Conventional electronics are rigid and unable to withstand the twisting and stretching movements that clothing experiences during typical daily activities. Due to their fluid nature and excellent conductivity, gallium-based liquid metals (LMs) are promising materials for flexible electronics. However, LMs do not stick well to tissues and their high surface tension causes them to bulge when 3D printed, rather than forming continuous circuits. Yong He and his colleagues wanted to develop a new type of conductive ink that could be 3D printed directly onto clothing in intricate patterns.
To make their ink, the researchers mixed LM and alginate. Stirring the solution and removing excess liquid yielded LM microdroplets coated in an alginate microgel envelope. The ink was very thick until it was pressed through a nozzle for 3D printing, which broke the hydrogen bonds in the microgel and made it more fluid. Once the ink reached the surface of the fabric, the hydrogen bonds reformed, allowing the printed design to retain its shape. The team 3D printed the new ink on a variety of surfaces, including paper, polyester fabrics, non-woven fabrics and acrylic-based tape. Although the printed patterns were not initially conductive, the researchers activated them by stretching, squeezing or freezing them, which broke up the dried alginate networks to connect the LM microdroplets.
After activation, the circuit boards had excellent electrical conductivity and strain sensing properties. Also, applying a small voltage to the ends of the circuit caused it to heat up, even in very cold temperatures. To demonstrate the ink’s capabilities, the team 3D printed a series of electronic components onto commercial garments. On a T-shirt, they printed a near-field communication tag that directed a nearby smartphone to open a website. Other sensors printed on the clothing monitored the movement of an elbow or knee joint. And a circuit powered by a small battery heated the printed pattern to over 120 F in less than a minute. LM-alginate ink can be recycled by soaking the fabric in a weak sodium hydroxide solution, recovering fresh liquid metal for new applications.
The authors acknowledge funding from the National Natural Science Foundation of China and the National Key Research and Development Program of Zhejiang Province.