When we recently did an overview of the evolution of bicycling technology, helmets were barely mentioned. They've been made out of the same materials for decades, and the only improvement they've seen in that time is a more efficient venting layout. But the timing of that article turned out to be propitious because, a few months later, Trek got in touch to let me know it was introducing the first major change in helmet technology in years.
Normally, emails like that are little more than marketing, or failing that, everything's proprietary and can't be talked about. But in this case, Trek promised that there was peer-reviewed science behind the announcement and I'd get the chance to talk to the scientists themselves. A few weeks later, I got the chance to check out the helmets and meet the scientists (though I narrowly missed my chance to shake hands with cycling legend Jens Voigt).
Whats a helmet actually do?
The obvious answer is that helmets are meant to protect your brain when your head experiences an impact. But the more detailed answer requires delving into a little bit of physics. On a simple level, an impact generates force that, if nothing is protecting you, is translated directly to your skull. A helmet's job is to dissipate that force. If a helmet could be arbitrarily large or heavy, this would not be a problem. But cyclists are notoriously picky about their equipment's weight and aerodynamics, which means that a helmet has to do all its redirection of forces in as little space as possible, using light materials.
Given this constraint, helmet manufacturers have settled on expanded polystyrene foam (EPS), the same material that's used for disposable ice chests. EPS was described to me as a bunch of polystyrene bubbles encased in a more diffuse mesh. On impact, those bubbles can rupture, diffusing some of the force, and the mesh can allow the bubbles to slide past each other, diffusing a bit more of it. It's simple and effective for straightforward impacts.
According to April Beard, Trek's helmet product manager, research focusing on football helmets began to indicate that EPS wasn't enough. Studies there had shown that many impacts didn't generate the sort of linear forces that EPS worked well against; instead, there were lots of twisting and off-axis forces involved. While these wouldn't necessarily translate into damage to the skull, they could put strain on the underlying brain, as different parts were subjected to distinct forces. This could cause breaks in the axons that provide connections among nerve cells, interfering with memories and other cognitive functions. Dissipating these off-balance forces is a much larger challenge.
Beard said that one idea was to provide helmets with internal liners that would slide on impact and handle some of the off-axis force. But then an engineer at Trek found a paper by a company called WaveCel that seemed to provide an even better option.
Materials meet science
WaveCel is the product of orthopedic surgeon Steve Madey and a biomedical engineer named Michael Bottlang. The two had been working on a variety of ideas related to medical issues and protective gear, funded in part by federal grant money. When considering the idea of a lightweight material that could evenly distribute forces, Bottlang told Ars that they first focused on a honeycomb pattern. But they found that it was actually too robust—the honeycomb wouldn't collapse until a lot of force had been applied, and then it would fail suddenly.
The design they eventually developed has a shape that allows flexing almost immediately when force is applied. "It starts to glide right away," Bottlang said. The manufacturing technique creates a clear point of failure that allows more extensive flexing once a certain level of force is exceeded—part of the structure will fold over rather than experiencing a complete failure. Then, once folded, the polymer it's made of will allow neighboring cells to glide over each other. This provides some resistance even after the structure has collapsed.
(The one thing that Trek kept proprietary was the identity of the polymer material.)
For the helmet, a patch of this material is attached to the inside of a more traditional EPS helmet, which provides impact resistance. But the WaveCel mesh is allowed to float within the helmet and can absorb much of the force of off-axis impacts. The thin strips of soft material found in more traditional helmets are attached directly to the WaveCel mesh.
It looks more uncomfortable than it is. Madey, the orthopedic surgeon, said they've done tests that show that, even if placed directly on the skin, the WaveCel mesh wouldn't break the skin under most impact forces.