COMMUNICATION © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 5) 1500289 wileyonlinelibrary.com Self-Healing Inks for Autonomous Repair of Printable Electrochemical Devices Amay J. Bandodkar, Vinu Mohan, Cristian S. López, Julian Ramírez, and Joseph Wang* A. J. Bandodkar, Dr. V. Mohan, C. S. López, J. Ramírez, Prof. J. Wang Department of NanoEngineering University of California San Diego La Jolla, CA 92093, USA E-mail: josephwang@eng.ucsd.edu DOI: 10.1002/aelm.201500289 leads to permanent failure. Therefore, it is critical to develop self-healing conductive inks for fabricating intelligent all-printed electrochemical devices that autonomously restore the lost elec- trical conductivity caused by mechanical damage, degradation, and failure. In the present work we report, for the first time, the synthesis of printable inks containing self-healing microcapsules for fab- ricating 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 struc- ture. 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 mechan- ical and electrical contacts. Since the capsules are loaded directly in the inks, the entire footprint of the printed electro- chemical 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. Ini- tially, 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 dis- played 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 self- healing 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 Device failure incurred due to mechanical fatigue and exces- sive strain is a major cause of concern in the field of electronics as this shortens a device’s lifespan and increases maintenance costs. [1,2] Additionally, in some scenarios, replacing dysfunc- tional devices may become complicated [3,4] and can have high environmental price tag. [5] Biological systems overcome this challenge of mechanical damage by utilizing unique self-healing processes that enable them to augment their lifespan. [6,7] Taking cues from nature, several groups have developed bio- mimetic materials that autonomously repair themselves when mechanically damaged. [8] Such self-healing materials rely on either capsule, vascular, or intrinsic methods, and have been developed for applications in construction, [9,10] corrosion, [11,12] prosthetics, [13,14] tissue engineering, [15] and electronics. [16–18] Printed electronics has garnered tremendous attention and its market size is expected to reach $300 billion over the next two decades. [19] Mechanical damage-induced device failure represents a major challenge hampering the progress of this growing field due to the fragile nature of the printed devices. Yet, little attention has been given to the preparation of self- healing inks for realizing smart printed electronics that will self-repair when damaged. [16–18] The reported conductive self- healing materials either require heat to initiate the healing process, [16] or rely on healing agent-filled capsules loaded in a separate nonconductive, elastomeric coating overlaying the conductive printed circuit. [18,20] In the former case, additional apparatus is mandated to initiate the healing process and thus such materials are less attractive when autonomous self-healing is required. In the latter case, the capsules may fail to release the healing agent (in response to mechanically induced cracks in the printed conductive trace) due to mismatch between the elastic properties of the rigid conductive circuit and the over- laying self-healing agent-loaded coating. Over the past two decades printed electrochemical devices have acquired remarkable importance in healthcare, [21] energy, [22,23] and security [24,25] domains. In several practical situations, elec- trochemical devices, such as wearables [26,27] or batteries, [28,29] face mechanical deformations that could potentially reduce their lifespan. In order to address this issue, researchers have fabricated devices on plastic, [30,31] paper, [32,33] and textile [34,35] substrates that can be easily bent and even stretched. [36] How- ever, any strain beyond the limits of these devices’ resiliency www.MaterialsViews.com www.advelectronicmat.de Adv. Electron. Mater. 2015, 1, 1500289