Here is an interesting idea for a helmet, designed for motorcyclists but that might work for cyclists and skateboarders down the road. 80% of fatal motorcycle accidents are caused by head trauma, and most of those are from "rotational acceleration" and subsequent "intracranial shearing"- the head is turned rapidly and the brain, floating loosely inside, sloshes around a few milliseconds later, tearing blood vessels and nerve fibers.
The Superskin helmet has a membrane on the outside, separated from the main body of the helmet by a lubricant, so that when the helmet hits the ground, the skin slides, absorbing the rotational forces.
Stephen Knowles of the International Design Consultancy (IDC) explains in Dexigner:
"Traditionally, motorcycle helmets have been rigid in design,We needed to introduce a dynamic element of movement to dramatically reduce the rotational impact which often causes life-threatening injuries. On impact, the outer membrane is able to stretch and slide over the main helmet shell to prevent these dangerous rotational forces being transmitted to the head and brain."
It is based on the work of Dr. Ken Phillips; according to Lazer Helmets,
Considering with good reason that the best helmet had already been invented by nature, he postulated that the skin on the skull, our scalp, played an all-important role in the event of an impact and was able to demonstrate that the same effect was achieved by putting a lubricated membrane on a helmet.
He sees applications for cyclists and riding helmets as well. An interesting exercise in biomimicry. More at Lazer Helmets