To pierce your skin, a porcupine quill needs only about half the force of a hypodermic needle, according to a new study. The work, which also explains why the quills are so hard to remove, could improve the design of a variety of medical instruments, from devices that poke us to those that help keep wounds shut.
Porcupines are famed for their quills, which are actually large, stiff hairs that help defend the animals against natural predators. Contrary to popular notions, the large rodents can't throw their quills at an enemy, says Jeffrey Karp, a bioengineer at Harvard Medical School in Boston. However, the quills are readily shed and can become firmly embedded in an unfortunate victim.
The North American porcupine has about 30,000 quills, each one adorned with between 700 and 800 barbs along the 4 millimeters or so nearest its tip. Although those barbs help the quills remain embedded in a victim's skin, scientists haven't studied the details of how they do so. To better understand the function of the barbs and to determine if they might be useful for medical devices, Karp and his colleagues conducted several lab tests—with, in some cases, unexpected results.
The researchers took barbed porcupine quills and plunged them into samples of pig skin, measuring how much force it took to pierce the flesh and then how much force was required to extricate the quill. The team then performed the same tests using quills whose barbs they had sanded off. They also tested an African porcupine's quills (which naturally have no barbs) and an 18-gauge hypodermic needle, which is approximately the same diameter as a quill from the North American porcupine.
To the scientists' surprise, barbed quills required approximately half the penetration force of the barbless quills—either those naturally barbless or those sanded clean—and only 56% of the force needed for the hypodermic needle to breach the skin. The researchers report their findings online today in the Proceedings of the National Academy of Sciences. Computer models suggest that the barbs ease the quill's penetration by concentrating force along the edges of the barbs, similar to how the serrations on a knife blade make cutting meat easier, Karp says.
Barbs render a quill about four times harder to pull out once they're embedded, the team found. The barbs at the tip of the quill were most effective at resisting removal. In fact, the barbs located within 1 millimeter of the tip contributed about half of the pull-out resistance—possibly because the flesh more tightly surrounded the tip than it did the rest of the quill, Karp says.
The team's findings could be used to enhance various biomedical devices or to design new ones, Karp says. For instance, he notes, rather than use wound dressings that rely on a chemical adhesive, which can trigger allergies or cause other problems, dressings could use tiny barbed needles to pierce the skin and then hold tight. Also, the staples now used to hold some surgical incisions shut—which largely rely on friction along their length to remain in the flesh—could be replaced by barbed staples that are shorter and have a smaller diameter. Even though porcupine quills cause some damage to flesh when they're pulled out, extricating smaller, quill-inspired staples would likely cause less overall damage than pulling out the staples currently in use, Karp suggests.
The team's findings "are just one more example of how what we see in nature can help us," says Anthony Atala, a biomedical researcher at Wake Forest Baptist Medical Center in Winston-Salem, North Carolina. Hollow versions of quill-inspired needles could help doctors better deliver drugs or chemicals through patches adhering to the skin, he notes. "Now that we know how these barbs work, we can modify them to make devices perform even better."