Chameleons are able to deliberately change their colour, as well as some species of squid. Butterflies and beetles shimmer in a stunning variety of colours. Products which change colour have long been a focus of research and development, the main goal in many cases being to use colour changes as an easily readable and clearly understandable signal of an invisible external influence occurring or of the presence of an invisible substance. Colour changes may be caused by fluctuations in temperature, humidity or light, and by mechanical pressure, chemical reaction with a substance and by electric current, among other factors. Indicator dyes having specific properties, colours or colour change characteristics can be synthesised in targeted fashion for a host of innovative applications.
Structural Colours like on Butterfly Wings
Polymers are significantly easier to process than metallic compounds, but they are generally colourless, requiring the addition of pigments. Inspired by the natural world, Andrew John Parnell of the University of Sheffield has developed colour-rich polymer materials, which need no pigments. This is accomplished through photonic block copolymers, which have a structure that produces colour in a manner similar to thermochromic liquid crystals, reflecting and interfering incident light at various surface areas. Having special structures like those found in peacock feathers and butterfly wings, block copolymers exhibit a metallic shimmer and different colours depending on the viewing angle. Highly regular step-like geometries create interference with adjacent light beams, which are reflected at various high levels. Photonic components based on block copolymers are especially attractive as they are able to form many of these structures having differing properties on their own through self-organisation. Additionally, the materials are easy to process. Initial attempts to commercialise the photonic block copolymers and use them for anti-counterfeiting purposes however have failed because it is not yet technologically possible to achieve such thin layer thicknesses. Parnell is now collaborating with the Natural History Museum in London to investigate further structures and colours like those found on butterflies, beetles and bird feathers to close the gap with the fine layer thicknesses found in the natural world.
Stretch-Induced Colour Changes
The interference effect is being exploited as well by Pete Vukusic of the Biological Photonics research group at the University of Exeter and by Mathias Kolle of Harvard University in efforts to develop textile fibres which change colour when stretched, inspired by the seeds of the bastard hogberry, which have a metallic blue shimmer. Vukusic has discovered that this particular colour is the result of interference caused by several layers of cells having a highly regular cylindrical structure. Kolle has succeeded in applying this structure onto textile fibre material by wrapping a double-layered material around a core filament. The bottom layer is made of the polymer PDMS, the upper layer of polystyrene PS-PI. Winding several times around gives the yarn the refractive indices, curved shape and cylindrical cross-section required. Stretching alters the layer thickness so the colour changes as other wavelengths interfere destructively and constructively. The advantage of the roll-up production technology is the precise creation of multiple layers from a double-layered starting material. The length of fibres produced is very limited at this time, but plans are to achieve the production of single fibres several kilometres in length. Existing extrusion methods for making continuous filaments are currently insufficient for producing such thin and precise coatings. In addition to colour-based strain sensors Vukusic sees a potential application in sports clothing, which renders muscle contractions more visible.
Pigment colours can be deliberately changed and change through natural processes as well, an effect, which is often undesirable such as yellowing and fading. Berlin designers Essi Johanna Glomb and Rasa Weber of design studio Blond and Bieber are making use of this reaction. With the support of the Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB) in Stuttgart they are developing a method to dye textiles using microalgae which change colour under light effects. Rather than simply getting brighter as one might think, their colour shifts, from green for example to an arctic turquoise. Another variety turns from an initial green into pastel yellow over time, while a red alga turns either pink or yellow depending on the binding agent used. Starting out the designer duo was exclusively concerned with using microalgae – a largely untapped but abundantly available resource – for dyeing textiles due to the fascinating spectrum of colours they exhibit. Their chief aim was to arrive at a production method, which is accessible and easy to understand for users and observers. The modest algae proved excellent to work with, as they grow in regular water inside a glass flask functioning as photobioreactor. The algae only have to be ventilated two to three times a day to ensure they have enough carbon dioxide. Once the algae concentration is high enough, as evident from the colour of the water, the algae are filtered on a specially built algae growing textile printing apparatus made like a pull-cart. Using a binding agent Blond and Bieber imprint the algae onto textiles without pulverising them using a print roller built into the cart or via screen printing. The designers are currently investigating a range of binding agents, both biological and chemical, but algae do not react that well with chemical ones. A binding agent typically used in the textile industry works better, which itself is made from algae. It was only noticed later that the algae had changed colour during the first weeks of processing, but this interested Blonde and Bieber, as they believe users become more attached to products, which change during use. The rate of colour change varies depending on the application and corresponding light exposure – shoes change colour more slowly than curtains, for example. In addition to optimising the manufacturing process, Blond and Bieber are also experimenting with colour changes for specific patterns by printing superimposed layers, partially pre-exposing prints or printing algae of the same colour which turn different colours under light exposure.
Mareike Gast is an industrial designer with her own business in Frankfurt, specialising in new materials and technologies. In close cooperation with industry and research, she develops innovative products and product strategies. In addition to her work in product development, she also regularly teaches at various international universities.