Insects can patch their broken ‘bones’

When a person breaks a leg, they might get a splint, cast or boot to cradle the bone as it heals. But what happens when a locust breaks a limb? Instead of a cast on the outside, the insect will patch itself up from the inside. These patches can restore up to 66 percent of a leg’s former strength, a new study finds.
The data also suggest new ideas for mending various types of pipes — from those in our homes to the living “pipes” inside our bodies.
Locusts and other insects rely on an exoskeleton — external support — made of cuticle. This material is made from a material called chitin. The cuticle has two layers. The outer one — or exocuticle — is tough and can be very thick. It forms a protective armor. The inner layer — or endocuticle — flexes much more.
When cut, the cuticle forms a clot to seal off the wound. Then cells on either side of the cut secrete new endocuticle. The secretion spreads across and under the cut. Eventually it turns hard. This creates a thick patch on the inside.
While scientists understood that insects patched themselves this way, Eoin Parle realized that no one knew just how strong the repaired sites were. He decided to find out. Parle is a bioengineer — a scientist who uses engineering to study living things. He began this research while working at Trinity College Dublin in Ireland (he now works at University College in Dublin).
“There’s a lot to learn from the natural world,” Parle says. An insect’s cuticle, for instance, is very light and hard-wearing, he explains. Strong and stiff, it tends to be very tough, he adds.
Desert locusts (Schistocerca gregaria) roam Asia, Africa and the Middle East, where swarms of the critters can devastate farmers’ crops. This species became Parle’s test subjects.

Leaping locusts

He brought the bugs into his lab. “You always get a few raised eyebrows walking through the bioengineering facility with a cage full of locusts,” he notes. But the insects provide a good opportunity to study healing. Their back legs have to withstand strong forces when they jump. Those limbs offered a chance to study just how well the cuticle would mend.
“An uninjured locust leg can withstand a bending pressure of about 172 megapascals before breaking. “Cuticle has a higher bending strength than wood,” Parle notes. “Their legs are unbelievably strong.” These limbs are “as strong or stronger than [human] bone — really impressive.”
To study what an injury would do, Parle carefully sliced into the legs of 32 locusts using a scalpel. Parle then let the legs heal. He left a further 64 locusts unharmed. They served as unaffected comparisons — or controls. Afterward, he measured leg strength in all of the bugs.
An injured leg lost about two-thirds of its former strength. In this state, Parle says, a locust risks snapping its leg right off during a jump.
After rest and repair, however, many of the locusts’ legs acquired a thick patch beneath the endocuticle. This mended the cut. Affected legs became about two-thirds as strong as they had been before the injury. That was good enough to let the bug safely resume jumping. As such, Parle concludes, that mending “is restoring fitness to the insect.”

Inspired by insects

Not all of the cuts healed, however. In fact, a bit fewer than half did. If the cut was jagged or too wide, cells around the wound could not secrete enough endocuticle to patch the gap. But Parle was surprised to find that even when cuts failed to heal, they didn’t get any bigger. The cuticle around them also did not crack.
This made the engineer wonder if cuticle-inspired materials might one day help make and repair pipes, such as those that carry water through a building. In pipes used today, a small crack can quickly grow and spread from the site of the initial break, he notes.
Parle thinks that an insect’s patch system might even inspire ways to repair burst blood vessels in people. Instead of stitches, we could “effectively restore strength and toughness by applying an internal patch,” he suggests. Parle and his colleagues published their findings April 6 in the Journal of the Royal Society Interface.
A study on broken locust legs is “exactly the kind of study we need,” says Marianne Alleyne. She was not involved with Parle’s research. Alleyne is an entomologist — someone who studies insects — at the University of Illinois in Champaign. “This is an exciting time to look at this stuff,” she says.
While it’s good to know locusts in a lab can heal busted limbs, no one knows if they also will do it in the wild. It took at least 10 days for a leg to heal. That’s a long time in the three-to-six-month lifespan of a locust.
“This proves they can do it,” Alleyne says. “But it doesn’t prove they do this in nature.” And, of course, when locusts do get injured in the wild, they probably don’t get a carefully controlled cut from a scalpel.
But Alleyne hopes that scientists might figure out how to use current technologies to make materials similar to an insect’s exoskeleton.
Plumbing pipes would benefit from being made from something that could be patched and wouldn’t continue to crack when broken. A cuticle-like material is “self-patching and it’s recyclable,” Alleyne notes. She adds that it’s also pretty tough. n

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