Plastic that mimics insect wings kills bacteria

The curved plastic may one day be used as an artificial cornea

Tiny pillars on insect wings inspired scientists and engineers to make polymer nanopillars that can kill bacterial cells on this artificial cornea.

A new plastic that mimics the surface of insect wings might help save people’s eyesight. More than 40,000 people each year need a transplant for the front part of the eye, called the cornea. But donors aren’t always available. Also, some people’s bodies won’t accept a replacement from someone else. And bacteria could easily infect materials for artificial ones — at least until now.  
Researchers at the University of California (UC), Irvine have made an antibacterial material with thousands of teeny, spike-like pillars. Each pillar is like one of the invisible hairs on a cicada wing. And like the insect wings, the surface kills various types of bacterial cells. Better still, such surfaces can be shaped in a curve, just like the eye is.

This cicada's wings may appear shiny and smooth. But they actually host germ-impaling hair-like spikes.

BRUCE MARLIN/WIKIMEDIA COMMONS

The idea for the project came about because scientists and engineers in different fields shared ideas. Albert Yee is a materials scientist at UC Irvine. He works in nanotechnology. That field deals with structures on the scale of less than 100 billionths of a meter. That’s roughly one ten-thousandth the diameter of a human hair. Yee had formed nanostructures for computer chips using polymers. Such materials, which include plastics, have molecules that are chains of repeating groups of atoms.
Yee learned about medical researchers wanting to mimic the surfaces of wings on cicadas and dragonflies. Bacteria die after landing on those surfaces. The microbes jab themselves into the nanospikes on those wings. Essentially, they spear themselves to death.
Yee and his colleagues decided to see if ideas from the earlier work might help keep plastic corneas similarly germ-free. His group chose a plastic called PMMA. It’s short for polymethylmethacrylate (POL-ee-METH-ul-meh-THAK-rih-LATE). They used a readily available mold to make it into spikey nanopillars. The flat mold had thousands of tiny indentations. Heated PMMA was pressed against the mold. Later, the cooled PMMA came out bearing nanospikes like those on cicada wings. In tests, this surface killed lots of thin-walled bacteria, such as E. coli.
But a cornea is curved, not flat. And just as a paper wrinkles as you press it around a baseball, flat polymers get distorted around curved shapes. To deal with that, the group made a new curved mold. Cooled PMMA comes out of this mold already curved.

The cornea is a clear protective outer covering of the front, vision-sensing structures on the eyeball.

“We’ve done initial testing to prove that it will work for the cornea device,” says Mary Nora Dickson. This chemical engineer is a graduate student at UC Irvine and a member of the research team. She and fellow graduate student Elena Liang reported on their team’s work on March 16 in San Diego at the spring meeting of the American Chemical Society.

The next challenge

The researchers now hope to tweak their material so that it will kill bacteria with cell walls thicker than E. coli. This would fight infections such as staph (caused by Staphylococcus germs). The trick requires building taller nanospikes, like those on dragonfly wings. But, notes Dickson, “It’s actually pretty difficult to make polymers into skinny shapes without damaging them.”
As molecules go, polymer chains are “kind of like spaghetti,” she explains. “It might be easy to put spaghetti into a big bowl. If you tried to put it into something skinny, like a vase, it will get harder and harder to push that spaghetti into that skinny shape.” Getting the material out of the mold also gets harder as the pillars grow taller.
Her team is now applying coatings to the molds for the plastic to see if that helps. A light spray of oil makes it easier to get cupcakes out of a tin. The group hopes its coatings might do the same thing for the nanospikes on their plastic.
This new research adds to other studies probing how to tweak the form and structure of materials at the smallest scales. Taken together, they show “you can profoundly affect the way cells interact with a given surface,” says Masaru Rao. He’s a materials engineer at the University of California, Riverside. That principle could work for other medical uses as well, he suspects.
Also, the ability to kill bacteria comes from the material’s structure. In contrast, most other antibacterial materials have a coating. So, Rao says, this new approach “may yield greater reliability and damage-resistance.” The process for making the nanospike-shaped plastic also can be scaled up. So low-cost production of medical devices might be possible.
Before that happens, additional tests must take place. The team needs more proof of the material’s germ-killing abilities, of how long the material can do its job, and if there are any side effects.
Meanwhile, Rao observes, “There’s still much we can learn from nature.
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