Double-layer liquid-surface printing of soft films for flexible electronics

Soft thin films are essential platforms for developing flexible electronics. However, producing thin films with uniform thickness, free residual stress while often followed by a subsequent defect-free transfer at high yield remains a significant fabrication challenge. Here, we present a liquid-surface printing technique that enables the fabrication of highly uniform, soft, thin films by leveraging controlled spreading of 3D-printed polymeric ink droplets on a double-layer immiscible liquid substrate. The inherent smoothness of the liquid surface together with the self-leveling-driven uniformity of the immiscible liquid layer yields precise control over ink droplet spreading and produces highly uniform soft films free of structural defects or surface irregularities. These films can be programmably transferred onto curvilinear substrates because of the fluidic nature of the underlying liquid substrate and its weak van der Waals interactions with the films while maintaining conformal contact. We demonstrate the versatility of this printing approach by fabricating a broad variety of geometric patterns and local features and also validate its robustness by their extensive mechanical and electrical performance measurement. We further fabricate flexible radio-frequency identification (RFID) tags and conformally deploy them to the "skin" of food or food packaging with diverse curvatures for real-time spoilage monitoring, showcasing this technological capability and potential for fabricating flexible and conformable electronics.