Concussions: Can cap keep football players' heads in game? – Orange County Register
The impact of 'Guardian Caps' on player safety
What are Guardian Caps? Shadow Hills football using new device to reduce head injuries
These puffy helmet caps are the next big thing in NFL player safety, National Sports
More padding the issue of concussions and better helmets
Brown] All the Bills' linemen are wearing an extra helmet protection this year. It's been done in college at least a few years. First for the Bills. Believe they are Football Guardian
NFL players wear Guardian Caps to reduce head trauma during training camp
Football helmet - Wikipedia