A novel direct writing of eutectic gallium indium (EGaIn) patterns on uneven surfaces including both inclined and curved substrates is reported. The approach relies on four degrees‐of‐freedom motion control of the pressurized EGaIn dispenser and precise sensing of the dispenser tip–substrate distance. An experimental hardware is built by using three motorized linear stages, a motorized rotation stage, two electronic pressure regulators, and a laser distance sensor and operating programs are developed. While the rotation stage makes the laser sensor always precede the dispenser tip by a predetermined value, the vertical stage maintains the dispenser tip–substrate distance by using the readout of the laser sensor recorded beforehand. By incorporating the time delay from the laser sensor preceding the dispenser tip for feedback control of the dispenser tip position, various EGaIn patterns are directly written on uneven substrates with their widths being 70–80 µm. Electrical connectivity and structural integrity of written EGaIn patterns are confirmed by the light‐emitting diode mounted between two end segments of EGaIn patterns. The maximum slope for reliable patterning is found to be ≈20°. To show practical applications of this new concept, a curved keypad and a glove‐type wearable device with strain sensors integrated are demonstrated.

Eutectic gallium indium (EGaIn) is actively investigated toward wearable and stretchable electronic devices due to the high fluidity, high electrical conductivity, and low toxicity. However, high surface tension along with spontaneous oxidation makes fine patterning below ≈10 µm challenging. In this paper, a novel manufacturing technique that enables EGaIn patterns of single-digit micrometer widths on planar elastomeric substrates is presented. First, a custom direct printing setup is constructed for continuous and uniform printing of EGaIn by feedback control of the distance between the dispensing needle and the substrate. With this setup, a 120 µm wide linear pattern is printed on the Ecoflex, a stretchable elastomer. Then, the initial printed line is stretched, frozen with deionized water, and transferred to an unstretched Ecoflex substrate. Upon gentle heating after the pick-nplace of the EGaIn line frozen with deionized water, only the stretched EGaIn line is left on the new Ecoflex substrate. The aforementioned pick-n-place transfer of the stretched EGaIn frozen with water is cascaded multiple times until a target width is obtained. Finally, a 2 µm wide linear pattern, 60-fold reduction with respect to the initial dimension, is acquired. For practical applications, strain and tactile sensors are demonstrated by width-reduced EGaIn patterns.

This paper reports a microfabrication-free approach to make hollow channel mass sensors by pulling a glass capillary and suspending it on top of a machined jig. A part of the pulled section makes simple contact with an actuation node and a quartz tuning fork (QTF) which acts as a sensing node. The two nodes define a pulled micro capillary tube resonator (PμTR) simply supported at two contacts. While a piezo actuator beneath the actuation node excites the PμTR, the QTF senses the resonance frequency of the PμTR. The proposed concept was validated by electrical and optical measurements of resonant spectra of PμTR. Then, different liquid samples including water, ethanol, glycerol, and their binary mixtures were introduced into the PμTR and the resonance frequency of the PμTR was measured as a function of liquid density. Density responsivity of −3,088 Hz-g−1 cm3 obtained is comparable to those of microfabricated hollow resonators. With a micro droplet generation chip configured in series with the PμTR, size distribution of oil droplets suspended in water was successfully measured with the radius resolution of 31 nm at the average droplet radius, 28.47 μm. Overall, typical off-the-shelf parts simply constitute a resonant mass sensing system along with a convenient electrical readout.

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In article number https://doi.org/10.1002/adfm.201800380, Sang Ken Kauh, Jungchul Lee, and co‐workers propose a novel method to enable fine liquid metal patterns without relying on soft‐lithographically prepared molds. The cover image depicts phase change mediated pick‐n‐place transfer of liquid metal, eutectic gallium indium (EGaIn), that combines direct printing of EGaIn and transfer of EGaIn patterns stretched on elastomer and then frozen with deionized water. Cascading this approach enables EGaIn patterns of single‐digit micrometer width.