Touchscreens could double as lab-grade sensors

Researchers from the University of Cambridge showed how a typical phone’s touchscreen could be used to identify common ionic contaminants in soil or drinking water by simply dripping liquid samples onto the screen. The sensitivity of the touchscreen sensor is comparable to typical lab-based equipment, rendering it useful in low-resource settings.

The researchers say their proof of concept could one day be expanded for a wide range of sensing applications, including for biosensing or medical diagnostics.

While other research teams have utilised the computational power of smartphones for sensing applications, this is the first study to use the screen itself, rather than the camera or peripheral devices or significant additions to the screen. A typical phone screen is covered with a grid of electrodes; when a finger causes a local disruption in the electric field of these electrodes, the phone interprets this input signal.

“We wanted to know if we could interact with the technology in a different way, without having to fundamentally change the screen,” said Dr Ronan Daly, who co-led the project. “Instead of interpreting a signal from your finger, what if we could get a touchscreen to read electrolytes, since these ions also interact with the electric fields?”

Daly and his colleagues began with computer simulations then validated these with a stripped-down, standalone touchscreen similar to those used in phones and tablets. They placed different liquids on the screen with pipettes and measured the resulting change in capacitance. Ions in the fluid all interact with the electric field of the screen differently, depending on the concentration of ions and their charge.

“Our simulations showed where the electric field interacts with the fluid droplet. In our experiments, we then found a linear trend for a range of electrolytes measured on the touchscreen," said PhD candidate Sebastian Horstmann. “The sensor saturates at an anion concentration of around 500 micromolar, which can be correlated to the conductivity measured alongside. This detection window is ideal to sense ionic contamination in drinking water.”

Among the early applications for this technology include detecting arsenic contamination in drinking water. Arsenic, a common contaminant in groundwater, is typically screened for and filtered before it reaches household plumbing. In parts of the world without water treatments plants, arsenic contamination of drinking water is a serious problem. In theory, a person could add a drop of water to their phone before drinking to check that it is safe.

While the sensitivity of phone and tablet screens is tuned for fingers at present, the sensitivity could be changed in a certain part of the screen by modifying the electrode design to optimise sensitivity.

“The phone's software would need to communicate with that part of the screen to deliver the optimum electric field and be more sensitive for the target ion, but this is achievable,” said Professor Lisa Hall, a Cambridge chemical engineer who co-led the research with Daly. “We're keen to do much more on this; it's just the first step.”

Hall, Daly and their colleagues hope to develop the technology to detect a wider range of molecules, opening up potential health applications. For instance, if the sensitivity reached a point at which heavy metals could be detected, it could be used to test for lead in drinking water, as well as opening up potential home health monitoring applications.

“This is a starting point for broader exploration of the use of touchscreen sensing in mobile technologies and the creation of tools that are accessible to everyone, allowing rapid measurements and communication of data,” said Hall.