More Than Sharp: The Science of Porcupine Quills

A close-up of a porcupine quill shows the difference in texture between the tip and the base. Credit: PNAS

If you?re walking through the woods and you see what looks like a small dog clinging to a high branch, don?t investigate. That?s probably a North American porcupine up to 3 feet long weighing 35 pounds, covered in 30,000 sharp spines. And he won?t hesitate to stick you if you?re acting suspicious.

Most people know to steer clear of these spiny rodents (though some curious dogs learned it the hard way). What you may not know is that there is more to these quills than meets the eye. In a study out today in the Proceedings of the National Academy of Sciences, Jeffrey Karp of the Center for Regenerative Therapeutics at Brigham and Woman?s Hospital and a team of Harvard Medical School researchers have comprehensively described these defensive hairs.

Their quills easily penetrate the skin of a predator but are much harder to get out. And unlike the African porcupine, hedgehog, and echidna?which have smooth spines?the North American porcupine?s quills are tipped with microscopic backward-facing barbs that act like grappling hooks and lodge in the skin of a threat, Karp says. Each quill has up to 800 barbs, all of which are located on the first 4 mm of the quill?s tip. The quills easily detach from the porcupine, allowing it to escape while the sharp hair remains painfully lodged in the enemy.

?We were most surprised to find that the barbs not only increased the force required for removal, but reduced the penetration force," Karp says.

He found that barbed quills require less energy to penetrate deeper into muscle than quills without barbs because the barbs concentrate the stress of surface tension on smaller points instead of the point of the entire quill. Less pressure on lots of little points gets the same result as lots of pressure on one big point.

It?s a mathematical concept we already use for cutlery: it takes less work to cut with a serrated blade, which localizes strain at points on the tips of the teeth, than with a straight blade. ?If you?re going to cut a tomato, use a serrated knife," Karp says.

His team also tested the findings beyond porcupines. The scientists created a prototype hypodermic needle with barbs, and found that it took 80 percent less force to penetrate skin than a traditional non-barbed needle. Karp says that the more pressure is required to insert a needle, the less tactile feedback the doctor or nurse gets. With reduced required force, needle placement would be more accurate with less chance for breakage. (PM reported on moving beyond traditional hypodermics earlier this year.)

Medical technology has also been moving away from sutures and staples, which require time and diligence, toward alternatives like mechanically interlocking adhesive patches. Porcupine-quill tech could improve these too a Band-Aid-like patch with tiny barbed points could be applied with minimal force and would seal the wound quickly and securely. Hernia mesh, long a painful treatment for patients, might be more comfortable, since barbed points don?t need to penetrate the skin as deeply, Karp says.

So if you ever see a porcupine in the forest, say thank you for inspiring some next-gen medical technology. Just don?t get too close.

Source: http://www.popularmechanics.com/how-to/blog/more-than-sharp-the-science-of-porcupine-quills-14841212?src=rss

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