Self-assembled aqueous liquid metal inks for stretchable conductors and circuits

Printable Metal-Polymer Conductors for Highly Stretchable Bio-Devices

Stretchable conductor is the key component of stretchable electronic devices which has become an important development direction of flexible electronic. Patterned stretchable conductor is one of the key technologies for successfully manufacturing stretchable electronic devices. In order to acquire stretchable conductor with good conductivity and stable mechanical stretchability, stretchable conductive ink based on liquid metal nanoparticles (LMNPs) and silver flakes was developed and printed on the modified PDMS substrate by screen printing technology. Liquid metal recovered the fluidity and connected with the silver flakes under pressure after drying. So the fluidity and conductivity of the liquid metal can be used to achieve the preparation of stretchable conductor and overcome the trade-off between conductivity and stretchability. The initial resistance of prepared stretchable conductor is 2 Ω, and the resistance change rate is 0.65 when the tensile strain is 110%. At the same time, it still has a low resistance of 42.7 Ω and resistance change rate of 20.35 when the tensile strain is 660%.KeywordsLiquid metalStretchable conductorConductivityTensile strainPrinting

Electrical and mechanical integration approaches are essential for emerging hybrid electronics that must robustly bond rigid electrical components with flexible circuits and substrates. However, flexible polymeric substrates and circuits cannot withstand the high temperatures used in traditional electronic processing. This constraint requires new strategies to create flexible materials that simultaneously achieve high electrical conductivity, strong adhesion, and processibility at low temperature. Here, an electrically conductive adhesive is introduced that is flexible, electrically conductive (up to 3.25×105 S m−1) without sintering or high temperature post‐processing, and strongly adhesive to various materials common to flexible and stretchable circuits (fracture energy 350 <Gc < 700 J m−2). This is achieved through a multiphase soft composite consisting of an elastomeric and adhesive epoxy network with dispersed liquid metal droplets that are bridged by silver flakes, which form a flexible and conductive percolated network. These inks can be processed through masked deposition and direct ink writing at room temperature. This enables soft conductive wiring and robust integration of rigid components onto flexible substrates to create hybrid electronics for emerging applications in soft electronics, soft robotics, and multifunctional systems.

We tackle two well-known problems in the fabrication of stretchable electronics: interfacing soft circuit wiring with silicon chips and fabrication of multi-layer circuits. We demonstrate techniques that allow integration of embedded flexible printed circuit boards (FPCBs) populated with microelectronics into soft circuits composed of liquid metal (LM) interconnects. These methods utilize vertical interconnect accesses (VIAs) that are produced by filling LM alloy into cavities formed by laser ablation. The introduced technique is versatile, easy to perform, clean-room free, and results in reliable multi-layer stretchable hybrid circuits that can withstand over 80% of strain. We characterize the fabrication parameters of such VIAs and demonstrated several applications, including a stretchable touchpad and pressure detection film, and an all-integrated multi-layer electromyography (EMG) circuit patch with five active layers including acquisition electrodes, on-board processing and Bluetooth communication modules.

Gallium-based liquid metals (LMs) are promising materials for stretchable electronics due to their metallic conductivity and deformability. However, the fabrication of large-area stretchable integrated electronics using LMs on various polymers remains challenging due to their high surface tension, fluidity, and poor wettability. Current techniques, such as selective wetting and lift-off processes, face limitations related to substrate compatibility and Ga/metal alloying, hindering their applicability in integrated electronic systems. To address these challenges, we developed a high-resolution top-down etching-based photolithography combined with a universal cryogenic transfer method for transferring patterned LM particles (LMPs) in various polymer substrates. The cryogenic environment modifies the interfacial bonding between the LMPs and substrates, resulting in a universal transfer. The resulting liquid metal particle network embedded polymer (LNEP) exhibits high electrical conductivity (~1.71 × 10⁶ S/m), stability, and strain-insensitive performance across various polymers. This process is scalable to large-area fabrication, overcoming the limitations of existing LM patterning techniques. Leveraging this approach, we demonstrated the use of LNEP ranging from skin-conformal wearable sensors to hybrid stretchable circuits and implantable devices, demonstrating the universality of the method. This technique establishes a scalable pathway for stretchable electronics in advanced applications.

Copper nanowire (CuNW) conductors have been considered to have a promising perspective in the area of stretchable electronics due to the low price and high conductivity. However, the fabrication of CuNW conductors suffers from harsh conditions, such as high temperature, reducing atmosphere, and time-consuming transfer step. Here, a simple and rapid one-step photonic sintering technique was developed to fabricate stretchable CuNW conductors on polyurethane (PU) at room temperature in air environment. It was observed that CuNWs were instantaneously deoxidized, welded and simultaneously embedded into the soft surface of PU through the one-step photonic sintering technique, after which highly conductive network and strong adhesion between CuNWs and PU substrates were achieved. The CuNW/PU conductor with sheet resistance of 22.1 Ohm/sq and transmittance of 78% was achieved by the one-step photonic sintering technique within only 20 μs in air. Besides, the CuNW/PU conductor could remain a low sheet resistance even after 1000 cycles of stretching/releasing under 10% strain. Two flexible electronic devices, wearable sensor and glove-shaped heater, were fabricated using the stretchable CuNW/PU conductor, demonstrating that our CuNW/PU conductor could be integrated into various wearable electronic devices for applications in food, clothes, and medical supplies fields.

Downsizing bulk liquid metals (LM) at the nanometer scale with biocompatibility and multifunction is a key process for electronic or medical applications. Here, we report a stable and green LM aqueous colloidal ink by wrapping eutectic gallium-indium alloys (EGaIn) with carboxymethyl glucomannan (CGM) derived from radiata pine chip, which is capable of being prepared into a free-standing, photothermal-actuating, and motion-monitoring Janus film. With the assistance of CGM, the bulk EGaIn was ultrasonicated into stable nanodroplets (∼500 nm) with a typical "core-shell" structure, in which the colloidal inks can be stored for more than 1 week under room temperature. The stable CGM/EGaIn inks can be patterned on different substrates to form coating layers or self-assembled into free-standing Janus films with high mechanical strength and modulus (∼94 MPa and ∼3.8 GPa) by density deposition. Such a Janus film with anisotropic thermal conductivity made it a potential photothermal actuator. In addition, the biocompatible film demonstrated both high conductivity and large resistance variation in response to strain change (gauge factor >500), allowing for human motion monitoring. This work provides a new prospect for the development of biocompatible and high-performance nano-LM materials.

Three-dimensional (3D) printing technology enables the rapid manufacturing of complex prototypes and customized soft electronics. However, the integrated manufacturing of multilayer, multimaterial, and multifunctional soft electronics via 3D printing remains challenging due to insufficient synergy between material development and process innovation, as well as inadequate compatibility between stretchable conductors and substrates. Herein, we propose a material-process-performance integrated manufacturing strategy for multilayer soft electronics. Firstly, we develop a novel stretchable conductor with strong interfacial bonding to stretchable substrates. By regulating its rheological properties, the material becomes compatible with 3D printing processes. The stretchable conductor employs 3D clustered silver nanoparticles with large surface area and low aspect ratio as fillers, effectively addressing the inherent trade-off in the performance of stretchable conductors. This design simultaneously achieves high stretchability, conductivity, and low hysteresis. Secondly, we propose a manufacturing strategy combining multi-material 3D printing with sacrificial layer assistance. By leveraging the good thixotropy of the stretchable conductor, the direct formation of 3D stretchable interconnects between layers is enabled, ultimately achieving customized and integrated manufacturing of multilayer soft electronics. Through applications such as multilayer infrared encryption devices and wearable wristbands, we demonstrate the feasibility of the proposed stretchable conductor and integrated manufacturing strategy.

A versatile stretching setup has been developed for in-situ microstructural testing of flexible conductors in combination with X-ray scattering studies at the synchrotron beamlines. The application of the in-situ stretching setup is demonstrated on a prototype of Ag flakes and liquid metals on stretchable elastomers. The new setup allows material structural changes to be monitored by X-ray scattering in direct combination with electrical response, allowing their direct correlation. The versatile setup could be operated with any type of flexible conductor with controlled stretching steps and rates, and up to a maximum stretching of 200 %. Microstructural changes are determined by grazing X-ray scattering technique during the whole stretching cycle: before, during and after the application of strain. The stretchable conductor (Ag flakes/EGaInPs) is shown to be stable up to a threshold strain of 15 %, followed by a phase transition and an increase in resistance at higher strains. Due to the compact and versatile design of the stretching setup, the experiment for direct microstructural and electrical correlation under applied strain could be easily adapted to different synchrotron beamlines. • Developed a compact in situ stretching setup enabling simultaneous structural and electrical measurements up to 200 % strain • Demonstrated stable microstructure and low electrical resistance in Ag-flake/EGaInPs elastomer composites up to a threshold strain of 15 % • Revealed strain-induced Ag 2 Ga intermetallic phase formation beyond 15 % strain, directly correlating with a sharp increase in resistance • Provided critical insights into real-time structure–property relationships under mechanical deformation for optimization of stretchable conductors

Soft, elastically deformable composites can enable new generations of multifunctional materials for electronics, robotics, and reconfigurable structures. Liquid metal (LM) droplets dispersed in elastomer matrices represent an emerging material architecture that has shown unique combinations of soft mechanical response with exceptional electrical and thermal functionalities. These properties are strongly dependent on the material composition and microstructure. However, approaches to control LM microdroplet morphology to program mechanical and functional properties are lacking. Here, this limitation is overcome by thermo‐mechanically shaping LM droplets in soft composites to create programmable microstructures in stress‐free materials. This enables LM loadings up to 70% by volume with prescribed particle aspect ratios and orientation, enabling control of microstructure throughout the bulk of the material. Through this microstructural control in soft composites, a material which simultaneously achieves a thermal conductivity as high as 13.0 W m−1 K−1 (>70 × increase over polymer matrix) with low modulus (<1.0 MPa) and high stretchability (>750% strain) is demonstrated in stress‐free conditions. Such properties are required in applications that demand extreme mechanical flexibility with high thermal conductivity, which is demonstrated in soft electronics, wearable robotics, and electronics integrated into 3D printed materials.

We demonstrate a magnetocaloric ferrofluid based on a gadolinium saturated liquid metal matrix, using a gallium-based liquid metal alloy as the solvent and suspension medium. The material is liquid at room temperature, while exhibiting spontaneous magnetization and a large magnetocaloric effect. The magnetic properties were attributed to the formation of gadolinium nanoparticles suspended within the liquid gallium alloy, which acts as a reaction solvent during the nanoparticle synthesis. High nanoparticle weight fractions exceeding 2% could be suspended within the liquid metal matrix. The liquid metal ferrofluid shows promise for magnetocaloric cooling due to its high thermal conductivity and its liquid nature. Magnetic and thermoanalytic characterizations reveal that the developed material remains liquid within the temperature window required for domestic refrigeration purposes, which enables future fluidic magnetocaloric devices. Additionally, the observed formation of nanometer-sized metallic particles within the supersaturated liquid metal solution has general implications for chemical synthesis and provides a new synthetic pathway toward metallic nanoparticles based on highly reactive rare earth metals.

Gallium-based liquid metals (LMs) are materials that possess some unique properties. Just like any other metal, they have high thermal conductivity and low interfacial resistance, but they are in a liquid phase at room temperature. In contrast to mercury alloys, gallium alloys are non-toxic. They don’t evaporate and they can’t be inhaled. The viscosity of those alloys is very similar to water, but they are six times as dense as water. All those properties make Gallium-based liquid metals very good candidates for thermal interface material (TIM) in electronics applications. On the other hand, the reaction and incompatibility of those alloys with some metals is one of the challenges for this type of TIM. The other challenge is that those materials are not just thermally, but also electrically conductive and that is not a desirable property for TIM. A suitable barrier that will prevent any leakage of LMs and the best way to apply the appropriate volume of LM in high-volume production (HVP) would be one of the most important things for any application. A key challenge is applying the liquid metal consistently through a traditional dispensing method due to its property and behavior which involves high surface tension. Through advanced dispensing techniques like jetting technology liquid metal can be applied reliably on a flat, uneven surface or in arrays of minuscule confined spaces or cavities. This paper highlights the dispensing quality, weight repeatability from one substrate to another, and valve hardware stability. This paper will also address the challenges faced during dispensing of liquid metal in high volume manufacturing, and how to achieve desired bondline thickness with jet dispensing for higher throughput and process reliability.

Spring-Type Electronics In article number 2101657, Tomoyuki Yokota, Takao Someya and colleagues develop soft spring-type electronic devices formed by core–sheath structures of liquid metals and 3D-printed elastomeric shells. The spring devices work not only as stretchable conductors with minor resistance change, but also as inductive strain/pressure sensors for robotic applications by using the change in the magnetic flux caused by the spring deformation.

Gallium‐based liquid metal (LM) is expected to be an ideal candidate for flexible electromagnetic interference (EMI) shielding materials in wearable electronics due to its excellent electrical conductivity and extraordinary fluidity. However, it is difficult to fabricate LM as a free‐standing material due to its high surface tension and unmanageable fluidity, as well as its poor compatibility with other materials. Herein, a processing strategy is developed to fabricate free‐standing LM‐based films by vacuum filtration with the assistance of dough‐like aramid nanofiber (ANF) putty. The ANF putty improves the compatibility of ANF and LM, and also greatly improves the stability, reliability, and processability of ANF. Rational design of the structure enables the preparation of LM/ANF films with ANF as the nanobridge connecting LM micro/nanodroplets, and ANF‐LM‐ANF sandwich films with ANF as the shells, LM layer as the core. The LM/ANF film (tensile strength: 5.4 MPa) exhibits an ultrahigh EMI SE of up to 105.9 dB (8.0–12.4 GHz) at a thickness of 60 μm. While the ANF‐LM‐ANF sandwich film has higher mechanical strength (33.1 MPa), and also has good EMI shielding properties. The average EMI SE of the 10 μm thick sandwich film exceeds 45 dB in the X‐band.

Conductive hydrogel materials are garnering attention for their high flexibility and biocompatibility, establishing them as promising candidates for use in wearable and stretchable electronics. Despite the high conductivity of many reported hydrogel materials, they generally lack intelligent properties such as degradability, stretchability, self-adhesion, and self-healing. This study proposes a stress-assisted conductive networking mechanism for liquid metal (LM)-based gel, which is attributed to the internal stress generated by volume shrinkage during the liquid–solid phase transition, and operates without the requirement of additional energy input. Based on this mechanism, LM particles are subsequently incorporated into a polyvinylpyrrolidone (PVP)-fructose composite (PFC) to fabricate a highly conductive LM-PFC gel. This gel not only achieves high conductivity but also demonstrates excellent intelligent properties, including high stretchability, degradability, high mechanical strength, self-adhesion, self-conforming ability, recyclability, and self-healing ability, which make it highly suitable for applications such as monitoring bioelectric signals, thereby highlighting its immense potential in stretchable electronics and wearable technologies.