Stretchable Circuits Research Articles

A Subtractive Method to Chemically Pattern Liquid Metal for Stretchable Circuits

AbstractAdvancements in biomedical research have spurred the development of stretchable electronic devices. While soft insulators are readily available, soft conductors with metal‐like electrical conductivity are rare. Gallium and its alloys, being nontoxic and intrinsically stretchable, are potentially ideal solutions. However, current additive liquid metal (LM) patterning methods face limitations in achieving high‐throughput, high‐resolution, and high‐density LM wiring. Here, a subtractive LM patterning method is developed to meet all these requirements simultaneously. The innovative method involves parallel filling a single continuous microfluidic mesh network with LM that short‐circuits all the pins and pads of a circuit, followed by parallel cutting of the unwanted short‐circuited interconnections using hydrochloric acid (HCl) vapor. Cutting locations are pre‐defined by designing narrower intersecting channels, leveraging capillary force for precise filling and cutting. The process is characterized using a multidimensional parametric study with varying LM line widths and HCl concentrations, and in situ impedance measurements to assess insulation performance. To showcase its high‐throughput capabilities, a mock circuit is used to successfully generate complex LM interconnects that connected hundreds of electrical pads. Finally, a stretchable LM circuit with a micro‐LED array is fabricated to demonstrate the practical application of this technology for massively parallel wiring in stretchable electronics.

Fully Flexible All‐in‐One Electronic Display Skin with Seamless Integration of MicroLED and Hydrogel Battery

AbstractVisualization is a cornerstone in human–computer interaction, evolving from large screens to mobile devices and now to wearable display technology. However, conventional batteries and displays lack the softness and stretchability inherent to human skin. Here, a fully flexible, all‐in‐one electronic display skin by integrating a flexible microLED display, a stretchable circuit, and a Zn|PAM|V2O5 hydrogel battery pack is developed. Vapor‐phase bulk transfer is used to efficiently and accurately transfer 10 000 microLEDs onto a flexible substrate, ensuring high flexibility and ultra‐thin (240 µm). The flexible hydrogel battery pack provides sustainable energy, with a high specific capacity of 331.3 mAh g−1. Additionally, a transparent stretchable circuit board (maximum stretch 40%) is made by 3D printing technology. The skin display exhibited exceptional stretchability and biocompatibility, conforming to movements while maintaining high‐resolution dynamic visual outputs. These innovations pave the way for advanced skin‐implanted displays, promising transformative applications in information interaction, healthcare, and artificial intelligence.

Breathable and stretchable circuit boards based on liquid metal nanofiber membranes

Breathable and stretchable circuit boards based on liquid metal nanofiber membranes

3D printing of liquid metal circuits on fabric for stretchable heater application

Abstract Liquid metals (LMs), due to their excellent stretchability, low resistance, and high thermal conductivity, has become an ideal material for preparing stretchable circuits. 3D printing technology can produce patterns with complex structures with high precision and is suitable for low-cost large-scale production of circuits. However, the high surface tension of liquid metals limits the practical application of 3D printing of LMs. In this work, we proposed a method for preparing a mixed ink based on thermoplastic polyurethane, Cyrene solvent, and LMs, and producing stretchable LMs-based electrodes through 3D printing on a fabric substrate. The stretchable LMs-based electrode prepared by this method shows a wide range of strain response and excellent tensile recovery performance, providing theoretical guidance for the application of LMs electrodes. In addition, the LMs-based heaters showed good Joule thermal response at low voltages, as well as good tensile stability, providing a new strategy for wearable heaters.

Hetero phase nanocomposite based posture sensor with stretchable connector-sensor interface

During recent years, stretchable and flexible sensors for wearable application have gained a lot of interest, but one of the grand challenges that yet to be addressed is the interface between the soft and stretchable sensor and the hard and rigid circuits. The mechanical mismatch between soft sensors and hard connectors to external circuits makes the interface prone to quick failure under mechanical strain and deformation.Here, we report a soft piezoresistive posture sensor made of a hetero-phase nanocomposite with the stretchable connector-sensor interface for the robust sensing of the posture. The hetero-phase nanocomposite is made of Gallium-indium (GaIn) in the liquid phase, carbon nano tube (CNT) and polydimethylsiloxane (PDMS) in the solid phase. The sensor with the Young’s modulus of 158 kPa shows comparable mechanical softness to the skin, which is essential for accurate sensing. Further, we developed liquid metal (LM) based stretchable connections between the soft sensor and the connectors to the external circuit to enable the stable operation of the sensor under deformation. The developed posture sensor has a gauge factor (GF) of 0.815 under 20 % and 0.32 between 20 % and 60 % of applied tensile strain. The sensor was successfully used for sensing of the posture with the sensitivity of 1 % change in its electrical resistance per 10 degrees of neck tilting and strain sensitivity of 2 % of applied strain. The fabrication of the CNT-LM-PDMS posture sensor employed a scalable and cost-effective process. The presented stretchable connection is a solution that can be applied to many soft and stretchable wearable sensors and circuit interfaces with various physiological sensing applications.

Highly robust soft-rigid connections via mechanical interlocking for assembling ultra-stretchable displays

Stretchable electronic circuits can seamlessly conform to irregular and dynamic surfaces with high integration. However, current stretchable configurations typically have limited stretchability due to the lack of robust connections between soft interconnects and rigid electronics. Here, we printed highly stretchable metal–polymer conductors on thermoplastic elastomers as interconnects. We developed electronic vests with porous surfaces for rigid electronics and introduced polyester hot-melt adhesives to strengthen connections between soft interconnects and rigid electronics. After thermal bonding, the adhesive penetrates the porous surface of electronic vests, creating a mechanical interlock and providing an adhesion force of 8.34 N/cm for the connection (3× higher than conductive adhesives). Thus, rigid electronics of different sizes and different pin counts can form strong connections to soft interconnects, achieving a maximum strain tolerance of ~700% (10× higher than conductive adhesives). We achieved highly integrated ultra-stretchable displays that can withstand stretching up to 220% without dead pixels.

Scalable Fabrication of Large-Scale, 3D, and Stretchable Circuits.

Stretchable electronics have demonstrated excellent potential in wearable healthcare and conformal integration. Achieving the scalable fabrication of stretchable devices with high functional density is the cornerstone to enable the practical applications of stretchable electronics. Here, a comprehensive methodology for realizing large-scale, 3D, and stretchable circuits (3D-LSC) is reported. The soft copper-clad laminate (S-CCL) based on the "cast and cure" process facilitates patterning the planar interconnects with the scale beyond 1 m. With the ability to form through, buried and blind VIAs in the multilayer stack of S-CCLs, high functional density can be achieved by further creating vertical interconnects in stacked S-CCLs. The application of temporary bonding substrate effectively minimizes the misalignments caused by residual strain and thermal strain. 3D-LSC enables the batch production of stretchable skin patches based on five-layer stretchablecircuits, which can serve as a miniaturized system for physiological signals monitoring with wireless power delivery. The fabrications of conformal antenna and stretchable light-emitting diode display further illustrate the potential of 3D-LSC in realizing large-scale stretchable devices.

Stretchable electronic strips for electronic textiles enabled by 3D helical structure

The development of stretchable electronic devices is a critical area of research for wearable electronics, particularly electronic textiles (e-textiles), where electronic devices embedded in clothing need to stretch and bend with the body. While stretchable electronics technologies exist, none have been widely adopted. This work presents a novel and potentially transformative approach to stretchable electronics using a ubiquitous structure: the helix. A strip of flexible circuitry (‘e-strip’) is twisted to form a helical ribbon, transforming it from flexible to stretchable. A stretchable core—in this case rubber cord—supports the structure, preventing damage from buckling. Existing helical electronics have only extended to stretchable interconnects between circuit modules, and individual components such as printed helical transistors. Fully stretchable circuits have, until now, only been produced in planar form: flat circuits, either using curved geometry to enable them to stretch, or using inherently stretchable elastomer substrates. Helical e-strips can bend along multiple axes, and repeatedly stretch between 30 and 50%, depending on core material and diameter. LED and temperature sensing helical e-strips are demonstrated, along with design rules for helical e-strip fabrication. Widely available materials and standard fabrication processes were prioritized to maximize scalability and accessibility.

Surface Embedded Metal Nanowire-Liquid Metal-Elastomer Hybrid Composites for Stretchable Electronics.

Both liquid metal (LM) and metallic filler-based conductive composites are promising stretchable conductors. LM alloys exhibit intrinsically high deformability but present challenges for patterning on polymeric substrates due to high surface tension. On the other hand, conductive composites comprising metallic fillers undergo considerable decrease in electrical conductivity under mechanical deformation. To address the challenges, we present silver nanowire (AgNW)-LM-elastomer hybrid composite films, where AgNWs and LM are embedded below the surface of an elastomeric matrix, using two fabrication approaches, sequential and mixed. We investigate and understand the process-structure-property relationship of the AgNW-LM-elastomer hybrid composites fabricated using two approaches. Different weight ratios of AgNWs and LM particles provide tunable electrical conductivity. The hybrid composites show more stable electromechanical performance than the composites with AgNWs alone. In particular, 1:2.4 (AgNW:LMP w/w) sequential hybrid composite shows electromechanical stability similar to that of the LM-elastomer composite, with a resistance increase of 2.04% at 90% strain. The sequential approach is found to form AgIn2 intermetallic compounds which along with Ga-In bonds, imparts large deformability to the sequential hybrid composite as well as mechanical robustness against scratching, cutting, peeling, and wiping. To demonstrate the application of the hybrid composite for stretchable electronics, a laser patterned stretchable heater on textile and a stretchable circuit including a light-emitting diode are fabricated.

Dopamine-induced high fiber wetness for improved conductive fiber bundles with striated polypyrrole coating toward wearable healthcare electronics

Stretchable conductive fibers have shown considerable promise in the field of wearable healthcare electronics, but their widespread application has been impeded by the complexity of their manufacturing processes and their susceptibility to electrical failure under significant deformations. In response to this challenge, this study proposes an approach that harnesses dopamine to enhance the wettability of commercial polyurethane (PU) multifilaments. This enhancement facilitates the penetration of aqueous solutions into the core layer of the multifilaments, thereby enabling the polymerization of a striated polypyrrole (PPy) coating on the monofilament surface of the multifilament core layer. The resulting dopamine-induced composite fiber bundles (DCFBs), drawing inspiration from the structure of striated muscle fiber bundles, demonstrates insensitive conductivity at a high tensile strain of 600% and exceptional durability exceeding 40,000 cycles, as the bionic structure mitigates the effect of PPy coating cracks on overall conductivity. The bionic DCFBs holds potential for applications in stretchable electrical circuits, all-weather wearable thermal therapy devices, and pressure sensors for Moss Code communication and human state detection. This work presents a straightforward and versatile technique for producing highly stretchable fibers with multilayer conductive coatings featuring microstructures in the context of wearable healthcare electronics, utilizing commercial fibers as the primary material source.

Ultra-flexible stretchable liquid metal circuits with antimicrobial properties through selective laser activation for health monitoring

Liquid metal (LM) was generally used to construct flexible wearable electronics to solve the stretchability-conductivity dilemma of traditional electronic devices. Herein, ultra-flexible and stretchable LM circuits with a unique layered structure (LM-Ag-Cu) are prepared by laser-induced selective metallization combined with the highly selective reaction wetting. The Ag-Cu layer is first prepared as templates using the laser-induced selective metallization technology on transparent flexible polymer films, and the LM circuit is obtained by brushing LM onto the Ag-Cu template. The prepared LM circuit demonstrates excellent conductivity and good electrical heating properties even under significant bending and stretching. Moreover, it also exhibits excellent antimicrobial resistance (>99 % antimicrobial activity against E. coli and S. aureus). When designing wearable bioelectrodes, the electrodes based on this LM circuit show accurate and stable real-time monitoring performance of biomechanical (motion detection) and bioelectrical signals (electrocardiography and electromyography signals). Because laser-induced selective metallization technology has characteristics of high-precision preparation of metal circuits, the obtained Ag-Cu layer is accurate and can be reactively infiltrated with LM, so that this new LM-Ag-Cu circuit is born with a layered structure. The LM circuit prepared through our strategy provides a practical and innovative approach to fabricating intelligent skin bioelectrodes.

Controllable Multimodal Actuation in Fully Printed Ultrathin Micro-Patterned Electrochemical Actuators.

Submillimeter or micrometer scale electrically controlled soft actuators have immense potential in microrobotics, haptics, and biomedical applications. However, the fabrication of miniaturized and micropatterned open-air soft actuators has remained challenging. In this study, we demonstrate the microfabrication of trilayer electrochemical actuators (ECAs) through aerosol jet printing (AJP), a rapid prototyping method with a 10 μm lateral resolution. We make fully printed 1000 × 5000 × 12 μm3 ultrathin ECAs, each of which comprises a Nafion electrolyte layer sandwiched between two poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) electrode layers. The ECAs actuate due to the electric-field-driven migration of hydrated protons. Due to the thinness that gives rise to a low proton transport length and a low flexural rigidity, the printed ECAs can operate under low voltages (∼0.5 V) and have a relatively fast response (∼seconds). We print all the components of an actuator that consists of two individually controlled submillimeter segments and demonstrate its multimodal actuation. The convenience, versatility, rapidity, and low cost of our microfabrication strategy promise future developments in integrating arrays of intricately patterned individually controlled soft microactuators on compact stretchable electronic circuits.

A Flexible and Electrically Conductive Liquid Metal Adhesive for Hybrid Electronic Integration

AbstractElectrical and mechanical integration approaches are essential for emerging hybrid electronics that must robustly bond rigid electrical components with flexible circuits and substrates. However, flexible polymeric substrates and circuits cannot withstand the high temperatures used in traditional electronic processing. This constraint requires new strategies to create flexible materials that simultaneously achieve high electrical conductivity, strong adhesion, and processibility at low temperature. Here, an electrically conductive adhesive is introduced that is flexible, electrically conductive (up to 3.25×105 S m−1) without sintering or high temperature post‐processing, and strongly adhesive to various materials common to flexible and stretchable circuits (fracture energy 350 <Gc < 700 J m−2). This is achieved through a multiphase soft composite consisting of an elastomeric and adhesive epoxy network with dispersed liquid metal droplets that are bridged by silver flakes, which form a flexible and conductive percolated network. These inks can be processed through masked deposition and direct ink writing at room temperature. This enables soft conductive wiring and robust integration of rigid components onto flexible substrates to create hybrid electronics for emerging applications in soft electronics, soft robotics, and multifunctional systems.

Self-Mixed Biphasic Liquid Metal Composite with Ultra-High Stretchability and Strain-Insensitivity for Neuromorphic Circuits.

Neuromorphic circuits that can function under extreme deformations are important for various data-driven wearable and robotic applications. Herein, biphasic liquid metal particle (BMP) with unprecedented stretchability and strain-insensitivity (ΔR/R0 = 1.4@ 1200% strain) is developed to realize a stretchable neuromorphic circuit that mimics a spike-based biologic sensory system. The BMP consists of liquid metal particles (LMPs) and rigid liquid metal particles (RLMPs), which are homogeneously mixed via spontaneous solutal-Marangoni mixing flow during coating. This permits facile single step patterning directly on various substrates at room temperature. BMP is highly conductive (2.3 × 106 S/m) without any post activation steps. BMP interconnects are utilized for a sensory system, which is capable of distinguishing variations of biaxial strains with a spiking neural network, thus demonstrating their potential for various sensing and signal processing applications.

Fast and Facile Liquid Metal Printing via Projection Lithography for Highly Stretchable Electronic Circuits.

Soft electronic circuits are crucial for wearable electronics, biomedical technologies, and soft robotics, requiring soft conductive materials with high conductivity, high strain limit, and stable electrical performance under deformation. Liquid metals (LMs) have become attractive candidates with high conductivity and fluidic compliance, while effective manufacturing methods are demanded. Digital light processing (DLP)-based projection lithography is a high-resolution and high-throughput printing technique for primarily polymers and some metals. If LMs can be printed with DLP as well, the entire soft devices can be fabricated by one printer in a streamlined and highly efficient process. Herein, fast and facile DLP-based LM printing is achieved. Simply with 5-10 s of patterned ultraviolet (UV)-light exposure, a highly conductive and stretchable pattern can be printed using a photo-crosslinkable LM particle ink. The printed eutectic gallium indium traces feature high resolution (≈20 µm), conductivity (3 × 106 S m-1), stretchability (≈2500%), and excellent stability (consistent performance at different deformation). Various patterns are printed in diverse material systems for broad applications including stretchable displays, epidermal strain sensors, heaters, humidity sensors, conformal electrodes for electrography, and multi-layer actuators. The facile and scalable process, excellent performance, and diverse applications ensure its broad impact on soft electronic manufacturing.

Liquid Metal Powders Wrapped with Robust Shell by Bimetallic Ions Chelation Strategy for Energy Harvesting and Flexible Electronics

AbstractExcellent electrical and fluidic properties render liquid metal (LM) a promising material for flexible electronics, soft robotics, and stretchable circuits. Although producing LM micro/nano droplets by nanometerization addresses the tricky issue of the processing result from high surface tension, it still remains challenging to obtain stable LM droplets because of their dynamic surfaces. In this study, LM powders wrapped with robust shells are fabricated by bimetallic ions chelation strategy. During sonicating LM (e.g., EGaIn) in an alginate solution, a microgel shell is first formed by the coordination of Ga3+ and carboxyl groups. Subsequently, LM powders are achieved by introducing Ca2+ as secondary metal ions to chelate with redundant carboxyl groups. Besides high chemical stability in water and air, these nonstick LM powders also exhibit superior photothermal effects for energy harvesting, being capable of recovering electrical conductivity by mechanical sintering for flexible electronics. These conductive circuits based on LM powders not only work as electro‐thermal heaters, and electromagnetic interference shielding material but also can be designed as smart actuators. Thus, this bimetallic ions chelation strategy may start a pathway to produce stable, transportable LM powders with multifunctional properties, applicable in stretchable electronics, smart actuators, and electric skins.

Tough soldering for stretchable electronics by small-molecule modulated interfacial assemblies

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.

Liquid-metal/NdFeB/Ni composites for high wettability, patternable, stretchable electronic wire

Room-temperature liquid-metal (LM) has high conductivity, which is an ideal material for manufacturing stretchable electronic wires. However, its high surface tension and fluidity make it extremely difficult to adhere to or encapsulate on flexible substrates. This work developed LM/NdFeB/Ni composites for stretchable electronic wires. The composites were composed of Ga68.5In21.5Sn10 (with a melting point at −19 °C) alloys, Nd2Fe14B (5 μm), and Ni (50–500 nm) microparticles. The SEM and EDS results showed the porous structure of the composites. Ni particles were added to improve the magnetic properties of LM/NdFeB composites, as well as the porous structure composed of magnetized NdFeB particles. Thereby the structure exhibited a heightened capacity for LM locking. Test results indicate that the liquid metal wires have good wettability and adhesion on different surfaces and could be manufactured using different patterning methods. Moreover, its anti-extrusion and anti-leakage characteristics were suitable for constructing stretchable circuits in open channels or on flat surfaces. The proposed LM/NdFeB/Ni composites are expected to manufacture low-cost, patternable, and anti-extrusion stretchable electronic wires.

Core–sheath structured polystyrene‐ethylene‐butene‐styrene/carbon nanotubes/carbonyl iron particlecomposite fiber with high stretchability for strain and magnetic field dual‐mode sensing

AbstractTo meet the requirement of high stretchability, high sensitivity, and multifunction for flexible sensors, core–sheath structured polystyrene‐ethylene‐butene‐styrene (SEBS)/carbon nanotubes (CNTs)/carbonyl iron particle (CIP) fiber with SEBS/CNTs as the sheath and SEBS/CIP as the core was prepared through wet spinning. The contents of CNTs and CIP in the composite fiber were adjusted by changing the extrusion speed of the SEBS/CNTs and SEBS/CIP spinning solutions. The morphology, electromechanical sensing, and magnetic precepting performances of the fiber were studied. The SEBS/CNTs/CIP fiber sensor showed high stretchability with a maximum strain sensing range of 0%–216.9%, high sensitivity with a gauge factor (GF) of up to 1068.28, as well as excellent durability. In addition, the SEBS/CNTs/CIP fiber sensor exhibited excellent magnetic sensing performance by changing the relative resistance of 35.2% with a magnetic flux density variation of 1.2 T. The applications of SEBS/CNTs/CIP fiber in stretchable circuits, human body motion monitoring, and magnetic field perception were demonstrated.Highlights SEBS/CNTs/CIP fiber for strain and magnetic field sensing was fabricated. The SEBS/CNTs/CIP fiber sensor could sense 216.9% strain with GF up to 1068.28. Relative resistance of SEBS/CNTs/CIP fiber sensor changed by 35.2% under 1.2 T. SEBS/CNTs/CIP fiber was used in human motion monitoring and magnetic perception.

Photothermal Lithography for Realizing a Stretchable Multilayer Electronic Circuit Using a Laser.

Photolithography is a well-established fabrication method for realizing multilayer electronic circuits. However, it is challenging to adopt photolithography to fabricate intrinsically stretchable multilayer electronic circuits fully composed of an elastomeric matrix, due to the opacity of thick stretchable nanocomposite conductors. Here, we present photothermal lithography that can pattern elastomeric conductors and via holes using pulsed lasers. The photothermal-patterned stretchable nanocomposite conductor exhibits 3 times higher conductivity (5940 S cm-1) and 5 orders of magnitude lower resistance change (R/R0 = 40) under a 30% strained 5000th cyclic stretch, compared to those of a screen-printed conductor, based on the percolation network formed by spatial heating of the laser. In addition, a 50 μm sized stretchable via holes can be patterned on the passivation without material ablation and electrical degradation of the bottom conductor. By repeatedly patterning the conductor and via holes, highly conductive and durable multilayer circuits can be stacked with layer-by-layer material integration. Finally, a stretchable wireless pressure sensor and passive matrix LED array are demonstrated, thus showing the potential for a stretchable multilayer electronic circuit with durability, high density, and multifunctionality.