Football helmet - Wikipedia
Thoughts on Guardian Caps for Concussion Prevention - Smith
Colleges add Guardian football protective helmet cap - Sports
Concussions among players who wore Guardian Caps over summer
NFL hopes to reduce head injuries with helmet experiment
Concussions among players who wore Guardian Caps over summer
How a soft-shell helmet is limiting the risk of concussions at
Guardian Caps Can Help Reduce Head Impacts - Goldberg, Persky
Guardian Caps - Are the soft-shelled football helmet covers
Puffy, spongy Guardian Caps could help curb NFL head injuries