Controlled Assembly of Liquid Metal Inclusions as a General Approach for Multifunctional Composites

Materials and Structural Designs toward Motion Artifact-Free Bioelectronics

Bioelectronics encompassing electronic components and circuits for accessing human information play a vital role in real-time and continuous monitoring of biophysiological signals of electrophysiology, mechanical physiology, and electrochemical physiology. However, mechanical noise, particularly motion artifacts, poses a significant challenge in accurately detecting and analyzing target signals. While software-based "postprocessing" methods and signal filtering techniques have been widely employed, challenges such as signal distortion, major requirement of accurate models for classification, power consumption, and data delay inevitably persist. This review presents an overview of noise reduction strategies in bioelectronics, focusing on reducing motion artifacts and improving the signal-to-noise ratio through hardware-based approaches such as "preprocessing". One of the main stress-avoiding strategies is reducing elastic mechanical energies applied to bioelectronics to prevent stress-induced motion artifacts. Various approaches including strain-compliance, strain-resistance, and stress-damping techniques using unique materials and structures have been explored. Future research should optimize materials and structure designs, establish stable processes and measurement methods, and develop techniques for selectively separating and processing overlapping noises. Ultimately, these advancements will contribute to the development of more reliable and effective bioelectronics for healthcare monitoring and diagnostics.

LM-Gel Plasticine Based on Binary Cooperative with Kneadable Shaping and Conductivity

Liquid metal (LM)-based polymers have received growing interest for wearable health monitoring, electronic skins, and soft robotics. However, fabricating multifunctional LM-based polymers, in particular, featuring a convenient shaping ability while offering excellent deformability and conductivity remains a challenge. To overcome this obstacle, here, we propose a strategy to prepare LM-Gel "plasticine" (LGP) with great deformability, which is composed of a PVA (poly(vinyl alcohol)) soft network and an LM conductive phase. LGP can be easily constructed into different shapes such as plasticine and can be applied to different conditions (such as building a 3D circuit, circuit repair, and switch). Meanwhile, LGP has great conductivity (2.3 × 104 S/m) after surface annealing. Besides, LGP has a good electric heating performance, which shows the potential for application in wearable heating devices. Thus, this approach not only provides a way to prepare LM-polymer plasticine but also provides a novel perspective toward extending the applied range of LM-polymer composites.

Liquid Metal-Polymer Conductor-Based Conformal Cyborg Devices

Gallium-based liquid metal (LM) exhibits exceptional properties such as high conductivity and biocompatibility, rendering it highly valuable for the development of conformal bioelectronics. When combined with polymers, liquid metal-polymer conductors (MPC) offer a versatile platform for fabricating conformal cyborg devices, enabling functions such as sensing, restoration, and augmentation within the human body. This review focuses on the synthesis, fabrication, and application of MPC-based cyborg devices. The synthesis of functional materials based on LM and the fabrication techniques for MPC-based devices are elucidated. The review provides a comprehensive overview of MPC-based cyborg devices, encompassing their applications in sensing diverse signals, therapeutic interventions, and augmentation. The objective of this review is to serve as a valuable resource that bridges the gap between the fabrication of MPC-based conformal devices and their potential biomedical applications.

Superstretchable Liquid-Metal Electrodes for Dielectric Elastomer Transducers and Flexible Circuits

Dielectric elastomer transducers (DETs), with a dielectric elastomer (DE) film sandwiched between two compliant electrodes, are highly sought after in the fields of soft robotics, energy harvesting, and human-machine interaction. To achieve a high-performance DET, it is essential to develop electrodes with high conductivity, strain-insensitive resistance, and adaptability. Herein, we design an electrode (Supra-LMNs) based on multiple dynamic bond cross-linked supramolecular networks (Ns) and liquid metal (LM), which realizes high conductivity (up to 16,000 S cm-1), negligible resistance changes at high strain (1.3-fold increase at 1000% strain), instantaneous self-healability at ambient temperature, and rapid recycling. The conductive pathway can be activated through simple friction by transmitting stress through the silver nanowires (AgNWs) and cross-linking sites of LM particles. This method is especially attractive for printing circuits on flexible substrates, especially DE films. Utilized as dielectric elastomer generator (DEG) electrodes, it reduces the charge loss by 3 orders of magnitude and achieves high generating energy density and energy conversion efficiency on a low-resistance load. Additionally, serving as sensor (DES) and actuator (DEA) electrodes, it enables a highly sensitive sensing capability and complex interaction.

Self-Healing Liquid Metal Magnetic Hydrogels for Smart Feedback Sensors and High-Performance Electromagnetic Shielding

Hydrogels exhibit potential applications in smart wearable devices because of their exceptional sensitivity to various external stimuli. However, their applications are limited by challenges in terms of issues in biocompatibility, custom shape, and self-healing. Herein, a conductive, stretchable, adaptable, self-healing, and biocompatible liquid metal GaInSn/Ni-based composite hydrogel is developed by incorporating a magnetic liquid metal into the hydrogel framework through crosslinking polyvinyl alcohol (PVA) with sodium tetraborate. The excellent stretchability and fast self-healing capability of the PVA/liquid metal hydrogel are derived from its abundant hydrogen binding sites and liquid metal fusion. Significantly, owing to the magnetic constituent, the PVA/liquid metal hydrogel can be guided remotely using an external magnetic field to a specific position to repair the broken wires with no need for manual operation. The composite hydrogel also exhibits sensitive deformation responses and can be used as a strain sensor to monitor various body motions. Additionally, the multifunctional hydrogel displays absorption-dominated electromagnetic interference (EMI) shielding properties. The total shielding performance of the composite hydrogel increases to ~ 62.5 dB from ~ 31.8 dB of the pure PVA hydrogel at the thickness of 3.0 mm. The proposed bioinspired multifunctional magnetic hydrogel demonstrates substantial application potential in the field of intelligent wearable devices.

Ultrahigh Strain-Insensitive Integrated Hybrid Electronics Using Highly Stretchable Bilayer Liquid Metal Based Conductor

Human-interfaced electronic systems require strain-resilient circuits. However, present integrated stretchable electronics easily suffer from electrical deterioration and face challenges in forming robust multilayered soft-rigid hybrid configurations. Here, a bilayer liquid-solid conductor (b-LSC) with amphiphilic properties is introduced to reliably interface with both rigid electronics and elastomeric substrates. The top liquid metal can self-solder its interface with rigid electronics at a resistance 30% lower than the traditional tin-soldered rigid interface. The bottom polar composite comprising liquid metal particles and polymers can not only reliably interface with elastomers but also help the b-LSC heal after breakage. The b-LSC can be scalably fabricated by printing and subsequent peeling strategies, showing ultra-high strain-insensitive conductivity (maximum 22 532 S cm^-1 ), extreme stretchability (2260%), and negligible resistance change under ultra-high strain (0.34 times increase under 1000% strain). It can act as stretchable vertical interconnect access for connecting multilayered layouts and can be scalably and universally fabricated on various substrates with a resolution of ≈200 µm. It is demonstrated that it can construct stretchable sensor arrays, multi-layered stretchable displays, highly integrated haptic user-interactive optoelectric E-skins, visualized heaters, robot touch sensing systems, and wireless powering for wearable electronics.

Thermal-Sinterable EGaIn Nanoparticle Inks for Highly Deformable Bioelectrode Arrays

Liquid metal (especially eutectic gallium indium, EGaIn) nanoparticle inks overcome the poor wettability of high surface tension EGaIn to elastomer substrates and show great potential in soft electronics. Normally, a sintering strategy is required to break the oxide shells of the EGaIn nanoparticles (EGaIn NPs) to achieve conductive paths. Herein, for the first time, thermal-sinterable EGaIn NP inks are prepared by introducing thermal expansion microspheres (TEMs) into EGaIn NP solution. Through the mechanical pressure induced by the expansion of the heated TEMs, the printed EGaIn NPs can be sintered into electrically conductive paths to achieve highly stretchable bioelectrode arrays, which exhibit giant electromechanical performance (up to 680% strain), good cyclic stability (over 2 × 10⁴  cycles), and stable conductivity after high-speed rotation (6000 rpm). Simultaneously, the recording sites are hermetically sealed by ionic elastomer layers, ensuring the complete leakage-free property of EGaIn and reducing the electrochemical impedance of the electrodes (891.16 Ω at 1 kHz). The bioelectrode is successfully applied to monitor dynamic electromyographic signals. The sintering strategy overcomes the disadvantages of the traditional sintering strategies, such as leakage of EGaIn, reformation of large EGaIn droplets, and low throughput, which promotes the application of EGaIn in soft electronics.

Ultrasoft and Ultrastretchable Wearable Strain Sensors with Anisotropic Conductivity Enabled by Liquid Metal Fillers

Herein, ultrasoft and ultrastretchable wearable strain sensors enabled by liquid metal fillers in an elastic polymer are described. The wearable strain sensors that can change the effective resistance upon strains are prepared by mixing silicone elastomer with liquid metal (EGaIn, Eutectic gallium-indium alloy) fillers. While the silicone is mixed with the liquid metal by shear mixing, the liquid metal is rendered into small droplets stabilized by an oxide, resulting in a non-conductive liquid metal elastomer. To attain electrical conductivity, localized mechanical pressure is applied using a stylus onto the thermally cured elastomer, resulting in the formation of a handwritten conductive trace by rupturing the oxide layer of the liquid metal droplets and subsequent percolation. Although this approach has been introduced previously, the liquid metal dispersed elastomers developed here are compelling because of their ultra-stretchable (elongation at break of 4000%) and ultrasoft (Young’s modulus of <0.1 MPa) mechanical properties. The handwritten conductive trace in the elastomers can maintain metallic conductivity when strained; however, remarkably, we observed that the electrical conductivity is anisotropic upon parallel and perpendicular strains to the conductive trace. This anisotropic conductivity of the liquid metal elastomer film can manipulate the locomotion of a robot by routing the power signals between the battery and the driving motor of a robot upon parallel and perpendicular strains to the hand-written circuit. In addition, the liquid metal dispersed elastomers have a high degree of deformation and adhesion; thus, they are suitable for use as a wearable sensor for monitoring various body motions.

3D Printing of Liquid Metal Embedded Elastomers for Soft Thermal and Electrical Materials

Liquid metal embedded elastomers (LMEEs) are composed of a soft polymer matrix embedded with droplets of metal alloys that are liquid at room temperature. These soft matter composites exhibit exceptional combinations of elastic, electrical, and thermal properties that make them uniquely suited for applications in flexible electronics, soft robotics, and thermal management. However, the fabrication of LMEE structures has primarily relied on rudimentary techniques that limit patterning to simple planar geometries. Here, we introduce an approach for direct ink write (DIW) printing of a printable LMEE ink to create three-dimensional shapes with various designs. We use eutectic gallium-indium (EGaIn) as the liquid metal, which reacts with oxygen to form an electrically insulating oxide skin that acts as a surfactant and stabilizes the droplets for 3D printing. To rupture the oxide skin and achieve electrical conductivity, we encase the LMEE in a viscoelastic polymer and apply acoustic shock. For printed composites with a 80% LM volume fraction, this activation method allows for a volumetric electrical conductivity of 5 × 10⁴ S cm^-1 (80% LM volume)─significantly higher than what had been previously reported with mechanically sintered EGaIn-silicone composites. Moreover, we demonstrate the ability to print 3D LMEE interfaces that provide enhanced charge transfer for a triboelectric nanogenerator (TENG) and improved thermal conductivity within a thermoelectric device (TED). The 3D printed LMEE can be integrated with a highly soft TED that is wearable and capable of providing cooling/heating to the skin through electrical stimulation.

Universal assembly of liquid metal particles in polymers enables elastic printed circuit board

An elastic printed circuit board (E-PCB) is a conductive framework used for the facile assembly of system-level stretchable electronics. E-PCBs require elastic conductors that have high conductivity, high stretchability, tough adhesion to various components, and imperceptible resistance changes even under large strain. We present a liquid metal particle network (LMP Net ) assembled by applying an acoustic field to a solid-state insulating liquid metal particle composite as the elastic conductor. The LMP Net conductor satisfies all the aforementioned requirements and enables the fabrication of a multilayered high-density E-PCB, in which numerous electronic components are intimately integrated to create highly stretchable skin electronics. Furthermore, we could generate the LMP Net in various polymer matrices, including hydrogels, self-healing elastomers, and photoresists, thus showing their potential for use in soft electronics.

Connecting liquid metals with sound

A stretchable conductive circuit is formed using a liquid metal-polymer composite

Conformally Adhesive, Large-Area, Solidlike, yet Transient Liquid Metal Thin Films and Patterns via Gelatin-Regulated Droplet Deposition and Sintering

Adhesion and spreading of liquid metals (LMs) on substrates are essential steps for the generation of flexible electronics and thermal management devices. However, the controlled deposition is limited by the high surface tension and peculiar wetting and adhesion behavior of LMs. Herein, we introduce gelatin-regulated LM droplet deposition and sintering (GLMDDS), for the upscalable production of conformally adhesive, solidlike, yet transient LM thin films and patterns on diverse substrates. This method involves four steps: homogeneous deposition of LM microdroplets, gelation of the LM-gelatin solution, toughening of the gelatin hydrogel by solvent displacement, and peeling-induced sintering of LM microdroplets. The LM thin film exhibits a three-layer structure, comprising an LM microdroplet-embedded tough organohydrogel adhesion layer, a continuous LM layer, and an oxide skin. The composite exhibits high stretchability and mechanical robustness, conformal adhesion to various substrates, high conductivity (4.35 × 10⁵ S·m^-1), and transience (86% LM recycled). Large-scale deposition (i.e., 5.6 dm²) and the potential for patterns on diverse substrates demonstrate its upscalability and broad suitability. Finally, the LM thin films and patterns are applied for flexible and wearable devices, i.e., pressure sensors, heaters, human motion tracking devices, and thermal management devices, illustrating the broad applicability of this strategy.

Influence of microstructural alterations of liquid metal and its interfacial interactions with rubber on multifunctional properties of soft composite materials

Liquid metal (LM)-based polymer composites are currently new breakthrough and emerging classes of soft multifunctional materials (SMMs) having immense transformative potential for soft technological applications. Currently, room-temperature LMs, mostly eutectic gallium‑indium and Galinstan alloys are used to integrate with soft polymer due to their outstanding properties such as high conductivity, fluidity, low adhesion, high surface tension, low cytotoxicity, etc. The microstructural alterations and interfacial interactions controlling the efficient integration of LMs with rubber are the most critical aspects for successful implementation of multifunctionality in the resulting material. In this review article, a fundamental understanding of microstructural alterations of LMs to the formation of well-defined percolating networks inside an insulating rubber matrix has been established by exploiting several existing theoretical and experimental studies. Furthermore, effects of the chemical modifications of an LM surface and its interfacial interactions on the compatibility between solid rubber and fluid filler phase have been discussed. The presence of thin oxide layer on the LM surface and the effects and challenges it poses to the adequate functionalization of these materials have been discussed. Plausible applications of SMMs in different soft matter technologies, like soft robotics, flexible electronics, soft actuators, sensors, etc. have been provided. Finally, the current technical challenges and further prospective to the development of SMMs using non‑silicone rubbers have been critically discussed. This review is anticipated to infuse a new impetus to the associated research communities for the development of next generation SMMs.

3R Electronics: Scalable Fabrication of Resilient, Repairable, and Recyclable Soft-Matter Electronics

E-waste is rapidly turning into another man-made disaster. It is proposed that a paradigm shift toward a more sustainable future can be made through soft-matter electronics that are resilient, repairable if damaged, and recyclable (3R), provided that they achieve the same level of maturity as industrial electronics. This includes high-resolution patterning, multilayer implementation, microchip integration, and automated fabrication. Herein, a novel architecture of materials and methods for microchip-integrated condensed soft-matter 3R electronics is demonstrated. The 3R function is enabled by a biphasic liquid metal-based composite, a block copolymer with nonpermanent physical crosslinks, and an electrochemical technique for material recycling. In addition, an autonomous laser-patterning method for scalable circuit patterning with an exceptional resolution of <30 µm in seconds is developed. The phase-shifting property of the BCPs is utilized for vapor-assisted "soldering" circuit repairing and recycling. The process is performed entirely at room temperature, thereby opening the door for a wide range of heat-sensitive and biodegradable polymers for the next generation of green electronics. The implementation and recycling of sophisticated skin-mounted patches with embedded sensors, electrodes, antennas, and microchips that build a digital fingerprint of the human electrophysiological signals is demonstrated by collecting mechanical, electrical, optical, and thermal data from the epidermis.

Patterned Magnetofluids via Magnetic Printing and Photopolymerization for Multifunctional Flexible Electronic Sensors

Liquid conductor-based flexible sensors with high mechanical deformability and reliable electrical reversibility have aroused great interest in electronic skin, soft robotics, environmental monitoring, and other fields. Herein, we develop a novel strategy to fabricate liquid conductor-based flexible sensors by combining ionic liquid-based magnetofluids (IL-MFs), magnetic printing, and photopolymerization techniques. The as-prepared sensors exhibit excellent electromechanical properties, such as a wide detection range, low hysteresis, fast response time, good durability, etc. Moreover, the gauge factors (GFs) of the sensor could be easily adjusted by changing the modulators with different line widths or patterns, and the strain sensors can also be designed for anisotropic monitoring. Apart from serving as strain sensors, the magnetofluid-based flexible sensors can be used to detect external pressure, human activities, and changes in temperature, illumination, and magnetic field as well. This work provides a facile strategy to fabricate liquid conductor-based multifunctional sensors. Such a magnetofluid-based sensor has a great promising future.

Rapid meniscus-guided printing of stable semi-solid-state liquid metal microgranular-particle for soft electronics

Liquid metal is being regarded as a promising material for soft electronics owing to its distinct combination of high electrical conductivity comparable to that of metals and exceptional deformability derived from its liquid state. However, the applicability of liquid metal is still limited due to the difficulty in simultaneously achieving its mechanical stability and initial conductivity. Furthermore, reliable and rapid patterning of stable liquid metal directly on various soft substrates at high-resolution remains a formidable challenge. In this work, meniscus-guided printing of ink containing polyelectrolyte-attached liquid metal microgranular-particle in an aqueous solvent to generate semi-solid-state liquid metal is presented. Liquid metal microgranular-particle printed in the evaporative regime is mechanically stable, initially conductive, and patternable down to 50 μm on various substrates. Demonstrations of the ultrastretchable (~500% strain) electrical circuit, customized e-skin, and zero-waste ECG sensor validate the simplicity, versatility, and reliability of this manufacturing strategy, enabling broad utility in the development of advanced soft electronics.

In this article, meniscus-guided printing of polyelectrolyte-attached liquid metal particles to simultaneously achieve mechanical stability and initial electrical conductivity at high resolution is introduced.

Processing of Self-Healing Polymers for Soft Robotics

Soft robots are, due to their softness, inherently safe and adapt well to unstructured environments. However, they are prone to various damage types. Self-healing polymers address this vulnerability. Self-healing soft robots can recover completely from macroscopic damage, extending their lifetime. For developing healable soft robots, various formative and additive manufacturing methods have been exploited to shape self-healing polymers into complex structures. Additionally, several novel manufacturing techniques, noted as (re)assembly binding techniques that are specific to self-healing polymers, have been created. Herein, the wide variety of processing techniques of self-healing polymers for robotics available in the literature is reviewed, and limitations and opportunities discussed thoroughly. Based on defined requirements for soft robots, these techniques are critically compared and validated. A strong focus is drawn to the reversible covalent and (physico)chemical cross-links present in the self-healing polymers that do not only endow healability to the resulting soft robotic components, but are also beneficial in many manufacturing techniques. They solve current obstacles in soft robots, including the formation of robust multi-material parts, recyclability, and stress relaxation. This review bridges two promising research fields, and guides the reader toward selecting a suitable processing method based on a self-healing polymer and the intended soft robotics application.

Tailorable, Lightweight and Superelastic Liquid Metal Monoliths for Multifunctional Electromagnetic Interference Shielding

A confined thermal expansion strategy to fabricate liquid metal (LM)-based monoliths with continuous LM network at ultra-low content. The results show a strong integration advantage of LM-based monoliths in density, mechanical strength, electromagnetic interference shielding effectiveness, and near field shielding effectiveness, as well as multi-functions such as magnetic actuation.

ABSTRACT

Liquid metal (LM) has become an emerging material paradigm in the electromagnetic interference shielding field owing to its excellent electrical conductivity. However, the processing of lightweight bulk LM composites with finite package without leakage is still a great challenge, due to high surface tension and pump-out issues of LM. Here, a novel confined thermal expansion strategy based on expandable microsphere (EM) is proposed to develop a new class of LM-based monoliths with 3D continuous conductive network. The EM/LM monolith (EM/LMm) presents outstanding performance of lightweight like metallic aerogel (0.104 g cm⁻¹), high strength (3.43 MPa), super elasticity (90% strain), as well as excellent tailor ability and recyclability, rely on its unique gas-filled closed-cellular structure and refined LM network. Moreover, the assembled highly conducting EM/LMm exhibits a recorded shielding effectiveness (98.7 dB) over a broad frequency range of 8.2–40 GHz among reported LM-based composites at an ultra-low content of LM, and demonstrates excellent electromagnetic sealing capacity in practical electronics. The ternary EM/LM/Ni monoliths fabricated by the same approach could be promising universal design principles for multifunctional LM composites, and applicable in magnetic responsive actuator. [Image: see text]

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40820-021-00766-5.

Textile-Based Mechanical Sensors: A Review

Innovations related to textiles-based sensors have drawn great interest due to their outstanding merits of flexibility, comfort, low cost, and wearability. Textile-based sensors are often tied to certain parts of the human body to collect mechanical, physical, and chemical stimuli to identify and record human health and exercise. Until now, much research and review work has been carried out to summarize and promote the development of textile-based sensors. As a feature, we focus on textile-based mechanical sensors (TMSs), especially on their advantages and the way they achieve performance optimizations in this review. We first adopt a novel approach to introduce different kinds of TMSs by combining sensing mechanisms, textile structure, and novel fabricating strategies for implementing TMSs and focusing on critical performance criteria such as sensitivity, response range, response time, and stability. Next, we summarize their great advantages over other flexible sensors, and their potential applications in health monitoring, motion recognition, and human-machine interaction. Finally, we present the challenges and prospects to provide meaningful guidelines and directions for future research. The TMSs play an important role in promoting the development of the emerging Internet of Things, which can make health monitoring and everyday objects connect more smartly, conveniently, and comfortably efficiently in a wearable way in the coming years.

Stimulus-driven liquid metal and liquid crystal network actuators for programmable soft robotics

Sophisticated soft matter engineering has been endorsed as an emerging paradigm for developing untethered soft robots with built-in electronic functions and biomimetic adaptation capacities. However, the integration of flexible electronic components into soft robotic actuators is challenging due to strain mismatch and material incompatibilities. Herein, we report a general strategy to integrate electrically conductive liquid metals (LMs) and shape-morphing liquid crystal networks (LCNs) towards multifunctional and programmable soft robotics. A unique colloidal LM ink with superior adhesion and photothermal conversion efficiency was judiciously designed and fabricated by ultrasonicating LMs and miniature carboxylated gold nanorods (MiniGNR-COOH) in an aqueous suspension of biological bacterial cellulose. The designed nanocellulose-based colloidal LM ink is used for shape-deformable and electrically conductive LM-LCN soft robots that can be electro- and photo-thermally actuated. As proof-of-concept demonstrations, we present a light-fueled soft oscillator, an inchworm-inspired soft crawler and programmable robotic Shadow Play exhibiting multifunctional controllability. The strategy disclosed here could open up a new technological arena for advanced multifunctional soft materials with potential utility in bioinspired soft machines, integrated soft electronics, human-computer interaction and beyond.

Recent advances in liquid-metal-based wearable electronics and materials

Soft wearable electronics are rapidly developing through exploration of new materials, fabrication approaches, and design concepts. Although there have been many efforts for decades, a resurgence of interest in liquid metals (LMs) for sensing and wiring functional properties of materials in soft wearable electronics has brought great advances in wearable electronics and materials. Various forms of LMs enable many routes to fabricate flexible and stretchable sensors, circuits, and functional wearables with many desirable properties. This review article presents a systematic overview of recent progresses in LM-enabled wearable electronics that have been achieved through material innovations and the discovery of new fabrication approaches and design architectures. We also present applications of wearable LM technologies for physiological sensing, activity tracking, and energy harvesting. Finally, we discuss a perspective on future opportunities and challenges for wearable LM electronics as this field continues to grow.

4D-Printable Liquid Metal-Liquid Crystal Elastomer Composites

Soft actuators that undergo programmable shape change in response to a stimulus are enabling components of future soft robots and other soft machines. Strategies to power these actuators often require the incorporation of rigid, electrically conductive materials into the soft actuator, thus limiting the compliance and shape change of the material. In this study, we develop a 4D-printable composite composed of liquid crystal elastomer (LCE) matrix with dispersed droplets of eutectic gallium indium alloy (EGaIn). Using deformable EGaIn droplets in place of rigid conductive fillers preserves the compliance and shape-morphing properties of the LCE. The process enables 4D-printed LCE actuators capable of photothermal and electrothermal actuation. At low liquid metal (LM) concentrations (71 wt %), the composite actuator exhibits a photothermal response upon irradiation of near-IR light. Printed actuators with a twisted nematic configuration are capable of bending angles of 150° at 800 mW cm^-2. At higher LM concentrations (88 wt %), the embedded LM droplets can form percolating networks that conduct electricity and enable electrical Joule heating of the LCE. Actuation strain ranging from 5 to 12% is controlled by the amount of electrical power that is delivered to the composite. We also introduce a method for multimaterial printing of monolithic structures where the LM filler loading is spatially varied. These multifunctional materials exhibit innate responsivity where the actuator behaves as an electrical switch and can report one of two states (on/off). These multiresponsive, 4D-printable composites enable multifunctional, mechanically active structures that can be powered with IR light or low DC voltages.

Pervasive liquid metal printed electronics: From concept incubation to industry

Electronic devices play vital role in modern civilization. Compared to conventional electronic manufacturing, the recently emerging liquid metal printed electronics (LMPE) is opening many extraordinary opportunities, such as large-area printing, pervasive adaptability, flexibility for personal use, low cost, high performance, and environmental friendliness. More uniquely, liquid metal printing allows customize electronic products on demand to fabricate electronics spanning from 2D plane surface to 3D structure and on any desired substrates. This deems it to reshape modern electronics and integrated circuits field. So far, a variety of technological breakthroughs in this new generation electronic engineering area have been made in the process of developing various liquid metal functional inks, printing machines and applications, which significantly stimulate the quick incubation and formation of a new electronic industry. Clearly, sorting out the major R&D directions and clarifying future challenges is crucial for the large scale industrialization of LMPE. This perspective article is dedicated to briefly outline the representative principles and key technologies lying behind, and illustrate the milestone products and equipment thus invented for the coming LMPE industry. In addition, we evaluate the corresponding industrialization trends and promising roadmap and interpret future prospects for the new era of pervasive electronics when anyone can freely use such a tool to print out himself functional electronic device to fulfill various purposes at anywhere and anytime.