Monolithically stacked VIA-free liquid metal circuit for stretchable electronics

Abstract

Researchers are eagerly developing stretchable conductors because they constitute basic building blocks for stretchable electronic devices in the fields of wearable electronics, soft robotics, and human–machine interfaces. Though various stretchable conductors with high stability are being devised, fabricating stretchable stacked circuits with them leads to new challenges. The most critical problem is evoked by vertical interconnection access (VIA) structures, which inevitably become stress-concentrated areas when stretched. Consequently, stretchable circuits become much more unstable when manufactured as stacked circuits than single-layered circuits. Here, we demonstrate a monolithically stacked VIA-free stretchable liquid metal circuit that is mechanically one-body-merged and electrically stacked-circuit-designed. The circuit has no vertical structures, which endows extreme electromechanical stability with various features. It is realized by unique combinations of liquid metal conductor and selective surface treatment. The stacked circuit can be elongated over 1,100 % strain, demonstrating negligible difference in stretchability compared to a single-layered circuit. The circuit also possesses various distinctive characteristics of security, choosability, and extendability. As proof, we demonstrate an encryption element, a choosable circuit, an extendable circuit, and other functional circuits. We expect the proposed stacked circuit to give directions to stretchable stacked electronics with various functionalities.