New helmet science could protect brain better
A new shock-absorbing football helmet system being developed at the University of Michigan could be applied to cycling helmets researchers say.
The new shock-absorbing helmet technology, called Mitigatium, effectively dissipates the energy from impact.
“Current bicycle helmets offer protection to the skull and the brain by dissipating energy through fracture of the helmets in some collisions, but that may be their only energy dissipating mechanism,” said Ellen Arruda, professor of mechanical engineering and biomedical engineering.
“The composite technology we developed can be used to better protect the brain in collisions in which the helmet doesn’t fracture, without sacrificing their existing energy dissipating mechanism. We believe this technology can be used to better protect the brain in all types of helmets.”
When a bike helmet breaks, it’s absorbing what’s called “impulse” – a secondary effect of an initial force. Impulse, which gives objects momentum, is what transmits kinetic energy through a system. It takes into account not just force, but also how long that force was applied. To calculate impulse, you multiply the average force by the length of time it was exerted on the subject.
For head protection to be most effective researchers say it has to block impulse.
“Everyone is focused on the force of an impact and only the force,” Arruda said. “But they’ve found that when they measure peak force on the surface of the skull, they can’t correlate that with brain injury. The reason is that force is only part of the story.”
Scientists and doctors don’t fully understand how a blow to the head translates to brain injury, but the U-M researchers say impulse is a big factor. Arruda and her colleagues have demonstrated this.
They’ve taken one of the first close looks at the mechanical features of impacts and blasts and how helmets and other armor might be designed to do a better job protecting sensitive structures. To do that, they built two-dimensional mock cross-sections of materials that stood in for the brain and skull in various helmet shells. Then they use a table-top collision simulator to test the different samples. They compared how much energy was transmitted through to the brain-type layer in their own helmet system and the status quo. They used a high-speed camera to help them observe how the brain model deformed in both systems
In their experiments, the conventional football helmet model did little to block impulse, however their prototype reduced impulse to just 20% of what got through to the brain model in the conventional helmet.
The prototype helmet is made up of three materials; the first layer is similar to the hard polycarbonate that’s the shell of present-day helmets. The second is a flexible plastic and the third, ‘visco-elastic’, layer has the consistency of dried tar.
“Together these substances reflect most of the initial shock wave from a collision – most of the initial force. They also do something else unique and important: They convert the frequency of that incoming pressure wave to a frequency that the next layer can, in essence, grab ahold of and dissipate by vibrating. This third ‘visco-elastic’ layer has the consistency of dried tar.”
“We’ve come up with a totally new concept of how to make efficient impact-mitigating structures that could dissipate energy without being damaged,”said Michael Thouless, the Janine Johnson Weins Professor of Engineering in mechanical engineering and materials science and engineering.
“And we used basic concepts of mechanics to develop a fundamental understanding of how to protect delicate structures such as the brain.”
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