Electrically conductive materials with improved flexibility and stretchability open new possibilities towards next-generation technologies for wearable, personal healthcare and soft robotics. In this regard, highly conductive and stretchable adhesives are essential components of soft electronic devices. Tin/lead (Sn/Pb) solders have been widely employed in the industry owing to their high electrical conductivity, low material cost, and simple fabrication process. However, the Sn-Pb solder is too rigid to be applied to flexible devices and lead discarded from electronic devices can be detrimental to the environment and human bodies. Epoxy-based silver adhesives have been suggested as a less deleterious alternative to the Sn/Pb soldering, but they not only exhibit poor adhesion, but also lack stretchability and flexibility to conform to flexible substrates. Therefore, upon stretching and bending, cracks are easily formed at the interconnects and the electrical conductivity is significantly sacrificed. Although numerous studies have been explored to improve the stretchability of the electrical interconnections, most of the methods require a large amount of silver particles added to the lead-free adhesives. Here, we present a novel approach to remarkably reduce the amount of conductive fillers in stretchable conductive adhesives and improve cyclic stability against stretching. Within a silicone matrix, boron nitrides (BNs) are used as auxiliary fillers that achieve a remarkable reduction of the percolation threshold concentration of silver particles by 12 wt% by enhancing the conductive filler dispersion. Decreasing the size of BNs can further reduce the percolation threshold concentration. As the size of BNs decreases, the relative surface area of silver particles compared to the size of BNs increases. In addition, the suggested adhesives show good electrical conductivity stability over 3000 stretching cycles at 50% strain and demonstrate excellent adhesion to various flexible substrates. Thereby, we propose an original means to considerably lower the amount of conductive silver particles in the stretchable conductive adhesives by introducing non-conductive auxiliary fillers.
