Pressure sensor reveals the colours of stress

Developed at the University of California, Riverside, the technology could also be applied to devices that rely on the application of pressure in order to operate.

The lab of Yadong Yin, an associate professor of chemistry, used a self-assembly method to string together gold nanoparticles which they then embedded into a polymer film. The film deformed when pressed, stretching the gold nanoparticle strings by increasing the separation between neighbouring gold nanoparticles.

‘This increased separation alters the way the nanoparticles interact with light,’ Yin said in a statement. ‘When linked together, the gold nanoparticles originally appear blue. But they gradually change to red with increasing pressure as the nanoparticles start disassembling. This easily and visually helps us figure out how much pressure has been applied.’

Study results appear this month in Nano Letters.

The sensor that Yin’s lab developed is said to differ from commercially available pressure sensor films as the latter indicate pressure by changing the intensity of just one colour. They tend to be difficult to interpret and have low resolution and contrast.

The new technology produces a mosaic of easy-to-distinguish colours and has the benefit of higher contrast and resolution. It can potentially be used in many safety devices for revealing pressure distribution over even very complex surfaces.

‘The many electronic stress sensors commercially available are bulky and not suitable for certain applications,’ Yin said. ‘For example, it is difficult to tell the stress distribution over a particular area if the contact surfaces are not flat and uniform. Our sensor films can be painted on the contact surfaces so that the colour variance in different areas clearly shows the stress distribution over the contact surface.’

While his lab used gold in the experiments, silver and copper could also work, Yin said. The sensor the lab developed is a solid plastic film. Under stress, it deforms like conventional plastics. The new colour that arises persists after the stress is removed.

‘This is why we are calling it a ’colorimetric stress memory sensor,’ Yin said.

One of the research interests of his lab is the design of materials with new properties via the self-assembly process. The lab first makes nanoparticles and then organises them together to produce new properties arising from particle-particle interactions.

‘In the case of our sensor, we initially found a way to organise gold nanoparticles together to form strings,’ Yin said. ‘That process is accompanied by a sharp colour-change from red to blue. We speculated that the reverse - disassembly - process might have the reverse colour change: from blue to red. We found to our surprise that mechanical force could achieve this disassembly. Considerable effort has been made by researchers to study nanoparticle self-assembly. Indeed, gold nanoparticles have conventionally been used as sensors based on the self-assembly process. What is novel about our work is that it shows that the disassembly process can also find great applications if the assembly is designed to be reversible.’