A type of plastic painted onto helmets could soon be used to warn sportsmen or soldiers if they have 
suffered a head injury.
Head injuries are common in sports as well as the military, and can cause severe health impacts such as memory loss, headaches and dementia.
In the 2014 season alone there were 123 recorded concussions in the American National Football League (NFL) and 47% of all reported sports concussions occur during high school american football.
Since these effects are often invisible or delayed, brain injuries can easily go unnoticed by both the victim and medical professionals. This can lead to further injury, and sports such as American Football can often face lawsuits due to injured sportsmen continuing play.
Now, a team of researchers led by Professor Shu Yang at the University of Pennsylvania have developed a material which changes its colour when a force is applied to it. The resulting colour depends on how hard it is hit.
The application of this is in integrating the material with protective headgear. If a rugby player wearing this helmet went into a violent scrum, the helmet would change colour. For example, orange could indicate a hard hit, and that you need to seek medical advice, whereas green would indicate that you could continue play.
The forces required to cause a colour change in this material are just in the right range for sports or military injuries. For example, a 30mN force (roughly equivalent to a family car crashing into a brick wall from 80mph) causes the polymer to change from red to green. A 90mN force (this time, equivalent of a truck) results in a purple colour.
An important aspect of this new material is that the colour is fixed. So after a force is applied, the resulting colour does not disappear. This would enable the victim to go into hospital, and the doctors would at a glance be able to tell the severity of the problem.
The material itself is polymer-based, and known as a "photonic crystal", meaning the underlying structure of the material causes light reflection. Shu was interested in developing specific nano-sized microstructures that reflect the light in a certain way. When the material is impacted with a force, the structure is deformed, resulting in a colour change.
The type of polymer used is called a thermoplastic - often used for door knobs and plastic boxes. This is a type of plastic which can change shape at higher temperatures, but when it is cooled, its shape is locked. This enables the structure (and corresponding colour change) to be locked in place after the force is applied.
The structure itself is termed "inverse opal", just like the gemstone. "What we tried to do is replicate this structure into a polymer" explains Shu, resulting in a porous structure which reflects light. The light coming off different pores interacts with each other, resulting in some wavelengths of light being enhanced, and some being reduced. This gives in a characteristic colour.
A key factor which will enable this technology to infiltrate the market sooner is its ability to be easily mass-produced. Polymer-based materials are relatively cheap to fabricate, and can be made using self-assembly. The polymer was simply heated and then poured into a mould made of different sized silica particles. These particles self-assemble into the required structure. When it has cooled, and the shape is therefore set, the silica mould is removed. This results in the "inverse" structure.
"We want to have a material that is power-free and film-based, so it's lightweight and mountable" said Yang. This is where this technology differs from what has been before. This material requires no batteries or external power to enable the colour changing technology. As it is a film, it can be easily incorporated onto helmets simply by using an adhesive, and so should be seen on the market relatively soon.
Traumatic brain injury caused the deaths of over 50,000 people in 2010. With a huge number of cases due to sports and the military, it is hoped that this new polymer will easily be able to knock this number right down.
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