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This paper demonstrates a method for the additive manufacturing of multilayer stretchable soft electronic circuits and sensors composed of printed liquid metal conductors and embedded surface mount components on each layer. Such stretchable devices are envisioned to find applications in wearable devices and soft robotics. The manufacturing method uses digital nozzle-based direct ink writing (DIW) or stencil printing to print a stretchable circuit and sensors composed of commercially available liquid metal conductive ink onto a stretchable thermoplastic polyurethane (TPU) sheet, followed by placement of components onto the liquid metal circuit, and encapsulation. Printing, placement and encapsulation are then repeated to build up multiple circuit and sensor layers, each layer with its own placed components. Liquid metal test traces and circuits were printed with widths and spacings down to 250 μm. Lamination of TPU sheets by hot-pressing was found to be the most robust encapsulation method for the liquid metal circuits, compared to the use of adhesive bond tape and casting silicone. Interlayer vias were created by laser-cutting via holes in the TPU encapsulant sheets prior to lamination, and filling these with liquid metal printed in the next circuit layer. Passive components (1608m and 1005m size) and packaged integrated circuits (ICs) - X2QFN-10 (eXtremely thin Quad Flat No-leads) were placed directly onto the liquid metal circuits with no additional adhesive or solder and were successfully encapsulated by conformally laminating a TPU sheet on top to form a robust assembly. The stretchable circuits were consistently able to withstand over 1000 cycles of stretching with up to 225% strain (the maximum tested) with average baseline resistance increasing from 0.58 Ω to 1.20 Ω for 5 cm-long 2 mm wide traces and from 2.17 Ω to 2.79 Ω for 3 cm long 250 μm wide traces. Circuits with vias and placed components had similar cyclic stretching performance with only modest additional resistances. Demonstrator circuits included a 2-layer circuit consisting of LEDs in both layers and a 2-layer circuit in which a packaged analog-to-digital converter IC reads out a resistive strain sensor that is monolithically printed on the same TPU substrate. While the demonstrated DIW and stencil printing processes are suitable for prototyping stretchable circuits, the overall process is also compatible with other printing methods including screen printing, which can be used for higher throughput production. 

Abstract Inkjet printing of electronic materials is of interest for digital printing of flexible electronics and sensors, but the width of the inkjet-printed lines is still large, limiting device size and performance. Decreasing the drop volume, increasing the drop spacing, and increasing the ink-substrate contact angle are all approaches by which the line width can be lowered, however these approaches are limited by the nozzle geometry, ink coalescence and bead instabilities, and contact angle hysteresis, respectively. Here we demonstrate a novel approach for stable inkjet printing of very narrow lines on ink-substrate combinations with a high contact angle, utilizing the de-wetting of the ink due to the decreased contact angle hysteresis. After printing and drying an initial layer of disconnected seed drops of silver nanoparticle ink, we print an additional layer of bridging drops of the same ink in between the dried seed drops. The bridging drops expand to touch the dried seed drops and then retract into a line, due to the pinning of the wet ink on the dried seed ink but not on the substrate, forming a continuous silver trace. The trace width is decreased from 60μm with a traditional printing approach down to 12.6μm with this seed-bridge approach. The electrical conductivity of the silver trace is similar to that of a conventionally printed trace. Due to poor adhesion on the print substrate, the trace was transferred to a separate polymer substrate with a simple hot-pressing procedure, which preserves the electrical conductivity of the trace.