Biodegradable sensors and displays via 3D printing

The researchers used hydroxypropyl cellulose (HPC), which is used as an excipient in pharmaceuticals, cosmetics, and foodstuffs, as a starting material. A special feature of HPC is that it forms liquid crystals after water is added. These have a remarkable property: depending on the crystal structure – which depends, among other things, on the HPC concentration – they shimmer in the most diverse colours, although they are actually colourless or pigmentless. This phenomenon is called structural colouration.

The structural colour of HPC changes not only with concentration, but also with temperature. To better exploit this property, the researchers at the Swiss Federal Laboratories for Materials Testing and Research (Empa) led by Gustav Nyström added 0.1 mass percent carbon nanotubes to the mixture of HPC and water. This makes the liquid electrically conductive and makes it possible to control the temperature – and thus the colour of the liquid crystals – by applying an electrical voltage. With another additive, a small amount of cellulose nanofibres, Nyström's team also managed to make the mixture 3-D printable without affecting colouring and conductivity.

Sustainable sensors and displays

Using 3D printing, the researchers produced various application examples from the novel cellulose mixture. These included a strain sensor that changes colour depending on mechanical deformation and a simple display consisting of seven electrically controlled segments. 

In the future, the cellulose-based ink could find many different applications, such as temperature and deformation sensors, food quality control, or biomedical diagnosis. "Sustainable materials that can be 3D printed are of great interest, among other things for applications in biodegradable electronics and for the Internet of Things," Nyström says.