Porcupine's prickly defence mystery solved

North American porcupine The North American porcupine is also known as a 'quill pig'

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Porcupine quills easily penetrate flesh but prove hard to remove due to microscopic barbs, say scientists.

North American porcupines are famed for their unique defence as they carry 30,000 quills on their backs.

Researchers studied the structures to understand how they effortlessly enter flesh but do not leave as smoothly.

Tiny backward-facing barbs studding the tip of each quill were found to reduce penetration force and maximise staying power.

The team, including Dr Jeffrey Karp from the Brigham and Women's Hospital, Boston, US and Dr Robert Langer from the Massachusetts Institute of Technology, US, suggests the discovery could revolutionise medical equipment for human use.

The findings are published in the Proceedings of the National Academy of Sciences.

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"We were surprised that no one had previously reported the forces required to remove quills from tissue," said Dr Karp.

"And I think [we were] most surprised to find that not only do the backwards-facing barbs make the quills difficult to remove, they significantly reduce the penetration force.

"To our knowledge this is the first demonstration of a highly engineered system that achieves polar-opposite dual functionality."

In terms of the potential applications of such barbed structures, Dr Karp suggested they could help in the design of both needles that cause less injury and adhesives to help hold tissue together.

"For needles, we envision we could use 'swell-able' or degradable barbs, to enable easy penetration and easy removal," said Dr Karp.

"Alternatively, our work could lead to next-generation medical adhesives that could replace staples or sutures.

"Tissue must be aligned with each pass of the suture or placement of staples, which can be time-consuming, and the greater the procedure time the greater the risk of complications," he explained.

Porcupine quill under the microscope with rhodamine dye. Scientists dyed the quill tips to reveal the barb structures under a microscope

"An adhesive porcupine mimetic patch could be quickly placed, which would be especially useful under emergency conditions."

In order to understand how the quills performed their remarkable dual function, researchers analysed them under a microscope.

Using a fluorescent dye to highlight the cell structure, they observed tiny barbs on the conical black tip of the quills.

To test how these barbs worked, the team used natural quills and synthetic replicas to pierce a variety of tissues.

Dr Karp compared the barbs to serrated knife blades that localise the penetration force at the tip of the teeth rather than along the length of the blade.

"Think of slicing a tomato - it is much easier to slide with a serrated versus non-serrated knife, and use of a serrated knife imposes less damage," he explained.

"We found the same result with the quills. We pushed them into tissue with and without barbs, left them in [and examined] the tissue.

North American porcupine North American porcupines sport a mixed coat of quills and fur

"We found far less tissue damage when the barbs were present."

The team found that the quills required 60-70% less force to penetrate muscle tissue than quills without barbs.

But when it came to removing the quills, the barbs played the opposite role with the microscopic structures acting as anchors in the tissue.

North American porcupines are the most northerly species of porcupine and are the second largest rodents found on the continent after beavers.

Their quills, which can reach 10cm in length, are shorter than those of their African cousins, which can be three times as long.

"Porcupines are slow-moving nocturnal creatures. Since they don't run towards a predator to attack, their quills must easily detach and penetrate tissue," explained Dr Karp.

"Importantly, porcupines do not shoot their quills through the air, which Aristotle once proclaimed."

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