T-Shirt Transfer Paper Provides Inspiration for a Novel Technique for Putting Circuits on Everything

Researchers from Tianjin University have come up with a technique for putting printable circuits onto fabric, plastics, and even fruit — taking inspiration from vintage iron-on t-shirt decal techniques.

"Low-cost, rapid patterning of liquid metal on various substrates is a key processing step for liquid metal-based soft electronics," the researchers explain in the abstract to their paper. "Current patterning methods rely on expensive equipment and specific substrates, which severely limit their widespread applications. Based on surface adhesion adjustment of liquid metal through thermal transferring toner patterns, we present a universal printing technique of liquid metal circuits."

That "universal method" owes much to the classic art of printing your own t-shirts: the circuit patterns are printed onto thermal transfer paper using a basic desktop laser printer, then transferred off to glass for application of a liquid-metal conductive ink before the item on which the circuit is to be built is pressed against the glass — from flexible plastics to curved fruit, with smooth or rough textures.

To prove the concept, the team built a range of test circuits with line-widths down as low as 50µm: LEDs, a microphone circuit applied to a wooden ukri instrument, a flexible electroluminescent display applied to a bottle, an electric heater, a radio-frequency identification (RFID) tag, and a 12-lead electrocardiogram (ECG) heart sensor for healthcare.

"The technique has yielded liquid metal circuits […] on various smooth, rough, and three-dimensional substrates with complex morphology," the team explains. "The liquid metal circuits can maintain their functions even under an extreme strain of 800 per cent. [We] indicate the great potential of such a technique to rapidly achieve large-area flexible circuits for wearable health monitoring, internet of things, and consumer electronics at low cost and high efficiency."

The team's work has been published under closed-access terms in the journal ACS Applied Materials & Interfaces.