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Liquid metal (LM) droplet‐embedded polymer composites with exceptional thermal, electrical, and mechanical characteristics are attractive for applications in soft electronics and thermal management. Generally, a high volume fraction of LMs is required for desirable thermal and electrical properties, and thus recycling of LMs is favorable and cost effective. However, recycling LMs from composites remains a formidable challenge because the LM droplets are typically dispersed into a permanent covalently crosslinked polymer matrix. Here, a facile synthesis of LM‐polymer composites with high electrical conductivity (1800 S cm−1), high stretchability (1400% tensile strain), as well as degradability and recyclability is reported. The composite comprises a poly(2‐hydroxyethyl acrylate) hydrogel that is physically crosslinked by LM nanodroplets as a polymer matrix and either LM microdroplets or silver microflakes as a conductive filler. A postdrying process improves the mechanical performance of the composite while creating percolating pathways via the rearrangement of the conductive fillers, imparting high electrical conductivity to the composite. Moreover, the composites are readily degraded and dissolved in aqueous alkali solutions at room temperature thanks to the reactive nature of LM crosslinks, enabling high recycling efficiency of ≈80% and 95% for composites with LM microdroplets and silver microflakes as conductive fillers, respectively. [ABSTRACT FROM AUTHOR]

Transient electronics with degradability or dissolvability on demand possess remarkable characteristics for wearable, bioelectronic, and implanted electronic devices. While synthetic materials have made significant progress in degradability and stretchability, they suffer from intrinsic limitations, including low degradation rates, poor mechanical compliance, and high production cost, which restrict the growth of transient electronic devices. Herein, a versatile fabrication strategy is presented for transient electronics that combine a resilient biopolymer substrate (gelatin biogels) with liquid metal (Galinstan). The circuits made from gelatin biogel‐based and liquid metal are mechanically resilient and durable, exhibiting good compliance with irregular and deformable surfaces, and thus can withstand long‐term exposure to dynamic deformation (about 60 000 cycles at a strain of 50%). More importantly, gelatin biogels are naturally derived and endowed with thermally reversible gelling, which enables on‐demand rapid dissolution and recycling of the materials. As a proof‐of‐concept, transient circuit‐based capacitive sensors, which can be degraded within 20 s, are fabricated for monitoring finger touching, morse code generation, and electrical touchpad. Such a strategy provides a guideline for designing transient electronics with a combination of biopolymer gels and liquid metal, and this approach is anticipated to find applications in human–machine interfaces and wearable devices. [ABSTRACT FROM AUTHOR]

Directed liquid metal (gallium‐based) manipulation and actuation are paramount for copious applications, including soft robotics, soft electronics, and targeted drug delivery. Although there are several strategies available to achieve mobility of liquid metals in a "wet" environment. Strategies to achieve and improve mobility of liquid metal droplets and puddles in a "dry" environment are scarce and rely on metallophobic surface design or liquid metal marbles. Here, high mobility of Galinstan is elucidated by combining metallophobic surface design and vertical vibrations. Vibration frequencies between 20 and 30 Hz are conducive to droplet movement and threshold inclination angles of 0.5° to 1° are observed upon actuation by these vibrations. The method itself is applicable for a wide range of droplet sizes (30 and 2000 µL) and very robust. The droplet movement typically comprises of periodic receding and advancing of the droplet and commences via a rolling mechanism rather than a gliding mechanism. Finally, it is shown that small (0.5 mm height) obstacles can be traversed by this method, indicating that it can be used in concert with other strategies, such as surface structuring strategies, which open up pathways for mobility and controlled actuation of liquid metal droplets in air. [ABSTRACT FROM AUTHOR]

The emergent transient electronics that can be destroyed or degraded harmlessly under an environmental stimulus are attracting extensive attentions toward eco‐friendly electronic devices and confidential communication applications. However, currently available transient electronics are limited by the poor recyclability of components, which fails to meet the ever‐increasing demands of advanced electronic devices. Here, this work reports a fully recyclable transient electronic device based on a multi‐layered architecture, comprising a liquid metal (LM) as the conductor and a NaOH solution‐embedded paraffin composite (NPC) as the degradation agent. Upon thermally triggered by the bottom heater, the middle‐layer LM circuit and the top NPC layer melt, releasing the aqueous alkali to destroy the circuit at a quick transient rate (46 ± 4 s). Afterward, both the free LM and paraffin are recycled at extremely high efficiencies, that is, 98% for LM owing to the self‐aggregation and fluidic nature, 95% for paraffin arising from the low melting point and good recoverability. The successful integration of transience and recycling abilities dramatically improves the feasibility of transient electronics in real applications, as further demonstrated in a radio‐frequency transmission device and a remote destructible memristor, which hold great promises for confidential data storage devices and next‐generation electronics. [ABSTRACT FROM AUTHOR]

A series of ester‐ether copolymers were obtained via the reaction between α,ω‐dihydroxyl poly(ϵ‐caprolactone) (PCL) and ethylene oxide (EO) or monosubstituted epoxides catalyzed by strong phosphazene bases. The two types of monomeric units were distributed in highly random manners due to the concurrence of epoxide ring‐opening and fast transesterification reactions. The substituent of epoxide showed an interesting bidirectional effect on the enzymatic degradability of the copolymer. Compared with PCL, copolymers derived from EO exhibited enhanced hydrophilicity and decreased crystallinity which then resulted in higher degradability. For the copolymers derived from propylene oxide and 1,2‐butylene oxide, the hydrophobic alkyl pendant groups also allowed lower crystallinity of the copolymers thus higher degradation rates. However, further enlarging the pendant groups by using styrene oxide or 2‐ethylhexyl glycidyl ether caused a decrease in the degradation rate, which might be ascribed to the higher bulkiness hindering the contact of ester groups with lipase. [ABSTRACT FROM AUTHOR]

Liquid metals are a class of materials with a millennia‐long history, albeit until 200 years ago, mostly centered on mercury‐based applications. The knowledge of mercury's toxicity and the discovery of it's bioaccumulation and biomagnification in the late 20th century resulted in phasing out mercury for copious application. Lead‐based, bismuth‐based, and sodium‐based technology has partially replaced mercury, but due to toxicity or reactivity issues, replacements for lead and sodium are sought. The discovery of indium and, especially, gallium has allowed for the third generation of alloys with a melting point close to room temperature. This review gives a comprehensive overview of the technological developments of fusible and liquid metals, i. e. metals and alloys with melting points below ca. 250 °C, from the early days to current research and industrial application. Special emphasis is focused on important steppingstones and recent developments of gallium‐based alloys as well as how they could be used for future applications. [ABSTRACT FROM AUTHOR]

Deformability and self‐adaptability are important for soft robots to deal with uncertain and varying situations and environments during movement and navigation. Droplet‐based robots are great candidates to travel inside narrow and constrained spaces without damaging the interfaces due to their extreme deformability and liquid nature, which enables smooth contact between robots and target spaces. Here, magnetic liquid metal droplet robots, comprising liquid metal and carbonyl iron, that can perform reversible telescopic deformation, bending, and on‐demand locomotion are proposed. The magnetic liquid‐metal‐based robots can perform on‐demand and reversible coalescence and splitting by intricately applying magnetic fields. Importantly, the liquid metal robot can perform phase transition to fix the desired shape after programmable shape encoding. The liquid‐metal‐based soft robots can serve as dynamic and recyclable switches for complex circuits, and are capable of repairing damaged sections of microcircuits by remote actuation, controllable coalescence, and on‐demand circuit welding. This technology provides a new application scenario of droplet‐based soft robots for on‐demand circuit welding and transient recyclable electronics. A preprint version of the article can be found at: https://www.authorea.com/doi/full/10.22541/au.165167005.56125584. [ABSTRACT FROM AUTHOR]

The booming field of wearable devices has nourished progress in developing multifunctional wearable energy sources that can withstand deformations while maintaining their electrochemical functions. Unlike energy storage systems such as rechargeable batteries and supercapacitors, wearable biofuel cells (w‐BFCs) generate green electricity from energy‐dense carbon‐neutral fuels via highly efficient bioelectrochemical reactions, delivering excellent biocompatibility, remarkable environmental sustainability, and exceptional capability of miniaturization. These desirable merits give w‐BFCs great potential in the field of wearable applications. Moreover, emerging studies of w‐BFCs in self‐powered biosensing, controlled drug delivery, and wound dressings have greatly expanded their possible fields of application. Recent progress and strategies to accomplish flexible and stretchable w‐BFCs are summarized here. Novel materials and configurations with tailored features that can be employed to fabricate w‐BFCs are elaborated and discussed. Current applications and near‐future applications of w‐BFCs in health‐monitoring and medical treatment fields are outlined. Furthermore, challenges and perspectives regarding this emerging field of materials science and engineering are also emphasized. [ABSTRACT FROM AUTHOR]

Liquid metals have aroused interest in the scientific communities of electronics, engineering and biomedicine. In particular, the stabilization of liquid metal nanodroplets by surfactants and macromolecules has been investigated in the past extensively. Herein, we investigate the change in interfacial tension of the liquid metal Galinstan in contact with aqueous electrolytes upon addition of surfactants in aqueous solutions. We show that is governed by the chemical environment, the structure of the electric double layer (EDL) and the surfactant charge. In most environments a critical micelle concentration (CMC) can be determined, the value of which is in favorable agreement with literature values. Furthermore, based on the selective adsorption of the surfactant, the structure of the EDL is predicted and discussed, and the adsorption of surfactants is employed to mitigate liquid metal droplet coalescence. In this regard, the data suggests that the packing parameter, change in IFT upon surfactant addition and charge density of EDL are the main contributors for liquid metal droplet stability. [ABSTRACT FROM AUTHOR]

Abstract: Photocleavable polymers have attracted much attention in drug delivery, photopatterning, and controlling cell behavior. Photolysis is usually induced by UV light. However, UV light cannot penetrate deeply into biological tissue and may damage biological components. Therefore, conventional UV‐light‐cleavable polymers are problematic for deep‐tissue biomedical applications. In this feature article, red and near‐infrared light‐cleavable polymers are reviewed, and their potential applications are highlighted. The remaining challenges in the field of photocleavable polymers are discussed. [ABSTRACT FROM AUTHOR]

Abstract: The mechanical properties (e.g., stiffness, stretchability) of prefabricated hydrogels are of pivotal importance for diverse applications in tissue engineering, soft robotics, and medicine. This study reports a feasible method to fabricate ultrasoft and highly stretchable structures from stiff and tough hydrogels of low stretchability and the application of these switchable hydrogels in programmable shape‐morphing systems. Stiff and tough hydrogel structures are first fabricated by the mechanical strengthening of Ca2+‐alginate/polyacrylamide tough hydrogels by addition of Fe3+ ions, which introduces Fe3+ ionically cross‐linked centers into the Ca2+ divalent cross‐linked hydrogel, forming an additional and much less flexible trivalent ionically cross‐linked network. The resulting stiff and tough hydrogels are exposed to an L‐ascorbic acid (vitamin C, VC) solution to rapidly reduce Fe3+ to Fe2+. As a result, flexible divalent ionically cross‐linked networks are formed, leading to swift softening of the stiff and tough hydrogels. Moreover, localized stiffness variation of the tough hydrogels can be realized by precise patterning of the VC solution. To validate this concept, sequential steps of VC patterning are carried out for local tuning of the stiffness of the hydrogels. With this strategy, localized softening, unfolding, and sequential folding of the tough hydrogels into complex 3D structures is demonstrated. [ABSTRACT FROM AUTHOR]

Tough hydrogels, polymeric network structures with excellent mechanical properties (such as high stretchability and toughness), are emerging soft materials. Despite their remarkably mechanical features, tough hydrogels exhibit two flaws (freezing around the icing temperatures of water and drying under arid conditions). Inspired by cryoprotectants (CPAs) used in the inhibition of the icing of water in biological samples, a versatile and straightforward method is reported to fabricate extreme anti-freezing, non-drying CPA-based organohydrogels with long-term stability by partially displacing water molecules within the pre-fabricated hydrogels. CPAbased Ca-alginate/polyacrylamide (PAAm) tough hydrogels were successfully fabricated with glycerol, glycol, and sorbitol. The CPA-based organohydrogels remain unfrozen and mechanically flexible even up to @70 8C and are stable under ambient conditions or even vacuum. [ABSTRACT FROM AUTHOR]

Abstract: Nontoxic liquid metals (conductive materials in a liquid state at room temperature) are an emerging class of materials for applications ranging from soft electronics and robotics to medical therapy and energy devices. Their sticky and corrosive properties, however, are becoming more of a critical concern for circuits and devices containing other metals as these are easily destroyed or contaminated by the liquid metals. Herein, a feasible method for fabricating highly conductive graphene‐coated liquid metal (GLM) droplets is reported and their application as nonstick, noncorrosive, movable, soft contacts for electrical circuits is demonstrated. The as‐prepared GLM droplets consist of a liquid‐phase soft core of liquid metal and a slippery outer layer of graphene sheets. These structures address the issue of simultaneous control of the wettability and conductivity of a soft electronic contact by combining extraordinary properties, i.e., nonstick, noncorrosive, yet exhibiting high electronic conductivity while in contact with metal substrates, e.g., Au, Cu, Ag, and Ni. As proof‐of‐concept, the as‐prepared GLM droplets are demonstrated as floating electrodes for movable, recyclable electronic soft contacts in electrical circuits. [ABSTRACT FROM AUTHOR]

This paper describes a fully aqueous- and air-compatible chemical approach to preparing high-performance conductive textiles. In this method, the surfaces of textile materials are first modified with an aqueous solution of double-bond-containing silane molecules to form a surface-anchoring layer for subsequent in situ free-radical polymerization of [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) in the air. Thin layers of poly-METAC (PMETAC) are therefore covalently grafted on top of the silane-modified textile surface. Cu- or Ni-coated textiles are finally fabricated by electroless deposition (ELD) onto the PMETAC-modified textiles. Parameters including polymerization time, temperature, and ELD conditions are studied to optimize the whole fabrication process. The as-made conductive textiles exhibit sheet resistance as low as 0.2 Ω sq−1, which makes them highly suitable for use as conductive wires and interconnects in flexible and wearable electronic devices. More importantly, the chemical method is fully compatible with the conventional 'pad-dry-cure' fabrication process in the textile manufacturing industry, thus indicating that it is very promising for high-throughput and roll-to-roll fabrication of high-performance metal-coated conductive textiles in the future. [ABSTRACT FROM AUTHOR]

Mechanically controlled polymerization that employs the mechanical energy to fabricate novel synthetic materials has attracted considerable interest. However, only a few examples have been achieved so far, owing to the limited choices of materials and strategies. Herein, a versatile, liquid metal (LM)‐mediated mechanochemical polymerization method (LMMMP) is developed for the air‐compatible, robust preparation of polymers in an aqueous solution. This method involves the simultaneous disruption of bulk LMs into micro‐ and nanodroplets and the combination of monomers into polymers during ultrasonic irradiation. The pristine and reactive LM surface continuously generated by ultrasound endows this polymerization method with excellent oxygen tolerance, high reaction rate, and the ability to produce polymers with high molecular weight from a wide variety of water‐soluble monomers. Besides, LM droplets are readily reclaimed and reused for polymerization. The authors envision that the LMMMP promotes the utilization of mechanical energy for the synthesis of functional polymers, constitutes a novel fabrication approach for polymer–LM nanocomposites, and provides new insight into the design of LM‐based platforms for polymerization. [ABSTRACT FROM AUTHOR]

Transient electronics, arising electronic devices with dissolvable or degradable features on demand, is still at an early stage of development due to the limited choices of materials and strategies. Herein, a facile fabrication method for transient circuits by the combination of room‐temperature liquid metals (RTLMs) as the electronic circuit and water‐soluble poly(vinyl alcohol) (PVA) as the packaging material is reported. The as‐made transient circuits exhibit remarkable durability and stable electric performance upon bending and twisting, while possessing short transience times, owing to the excellent solubility of PVA substrates and the intrinsic flexibility of RTLM patterns. Moreover, the RTLM‐based transient circuit shows an extremely high recycling efficiency, up to 96% of the employed RTLM can be recovered. As such, the economic and environmental viability of transient electronics increases substantially. To validate this concept, the surface patterning of RTLMs with complicated shapes is demonstrated, and a transient antenna is subsequently applied for passive near‐field communication tag and a transient capacitive touch sensor. The application of the RTLM‐based transient circuit for sequentially turning off an array of light‐emitting‐diode lamps is also demonstrated. The present RTLM‐based PVA‐encapsulated circuits substantially expand the scope of transient electronics toward flexible and recyclable transient systems. Flexible and recyclable transient circuits are fabricated by employing room‐temperature liquid metals as the electronic circuit and water‐soluble poly(vinyl alcohol) as the packaging material. The as‐made transient circuits are successfully applied to various electronic devices, including near‐field communication, capacitive touch sensors, and transient circuits to allow sequentially turning off an array of light‐emitting‐diode lamps. [ABSTRACT FROM AUTHOR]

Living biological tissues are made of structures with properly defined mechanical properties (toughness and stiffness) toward specific biological functions. Herein, a chemical manipulation strategy is developed to locally vary the oxidation state of Fe ions from divalent to trivalent in the tough hydrogels. The resultant trivalent ionically cross‐linked networks become less flexible and lead to a significant enhancement of the stiffness of the tough hydrogels. The mechanical strengthening of Fe2+/Ca2+‐alginate/polyacrylamide tough hydrogels is demonstrated by the oxidation with ammonium persulfate (APS). Moreover, by applying surface patterning, the mechanical properties of the tough hydrogels are spatially stiffened and thus can serve as anisotropic elements to guide the shape morphing of tough hydrogels into complex 3D structures. This method opens up a simple strategy not only to dynamically vary the mechanics of tough hydrogels, both in bulk and locally from prefabricated soft tough hydrogels, but also toward their shape morphing behaviors on demand. Stiffening and shape morphing of soft Fe2+/Ca2+‐alginate/polyacrylamide tough hydrogels is achieved by applying a chemical manipulation method to localized varying the oxidation state of Fe ions from divalent to trivalent. With this method, 3D molding or shape morphing of tough hydrogels is realized by the as‐encoded mechanical anisotropy. [ABSTRACT FROM AUTHOR]

Review: 74 refs. [ABSTRACT FROM AUTHOR]