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Hydrogel-based flexible electronics have been widely investigated in electronic skin and wearable sensors. However, the challenge of matching the modulus between the hydrogel and the electrode underscores the critical importance of flexibility of the electrode. Gallium-based liquid metals (GaLMs) are ideal electrode materials for flexible substrates due to their high conductivity and stretchability. However, the ease of aggregation and lack of adhesion happen when patterning GaLMs on hydrogel surfaces. This work proposes a direct ink writing (DIW) of highly oxidized EGaIn (hoEGaIn) on an acrylamide (AAm) hydrogel. The interface is modulated by increasing the oxide content to improve the printability. Compared to EGaIn with an oxide layer, hoEGaIn displays a lower surface tension dropped by about 28.5%, higher adhesion (an increase of about 24.4%), and lower contact angles. These optimized interface properties significantly improve its wettability and DIW stability on AAm hydrogel substrates. A minimum line width of 65 μm is obtained by regulating DIW parameters. Meanwhile, hoEGaIn exhibits impressive multisubstrate printability and conductivity of up to 2.22 × 10
Jiang et al. (Wed,) studied this question.