NFL hopes to reduce head injuries with helmet experiment - The San Diego Union-Tribune
NFL players wear Guardian Caps to reduce head trauma during training camp
EXPLAINER: NFL using helmet caps for player safety, research
Guardian Caps Guardian Sports
Built By Data: NFL Helmet Innovation
NFL's use of Guardian padded helmet caps in camp draws support and skepticism - The Athletic
Large Bubble Helmets Are Guardian Caps, NFL's New Safety Measure
NFL Presents: League-wide Guardian Cap Use to Reduce Head Impacts
NFL executive hints that Guardian Caps could eventually be worn in games, along with position-specific helmets