Ag flake/silicone rubber composite with high stability and stretching speed insensitive resistance via conductive bridge formation

www.nature.com/scientificreports OPEN Ag flake/silicone rubber composite with high stability and stretching speed insensitive resistance via conductive bridge formation In Seon Yoon1,3, Sun Hong Kim2, Youngsu Oh1,3, Byeong-Kwon Ju1* & Jae-Min Hong3,4* High stability, stretchable speed insensitive properties, high stretchability, and electrical conductivity are key characteristics for the realisation of wearable devices. However, conventional research is mainly focused on achieving only high stretchability and electrical conductivity. Studies on the stability and stretching speed insensitive properties generally require complex fabrication processes, which are in need of further improvement. In this study, we propose a facile formation of a conductive bridge in composites by using surface damage and the viscoelastic property of the polymer. Surface cracks due to repeated stretching cycles formed conductive bridges via stress relaxation of the viscoelastic polymer matrix. The conductive bridge resulted in the conductor having highly stable resistance values at target strains and stretching speed insensitive resistance, even at stretching speeds that were 20 times faster than the minimum. There have been great interests in stretchable conductors for the past few decades; as such, many methods for the fabrication of highly stretchable and conductive conductors have been researched. Instead of conventional rigid substrates and electrodes, stretchable substrates and conductors have been studied via various methods such as printing with composite ink1–4, prestraining5–8, and embedding nanowires in a stretchable matrix9–12. In addition, practical applications such as stretchable displays13–17, wearable motion sensors18–21, and stretchable energy storage devices22–25 have been studied using the stretchable conductors mentioned above. Furthermore, stretchable conductors are studied as an interconnection electrode for future transparent devices, such as flexible photodetectors26,27, and tiny devices, such as memristors and memory28,29. These researchers have made great progress in making highly stretchable and conductive conductors. However, for practical uses of these stretchable devices, there are new stability conditions, where the conductors are required to withstand thousands of stretching/contraction cycles and be independent of the stretching speed. From this point of view, self-healing polymer conductors are gaining more interest as a promising method to modify the stability of conductors30–32. Cracks are generated by repeated stretching/contraction cycles; this can make the conductive pathway longer, or even break it, making the conductor dielectric. Even after the conductors are separated, a self-healing polymer conductors can reform the network and make it conductive again. Although this healing process restores mechanical and electrical properties, there are still problems to be solved. Self-healing polymer conductors require many conditions such as broken conductors should be placed to contact each other and number of restores are limited. So, forming conductive bridge along the broken conductors can be an alternative for highly stable and stretching speed insensitive conductor. Recently, the concept of conductive bridges in stretchable conductors was first introduced by Lee33. Carbon black conductors in conducting composites were damaged due to stretching, resulting in a rapid increase of electrical resistance. Lee found that adding carbon nanotubes increased the electrical conductivity via the formation of conductive bridges between separated carbon black clusters. Thus, the formation of conductive bridges is 1 Display and Nanosystem Laboratory, Department of Electrical Engineering, Korea University, Seoul, 02841, Republic of Korea. 2Department of Electrical and Computer Engineering, Inter-University Semiconductor Research Centre, Seoul National University, Seoul, 08826, Republic of Korea. 3Photo-Electronic Hybrids Research Centre, Korea Institute of Science and Technology (KIST), Seoul, 02792, Republic of Korea. 4Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Jeonbuk, 55324, Republic of Korea. *email: ; Scientific Reports | (2020) 10:5036 | https://doi.org/10.1038/s41598-020-61752-2 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. (a) Schematic of a stretchable Ag flake/silicone rubber conductor on a silicone substrate. The cube illustrates the microstructure of the conductor. The inset shows the conductive ink before printing. (b) Photographs of the stretchable conductor at relaxed (0% strain) and stretched (400% strain) states. (c) Schematic illustration of a conductive bridge and microstructure in the conductor. (d) Scanning electron microscope (SEM) image (top view) of the stretchable conductor at a generated crack. advantageous in terms of the ease of fabrication and ease of preserving the conductive path in comparison with self-healing polymers. However, position and direction of the carbon nanotubes could not be changed arbitrary, also the composite showed poor electrical property. Therefore, the concept of conductive bridge needed further improvements in terms of forming process and consisting materials. In this paper, we suggest new method to fabricate stretchable conductors with high stability and strain-insensitive resistance due to the formation of conductive bridges induced by surface damage. Cracks are usually fatal defects for stretchable conductors. However, in the case of conductors with a viscoelastic polymer matrix, cracks can work as driving force for the formation of conductive bridges. At the crack, the composite curls up and down due to its viscoelastic property, and the closely packed conductive filler network forms a conductive bridge. In our study, this conductive bridge resulted in high stability for the conductor, with stable electrical resistance for 1000 cycles and stretching speed insensitive electrical resistance. Results and discussion Figure 1 shows schematics and samples of the stretchable conductor, composed of silicone rubber (Ecoflex) and Ag flakes, and the cracks induced by multiple elongation and concept of the conductive bridge. The inset of Fig. 1a shows a conductive ink comprising a conductive filler (Ag flakes), silicone rubber, and a solvent (4-methyl2-pentanone), which is used to enable the uniform dispersion of Ag flakes in the silicone rubber matrix. The conductor showed a high stretchability of 400–500%, even after sintering at 130 °C, as shown in Fig. 1b. The same silicone rubber material for both the matrix and substrate to eliminate the modulus mismatch34,35. The prepared samples went through a number of stretching/contraction cycles, creating cracks in the stretchable conductors as shown in Fig. 1c,d. The Ag flakes at the cracks curl up and down, forming a closely-packed conductive network i.e., a “conductive bridge.” Due to the v (...truncated)