EXPLAINER: NFL using helmet caps for player safety, research
What's with padded helmet caps teams are wearing
Large Bubble Helmets Are Guardian Caps, NFL's New Safety Measure
Puffy, spongy Guardian Caps could help curb NFL head injuries - The Washington Post
NFL Presents: League-wide Guardian Cap Use to Reduce Head Impacts
Guardian Caps: NFL requires use of helmet cover at training camps to reduce head injuries
Xaverian High in Mass. introduces Guardian Caps for football players
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
NFL executive hints that Guardian Caps could eventually be worn in games, along with position-specific helmets
Padded helmet cover shows little protection for football players - Scope
Guardian Caps: The new helmets worn by several NFL teams