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Demonstration of the seven‐segment stretchable circuit with LM soldering and the corrugated SWCNT/AgNP bilayer. a,b) Optical images of the stretchable seven‐segment circuit. c) Operation of the seven‐segment circuit 10 d after fabrication. d) Operation of the seven‐segment circuit during 2D stretching. e) I–V characteristics of the LM‐soldered three‐color LED chip with four contact pads.
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Liquid metals (LMs) are a special case of metals that exist in a liquid at room temperature, making them one of the most attractive conductive materials in stretchable electronics. In many cases, however, the LM attacks other metals in contact with the LM through penetration, embrittlement, and alloying. To address these critical issues, there have...
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Citations
... Flexible and stretchable electronics, the cutting edge of wearable (1), on-skin (2), robotics (3), biomedical (4), and bioelectronic (5) techniques, have gained persistent developments in both materials and structural layouts (6,7). Nonconductive polymers (8), polymeric conductors (9) and semiconductors (10), liquid metals (11), gel (12) and elastomer (2) electrolytes, and conductive elastomers (CEs) (13) that are soft, deformable, and shape-adaptable, have been widely explored for the substrates, circuits, sensors, and electronic components of flexible and stretchable electronic devices. Alternatively, structural layouts including serpentine (14), mesh (15), microcrack (16), and longitudinal wave (17) provide possibilities for achieving device flexibility and stretchability from rigid metals and silicon-based electronic components. ...
... These methods usually suffer from the problems of low interfacial adhesion strength, mechanical mismatch, or complicated postprocessing. Self-healable materials (44,45), liquid metals (11,(46)(47)(48), and interlocking microbridges (49) facilitate the self-connection of CE circuits, while they show limited adhesiveness toward independently fabricated electronic components. There are also attempts of using nonconductive polymer adhesives to enhance the interfacial adhesion between electronic components and CE circuits (50,51). ...
Flexible and stretchable electronic devices are subject to failure because of vulnerable circuit interconnections. We develop a low-voltage (1.5 to 4.5 V) and rapid (as low as 5 s) electric welding strategy to integrate both rigid electronic components and soft sensors in flexible circuits under ambient conditions. This is achieved through the design of conductive elastomers composed of borate ester polymers and conductive fillers, which can be self-welded and generate welding effects to various materials including metals, hydrogels, and other conductive elastomers. The welding effect is generated through the electrochemical reaction–triggered exposure of interfacial adhesive promotors or the cleavage/reformation of dynamic bonds. Our strategy can ensure both mechanical compliance and conductivity at the circuit interfaces and easily produce welding strengths in the kilopascal to megapascal range. The as-designed conductive elastomers in combination with the electric welding technique provide a robust platform for constructing standalone flexible and stretchable electronic devices that are detachable and assemblable on demand.
... This study originates from research on the interfacing of liquid metal with diamond surfaces published previously [28,29]. The corrosion of liquid metals on several metal substrates and substrates protected by non-metal materials have been shown [30][31][32]. In these cases, the liquid metal penetrated the non-metal coating, perhaps by force. ...
Thermal transport is of grave importance in many high-value applications. Heat dissipation can be improved by utilizing liquid metals as thermal interface materials. Yet, liquid metals exhibit corrosivity towards many metals used for heat sinks, such as aluminum, and other electrical devices (i.e., copper). The compatibility of the liquid metal with the heat sink or device material as well as its long-term stability are important performance variables for thermal management systems. Herein, the compatibility of the liquid metal Galinstan, a eutectic alloy of gallium, indium, and tin, with diamond coatings and the stability of the liquid metal in this environment are scrutinized. The liquid metal did not penetrate the diamond coating nor corrode it. However, the liquid metal solidified with the progression of time, starting from the second year. After 4 years of aging, the liquid metal on all samples solidified, which cannot be explained by the dissolution of aluminum from the titanium alloy. In contrast, the solidification arose from oxidation by oxygen, followed by hydrolysis to GaOOH due to the humidity in the air. The hydrolysis led to dealloying, where In and Sn remained an alloy while Ga separated as GaOOH. This hydrolysis has implications for many devices based on gallium alloys and should be considered during the design phase of liquid metal-enabled products.
... Another surface coating to protect electrodes from sticking of LMs is a layer of carbon. Oh et al. presented spraying carbon nanotubes (CNTs) on an Ag thin film surface as a barrier for reliable and stretchable electronics (Oh et al. 2018). Combining the alloying barrier properties of a commercially available carbon ink with a simple process like screen printing would allow for scalability. ...
Direct 3D printing of active microfluidic elements on PCB substrates enables high-speed fabrication of stand-alone microdevices for a variety of health and energy applications. Microvalves are key components of microfluidic devices and liquid metal (LM) microvalves exhibit promising flow control in microsystems integrated with PCBs. In this paper, we demonstrate LM microvalves directly 3D printed on PCB using advanced digital light processing (DLP). Electrodes on PCB are coated by carbon ink to prevent alloying between gallium-based LM plug and copper electrodes. We used DLP 3D printers with in-house developed acrylic-based resins, Isobornyl Acrylate, and Diurethane Dimethacrylate (DUDMA) and functionalized PCB surface with acrylic-based resin for strong bonding. Valving seats are printed in a 3D caterpillar geometry with chamber diameter of 700 µm. We successfully printed channels and nozzles down to 90 µm. Aiming for microvalves for low-power applications, we applied square-wave voltage of 2 Vpp at a range of frequencies between 5 to 35 Hz. The results show precise control of the bistable valving mechanism based on electrochemical actuation of LMs.
... However, it is still challenging to integrate LM composites with rigid electronic components because of the fluidity and poor adhesion of LM. There are a couple of pioneering attempts showing potential of solving these interfacial issues, such as adding additional encapsulation layers 23 , providing proper barriers 24 for generating interlocking effect between LM and components, or adding interface binder to make temporary connections by virtue of physicochemical bonding 25,26 , but these methods cannot form a robust interface between the soft conductive composites and rigid electronics directly. ...
The rapid-developing soft robots and wearable devices require flexible conductive materials to maintain electric functions over a large range of deformations. Considerable efforts are made to develop stretchable conductive materials; little attention is paid to the frequent failures of integrated circuits caused by the interface mismatch of soft substrates and rigid silicon-based microelectronics. Here, we present a stretchable solder with good weldability that can strongly bond with electronic components, benefiting from the hierarchical assemblies of liquid metal particles, small-molecule modulators, and non-covalently crosslinked polymer matrix. Our self-solder shows high conductivity (>2×105 S m⁻¹), extreme stretchability (~1000%, and >600% with chip-integrated), and high toughness (~20 MJ m⁻³). Additionally, the dynamic interactions within our solder’s surface and interior enable a range of unique features, including ease of integration, component substitution, and circuit recyclability. With all these features, we demonstrated an application as thermoforming technology for three-dimensional (3D) conformable electronics, showing potential in reducing the complexity of microchip interfacing, as well as scalable fabrication of chip-integrated stretchable circuits and 3D electronics.
... Therefore, some of the advantages of this device include its elasticity for making athletic clothing [232], nanoenergy production [233], and implanted medical equipment [234]. CNTs have already been isolated [235], combined [236] into polymer based composites for strain-sensors [237], the energy production using triboelectric [238], systems of checking [239] because of their exceptional mechanical and electrical capabilities. Additionally, the adaptable integrated circuits based on CNT system shows the benefit of small energy usage [240]. ...
The research interest in the field of carbon nanotube field effect transistors (CNTFETs) in the post Moore era has witnessed a rapid growth primarily due to the fact that the conventional silicon based complementary metal oxide semiconductor (CMOS) devices are approaching its fundamental scaling limits. This has led to significant interest among the researchers to examine novel device technologies utilizing different materials to sustain the scaling limits of the modern day integrated circuits. Among various material alternatives, carbon nanotubes (CNTs) have been extensively investigated owing to their desirable properties such as minimal short channel effects, high mobility, and high normalized drive currents. CNTs form the most important component of CNTFETs, which are being viewed as the most feasible alternatives for the replacement of silicon transistors. In this manuscript, detailed description of the recent advances of state of the art in the field of CNTFETs with emphasis on the most broadly impactful applications for which they are being employed is presented. The future prospects of CNTFETs while considering aggressively scaled transistor technologies are also briefly discussed.
... Wearable electronic devices (WEDs) have received considerable attention owing to the growing interest in soft robotics [1][2][3], biomedical treatments [4][5][6][7], electronic skin [8][9][10][11], wireless communication [12], and conformable sensors [13][14][15][16]. WEDs are typically manufactured using elastic polymers as matrices incorporated with conductive fillers to establish electrical pathways [17][18][19]. ...
Wearable electronic devices (WEDs) are receiving significant attention because of the increasing interest in soft robotics, electronic skin, and wearable sensors. Liquid metals (LMs) are compelling for WEDs owing to their metallic conductivity, fluidic nature, and low toxicity. Herein, we fabricated stretchable and soft WEDs using LM (eutectic gallium-indium alloy) wires (LMWs) patterned via force wetting through custom-made stencils on an elastic substrate. LMWs can generate thermal energy via Joule heating upon current application and deliver it to the substrate, resulting in wearable polymeric heaters. The LM mixed with carbonyl iron particles (CIPs) can also be patterned while preserving fluidic behavior. The degree of thermal energy generated through the LMWs can be manipulated as a function of the CIP concentration in the LM and geometrical factors of the electrode patterns, i.e., width and length. An elastic film patterned with LMWs attached to the human body exhibits changes in the effective electrical resistance depending on applied strain, demonstrating potential as a wearable strain sensor. This LM utilized WED that can generate thermal energy upon current application through the LMWs and detect the bodily motion has significant potential for application in wearable thermotherapy, electronic skin, and soft sensors.
... [98] However, since the metal films such as Cu, Au, and Ag have much lower tensile limit than intrinsically stretchable electrodes, specific structural designs to dissipate localized strain should be employed. Therefore, many researchers have developed representative structural designs such as serpentine [99][100][101][102] and wrinkled electrodes, [98,[103][104][105] which can maintain electrical conductivity by changing their geometrical shapes. ...
... Reproduced with permission. [105] Copyright 2018, Wiley-VCH. ...
... To further improve mechanical reliability, Oh et al. suggested a corrugated SWCNT diffusion barrier for the wrinkled electrodes. [105] LM was used as contact materials, which could dissipate concentrated mechanical strain at the boundary between LEDs and wrinkled electrodes. However, the LM could react with the wrinkled electrodes by penetrating grain boundaries in metals. ...
A stretchable display would be the ultimate form factor for the next generation of displays beyond the curved and foldable configurations that have enabled the commercialization of deformable electronic applications. However, because conventional active devices are very brittle and vulnerable to mechanical deformation, appropriate strategies must be developed from the material and structural points of view to achieve the desired mechanical stretchability without compromising electrical properties. In this regard, remarkable findings and achievements in stretchable active materials, geometrical designs, and integration enabling technologies for various types of stretchable electronic elements have been actively reported. This review covers the recent developments in advanced materials and feasible strategies for the realization of stretchable electronic devices for stretchable displays. In particular, representative strain‐engineering technologies for stretchable substrates, electrodes, and active devices are introduced. Various state‐of‐the‐art stretchable active devices such as thin‐film transistors and electroluminescent devices that consist of stretchable matrix displays are also presented. Finally, the future perspectives and challenges for stretchable active displays are discussed.
... 53 Another approach involves contacting liquid metals to conductive carbon materials, such as graphene or carbon nanotubes 54,55 or conductive polymers (e.g., PEDOT:PSS). 56 If the carbon is placed between the liquid metal and the metal, 57 it can prevent or minimize diffusion of gallium into the metallic contact pad. 58 While these prior studies focus on electrical connections, this paper describes and characterizes a simple way to create mechanically robust electrical interconnects directly between non-stretchable and stretchable electronic devices. ...
Stretchable electronic devices that maintain electrical function when subjected to stress or strain are useful for enabling new applications for electronics, such as wearable devices, human−machine interfaces, and components for soft robotics. Powering and communicating with these devices is a challenge. NFC (near-field communication) coils solve this challenge but only work efficiently when they are in close proximity to the device. Alternatively, electrical signals and power can arrive via physical connections between the stretchable device and an external source, such as a battery. The ability to create a robust physical and electrical connection between mechanically disparate components may enable new types of hybrid devices in which at least a portion is stretchable or deformable, such as hinges. This paper presents a simple method to make mechanical and electrical connections between elastomeric conductors and flexible (or rigid) conductors. The adhesion at the interface between these disparate materials arises from surface chemistry that forms strong covalent bonds. The utilization of liquid metals as the conductor provides stretchable interconnects between stretchable and non-stretchable electrical traces. The liquid metal can be printed or injected into vias to create interconnects. We characterized the mechanical and electrical properties of these hybrid devices to demonstrate the concept and identify geometric design criteria to maximize mechanical strength. The work here provides a simple and general strategy for creating mechanical and electrical connections that may find use in a variety of stretchable and soft electronic devices.
... Inert carbon materials, including carbon nanotubes and graphene, have served as the interlayer to prevent the penetration of liquid metal into other metals. 100,101 The success of liquid metal will change the way stretchable electronics are manufactured. When considering biomedical applications, conductive materials need a much higher resilience to oxidation and corrosion. ...
Recent decades have witnessed the booming development of stretchable electronics based on nano/micro composite inks. Printing is a scalable, low-cost, and high-efficiency fabrication tool to realize stretchable electronics through additive processes. However, compared with conventional flexible electronics, stretchable electronics need to experience more severe mechanical deformation which may cause destructive damage. Most of the reported works in this field mainly focus on how to achieve a high stretchability of nano/micro composite conductors or single working modules/devices, with limited attention given to the reliability for practical applications. In this minireview, we summarized the failure modes when printing stretchable electronics using nano/micro composite ink, including dysfunction of the stretchable interconnects, the stress-concentrated rigid-soft interfaces for hybrid electronics, the vulnerable vias upon stretching, thermal accumulation, and environmental instability of stretchable materials. Strategies for tackling these challenges to realize reliable performances are proposed and discussed. Our review provides an overview on the importance of reliable, printable, and stretchable electronics, which are the key enablers in propelling stretchable electronics from fancy demos to practical applications.
... In contrast, the liquid metal transition connection showed stable conductivity under various of vibration conditions ( Supplementary Fig. 7e, f), demonstrating the possibilities in mobile wearable ECG monitoring. Even though the liquid metal is corrosive against metals 42,43 , the presence of the generated alloy maintains a decent electrical conductivity after 12 h of corrosion (Supplementary Fig. 8), which is acceptable for disposable epidermal electrodes. ...
Six chest leads are the standardized clinical devices of diagnosing cardiac diseases. Emerging epidermal electronics technology shift the dangling wires and bulky devices to imperceptible wearing, achieving both comfortable experience and high-fidelity measuring. Extending small areas of current epidermal electronics to the chest scale requires eliminating interference from long epidermal interconnects and rendering the data acquisition (DAQ) portable. Herein, we developed a chest-scale epidermal electronic system (EES) for standard precordial-lead ECG and hydration monitoring, including the only μm-thick substrate-free epidermal sensing module and the soft wireless DAQ module. An electrical compensation strategy using double channels within the DAQ module and epidermal compensated branches (ECB) is proposed to eliminate unwanted signals from the long epidermal interconnects and to achieve the desired ECG. In this way, the EES works stably and precisely under different levels of exercise. Patients with sinus arrhythmias have been tested, demonstrating the prospect of EES in cardiac diseases.
