Four Degrees‐of‐Freedom Direct Writing of Liquid Metal Patterns on Uneven Surfaces

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.