A first step has been taken towards developing crash helmets that change colour when the wearer suffers a brain injury.
Scientists have produced a material containing opal-like polymer crystals that produce different colours according to how hard they are hit.
They hope in future the crystals will be incorporated into head gear that provides an immediate indication of trauma after a violent impact.
Serious head injuries can be disabling and life-changing, yet the effects are often hidden at the time they occur.
The new technology is aimed at turning a helmet into a trauma "traffic light" that can give paramedics and doctors an idea of the damage suffered.
US researcher Dr Shu Yang, from the University of Pennsylvania, said: "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.
"But a force-responsive, colour-changing patch could prevent additional injury."
In tests, the scientists applied varying amounts of force to the polymer crystal and observed colour changes.
Applying a 30mN force - roughly equivalent to a car travelling at 80mph crashing into a brick wall - caused the crystals to change from red to green.
A force of 90mN - the equivalent of a speeding truck hitting a wall - turned them purple.
"This force is right in the range of a blast injury or a concussion," said Dr Yang.
The research was presented at a meeting of the American Chemical Society in Boston, US.