Search for: All records

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. 

The integration of electronics into compliant materials is typically complex, cumbersome, and jeopardizes system-level compliance. Using multi-material fused deposition modeling, we introduce a framework in which components of a soft robot and conductive traces are deposited in a single print. Our novel procedure for attaching discrete electronic components to printed conductive traces using toluene solvent ensures reliable electrical connections by significantly reducing contact resistance by over an order of magnitude compared to existing methods. This fabrication pipeline is an additional key component that contributes to the broader objective of establishing a fully automated fabrication process for soft robots with integrated electronics. We demonstrate a complete assembly of a terrestrial soft robot and showcase its resilience against physical impacts. 

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. 

This letter describes the manufacturing and experimental characterization of novel stretchable strain sensors for continuum robots. The overarching goal of this research is to provide a new solution for the shape sensing of these devices. The sensors are fabricated via direct ink writing, an extrusion-based additive manufacturing technique. Electrically conductive material (i.e., the ink) is printed into traces whose electrical resistance varies in response to mechanical deformation. The principle of operation of stretchable strain sensors is analogous to that of conventional strain gauges, but with a significantly larger operational window thanks to their ability to withstand larger strain. Among the different conductive materials considered for this study, we opted to fabricate the sensors with a high-viscosity eutectic Gallium-Indium ink, which in initial testing exhibited high linearity (R2≈ 0.99), gauge factor ≈ 1, and negligible drift. Benefits of the proposed sensors include (i) ease of fabrication, as they can be conveniently printed in a matter of minutes; (ii) ease of installation, as they can simply be glued to the outside body of a robot; and (iii) ease of miniaturization, which enables integration into millimeter-sized continuum robots. 

CuBiW 2 O 8 (CBTO), with a band gap of 1.9–2.0 eV, responds to a wide region of the electromagnetic spectrum, which makes it a good candidate for solar-driven photocatalytic energy conversion and water treatment. We have previously demonstrated a Cu-rich solid state approach that enables the synthesis of CBTO accompanied by thermodynamically stable Bi 2 WO 6 impurity. Here, we describe an improved synthesis protocol with decreased impurity and synthesis time, and the first demonstration of CBTO as a functional material using photocatalytic Cr( vi ) photoreduction as a probe reaction. Transient absorption spectroscopy (TAS) was performed to investigate the ultrafast dynamics of the charge carriers after photoexcitation. The presence of two populations of photoexcited carriers was found, including short-lived free carriers with ∼10 ps lifetime and long-lived shallowly-trapped carriers with ∼1 ns lifetime. Together with carrier mobilities measured in our previous study, the new TAS results indicate that the long-lived charges have diffusion lengths similar to the CBTO particle size and were likely responsible for the majority of the photocatalytic activity. High activity of CBTO for Cr( vi ) photoreduction (∼100% reduction of 5 mg L −1 of Cr( vi ) in 15 minutes) was demonstrated, which clearly establishes the promise of this novel oxide for visible light-driven photocatalytic applications. Radical quenching experiments indicate that both ˙OH radicals and O 2 ˙ − radicals are produced by CBTO and are involved in the photoreduction of Cr( vi ). Repeated photocatalysis tests and analysis of the surface after the reaction show that CBTO is a stable and potentially reusable catalyst. Insights gained from correlating the synthesis conditions, carrier dynamics, and reactive species suggests that CBTO prepared with the improved protocol would be a favorable choice for photocatalytic reactions such as water decontamination from organic pollutants, water splitting, and solar fuel generation using visible light.