Stretchable Interconnects Research Articles

Milliwatt-Scale Body-Heat Harvesting Using Stretchable Thermoelectric Generators for Fully Untethered, Self-Sustainable Wearables

Stretchable thermoelectric generators (s-TEGs) have been regarded as promising energy harvesters for self-powered wearable electronics. However, previous s-TEGs show low power generation capacity due to their high module resistances, originating from the poor electromechanical interfaces between rigid–soft components and the high electrical resistances of stretchable interconnects. Herein, we report strategies to boost thermoelectric performance, which allows us to operate wireless communication systems from body heat by generating a power of 2.6 mW. Electromechanically graded interlayers that mediate discrete functionalities at the interfaces effectively reduce junction resistances, and solution-based welding that transforms scattered networks into mesh-like structures produces highly conductive and strain-resilient interconnects, respectively. Soft heat conductors are included to improve thermal interfaces, minimizing thermal impedance of elastomeric substrates. Consequently, the power generation capacity is significantly enhanced, exhibiting the highest normalized power density of 1.48 μW cm–2 K–2 among reported high-performance s-TEGs. Our s-TEGs provide realistic solutions for sustainable self-powered electronics.

A concept of stretchable coaxial cable based on one-body Au nanonetworks

Recently, the demand for wearable electronics has sharply been increasing for diverse purposes. For the entire system to process enormous data simultaneously between the devices, the importance of stretchable interconnects with large bandwidth has been emphasized. However, achieving an appropriate attenuation level as well as stable transmission performance against elongation is still challenging. This report suggests the stretchable coaxial cable based on the one-body metal network, which is intrinsically stretchable and highly conductive. By multiple wrapping of the one-body metal network, excellent connectivity and electrical conductivity were realized. Its sheet resistance reached as low as that of the solid cylindrical metal. The low-loss cable was obtained through a simple fabrication procedure; its attenuation at 1 GHz remained under 0.011 and 0.033 dB/mm against 0% and 20% elongation, respectively. Also, it only experienced an insignificant change of the attenuation level (∼3.0%) during 100 stretching/releasing cycles, implying its feasibility of the dynamic connections in prospective applications.

High‐resolution active‐matrix micro‐LED stretchable displays

AbstractThis paper describes the development of 12‐in. 100 PPI active‐matrix micro‐LED stretchable displays, capable of being stretched up to 20%. Our work has involved the development of key technologies such as stretchable interconnects, stretchable films, and micro‐LEDs and the integration process for these components. The stretchable displays exhibited various types of deformations, suggesting the potential for new applications in information display.

Interfacial Capillary Spooling of Conductive Polyurethane-Silver Core-Sheath (PU@Ag) Microfibers for Highly Stretchable Interconnects.

Conductive fibers are core materials in textile electronics for the sustainable operation of devices under mechanical stimuli. Conventional polymer-metal core-sheath fibers were employed as stretchable electrical interconnects. However, their electrical conductivity is severely degraded by the rupture of metal sheaths at low strains. Because the core-sheath fibers are not intrinsically stretchable, designing a stretchable architecture of interconnects based on the fibers is essential. Herein, we introduce nonvolatile droplet-conductive microfiber arrays as stretchable interconnects by employing interfacial capillary spooling, motivated by the reversible spooling of capture threads in a spider web. Polyurethane (PU)-Ag core-sheath (PU@Ag) fibers were prepared by wet-spinning and thermal evaporation. When the fiber was placed on a silicone droplet, a capillary force was generated at their interface. The highly soft PU@Ag fibers were fully spooled within the droplet and reversibly uncoiled when a tensile force was applied. Without mechanical failures of the Ag sheaths, an excellent conductivity of 3.9 × 104 S cm-1 was retained at a strain of 1200% for 1000 spooling-uncoiling cycles. A light-emitting diode connected to a multiarray of droplet-PU@Ag fibers exhibited stable operation during spooling-uncoiling cycles.

Silver-Nanowire-Based Elastic Conductors: Preparation Processes and Substrate Adhesion.

The production of flexible electronic systems includes stretchable electrical interconnections and flexible electronic components, promoting the research and development of flexible conductors and stretchable conductive materials with large bending deformation or torsion resistance. Silver nanowires have the advantages of high conductivity, good transparency and flexibility in the development of flexible electronic products. In order to further prepare system-level flexible systems (such as autonomous full-software robots, etc.), it is necessary to focus on the conductivity of the system's composite conductor and the robustness of the system at the physical level. In terms of conductor preparation processes and substrate adhesion strategies, the more commonly used solutions are selected. Four kinds of elastic preparation processes (pretensioned/geometrically topological matrix, conductive fiber, aerogel composite, mixed percolation dopant) and five kinds of processes (coating, embedding, changing surface energy, chemical bond and force, adjusting tension and diffusion) to enhance the adhesion of composite conductors using silver nanowires as current-carrying channel substrates were reviewed. It is recommended to use the preparation process of mixed percolation doping and the adhesion mode of embedding/chemical bonding under non-special conditions. Developments in 3D printing and soft robots are also discussed.

Silicon-Based Stretchable Structure via Parylene Kirigami Interconnection

High-performance stretchable electronics are attracting great attention due to the mechanical adaptability, but are still limited to achieving highly-integrable, high density, high-performance stretchable silicon-based electronics. This paper proposed a Parylene Kirigami structure as a stretchable interconnection to connect silicon (Si) units in an array. The equivalent Young’s modulus and stretchability model of the Parylene Kirigami were developed and verified numerically and experimentally. As a preliminary demonstration, a Parylene Kirigami structure-based stretchable Si array (Parylene KiSS array) was successfully fabricated by the wafer-scale microelectromechanical systems (MEMS) process with a high Si areal coverage of 68.8 % and the stretchability of 31.0 ± 1.1%. The equivalent Young’s modulus of this Parylene KiSS array was 130.88 ± 6.75 MPa, which was estimated precisely by the proposed model and was around 1000 times smaller than that of the intrinsic silicon. This MEMS process compatible stretchable electronics strategy can integrate Si-based high-performance chiplets, sensors and actuators to realize high-performance stretchable microsystems. [2022-0157]

Flexible-to-Stretchable Mechanical and Electrical Interconnects

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.

3D Printing of Conductive Flexible Filaments for E-Textile Applications

Electronic textiles (e-textiles) can incorporate conductive materials at all levels of integration, from fibers to yarns to fabric itself. There are many ways to connect the textile elements to electronic components with interconnect mechanisms from mechanical gripping to welding, to gluing, to printing, to embroidery, knitting, and weaving. 3D printing method offers the possibility of creating flexible and stretchable interconnects for e-textiles applications. This study explored 3D printing of flexible conductive filaments on fabric to create interconnects for hard electrical components as well as transmission lines and switches for electronic textile applications. NinjaTek Eel and Palmiga TPU based conductive filaments were printed on polyester knit fabric. Electrical characterization measurements as well as visual and haptic analysis of printed samples were conducted. The results showed that TPU based flexible conductive filaments offer possibilities of direct 3D printing onto textiles for electronic textile applications.

Reliability of printed stretchable electronics based on nano/micro materials for practical applications.

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.

Ultrahigh Strain-Insensitive Integrated Hybrid Electronics Using Highly Stretchable Bilayer Liquid Metal Based Conductor.

Human-interfaced electronic systems require strain-resilient circuits. However, present integrated stretchable electronics easily suffer from electrical deterioration and face challenges in forming robust multilayered soft-rigid hybrid configurations. Here, a bilayer liquid-solid conductor (b-LSC) with amphiphilic properties is introduced to reliably interface with both rigid electronics and elastomeric substrates. The top liquid metal can self-solder its interface with rigid electronics at a resistance 30% lower than the traditional tin-soldered rigid interface. The bottom polar composite comprising liquid metal particles and polymers can not only reliably interface with elastomers but also help the b-LSC heal after breakage. The b-LSC can be scalably fabricated by printing and subsequent peeling strategies, showing ultra-high strain-insensitive conductivity (maximum 22532Scm-1 ), extreme stretchability (2260%), and negligible resistance change under ultra-high strain (0.34 times increase under 1000% strain). It can act as stretchable vertical interconnect access for connecting multilayered layouts and can be scalably and universally fabricated on various substrates with a resolution of ≈200µm. It is demonstrated that it can construct stretchable sensor arrays, multi-layered stretchable displays, highly integrated haptic user-interactive optoelectric E-skins, visualized heaters, robot touch sensing systems, and wireless powering for wearable electronics.

Stretchable and Transparent Interconnects With Hybrid Ag-Nanomesh/Indium Tin Oxide Structure

We demonstrate a hybrid structure for both transparent and stretchable interconnects, that provides high conductance, stretchability, and transparency simultaneously. In this structure, a serpentine-shaped discrete Ag nanomesh formed by the nanotransfer printing technique works as the primary conducting layer, while an indium tin oxide thin film covering it allows planar contact with other devices for proper functions as interconnects. We have confirmed that the hybrid serpentine interconnects provide high conductance and stretching reliability comparable to that of opaque ductile metals, and the transmittance and sheet resistance can be controlled independently by modifying the structural parameters of the mesh layer. Based on conventional photolithography and etching processes compatible with the current flexible circuit technology, the new hybrid interconnect can be utilized in various devices requiring highly integrated transparent and stretchable circuits.

Intrinsically Conductive Liquid Metal‐Elastomer Composites for Stretchable and Flexible Electronics

AbstractLiquid metal‐embedded elastomers (LMEEs) are a class of deformable composites made of particles of liquid metal dispersed in an elastomeric matrix. Although these composites possess high thermal conductivity, they are not intrinsically electrically conductive unless a stimulus is applied to join the liquid metal inclusions into a conductive pathway. LMEEs with intrinsic conductivity, especially with a conductive surface, have great potential uses in flexible and stretchable electronics as soft, nondamaging contacts for device characterization, stretchable interconnects for deformable circuits, and as a “soft solder” to electrically connect devices to flexible and stretchable substrates. Here, a simple process is introduced to fabricate intrinsically conductive LMEEs (iLMEEs) with conductive surfaces through the sedimentation of microparticles of eutectic gallium‐indium alloy (EGaIn) in the elastomer poly(dimethylsiloxane). During this sedimentation process, an EGaIn‐rich 3D percolation network forms at the bottom surface. The resulting iLMEE possesses a conductive surface comprising a mosaic of EGaIn particles embedded in PDMS, with a low sheet resistance of 0.63 ± 0.04 Ω sq–1. iLMEE is soft, stretchable, and exhibits stable conductivity to 100% strain. We demonstrate the use of iLMEE as nondamaging, reusable soft electrical contact probes and as mechanically robust electrical connections between light‐emitting devices and flexible plastic substrates.

Thin PDMS-on-Sacrificial-PCB Devices

Poly(dimethylsiloxane) (PDMS) devices must often be so thin (e.g., <100 μm) and, therefore, so fragile that their mechanical release becomes challenging. As a simple solution, PDMS devices are frequently released by dissolving an underlying sacrificial resist. However, there may be several other issues, including temperature gradients during fabrication, electrical characterization, handling, co-integration with electronics, fabrication time, and cost. Here, we show that conventional printed circuit boards (PCBs) can be ideal sacrificial carriers, which can also perform useful functions during fabrication. The thick (e.g., 35 μm) and thermally conductive PCB copper layer behaves as an excellent heat spreading plate during temperature-sensitive process steps (e.g., PDMS curing and sintering of conductive inks) and enables pre-release electrical stimulations and characterizations of PDMS, which may help during process development. Experiments and finite element method (FEM) simulations confirm that the PCB copper layer can improve temperature uniformity. The UV-protecting film attached to the PCB can be cut to constitute a frame for easy handling. The PCB photoresist enables the straightforward release of PDMS by acetone, which is among the most environmentally friendly chemicals. Contacts can be opened by a simple bump-cut-peel strategy. As proofs of concept, we demonstrate pre-release capacitive characterization of PDMS for evaluating the ability to fabricate thin, but uniform and robust devices and post-release resistive characterization of stretchable strain sensor or interconnects encapsulated in PDMS. The proposed approach is simple, cheap, and environmentally friendly, can improve temperature uniformity throughout the pre-release process steps, enables pre-release electrical characterizations, can simplify co-integration with electronics and can be generalized to other elastomers.

Silicone Rubber Based-Conductive Composites for Stretchable “All-in-One” Microsystems

Wearable electronics with development trends such as miniaturization, multifunction, and smart integration have become an important part of the Internet of Things (IoT) and have penetrated various sectors of modern society. To meet the increasing demands of wearable electronics in terms of deformability and conformability, many efforts have been devoted to overcoming the nonstretchable and poor conformal properties of traditional functional materials and endowing devices with outstanding mechanical properties. One of the promising approaches is composite engineering in which traditional functional materials are incorporated into the various polymer matrices to develop different kinds of functional composites and construct different functions of stretchable electronics. Herein, we focus on the approach of composite engineering and the polymer matrix of silicone rubber (SR), and we summarize the state-of-the-art details of silicone rubber-based conductive composites (SRCCs), including a summary of their conductivity mechanisms and synthesis methods and SRCC applications for stretchable electronics. For conductivity mechanisms, two conductivity mechanisms of SRCC are emphasized: percolation theory and the quantum tunneling mechanism. For synthesis methods of SRCCs, four typical approaches to synthesize different kinds of SRCCs are investigated: mixing/blending, infiltration, ion implantation, and in situ formation. For SRCC applications, different functions of stretchable electronics based on SRCCs for interconnecting, sensing, powering, actuating, and transmitting are summarized, including stretchable interconnects, sensors, nanogenerators, antennas, and transistors. These functions reveal the feasibility of constructing a stretchable all-in-one self-powered microsystem based on SRCC-based stretchable electronics. As a prospect, this microsystem is expected to integrate the functional sensing modulus, the energy harvesting modulus, and the process and response modulus together to sense and respond to environmental stimulations and human physiological signals.

Combination of Micro-Corrugation Process and Pre-Stretched Method for Highly Stretchable Vertical Wavy Structured Metal Interconnects.

Metal interconnects with a vertical wavy structure have been studied to realize high-density and low-electric-resistance stretchable interconnects. This study proposed a new method for fabricating vertical wavy structured metal interconnects that comprises the pre-stretch method and the micro-corrugation process. The pre-stretch method is a conventional method in which a metal film is placed on a pre-stretched substrate, and a vertical wavy structure is formed using the return force of the substrate. The micro-corrugation process is a recent method in which a metal foil is bent vertically and continuously using micro-gears. In the proposed method, the pitch of the vertical wavy structured interconnect fabricated using the micro-corrugation process is significantly narrowed using the restoring force of the pre-stretched substrate, with stretchability improvement of up to 165%, which is significantly higher than that of conventional vertical wavy structured metal interconnects. The electrical resistance of the fabricated interconnect was low (120–160 mΩ) and stable (±2 mΩ or less) until breakage by strain. In addition, the fabricated interconnect exhibits durability of more than 6500 times in a 30% strain cycle test.

Printing Liquid Metal Elastomer Composites for High‐Performance Stretchable Thermoelectric Generators

AbstractContinuous powering of wearable electronics and personalized biomonitoring systems remains a great challenge. One promising solution is the use of thermoelectric generators (TEGs) that convert body heat to electricity. These energy harvesters must conform to curved surfaces and minimize thermal barriers to maintain efficiency while still exhibiting durability under large deformations. Here, highly efficient, stretchable thermoelectric generators made of inorganic semiconductors and printed multifunctional soft matter are introduced. Liquid metal elastomer composites with tailored microstructures are printed as highly conductive thermal interface materials and stretchable interconnects. Additionally, elastomer composites with hollow microspheres are formulated to print a deformable and lightweight thermal insulator within the device. These stretchable thermoelectric wearables show an excellent performance by generating an open‐circuit voltage of 392 mV and a power density of ≈650 µW cm−2 at ∆T = 60 °C and withstanding more than 15 000 stretching cycles at 30% strain. Furthermore, the additive manufacturing process is leveraged by direct writing of the TEGs on textiles to demonstrate their seamless integration and by 3D printing of stretchable heatsinks to maintain a large temperature gradient across the device and to study the effect of convective heat transfer on device performance.

Reliable Lego®-style assembled stretchable photovoltaic module for 3-dimensional curved surface application

To extend the applications of photovoltaics in the urban environment, the one-size-fits-all strategies based on watt-per-cost concepts should shift toward customized energy-yield-per-watt strategies to meet various requirements for new applications such as vehicle-integrated photovoltaics (VIPVs) and building-integrated photovoltaics (BIPVs). From this viewpoint, a customizable lightweight photovoltaic module with high performance and reliability and that can be applied to curved surfaces has attracted much attention. To satisfy the requirements for such devices, we have introduced a Lego®-style assembled module that can be assembled on curved surface using glass-free encapsulated solar-cell units and stretchable interconnection units similar to those of Lego® block construction. The developed modules exhibit 20% energy conversion efficiency and high reliability, including the ability to endure a compressive force of 5000 N and to maintain photovoltaic performance in an environment at 85 ℃ and 85% relative humidity for 500 h. The proposed module concept is promising for satisfying the requirements for next-generation photovoltaic applications, including BIPVs and VIPVs, because of the modules’ free-design possibilities, which make them compatible with three-dimensional curved surfaces, and good performance with high reliability.

High‐Conductivity Crack‐Free 3D Electrical Interconnects Directly Printed on Soft PDMS Substrates

AbstractNanoparticle 3D printing and sintering is a promising method to achieve freeform interconnects on compliant substrates for applications such as soft robotics and wearable healthcare devices. However, previous strategies to sinter metallic nanoparticles while preserving the soft polymer substrate are rife with problems such as cracking and low conductivity of the metallic features. In this paper, the mechanisms of cracking in nanoparticle‐based 3D printed and sintered stretchable interconnects are identified and architecture and processing strategies are demonstrated to achieve crack‐free interconnects fully embedded in thin (&lt;100 μm in thickness) stretchable polydimethylsiloxane (PDMS) with external connectivity. Capillary forces between nanoparticles developed through rapid solvent evaporation in the colloidal ink is hypothesized to initiate cracking during drying. Additionally, the presence of oxygen promotes the removal of organic surfactants and binders in the nanoparticle ink which increases nanoparticle agglomeration, grain growth, and subsequently conductivity. An experimental step‐wise variation of the thermal/atmospheric process conditions supports this hypothesis and shows that the presence of air during a low temperature drying step reduces the capillary stress to produce crack‐free interconnects with high conductivities (up to 56% of bulk metal) while having an excellent compatibility with the underlying polymer materials. Finally, stretchable interconnects fully‐encapsulated in PDMS polymer, with 3D pillar architectures for external connectivity are demonstrated, thus also solving an important “last‐mile” problem in the packaging of stretchable electronics.

46‐4: Student Paper: Reconfigurable and Reusable Soft Modular LED Blocks Assembly

We present a soft modular LED blocks assembly approach that is reconfigurable and reusable based on sticky elastomeric layers and stretchable interconnectors. The soft modular LED blocks are electrically connected by stretchable interconnects. We demonstrated various reconfigurations of modular LED blocks, including three by three LEDs onto a curvilinear surface.

41‐2: Invited Paper: Active Matrix Micro‐LED Stretchable Display and Technical Challenges

In this paper, we report a 13‐inch active matrix (AM) micro‐LED stretchable display, which can be stretched by up to 20% with a 72 ppi resolution. It features a unique panel de sign, including stretchable interconnects fabricated via the conventional LTPS backplane process and micro‐LED for a light emitting device. This paper will discuss methods for realizing a stretchable panel and practical problems in stretchable display engineering.