Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
The story behind those soft-shell puffy Guardian Caps players have been wearing
What are NFL players wearing on their helmets at training camp and how do they work? - The Mirror US
Giants react to NFL's new 'Guardian Caps' at training camps - Big Blue View
NFL hopes to reduce head injuries with helmet experiment
NFL players don Guardian Caps, designed to protect from concussion
Vikings Introduce Soft-Padded Helmets for Parts of OTA Practices
Why are players wearing padded helmets at NFL training camps?
Here's why some Patriots players are wearing caps over their helmets during training camp
Guardian Cap Debuts at Training Camps League-wide
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
Cowboys seen wearing protective, NFL mandated Guardian Caps at OTAs
Browns wearing Guardian Caps during training camp
Guardian Cap Use a Concern for Watt, Saleh Early in Training Camp - Sports Illustrated
Puffy, spongy Guardian Caps could help curb NFL head injuries - The Washington Post