Head injuries can be particularly sinister -- unless it's obvious that you took a bad blow, you might not get the help you need in time to mitigate the damage. University of Pennsylvania scientists may have an easy way to raise the red flag when you're hurt, however. They've crafted a polymer that changes color depending on the amount of force, giving anyone around you a sign that you've suffered some trauma. The key was to use holographic lithography to create photonic crystals whose structures change color when they deform. They don't require any power to work, and they're light enough that you could apply them to a helmet without noticing a significant weight difference.