Color-changing inks for money and fashion

Inks that adapt to their environment are not the stuff of science fiction anymore. Thermochromic inks, sensing inks and inks that react to changes in CO2-concentration are being invented by scientists around the world. Two recent examples of this new field are color-changing textile inks by designer Lauren Bowker and humidity-sensing inks perfected by a team of Chinese scientists. The humidity-sensing inks can be printed on an inkjet printer and may help detect counterfeit money.

Fashion designer Lauren Bowker made a splash when she first presented her collection of clothes that change color with the wind. The self-described alchemist graduated from the Manchester School of Art after she invented the award-winning carbon emission sensing ink PdCl2 and subsequently studied printed textiles at the Royal College of Art in London. Since then, she has worked on developing her multi-sensory chromic color-change inks even further. She founded UNSEEN, a design firm that specialized in biological and chemical technology.

Her first success was a color-changing ink that transformed from yellow to black based on the pollution particles in the air; it changed back to yellow when the air was clean. One of her most recent pieces is a cloak made of feathers coated in inks that respond to UV light, moisture, heat and friction. The ink can be screen-printed, painted, sprayed or used as a dye. While she displays her creations at events like Fashion Week 2014, Bowker has received multiple inquiries from the health care and auto industries that see a variety of applications for the color-changing inks.

One project that Bowker likes to cite as an example of how the inks may be used in the future is a T-shirt that could indicate the severity of an asthma attack by changing colors. While this kind of textile ink has yet to be developed, Bowker clearly sees a range of worthwhile application of her technology in the medical industry. “Over the next six months, we have a research and development phase in laundering these inks and getting them to a point where they can be implemented onto vast amounts of surfaces and have passed all the sort of rules and regulations that an ink should”, Bowker said in a recent interview with WILD magazine. “Going forward, we’ll also be looking at the digital linkup of this technology”, she adds.

Meanwhile, a team of Chinese scientists has invented an ink that changes color when the humidity in the environment changes. The ink could be used to detect counterfeit money simply by breathing on it. This is not the first time that color-changing money has been experimented with to prevent fraud but previous attempts were too expensive and not as effective as expected.

Now researchers Zhongze Gu, Zhuoying Xie, Chunwei Yuan and their team published an article in the scientific journal ACS Nano that seems to have solved the problems that plagued previous attempts. Their approach was inspired by a species of longhorn beetle called Tmesisternus isabellae that changes its color from gold to red and back again depending on the humidity in the environment.

The ink contains colloidal photonic crystals responsible for its unique properties. The ability of photonic crystals to control the flow of light makes them a suitable material for diverse applications including optical communications, biosensors and solar cells.

So far, however, printing photonic crystals has been expensive. The researchers from Southeast University in Nanjing, China, now managed to use them in an inkjet printing process while maintaining the properties of the ink. The color of the printed ink changed when exposed to nitrogen and ethanol vapors. It even reacted when a person simply blew air on a printed piece of paper. If this new ink proves to be as good as expected, detecting fake money could become as easy as exhaling after a deep breath.