Microfluidic Mass Production of Stabilized and Stealthy Liquid Metal Nanoparticles

Functional nanoparticles comprised of liquid metals, such as eutectic gallium indium (EGaIn) and Galinstan, present exciting opportunities in the fields of flexible electronics, sensors, catalysts, and drug delivery systems. Methods used currently for producing liquid metal nanoparticles have significant disadvantages as they rely on both bulky and expensive high‐power sonication probe systems, and also generally require the use of small molecules bearing thiol groups to stabilize the nanoparticles. Herein, an innovative microfluidics‐enabled platform is described as an inexpensive, easily accessible method for the on‐chip mass production of EGaIn nanoparticles with tunable size distributions in an aqueous medium. A novel nanoparticle‐stabilization approach is reported using brushed polyethylene glycol chains with trithiocarbonate end‐groups negating the requirements for thiol additives while imparting a “stealth” surface layer. Furthermore, a surface modification of the nanoparticles is demonstrated using galvanic replacement and conjugation with antibodies. It is envisioned that the demonstrated microfluidic technique can be used as an economic and versatile platform for the rapid production of liquid metal‐based nanoparticles for a range of biomedical applications.     

Directed Assembly of Liquid Metal–Elastomer Conductors for Stretchable and Self-Healing Electronics

Growing interest in soft robotics, stretchable electronics, and electronic skins has created demand for soft, compliant, and stretchable electrodes and interconnects. Here, dielectrophoresis (DEP) is used to assemble, align, and sinter eutectic gallium indium (EGaIn) microdroplets in uncured poly(dimethylsiloxane) (PDMS) to form electrically conducting microwires. There are several noteworthy aspects of this approach. 1) Generally, EGaIn droplets in silicone at loadings approaching 90 wt% remain insulating and form a conductive network only when subjected to sintering. Here, DEP facilitates assembly of EGaIn droplets into conductive microwires at loadings as low as 10 wt%. 2) DEP is done in silicone for the first time, enabling the microwires to be cured in a stretchable matrix. 3) Liquid EGaIn droplets sinter during DEP to form a stretchable metallic microwire that retains its shape after curing the silicone. 4) Use of liquid metal eliminates the issue of compliance mismatch observed in soft polymers with solid fillers. 5) The silicone–EGaIn “ink” can be assembled by DEP within the crevices of severely damaged wires to create stretchable interconnects that heal the damage mechanically and electrically. The DEP process of this unique set of materials is characterized and the interesting attributes enabled by such liquid microwires are demonstrated.     

Plasma‐Induced Hetero‐Nanostructures on a Polymer with Selective Metal Co‐Deposition

Plasma‐induced pattern formation is explored on polyethylene terephthalate (PET) using an oxygen plasma glow discharge. The nanostructures on PET are formed through preferential etching directed by the co‐deposition of metallic elements, such as Cr or Fe, sputtered from a stainless‐steel cathode. The local islands formed by metal co‐deposition have significantly slower etching rates than those of the pristine regions on PET, generating anisotropic nanostructures in pillar‐ or hair‐like form during plasma etching. By covering the cathode with the appropriate material, the desired metallic or polymeric elements can be co‐deposited onto the target surfaces. When the cathode is covered by a relatively soft material composed of only carbon and hydrogen, such as polystyrene, nanostructures typically induced by preferential etching are not observed on the PET surface, and the surfaces are uniformly etched. A variety of metals, such as Ag, Cu, Pt, or Si, can be successfully co‐deposited onto the PET surfaces by simply using a cathode covered in the desired metal; high‐aspect‐ratio nanostructures coated with the co‐deposited metal are subsequently formed. Therefore this simple single‐step method for forming hetero‐nanostructures—that is, nanoscale hair‐like polymer structures decorated with metals—can be used to produce nanostructures for various applications, such as catalysts, sensors, or energy devices.     

Autonomous Surface Reconciliation of a Liquid-Metal Conductor Micropatterned on a Deformable Hydrogel

Spreading liquid droplets on solid surfaces is a core topic in physical chemistry with significant technological implications. Liquid metals, which are eutectic alloys of constituent metal atoms with low melting temperatures, are practically useful, but difficult to spread on solid surfaces because of their high surface tension. This makes it difficult to use liquid metals as deformable on-board microcircuitry electrodes, despite their intrinsic deformability. In this study, it is discovered that eutectic gallium–indium (EGaIn) can be spread onto the surface of chemically cross-linked hydrogels consisting of aliphatic alkyl chains with numerous hydroxyl groups ( OH), thus facilitating the development of directly micropatterned EGaIn electrodes. More importantly, EGaIn patterned on a hydrogel autonomously reconciliates its surface to form a firm hydrogel interface upon mechanical deformation of the hydrogel. This autonomous surface reconciliation of EGaIn on hydrogels allows researchers to reap the benefits of chemically modified hydrogels, such as reversible stretching, self-healing, and water-swelling capability, thereby facilitating the fabrication of superstretchable, self-healable, and water-swellable liquid-metal electrodes with very high conductance tolerance upon deformation. Such electrodes are suitable for a variety of deformable microelectronic applications.     

Non‐Magnetic Injectable Implant for Magnetic Field‐Driven Thermochemotherapy and Dual Stimuli‐Responsive Drug Delivery: Transformable Liquid Metal Hybrid Platform for Cancer Theranostics

Transformable liquid metal (LM)‐based materials have attracted considerable research interest in biomedicine. However, the potential biomedical applications of LMs have not yet been fully explored. Herein, for the first trial, the inductive heating property of gallium–indium eutectic alloy (EGaIn) under alterative magnetic field is systematically investigated. By virtue of its inherent metallic nature, LM possesses excellent magnetic heating property as compared to the conventional magnetite nanoparticles, therefore enabling its unique application as non‐magnetic agents in magnetic hyperthermia. Moreover, the extremely high surface tension of LM could be dramatically lowered by a rather facile PEGylation approach, making LM an ideal carrier for other theranostic cargos. By incorporating doxorubicin (DOX)‐loaded mesoporous silica (DOX‐MS) within PEGylated LM, a magnetic field‐driven transformable LM hybrid platform capable of pH/AFM dual stimuli‐responsive drug release and magnetic thermochemotherapy are successfully fabricated. The potential application for breast cancer treatment is demonstrated. Furthermore, the large X‐ray attenuation ability of LM endows the hybrid with the promising ability for CT imaging. This work explores a new biomedical use of LM and a promising cancer treatment protocol based on LM hybrid for magnetic hyperthermia combined with dual stimuli‐responsive chemotherapy and CT imaging.     

Fabrication of Lotus‐type Porous Metals and their Physical Properties

Lotus‐type porous metals whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification in a pressurized gas atmosphere. The pores are formed as a result of precipitation of supersaturated gas when liquid metal is solidified. The lotus‐type porous metals with homogeneous size and porosity of the evolved pores produced by a mould casting technique are limited to the metals with high thermal conductivity. On the other hand, the pores with inhomogeneous pore size and porosity are evolved for metals and alloys with low thermal conductivity such as stainless steel. In order to obtain uniform pore size and porosity, a new “continuous zone melting technique” was developed to fabricate long rod‐ and plate‐shape porous metals and alloys even with low thermal conductivity. Mechanical properties of tensile and compressive strength of lotus‐type porous metals and alloys are described together with internal friction, elasticity, thermal conductivity and sound absorption characteristics. All the physical properties exhibit significant anisotropy. Lotus‐type porous iron fabricated using a pressurized nitrogen gas instead of hydrogen exhibits superior strength.     

Electro‐Assisted Bioprinting of Low‐Concentration GelMA Microdroplets

Low‐concentration gelatin methacryloyl (GelMA) has excellent biocompatibility to cell‐laden structures. However, it is still a big challenge to stably fabricate organoids (even microdroplets) using this material due to its extremely low viscosity. Here, a promising electro‐assisted bioprinting method is developed, which can print low‐concentration pure GelMA microdroplets with low cost, low cell damage, and high efficiency. With the help of electrostatic attraction, uniform GelMA microdroplets measuring about 100 μm are rapidly printed. Due to the application of lower external forces to separate the droplets, cell damage during printing is negligible, which often happens in piezoelectric or thermal inkjet bioprinting. Different printing states and effects of printing parameters (voltages, gas pressure, nozzle size, etc.) on microdroplet diameter are also investigated. The fundamental properties of low‐concentration GelMA microspheres are subsequently studied. The results show that the printed microspheres with 5% w/v GelMA can provide a suitable microenvironment for laden bone marrow stem cells. Finally, it is demonstrated that the printed microdroplets can be used in building microspheroidal organoids, in drug controlled release, and in 3D bioprinting as biobricks. This method shows great potential use in cell therapy, drug delivery, and organoid building.     

Liquid Metal Droplet and Graphene Co‐Fillers for Electrically Conductive Flexible Composites

Colloidal liquid metal alloys of gallium, with melting points below room temperature, are potential candidates for creating electrically conductive and flexible composites. However, inclusion of liquid metal micro‐ and nanodroplets into soft polymeric matrices requires a harsh auxiliary mechanical pressing to rupture the droplets to establish continuous pathways for high electrical conductivity. However, such a destructive strategy reduces the integrity of the composites. Here, this problem is solved by incorporating small loading of nonfunctionalized graphene flakes into the composites. The flakes introduce cavities that are filled with liquid metal after only relatively mild press‐rolling (<0.1 MPa) to form electrically conductive continuous pathways within the polymeric matrix, while maintaining the integrity and flexibility of the composites. The composites are characterized to show that even very low graphene loadings (≈0.6 wt%) can achieve high electrical conductivity. The electrical conductance remains nearly constant, with changes less than 0.5%, even under a relatively high applied pressure of >30 kPa. The composites are used for forming flexible electrically‐conductive tracks in electronic circuits with a self‐healing property. The demonstrated application of co‐fillers, together with liquid metal droplets, can be used for establishing electrically‐conductive printable‐composite tracks for future large‐area flexible electronics.     

Morphology‐Controlled Metal Sulfides and Phosphides for Electrochemical Water Splitting

Because H₂ is considered a promising clean energy source, water electrolysis has attracted great interest in related research and technology. Noble‐metal‐based catalysts are used as electrode materials in water electrolyzers, but their high cost and low abundance have impeded them from being used in practical areas. Recently, metal sulfides and phosphides based on earth‐abundant transition metals have emerged as promising candidates for efficient water‐splitting catalysts. Most studies have focused on adjusting the composition of the metal sulfides and phosphides to enhance the catalytic performance. However, morphology control of catalysts, including faceted and hollow structures, is much less explored for these systems because of difficulties in the synthesis, which requires a deep understanding of the nanocrystal growth process. Herein, representative synthetic methods for morphology‐controlled metal sulfides and phosphides are introduced to provide insights into these methodologies. The electrolytic performance of morphology‐controlled metal sulfide‐ and phosphide‐based nanocatalysts with enhanced surface area and intrinsically high catalytic activity is also summarized and the future research directions for this promising catalyst group is discussed.     

Liquid Metal Initiator of Ring-Opening Polymerization: Self-Capsulation into Thermal/Photomoldable Powder for Multifunctional Composites

Liquid metal nanodroplets not only share similar metallic properties and nanoscale effect with solid metal nanoparticles, but also possess the additional uniqueness in nonvolatile fluidity and ambient sintering ability into continuous conductors. In most cases, liquid metal nanodroplets are encapsulated into ultrathin and fragile shells of oxides and amphiphile monolayers, and may be hindered from incorporating homogeneously into various composites through conventional processing methods. In this study, ring-opening polymerization is found to be initiated by sonicating the liquid metal EGaIn in fluidic lactones. By this in situ polymerization, EGaIn nanodroplets are encapsulated into polylactone shells with tunable thickness, which can further be dried into a solid powder. Besides high chemical stability and dispersibility in organic solvents, the powder of the EGaIn capsules combines the exceptional properties of the EGaIn droplets (e.g., photothermal effect) and the polylactone shells (e.g., biocompatibility, biodegradability, and compatibility with different polymer matrixes), being capable of being introduced into thermoplastic composites through liquid casting and thermal- or photomolding for the notch-insensitive tearing property, sintering-induced electric conductivity, and photothermal effect. Thus, the EGaIn initiator of ring-opening polymerization may start a pathway to produce stable andthermal/photomoldable powders of EGaIn capsules and their multifunctionalcomposites, applicable in biomedicines, soft electronics, and smart robots.     

Architected Polymer Foams via Direct Bubble Writing

Polymer foams are cellular solids composed of solid and gas phases, whose mechanical, thermal, and acoustic properties are determined by the composition, volume fraction, and connectivity of both phases. A new high‐throughput additive manufacturing method, referred to as direct bubble writing, for creating polymer foams with locally programmed bubble size, volume fraction, and connectivity is reported. Direct bubble writing relies on rapid generation and patterning of liquid shell–gas core droplets produced using a core–shell nozzle. The printed polymer foams are able to retain their overall shape, since the outer shell of these bubble droplets consist of a low‐viscosity monomer that is rapidly polymerized during the printing process. The transition between open‐ and closed‐cell foams is independently controlled by the gas used, while the foam can be tailored on‐the‐fly by adjusting the gas pressure used to produce the bubble droplets. As exemplars, homogeneous and graded polymer foams in several motifs, including 3D lattices, shells, and out‐of‐plane pillars are fabricated. Conductive composite foams with controlled stiffness for use as soft pressure sensors are also produced.     

A Liquid‐Metal–Elastomer Nanocomposite for Stretchable Dielectric Materials

Stretchable high‐dielectric‐constant materials are crucial for electronic applications in emerging domains such as wearable computing and soft robotics. While previous efforts have shown promising materials architectures in the form of dielectric nano‐/microinclusions embedded in stretchable matrices, the limited mechanical compliance of these materials significantly limits their practical application as soft energy‐harvesting/storage transducers and actuators. Here, a class of liquid metal (LM)–elastomer nanocomposites is presented with elastic and dielectric properties that make them uniquely suited for applications in soft‐matter engineering. In particular, the role of droplet size is examined and it is found that embedding an elastomer with a polydisperse distribution of nanoscale LM inclusions can enhance its electrical permittivity without significantly degrading its elastic compliance, stretchability, or dielectric breakdown strength. In contrast, elastomers embedded with microscale droplets exhibit similar improvements in permittivity but a dramatic reduction in breakdown strength. The unique enabling properties and practicality of LM–elastomer nanocomposites for use in soft machines and electronics is demonstrated through enhancements in performance of a dielectric elastomer actuator and energy‐harvesting transducer.     

Arrays of Hollow Silica Half‐Nanospheres Via the Breath Figure Approach

Breath figures (BFs) are patterns of liquid droplets that usually form upon condensation on a cold surface. Earlier work has shown that BFs can be used to produce continuous films of porous honeycomb‐structured patterns on various types of materials, paving the path to a number of important applications such as the manufacturing of highly ordered nano‐ and micron‐sized templates, micro lenses, and superhydrophobic coatings. It is worth noting, however, that few new findings have been reported in this area in recent years, limiting pursuits of novel architectures and key applications. In this report, an alternative method is described by which arrays of hollow silica half‐nanospheres can be produced via BF templates. In the present method, a chemical vapor deposition (CVD) protocol performed while the BF is formed on a glass substrate yields a nanostructured pattern of silica half‐spheres, which size (100–700 nm) and density across the glass surface vary with substrate modification and with the relative rates of water condensation and hydrolysis from silica precursors (a process carried out at room temperature). This method of forming arrays of hollow half‐nanospheres via the BF approach may be applicable to various other oxides and a broad range of substrates including large‐area flexible plastics.     

Thermally Stable Metal‐Organic Framework‐Templated Synthesis of Hierarchically Porous Metal Sulfides: Enhanced Photocatalytic Hydrogen Production

Porous nanostructured materials are demonstrated to be very promising in catalysis due to their well accessible active sites. Thermally stable metal‐organic frameworks (MOFs) as hard templates are successfully utilized to afford porous metal oxides and subsequently metal sulfides by a nanocasting method. The resultant metal oxides/sulfides show considerable Brunauer–Emmett–Teller (BET) surface areas, by partially inheriting the pore character of MOF templates. Preliminary investigation on the obtained hierarchically porous CdS for water splitting, as a proof of concept, demonstrates its much higher activity than both corresponding bulk and nanosized counterparts, under visible light irradiation. Given the structural diversity and tailorability of MOFs, such synthetic approach may open an avenue to the synthesis of advanced porous materials for functional applications.     

EGaIn‐Assisted Room‐Temperature Sintering of Silver Nanoparticles for Stretchable,Inkjet‐Printed,Thin‐Film Electronics

Coating inkjet‐printed traces of silver nanoparticle (AgNP) ink with a thin layer of eutectic gallium indium (EGaIn) increases the electrical conductivity by six‐orders of magnitude and significantly improves tolerance to tensile strain. This enhancement is achieved through a room‐temperature “sintering” process in which the liquid‐phase EGaIn alloy binds the AgNP particles (≈100 nm diameter) to form a continuous conductive trace. Ultrathin and hydrographically transferrable electronics are produced by printing traces with a composition of AgNP‐Ga‐In on a 5 µm‐thick temporary tattoo paper. The printed circuit is flexible enough to remain functional when deformed and can support strains above 80% with modest electromechanical coupling (gauge factor ≈1). These mechanically robust thin‐film circuits are well suited for transfer to highly curved and nondevelopable 3D surfaces as well as skin and other soft deformable substrates. In contrast to other stretchable tattoo‐like electronics, the low‐cost processing steps introduced here eliminate the need for cleanroom fabrication and instead requires only a commercial desktop printer. Most significantly, it enables functionalities like “electronic tattoos” and 3D hydrographic transfer that have not been previously reported with EGaIn or EGaIn‐based biphasic electronics.     

Dendrite‐Free Sodium‐Metal Anodes for High‐Energy Sodium‐Metal Batteries

Sodium (Na) metal is one of the most promising electrode materials for next‐generation low‐cost rechargeable batteries. However, the challenges caused by dendrite growth on Na metal anodes restrict practical applications of rechargeable Na metal batteries. Herein, a nitrogen and sulfur co‐doped carbon nanotube (NSCNT) paper is used as the interlayer to control Na nucleation behavior and suppress the Na dendrite growth. The N‐ and S‐containing functional groups on the carbon nanotubes induce the NSCNTs to be highly “sodiophilic,” which can guide the initial Na nucleation and direct Na to distribute uniformly on the NSCNT paper. As a result, the Na‐metal‐based anode (Na/NSCNT anode) exhibits a dendrite‐free morphology during repeated Na plating and striping and excellent cycling stability. As a proof of concept, it is also demonstrated that the electrochemical performance of sodium–oxygen (Na–O₂) batteries using the Na/NSCNT anodes show significantly improved cycling performances compared with Na–O₂ batteries with bare Na metal anodes. This work opens a new avenue for the development of next‐generation high‐energy‐density sodium‐metal batteries.     

Soft Multifunctional Composites and Emulsions with Liquid Metals

Binary mixtures of liquid metal (LM) or low‐melting‐point alloy (LMPA) in an elastomeric or fluidic carrier medium can exhibit unique combinations of electrical, thermal, and mechanical properties. This emerging class of soft multifunctional composites have potential applications in wearable computing, bio‐inspired robotics, and shape‐programmable architectures. The dispersion phase can range from dilute droplets to connected networks that support electrical conductivity. In contrast to deterministically patterned LM microfluidics, LMPA‐ and LM‐embedded elastomer (LMEE) composites are statistically homogenous and exhibit effective bulk properties. Eutectic Ga‐In (EGaIn) and Ga‐In‐Sn (Galinstan) alloys are typically used due to their high conductivity, low viscosity, negligible nontoxicity, and ability to wet to nonmetallic materials. Because they are liquid‐phase, these alloys can alter the electrical and thermal properties of the composite while preserving the mechanics of the surrounding medium. For composites with LMPA inclusions (e.g., Field's metal, Pb‐based solder), mechanical rigidity can be actively tuned with external heating or electrical activation. This progress report, reviews recent experimental and theoretical studies of this emerging class of soft material architectures and identifies current technical challenges and opportunities for further advancement.     

Stimuli‐Driven Control of the Helical Axis of Self‐Organized Soft Helical Superstructures

Supramolecular and macromolecular functional helical superstructures are ubiquitous in nature and display an impressive catalog of intriguing and elegant properties and performances. In materials science, self‐organized soft helical superstructures, i.e., cholesteric liquid crystals (CLCs), serve as model systems toward the understanding of morphology‐ and orientation‐dependent properties of supramolecular dynamic helical architectures and their potential for technological applications. Moreover, most of the fascinating device applications of CLCs are primarily determined by different orientations of the helical axis. Here, the control of the helical axis orientation of CLCs and its dynamic switching in two and three dimensions using different external stimuli are summarized. Electric‐field‐, magnetic‐field‐, and light‐irradiation‐driven orientation control and reorientation of the helical axis of CLCs are described and highlighted. Different techniques and strategies developed to achieve a uniform lying helix structure are explored. Helical axis control in recently developed heliconical cholesteric systems is examined. The control of the helical axis orientation in spherical geometries such as microdroplets and microshells fabricated from these enticing photonic fluids is also explored. Future challenges and opportunities in this exciting area involving anisotropic chiral liquids are then discussed.     

Synthesis and Applications of Graphdiyne‐Based Metal‐Free Catalysts

The development of carbon materials offers the hope for obtaining inexpensive and high‐performance alternatives to substitute noble‐metal catalysts for their sustainable application. Graphdiyne, the rising‐star carbon allotrope, is a big family with many members, and first realized the coexistence of sp‐ and sp²‐hybridized carbon atoms in a 2D planar structure. Different from the prevailing carbon materials, its nonuniform distribution in the electronic structure and wide tunability in bandgap show many possibilities and special inspirations to construct new‐concept metal‐free catalysts, and provide many opportunities for achieving a catalytic activity comparable with that of noble‐metal catalysts. Herein, the recent progress in synthetic methodologies, theoretical predictions, and experimental investigations of graphdiyne for metal‐free catalysts is systematically summarized. Some new perspectives of the opportunities and challenges in developing high‐performance graphdiyne‐based metal‐free catalysts are demonstrated.     

Fabrication of Micropatterned Dipeptide Hydrogels by Acoustic Trapping of Stimulus‐Responsive Coacervate Droplets

Acoustic standing waves offer an excellent opportunity to trap and spatially manipulate colloidal objects. This noncontact technique is used for the in situ formation and patterning in aqueous solution of 1D or 2D arrays of pH‐responsive coacervate microdroplets comprising poly(diallyldimethylammonium) chloride and the dipeptide N‐fluorenyl‐9‐methoxy‐carbonyl‐D‐alanine‐D‐alanine. Decreasing the pH of the preformed droplet arrays results in dipeptide nanofilament self‐assembly and subsequent formation of a micropatterned supramolecular hydrogel that can be removed as a self‐supporting monolith. Guest molecules such as molecular dyes, proteins, and oligonucleotides are sequestered specifically within the coacervate droplets during acoustic processing to produce micropatterned hydrogels containing spatially organized functional components. Using this strategy, the site‐specific isolation of multiple enzymes to drive a catalytic cascade within the micropatterned hydrogel films is exploited.