The Empa logo 3D-printed (L) from the new HPC mixture changes colour when it gets warmer. The display consists of seven electrically conductive segments (R) that change colour when a voltage is applied. Pix: Empa.

Researchers in cellulose and wood materials have developed a cellulose-based material that prints bio-degradable 3D sensors and displays. This comes from Empa, the Swiss Federal Laboratories for Material Sciences and Technology.

The mixture of hydroxyl propyl cellulose with water, carbon nano-tubes and cellulose nano-fibrils changes colour when heated or stretched. This is done without the addition of any pigments.

The researchers started with hydroxyl propyl cellulose (HPC), which is commonly used as an excipient in pharmaceuticals, cosmetics and foodstuffs, among other things.

Water mixed with HPC forms liquid crystals. These crystals have a remarkable property; depending on their structure – which itself depends on the concentration of HPC, among other things – they shimmer in different colours, although they themselves have no colour or pigment.

This phenomenon is called ‘structural colouring’ and is known to occur in nature. It is found in peacock feathers, butterfly wings and chameleon skin.

They get all or part of their brilliant coloration not from the microscopic structures that “split” the (white) daylight into spectral colours and reflect only the wavelengths for specific colours.

The structural colouring of HPC changes not only with concentration but also with temperature. To better exploit this property, the researchers, led by Gustav Nyström, added 0.1 weight percent carbon nano-tubes to the mixture of HPC and water.

This renders the liquid to electrically conduct and allow the temperature, and thus the control the colour of the liquid crystals, by applying a voltage. By doing this, the carbon acts as a broadband absorber that makes the colours deeper.

By incorporating a small amount of cellulose nano-fibers into the mixture, Nyström’s team was also able to make 3D printable without affecting structural colouring and electrical conductivity.

The researchers used the novel cellulose mixture to 3D print various potential applications of the new technology. These included a strain sensor that changes colour in response to mechanical deformation and a simple seven-segment display.

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

In future, cellulose-based ink would have applications such as temperature and strain sensors in food quality control or biomedical diagnostics. “Sustainable materials that can be 3D printed are of great interest, especially for applications in bio-degradable electronics and the Internet of Things,” says Nyström.

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