We are realising now that many sports, such as Rugby, Soccer and American Football, entail brain damage by repeated, low-level impacts.
Today’s invention takes the form of leather headgear, like a sparring helmet, which contains a number of packets of dyed fluid, distributed over the interior surface (something like paintballs).
These would be designed so as to burst when subject to either a fixed peak force or to leak after a known number of such impacts.
The fluid could be some bright (but not red) colour in order to be easily seen and not confused with actual bleeding.
This would allow players who had been hit to be withdrawn from play. If causing someone’s helmet to bleed resulted in a penalty against your team, it might also provide a way to encourage players not to target the heads of opponents.