All-Printed Flexible and Stretchable Electronics with Pressing or Freezing Activatable Liquid-Metal-Silicone Inks

Liquid-metal (LM)-based flexible and stretchable electronics have attracted widespread interest in wearable computing, human-machine interaction, and soft robotics. However, many current examples are one-off prototypes, whereas future implementation requires mass production. To address this critical challenge, an integrated multimaterial 3D printing process composed of direct ink writing (DIW) of sealing silicone elastomer and special LM-silicone (LMS) inks for manufacturing high-performance LM-based flexible and stretchable electronics is presented. The LMS ink is a concentrated mixture of LM microdroplets and silicone elastomer and exhibits excellent printability for DIW printing. Guided by a verified theoretical model, a printing process with high resolution and high speed can be easily implemented. Although LMS is not initially conductive, it can be activated by pressing or freezing. Activated LMS possesses good conductivity and significant electrical response to strain. Owing to LMS's unique structure, LMS-embedded flexible electronics exhibit great damage mitigation, in that no leaking occurs even when damaged. To demonstrate the flexibility of this process in fabricating LM-based flexible electronics, multilayer soft circuits, strain sensors, and data gloves are printed and investigated. Notably, utilizing LMS's unique activating property, some functional circuits such as one-time pressing/freezing-on switch can be printed without any structural design.