Stretchable Electronic Circuits Research Articles

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.

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.

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.

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.

A micro-vibration-driven direct ink write printing method of gallium–indium alloys

Combining liquid fluidity and metallic conductivity, gallium–indium (Ga–In) alloys are making a splash in areas such as stretchable electronic circuits and wearable medical devices. Due to high flexibility, direct ink write printing is already widely employed for printing Ga–In alloys. Currently, pneumatic extrusion is the main method of direct ink write printing, but the oxide skin and low viscosity of the Ga–In alloys make it challenging to control after extrusion. This work proposed a method for direct ink write printing of Ga–In alloys utilizing micro-vibration-driven extrusion. Micro-vibration reduces the surface tension of Ga–In alloy droplets and avoids the appearance of random droplets during printing. Under micro-vibration, the nozzle tip pierces the oxide skin to form small droplets which have a high moldability. The droplet growth process is significantly slowed down by optimizing suitable micro-vibration parameters. Therefore, the Ga–In alloy droplets with high moldability can be maintained at the nozzle for a long period, which improves printability. Furthermore, better printing outcomes were obtained with micro-vibrations by choosing the proper nozzle height and printing speed. Experiment results demonstrated the superiority of the method in terms of Ga–In alloys extrusion control. With this method, the printability of the liquid metals is enhanced.

Ultraelastic and High-Conductivity Multiphase Conductor with Universally Autonomous Self-Healing.

Next-generation, truly soft, and stretchable electronic circuits with material level self-healing functionality require high-performance solution-processable organic conductors capable of autonomously self-healing without external intervention. A persistent challenge is to achieve required performance level as electrical, mechanical, and self-healing properties optimized in tandem are difficult to attain. Here heterogenous multiphase conductor with cocontinuous morphology and macroscale phase separation for ultrafast universally autonomous self-healing with full recovery of pristine tensile and electrical properties in less than 120 and 900 s, respectively, is reported. The multiphase conductor is insensitive to flaws under stretching and achieves a synergistic combination of conductivity up to ≈1.5 S cm-1 , stress at break ≈4MPa, toughness up to >81MJ m-3 , and elastic recovery exceeding 2000% strain. Such properties are difficult to achieve simultaneously with any other type of material so far. The solution-processable multiphase conductor offers a paradigm shift for damage tolerant and environmentally resistant soft electronic components and circuits with material level self-healing.

A highly stretchable and conductive continuous composite filament with buckled polypyrrole coating for stretchy electronic textiles

Highly stretchable and conductive fibers as crucial components for stretchy electronic textiles have developed rapidly in recent years, benefiting from the design of stretchable structures. However, routes toward continuous production via simpler fabrication processes remain a huge challenge that has greatly limited its widespread application. In this work, we reported an intrinsically stretchable multifilament with both highly stretchable and conductive (537 S m−1 under 100 % strain) polypyrrole (PPy) coating enhanced by sodium sulfosalicylate (NaSSA) as a dopant. Based on this, a post-drawing method was further proposed to continuously fabricate stretchable conductors with PPy buckling on surface. The PU multifilament with buckle PPy coating (PU@bPPy) exhibited a high and ultra-stable conductance, showing initial conductivity at 529 S m−1 and only 3.5 % change in resistance after > 6500 stretching cycles at 50 % strain. Moreover, the PU@bPPy could be easily integrated into electronic textiles as stretchable electronic circuits and stretchable joule heaters.

Tuning the Mechanical and Electric Properties of Conjugated Polymer Semiconductors: Side‐Chain Design Based on Asymmetric Benzodithiophene Building Blocks

AbstractUnderstanding molecular design rules for stretchable polymer semiconductors is important for enabling next generation stretchable electronic circuits. To simultaneously improve both electrical properties and mechanical stretchability, a design strategy is reported in introducing conjugated rigid fused‐rings with bulky side groups in semiconducting polymers. In this work, the understanding of this design concept is improved by systematically investigating the effect of different types of bulky side groups asymmetrically substituted on conjugated polymer semiconductor backbones. Specifically, four types of side groups are investigated, including naphthalene (NaPh), biphenyl (PhPh), thienylphenyl (ThPh), and alkylphenyl (C4Ph), asymmetrically substituted on benzodithiophene units, namely asy‐BDT. With the four types of side groups installed on BDT‐containing conjugated polymers in an asymmetrical fashion, it is observed that they reduced the polymer chain aggregation and film crystallinity, hence improving the film stretchability. Furthermore, the fully conjugated polymer backbone allows maintenance of good charge carrier mobilities. Specifically, polymer PDPP‐C4Ph (with C4Ph side groups) shows the highest mobility in the fully stretchable transistor and maintained its mobility even after being subjected to hundreds of stretching‐releasing cycles at 25% strain. Overall, the results provide anunderstanding of the use of asymmetrically substituted fused‐ring conjugated polymer structures to tune mechanical and charge transport properties.

Printed and Laser-Activated Liquid Metal-Elastomer Conductors Enabled by Ethanol/PDMS/Liquid Metal Double Emulsions.

Soft electronic systems require stretchable, printable conductors for applications in soft robotics, wearable technologies, and human-machine interfaces. Gallium-based room-temperature liquid metals (LMs) have emerged as promising candidates, and recent liquid metal-embedded elastomers (LMEEs) have demonstrated favorable properties such as stable conductivity during strain, cyclic durability, and patternability. Here, we present an ethanol/polydimethylsiloxane/liquid metal (EtOH/PDMS/LM) double emulsion ink that enables a fast, scalable method to print LM conductors with high conductivity (7.7 × 105 S m-1), small resistance change when strained, and consistent cyclic performance (over 10,000 cycles). EtOH, the carrier solvent, is leveraged for its low viscosity to print the ink onto silicone substrates. PDMS resides at the EtOH/LM interface and cures upon deposition and EtOH evaporation, consequently bonding the LM particles to each other and to the silicone substrate. The printed PDMS-LM composite can be subsequently activated by direct laser writing, forming high-resolution electrically conductive pathways. We demonstrate the utility of the double emulsion ink by creating intricate electrical interconnects for stretchable electronic circuits. This work combines the speed, consistency, and precision of laser-assisted manufacturing with the printability, high conductivity, strain insensitivity, and mechanical robustness of the PDMS-LM composite, unlocking high-yield, high-throughput, and high-density stretchable electronics.

Cellulose Nanofiber Coatings on Cu Electrodes for Cohesive Protection against Water-Induced Short-Circuit Failures

Water is detrimental to electronic devices because it easily causes short circuits. The use of sealing to prevent water permeation is considered as the current conventional solution; however, the trend for developing flexible and stretchable electronic circuits has placed severe demands on waterproof technologies. This report describes a coating that protects circuits and prevents between-wire short circuits, even if the waterproof seals are damaged and the circuit becomes wet. We show that when Cu electrodes are coated with cellulose nanofibers (carboxylate content of 1.8 mmol/g), short circuits between the electrodes do not occur, even if the circuit is submerged in water for 24 h. The cellulose nanofibers accumulate at the anode because of electrophoresis, thereby forming a cohesive cellulose nanofiber layer that prevents short circuits between electrodes. Even if the cellulose nanofiber coating cracks because of external factors, the electrophoretic effect repairs the coating. This failure-containment mechanism is expected to be used in combination with existing waterproofing technology to dramatically improve the reliability of next-generation electronic devices under extreme operating conditions.

Highly stretchable multilayer electronic circuits using biphasic gallium-indium.

Stretchable electronic circuits are critical for soft robots, wearable technologies and biomedical applications. Development of sophisticated stretchable circuits requires new materials with stable conductivity over large strains, and low-resistance interfaces between soft and conventional (rigid) electronic components. To address this need, we introduce biphasic Ga-In, a printable conductor with high conductivity (2.06 × 106 S m-1), extreme stretchability (>1,000%), negligible resistance change when strained, cyclic stability (consistent performance over 1,500 cycles) and a reliable interface with rigid electronics. We employ a scalable transfer-printing process to create various stretchable circuit board assemblies that maintain their performance when stretched, including a multilayer light-emitting diode display, an amplifier circuit and a signal conditioning board for wearable sensing applications. The compatibility of biphasic Ga-In with scalable manufacturing methods, robust interfaces with off-the-shelf electronic components and electrical/mechanical cyclic stability enable direct conversion of established circuit board assemblies to soft and stretchable forms.

Design and analysis of Cu circuit for stretchable electronic circuits using finite element analysis

Stretchable electronic devices are desirable for the next application of electronic devices such as stretchable sensors, stretchable energy storage devices, and humanmachine interfaces. Also, stretchable electronic systems require a stretchable metal circuit on the elastomeric substrate for electrical connection to components. The stretchable circuits fabricated by Cu foil lamination on the thermoplastic polyurethane (TPU) film are investigated with various zigzag-patterned circuit designs. The various circuit designs are simulated by finite element analysis (FEA) for fatigue life prediction and the simulation results are compared with experimental results. The 6 types of zigzag-shaped circuit designs with different corner patterns are investigated. The thicknesses of the Cu electrode and TPU substrate are 18 μm and 50 μm, respectively. The stretchability of Cu stretchable circuits is evaluated by the uniaxial tensile test with 10% elongation. Also, the equivalent plastic strain is calculated for fatigue life prediction. The number of stretch cycles to failure increased as decreasing the equivalent plastic strain obtained by FEA. The tendency of crack propagation and fatigue life prediction are well agreed with FEA results. Thus, it is considered that modifying the circuit patterns of the strain concentration area is the most important to enhance the mechanical reliability of stretchable circuits.

Fabrication of High-resolution Graphene-based Flexible Electronics via Polymer Casting

In this study, a novel method based on the transfer of graphene patterns from a rigid or flexible substrate onto a polymeric film surface via solvent casting was developed. The method involves the creation of predetermined graphene patterns on the substrate, casting a polymer solution, and directly transferring the graphene patterns from the substrate to the surface of the target polymer film via a peeling-off method. The feature sizes of the graphene patterns on the final film can vary from a few micrometers (as low as 5 µm) to few millimeters range. This process, applied at room temperature, eliminates the need for harsh post-processing techniques and enables creation of conductive graphene circuits (sheet resistance: ~0.2 kΩ/sq) with high stability (stable after 100 bending and 24 h washing cycles) on various polymeric flexible substrates. Moreover, this approach allows precise control of the substrate properties such as composition, biodegradability, 3D microstructure, pore size, porosity and mechanical properties using different film formation techniques. This approach can also be used to fabricate flexible biointerfaces to control stem cell behavior, such as differentiation and alignment. Overall, this promising approach provides a facile and low-cost method for the fabrication of flexible and stretchable electronic circuits.

Mechanically Tunable Single-Walled Carbon Nanotube Films as a Universal Material for Transparent and Stretchable Electronics.

Soft, flexible, and stretchable electronic devices provide novel integration opportunities for wearable and implantable technologies. Despite the existing efforts to endow electronics with the capability of large deformation, the main technological challenge is still in the absence of suitable materials for the manufacturing of stretchable electronic circuits and devices with active (sensitive) and passive (stable) components. Here, we present a universal material, based on single-walled carbon nanotube (SWCNT) films deposited on a polydimethylsiloxane (PDMS) substrate, which can act as a material being both sensitive and insensitive to strain. The diverse performance of SWCNT/PDMS structures was achieved by two simple dry-transfer fabrication approaches: SWCNT film deposition onto the as-prepared PDMS and on the prestretched PDMS surface. The correlation between applied strain, microstructural evolution, and electro-optical properties is discussed on the basis of both experimental and computational results. The SWCNT/PDMS material with the mechanically tunable performance has a small relative resistance change from 0.05 to 0.07, while being stretched from 10 to 40% (stable electrode applications). A high sensitivity of 20.1 of the SWCNT/PDMS structures at a 100% strain was achieved (strain sensing applications). Our SWCNT/PDMS structures have superior transparency and conductivity compared to the ones reported previously, including the SWCNT/PDMS structures, obtained by wet processes.

3D-Printed Liquid Metal Interconnects for Stretchable Electronics

Stretchable electronic circuits and systems will be critical for future wearable devices and smart textiles, where existing flexible printed circuit board techniques severely limit conformal deformation. In this paper, we present a scalable fabrication approach for making robust interconnects for stretchable electronics using 3D-printed liquid metal paste. Direct extrusion of the liquid metal paste eliminates the need for expensive lithographic molds while offering improved conductivity over traditional carbon- and metal-based inks. As a proof of concept, we demonstrate a multi-layer stretchable circuit, including both active and passive electronic components. We additionally present highly stretchable strain sensors, demonstrating stable conductivity of the liquid metal paste circuit interconnects with minimal hysteresis over more than 350 cycles at 200% strain. These two devices are combined to demonstrate a printed stretchable strain sensor with integrated readout circuitry.

Liquid electromigration in gallium-based biphasic thin films

Liquid metals have recently gained interest as a material of choice for soft and stretchable electronic circuits, thanks to their virtually infinite mechanical failure strain and high electrical conductivity. Gallium-based thin films are obtained by depositing gallium in the vapor phase to form a class of liquid metal conductors. The films, with an average thickness below 1 µm, withstand mechanical strain in excess of 400%. However, modes of failure other than mechanical ones have not yet been thoroughly investigated. In particular, electromigration, a well-known cause of failure in solid thin film traces for integrated circuits, also occurs in bulk liquid metals. In this work, microscopic observation of the thin conductive traces reveals that gallium is displaced from the anode terminal toward the cathode terminal after direct current stressing. This results in a catastrophic increase in the trace resistance and electrical failure. The mean time to failure decreases with increasing current density, following Black’s equation, an empirical mathematical model originally developed to describe failure in solid metal thin-film tracks due to electromigration. We show that using alternating current, e.g., symmetric square wave, rather than direct current can extend the lifetime of the thin liquid metal film conductor by several orders of magnitude. These results may help stretchable circuit designers who select liquid metal thin-film conductors as the stretchable interconnect technology to predict devices’ lifetime and implement mitigation strategies at the system level or at the material level.

Soft elastomeric composite materials with skin-inspired mechanical properties for stretchable electronic circuits.

The stretchable version of electronic circuits harnesses commercial chip scale components to achieve complex functionality and mechanical deformability, which represents an emerging technology to expand the application of conventional electronics on rigid wafers. The bottleneck lies in the lack of a robust approach for the collective integration of off-the-shelf components into a reliable system. In this study, an elastomeric composite material with skin-like mechanical responses and spatially heterogeneous rigidity is reported as an attractive platform for stretchable circuit systems. The approach utilizes a high modulus microstructure embedded in the matrix of a soft elastomer to achieve programmable mechanical properties, thereby offering selective strain isolation for fragile components and overall protection against excessive loads. A low cost procedure involving laser ablation and blade coating is established to create the composite material matching with the circuit design. In addition, ultrasonic atomization of liquid metal into microparticles allows flexible preparations of deformable conductors in the forms of interconnects and contacts. An LED matrix is demonstrated as a prototype circuit system with excellent durability to withstand repetitive stretching and external impacts. Stretchable circuit systems based on soft elastomeric composite materials may find potential uses in health monitoring, mechatronic prosthetics, and soft robotics.

Geometry Analysis in Screen-Printed Stretchable Interconnects

Deformability of interconnects and conductors as the skeleton of soft electronic devices plays an important role in the stretchability of the whole system. To make the interconnects stretchable, either employing deformable materials or tailoring the design are the primary approaches. It is also possible to combine the two strategies. In advanced stretchable electronic circuits, there is a frequent need for the interconnects to transform from a narrow to a wide geometry. Therefore, this paper assesses three different geometries that accommodate a narrow-to-wide transition along the interconnects. First, the geometry is modeled using finite-element (FE) analysis. Second, in order to verify the accuracy of the FE model, screen-printed interconnects are fabricated accordingly by using silver flake ink on a deformable substrate. The geometrical modification shows a considerable improvement in the stretchability of the whole system. In addition, the effect of encapsulation with thermoplastic polyurethane on the performance of stretchable interconnects is investigated.

A stretchable flexible electronic platform for mechanical and electrical collaborative design

With the extensive application of stretchable flexible electronic circuits to wearable equipment and biomedicine, research on the electrical properties of such circuits has become a hot topic. In this paper, a flexible electronic design platform is established for the first time, realizing co-simulation of the mechanical and electronic aspects. The main functions of this platform include the design of stretchable interconnects, flexible devices, stretchable flexible electronic circuits, and stretchable flexible circuit-layout wiring. The following conclusions can be obtained: (1) With increased applied strain, the inductance of a stretchable interconnect will increase, while the delay and crosstalk will become non-negligible. (2) Although the performance of flexible passive devices does not change after transfer printing, the gate capacitor of the flexible MOS at the cut-off area does decrease. (3) Taking a flexible comparator as an example, the function and electrical performance of a flexible circuit are verified. (4) Taking a flexible amplifier circuit as an example, the flexible interconnection layout and wiring are simulated and verified.

A Platform for Mechano(-Electrical) Characterization of Free-Standing Micron-Sized Structures and Interconnects.

A device for studying the mechanical and electrical behavior of free-standing micro-fabricated metal structures, subjected to a very large deformation, is presented in this paper. The free-standing structures are intended to serve as interconnects in high-density, highly stretchable electronic circuits. For an easy, damage-free handling and mounting of these free-standing structures, the device is designed to be fabricated as a single chip/unit that is separated into two independently movable parts after it is fixed in the tensile test stage. Furthermore, the fabrication method allows for test structures of different geometries to be easily fabricated on the same substrate. The utility of the device has been demonstrated by stretching the free-standing interconnect structures in excess of 1000% while simultaneously measuring their electrical resistance. Important design considerations and encountered processing challenges and their solutions are discussed in this paper.