If you’re unlucky enough to have met the business end of a porcupine—or if you have a pesky dog that has—you probably know a thing or two about porcupine quills. Scientific research is now catching up: a new study in PNAS takes a closer look at porcupine quills. It illustrates why they penetrate skin and muscle so easily, and why they are so difficult to remove. Furthermore, it suggests the structure of these quills may have useful medical applications.
North American porcupines have as many as 30,000 quills that are ready to be deployed when a predator comes into contact with the porcupine (contrary to popular belief, the quills can't be shot through the air). Unlike the quills of hedgehogs, echidna, and even the African porcupine, the tips of the quills of North American porcupines are covered in overlapping backward-facing barbs.
To figure out how these barbs affected quill function, the researchers compared normal North American porcupine quills to quills that had been carefully sanded until they were smooth and barb-free. The smooth quills required more than fifty percent additional force to penetrate tissue than barbed quills did, and they actually caused more damage as they entered tissue. Barbs help quills enter cleanly, since they create high stress concentration points in the tissue, lessening the overall force needed for penetration. Serrated knives operate under a similar principle, helping them cut cleanly and easily through food (or skin, if you’re a surgeon or serial killer).
Once a North American porcupine quill is embedded in tissue, the barbs make it difficult to remove. Quills with barbs required 0.33 N more force to remove than those without barbs. It turns out the tiny barbs are either deployed or bent once the quill is in the tissue, interlocking with the tissue fibers and increasing resistance. The researchers found that the barbs within 1mm of the tip of the quill make the most difference in the “pull-out force” needed, although barbs all along the first 4mm of the quill cooperate in increasing the difficulty of extraction.
The researchers realized they could apply these findings to medical technology to increase the efficacy of various medical devices. Natural innovations have a long history of inspiring man-made materials. Velcro was inspired by the sticky burrs of the burdock plant, and high-performance swimsuits are based on the hydrodynamic qualities of shark skin. Similarly, porcupine quills could serve as models for materials that need to either enter the skin cleanly and easily, or stay adhered to the skin.
The researchers actually built two medical prototypes to illustrate the efficacy of the porcupine quills. First, they created a polyurethane hypodermic needle with microscopic barbs; this needle required 80 percent less force to penetrate muscle tissue than a similarly shaped needle without barbs. Then they created a patch with microscopic quills on its underside, which was able to adhere incredibly tightly to pig skin by interlocking with the tissue. It took thirty times more work to remove the barbed patch than a similar patch without barbs. Patches like these could be extremely effective in delivering anesthesia or other medication, or in joining tissue together.
The study, however, doesn't address the amount of tissue damage that occurs when the quills—or the man-made barbed patch—are pulled out of skin or muscle.
PNAS, 2012. DOI: 10.1073/pnas.1216441109 (About DOIs).
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