NFL hopes to reduce head injuries with helmet experiment
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
Keep Your Team Fresh For Gameday With Guardian Caps
If a player wanted to wear a Guardian Cap during a game, the league would prohibit it - NBC Sports
Guardian Caps Exceed NFL's Expectations - Guardian Sports
What are Guardian Caps and how are they helping football teams?
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
Guardian Sports - The Science Behind The Guardian Cap
NFL mandates extra head protection for training camp
Guardian cap' to prevent head injuries mandated for practices, CFL says
NFL introduces Guardian Caps across league to limit number of brain injuries in preseason
Puffy, spongy Guardian Caps could help curb NFL head injuries - The Washington Post