Direct Ink\nWriting of Liquid Metal on Hydrogel through\nOxides Introduction

Hydrogel-based flexible electronics have been widely\ninvestigated\nin electronic skin and wearable sensors. However, the challenge of\nmatching the modulus between the hydrogel and the electrode underscores\nthe critical importance of flexibility of the electrode. Gallium-based\nliquid metals (GaLMs) are ideal electrode materials for flexible substrates\ndue to their high conductivity and stretchability. However, the ease\nof aggregation and lack of adhesion happen when patterning GaLMs on\nhydrogel surfaces. This work proposes a direct ink writing (DIW) of\nhighly oxidized EGaIn (hoEGaIn) on an acrylamide (AAm) hydrogel. The\ninterface is modulated by increasing the oxide content to improve\nthe printability. Compared to EGaIn with an oxide layer, hoEGaIn displays\na lower surface tension dropped by about 28.5%, higher adhesion (an\nincrease of about 24.4%), and lower contact angles. These optimized\ninterface properties significantly improve its wettability and DIW\nstability on AAm hydrogel substrates. A minimum line width of 65 μm\nis obtained by regulating DIW parameters. Meanwhile, hoEGaIn exhibits\nimpressive multisubstrate printability and conductivity of up to 2.22\n× 10<sup>6</sup> S·m<sup>–1</sup>. Furthermore, a\ncantilever beam strain sensor is manufactured by DIW hoEGaIn on an\nAAm hydrogel, which exhibits fast response and recovery, excellent\ndynamic response, and stability. This study demonstrates a potential\nmethod for the DIW of GaLMs on hydrogels.