Potential for the development of biodegradable sensors and displays: Researchers have developed a material from the natural substance cellulose that conducts electricity and can change its color when heated or stretched. It can also be used as ink for 3D printing. The scientists see possible future applications, for example, for temperature and deformation sensors, for monitoring food quality or for biomedical diagnosis.
Artificial, difficult to degrade and in some cases harmful: many materials used in human engineering have problematic properties, as is well known. The "technology of nature", on the other hand, is predominantly characterized by sustainable, harmless and biodegradable substances. The most common natural substance is cellulose. This complex multiple sugar compound (polysaccharide) is the main component of plant cell walls and is responsible for the strength of fibers or wood, among other things. Man has been using this material in the traditional way as a construction or raw material for a long time.
Natural made technically usable
The interdisciplinary research team "Cellulose & Wood Materials" from the Swiss Federal Laboratories for Materials Testing and Research (Empa) in Dübendorf, on the other hand, is exploring the potential of the natural substance for modern technology. They can already point to exciting successes: “In our laboratory we have already developed various electronic components based on cellulose – such as batteries and sensors. Now, for the first time, we have also been able to develop a cellulose-based display,” says team member Xavier Aeby. As reported by Empa, the scientists used a derivative of the biomolecule as the starting material for this project. The so-called hydroxypropyl cellulose (HPC) has special features due to its structural features, but remains harmless: HPC is used in medicines, cosmetics and food, among other things.
Adjustable structure colors
The researchers have now taken advantage of the fact that HPC forms liquid crystals in water. Other versions of such structures are already known to be used in LCD screens. As the researchers explain, the hydroxypropyl cellulose liquid crystals also have interesting potential: Depending on the crystal structure, which depends, among other things, on the HPC concentration, they shimmer in different colors. This is not based on color pigments, but on the effect of the so-called structural coloring. Microscopic structures split the incident light into spectral colors and reveal certain wavelengths. This effect also causes the iridescent color phenomena of some bird feathers or butterfly wings, for example.
The team also took advantage of the fact that the color of HPC is influenced not only by concentration but also by structural effects such as strain and temperature changes. To control this, the researchers added 0.1 percent by mass of carbon nanotubes to the mixture of HPC and water. As they explain, this makes the substance electrically conductive, which means that it can be heated by applying voltage. The color of the liquid crystals can then be adjusted in this way. The carbon even provided a beneficial additional effect: the colors glowed more intensely, the team found. With a further addition, the scientists were then able to optimize the properties of the liquid to such an extent that it is also suitable for use in 3D printers: A small amount of cellulose nanofibers led to a favorable consistency without affecting coloration and conductivity.
Promising potential
The team now sees considerable potential in the concept. As a demonstration, they have already produced various application examples from the new cellulose mixture. These include a strain sensor that changes color depending on mechanical deformation, and an element that changes color when it is heated through skin contact. They also demonstrated the potential for biodegradable electronic units: they built a simple display made of seven conductive segments that change color when subjected to electrical heat. The researchers are convinced that there are various possible uses for the cellulose-based mixture: for example for temperature and deformation sensors, for monitoring food quality or for biomedical diagnosis.
"Sustainable materials are of great interest," emphasizes laboratory manager Gustav Nyström from Empa. As the scientists emphasize, however, they are only at the beginning of the development of the concept: "There are still many unanswered questions about how the structural colors come about in the first place and how they can be changed by different additives or environmental influences," says Nyström. Through their continued work, he and his colleagues hope to uncover even more interesting phenomena and possible applications.