Significance Efficient thermal transport is critical for applications ranging from electronics and energy to advanced manufacturing and transportation; it is essential in emerging domains like wearable computing and soft robotics, which require thermally conductive materials that are also soft and stretchable. However, heat transport within soft materials is limited by the dynamics of phonon transport, which results in a trade-off between thermal conductivity and compliance. We overcome this by engineering an elastomer composite embedded with elongated inclusions of liquid metal (LM) that function as thermally conductive pathways. These composites exhibit an extraordinary combination of low stiffness (<100 kPa), high strain limit (>600%), and metal-like thermal conductivity (up to 9.8 W⋅m−1⋅K−1) that far exceeds any other soft materials. Soft dielectric materials typically exhibit poor heat transfer properties due to the dynamics of phonon transport, which constrain thermal conductivity (k) to decrease monotonically with decreasing elastic modulus (E). This thermal−mechanical trade-off is limiting for wearable computing, soft robotics, and other emerging applications that require materials with both high thermal conductivity and low mechanical stiffness. Here, we overcome this constraint with an electrically insulating composite that exhibits an unprecedented combination of metal-like thermal conductivity, an elastic compliance similar to soft biological tissue (Young’s modulus < 100 kPa), and the capability to undergo extreme deformations (>600% strain). By incorporating liquid metal (LM) microdroplets into a soft elastomer, we achieve a ∼25× increase in thermal conductivity (4.7 ± 0.2 W⋅m−1⋅K−1) over the base polymer (0.20 ± 0.01 W⋅m−1·K−1) under stress-free conditions and a ∼50× increase (9.8 ± 0.8 W⋅m−1·K−1) when strained. This exceptional combination of thermal and mechanical properties is enabled by a unique thermal−mechanical coupling that exploits the deformability of the LM inclusions to create thermally conductive pathways in situ. Moreover, these materials offer possibilities for passive heat exchange in stretchable electronics and bioinspired robotics, which we demonstrate through the rapid heat dissipation of an elastomer-mounted extreme high-power LED lamp and a swimming soft robot.

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High thermal conductivity in soft elastomers with elongated liquid metal inclusions.

Summary Liquid metal (LM) with high electrical conductivity, thermal conductivity, excellent biocompatibility, and extraordinary fluidity has emerged as a promising class of functional materials. However, such materials still encounter many practical challenges due to the rather limited forms available so far. As a promising remedy, LM composites in synergy with other materials would open tremendous opportunities for fundamental research or practical applications. This is because controllable integration of base LM with functional materials (e.g., metal nanoparticles, polymers, and drug molecules) would significantly tune the intrinsic properties of LM as desired, enabling it to offer further major potential in tackling various sectors' challenging issues, including thermal management, biomedicine, chemical catalysis, flexible electronics, and soft robots. Here, we systematically summarize and review the fundamental progress in pursuing LM composites. The basic composite strategies are outlined in three categories: LM composites with core-shell structure, LM-polymer composites, and LM-particle composites. The effectiveness of the composite strategy is illustrated via the typical applications of LM composites in representative fields. The challenges and perspectives in developing LM composites are also identified and interpreted to better guide future research. It is expected that the coming LM era will witness a new world of fruitful composites thereby discovered or invented.

Liquid Metal Composites

There is little question that the "electronic revolution" of the 20th century has impacted almost every aspect of human life However, the emergence of solid-state electronics as a ubiquitous feature of an advanced modern society is posing new challenges such as the management of electronic waste (e-waste) that will remain through the 21st century In addition to developing strategies to manage such e-waste, further challenges can be identified concerning the conservation and recycling of scarce elements, reducing the use of toxic materials and solvents in electronics processing, and lowering energy usage during fabrication methods In response to these issues, the construction of electronic devices from renewable or biodegradable materials that decompose to harmless by-products is becoming a topic of great interest Such "green" electronic devices need to be fabricated on industrial scale through low-energy and low-cost methods that involve low/non-toxic functional materials or solvents This review highlights recent advances in the development of biodegradable materials and processing strategies for electronics with an emphasis on areas where green electronic devices show the greatest promise, including solar cells, organic field-effect transistors, light-emitting diodes, and other electronic devices

Biodegradable Materials and Green Processing for Green Electronics.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/inf2.12060

Physical sensors for skin-inspired electronics

Flexible materials with the ability to be bent, strained, or twisted play a critical role in soft robots and stretchable electronics. Although tremendous efforts are focused on developing new stretchable materials with excellent stability, inevitable mechanical damage due to long term deformation is still an urgent problem to be tackled. Here, a magnetic healing method based on Fe-doped liquid metal (Fe-GaIn) conductive ink via a noncontact way is proposed. Further, multifunctional flexible electronics are designed with combined performances of superior remote self-healing under magnetic field, water-degradable, and thermal transfer printing, which attribute to three parts of the materials including Fe-GaIn conductive ink, degradable PVA substrate, and adhesive fructose. The as-made light emitting diodes (LED) circuit is demonstrated with both structural and functional repairing after single or multipoint damage. The self-healing time from multipoint damage is pretty fast within 10 s. Due to the water-soluble PVA film, the recycling process is simple via immersing into water. Through heating, the electric circuit on fructose can be transferred to other flexible substrate with high efficiency, which broadens the practical applications of the present system. The novel and multifunctional electronics hold great promise for self-healing electronics, transient electronics, and soft robots.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201901478

Magnetic Liquid Metal (Fe-EGaIn) Based Multifunctional Electronics for Remote Self-Healing Materials, Degradable Electronics, and Thermal Transfer Printing.

Liquid Metal‐Based Transient Circuits for Flexible and Recyclable Electronics

Patterning of liquid metals (LMs) at different length scales remains one of the issues hindering their practical applications. Herein, we report a robust LM patterning method enabled by transiently sealing and subsequently dissolving a sacrificial layer (a polyvinyl alcohol (PVA) thin film or an adhesive tape). In this method, the LM was filled into the channels, which were temporarily formed by sealing with a sacrificial layer, by either negative or positive pressure. Afterward, LM patterns were obtained by the dissolution of the sacrificial layer in water. This method affords a robust generation of micro- and macro-patterns of LMs in a wide range from 30 μm to 1200 μm with high reliability. To demonstrate the potential application of this method, flexible and wearable LM-based wireless charging devices were fabricated, optimized and characterized. A power transfer efficiency (PTE) up to 60% was achieved while maintaining their function even at a bending radius of 12.5 mm. As a proof-of-concept, flexible LM-based wireless charging devices were employed to power passive, implantable and wearable devices.

Robust, multiscale liquid-metal patterning enabled by a sacrificial sealing layer for flexible and wearable wireless powering

The dynamic growth process of gold nanoflowers (AuNFs), which have ample distinct optical properties important for chemical and biosensing as well as high catalytic efficiency, is essential for the controlled synthesis of AuNFs. Here, a simple and rapid microfluidic strategy is developed not only to synthesize AuNFs of various sizes but also to investigate the growth process of AuNFs, which surprisingly allows for the observation of previously unidentifiable processes. These AuNFs are quasi-spherical with many branches in various orientations and poly-crystalline with different crystal faces. The detailed process about the aggregation of primary nanoparticles in the growth process of AuNFs is deconvoluted into a dynamic self-limiting growth process, which is controlled by electrostatic repulsion and van der Waals attraction between building blocks. Monte Carlo simulation confirms the self-limiting growth process. Meanwhile, the as-prepared AuNFs show a high catalytic activity for the reduction of 4-nitrophenol (4-NP) by NaBH4 while not necessitating post-washing treatments.

Rapid synthesis and growth process deconvolution of Au nanoflowers with ultrahigh catalytic activity based on microfluidics

The invention discloses a liquid metal wireless charging coil and a preparation method thereof. The wireless charging coil comprises a spiral flexible pipeline and liquid metal for filling the flexible pipeline; the inner diameter cross section of the flexible pipeline is 1-4mm; and the two ends of the flexible pipeline are connected with a PCB. By adopting the millimeter-level and large-dimensionflexible pipeline, and by injecting the flexible pipeline with the liquid metal, the liquid metal wireless charging coil is manufactured, so that the cross section of the coil is enlarged, the directcurrent resistance of the coil is lowered, the charging efficiency is highly increased, and energy consumption caused by heating is lowered in a condition of not influencing the inductance value; andin addition, the liquid metal wireless charging coil is high in flexibility, easy to bend and fold, convenient to carry, low in construction cost, recyclable and high in applicability.

Liquid metal wireless charging coil and preparation method thereof

The invention discloses a liquid metal structure and an instantaneous patterning packaging method thereof. The method comprises the following steps: preparing a pattern template, wherein the pattern template is provided with a groove; sealing the groove in the pattern template by adopting a water-soluble film; filling the groove with liquid metal through a positive pressure filling method or a negative pressure filling method; and putting the pattern template filled with the liquid metal into deionized water, and removing the water-soluble film to obtain a liquid metal structure. According tothe invention, the water-soluble film is arranged on the substrate, the groove is sealed, the liquid metal is injected into the groove by using the positive pressure filling method or the negative pressure filling method, finally, the pattern template filled with the liquid metal is put into the deionized water to remove the water-soluble film, and because the liquid metal is always deposited in the groove, the water-soluble film on the surface does not influence the pattern, so that the patterning resolution of the liquid metal is very high.

Teng Long

Papers

Liquid Metal‐Based Transient Circuits for Flexible and Recyclable Electronics

Robust, multiscale liquid-metal patterning enabled by a sacrificial sealing layer for flexible and wearable wireless powering

Rapid synthesis and growth process deconvolution of Au nanoflowers with ultrahigh catalytic activity based on microfluidics

Liquid metal wireless charging coil and preparation method thereof

Liquid metal structure and instantaneous patterning packaging method thereof