an elastomer sponge, which exhibits low sheet resistance (∼0.5 Ω sq-1), high stretchability (400% strain), and excellent durability. The porous micromesh shows textile-level permeability to achieve long-term wearing comfort. The conformal interaction of the liquid metal micromesh with the skin gives rise to a low contact impedance. An integrated epidermal sensing sleeve is demonstrated as a human-machine interface to distinguish different hand gestures by recording muscle contractions. The reported stretchable and permeable liquid metal conductor shows promising potentials in next-generation epidermal electronics.

Carbon Nanotube/Polymer Coaxial Cables with Strong Interface for Damping Composites and Stretchable Conductors

AbstractCarbon nanotube (CNT) fibers have the potential to serve as continuous nano‐fillers for structural and functional composite materials, yet there exists the interfacial problem that has hindered the development of continuous fiber reinforced composites for a long time. Here, a method to overcome this challenge by coating a thick CNT sponge around a single‐walled nanotube (SWNT) fiber is presented, and then infiltrating polymers into the sponge to construct a SWNT@polymer coaxial cable. It has been found that the residual stress‐induced deposition of amorphous carbon as an intermediate layer between the underlying SWNT fiber and the subsequently coated multi‐walled nanotube (MWNT) sponge plays a key role in interfacial properties, and together with well‐dispersed CNT in the polymer matrix, a superior interfacial shear strength (>12 MPa), enhanced tensile strength and toughness is achieved. These coaxial cables demonstrate high mechanical damping ability and can serve as stretchable conductors maintaining high electrical conductivity during large‐strain cyclic deformation. These results indicate a promising route toward developing continuous CNT fiber‐based composites with wide applications as structural and multi‐functional composite materials.

Stretchable 3D Wideband Dipole Antennas from Mechanical Assembly for On-Body Communication.

The development of wearable/stretchable electronics could largely benefit from advanced stretchable antennas with excellent on-body performance upon mechanical deformations. Despite recent developments of stretchable antennas based on intrinsically stretchable conductors, they are often affected by lossy human tissues and exhibit resonant frequency shifts upon stretching, preventing their applications in on-body wireless communication and powering. This work reports a three-dimensional (3D) stretchable wideband dipole antenna from mechanical assembly to simultaneously reduce the frequency detuning and enhance on-body performance. The large bandwidth is achieved by coupling two resonances from two pairs of radiation arms, which is well-maintained even when the antenna is directly placed on human bodies or stretched over 25%. Such an excellent on-body performance allows the antenna to robustly transmit the wireless data and energy. The design of the 3D stretchable wideband dipole antenna with significantly enhanced on-body wireless communication performance was validated by an experimental demonstration that features a small difference in the wirelessly received power between the on-body and off-body use. The combination of the mechanically assembled 3D geometries and the coupled mechanical-electromagnetic properties can open up new opportunities in deformable 3D antennas and other microwave devices with excellent on-body performance and tunable properties.

Transparent, stretchable and anti-freezing hybrid double-network organohydrogels

Stretchable ionic conductors with high transparency and excellent resilience are highly desired for flexible electronics, but traditional ionic conductive hydrogels are easy to dry and freeze. Herein, a newly hybrid crosslinking strategy is presented for preparing a stretchable and transparent hydrogel by using sodium alginate (SA) and acrylamide based on the unique physically and covalently hybrid crosslinking mechanism, which is transformed into organohydrogel by simple solvent replacement. Due to the combination of hybrid crosslinking double network and hydrogen bond interactions introduced by the glycerin-water binary solvent, the SA-poly (acrylamide)-organohydrogel (SPOH) demonstrates excellent anti-freezing (−20°C) property, stability (>2 days), transparency, stretchability (∼1600%) and high ionic conductivity (17.1 mS cm−1). Thus, a triboelectric nanogenerator made from SPOH (O-TENG) shows an instantaneous peak power density of 262 mW m−2 at a load resistance of 10 M∛ and efficiently harvests biomechanical energy to drive an electronic watch and light-emitting diode. Moreover, The O-TENG exhibits favorable long-term stability (2 weeks) and temperature tolerance (-20°C). In addition, the raw materials can be prepared into SPOH fibers by a simple tubular mold method, exhibiting high transparency, which can be used for laser transmission. The various abilities of the SPOH promise the application of energy harvesting and laser transmission for wearable electronics and biomedical field.

A Waterproof Ion‐Conducting Fluorinated Elastomer with 6000% Stretchability, Superior Ionic Conductivity, and Harsh Environment Tolerance

AbstractThe development of ionic conductors with extreme stretchability, superior ionic conductivity, and harsh‐environment resistance is urgent while challenging because the tailoring of these performances is mutually exclusive. Herein, a hydrophobicity‐constrained association strategy is presented for fabricating a liquid‐free ion‐conducting fluorinated elastomer (ICFE) with microphase‐separated structures. Hydrophilic nanodomains with long‐range ordering and selectively enriched Li ions provided high‐efficient conductive pathways, yielding impressive room‐temperature ionic conductivity of 3.5 × 10–3 S m–1. Hydrophobic nanodomains with abundant and reversible hydrogen bonds endow the ICFE with superior damage‐tolerant performances including ultrastretchability (>6000%), large toughness (17.1 MJ m–3) with notch insensitivity, antifatigue ability, and high‐efficiency self‐healability. Due to its liquid‐free characteristic and surface‐enriched hydrophobic nanodomains, the ICFE demonstrates an extreme temperature tolerance (−20 to 300 °C) and unique underwater resistance. The resultant ICFE is assembled into a proof‐of‐concept skin‐inspired sensor, showing impressive capacitive sensing performance with high sensitivity and wide‐strain‐range linearity (gauge factor to 1.0 in a strain range of 0–350%), excellent durability (>1000 cycles), and unique waterproofness in monitoring of complex human motions. It is believed that the hydrophobicity‐constrained association method boosts the fabrication of stretchable ionic conductors holding a great promise in skin‐inspired ionotronics with harsh‐environment tolerance.

3D Printed Spring‐Type Electronics with Liquid Metals for Highly Stretchable Conductors and Inductive Strain/Pressure Sensors

AbstractThe spring structure is stretchable and resilient, enabling electronic devices to be highly stretchable and durable. However, spring objects composed of only soft materials cannot be maintained without a supporting rod inside the coil owing to self‐weight deformation. Therefore, a design strategy should be investigated. In this study, self‐maintaining spring structures with soft materials are designed using physical simulations and soft spring‐type electronic devices (a stretchable conductor, strain sensor, and pressure sensor) with little hysteresis are developed. The spring‐type electronic devices consist of a core–sheath structure, which is a core channel filled with liquid metal and a 3D‐printed elastomeric sheath. The spring‐type conductor can be stretched with a small force (1.25 N) and has a negligible resistance change for up to 400% strain and ≈5.5% resistance increase at 640% strain. Moreover, the inductance induced by the coil structure monotonically decreases in the range of 200% stretching. The device also has high durability with little change in the electrical characteristics during 1000 cycles of 100% stretching. Furthermore, an inductive pressure sensor can measure <250 Pa (50 mN) by compressing the spring. Spring‐type electronic devices composed of soft materials contribute to highly durable and stretchable sensors for robotic applications.

Hierarchical Response Network Boosts Solvent-Free Ionic Conductive Elastomers with Extreme Stretchability, Healability, and Recyclability for Ionic Sensors.

The construction of solvent-free ionic conductive elastomers with high mechanical stretchability and large dynamic reversibility of chain segments is highly desired yet challenging. Here, a hierarchical response network strategy is presented for preparing highly stretchable yet mechanical robust ionic conductive elastomer composites (ICECs), among which poly(ethylene oxide) (PEO) microcrystalline serves as a physical cross-linking site providing high mechanical strength and elasticity, while dense hydrogen bonds endow superior mechanical toughness and dynamic reversibility. Due to the formation of the hierarchical response network, the resultant ICECs exhibit intrinsically high stretchability (>1500%), large tensile strength (∼2.1 MPa), and high fracture toughness (∼28 MJ m-3). Intriguingly, due to the high reversibility of hydrogen-bonded networks, the ICECs after being crushed are capable of healing and recycling by simple hot-pressing for multiple cycles. Moreover, the ICECs are dissolvable under an alkaline condition and easily regenerated in an acid solution for manifold cycles. Importantly, the healed, recycled, and regenerated ICECs are capable of maintaining their initial mechanical elasticity and ionic conducting performance. Due to the integration of high stretchability, fatigue resistance, and ionic conductivity, the ICECs can readily work as a stretchable ionic conductor for skin-inspired ionic sensors for real-time and accurately sensing complex human motions. This study thus provides a promising strategy for the development of healable and renewable ionic sensing materials with high stretchability and mechanical robustness, demonstrating great potential in soft ionotronics.

Highly efficient self-healable and robust fluorinated polyurethane elastomer for wearable electronics

• The polyurethane elastomers with multiple functions were successfully prepared. • The polyurethane elastomer has excellent self-healing properties and mechanical robustness. • Fluorinated polyurethane elastomers can be used to construct wearable electronic devices. Self-healable polyurethanes (PUs) containing reversible dynamic bonds have been widely recognized and rapidly developed in recent years. Because the self-heal capability and mechanical properties of materials are mutually exclusive in nature, it is still a formidable challenge to design a highly efficient self-healable and robust PUs, especially in wearable electronics field. Herein, a series of self-healable thermosetting PU elastomer based on widely used phenolic urethane bond was prepared to achieve above requirements. In order to investigate the effect of introducing different bisphenol chain extenders into the hard segment domain on the dissociation temperature of phenolic carbamate bond, both bisphenol A (BPA) and bisphenol AF (BPAF) were used to synthesize thermosetting PU elastomers (PU-BPA and PU-BPAF). The healing efficiency of PU-BPA was 95% with a thermal treatment at 100 °C for 1 h, and its initial dissociation temperature was 90 °C. Due to the presence of trifluoromethyl (-CF 3 ) group, the thermal self-healing efficiency of PU-BPAF was improved, which could be completely healed (100%) at the same conditions and its initial dissociation temperature was reduced to 80 °C. PU-BPAF were used to prepare a self-healable triboelectric nanogenerator (SH-TENG) to convert mechanical movements into electrical energy that can be applied to next-generation wearable devices.Their measured open circuit voltage, short circuit current, short-circuit charge quantity were 240 V, 2.20 μA, 20 nC, respectively. In addition, the PU-BPAF based SH-TENG exhibited better self-cleaning ability (83%) than that of PU-BPA elastomer, which was attributed to the introduction of trifluoromethyl reducing the surface energy of the material. Consequently, PU-BPAF showed great potentials as a self-healable and stretchable conductor to be applied to flexible electronic devices.

3D reactive printing of polyaniline hybrid hydrogel microlattices with large stretchability and high fatigue resistance for wearable pressure sensors

Conducting polymer hydrogels face difficulties in the combination of high elasticity, large sensitivity and wide response range for flexible pressure sensing. Herein, an intrinsically stretchable polyaniline hybrid hydrogel (PHH) is prepared through a reactive shaping method including two steps. First, a high-viscosity precursor solution containing poly(acrylic acid) (PAA), poly(ethylene oxide) (PEO) and aniline monomers is submerged into an acid solution containing initiators for the polymerization of aniline, and gelation is quickly formed due to the formation of dense hydrogen bonds between the PAA and PEO chains. Second, the initiators in the acid solution facilitate subsequent in-situ polymerization of aniline confined in the PAA/PEO framework into a 3D polyaniline (PANI) network. Benefiting from the formation of reversible hydrogen bonds and electrostatic interactions between the PAA/PEO and PANI networks, the resultant PHH exhibits large stretchability and high fatigue resistance. The reactive shaping method facilitates a 3D reactive printing of PHH into pre-designed microlattice structures with high elasticity and electron/ion dual-conducting pathways, which is promising as a stretchable conductor for capacitive pressure sensors. The sensors therefore display high sensitivity (7.10 kPa−1) in a wide detecting range and are capable of detecting complex human motions for wearable sensing applications.

Electrical Failure Mechanism in Stretchable Thin-Film Conductors.

Stretchable thin-film conductors are basic building blocks in advanced flexible and stretchable electronics. Current research mainly focuses on strategies to improve stretchability and widen the range of applications of stretchable conductors. However, stability should not be neglected, and the electrical failure mode is one of the most common stability issues that determines the current range and duration in a circuit. In this work, we report the electrical failure mechanism of stretchable conductors. We find a special failure mode for the stretchable conductors, which can be attributed to the coupling effect between local thermal strains and dynamic resistance changes of the thin film. This creates a vicious circle that significantly differs from traditional conductors. Physical parameters related to this special failure mode are investigated in detail. It is found that this mechanism is applicable to different kinds of stretchable conductors. Based on this finding, we also explore methods to modulate the failure of stretchable conductors. The failure mechanism found here provides a fundamental understanding of the current effect of stretchable circuits and is crucial for designing stable stretchable bioelectrodes and circuits.

Interfacial assembly of metallic nanomembranes for highly stretchable conductors

Stretchable conductors that can simultaneously achieve metallic conductivity and high stretchability are an essential component for almost all soft, wearable electronics. An interfacial assembly method at the water/oil interface was recently reported to fabricate a nanomembrane with high conductivity, high stretchability, ultrathin thickness, and facile patternability. Stretchable conductors that can simultaneously achieve metallic conductivity and high stretchability are an essential component for almost all soft, wearable electronics. An interfacial assembly method at the water/oil interface was recently reported to fabricate a nanomembrane with high conductivity, high stretchability, ultrathin thickness, and facile patternability.

An ultra-low hysteresis, self-healing and stretchable conductor based on dynamic disulfide covalent adaptable networks

A new strategy to achieve self-healing, anti-hysteresis and stretchable conductors was developed via the design of chemical structures and activity of dynamic bonds in covalent adaptable networks (CANs).

PEDOT:PSS-based intrinsically soft and stretchable bioelectronics

Intrinsically soft and stretchable bioelectronics exhibit tissue-like mechanical behavior that enables the seamless integration of electronic devices with the human body to achieve high-quality biosignal recording and high-efficacy neural modulation. The conducting polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) shows significant promise in this field because of its high conductivity, excellent biocompatibility and commercial availability. However, pristine PEDOT:PSS is brittle and rigid and thus cannot be used in soft and stretchable electronics. More effort is therefore required to engineer PEDOT:PSS into a stretchable conductor that meets the demands of bioelectronics. In this perspective, we review the recent progress and propose the possible future directions of PEDOT:PSS-based bioelectronics.

Stretchable solvent-free ionic conductor with self-wrinkling microstructures for ultrasensitive strain sensor.

Stretchable solvent-free ionic conductors with outstanding physicochemical stability are attractive in emerging sensing devices. However, existing ionic conductors struggle to keep high strain sensitivity with a gauge factor (GF) smaller than 4 at a strain range of up to 100%, which seriously affects the reliability of the signal output. Herein, we prepared a novel solvent-free ionic conductor, consisting of a hybrid cross-linked polymer network and mobile ions. Exquisite reticular wrinkling microstructures with ion channels were formed spontaneously by utilizing modulus mismatch in shrinkage during polymerization. The ion channels promote the ionic transport in the polymer, and the reticular wrinkling microstructures dramatically reinforce the mechanical stability and enhance energy dissipation. After being carefully optimized, the as-prepared ionic conductor demonstrated many unique advantages including perfect physicochemical stability, wide operating temperature range, the high ionic conductivity of 1.17 mS cm-1 at 15 °C and excellent sensitivity with GF as high as 7.03 at 100% strain. Moreover, benefiting from the abundant hydrogen bonds and ionic interactions in the polymer network, the ionic conductor also possesses self-healable and adhesion ability. The resulting devices can be used for quantitative evaluation of human joint motion and exhibit great potential in soft electronics or iontronics.

3D printing of stretchable, ionic conductive, adhesive hydrogel with enhanced water retention for motion detection

Stretchable ionic conductors, including ionic conductive hydrogels, are ideal for flexible electronics. However, due to poor adhesion to human skin and quick drying in ambient conditions, the development of hydrogel-based electronics is currently constrained. This research used a digital light processing (DLP) technique to make a 3D printed ionic conductive hydrogel. The novel hydrogel's high ZnSO4 salt content improves interfacial adhesion and water retention. A sandwiched hydrogel-elastomer capacitive sensor was created to show its ability to collect body signals, which opens a new route in flexible sensors.