Liquid Metal Particles Decorated by Poly(imidazole-urea) as Versatile Adhesive and Recyclable Inks for Substrate-Irrelevant Direct Writing

Liquid metals (LMs) with fluidity and conductivity are widely applied in flexible electronics. However, the surface patterning of liquid metals (LMs) is restricted by the low adhesion effect on the substrates because of the intrinsic high surface tension. In this study, a versatile adhesive conductive poly(imidazole-urea)/eutectic Ga_75.5In_24.5 alloy (PIU/EGaIn) ink is proposed by wrapping the EGaIn particles with PIU through metal coordination to realize substrate-independent direct writing. The adhesion of PIU guarantees that the PIU/EGaIn ink can be written smoothly on different substrates, ranging from flexible to rigid and plane to camber. Complex patterns can also be stamped on the substrate by transfer printing. The maximum conductivity of the handwriting trace can reach as high as 1.3 × 10⁶ S/m due to the highly efficient stability of EGaIn particles with a low content of residue PIU. The written circuit demonstrates high stability and maintains constant conductivity after 500 cycles of deformations (folding, bending, and stretching), thanks to the good adhesion effect of PIU with substrates. In addition, the resistance touch sensor was patterned to detect finger contact as a demonstration of potential application. The PIU/EGaIn ink waste can be recycled using an alkaline solution owing to the intrinsic degradability of PIU. This strategy offers a new choice for universal adhesive conductive ink that is suitable for environmentally friendly flexible electronics.