Stretchable Electronic Circuits Research Articles

(Invited) Fully-Printed Stretchable Electronic Devices and Circuits

Flexible electronics has generated great amount of interest due to its wide range of potential applications in flexible displays, wearable electronics and sensors, implantable biomedical devices, and many more. Despite significant amount of success and progress being made in the field, mechanical flexibility alone may not be sufficient for certain applications. For instance, in order to conformally wrap a surface with nonzero Gaussian curvature, stretchable electronic system is needed. In this talk, I will present our recent work on addressing the two major challenges faced by stretchable electronics - the development of intrinsically-stretchable electronic materials and the need for scalable fabrication processes. We have developed nanomaterials-based metal, semiconductor, and dielectric materials with superior electronic property, stretchability, and inter-layer adhesion. Such materials are formulated as electronic inks to allow highly uniform and scalable material patterning using a fully-printed process, allowing us to directly print high-performance thin-film transistors (TFTs) and logic circuits onto ultrathin elastomer substrates. Electrical and mechanical characterizations reveal that the TFTs and inverters can withstand biaxial strain of up to 125% for hundreds of cycles. The TFTs also exhibit respectable performance with field-effect mobility up to 10 cm2/Vs. This work represents a major step forward in moving the stretchable electronics research from proof-of-concept demonstrations toward practical applications in wearable devices or stretchable displays.

A highly stretchable, helical copper nanowire conductor exhibiting a stretchability of 700%

Jooho Moon, Sunho Jeong and colleagues in South Korea have fabricated conducting meshes and extremely stretchable helices from copper nanowires. Metallic nanowires that are randomly entangled with each other in flexible meshes are of interest as electrical conductors for stretchable electronics. The researchers from Yonsei University and the Korea Research Institute of Chemical Technology fabricated nanowires made of copper with a simple and scalable process based on room-temperature chemical synthesis. When the nanowires were deposited on a flexible polymer substrate, the resulting meshes could stretch to up to twice their original size. When shaped as a helix, the nanowires showed an even greater stretchability of up to 700%. Being more cost-efficient than the silver nanowires used previously for similar applications, these copper nanowires hold a great promise for stretchable electronic circuits or in wearable electronics.

Fabrication of well-controlled wavy metal interconnect structures on stress-free elastomeric substrates

We propose a new technique for fabricating well-controlled wavy surface structures on an elastomeric substrate at a few-micrometer scale without any pre-stretching and deposition steps, as the platform for stretchable metal interconnects. In this process, the wavy structure is defined by photolithography on a stress-free elastomeric substrate, so that we can provide various types of wavy profiles for metal interconnects with arbitrary sizes and orientations within a single substrate. As the wavy structures can be formed only within selected regions while keeping the whole substrate area free of strain, it may be possible to fabricate entire circuitry including active devices directly on the elastomeric substrate with no need for mechanical transfer steps. The present technique can provide a practical strategy for realizing large-area stretchable electronic circuits, for various applications such as stretchable display or wearable electronic systems.

Intrinsically stretchable and rechargeable batteries for self-powered stretchable electronics

Stretchable electronic circuits conform to irregular three dimensional surfaces. They are formed with soft materials and contain electronic circuits, sensors, and other components. We report on a soft matter based rechargeable electrochemical power storage element for such devices. The chemistry is based on a rechargeable alkaline manganese battery concept. The cells withstand more than 700 mechanical stretch relaxation cycles up to 25% strain, with an average cell capacity of 6.5 mA h. Combined with wireless power transmission or stretchable solar cells, the rechargeable battery can be used to store and supply energy in stretchable electronic devices.

SCB and SMI: two stretchable circuit technologies, based on standard printed circuit board processes

PurposeIn the past 15 years stretchable electronic circuits have emerged as a new technology in the domain of assembly, interconnections and sensor circuits and assembly technologies. In the meantime a wide variety of processes with the use of many different materials have been explored in this new field. The purpose of the current contribution is for the authors to present an approach for stretchable circuits which is inspired by conventional rigid and flexible printed circuit board (PCB) technology. Two variants of this technology are presented: stretchable circuit board (SCB) and stretchable mould interconnect (SMI).Design/methodology/approachSimilarly as in PCB 17 or 35 μm thick sheets of electrodeposited or rolled‐annealed Cu are structured to form the conductive tracks, and off‐the‐shelf, standard packaged, rigid components are assembled on the Cu contact pads using lead‐free solder materials and reflow processes. Stretchability is obtained by shaping the Cu tracks not as straight lines, like in normal PCB design, but as horseshoe shaped meanders. Instead of rigid or flexible board materials, elastic materials, predominantly PDMS (polydimethylsiloxane), are used to embed the conductors and the components, thus serving as circuit carrier. The authors include some mechanical modeling and design considerations, aimed at the optimization of the build‐up and combination of elastic, flexible and rigid materials towards minimal stress and maximum mechanical reliability in the structures. Furthermore, details on the two production processes are given, reliability findings are summarised, and a number of functional demonstrators, realized with the technologies, are described.FindingsKey conclusions of the work are that: supporting the metal meanders with a flexible carrier prior to embedding in an elastic substrate substantially increases the reliability under mechanical stress (cyclic uniaxial stretching) of the stretchable interconnect and the transition areas between rigid components and stretchable interconnects are the zones which are most sensitive to failure under mechanical stress. Careful design and technology implementation is necessary, providing a gradual transition from rigid to flexible to stretchable parts of the circuit.Originality/valueTechnologies for stretchable circuits, with the same level of similarity to standard PCB manufacturing and assembly, and thus with the same high potential for transfer to an industrial environment and for mass production, have not been shown before.

Integration of stretchable and washable electronic modules for smart textile applications

Today electronics in “wearable systems” or “smart textiles” are mainly realised on traditional interconnection substrates, like rigid printed circuit boards or mechanically flexible substrates. The electronic modules are detachable to allow cleaning and washing of the textile. In order to achieve a higher degree of integration and user comfort, a technology for flexible and stretchable electronic circuits was developed. The electronic system is completely embedded in an elastomer material like polydimethylsiloxane (PDMS, silicone), resulting in soft and stretchable electronic modules. The technology uses standard packaged components (ICs) and meander-shaped copper tracks, so that stretchable systems with complex functionality can be achieved. This paper describes how these stretchable modules can be integrated in textiles. Also the influence of the PDMS on the stretchability of the textile has been examined. A basic electronic module with blinking light emitting diodes (LEDs) was designed and produced to illustrate the textile integration and to perform initial washing tests.

Printed circuit board technology inspired stretchable circuits

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Stretchable organic integrated circuits for large-area electronic skin surfaces

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Thin-film stretchable electronics technology based on meandering interconnections: fabrication and mechanical performance

A new fabrication technology for stretchable electrical interconnections is presented. This technology can be used to connect various non-stretchable polyimide islands hosting conventional electronic components. The interconnections are realized by patterning a 200 nm thick sputter-deposited gold film into meandering horseshoe shapes, functioning as ‘two-dimensional springs’ when embedded in a silicone elastomer. Polyimide support is introduced around the meandering conductors as a means to improve the mechanical performance. Processing is done on a temporary carrier; the islands and interconnections are embedded in polydimethylsiloxane in a final stage. To this end, a release technique compatible with high temperatures up to 350 based on the evaporation of a 400 nm thick layer of potassium chloride is developed. Test structures consisting of stretchable interconnections with a varying polyimide support width were fabricated. These were strained up to twice their original length without compromising their functionality. Also cyclic mechanical loading at various strains was performed, indicating the influence of the polyimide support width on the lifetime. At strains of 10%, a minimum lifetime of 500 000 cycles is demonstrated. The presented technology thus provides a promising route towards the fabrication of stretchable electronic circuits with enhanced reliability.

Stretching-induced interconnect delamination in stretchable electronic circuits

Stretchable electronics offer increased design freedom of electronic products. Typically, small rigid semiconductor islands are interconnected with thin metal conductor lines on top of, or encapsulated in, a highly compliant substrate, such as a rubber material. A key requirement is large stretchability, i.e. the ability to withstand large deformations during usage without any loss of functionality. Stretching-induced delamination is one of the major failure modes that determines the amount of stretchability that can be achieved for a given interconnect design. During peel testing, performed to characterize the interface behaviour, the rubber is severely lifted at the delamination front while at the same time fibrillation of the rubber at the peel front is observed by ESEM analyses. The interface properties are established by combining the results of numerical simulations and peeling experiments at two distinct scales: the global force–displacement curves and local rubber lift geometries. The thus quantified parameters are used to predict the delamination behaviour of zigzag- and horseshoe-patterned interconnect structures. The accuracy of these finite element simulations is assessed by a comparison of the calculated evolution of the shape of the interconnect structures and the fibrillation areas during stretching with experimental results obtained by detailed in situ analyses.

Stretchable and Washable Electronics for Embedding in Textiles

AbstractElectronics in “wearable systems” or “smart textiles” are nowadays mainly realized on traditional interconnection substrates, like rigid Printed Circuit Boards (PCB) or mechanically flexible substrates. The electronic modules are detachable to allow cleaning and washing of the textile. In order to achieve a higher degree of integration and user comfort, IMEC-UGent/CMST developed a technology for flexible and stretchable electronic circuits. The electronic system is completely embedded in an elastomer material like PDMS (silicone), resulting in soft and stretchable electronic modules. The technology uses standard packaged components (IC's) and meander shaped copper tracks, so that stretchable systems with complex functionality can be achieved. Testing methods for washability were selected and developed. First tests are showing promising results, leveling the path to washable electronics in textiles. In order to show the possibilities of the technology in the field of textile applications a 7x8 single color stretchable LED-matrix was designed and integrated in textile. This LED-matrix can be applied for example in wearable signage applications.

Design and performance of metal conductors for stretchable electronic circuits

PurposeThe purpose of this paper is to present an update on the progress of the design and reliability of stretchable interconnections for electronic circuits.Design/methodology/approachFinite element modelling (FEM) is used to analyse the physical behaviour of stretchable interconnects under different loading conditions. The fatigue life of a copper interconnect embedded into a silicone matrix has been evaluated using the Coffin‐Manson relation and FEM.FindingsThe mechanical properties of the substrate and the design of the metal interconnection play an important role on the fatigue lifetime of circuit. In the case of copper embedded into a PDMS Sylgard 186, more than 2,500 tensile cycles have been observed for a periodic deformation of 10 per cent.Research limitations/implicationsReliability results are limited and need further work to create a more accurate empirical model to estimate the lifetime of stretchable interconnections.Originality/valueThe combined use of FEM and experimental analysis enable a more reliable design of the stretchable metal interconnections. The proposed horseshoe design offers the benefit of reduced permanent damage during elongation.

Design and Fabrication of Elastic Interconnections for Stretchable Electronic Circuits

For biomedical and textile applications, the comfort of the user will be enhanced if the electronic circuits are not only flexible but also elastic. This letter reveals a simple moulded-interconnect-device technology for the construction of elastic point-to-point interconnections, based on 2-D spring-shaped metallic tracks, which are embedded in a highly elastic silicone film. Metal interconnections of 3-cm long were constructed with an initial resistance of about 3Omega , which did not significantly increase (<5%) when stretched. A stretchability above 100% in one direction has been demonstrated.

High‐Conductivity Elastomeric Electronics

Stretchable electronic circuits have been microfabricated using tortuous gold wires embedded in silicone elastomer. With appropriate geometric parameters, the wires could elongate by more than 50 % while maintaining stable conductivity (see Figure). This technology represents a key development towards conductive fabrics, flexible displays, and other applications that require electronics to withstand unprecedented degrees of stress and vibration.

Super-elastic Gold Conductors on Elastomeric Substrates

Abstract3-D displays, sensor skins, mechatronic structures, and e-textiles will rely on deformable and stretchable electronic circuits. It is likely that such circuits will be made of rigid semiconductor islands interconnected with one-time deformable or even elastic metallization. However, free-standing metal films fracture at tensile strains in the order of one percent, well short of the approximately ten-percent extension needed for deformable circuits. We have discovered that flat metal lines made on an elastomeric substrate can be stretched reversibly by ten percent without losing electrical conduction. While this phenomenon of “super-elastic” conductive films is as of yet unexplained, it appears to originate in the diversion of mechanical strain around cracks in the film and through the elastomer substrate. We fabricated 1mm thick poly-dimethylsiloxane (PDMS) membranes with up to 50-nm thick, 1-mm wide gold lines deposited by electron beam evaporation. Then we evaluated the structure of the gold films by optical and scanning electron microscopy, and measured the electro-mechanical characteristics in a strain tester, with contact electrodes applied to the film. We find that 50 nm thick lines retain their electrical conduction up to 30 percent strain. Also, when the tensile strain is cycled between 0 and 10 percent, the electrical resistance in the stressed and relaxed states are reproducible. We will describe substrate and conductor preparation, and their structural and electro-mechanical properties.