In this case, they became interested in the North American porcupine, which has about 30,000 barbed quills to defend against predators. Each quill is several centimeters long; the four millimeters at the very tip are covered in microscopic barbs.
To their surprise, the researchers found that despite the difficulty of removing the quills, they require very little force to penetrate tissue. Compared to quills with no barbs, the barbed quills require 60 to 70 percent less force to penetrate muscle tissue.
The team then set out to determine how the quills achieve this unique combination of easy penetration and difficult removal. "By understanding the mechanism, we can design an artificial system in the right way," Cho says.
They found that the tiny barbs at the end of the quill are the key to both ease of penetration and resistance to removal. While the quill is entering tissue, the barbs act to localize the penetration forces, allowing them to tear through tissue fibers much more easily just as a serrated knife cuts through tomato skin far more cleanly than a straight-edged knife.
When it comes to the force required for pullout, the barbs act like anchors that keep the quill from coming out. The force required to pull out barbed quills is four times that required to remove barbless quills.
Toward new adhesives
To explore the possibility of making stronger adhesives, the researchers created a patch with an array of barbed quills on one side. They found that the energy required to remove this patch was 30 times greater than that needed for a control patch, which had quills but no barbs.
The system could also be tweaked so that it penetrates tissue easily but is not as difficult to remove as a porcupine quill, enabling design of less-painful needles for injections. "If you can still create the stress concentrations but without having a
|Contact: Sarah McDonnell|
Massachusetts Institute of Technology