Large Bubble Helmets Are Guardian Caps, NFL's New Safety Measure
Funky-Looking Guardian Caps Rejected by NFL for Regulation Play, OK'd for Practice - Core77
The NFL Has Introduced 'Guardian Caps' to Reduce Concussions, and Made Them Mandatory. But Only for Some Positions. And Only Through the 2nd Preseason Game.
NFL Presents: League-wide Guardian Cap Use to Reduce Head Impacts
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
We just don't have it': On matter of helmet safety, money separates local football programs
What are Guardian Caps? NFL's latest padded helmets for added safety
NFL report: concussions dropped 50 PERCENT when players wore new 'Guardian Caps' over the summer
NFL, NFLPA approve concussion-reducing helmet for quarterbacks
Guardian Caps: Why some NFL players are wearing them in training camps
Guardian Cap Debuts at Training Camps League-wide
Funky-Looking Guardian Caps Rejected by NFL for Regulation Play, OK'd for Practice - Core77
NFL Reveals What is Preventing Concussions During Offseason Training
Football Guardian Cap draws mixed reviews from players, experts for safety effectiveness - The Daily Tar Heel
NFL rejects polyurethane safety caps for football helmets