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Colour-changing polymer indicates extreme impacts

16 August 2015

A team of researchers at the University of Pennsylvania has developed a polymer-based material that changes colour when subjected to high impacts.

Recent research has indicated that soldiers and professional athletes may suffer long-term complications — such as memory loss, headaches and dementia — stemming from past head trauma. There is no easy way to tell if someone has just sustained a brain injury, so soldiers and athletes may unknowingly continue to do the very activity that caused the damage and potentially cause more harm.

However, a force-responsive, colour-changing patch could prevent additional injury, says University of Pennsylvania researcher, Shu Yang “If the force was large enough, and you could easily tell that, then you could immediately seek medical attention,” she says.

Yang’s team used holographic lithography (HL) to create photonic crystals with carefully designed structures to give them a particular colour, just like opals. Deforming the crystals with an applied force changes their internal structures, and thus the crystal’s colour.

The material does not require power to detect forces and is lightweight, thus offering an attractive way for medical personnel to identify a damaging force on-site without the use of expensive tools. However, making these crystals is an expensive process that isn’t suitable for mass production.

So the team turned to self-assembly and polymer-based materials that are cheaper to produce over a large area than the earlier HL method. The first step was to mould the polymer into a structure that worked like the specialised photonic crystals. To make a mould, the researchers mixed silica particles of various sizes and allowed them to self-assemble into crystals with the desired pattern.

They heated the polymer, which infiltrated the mould, allowed it to solidify and then removed the silica mould, leaving behind the inversed polymer crystals.

The researchers then applied varying amounts of force to the polymer crystal and recorded the colour change. The results were encouraging. “We were able to change the colour consistently with certain forces,” Yang says.

For example, applying a 30MN force — approximately the force of a car moving at 80mph crashing into a brick wall — caused the crystal to change from red to green. A force of 90MN — the equivalent of a speeding lorry hitting the same wall — turned the polymer purple.

“This force is right in the range of a blast injury or a concussion,” Yang says.

In future studies, Yang plans to develop materials that can indicate how quickly a force is applied, which affects how damaging a particular trauma is on the brain.

A video explaining the discovery and its background is available to view here.


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