NFL introduces Guardian Caps across league to limit number of brain injuries in preseason
Guardian Caps: Why some NFL players are wearing them in training camps
NFL training camps: Guardian Cap players are wearing over helmets, why
Guardian Caps debut at training camps throughout NFL
NFL hopes to reduce head injuries with helmet experiment
What is the NFL guardian cap and how does it work? – NBC Sports Bay Area & California
Guardian Caps: Why some NFL players are wearing them in training camps
NFL executive hints that Guardian Caps could eventually be worn in games, along with position-specific helmets
NFL's stern stance on concussion-preventing helmets, revealed
If a player wanted to wear a Guardian Cap during a game, the league would prohibit it
In fall camp, Montana State using Guardian Caps for safety, preservation of game
What are Guardian Caps and how are they helping football teams?
Truth About Guardian Caps
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
EXPLAINER: NFL using helmet caps for player safety, research