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Due to constant advancements in materials research, conductive textile-based materials have been used increasingly in textile-based wearables. However, due to the rigidity of electronics or the need for their encapsulation, conductive textile materials, such as conductive yarns, tend to break faster around transition areas than other parts of e-textile systems. Thus, the current work aims to find the limits of two conductive yarns woven into a narrow fabric at the electronics encapsulation transition point. The tests consisted of repeated bending and mechanical stress, and were conducted using a testing machine built from off-the-shelf components. The electronics were encapsulated with an injection-moulded potting compound. In addition to identifying the most reliable conductive yarn and soft–rigid transition materials, the results examined the failure process during the bending tests, including continuous electrical measurements. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach for removing out-of-plane deformation in stretched metal interconnects by adding a fractal structure to the original meander shape and using an optimized fabrication stack. In thermoformed electronics, in cases where copper is used as conductor, the twisting of meander-shaped structures caused by excessive mechanical stress can cause a non-uniform surface, delamination of the metal interconnect from the substrate, and in some cases, a short circuit to the adjacent tracks. Typically, designers of stretchable electronics use various shapes and widths of the copper interconnect to tackle this issue. Using conventional meander shapes such as horseshoes and U-shapes is not universally practical, especially when stretching is higher than 30 percent leading to significant out-of-plane buckling. Limiting this out-of-plane buckling by reducing the track width is not always applicable, as a minimum width is needed from a technology and conductivity perspective. The presented approach is inspired by computational and experimental studies of multiple meander shapes and fabrication methods. A geometry- and fabrication-based approach is presented, reducing the mechanical stress of almost all possible meander shapes by increasing the meander's path length to accommodate the metal track's produced torque during stretching. An analytical approach is provided for calculating the optimal meander parameters and the optimal fabrication stack is achieved based on simulation results. Experiments and finite element modeling for a case study show the improvement in the stress distribution and reduction of out-of-plane buckling. [ABSTRACT FROM AUTHOR]

The smart textiles and wearable technology markets are expanding tirelessly, looking for efficient solutions to create long-lasting products. The research towards novel integration methods and increasing reliability of wearables and electronic textiles (e-textiles) is expanding. One obstacle to be tackled is the washability and the endurance to mechanical stresses in the washing machine. In this article, different layering of thermoplastic polyurethane (TPU) films and knit fabrics are used to integrate three different designs of stretchable copper-based meander tracks with printed circuit boards. The various combinations are washed according to the ISO 6330-2012 standard to analyze their endurance. Results suggest that one meander design withstands more washing cycles and indicate that the well-selected layer compositions increase the reliability. Higher stretchability together with greater durability is accomplished by adding an extra meander-shaped TPU film layer. [ABSTRACT FROM AUTHOR]

Electronic textiles (e-textiles) and wearable computing have been emerging increasingly during the last decade. Since the market interest and predictions have grown, the research into increasing reliability and durability of wearables and e-textiles is developing rapidly. The washability of different integration methods and resistance to mechanical stress are the main obstacles being tackled. However, the freedom of movement and overall comfort is still often overlooked during the development phase. It is essential to see the e-textile product as a whole and consider several aspects of user experience. This work will focus on developing and improving the thermoplastic polyurethane (TPU) lamination integration method for e-textiles. In the work, a stretchable copper-polyimide based circuit was laminated onto knit fabric using various TPU films and stacks. The study shares measurable characteristics to determine which material assembly and design would ensure the highest durability for the electronics part without losing its original textile softness, flexibility and stretchability. [ABSTRACT FROM AUTHOR]

The development and mechanical characterization of a novel technology for stretchable electronics is presented, which can be used for the realization of wearable textile electronics and biomedical implants. The stretchable devices consist of rigid or flexible component islands interconnected with stretchable meander-shaped copper conductors embedded in a stretchable polymer, polydemethylsiloxane. The technology uses standard printed circuit board manufacturing steps and liquid injection molding techniques to achieve a robust and reliable product. The conductors in the device are designed to accommodate strains up to 10–15%. Spin-on photo-definable polyimide as mechanical support for the stretchable interconnects and the functional flexible islands are introduced. By use of polyimide, the reliability of the stretchable interconnects, the straight interconnects on the flexible islands and the transitions between the stretchable and nonstretchable parts are improved. Long-term endurance behavior of the stretchable interconnects is studied by cyclic elongation at strain ranges of up to 20% while monitoring the electrical connectivity. It's shown that the lifetime of the polyimide supported interconnects is at least two times better compared to the nonsupported. [ABSTRACT FROM AUTHOR]

This paper discusses the optimization of a stretchable electrical interconnection between integrated circuits in terms of stretchability and fatigue lifetime. The interconnection is based on Cu stripes embedded in a polyimide-enhanced (PI-enhanced) layer. Design-of-experiment (DOE) methods and finite-element modeling were used to obtain an optimal design and to define design guidelines, concerning both stripe and layer dimensions and material selection. Stretchable interconnects with a PI-enhanced layer were fabricated based on the optimized design parameters and tested. In situ experimental observations did validate the optimal design. Statistical analysis indicated that the PI width plays the most important role among the different design parameters. By increasing the PI width, the plastic strain in the Cu stripes is reduced, and thus, the stretchability and fatigue lifetime of the system is increased. The experimental results demonstrate that the PI-enhanced stretchable interconnect enables elongations up to 250% without Cu rupture. This maximum elongation is two times larger than the one in samples without PI enhancement refid="ref1"/ . Moreover, the fatigue life at 30% elongation is 470 times higher refid="ref2" . [ABSTRACT FROM PUBLISHER]

Electronic devices capable of performing in extreme mechanical conditions such as stretching, bending, or twisting will improve biomedical and wearable systems. The required capabilities cannot be achieved with conventional building geometries, because of structural rigidity and lack of mechanical stretchability. In this article, a zigzag-patterned structure representing a stretchable interconnect is presented as a promising type of building block. In situ experimental observations on the deformed interconnect are correlated with numerical analysis, providing an understanding of the deformation and failure mechanisms. The experimental results demonstrate that the zigzag-patterned interconnect enables stretchability up to 60% without rupture. This stretchability is accommodated by in-plane rotation of arms and out-of-plane deformation of crests. Numerical analysis shows that the dominating failure cause is interfacial in-plane shear stress. The plastic strain concentration at the arms close to the crests, obtained by numerical simulation, agrees well with the failure location observed in the experiment. [ABSTRACT FROM AUTHOR]

Abstract: Tactile sensors are often used on irregular or moving surfaces, for example as artificial skin on a robotic hand or inside a prosthetic socket. In addition to tactile sensors able to detect pressure, there is also a need to detect shear stress, associated with slippage or friction. Therefore, this paper demonstrates a technology for fabricating unobtrusive flexible tactile shear sensors using a thin and mechanically strong polyimide substrate. The operation of the sensor is based on the changing optical coupling between a Vertical-Cavity Surface-Emitting Laser (VCSEL) and photodiode. Since this sensor is based on an optical principle, it is less susceptible to environmental influences and electromagnetic interference (EMI). Furthermore, a new design of this sensor was developed and tested to enable the detection of both shear force magnitude and direction (two axis sensor). [Copyright &y& Elsevier]

Abstract: The trend of microelectronic products in the textile or medical field is toward higher functionality, miniaturization, application of new materials and a necessity for deformable electronic circuits for improving the comfort control. In this work, the design of flexible and stretchable interconnections is presented. These interconnections are done by embedding sinuous electroplated metallic wires in a stretchable substrate material. A silicone material was chosen as substrate because of its low stiffness and high elongation before break. Common metal conductors used in the electronic industry have very limited elastic ranges; therefore a metallization design is crucial to allow stretchability of the conductors going up to 100%. Different configurations were simulated and compared among them and based on these results, a horseshoe like shape was suggested. This design allows a large deformation with the minimum stress concentration. Moreover, the damage in the metal is significantly reduced by applying narrow metallization schemes. In this way, each conductor track has been split in four parallel lines of 15μm and 15μm space in order to improve the mechanical performance without limiting the electrical characteristics. [Copyright &y& Elsevier]