The breakthrough, which could see electronic shirts capable of keeping the wearer comfortably warm or cool, or medical fabrics that are able to deliver drugs, involves inkjet printing and materials with a crystal structure discovered nearly two centuries ago. The researchers have been able to apply circuitry, with precision and at low processing temperatures, directly onto cloth - a promising potential solution to the longstanding trade-off between performance and fabrication costs.
"Much effort has gone into integrating sensors, displays, power sources and logic circuits into various fabrics for the creation of wearable, electronic textiles," said Chih-Hung Chang, professor of chemical engineering at Oregon State. "One hurdle is that fabricating rigid devices on cloth, which has a surface that's both porous and non-uniform, is tedious and expensive, requiring a lot of heat and energy, and is hard to scale up. And first putting the devices onto something solid, and then putting that solid substrate onto fabric, is problematic too - it limits the flexibility and wearability of the fabric and also can necessitate cumbersome changes to the fabric manufacturing process itself."
Chang and collaborators in the OSU College of Engineering and at Rutgers University have been able to tackle those challenges by coming up with a stable, printable ink, based on binary metal iodide salts, that thermally transforms into a dense compound of caesium, tin and iodine.
The resulting film of Cs2SnI6 has a crystal structure that makes it a perovskite and, as a consequence, Chang's team have been able to print negative-temperature-coefficient thermistors directly onto woven polyester at temperatures as low as 120 Celsius - just 20 degrees higher than the boiling point of water.
A thermistor is a type of electrical component known as a resistor, which controls the amount of current entering a circuit. Thermistors are resistors whose resistance is temperature dependent, and this research involved negative-temperature-coefficient, or NTC, thermistors - their resistance decreases as the temperature increases.
"A change in resistance due to heat is generally not a good thing in a standard resistor, but the effect can be useful in many temperature detection circuits," Chang said. "NTC thermistors can be used in virtually any type of equipment where temperature plays a role. Even small temperature changes can cause big changes in their resistance, which makes them ideal for accurate temperature measurement and control."
The research demonstrates directly fabricating high-performance NTC thermistors onto fabrics at half the temperature used by current state-of-the-art manufacturers, Chang said.
"In addition to requiring more energy, the higher temperatures create compatibility issues with many fabrics," he said. "The simplicity of our ink, the process' scalability and the thermistor performance are all promising for the future of wearable e-textiles."
