Padded helmet cover shows little protection for football players - Scope
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
FOOTBALL: NFL following Sequim's lead with protective caps
Puffy, spongy Guardian Caps could help curb NFL head injuries - The Washington Post
NFL players don Guardian Caps, designed to protect from concussion
Thoughts on Guardian Caps for Concussion Prevention - Smith Physical Therapy & Running Academy
EXPLAINER: NFL using helmet caps for player safety, research
Guardian Caps Guardian Sports
NFL stars could wear motorcycle-style helmets as the sport searches for new ways to protect players from head injury and concussion: 'There are big strides we can still take'