Structural Colour
Brilliant by nature
Explore the hidden beauty of one of nature’s most abundant materials — cellulose. Discover how this plant-derived biopolymer can create shimmering structural colours, inspire sustainable innovation, and build bridges between science and art.
What is cellulose?
Cellulose is a long chain of glucose units produced by virtually all plants and many other organisms, making it the most abundant biopolymer on Earth. In its native form, the chains are tightly stacked together, forming semi-crystalline fibrils that provide mechanical strength to plant cell walls. When arranged in a helicoidal structure, cellulose fibrils create a phenomenon called structural colouration, which produces some of the most vibrant and dazzling colours found in nature, such as those seen in the fruits of Pollia condensata and Margaritaria nobilis. These natural examples inspire us to explore ways to use biopolymers as a sustainable source of colouration.
Why cellulose?
- Renewable—sourced from plants
- Biodegradable—breaks down naturally
- Versatile—chemically modifiable for different applications
The origin of colours
Nature creates colour in two main ways: through conventional pigments or dyes, which absorb specific wavelengths of light, and through structural colour, which arises from the interference of light with periodic nanoscale structures. In structural colouration, the periodic arrangement of the material causes interferences between light waves that are either constructive or destructive, leading to the selective reflection of a given wavelength. Therefore, although cellulose itself is transparent, it can generate colour upon forming a highly ordered structure with dimensions comparable to the wavelengths of visible light. Unlike conventional colouring agents, which degrade upon interacting with light, structural colours do not fade.
Cellulose-based materials
Colourful films made from cellulose
Cellulose can be extracted from natural sources in the form of thin, rigid rods known as cellulose nanocrystals (CNCs). CNCs are less than a micron long—too small to be seen by the human eye, even under an optical microscope. Still, upon drying, they can form films with dazzling colours. This phenomenon arises from the spontaneous self-assembly of the particles into periodic structures called liquid crystal phases. More precisely, the CNCs form a stair-like (helical) structure made from rotating layers of locally aligned CNCs.
Light interference within this periodic structure leads to the selective reflection of certain wavelengths and not others. The reflected wavelength is proportional to the spacing between the CNC layers: a larger spacing reflects longer wavelengths, so the colour is red-shifted, while a smaller spacing reflects shorter wavelengths, causing a blue shift.
In recent years, researchers have developed techniques to guide the self-assembly of CNCs, enabling precise control over film colouration. Today, CNCs are commercially available and can be used to create colourful films, droplets and particles derived from natural feedstocks.
In our lab, we have made significant advances in scalable pigment production through the use of an industrial roll-to-roll coating system. This technology allows the fabrication of cellulose-based photonic films from nano to meter-scale, which can be ground into vibrant pigments and glitter. These innovations offer a sustainable alternative to conventional pigments, delivering intense colour and shimmer while being biodegradable and biocompatible. They can replace plastic-based glitter in textiles and household products, and can compete with environmentally problematic titania and mica-based effect pigments in cosmetics, food, paints, and packaging.
While significant progress has been made, further research is essential to fully realize the potential of these innovative materials. Ongoing work focuses on improving the sustainability of CNC production and enhancing the homogeneity of their resulting structural colouration.
Hydroxypropyl cellulose
Hydroxypropyl cellulose (HPC) is another cellulose derivative that exhibits vibrant colours in its liquid crystalline phase and responds to external stimuli. As with the CNCs, the bright colours exhibited by HPCs are due to structural colouration. When mixed with water in the right proportions, the polymer chains self-assemble into a twisted stack that selectively reflects light within a narrow range of wavelengths matching the periodic dimensions of the structure.
This property makes it an easily fabricated, processable coloured material, with colour tuning achievable simply by altering the water content. Additionally, HPC is responsive to changes in pressure and temperature. HPC offers the benefits of being biocompatible and edible, making it a common thickening agent in pharmaceuticals and food products (for example, it serves as a thickener in sauces and dips, as well as a stabiliser in whipped cream).
In our lab, we have explored how helicoidal structures in hydroxypropyl cellulose respond to vertical compression. Using a roll-to-roll machine, we developed a large-scale pressure sensor that visually maps pressure in a way that is intuitive and easy to interpret.
Art
The artist Lidia Sigle is collaborating with the Department of Sustainable and Bio-Inspired Materials. Inspired by photonic architectures that some plants evolved, Sigle is working with cellulose to create iridescent structural colour.
Iridescence is the phenomenon of certain surfaces appearing to change colour as the angle of view or the angle of illumination changes. Constructive and destructive interference result in different colours appearing at different angles. At certain angles the waves add, giving a strong reflection of just one wavelength—a pure colour.
Want to Learn More?
Visit our group websites:
https://www.ch.cam.ac.uk/group/vignolini
https://www.mpikg.mpg.de/sbm
Detailed overview of CNC self-assembly in our comprehensive review article