Making artificial colour-changing skin

Materials World magazine
,
27 Sep 2019

An artificial material that mimics the colour-changing ability of cuttlefish skin has been developed as a potential camouflage. Idha Valeur finds out more.

An artificial replica of cuttlefish skin, which is able to change colour depending on temperature, has been developed by researchers from the University of Cambridge, UK. Applications for the material could range from camouflage or large-scale dynamic displays.

Cuttlefish and zebrafish can change colour due to their skin cells, which have fibres that move pigments around as the skin contracts. When spread out the pigment is visible, and when contracted the cell appears blank. The artificial skin was developed based on this process.

The material is made using a base of gold nanoparticles with a polymer shell coating, according to University of Cambridge co-author, Dr Andrew Salmon.

The gold nanoparticles have a poly(N-isopropylacrylamide) (pNIPAM) polymer shell coating and are suspended in water. ‘This polymer has a temperature-dependent phase change. Below 32°C it is hydrophilic, causing its chains to be extended, while above 32°C the polymer becomes more hydrophobic and the chains collapse,’ Salmon told Materials World. ‘This is an entropy effect. At high temperatures the higher entropy state is more favoured, which means releasing the water that is geometrically ordered around the polymer chains,’ he added.

‘We coat gold nanoparticles with a shell of this switchable polymer. Below 32°C, the polymer stabilises the particles, above 32°C the shell collapses and causes the particles to stick together. We put these switchable nanoparticles at high concentration, 1,014ppm, into water-in-oil droplets.’

Salmon explained that two main occurrences enabled the material’s colour change. Firstly, the nanoparticles’ resonance frequency changes when they are stuck together. The oscillation modes couple and transition from transmitting red light to blue light.

‘Secondly, the droplets help to localise the nanoparticle aggregates together, and the nanoparticles increasingly shadow each other. This makes the absorption of light less efficient and increases the transmission. So when the nanoparticles aggregate together in the hot state, the overall the transparency increases,’ he said.

Material choice

Salmon explained that the gold nanoparticles are especially beneficial due to their strong colouration. ‘This is because the free electrons in the metal have a natural frequency at which they oscillate in resonance with the electric field of light,’ he said adding that, ‘in principle, there
is nothing stopping us using other nanoparticles, which is, in fact, how we plan to achieve different colours. Carbon for black and silver for yellow, for example.’

From single to multiple layers

At present, the material developed has a single layer, so it can only emit a single colour. The plan is to create a fully dynamic material by stacking several chromatophore layers on top of each other. ‘If we can switch those independently between the transparent and coloured states then it increases the range of colours you can achieve,’ Salmon said. T’his is the same trick that animals use. They have layers of chromatophores and they might, for example, make a top layer transparent to show another coloured layer beneath.’

According to Salmon, the trick is to make several layers that each respond to different factors, such as temperature, pH levels or ionic strength, when independently triggered. This can be achieved by changing the material of the polymer shell, which is a key aspect the team is working on.     

Colourful applications

Applications for the responsive material include smart colour displays. ‘Think colour-changing wallpaper. For small displays, LCD screens are obviously extremely good, but for large areas, those become hard to make and would use a lot of power,’ Salmon explained.

‘Less obvious, but something we are really interested in is integrating chemical triggers into the switching mechanism. If we could combine this with established biochemistry then the colour change could be used as a low-cost readout for disease assays,’ he added.

The team is currently conducting follow-up work and investigating opportunities for commercialisation. The research has received funding from the European Research Council (ERC) and the Engineering and Physical Sciences Research Council (EPSRC).