What is the NFL Guardian Cap? Will it do anything to reduce concussion? — Dr. Mark Heisig
NFL announces more steps to reduce head injuries
NFL players experimenting with 'Guardian Caps' to prevent head injuries
EXPLAINER: NFL using helmet caps for player safety, research - The San Diego Union-Tribune
The Coat of Arms Added Protection or False Security: Football Tests New Guardian Caps to Prevent Concussions
NFL hopes to reduce head injuries with helmet experiment
What Pros Wear: Top 5 Helmets Rated by the NFL & NFLPA - What Pros Wear
NFL's use of Guardian padded helmet caps in camp draws support and skepticism - The Athletic
Guardian cap' to prevent head injuries mandated for practices, CFL says
Eagles players on Guardian Caps: 'We look a little goofy,' but all for safety
NFL says Guardian Caps reduced concussions by over 50%
NFL hopes to reduce head injuries with helmet experiment