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Thursday, December 13, 2012

The secret of the porcupine’s quill

North American porcupine quills are notoriously easy to stick into something but hard to pull back out. A dozen researchers from MIT, Harvard, Xi’an Jiaotong University and Trinity College have now cracked the puzzle as to why this is so. And in case you’re wondering why you should care, consider that there are a lot of medical devices that also function by being stuck into people.

Porcupine quills are composed of a black tip, typically a few millimeters long, and a much longer white shaft. The tip contains microscopic backward-facing barbs, each less than 50 microns (millionths of a meter) across. The researchers compared normal porcupine quills to quills that had been carefully sanded down to remove those tiny barbs without altering the overall shape of the quill. The barbed quills required 54% less penetration force than the sanded quills. Interestingly, it’s the region 2-4 mm from the tip that’s critical for reducing penetration force, and not the very tip. Quills with the barbs sanded down only in this region required significantly more force to penetrate flesh regardless of whether the barbs in the first 2 mm were intact.

It turns out that a barbed porcupine quill requires less penetration force than an 18 gauge hypodermic needle. Though, to be fair, you’ll rarely need to be stuck with such a large needle unless you’re giving blood. So, I don’t think you’ll be seeing your doctor pull out a porcupine quill the next time you get your flu shot. On the other hand, the backward facing barbs somewhat counterintuitively result in a cleaner cut with less tissue damage. This is despite the fact that the quills with barbs penetrate deeper into flesh and are harder to remove. This characteristic may prove useful for implanting medical devices.

And now, a random porcupine.

Woo Kyung Cho, James A. Ankrum, Dagang Guoa, Shawn A. Chester, & et al (2012). Microstructured barbs on the North American porcupine quill enable easy tissue penetration and difficult removal Proceedings of the National Academy of Sciences of the United States of America : doi: 10.1073/pnas.1216441109.