Researchers have made a thin multi-layer plastic film that changes color in response to humidity changes. By combining this film with photonic sensors, they have made a touchless control system for electronic devices.

In an article recently published in Material Horizons, the researchers describe how the touchless control system allows users to switch a light-emitting diode using their breath or holding their fingertip in close proximity. The device also allows performing simple tasks on a computer like making phone calls or playing music. The system could be useful for surgeons and drivers or users with mobility impairments.

Humidity sensors typically contain moisture-absorbing polymers sandwiched between electrodes. The devices measure tiny fluctuations in conductivity or capacitance due to changes in humidity levels.

The new sensor relies on the change in the wavelengths of visible light as it bounces off a thin film made of multiple polymer layers. “This offers a visible humidity-detection platform without the requirement of complicated electronic devices,” says Xiaokong Liu, a polymer chemist at the Future Industries Institute at the University of South Australia. The film shows a response in 35 milliseconds, 10 times faster than conventional electronic humidity sensors.

Liu, George Chen, and their colleagues made the film by depositing alternate layers of oppositely charged polyelectrolytes deposited on silicon wafers. They chose positively charged poly(diallyldimethylammonium) (PDDA) and negatively charged poly(styrenesulfonate) (PSS), “because they are the most common, commercially available, and cheap polyelectrolytes,” Liu says.

The film exhibits different colors at different thicknesses because its layers cause interference of light waves. When the film absorbs moisture and swells, the change in thickness causes a visible color change. For example, a 16-layer (made of eight pairs of oppositely charged layers) film goes from a thickness of 295.2 nm at 0% relative humidity to 482.6 nm thick at 100% relative humidity, changing in color from dark blue to a light purplish pink.

To convert the color change into an electrical signal, the researchers reflected a beam of laser light off the film onto a photodetector. As the thickness increases and the reflected light’s color changes, the output voltage of the photodetector goes up.

The team built the touchless control system by feeding the photodetector’s output to a computer. In the study, a user exhaled on the sensor a certain number of times in a given timeframe, and software converted that input into commands such as playing music or opening a web browser. By controlling the strength and duration of breaths, Liu says that users could generate more complex commands that are typically translated through a software interface.

Jian Ji, a polymer science engineer at Zhejiang University, says that the quick response and dramatic color change that the humidity sensor shows are impressive. “Similar photonic-material technologies reported before tend to lack one or the other,” he says. “This is crucial for practical touchless-control systems where latency and sensitivity are the key factors.” The technology is simple and potentially low-cost, which should make it easy to produce on a larger scale, he adds.

Read the abstract in Material Horizons.