1 Introduction
Traditional electronic devices are usually composed of rigid materials, which have poor flexibility and stretchability, and have significant application limitations when in contact with curved surfaces
［1-3］. In recent years, flexible and stretchable conductive materials have attracted much attention due to their great potential applications in the field of smart devices
［4-6］. With the continuous development of this research field, traditional electronic technology has also gradually expanded into new application areas, such as human-computer interaction
［7-8］ and artificial intelligence
［9-11］. To meet the preparation needs of high-performance flexible electronic devices, various methods have been proposed to form flexible and stretchable circuits. The basic construction strategy is to combine the nano or specific geometric structures of intrinsic conductive materials with elastomers. Currently, a variety of conductive nanomaterials such as carbon-based materials
［12-14］ (carbon nanotubes, graphene, etc.), conductive polymers
［15-16］, transition metal carbides and nitrides (MXene)
［17-19］, and metal-based materials
［20-21］ (metal nanowires, metal grids, etc.) have been selected to prepare electrodes or circuits for use in flexible electronic devices. Although these materials can achieve bending and twisting, they still struggle to handle large tensile strains. To achieve the preparation of stretchable electronic devices, people have shifted their focus to deterministic circuit architectures, in which the stretching function is realized through wavy or serpentine geometric circuit traces
［22-24］. Although these methods can prepare electronic devices with stable performance, they usually rely on cleanroom photolithography or specialized processing steps, which are difficult to be compatible with low-cost solution processing techniques for flexible electronic devices. In contrast, liquid metals, especially gallium and its alloys, whether in bulk or particulate form, exhibit ductility surpassing other conductive materials. The inherent high conductivity, fluidity, ductility, low viscosity, and spontaneous formation of an oxide layer on the surface of liquid metals impart them with excellent stretchability and tunable surface properties, making them ideal materials for soft electronic conductors in stretchable electronic devices
［25-27］. Liquid metals have been used to fabricate various flexible and stretchable devices and equipment such as stretchable electrodes
［28-29］, electronic skin
［30-32］, electronic textiles
［33-34］, stretchable displays
［35-37］, and sensors
［38-39］.
Based on their superior electrical conductivity and mechanical deformability, liquid metals can form the required circuits or interconnect structures in various elastomers in a variety of ways. To date, the commonly used processing methods for liquid metal-based conductive composites are mainly photolithography
［40-41］, screen printing
［29, 42］, and laser ablation
［43-44］. However, these methods have many shortcomings such as complex preparation processes, time-consuming and energy-intensive procedures, low pattern resolution, and low material utilization. The root causes of these problems are the excessive surface tension of liquid metals and poor surface selective wetting performance, which pose challenges to the manufacture of liquid metal-based conductive circuits
［45-46］. To overcome these drawbacks, in recent years, people have mainly designed various preparation methods for liquid metals from two research directions: intermediate embedding and surface printing, including using mold injection to shape liquid metals
［47-49］, freeze casting or post-encapsulation under magnetic assistance on rigid substrates
［50-51］, using stretchable materials to wrap liquid metals into fibers
［33, 52］, and preparing liquid metal conductive inks for screen printing
［53-54］, 3D printing
［55-57］ and direct writing printing
［58］, wettability modification of stretchable substrate surfaces
［59-61］, and using fiber elastic substrates
［62-63］. The use of liquid metal composites and multiple process printing provides a powerful universal platform for directly building multifunctional integrated products, and has become a promising method for the rapid manufacturing of flexible and stretchable electronics. Compared with traditional electronic circuits, stretchable circuits made of liquid metals can still maintain excellent conductivity under large deformation.
This article will focus on introducing the material properties of conductive materials and substrate materials in liquid metal-based stretchable composite electrodes, as well as the patterning preparation technology of liquid metals. It summarizes the current status and challenges faced by liquid metal-based stretchable circuits and provides an outlook on their future development trends.
2 Material Composition of Liquid Metal-Based Flexible Devices
In stretchable electronic devices, stretchable electrodes are generally fabricated by depositing patterned wires on insulating elastomers, serving to connect power sources, transmit electrical energy, and conduct signals. Among these, the conductivity of the conductive materials should be as high as possible, usually measured by electrical conductivity, while the substrate materials need to possess good stretching properties.
2.1 Liquid Metal and Its Composites
Liquid metals are a class of metals or alloys with relatively low melting points compared to traditional solid metals, possessing metallic and fluid properties at room temperature or near room temperature. Liquid metals mainly include mercury (Hg, melting point -38.8 ℃), gallium (Ga, melting point 29.8 ℃), rubidium (Rb, melting point 38.9 ℃), cesium (Cs, melting point 28.4 ℃), and francium (Fr, melting point 27 ℃). Among them, mercury, the most common liquid metal in daily life, has good adhesion, but its high volatility and extreme toxicity are fatal defects. Rubidium, cesium, and francium have limited applications due to their high radioactivity and low chemical stability. In contrast, gallium's low melting point (29.8 ℃) and high boiling point (2204 ℃) can maintain its structural stability while having low viscosity and high conductivity (3.4×10
4 S/cm)
［65-67］, thus gaining wide application and research. The liquid metals discussed in this article are mainly pure gallium, gallium-based liquid metal alloys, especially eutectic gallium-indium (EGaIn, 75.5% Ga, 24.5% In) and eutectic gallium-indium-tin (Galinstan, 68.5% Ga, 21.5% In, 10% Sn), both with similar performance. The physical properties of common liquid metals are shown in
Table 1［64］. Taking EGaIn as an example, its melting point is 15.5 ℃, lower than that of gallium, making it the most common alloy with a melting point below room temperature, featuring excellent high conductivity (DC conductivity of 3.4×10
6 S/m), low viscosity, deformability, non-toxicity, and other physicochemical properties
［65, 68］, making it an ideal choice for manufacturing conductive circuits in stretchable devices. Unlike solid metal films deposited on elastomers which crack with increasing strain, liquid metals can change shape following the bending or stretching of elastomers
［69］.
There are certain challenges in preparing stretchable electronic devices using EGaIn. The viscoelastic and wetting properties of liquid metal are strongly influenced by the rapidly formed oxide layer at the interface. Even with trace amounts of oxygen (>1 ppm), the surfaces of pure gallium or eutectic gallium-based alloys are extremely prone to oxidation, forming an amorphous gallium oxide layer
［70-75］. The thickness of the formed gallium oxide surface layer is 0.5~3 nm, which does not significantly affect electrical transport at the interface
［76］. Although the oxide layer is only a few nanometers thick, it can provide mechanical stability for small liquid metal droplets while also making the droplets ductile, influencing the rheology of the liquid metal on a macroscopic scale
［77］, increasing the wettability of the liquid metal so that it can adhere to many surfaces. Complete control of fluidity can be achieved by applying an electrochemical reduction potential or simply by eliminating the surface oxide layer through dilute acid/strong alkali media
［78］.
2.2 Flexible Substrate Materials
To meet the requirements of flexible and stretchable electronic devices, researchers have made efforts to prepare stretchable electrodes on various stretchable elastic substrates, such as Ecoflex
［47, 79-81］, polydimethylsiloxane (PDMS)
［29, 82-84］, poly［styrene-
b-(ethylene-co-butylene)-
b-styrene］(SEBS)
［60, 85］, styrene-isoprene-styrene (SIS)
［86-87］, hydrogel
［88-91］, fiber fabric
［63, 92-94］, etc. However, the process of preparing liquid metal-based stretchable electrodes on these elastomers is relatively complex, requiring the combination of multiple processes to finally obtain liquid metal-based stretchable electrodes. The main reason for this result is that the high surface tension and easy oxidation properties of liquid metals are incompatible with elastomers, making it difficult to achieve patterning of the conductive layer and maintain high electrical and mechanical stability. Therefore, it is necessary to perform surface modification on elastomers or dope liquid metals to adjust the compatibility between the two.
3 Preparation Methods of Liquid Metal-Based Flexible Conductive Composites
The fluidic properties of liquid metals allow for the adoption of novel and pioneering processing methods to address issues related to their incompatibility with flexible substrates by controlling oxide layers and regulating surface tension. There are various ways to combine liquid metals with stretchable substrates, ranging from traditional photolithography
［41, 95］ to non-traditional microfluidic channel injection
［96-98］, elastomer molding
［99-100］, 3D printing
［101-102］, elastomer surface modification
［59, 103-104］, fiber infiltration
［62, 105-106］, and even fabricating liquid metals into elastic fibers to form stretchable conductors
［52, 107］. Here, we have summarized and reviewed representative works based on the positional structure of liquid metals and stretchable substrates, mainly divided into four categories, and briefly outlined their latest progress. The first two categories discuss whether the material surface has been modified or not: one category involves passive methods without modifying the materials, embedding liquid metal into the interior of stretchable substrates to form conductive pathways, including embedded injection encapsulation, post-encapsulation, and coaxial elastic fibers; the other category involves active methods modifying the materials, forming circuits on the surface of stretchable substrates, which is further divided into modifying liquid metals or elastomer surfaces, specifically elaborated through methods such as conductive ink formulation, interface modification, and conductive fabrics. The third category involves modifying liquid metal materials and dispersing them in non-conductive elastic materials to give them intrinsic conductivity and stretchability. The fourth category involves the modification and patterning methods of new liquid metal materials.
3.1 Passive Internal Embedding Method
3.1.1 Embedded Injection Encapsulation
The most typical method of embedded injection encapsulation is microchannel injection. By embedding liquid metal in the middle of an elastomer and injecting the liquid metal into microchannels, stretchable conductive pathways can be manufactured. Due to the low viscosity of liquid metal, sufficient pressure can be applied at the channel entrance to quickly fill the microchannels. Since a certain amount of oxide is produced during this process, the quality of the conductive pathway is inversely proportional to the channel diameter
［108］. Liquid metal is injected into 150 nm capillaries using a high-pressure syringe
［109］. Chen et al.
［110］ used sinusoidal wave microchannels to fabricate stretchable strain sensors. This microfluidic strain sensor is made by embedding liquid metal into patterned microchannels. As shown in
Fig. 1a, a SU-8 photoresist mold was first fabricated using soft lithography, then uniformly mixed Ecoflex 00-30 was poured into the mold for shaping, and finally EGaIn was injected into the microfluidic channels using a syringe. The optical images of sensors with straight and wavy channels are shown in
Fig. 1b. The strain sensor samples were stretched gradually from 0% to 320% without failure, and EGaIn could fully follow the elastic deformation of the microchannels, as shown in
Fig. 1c. Additionally, Bhuyan et al.
［47］ introduced a simpler non-lithographic method by replicating the microchannel structure in the silicone film body using copper wire, and manufacturing microfluidics using soft and stretchable silicone. Then liquid gallium was injected into the resulting channels to create simple geometries, as shown in
Fig. 1d. The liquid wires remained conductive under stretching conditions, as shown in
Fig. 1f. Similarly, Wu et al.
［111］ fabricated molds through 3D printing, evacuated Ecoflex 00-10 AB mixture to remove bubbles and injected it into the molds to obtain flexible films. Finally, liquid metal was injected into the silicone cavity using a syringe, and electrical connections were made by connecting wires to the liquid metal, as shown in
Fig. 1e. Pan et al. used the liquid metal injection method to manufacture LED line units that can operate normally under 260% strain. Through a surface-mount integration process, electronic devices were integrated onto flexible substrates to achieve a flexible circuit system for lighting LED lamps
［112］, as shown in
Fig. 1g. Kim et al. prepared flexible breadboards by injecting liquid metal into sponge electrodes, using salt immersion methods in PDMS for patterning
［113］, as shown in
Fig. 1h.
Although the microchannel injection method is simple and easy to operate, the produced stretchable circuits have certain limitations: the different thermal expansion coefficients of liquid metal and stretchable elastomer cause bubbles in the cavity when excessive load generates heat; when the stretching strain is large, the mismatch between the two leads to problems such as uneven distribution and leakage of liquid metal.
3.1.2 Post-Packaging
Post-encapsulation refers to the process where liquid metal is first deposited into the mold's channels or liquid metal circuits are printed on the surface, and after the conductive pathways are manufactured, they are encapsulated using elastomers. Common methods include elastomer molding and compression film printing
［114-120］.
Helseth
［51］ prepared 100 µm thick interdigital copper electrodes on an epoxy substrate using standard photolithography techniques, as shown in
Figure 2a. The elastomer (Sylgard 184) was poured on top of the copper mask, degassed under mild vacuum, and cured at 11 ℃ for 30 minutes. After peeling off the cured elastomer, EGaIn was deposited into the channels. After creating the EGaIn pattern, PDM was poured on top and cured. Examples of elastomer embedded liquid metal electrodes and capacitances with different curvature diameters are shown in
Figure 2b. Li et al.
［121］ developed a liquid metal wire based on soft stamp printing process and used it for stretchable conductors and capacitors. The Ecoflex solution was spin-coated onto a silicon wafer and cured, and the bottom liquid metal circuit was printed on the Ecoflex film using a soft stamp printing process. Then the liquid metal layer was covered with an Eco-flex solution. Subsequently, liquid metal circuits were printed on the elastomer. Finally, electronic chips were implanted on the top liquid metal wires and connected to the bottom and top liquid metal wires. Then, the silicon wafer was covered again with Ecoflex solution, cured, and separated. A wearable multi-layer liquid metal-based pulse sensor that can conform to the skin was successfully fabricated, as shown in
Figure 2c. Chen et al.
［122］ fabricated a self-healing, robust conductive and stretchable conductor by embedding liquid metal patterns into an imprintable self-healing elastomer, as shown in
Figure 2d. They designed a PDMS/MW-CNT self-healing elastomer with reversible imine bonds as self-healing points and multi-walled carbon nanotubes as reinforcements. This elastomer has high stretchability (500%), good elastic recovery, and room temperature self-healing ability (94.3%). Utilizing the excellent imprinting performance of the self-healing elastomer, this work proposed a structurally confined filling strategy combining nanoimprint and printing technologies to fabricate embedded liquid metal circuits with controllable line widths in various geometric patterns. The resulting liquid metal-embedded structure not only provides reliable external circuit interfaces, achieving stable conductivity resistant to scratches and peeling, but also provides flow channels for fluidic liquid metals, allowing them to automatically merge together when disconnected to re-establish conductive paths. Lim et al.
［123］ first utilized the unique structural properties of gallium oxide to create 3D micro-nano wrinkle structures on the surface of gallium, then successively deposited gold nanoparticles and biostable poly(3,4-ethylenedioxythiophene) on the wrinkled liquid metal surface for encapsulation, and finally tested and proved that the performance of the encapsulated liquid metal device is superior to the bare liquid metal device, as shown in
Figure 2e. Munirathinam et al.
［124］ fabricated a liquid metal-based triboelectric nanogenerator (LM-TENG) for harvesting energy from running water and flow sensors. The proposed LM-TENG mainly consists of a Galinstan working electrode encapsulated in a PDMS friction layer. At a flow rate of 2.5 L/min, the output performance of LM-TENG (6.2 V and 3.6 μA) is superior to copper electrode-based TENG (C-TENG, 2.4 V and 0.8 μA). The fabricated LM-TENG has been successfully applied to a self-powered flow sensor for remote monitoring of water flow in pipelines. In addition, LM-TENG can power more than 30 LEDs, enabling direct powering of low-power electronic devices, as shown in
Figure 2f.
3.1.3 Core Layer Coated Spinning
E-textiles
［125-127］ have been demonstrated to be a customizable and multifunctional assembly strategy for next-generation wearable devices, in which stretchable fiber electronics with tough one-dimensional characteristics can be fabricated into multidimensional configurations through textile strategies for better wearability. Elastic fibers are generally composed of elastic microtubes with cavities and liquid metal, possessing high stretchability and flexibility
［52］. Compared with planar liquid metal devices manufactured through injection and printing routes, fibrous hollow liquid metal electronics are more suitable for detecting deformations caused by axial stretching and radial compression
［92,128-129］.
Qi et al
［33］ developed an underwater-useable electronic textile with significant sensing and electrothermal performance, which is prepared by coaxial wet spinning using liquid metal-based stretchable core-sheath fibers, as shown in
Figure 3a. The fiber exhibits excellent specific resistance (≈ 0.05 Ω/cm), strain limit (approximately 600% elongation), and pressure tolerance limit (approximately 30 MPa), making it capable of a wide sensing detection range, along with fast response time (30 ms). Glove controllers fabricated through different textile methods (weaving, knitting, and braiding) are shown in
Figure 3b, where the gloves feature stretch sensing, reversible press-type switching, and low-power electrothermal management functions. Wang et al
［128］ created a large-scale, robust textile-based triboelectric nanogenerator (t-TENG). The t-TENG consists of liquid metal/polymer core/shell fibers (LCF), obtained by continuously pumping liquid metal into polymer hollow fibers, as shown in
Figure 3c. Dong et al
［107］ first manufactured a thermoplastic elastomer whose rheological properties are compatible with highly versatile fiber thermal drawing technology. Based on this, kilometer-long microstructured TENG fibers were produced, featuring complex cross-sectional structures integrating multiple liquid metal electrodes and micron-scale surface textures. The resulting fibers exhibit high deformability, capable of withstanding complex deformations and extreme stretching up to 560%. Lee et al
［130］ prepared ultra-stretchable elastic fibers with a liquid metal (EGaIn, eutectic alloy of gallium and indium) core, fabricating two-dimensional capacitive sensors by arranging fibers in a cross structure. EGaIn was injected into the inner core of hollow elastic fibers to create 2D capacitive sensors. Due to the fluid nature of the metal, this method of manufacturing 2D capacitive sensors using one-dimensional fibers with liquid metal cores can be applied in electronic textiles, wearable sensors, and soft robotics, as shown in
Figure 3d. Ma et al utilized the phase change of gallium and coated wires with polyurethane before liquefying the metal to create shape-programmable liquid metal (LM) fibers, forming 3D helical structures. The 3D helical LM fibers enhance stretchability, with fracture strain reaching 1273%, and exhibit invariant conductivity when strain exceeds 283%, as shown in
Figure 3e. Zhang et al
［131］ proposed a simple strategy for fabricating stretchable conductive fibers with binary rigid-soft conductive components and dynamic compensatory conductivity. Wet-spun thermoplastic polyurethane (TPU)/silver flake (AgFKs) (TA) composite fibers were coated with an aqueous polyurethane (WPU) thin layer, followed by applying an LM coating to obtain TPU/AgFKs/WPU/LM (TAWL) fibers. The TAWL fibers demonstrate excellent elongation (~600% strain), ~2 Ω/cm (~3125 S/cm) conductivity, and reversible resistance response within 70% tensile strain, as shown in
Figure 3f.
3.2 Active Surface Structure Modification Method
Surface printing refers to the patterning of liquid metal on the surface of an elastomer where the pattern and outline of the liquid metal are formed by the interaction between adhesion and cohesion at the interface and the stiffness of the formed solid oxide skin. Due to the excessive surface tension of the liquid metal and poor surface selective wetting properties researchers in recent years have addressed this issue by developing various methods for fabricating conductive pathways of liquid metals through three main approaches: surface modification of liquid metals surface modification of elastomers and physical structure modification of substrates.
3.2.1 Conductive Ink and Its Printing Technology
3.2.1.1 Conductive Ink
To overcome the limitations of bulk liquid metals and quickly achieve simple and scalable patterns of conductive features, liquid metals can be combined with polymers or other metal particles to configure various ink dispersions, preparing a new generation of electronic inks and improving their wettability on various materials
［102-133］, making them easy to write on soft or hard substrates such as epoxy boards, glass, plastic, silicone, paper, cotton, textiles, and cloth. To further improve processability, achieve better dispersion states, and avoid the oxidation of liquid metals, significant efforts have been made to modify the surface of liquid metal particles, which is conducive to the fabrication of liquid metal-based conductive circuits
［134］. Zhang et al.
［135］ used ethanol (EtOH) as a solvent and polyvinylpyrrolidone (PVP) as a stabilizer to prepare liquid metal nano-ink. The PVP/liquid metal nano-ink exhibits excellent colloidal stability and biocompatibility, along with good wettability on various rigid/flexible substrates. Mixing liquid metals with other metal powders (such as nickel and iron) makes it easier to process, but metal composites are prone to forming galvanic cells, leading to poor long-term stability of the composite
［53］. Although liquid metal micro/nano droplets can enhance the wettability of liquid metals on various substrates, it remains difficult to prepare liquid metal nano-inks that maintain both excellent stability and biocompatibility. Additionally, developing different patterning methods for fabricating flexible electronic devices based on liquid metal nano-inks also requires research.
Embedding liquid metal in elastomers is an ideal platform for achieving stretchable electronic devices with deformation capabilities. Although the aforementioned methods for preparing conductive pathways of liquid metal are promising, the preparation process is time-consuming and labor-intensive. Therefore, there is a growing interest in using industrial manufacturing methods for rapid and automated printing of liquid metal patterns. Liquid metal inks must not only have good wettability with the elastomer substrate surface to retain printed patterns but also be compatible with common patterning methods, enabling the creation of uniform, smooth, and stable conductive pathways over large areas, such as screen printing, direct writing, and 3D printing.
3.2.1.2 Coating and Printing Type
Coating printing, as a mature printing technology in the printing industry, is commonly used for the preparation of flexible electronic devices, especially screen printing. With the extensive research on stretchable electronics today, screen printing can be performed on a large scale and size with a relatively simple process and lower cost. This two-dimensional printing process mainly involves depositing a layer of ink on a stencil steel plate with a certain pattern and then applying pressure through a scraping method to pass the ink through the patterned holes of the stencil onto the substrate, thereby forming the desired pattern on the substrate
［136-147］. Dong et al.
［83］ fabricated liquid metal-based electrode arrays on PDMS substrates using screen printing and microfabrication, as shown in
Figure 4a. They prepared EGaIn microdroplets using ultrasonic dispersion and dispersed EGaIn droplets into nanoparticles (300 nm) using ethanol. After ultrasonic treatment, a core-shell structure appeared, featuring an EGaIn core and a thin oxide layer of Ga
2O
3. Smaller-sized EGaIn ink is more suitable for high-resolution printing, and
Figure 4b demonstrates its high flexibility (middle) and stretchability (right). Dou et al.
［54］ introduced carbon nanotubes into PDMS elastomer and liquid metal dispersed particles, preparing liquid metal ink through ultrasonic treatment and fabricating high-performance stretchable conductive adhesives (SCA) that connect rigid electronic devices and deformable circuits. Screen printing was performed on Ecoflex films for patterning, and holes were drilled in the film substrate before patterning, as shown in
Figure 4c. The prepared SCA exhibited excellent stretchability, stable conductivity, and strong adhesion to polymer substrates. Additionally, flexible circuits could be made using liquid metal particles through screen printing
［53］, offering outstanding stability and durability due to the functional core-shell structure (after 10,000 bending cycles with a radius of 0.5 mm,
R/
R0 < 1.65). Zhou et al.
［148］ constructed an all self-powered mechanoluminescent triboelectric sensor based on micro-nanostructured mechanoluminescent elastomers, which could display force trajectories via patterning. The stretchable electrodes’ deformable liquid metal was prepared through screen printing, achieving stable stress transfer through the entire device and demonstrating excellent mechanoluminescence (grayscale value of 107 at a stimulus force as low as 0.3 N, with repeatability over 2000 times). Moreover, a microstructured surface was built, significantly improving the triboelectric performance of the resulting composites (voltage increased from 8 V to 24 V), as shown in
Figure 4d. Carneiro et al.
［149］ proposed a novel material and fabrication technology architecture as a general approach to achieve thin-film biostickers for high-resolution electrophysiological monitoring. They used printable biphasic liquid metal silver composites as electrical interconnects and electrodes, as shown in
Figure 4e.
Although the screen printing process is simple and low-cost, each pattern in this process requires a fixed mask, and selecting different patterns requires the fabrication of a new mask. Moreover, it has high requirements for the mechanical properties such as the rheological performance of the ink. For instance, the viscosity, concentration, shear rate, etc., of the ink all need to be controlled within a certain range. These various drawbacks have restricted the development of screen printing in the preparation of liquid metal electrodes.
3.2.1.3 Direct Writing Print Type
Direct writing, as the name implies, uses widely available extrusion printers to create highly scalable and electromechanically stable soft circuits through high-resolution direct writing. This method does not require a photomask and prepares patterns by setting parameters via a computer terminal. It has high requirements for ink, including strict requirements on the size of ink particles, concentration, viscosity, and wettability with the elastomer surface
［150-156］. Lopes et al.
［58］ developed a new type of biphasic Ag-In-Ga ink, which is extruded and printed at high resolution using an extrusion printer. The process is shown in
Figure 5b, which exhibits ideal electromechanical properties similar to liquid metal alloys. The Ag-In-Ga material system demonstrates a unique combination of fluid-like deformability, the ability to withstand significant tensile strain with only a slight increase in resistance, and solid-like integrity that prevents smudging or marking and allows robust printing. Microchips are connected to the circuit using anisotropic conductive film, as shown in
Figure 5c. Circuits printed with this method have high conductivity (7.02×10
5 S/m), high stretchability (strain > 600%), and minimal changes in conductivity after 1000 cycles. Similarly, Lopes et al.
［157］ also used Ag-EGaIn-SIS ink and a desktop extrusion printer (Voltera V-one) to print the desired circuits, as shown in
Figure 5a.
Figure 5d shows an example of a stretchable circuit with integrated LEDs. Based on this, the conductivity of printed interconnects was increased more than twofold by improving the percolation of micro-fillers, and microcracks in the substrate were repaired, thereby increasing the strain limit of printed interconnects to approximately 1200%, as shown in
Figure 5e. Zu et al.
［158］ further improved the ink formulation, focusing on enhancing the impact of silver microflake selection on the electrical and electromechanical performance of composites. By using specific Ag microflakes, AgInGa-SIS ink with a conductivity of up to 6.38×10
5 S/m, a strain limit exceeding 1000%, and low electromechanical coupling can be synthesized, as shown in
Figure 5j. However, if the ink is not well controlled, it may cause nozzle clogging, low pattern accuracy, or even aggregation between ink droplets, thus failing to successfully form the desired pattern.
Direct writing can also be combined with 3D printing, which is a technology that constructs objects based on digital model files and uses appropriate materials to print layer by layer. The ink used in 3D printing involves multiple materials, and the wettability of liquid metal ink on elastomer surfaces is improved by doping with other materials. Xu et al
［56］ addressed the poor wettability between pure liquid metal and various materials by introducing polyvinyl alcohol (PVA) solution as a "bridge" connecting the substrate and the surface oxide film of the liquid metal, allowing PVA-liquid metal ink to be used on various substrates. The process is shown in
Figure 5f, which can generate high-resolution patterns and maximize material utilization. The PVA-liquid metal ink has excellent electrical conductivity (1.3×10
5 S/m) and can be used to design alarm systems and object locators, and even made into flexible sensors to monitor human motion. The difference in surface wettability between liquid metal ink and elastomers doped with PVA is shown in
Figure 5g. Park et al
［57］ demonstrated their use of high-resolution, reconfigurable 3D printing with liquid metal and its application in stretchable 3D integration that is difficult to achieve with traditional manufacturing processes, as shown in
Figure 5h. It can achieve a minimum line width of 1.9 mm, and the printing resolution can be controlled using different nozzle diameters. Compared with traditional 3D printing, this method can form fine, freestanding 3D electrode structures with pattern reconfigurability, as shown in
Figure 5i.
3.2.2 Modification of Wettability at the Elastomer Interface
Interfacial modification refers to the initial direct patterning modification of wettability on a stretchable elastomer, with common methods including selective metal alloy wetting, utilizing differences in substrate hydrophobicity to achieve patterned printing. A solid metal thin layer is laid on the elastomer, and the liquid metal wets this thin layer; excess liquid metal can be removed by briefly exposing it to acid or alkali, thereby forming patterned conductive pathways of liquid metal. This technique is very simple and can utilize traditional patterning methods to create a solid metal thin layer on a precise position of the substrate
［159-166］. Zhang et al.
［167］ used evaporation to deposit Cr/Cu thin films on a styrene-isoprene block copolymer (SIS) elastic substrate, as shown in
Figure 6a. Subsequently, with the help of dilute hydrochloric acid (HCl), liquid metal droplets easily spread onto the metallized substrate, and the copper film dissolves into the liquid metal through an alloying reaction. The prepared liquid metal electrodes exhibit low surface resistance (0.15 Ω/sq), high optical reflectivity (95% at 550 nm), and ultra-high stretchability (500% strain), comparable to silver and aluminum films fabricated using physical vapor deposition techniques. The liquid metal electrodes produced by this method possess unique properties, capable of accommodating large repeated strains without forming cracks. Ozutemiz et al.
［168］ were the first to lithographically pattern a nanoscale copper layer with the desired circuit geometry onto a wafer coated with an elastic substrate, as shown in
Figure 6b, then used a liquid metal dip-coating method to deposit EGaIn onto the patterned copper wetting layer. When the patterned wafer was immersed in a cleaning bath, the NaOH solution helped to remove impurities and any oxides from the copper thin layer surface. Finally, PDMS was cured on the top layer to complete the circuit fabrication. Dong et al.
［169］ successfully printed stretchable liquid metal (EGaIn) circuits on ZnO NPs-anchored ultrafine fibers, simultaneously imparting triple-layer superfabric with antibacterial capability, heating ability, and high-fidelity detection of surface electromyography signals for various physical activities. Additionally, incorporating thermochromic microcapsules into the outermost fibers also endowed the fabric Joule heater with visual indication capability of reversible color switching, as shown in
Figure 6c. Choi et al.
［170］ extended the stretchability of electrodes by introducing liquid metal-based elastic origami electrodes (LM-eKE), where the soft elastomer of the origami pattern is coated with a eutectic gallium-indium (EGaIn) alloy and anchored with a gold layer. This overcomes previous mechanical and electrical limitations of using origami-like structures, allowing the fully soft LM-eKE to stretch up to 820% strain while increasing resistance by only 33%, as shown in
Figure 6d. Han et al.
［171］ prepared a novel stretchable thin film based on stacked liquid metal networks. An oxidized interfacial layer helps construct uninterrupted indium and gallium nanoclusters and creates additional electrical pathways between two metallic networks under mechanical deformation. These films exhibit giant negative piezo-resistance (G-NPR), reducing resistance by 85% during the first 50% stretching. This G-NPR characteristic is attributed to the rupture of metal oxides, thereby forming a liquid eutectic gallium-indium (EGaIn) and connecting stacked networks to build new electrical paths. Electrodes with G-NPR are complementarily combined with traditional electrodes to enhance their performance or achieve some unique operations. This method is a scalable approach to manufacturing stretchable electronics, achieving scalability, repeatable fabrication, precision, and microelectronics compatibility. Notably, the selective metal alloy wetting method has realized wafer-level applications, enabling parallel production of multiple stretchable electronic devices. Compared with 3D printing and physical deposition methods, this method offers high efficiency and repeatability. In future work, process parameters and the geometry of the wetting layer will be further optimized.
3.2.3 Infiltration and Permeation of Fiber Structures
In addition to the above methods, researchers have designed a new process for manufacturing liquid metal electrodes by simply treating liquid metals and coating them on fabric materials to prepare flexible, stretchable, and wearable conductive components. This method is achieved by infiltrating the fabric with liquid metal
［172-174］. Ou et al.
［63］ introduced a unique vibration-assisted printing method for oxidized Galinstan to ensure the formation of a robust liquid metal network in nylon Lycra fabric (NLF). Initially, partial oxidation is used to improve the high surface tension and poor wettability of liquid metals. Subsequently, partially oxidized liquid metals (POLMs) are vibrated to fill the internal spaces of fabric fibers and form a network to address their susceptibility to external breaking forces, as shown in
Figure 7a. This network will be protected by the NLF fiber network, maintaining its integrity under external forces. This electrode not only exhibits high metallic conductivity but also strong durability and does not add extra thickness to the fabric. The maximum tensile strain of POLMs/NLF is 250%, and it can achieve stable electrothermal applications even under intense stretching and twisting conditions, as shown in
Figure 7b. Additionally, Jia et al.
［62］ prepared PDMS-LM/textiles using a liquid metal solution coating method, as shown in
Figure 7c. Conductive fabrics based on liquid metals are made by depositing highly conductive liquid metal coatings on textiles, as shown in
Figure 7d, followed by depositing a polydimethylsiloxane (PDMS) encapsulation layer to secure the liquid metal network. Dong et al.
［175］ precisely printed high-conductivity liquid metal (LM) circuits onto substrates through electrospinning-assisted face-to-face assembly of all-SEBS microfibers with different diameters and compositions, creating porosity and wettability asymmetry throughout the textile to provide anti-gravity water transport capability for continuous sweat release. Moreover, phosphor particles uniformly wrapped in elastic fibers exhibit stable luminescence under extreme stretching in dark environments, as shown in
Figure 7e. Bhuyan et al.
［176］ achieved enhanced dielectric properties by dispersing graphite nanofiber (GNF) fillers in a polydimethylsiloxane (PDMS) substrate, exhibiting excellent wettability on the composite surface through oxidized gallium-based liquid metals and template printing for patterning electrodes. Utilizing the fluid behavior of liquid metal electrodes and the tunable dielectric properties of composites (k = 6.41±0.092@6 wt% at 1 kHz), they fabricated stretchable soft capacitive sensors capable of distinguishing various hand movements, as shown in
Figure 7f.
3.3 Direct Co-blending Method
In addition to the above-mentioned manufacturing methods, the direct blend composite of liquid metal and elastomer has also received extensive attention from researchers
［177-182］, primarily through improving electromechanical performance under strain
［183-185］. This composite material is made by blending liquid metal into a silicone rubber matrix. For example, uncured elastic PDMS is mixed with bulk liquid metal, during which the bulk liquid metal is broken down into small droplets, and flexible and stretchable microfilaments are fabricated by dielectrophoresis (DEP) assisted dispersion of EGaIn microdroplets in a PDMS matrix, as shown in
Figure 8a. After crosslinking of the PDMS matrix, the EGaIn microfilaments retain their conductivity without requiring additional post-processing (such as mechanical sintering)
［186］. Besides, by mixing 11-mercaptoundecanoic acid (MUA) modified liquid metal nanoparticles with polystyrene-block-polybutadiene-block-polystyrene (SBS), a new type of conductive nanocomposite can be prepared, which is biocompatible (in vivo and in vitro), conductive (12,000 S/cm), and stretchable (elongation of 800%), as shown in
Figure 8b. In addition to its excellent performance, this material can also be mass-produced using commercial polymer products and simple production processes. MUA is used to break the dense "gallium oxide shell" of liquid metal nanoparticles, thus enabling the entire composite to conduct electricity. By modifying this composite with rubber, this new conductive material can be adhesive and highly conductive, and can serve as a stable and efficient connector between soft conductors and rigid components
［187］. Li et al.
［188］ fabricated an LM electrode called Kirigami-structured LM paper (KLP), which has excellent properties such as self-supporting, exposed conductor, stretchable, ultra-thin, and recyclable. KLP is made using the art of kirigami and has three structures: uniaxial, biaxial, and square spiral. It can be stretched in various directions, as shown in
Figure 8c. Pei et al.
［189］ uniformly dispersed EGaIn droplets into an elastomer while incorporating dynamic disulfide bonds into the elastomer, making the composite elastomer thermally processable, recyclable, reversibly wet-adhesive, and self-healing. When the EGaIn content is ≥40 vol%, the resulting composite elastomer has an electrical conductivity of 1.3×10
4 S/m, self-healing time of 8.0 hours, and reversible adhesion strength of up to 670 kPa after curing for 2.0 hours. When used as a conductive adhesive, it can easily adhere to metal electrodes to light up LEDs even when stretched to 50%. When used as a self-adhesive bioelectrode, it can also detect human electromyography signals, as shown in
Figure 8d. Zhang et al.
［190］ used cellulose nanofiber-stabilized liquid metal droplets to initiate polymerization and simultaneously served as solid conductive fillers to construct polyacrylamide/MXene/glycerol hydrogels, which have qualified stretchability (1000%) and high environmental adaptability (-25~80 ℃) and can be used for multifunctional sensing, as shown in
Figure 8e. Bhuyan et al.
［191］ designed an ultra-soft, self-healing, and portable liquid metal dispersed gel device. This soft device consists of an ultra-soft dielectric silicone gel matrix and liquid metal alloy (EGaIn, eutectic gallium-indium) microdroplet filler, which can reduce the dielectric loss of the system through the polarization of liquid metal droplets, thereby allowing electrostatics to transfer current to the connected electrodes, as shown in
Figure 8f.
3.4 Preparation and Patterning Methods of Novel Liquid Metal Electrodes
With the further application of liquid metal in the preparation of stretchable electrodes, many new preparation and patterning processes have emerged, which will be reviewed in categories below.
3.4.1 Laser Thermal Effect Patterning
The most mature patterning method in silicon-based semiconductor processing technology currently revolves around laser light sources for patterning. There are two core methods: one is through photolithography, which involves selective light exposure to modify the material, thereby achieving patterning. This process can be roughly divided into two types: positive photoresist and negative photoresist. The other method involves direct writing using a laser beam to etch patterns directly onto the material. One type uses laser vaporization of the material to achieve patterning, while the other utilizes the thermal effects of the laser to impact the material. Laser-based patterning methods
［192-194］ are also employed in the fabrication of patterned liquid metal electrodes. These methods are typically convenient, fast, and compatible with traditional semiconductor processing techniques, but the downside is that they usually require specific materials and equipment.
Kim et al
［195］ developed freestanding patterned liquid metal film conductors (FS-GaIn). FS-GaIn is achieved by introducing metal nanowires into liquid metal and subsequent sequential selective laser processing and etching of directly patterned traces. FS-GaIn can be directly applied to non-flat surfaces without a substrate. When integrated into circuits, FS-GaIn exhibits high conductivity, stretchability, and stability, as shown in
Figure 9a. Cho et al
［196］ developed a novel liquid metal-based fragmentary eutectic gallium-indium alloy (EGaIn) and silver nanowire (NW) backbone electrode, with connection aggregation controlled by laser-induced photothermal reactions to achieve instant and direct patterning of stretchable electrodes with spatially programmed anti-strain properties. The coexistence of fragmented EGaIn and AgNW backbone (i.e., biphasic metal composite (BMC)) primarily aims at the uniform and durable formation of conductive layers on stretchable substrates. The laser-induced photothermal reaction not only promotes adhesion between the BMC layer and the substrate but also alters the structure of the laser-irradiated BMC. By controlling the degree of connection between fragmented EGaIn and AgNW, the initial conductivity and local gauge factor are adjusted, making the electrode insensitive to applied strain, as shown in
Figure 9b. Luo et al
［197］ proposed an all-soft self-powered vibration sensor (SSVS) designed through laser-assisted fabrication. Unlike rigid counterparts, the device is made entirely of extensible materials, including a liquid metal core and an elastic shell. Additionally, laser direct writing enables rapid and maskless processing to obtain complex patterns and functional surfaces of SSVS, as shown in
Figure 9c.
3.4.2 Electrostatic Synchronous Spinning
For flexible and stretchable films, they can be prepared by electrospinning, and the synchronous spinning method can be used to modify the material by introducing liquid metal simultaneously during spinning.
Cao et al
［25］ fabricated a highly robust stretchable electrode (NHSE) based on nanoscale liquid metal (LM) with an adaptive interface that mimics water-mesh interactions. The fabrication involves the in-situ assembly of electrospun elastic nanofiber scaffolds and electrosprayed LM nanoparticles, and the NHSE exhibits an extremely low sheet resistance of 52 mΩ/sq. It is not only insensitive to a wide range of mechanical stretching (i.e., 350% resistance change at 570% elongation) but also to cyclic deformation (i.e., 5% resistance increase after 100% stretching cycles). Its robustness and stability are verified under various conditions, including long-term exposure to air (420 days), cyclic immersion (30,000 cycles), and recovery from mechanical damage, as shown in
Figure 10a. Ma et al
［198］ synthesized a lightweight and highly conductive composite embedded with a liquid metal fiber network. This new paradigm of liquid metal composite consists of interconnected liquid metal fiber networks embedded in a compliant rubber matrix, where the liquid metal fiber network serves as an ultra-lightweight conductive pathway for electrons. Electrode thin films can be prepared using the electrospinning method, as shown in
Figure 10b. Li et al
［199］ developed a microfiber membrane (SPSM) with omnidirectional superelasticity, permeability, and superhydrophobicity through simultaneous electrospinning of styrene-isoprene (SIS) block copolymers and electrospraying of fluorinated SiO
2 nanoparticles.
3.4.3 Electrochemical Selective Deposition
Liquid metal materials can also be selectively deposited on the surface of flexible substrates by electrochemical methods. Under normal circumstances, the high surface tension of liquid metals makes it challenging to form patterns with sub-micron resolution.
In the work of Monnens et al
［200］, this limitation was overcome by electrodepositing EGaIn using a non-aqueous acetonitrile-based electrolyte with high electrochemical stability and chemical orthogonality, producing low-resistance wires that remain stable when (repeatedly) stretched to 100% strain. High-density integration of regular EGaIn lines with a thickness of 300 nm was achieved, as shown in
Figure 11a. Sanati et al
［201］ enhanced surface stability by coating nanoscale EGaIn droplets with a small amount of graphene oxide (GO), thereby improving electrochemical energy storage capacity. Using this, thin-film supercapacitors were fabricated, as shown in
Figure 11b.
4 Conclusions and Prospects
In this review, based on the various excellent properties of liquid metals such as high conductivity, fluidity, ductility, and low viscosity, the latest research on reproducible manufacturing methods for patterned preparation of stretchable liquid metal conductors is studied. To address the incompatibility between liquid metals and flexible substrates, we have mainly organized various preparation methods of liquid metals from two major perspectives: intermediate embedding and surface printing. One category is the passive method that does not modify the material, which involves embedding the liquid metal into the interior of a stretchable substrate to form conductive pathways; the other category is the active method that modifies the material, forming circuits on the surface of the stretchable substrate by modifying the liquid metal or the elastomer surface. At the same time, the blending modification for manufacturing intrinsically conductive elastic materials and new modification and patterning methods are also introduced. To date, gallium-based liquid metals have been proposed and successfully demonstrated in various applications, and many future opportunities still exist, while certain limitations remain to be resolved.
Liquid metal stretchable electronics is a cutting-edge and rapidly developing field involving various interdisciplinary technologies, including materials science, electronic engineering, and biomedical engineering. In the future, the development prospects of this field may focus on the following directions: First, material innovation, developing new liquid metal alloys to enhance their conductivity, stretchability, and biocompatibility. New materials may have higher stability and lower toxicity, making them safer for a wider range of applications; Second, manufacturing processes, improving and optimizing manufacturing technologies such as 3D printing and micro-nano fabrication techniques to make the production of liquid metal electronics more efficient and precise. This will help achieve large-scale production and customized design; Third, flexible displays and sensors, the high conductivity and stretchability of liquid metals make them ideal materials for flexible displays and sensors. Future research may focus on improving the resolution, response speed, and durability of these devices; Fourth, medical and biomedical applications, liquid metal stretchable electronics have great potential in wearable devices, implantable devices, and biosensors. For example, implantable cardiac monitors, EEG sensors, and smart bandages can be developed to provide more accurate health monitoring and treatment; Fifth, energy harvesting and storage, exploring the application of liquid metals in flexible solar cells, wearable batteries, and other energy harvesting and storage devices. The high conductivity and stretchability of liquid metals make them a potential material for improving the performance of these devices; Sixth, human-computer interaction interfaces, developing flexible electronic skins and tactile sensors based on liquid metals to enhance the interactive experience of virtual reality and augmented reality devices. These devices can provide more natural and precise feedback, improving user experience; Seventh, environmental monitoring, liquid metal sensors can be used to detect harmful substances in the environment, such as heavy metal pollution and organic pollutants. Future research may focus on improving the sensitivity and selectivity of these sensors; Eighth, robotics, in the field of soft robotics, liquid metal stretchable electronics can be used to manufacture flexible sensors and actuators, enhancing the flexibility and adaptability of robots. These development directions will not only help advance the technical level of liquid metal stretchable electronics but also broaden their application fields, thereby bringing more innovation and commercial opportunities.
Today, many functional devices based on liquid metals have been developed. Unlike common and mass-produced flexible printed circuits (such as RFID tags), the manufacturing methods for stretchable circuits have not yet matured and are still limited to laboratory scale. Scaling up these methods for industrial applications still faces considerable challenges. Moreover, due to the liquid nature of gallium-based liquid metals, miniaturization and multi-layer patterning integration of devices made from them remain challenging, making the integration of various functional components difficult to achieve. However, we believe that in the future, a steady stream of new methods will be researched and developed to meet the requirements for device miniaturization, reliability, and long-term stability, paving new avenues in the field of stretchable electronics.
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