In-Fiber Micro-Devices and Stretchable Interconnects for Textile-Based Electronics

Textile-based electronics hold promise for wearable sensors and devices, as well as large area flexible and conformable electronics. Their outstanding mechanical properties arise from a combination of two elements: the macroscopic structure of the knit or weave and the microscopic mechanical properties of the yarns that constitute the fabric. In particular, yarns in an electronic textile must withstand the large strains and bending associated with weaving, knitting, and daily use, while maintaining their functional integrity.<br/>Thanks to the miniaturization of rigid, silicon-based technologies, high performance microscopic chips can be embedded into polymeric fibers to bring sensing, communication, or data storage capabilities into a yarn, while minimally affecting its mechanical properties. However, this approach presents an outstanding design and manufacturing challenge which is building soft and stretchable interconnects that are constrained within the one-dimensional geometry of a yarn and exhibit high conductivity to connect devices over hundreds of meters of fiber. This study focuses on solid metal conductors, which exhibit conductivities orders of magnitude higher than polymer nanocomposites or conductive polymers and eliminate challenges associated with liquid metals such as potential leakage. Structural elasticity, which relies on bending deformation of curved shapes, has allowed for stretchable metal electrodes in planar and three-dimensional devices. However, because the processes used to make textile fibers inherently involve high elongation and axial alignment, building structural elasticity into a fiber, at first, appears impossible. To meet this challenge, metal electrodes are incorporated into hollow channels of thermally drawn elastomer fibers. Then, buckling induced in the microwires results in tens of meters of fibers with highly conductive wavy electrodes and up to 50% elasticity. After validation of the mechanical and electrical properties of the electrodes, a process is developed to connect microchips to the wires and draw them inside a fiber in a single step. One, two, or four buckled electrodes are connected to a single device. Device density and fiber elasticity are precisely tuned.<br/>Two electronic fabrics are demonstrated applying this process. A network of woven fibers with thermistor chips make up a localized temperature sensing fabric, while integrating microscale light emitting diodes in fibers enables the first soft and stretchable LiFi fabric antenna. The fibers are incorporated into the fabrics using automated knitted and weaving machines, thus proving that the fibers are robust enough to undergo large-scale textile processes.