Large Bubble Helmets Are Guardian Caps, NFL's New Safety Measure
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
NFL's use of Guardian padded helmet caps in camp draws support and skepticism - The Athletic
Adrian Peterson set a high bar for Robert Griffin III after ACL tear
EXPLAINER: NFL using helmet caps for player safety, research
What Are the Experts Saying About Guardian Caps – The Concussion Blog
NFL hopes to reduce head injuries with helmet experiment
Shell game: Player safety looming concern as new season approaches, Local Sports
Guardian Caps Guardian Sports
Football helmet - Wikipedia
Damar Hamlin and Sam Peltzman: Where Football Meets Economics - Econlib