Open-loop printing of liquid metal for the low-cost rapid fabrication of soft sensors

Soft sensors consisting of highly conductive liquid metal, eutectic gallium-indium (eGaIn), embedded in a soft elastomer have been of great interest recently due to their stretchability for sensing temperature, pressure, and strain for a variety of applications. Previous studies have explored methods for patterning the liquid metal during the manufacture of soft sensors. Among these, direct printing of eGaIn is attractive as it is easily re-configurable for fabricating custom sensors. However, to print liquid metal traces with consistent widths, previous approaches require combinations of 1) high-precision feedback control of the stand-off distance and the extrusion rate, 2) a very low stage velocity, and/or 3) modification of the properties of liquid metal with additional components or processing steps. To address these limitations, we present two open-loop liquid metal printing methods to achieve high precision using a low-cost, open-source system without the need to optimize the properties of the liquid metal. We discuss how key print parameters—as well as a novel print head design inspired by a paintbrush—affect the printing quality, and how well the printing methods can resist variation in the height of the substrate. We then apply these methods to manufacture soft strain sensors, which we test and calibrate. In general, these printing methods represent easily accessible methods for manufacturing liquid metal sensors without the need for complex equipment or materials processing.