Puffy, spongy Guardian Caps could help curb NFL head injuries - The Washington Post
Guardian cap' to prevent head injuries mandated for practices, CFL says
NFL Health and Safety Fact Sheet
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
Why Alabama football players are wearing padded helmets in practice
Guardian Caps NFL: What player positions are required to wear them?
What are those funny-looking soft caps on NFL helmets during camp?, Sports
Guardian Caps debut at training camps throughout NFL
NFL players experimenting with 'Guardian Caps' to prevent head injuries
Guardian Cap Debuts at Training Camps League-wide