December 11, 2012
Inspiration From Mother Nature – Bioengineered Devices Inspired By Porcupine Quills
April Flowers for redOrbit.com - Your Universe Online
A research team from Brigham and Women's Hospital (BWH) and the Massachusetts Institute of Technology (MIT) has discovered how North American porcupine quills easily penetrate tissues, such as skin, and why, once lodged there, they are quite difficult to remove.
This discovery could prompt device makers to design medical needles that easily penetrate surfaces and resist buckling, the scientists say. It might even lead to the next generation of medical adhesives.
Both natural porcupine quills and replica-molded synthetic polyurethane quills were used by the research team to understand the physical forces at play when the quills penetrate and are removed from a variety of tissues. These tissues include muscle and skin.
The North American porcupine has a formidable defense system, including approximately 30,000 quills on its back, each of which contains a conical black tip studded with a layer of microscopic, backward-facing barbs and a cylindrical base with smooth, scale-like structures. Each quill, which is a long, stiffened hair, has between 700 and 800 barbs along the last 4 centimeters of its length. When the porcupine encounters a predator, it releases the quills.
The researchers found that it is the quill's geometry that allows it to penetrate so easily and maintain such high adhesion once in the tissue.
"The philosopher Aristotle who was clearly misinformed warned that porcupines could shoot their quills over great distances, which is completely untrue," said Jeffrey Karp, PhD, BWH Division of Biomedical Engineering, Department of Medicine. "In fact there are many misconceptions about porcupines and their quills. We were most surprised to find that the barbs on quills serve a dual function. Namely, the barbs reduce the penetration force for easy insertion into tissue and maximize the holding force to make the quills incredibly difficult to remove."
"We were surprised that no one had previously reported the forces required to remove quills from tissue," said Dr Karp. "To our knowledge this is the first demonstration of a highly engineered system that achieves polar-opposite dual functionality."
Since porcupine quills do not shoot through the air, the researchers assert that the quills must have evolved a specialized mechanism to achieve their easy penetration that depends on the backwards-facing barbs. The barbs are similar to serrated blades that cut through tissues easier by localizing forces at the tips of the teeth of the blade. The serrated blade provides a cleaner cut in tissue, likewise, the barbs appear to minimize the penetration force by reducing the deformation of the tissue.
"By carefully removing the barbs' from the quill, we discovered that in addition to their physical features, the location of barbs on the quill played a major role in minimizing penetration forces and maximizing the work needed to yank them from the tissue," said Woo Kyung Cho, PhD, BWH Division of Biomedical Engineering, Department of Medicine.
"By understanding the mechanism, we can design an artificial system in the right way," Woo Kyung Cho, a postdoc in the Harvard-MIT Division of Health Sciences and Technology, said.
The potential of this finding is comparable to other bioengineered devices inspired by nature, such as Velcro hook-and-loop fasteners and gecko inspired tape adhesives — which Robert Langer and Karp introduced in 2008. The team expects many implications across multiple disciplines, including medicine.
"This is especially true given that quills can strongly grip tissue with minimal depth of penetration, less than half a centimeter is enough and they don't need to bend like staples to achieve secure fixation," said James Ankrum, PhD, MIT graduate student.
"Towards medical applications we developed plastic replicas that remarkably mimicked the reduced penetration force and increased pullout. This should be useful to develop next generation medical adhesives and potentially design needles with reduced pain," added Karp.
The next step, according the team, is to test the synthetic quill in a variety of medical applications.
"This work is a valuable addition to our understanding that bio-inspired materials or devices have great potential to revolutionize the existing biomedical materials and tools from drug delivery to tissue engineering," said Kahp-Yang Suh, PhD, School of Mechanical and Aerospace Engineering, Seoul National University, and an expert in biomechanical innovations.
One innovation that is possible from this discovery is in the field of internal adhesives that can bind tissues more securely. Such adhesives are needed, especially for patients who have undergone gastric bypass surgery or other types of gastric or intestinal surgery. Currently, such incisions are sealed with sutures or staples, both of which can leak and cause complications.
"With further research, biomaterials modeled based on porcupine quills could provide a new class of adhesive materials," said Langer, the David H. Koch Institute Professor at MIT.
The research team created a patch with an array of barbed quills on one side to test the possibility of making such adhesives. The energy required to remove this patch was 30 times greater than that needed to remove a control patch that had quills but no barbs.
They envision tweaking the system to penetrate tissue easily without the extra effort needed to remove it, thus enabling a design of less-painful needles for medical use.
"For needles, we envision we could use 'swell-able' or degradable barbs, to enable easy penetration and easy removal," said Dr Karp.
"If you can still create the stress concentrations but without having a barb that catches tissue on removal, potentially you could create something with just easy insertion, without the adhesion," said Ankrum.
The findings of the study are published in Proceedings of the National Academy of Sciences.
Image 2 (below): This fluorescent microscope image shows the tiny barbs that coat the tip of a porcupine's quills. Image: Woo Kyung Cho