Imagine if there were inexpensive 3D-printed sensors that changed color to show us if something had gotten too warm or was subjected to too much stress. Well, there soon could be – and they'll biodegrade once discarded, to boot.

The sensors are being developed by scientists at Switzerland's Empa research institute, led by Dr. Gustav Nyström. At the heart of the technology is a substance known as hydroxypropyl cellulose (HPC), which is already utilized as a carrier for active ingredients in things like pharmaceuticals and foods.

HPC forms liquid crystals when mixed with water. Thanks to the microstructure of those crystals, they only reflect certain wavelengths of the visible light spectrum, causing them to appear as those colors when viewed by the human eye. The same principle is naturally utilized by vividly colored butterfly wings, among other things.

It's possible to change the "structural color" of the liquid crystals simply by changing their temperature. With that fact in mind, the Empa scientists added a small amount of carbon nanotubes to an HPC/water mixture, boosting its electrical conductivity. As a result, when voltage was applied to the material, its temperature rose and the crystals changed color accordingly.

After "bulking up" the biodegradable mixture by adding cellulose nanofibers to it, it was possible to 3D-print the material without altering either its structural color or its conductivity. The scientists have so far utilized the technology to produce a seven-segment numeric display, along with a strain sensor that changes color in response to a piezoelectric current generated by mechanical deformation.

"Our lab has already developed different disposable electronic components based on cellulose, such as batteries and sensors," said Xavier Aeby, co-author of the study. "This is the first time we were able to develop a cellulose-based display."