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'Slippery steel' holds promise for non-fouling tools and devices

21 October 2015

Researchers at the Harvard School of Engineering and Applied Sciences (SEAS) have demonstrated a way to make steel stronger, safer and more durable.

The coating, made from rough nanoporous tungsten oxide, is capable of repelling any kind of liquid even after sustaining intense structural abuse (image: Aizenberg Lab/Harvard SEAS)

Their new surface coating, made from rough nanoporous tungsten oxide, is claimed to be the most durable anti-fouling and anti-corrosive material to date, capable of repelling any kind of liquid even after sustaining intense structural abuse.

The new material joins the portfolio of other non-stick, anti-fouling materials developed in the lab of Professor Joanna Aizenberg of the Wyss Institute for Biologically Inspired Engineering at Harvard University. Aizenberg's team developed Slippery Liquid-Infused Porous Surfaces in 2011 and since then has demonstrated a broad range of applications for the super-slick coating, known as SLIPS. The new SLIPS-enhanced steel is described in Nature Communications.

"Our slippery steel is orders of magnitude more durable than any anti-fouling material that has been developed before," says Aizenberg. "So far, these two concepts - mechanical durability and anti-fouling - were at odds with each other. We need surfaces to be textured and porous to impart fouling resistance but rough nanostructured coatings are intrinsically weaker than their bulk analogues. This research shows that careful surface engineering allows the design of a material capable of performing multiple, even conflicting, functions, without performance degradation."

The material could have far-ranging applications and avenues for commercialization, including non-fouling medical tools and devices, such as implants and scalpels, nozzles for 3D printing and, potentially, larger-scale applications for buildings and marine vessels.

The biggest challenge in the development of this surface was to figure out how to structure steel to ensure its anti-fouling capability without mechanical degradation. The team solved this by using an electrochemical technique to grow an ultra-thin film of hundreds of thousands of small and rough tungsten-oxide islands directly on a steel surface.

"If one part of an island is destroyed, the damage doesn't propagate to other parts of the surface because of the lack of interconnectivity between neighbouring islands," says Alexander Tesler, a research fellow at Weizmann Institute of Science in Israel and the paper's first author. "This island-like morphology combined with the inherent durability and roughness of the tungsten oxide allows the surface to keep its repellent properties in highly abrasive applications, which was impossible until now."

Electrochemical deposition is already a widely used technique in steel manufacturing, said Aizenberg. "I don't want to create another line that would cost millions and millions of dollars and that no one would adopt," she says. Rather the goal is to be scalable, but not disruptive to current industry practices.

The team tested the material by scratching it with stainless steel tweezers, screwdrivers, diamond-tipped scribers, and pummelling it with hundreds of thousands of hard, heavy beads. Then, the team tested its anti-wetting properties with a wide variety of liquids, including water, oil, highly corrosive media, biological fluids containing bacteria and blood. Not only did the material repel all the liquid and show anti-biofouling behaviour but the tungsten oxide actually made the steel stronger than steel without the coating.


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