In the present work we report, for the first time, the synthesis of printable inks containing self-healing microcapsules for fabricating self-repairable inexpensive electrochemical devices. Unlike previous work,[18, 20] the new tailor-made conductive inks contain the self-healing capsules and do not require a separate coating of the microcapsules over the printed structure. By judiciously identifying the binder and thinner, we were able to synthesize conductive inks that could be easily loaded with the healing capsules while enabling convenient printing (Figure 1A). When the printed device is damaged, the capsules release the hexyl-acetate healing solvent to restore the mechanical and electrical contacts. Since the capsules are loaded directly in the inks, the entire footprint of the printed electrochemical devices has the ability to self-heal upon mechanical damage. By leveraging printing technology and the self-healing inks, we demonstrate smart electrochemical devices that rapidly self-repair mechanical damage at ambient temperature, and restore electrochemical performance. A typical screen-printed conductive ink is composed of the conductor particles, polymeric binder, and other additives.[37] Several binders and solvents were evaluated toward successful preparation of capsule-loaded self-healing carbon ink. Initially, capsules were directly loaded in commercial carbon inks obtained from different sources (Ercon Inc., Gwent Group, and Henkel Inc.). However, either these inks could not be printed efficiently or the healing solvent failed to restore the mechanical damage. Attempts were also made by dispersing the capsules and graphite powder in commercial insulating ink (DuPont 5036). In this case, the conductivity was restored only after several minutes and the process was irreproducible. Subsequently, self-healing carbon inks based on polystyrene (poly (styrene-co-methyl methacrylate)) and acrylic (Speedball, Art Products Inc.) binders were also explored. Printing of the polystyrene-based carbon ink was a major hurdle. In contrast, capsule-loaded acrylic-based carbon inks printed readily but displayed an unstable electrochemical behavior. The study revealed that absorption of water by the acrylic binder led to its poor electrochemical stability. Therefore, a water-resistant acrylic varnish binder (Liquitex Inc.) was used to synthesize the selfhealing carbon ink.

Inks with varying carbon and capsule loadings were prepared to optimize the ink composition. Low loading of the capsules led to their spare distribution within the ink and hence healing could occur only at locations where the capsules were present. Alternatively, high loading of capsules led to highly viscous