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Materials with programmable conductivity and stiffness offer new design opportunities for next-generation engineered systems in soft robotics and electronic devices. However, existing approaches fail to harness variable electrical and mechanical properties synergistically and lack the ability to self-respond to environmental changes. We report an electro-mechano responsive Field’s metal hybrid elastomer exhibiting variable and tunable conductivity, strain sensitivity, and stiffness. By synergistically harnessing these properties, we demonstrate two applications with over an order of magnitude performance improvement compared to state-of-the-art, including a self-triggered multiaxis compliance compensator for robotic manipulators, and a resettable, highly compact, and fast current-limiting fuse with an adjustable fusing current. We envisage that the extraordinary electromechanical properties of our hybrid elastomer will bring substantial advancements in resilient robotic systems, intelligent instruments, and flexible electronics.

Flexible pressure sensors based on polymer elastomers filled with conductive fillers show great advantages in their applications in flexible electronic devices. However, integratable high-sensitivity pressure sensors remain understudied. This work improves the conductivity and sensitivity of PDMS-Fe/Ni piezoresistive composites by introducing silver flakes and magnetic-assisted alignment techniques. As secondary fillers, silver flakes with high aspect ratios enhance the conductive percolation network in composites. Meanwhile, a magnetic field aligns ferromagnetic particles to further improve the conductivity and sensitivity of composites. The resistivity of the composite decreases sharply by 1000 times within a tiny compression strain of 1%, indicating excellent sensing performance. On the basis of this, we demonstrate an integratable miniature pressure sensor with a small size (2 × 2 × 1 mm), high sensitivity (0.966 kPa−1), and wide sensing range (200 kPa). Finally, we develop a flexible E-skin system with 5 × 5 integratable sensor units to detect pressure distribution, which shows rapid real-time response, high resolution, and high sensitivity.

Soft crawling robots have potential applications for surveillance, rescue, and detection in complex environments. Despite this, most existing soft crawling robots either use nonadjustable feet to passively induce asymmetry in friction to actuate or are only capable of moving on surfaces with specific designs. Thus, robots often lack the ability to move along arbitrary directions in a two-dimensional (2D) plane or in unpredictable environments such as wet surfaces. Here, leveraging the electrochemically tunable interfaces of liquid metal, we report the development of liquid metal smart feet (LMSF) that enable electrical control of friction for achieving versatile actuation of prismatic crawling robots on wet slippery surfaces. The functionality of the LMSF is examined on crawling robots with soft or rigid actuators. Parameters that affect the performance of the LMSF are investigated. The robots with the LMSF prove capable of actuating across different surfaces in various solutions. Demonstration of 2D locomotion of crawling robots along arbitrary directions validates the versatility and reliability of the LMSF, suggesting broad utility in the development of advanced soft robotic systems.

Materials with programmable conductivity and stiffness offer new design opportunities for next-generation engineered systems in soft robotics and wearable devices. However, existing approaches fail to harness variable electrical and mechanical properties synergistically, and lack the ability to self-respond to environmental changes. We report an electro-mechano responsive Field’s metal hybrid elastomer (FMHE) exhibiting variable and tuneable conductivity, strain sensitivity, and stiffness. By synergistically harnessing these properties, we demonstrate two applications with over an order of magnitude performance improvement compared to state-of-the-art, including a self-triggered multi-axis compliance compensator for robotic manipulators, and a resettable, highly compact, and fast current-limiting fuse with adjustable fusing current. We envisage that the extraordinary electromechanical properties of our hybrid elastomer will bring significant advancements in resilient robotic systems, intelligent instruments, and flexible electronics.

Nanoparticles (NPs) of gallium-based liquid metal (LM) alloys have potential applications in flexible electronics, drug delivery, and molecular imaging. They can be readily produced using top-down methods such as sonication. However, the sonication process generates heat that can cause dealloying of NPs through hydrolysis and oxidation of gallium. This limits the sonication power and period that can be applied for disrupting LM into smaller particles with high concentrations. Also, it remains challenging to achieve long-term colloidal stability of NPs in biological buffers. Here, we develop a dynamic temperature control system for improving the production performance of LM NPs. The enhanced performance is reflected by the significantly increased particle concentration, the decreased overall particle size, the prevention of the formation of oxide nanorods, and the versatility of producing NPs of different types of alloys. In addition, we design a brushed polyethylene glycol polymer with multiple phosphonic acid groups for effectively anchoring the NPs. More importantly, we discover that phosphate can effectively passivate the surface of NPs to further improve their stability. Using these strategies, the produced NPs remain stable in biological buffers for at least six months. Thus, the proposed methods can unleash the vast potential of LM NPs for biomedical applications.

Nanoparticles produced from gallium-based liquid metal alloys have been explored for developing applications in the fields of electronics, catalysis, and biomedicine. Nonetheless, physical properties, such as phase behavior at micro-/nanosize scale, are still significantly underexplored for such nanoparticles. Here, we conduct an in situ investigation of phase behavior for gallium-based liquid metal nanoparticles and discover the unprecedented coexistence of solid particles in spherical liquid metal shells without the support of a crystalline substrate. The particles can also transform into solid Janus nanoparticles after temperature cycling. In addition, we investigate the optical properties of the nanoparticles before and after phase separation using in situ electron energy-loss spectroscopy. Most importantly, we discover that increasing the content of indium within the nanoparticle can stabilize the solid-core/liquid-shell structure at room temperature. This study provides a foundation to engineer liquid metal nanoparticles for developing new applications in nanoscale optical platforms and shape-configurable transformers.Although various electronic, chemical, and biomedical applications have been demonstrated for nanoparticles made from gallium-based liquid metal alloys, fundamental physical properties such as phase behavior of such nanoparticles are still significantly underexplored. Here, Tang and coworkers present the in situ investigation of phase separation in binary and ternary spherical liquid metal nanoparticles upon cooling, and demonstrate the coexistence of solid core/liquid shell without the support of a crystalline substrate. This study provides insight into engineering such nanoparticles for the development of new applications.Despite gallium-based liquid metal alloys attracting extensive attention for various applications, their phase behavior at the nanoscale is still underexplored. Understanding the impact of phase separation in nanoparticles can be extremely useful for developing new structures and exploring suitable applications. Here, we report on the investigation of phase behavior for spherical nanoparticles made from gallium-based liquid metal alloys upon cooling. We discover the thermally stable coexistence of solid cores in spherical liquid metal shells without the support of a crystalline substrate at room temperature. Given the unique properties of liquid metal, together with the facile process for producing core-shell and Janus nanoparticles, this work will encourage further investigation of the properties of such nanoparticles for developing applications in the fields of electronics, catalysis, nanomedicine, and beyond.

Conductive elastic composites have been used widely in soft electronics and soft robotics. These composites are typically a mixture of conductive fillers within elastomeric substrates. They can sense strain via changes in resistance resulting from separation of the fillers during elongation. Thus, most elastic composites exhibit a negative piezoconductive effect, i.e. the conductivity decreases under tensile strain. This property is undesirable for stretchable conductors since such composites may become less conductive during deformation. Here, we report a liquid metal-filled magnetorheological elastomer comprising a hybrid of fillers of liquid metal microdroplets and metallic magnetic microparticles. The composite’s resistivity reaches a maximum value in the relaxed state and drops drastically under any deformation, indicating that the composite exhibits an unconventional positive piezoconductive effect. We further investigate the magnetic field-responsive thermal properties of the composite and demonstrate several proof-of-concept applications. This composite has prospective applications in sensors, stretchable conductors, and responsive thermal interfaces., Liquid metal-filled elastic composites for strain sensing devices exhibit reduced conductivity under strain, which limits their usefulness. Here, the authors report a positive piezoconductive effect in liquid metal-filled magnetorheological elastomers and illustrate proof-of concept applications.

Although droplet-based microfluidics has been broadly used as a versatile tool in biology, chemistry, and nanotechnology, its rather complicated microfabrication process and the requirement of specialized hardware and operating skills hinder researchers fully unleashing the potential of this powerful platform. Here, we develop an integrated microdroplet generator enabled by a spinning conical frustum for the versatile production of near-monodisperse microdroplets in a high-throughput and off-chip manner. The construction and operation of this generator are simple and straightforward without the need of microfabrication, and we demonstrate that the generator is able to passively and actively control the size of the produced microdroplets. In addition to water microdroplets, this generator can produce microdroplets of liquid metal that would be difficult to produce in conventional microfluidic platforms as liquid metal has high surface tension. Moreover, we demonstrate that this generator can produce solid hydrogel microparticles and fibers using integrated ultraviolet (UV) light. In the end, we further explore the ability of this generator for forming double emulsions by coflowing two immiscible liquids. Given the remarkable abilities demonstrated by this platform and the tremendous potential of microdroplets, this user-friendly method may revolutionize the future of droplet-based chemical synthesis and biological analysis.

Microalgae cells have been recognized as a promising sustainable resource to meet worldwide growing demands for renewable energy, food, livestock feed, water, cosmetics, pharmaceuticals, and materials. In order to ensure high-efficiency and high-quality production of biomass, biofuel, or bio-based products, purification procedures prior to the storage and cultivation of the microalgae from contaminated bacteria are of great importance. The present work proposed and developed a simple, sheathless, and efficient method to separate microalgae Chlorella from bacteria Bacillus Subtilis in a straight channel using the viscoelasticity of the medium. Microalgae and bacteria migrate to different lateral positions closer to the channel centre and channel walls respectively. Fluorescent microparticles with 1 μm and 5 μm diameters were first used to mimic the behaviours of bacteria and microalgae to optimize the separating conditions. Subsequently, size-based separation in Newtonian fluid and in viscoelastic fluid in straight channels with different aspect ratios was compared and demonstrated. Under the optimal condition, the removal ratio for 1 μm microparticles and separation efficiency for 5 μm particles can reach up to 98.28% and 93.85% respectively. For bacteria and microalgae cells separation, the removal ratio for bacteria and separation efficiency for microalgae cells is 92.69% and 100% respectively. This work demonstrated the continuous and sheathless separation of microalgae from bacteria for the first time by viscoelastic microfluidics. This technique can also be applied as an efficient and user-friendly method to separate mammalian cells or other kinds of cells.

Portability and low-cost analytic ability are desirable for point-of-care (POC) diagnostics, however, current POC testing platforms often require time-consuming multiple microfabrication steps and rely on bulky and costly equipment. This hinders the capability of microfluidics to prove its power outside of laboratories and narrows the range of applications. This paper details a self-contained microfluidic device, which does not require any external connection or tubing to deliver insert-and-use image-based analysis. Without any microfabrication, magnetorheological elastomer (MRE) microactuators including pumps, mixers and valves are integrated into one modular microfluidic chip based on novel manipulation principles. By inserting the chip into the driving and controlling platform, the system demonstrates sample preparation and sequential pumping processes. Furthermore, due to the straightforward fabrication process, chips can be rapidly reconfigured at a low cost, which validates the robustness and versatility of an MRE-enabled microfluidic platform as an option for developing an integrated lab-on-a-chip system.

Microfluidic chips have demonstrated unparalleled abilities in droplet generation, including precise control over droplet size and monodispersity. And yet, their rather complicated microfabrication process and operation can be a barrier for inexperienced researchers, which hinders microdroplets from unleashing their potential in broader fields of research. Here, we attempt to remove this barrier by developing an integrated and modular revolving needle emulsion generator (RNEG) to achieve high-throughput production of uniformly sized droplets in an off-chip manner. The RNEG works by driving a revolving needle to pinch the dispersed phase in a minicentrifuge tube. The system is constructed using modular components without involving any microfabrication, thereby enabling user-friendly operation. The RNEG is capable of producing microdroplets of various liquids with diameters ranging from tens to hundreds of micrometres. We further examine the principle of operation using numerical simulations and establish a simple model to predict the droplet size. Moreover, by integrating curing and centrifugation processes, the RNEG can produce hydrogel microparticles and transfer them from an oil phase into a water phase. Using this ability, we demonstrate the encapsulation and culture of single yeast cells within hydrogel microparticles. We envisage that the RNEG can become a versatile and powerful tool for high-throughput production of emulsions to facilitate diverse biological and chemical research.

Nanoparticles (NPs) and microparticles (MPs) have been widely used in different areas of research such as materials science, energy, and biotechnology. On-demand synthesis of NPs and MPs with desired chemical and physical properties is essential for different applications. However, most of the conventional methods for producing NPs/MPs require bulky and expensive equipment, which occupies large space and generally need complex operation with dedicated expertise and labour. These limitations hinder inexperienced researchers to harness the advantages of NPs and MPs in their fields of research. When problems individual researchers accumulate, the overall interdisciplinary innovations for unleashing a wider range of directions are undermined. In recent years, modular and integrated systems are developed for resolving the ongoing dilemma. In this review, we focus on the development of modular and integrated systems that assist the production of NPs and MPs. We categorise these systems into two major groups: systems for the synthesis of (1) NPs and (2) MPs; systems for producing NPs are further divided into two sections based on top-down and bottom-up approaches. The mechanisms of each synthesis method are explained, and the properties of produced NPs/MPs are compared. Finally, we discuss existing challenges and outline the potentials for the development of modular and integrated systems.

Summary Anisotropic elastic composites—that is, elastomers containing aligned fillers—often have enhanced properties in the direction of alignment. Depending on the fillers, these composites can have desirable electrical, thermal, or mechanical properties. Here, a silicone composite filled with both solid magnetic metal microparticles and liquid metal microdroplets has been developed. Aligning the solid particles within a magnetic field during curing imparts anisotropy in several properties. Thus, the composite is called an anisotropic liquid metal-filled magnetorheological elastomer (ALMMRE). Compared with isotropic liquid metal-filled composites, the conductivity of the ALMMRE is significantly enhanced in all directions. The ALMMRE also exhibits anisotropic piezoconductivity and a significantly enhanced electrical anisotropy under mechanical deformation; these properties are not observed in conventional anisotropic composites. The ALMMRE also shows anisotropic mechanical, thermal, and magnetic properties and demonstrates its several proof-of-concept applications. The sensitivity of the ALMMRE's properties to strain may help advance future flexible sensors and soft electronics.

In recent years, the demand for wearable strain sensors is growing due to their motion monitoring and clinical data collection capabilities. The sensors with positive piozeconductivity, that is, their conductivity increases when stretched, are more favored in wearable electronics because they can maintain high conductivity under stretching. However, reported composites with positive piezoconductivity mostly show low sensitivity. Furthermore, few wearable electronic systems based on strain sensors are built completely with data processing capabilities. Herein, we present a wearable sensor based on a positive piezoconductive composite composed of liquid metal microdroplets, metal microparticles, and elastic matrix. The resistance of the composite is extremely high at the relaxed state but drops drastically under any deformation. It has high stretchability (fracture strain > 200%) and sensitivity (gauge factor ~ 100), which shows excellent performance in detecting human motions such as joint bending. Based on this composite, we demonstrate a wearable and portable sign language communication device with a fully programmable and configurable system to help people with language disabilities communicate effectively with others from the visual and auditory perspectives. By bending the fingers, the device can type words or sentences on screen and speak relevant letters through the audio.

Stretchable conductive composites (SCCs) are generally elastomer matrices filled with conductive fillers. They combine the conductivity of metals and carbon materials with the flexibility of polymers, which are attractive properties for applications such as stretchable electronics, wearable devices, and flexible sensors. Most conventional conductive composites that are filled with only one type of conductive filler face issues in mechanical and electrical properties. Recently, some studies introduced secondary fillers to create hybrid‐filler SCCs to solve these problems. The secondary fillers produce a synergistic effect with the primary fillers to enhance the electrical conductivity of the composites. They also improve the thermal conductivity and mechanical properties or impart composites with special functions like catalysis and self‐healing. Herein, the fabrication methods, stretchability enhancement strategies, and piezoresistivity of SCCs are analyzed, and their latest applications in stretchable electronics are introduced. Finally, the challenges and prospects of their development are discussed.

Correction for ‘Modular off-chip emulsion generator enabled by a revolving needle’ by Yuxin Zhang et al., Lab Chip, 2020, 20, 4592–4599, DOI: 10.1039/D0LC00939C.

In the regime of inertial microfluidics, the secondary flow is widely adopted to reduce the number of the equilibrium position and improve the focusing performance of particles. At the same time, secondary flow can also enhance the mixing effect and may deteriorate particle focusing due to the induced rotating streams, especially for the particle with a small size. In a double-layered microchannel with slanted groove structures, it has been demonstrated that the generated secondary flow at a high flow rate could focus the particle with a large size (> 8 µm). However, the manipulation of small-size particles (

In this work, a novel double-layer microfluidic device for enhancing particle focusing was presented. The double-layer device consists of a channel with expansion-contraction array and periodical slanted grooves. The secondary flows induced by the grooves modulate the flow patterns in the expansion-contraction-array (ECA) channel, further affecting the particle migration. Compared with the single ECA channel, the double-layer channel can focus the particles over a wider range of flow rate. Due to the differentiation of lateral migration, the double-layer channel is able to distinguish the particles with different sizes. Furthermore, the equilibrium positions could be modulated by the orientation of grooves. This work demonstrates the possibility to enhance and adjust the inertial focusing in an ECA channel with the assistance of grooves, which may provide a simple and portable platform for downstream filtration, separation, and detection.

Numerous lab-on-a-chip applications benefit from channels with complex structures and configurations in the areas of tissue engineering and clinical diagnostics. The current fabrication approaches require time-consuming, complicated processes and bulky, expensive facilities. In this work, we propose a novel method for the fabrication of complex channels with the assistance of amalgamation of liquid metal with copper tape. This new technique enables the rapid fabrication of liquid metal molds with various dimensions and diverse structures. Two proof-of-concept experiments were conducted to verify the utilization of this method. First, the channel replicated from the liquid metal mold is used to enhance the mixing performance of liquids flowing through the channel. Second, a channel with a semicircular cross-section is fabricated to achieve 3D focusing in a simple way. This proposed technique can be readily used for fabricating complex channels for a wide range of applications.

This work presents a simple, low-cost method to fabricate semi-circular channels using solder paste, which can amalgamate the cooper surface to form a half-cylinder mold using the surface tension of Sn-Pd alloy (the main component in solder paste). This technique enables semi-circular channels to be manufactured with different dimensions. These semi-circular channels will then be integrated with a polymethylmethacrylate frame and machine screws to create miniaturized, portable microfluidic valves for sequential liquid delivery and particle synthesis. This approach avoids complicated fabrication processes and expensive facilities and thus has the potential to be a useful tool for lab-on-a-chip applications.

Proteinuria is an established risk marker for progressive renal function loss and patients would significantly benefit from a point-of-care testing. Although extensive work has been done to develop the microfluidic devices for the detection of urinary protein, they need the complicated operation and bulky peripherals. Here, we present a rapid, maskless 3D prototyping for fabrication of capillary fluidic circuits using laser engraving. The capillary circuits can be fabricated in a short amount of time (

Microdroplets of gallium-based liquid metal alloys have enabled various applications in the fields of biomedicine, electronics, and chemistry. However, due to the high surface tension of liquid metal, high-throughput production of uniformly sized liquid metal microdroplets is challenging using conventional acoustic or microfluidic methods. Here, adapting the submerged electrodispersion technique that has conventionally been used for generating water-based microdroplets, we develop a simple and straightforward platform for the high-throughput production of near-monodisperse (coefficient of variation less than 5%) liquid metal microdroplets in oil without using microfluidic devices. We demonstrate the capabilities of this method for producing liquid metal microdroplets (diameters ranging from tens to hundreds of micrometers) and introduce a spinning disk to induce a flow of oil phase for preventing the coalescence of the microdroplets. The simplicity and remarkable abilities demonstrated for this method may pave the path for the development of future innovative applications based on liquid metal microdroplets.

Functional liquid metal nanoparticles (NPs), produced from eutectic alloys of gallium, promise new horizons in the fields of sensors, microfluidics, flexible electronics, catalysis, and biomedicine. Here, the development of a vapor cavity generating ultrasonic platform for nebulizing liquid metal within aqueous media for the one-step production of stable and functional liquid metal NPs is shown. The size distribution of the NPs is fully characterized and it is demonstrated that various macro and small molecules can also be grafted onto these liquid metal NPs during the liquid-based nebulization process. The cytotoxicity of the NPs grafted with different molecules is further explored. Moreover, it is shown that it is possible to control the thickness of the oxide layer on the produced NPs using electrochemistry that can be embedded within the platform. It is envisaged that this platform can be adapted as a cost-effective and versatile device for the rapid production of functional liquid metal NPs for future liquid metal-based optical, electronic, catalytic, and biomedical applications.

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.

Microfluidic systems enable rapid diagnosis of diseases, biological analysis, drug screening, and high-precision materials synthesis. In spite of these remarkable abilities, conventional microfluidic systems are microfabricated monolithically on a single platform and their operations rely on bulky expensive external equipment. This restricts their applications outside of research laboratories and prevents development and assembly of truly versatile and complex systems. Here, novel magnetorheological elastomer (MRE) microactuators are presented including pumps and mixers using an innovative actuation mechanism without the need of delicate elements such as thin membranes. Modularized elements are realized using such actuators, which can be easily integrated and actuated using a single self-contained driving unit to create a modular, miniaturized, and robust platform. The performance of the microactuators is investigated via a series of experiments and a proof-of-concept modular system is developed to demonstrate the viability of the platform for self-contained applications. The presented MRE microactuators are small size, simple, and efficient, offering a great potential to significantly advance the current research on complex microfluidic systems.