Staying Flexible

Crystal structure stretches wearable electronics’ possibilities

If you’ve ever changed clothes to feel warmer or cooler — and, of course, who hasn’t? — rest assured that researchers in the College of Engineering are working on solving that small problem for you, and in the process many other, much larger ones too.

Electronic shirts that keep the wearer comfortable no matter the ambient temperature, as well as medical fabrics that deliver drugs, monitor the condition of a wound, and perform other tasks, may one day be manufactured more efficiently, thanks to a key advance involving materials with a crystal structure discovered nearly two centuries ago.

The breakthrough also involves inkjet printing, and the upshot is the ability to apply circuitry, with precision and at low processing temperatures, directly onto cloth — a promising potential solution to the longstanding tradeoff 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 University. “One hurdle is that fabricating rigid devices on cloth, which has a surface that’s both porous and nonuniform, is tedious and expensive. It requires a lot of heat and energy, and it is hard to scale up. One alternative approach — first putting the devices onto something solid, 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.”

Chang and collaborators at Oregon State and at Rutgers University tackled those challenges by coming up with a stable, printable ink, based on binary metal iodide salts, that thermally transforms into a dense compound of cesium, tin, and iodine (Cs2SnI6).

The resulting film has a particular crystal structure that places it within a class of materials known as perovskites.

The first perovskite was discovered in 1839 by German mineralogist Gustav Rose, who came across an oxide of calcium and titanium in the Ural Mountains, noted its intriguing crystal structure, and named it in honor of Russian nobleman Lev Perovski.

The term perovskite now refers to a range of materials that share the crystal lattice of the original. Interest began to accelerate in 2009, after Japanese scientist Tsutomu Miyasaka discovered that some perovskites are effective absorbers of light. Perovskites based on a metal and a halogen, such as iodine, are semiconductors, essential components of most electrical circuits.

Thanks to the perovskite film, Chang’s team was able to print negative-temperature-coefficient thermistors directly onto woven polyester at temperatures as low as 120 degrees 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 vary their resistance according to temperature; with negative-temperature-coefficient thermistors, resistance decreases as 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, which included Shujie Li and Alex Kosek of the Oregon State College of Engineering and Mohammad Naim Jahangir and Rajiv Malhotra of Rutgers University, demonstrates direct fabrication of high-performance NTC thermistors onto fabrics at half the temperature used by current state-of-the-art manufacturers.

“In addition to requiring more energy, the higher temperatures create compatibility issues with many fabrics,” Chang said. “The simplicity of our ink, the process’ scalability, and the thermistor performance are all promising for the future of wearable e-textiles.”

The Walmart Manufacturing Innovation Foundation and National Science Foundation supported this study. Findings were published in Advanced Functional Materials.