That's a very specific application.

Isn't there a bound on the efficacy of a helmet based on its size / thickness? I.e. your head's initial velocity and the thickness of the helmet constrain the distance over which your head's velocity must drop to zero, so there's some minimum force that must be applied to your head no matter what the helmet's material is?

Helmets have an interesting correlation with concussion. They can help a lot in cycling, hurling, and f1, but can encourage reckless collision seeking in american football for example. Contact sports are intrinsically dangerous and introducing protection from one type of damage can allow increase in other damage forms.

Avoid collisions

I reckon rugby must change the most, which is a real shame because rugby at the highest level is amazing

Though ice hockey is pretty daft

Apparently people have been experimenting with magnets in football helmets for several years now, albeit much larger ones: https://leaps.org/this-brain-doc-has-a-repulsive-idea-to-mak...

There is a strong correlation between neck weakness and brain damage in contact sports

Search "NFL neck" in a search engine and look at the images