An electron microscope image of banana-shaped nanofibers templated with a liquid crystal. Image: Kenneth Cheng, Lahann Lab, Michigan Engineering.
An electron microscope image of banana-shaped nanofibers templated with a liquid crystal. Image: Kenneth Cheng, Lahann Lab, Michigan Engineering.

Inspired by the extraordinary characteristics of polar bear fur, lotus leaves and gecko feet, engineering researchers have developed a new way to make arrays of nanofibers that can form coatings that are sticky, repellent, insulating or light emitting, among other possibilities.

"This is so removed from anything I've ever seen that I would have thought it was impossible," said Joerg Lahann, a professor of chemical engineering at the University of Michigan (U-M) and senior author of a paper on this work in Science.

The somewhat serendipitous discovery was made by researchers at U-M and the University of Wisconsin (U-W), who revealed a new and powerful method for making arrays of fibers that are hundreds of times thinner than a human hair.

Polar bear hairs are structured to let light in while keeping heat from escaping. Water-repelling lotus leaves are coated with arrays of microscopic waxy tubules. And the nanoscale hairs on the bottom of the feet of gravity-defying geckos get so close to other surfaces that atomic forces of attraction come into play. Researchers looking to mimic these superpowers and more have needed a way to create the minuscule arrays that do the work.

"Fundamentally, this is a completely different way of making nanofiber arrays," Lahann said.

The researchers have shown that their nanofibers can repel water like lotus leaves. They also grew straight and curved fibers and tested how they stuck together – finding that clockwise and counterclockwise twisted fibers knitted together more tightly than two arrays of straight fibers.

They also experimented with the optical properties of the fibers, by making a material that glowed. They believe it will be possible to make a structure that works like polar bear fur, with individual fibers structured to channel light.

But molecular carpets weren't the original plan. Lahann's group was working with the group of Nicholas Abbott, at the time a professor of chemical engineering at UW-Madison, to put thin films of polymers on top of liquid crystals. Liquid crystals are best known for their use in displays such as televisions and computer screens, but the researchers wanted to employ them to make sensors that could detect single molecules.

Lahann brought expertise in producing thin films, while Abbott led the design and production of the liquid crystals. In typical experiments, Lahann's group evaporated single links in the polymers and then coaxed them to condense onto the surface of the liquid crystals. But the thin polymer films sometimes didn't materialize as expected.

"The discovery reinforces my view that the best advances in science and engineering occur when things don't go as planned," Abbott said. "You just have to be alert and view failed experiments as opportunities."

Instead of coating the top of the liquid crystal, the links slipped into the fluid and connected with each other on the glass slide. The liquid crystal then guided the shapes of the nanofibers as they grew up from the bottom, creating nanoscale carpets.

"A liquid crystal is a relatively disordered fluid, yet it can template the formation of nanofibers with remarkably well-defined lengths and diameters," Abbott said.

And the liquid crystals didn't just make straight strands. Depending on the liquid crystal, they could generate curved fibers, like microscopic bananas or staircases.

"We have a lot of control over the chemistry, the type of fibers, the architecture of the fibers and how we deposit them," Lahann said. "This really adds a lot of complexity to the way we can engineer surfaces now; not just with thin two-dimensional films but in three dimensions."

This story is adapted from material from the University of Michigan, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.