Say you’re looking to make the next generation of medical tape. You want something that will hold skin and other organs together while they heal. You want it to be more convenient than sutures and less brutal than staples. It has to stick easily, hold on tightly, and come off painlessly.
There are worse places to search for inspiration than the guts of a fish.
Fish intestines are home to a group of parasites called spiny-headed worms, or acanthocephalans. Their most distinctive feature is a spine-covered snout that the worm stabs into the gut walls of its host. Once inside, it contracts two muscles and the long snout rapidly swells into a bulb, anchoring the worm in place. The fastened parasite can now drink deeply from the river of nutrients washing over it, absorbing them through its skin.
To the fish, the worm’s spiny head is a health hazard. To Jeffrey Karp, it was something to emulate. His team at the Brigham and Women’s Hospital in Boston have spent many years developing medical adhesives, constantly looking to nature for inspiration. In 2008, for example, they developed a sticky tape based on the feet of a gecko. And last year, they created artificial microneedles based on a porcupine’s quills, whose structure allows them to easy to stab into flesh but hard to pull back out.
Geckos are famously sticky, and porcupines are famously stabby, but Karp also realised that parasites must have fantastic ways of fastening themselves to their hosts. That’s how he came across the spiny-headed worms and one species in particular—Pomphoryhnchus laevis.
Karp’s team member Seung Yun Yang mimicked P.laevis’s hooks by creating two-layered microneedles. They have a stiff, cone-shaped core made of polystyrene, covered by a soft outer layer made of polyacrylic acid—an absorbent chemical used in disposable diapers. When the needles pierce flesh, the cores stay the same while the outer layers quickly absorb water and swell, just like P.laevis’s snout. The swollen tips are like arrow-heads, locking the entire needles into place. If a bandage or a piece of tape is coated with these needles, they’d easily hold two pieces of tissue together.
To Karp, the most obvious application for the worm-based needles is to hold skin grafts in place. These patches of skin are often used to close up gaping wounds from burns, trauma or major surgeries. For now, most surgeons fasten them to patients by stapling or stitching them around their edges. But these methods have problems.
Staples, in particular, go in so deeply that they can damage tissue, blood vessels and nerves. The torn tissue creates a hole that’s slightly wider than the width of the staple, creating an easy entrance for infectious bacteria. And since the staples are only applied around the edges, fluid can pool into the central space and prevent the graft from fusing to the underlying skin.
By contrast, the worm-based patches made continuous contact with any flesh they sit over. They barely damage the underlying tissue and the swollen tips automatically seal any holes they create, preventing bacteria from getting in. They’re also stronger than current adhesives. When Yang built 100-needle patches and tested them on the skins and intestines of dead pigs, they took more than three times as much force to pull out as regularly stapled grafts.
Karp’s needles have other advantages. Unlike sutures, they are easy to apply, and unlike most bandages and plasters, they work equally well on dry and wet surfaces. This means that they could also be used inside the body to hold tendons or ligaments in place, or to seal leaks in intestines of lungs. “This could be a universal adhesive for soft tissues,” says Karp.
Other groups have tried to create patches of microneedles that do the same job, but these tend to be stiff so they can penetrate the skin. This means that awkward movements can break the needles off inside a patient’s fleshBy contrast, Karp says, “The worm system is stiff going in but, after swelling, one can rotate the patches without any breakage of microneedles.”
And unlike most microneedles, the worm-based ones stick in place because of swollen heads rather than backwards-pointing barbs. When they are peeled away, the tips can squeeze down to fit through the holes that have already been made and since they are barb-free they don’t snag on tissue on the way out. Once the needles are removed, they return to their original shape within just 15 minutes, while the holes they leave close up in an hour.
Microneedles can also be used to inject medicine as well as fasten tissues. Just dip their tips in your drug-of-choice, and they will slowly release it into the surrounding flesh when they swell. And once you remove them, you can use them again.
And what if you don’t want to remove them at all? “We are currently developing a degradable version,” says Karp.
Reference: Yang, Cearbhaill, Sisk, Park, Cho, Villiger, Bouma, Pomahac & Karp. 2013. A bio-inspired swellable microneedle adhesive for mechanical interlocking with tissue. Nature Communications. http://dx.doi.org/10.1038/ncomms2715