Biology – Hedgehog
Editor | On 27, Jan 2019
We love hedgehogs here at SI HQ. Despite the fact that helping to feed them can get expensive when a family of twenty-plus taking refuge in our garden decide they like only the best cat food, and only top-quality meal-worms to eat. Still, the sight of them all scuttling around after dark is something of a joy. And the noises during mating season are also pretty amusing.
If you ever find yourself watching hedgehog go about its day, you’ll notice that they tend to fall out of trees — a lot. Wild hedgehogs climb trees as high as 30 feet, looking for insects and food to eat. Sometimes they fall by accident, other times they fall on purpose to evade a predator or because falling is a lot faster than climbing down.
As a hedgehog falls toward the ground, it keeps itself safe by rolling into a ball to surround itself with “spines” that absorb the impact. (Hedgehog spines are colloquially referred to as “quills,” which is the official term for what porcupines have. Hedgehog spines function differently, however, than porcupine quills.) It’s an effective method of protection — and one that humans want to steal.
“The animal walks away uninjured,” says Nathan Swift, chief operating officer at Hedgemon, a company that designs hedgehog-inspired helmets. Hedgemon is one of a growing number of organisations whose inspiration comes from biomimicry. The project to use hedgehog-inspired biomimicry to craft better helmets began about four years ago with a group of students in a class co-taught by professors from the University of Akron and the Cleveland Institute of Art. The idea was to find a biological model to address impact protection, says Swift.
He explains that in any helmet, such as for football or cycling, there are usually three main layers: an outer shell, often made of polycarbonate, a middle layer for shock absorbency, and an innermost layer for padding. Hedgemon is working to improve is the middle layer. “Today’s helmets generally are inadequate when it comes to multi-hit durability to withstand multiple impacts and perform at the same level every time,” says Swift.
Moreover, he adds, they do little to protect against concussions.
A concussion happens when the brain not only knocks against one side of the skull (which happens in a straight-on hit to the head), but also when the brain bounces around, twists, turns, and gets dismantled from its regular position. “Most hits are not perfectly straight. You get those off-kilter hits that cause twisting or rotational movement,” Swift says.
The rotational movement of the brain is much more damaging than linear movement, but none of today’s helmets have any kind of rotational give. Though they can absorb impact head on, if a person hits their head hit off-center, the helmet fails to absorb that rotational energy and passes it onto the victim’s head.
Hedgemon wants to add a rotational component to helmets to fix this, and to improve general impact absorbency and multi-hit durability. Their solution is a middle layer that emulates a hedgehog’s spines. “We’re not planning to make a full helmet, we’re just making the liner,” says Swift, adding that they’ll sell that liner to other helmet companies.
Currently, Hedgemon is testing four by four squares of the liner, which will eventually be integrated into helmets. The liner itself has quills on it that bend and twirl around each other in order to not only absorb impact, but also to lessen the impact of a hit that would ordinarily cause the brain to rotate within the skull. The quills also are able to go in all different directions, depending on the hit.
“Each individual quill has an intricate, internal structure that reinforces it and allows it to buckle elastically and return back to its original state,” Swift explains. That’s just part of how the helmet layer emulates an actual hedgehog. “The other part has to do with layout: the hedgehog has about 7,000 spines. That’s not just strength in numbers, but the spines also overlap and interact with each other,” he adds. That helps spread the impact of the hit: if you push one spine, it pushes on three of its neighbors, which push on their neighbors, causing a domino effect. “It helps alleviate a lot of the force, but also strengthens the material and helps with absorption in that way,” Swift says. The helmet quills/spines will be made out of a polymer material that is similar in structure and layout to what’s on a hedgehog, he says.
To test the material, the researchers at Hedgemon drop a weighted piece of metal from a certain height straight onto the middle layer material they’re crafting. They measure the force return and use a high-speed camera to see how the material reacts, says Swift.
Hedgemon’s study, entitled “Dynamic impact testing of hedgehog spines using a dual-arm crash pendulum,” was the first to look at “the impact energy absorption capability of hedgehog spines beyond anecdotal biological evidence.” They found that greater impact speed decreases durability but not initial energy absorption. “When samples are arranged in an orientation analogous to the natural model, hedgehog spines demonstrate impact absorption capabilities that confirm their role in the protection of hedgehogs during falls,” the researchers wrote. “This study demonstrates that in certain conditions, hedgehog spines can absorb as much, if not more, than industry standard impact-absorbing foam.”
Hedgehog-inspired helmets made to absorb rotational hits and protect against concussions would be particularly useful for football players. “The NFL has a gigantic problem right now. The concussion rate is through the roof, and we’re continuously seeing the detrimental long-term effects on retired players,” says Swift. “There are numerous lawsuits involving both the NFL and the helmet manufacturers, and the sport is losing both players and viewers, not to mention all the bad press the NFL gets. If this works, they’ll be interested, they want nothing more than for this problem to go away as quickly as possible.”
Here’s the hedgehog’s problem expressed as a conflict: the need to be able to reliably and safely drop from a large height is a robustness-versus-force problem, with an element of shape thrown in to the mix:
All in all, a pretty good proxy for the hedgehog evolved solution – lots of spines (Principle 1, Segmentation), spines at different angles and orientations (Principles 3, 4 and 17), curved cross-section and profile (Principle 14), thin and flexible (30), dynamic – i.e. they deflect (asymmetrically) when impacted (Principle 15). Only the very elegant concept of successive deflection – the first deflected spine impacts the next ones to progressively cushion the impact – is probably missing from the list of Matrix recommendations, but even there its possible to make an argument that Principle 8, Anti-weight, is analogous to what’s happening.
All in all, I think I’m impressed enough to keep buying the mealworms and expensive cat food in the Autumn.