Bicycle helmets may be increasingly packed with technology like LED turn signals, radio communication and blind spot detection to help prevent accidents, but once a rider is involved in one, the method of protecting the head from hitting the ground hasn't really changed much. Now a team at Stanford has put self-inflating airbag helmets through their paces, with drop tests showing that they may be up to six times better at cushioning impact than traditional foam helmets.

The airbag helmet in question is the Hövding, which, most of the time, is a collar waiting to be a helmet. It is designed to sit around a rider's neck like a puffy scarf, but when it detects a collision, it quickly inflates and wraps around the head in time to soften the blow. The Hövding is already available in parts of Europe, but hasn't yet made it to the US.

CE-standard tests suggest that this design perform well, but to see how it might improve on conventional helmets, Stanford bioengineer David Camarillo and his team put the airbag helmet through standard drop tests. The pre-inflated helmet was placed on a dummy head fitted with accelerometers, and dropped onto a metal platform from different heights ranging between 0.8 and 2 m (2.6 and 6.6 ft). The linear acceleration of the impact was measured as the dummy struck the ground with either the top or side of the head.

"We conducted drop tests, which are typical federal tests to assess bicycle helmets, and we found that airbag helmets, with the right initial pressure, can reduce head accelerations five to six times compared to a traditional bicycle helmet," said Mehmet Kurt, one of the study's authors.

Linear acceleration is a good measure of the risk of a skull fracture and other head injuries from a given impact, but it doesn't necessarily say anything about the wearer's chances of sustaining a concussion. Studying this was a particularly personal goal of Camarillo's, having suffered two cycling-induced concussions himself.

"Foam bike helmets can and have been proven to reduce the likelihood of skull fracture and other, more severe brain injury," Camarillo says. "But, I think many falsely believe that a bike helmet is there to protect against a concussion. That's not true."

The reason for that is that scientists still don't fully understand specifically what about a blow to the head causes a concussion. Currently, the leading theory is that a twisting motion, more so than a linear one, is responsible, and although protection against this kind of injury wasn't directly measured in the team's tests, they believes the airbag helmets would probably still perform better than their foam counterparts thanks to their larger size and softer surface.

"There are many theories as to why concussion happens, but the predominant one is that, as your head rotates very quickly, the soft tissue within your brain contorts and, essentially, what you get is a stretching of the axons, which are the wiring of the brain," says Camarillo.

Next up, the team wants to measure how much the airbag helmets defend against these unwanted brain contortions, and determine how that can be improved, with an ultimate goal of improving the safety laws around helmet testing to cover concussion protection. Studying how the helmets fare from higher falls and what levels of inflation are necessary are other subjects the team has slated for further research.

In the lab, the helmets were pre-inflated to optimum pressure, but in the real-world, they're designed to inflate automatically when a collision is detected. This process may not guarantee that the airbag will inflate to the safest pressure, in which case the wearer's noggin may be worse off than if it was wrapped in a regular helmet. However, future versions may be able to predict how bad an impact is going to be and inflate itself to the optimum pressure.

The research was published in the Annals of Biomedical Engineering.

The team discusses the project and demonstrates the drops in the video below.

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