Impact test comparisons of 20th and 21st century American football helmets in: Journal of Neurosurgery Volume 116 Issue 1 (2012) Journals
Technology can't save football players' brains
Built By Data: NFL Helmet Innovation
Helmet safety: Inside the complicated equipment industry - Sports Illustrated
Helmet manufacturers, coaches try to protect players with advanced technology
Football helmets have improved but are far from 'concussion proof' – San Bernardino Sun
FOOTBALL: Guardian Caps gaining ground as concussion-prevention aid
Football helmet - Wikipedia
New Helmet Technology Aimed to Make Football and Other Sports Safer
More padding the issue of concussions and better helmets
How football helmets fail to protect against some of the most dangerous hits in the game - The Verge
Do helmets protect against concussion? - Queensland Brain Institute - University of Queensland
Helmet Safety Unchanged as Injury Concerns Rise - The New York Times
New tools in fight against concussions – The Denver Post
NFL Studying Helmet Face Guard That Works Like Surgical Mask – NBC4 Washington