<p>Driven by the increasing demand for continuous health monitoring, fitness tracking, and virtual reality, among others, there has been a fast-growing market for wearable electronics. Large stretching of the human body during daily movement requires wearable devices to have an appropriate stretchability to ensure mechanical robustness, signal quality, and wear comfort. This requests that stretchable conductors working as electrodes or interconnects should exhibit nearly constant electrical properties when undergoing large deformation and repeated loading cycles. There are generally two strategies to achieve stretchability for a device: material enabled stretchability and structural design enabled stretchability. The former use stretchable materials to fabricate device while the latter uses structure design to accommodate strain on rigid materials. Metallic nanowires have emerged as a promising candidate material for fabricating stretchable conductors. However, the electrical performance of these conductors still needs to be improved under large stretching. A number of mechanics design principles have been applied to metal film based stretchable conductors, but it is not straightforward to extend such principles to metallic nanowire-based ones.</p><p>In this work, we combine these two strategies and design a horseshoe-shaped composite conductor on an elastomer substrate.&nbsp; The conductor is composed of the silver nanowire (AgNW) and polyimide (PI) composite, then the horseshoe pattern is adhered onto the elastomer substrate by van der Waals force. By adjusting the thickness of the substrate with a fixed AgNW/PI composite thickness, different stretchability can be achieved. We further investigate the effect of adhesion between the AgNW/PI composite and elastomer substrate by controlling the interface modification. With the varied thickness of the substrate and the interface strength, several deformation modes of the AgNW/PI composite are observed, including local wrinkling, global buckling, and partial delamination. When the device is stretched, the thinner substrate results in global buckling in the perpendicular direction, while the weaker interfacial adhesion gives rise to partial delamination of the pattern, both of which contribute to accommodating large stretchability. As a result, the resistance change of the stretchable conductor was within 1% under a stretching up to 100%. Finite element analysis (FEA) is carried out to provide more insights into the mechanism of these deformation modes. Potential applications of the stretchable conductors are demonstrated in a wearable epidermal temperature sensor and the interconnects for stretchable LED arrays. This work provides reliable mechanical design strategies for nanomaterial-enabled stretchable devices, which can further enhance the stretchability of these devices for the application of wearable electronics.</p>

Stretchability of Horseshoe-Shaped Silver Nanowire Composite:  Experiments and Modeling

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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