Inspired by the aquatic spider (Argyroneta aquaticaspider), researchers have developed a superhydrophobic surface with a stable chassis that can last for months underwater. Such surfaces can be used in biomedical fields, such as reducing surgical infections, and in industrial fields, such as preventing pipe corrosion.
One species of spider lives its entire life underwater, although its lungs can only breathe atmospheric oxygen. How is it done? The spider, called Argyroneta aquatica, has millions of rough, hydrophobic hairs on its body. These hairs trap the air around its body, forming an oxygen reservoir and acting as a barrier between the spider's lungs and water.
Materials scientists have been trying to exploit this thin protective layer of air for decades. Doing so creates underwater superhydrophobic surfaces that prevent corrosion, bacterial growth, marine life adhesion, chemical fouling, and other harmful effects of liquids on surfaces. But it turns out that plastrons are extremely unstable underwater and can only stay dry on the surface for a few hours in the lab.
Now, a team of researchers led by the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), the Wyss Institute for Biologically Inspired Engineering at Harvard University, the Friedrich-Alexander University of Erlangen-Nuremberg in Germany, and Aalto University in Finland has developed a superhydrophobic surface with a stable plasma membrane that can remain in water for months. The team's overall strategy is to create long-lasting underwater superhydrophobic surfaces that repel blood and greatly reduce or prevent the attachment of bacteria and marine organisms such as barnacles and mussels, opening up a range of applications in biomedicine and industry.
"Bioinspired materials research is an extremely exciting field that continues to bring elegant solutions evolved in nature into the realm of man-made materials, allowing us to introduce new materials with never-before-seen properties," said Joanna Aizenberg, the Amy Smith Berylson Professor of Materials Science and professor of chemistry and chemical biology at SEAS, a co-author of the paper. "This study demonstrates that uncovering these principles can lead to the development of surfaces that remain superhydrophobic in water."
Aizenberg is also an associate faculty member at the Wyss Institute. The research was published in the journal Nature Materials.
Researchers have known for 20 years that a stable underwater chassis is theoretically possible, but until now, they had not been able to prove it experimentally.
One of the biggest problems with plastrons is that they require a rough surface to form, like the hairs of Argyroneta aquatica. But this roughness makes the surface mechanically unstable, susceptible to any tiny perturbations in temperature, pressure, or tiny imperfections.
The aerophilic surface, made from a commonly used and inexpensive titanium alloy, has a durable plasma membrane and remains dry after being soaked hundreds of times in blood culture dishes. Image source: Alexander B. Tesler/Friedrich-Alexander-Universität Erlangen-Nürnberg
Innovative technologies and discoveries
Current techniques for evaluating artificially produced superhydrophobic surfaces only consider two parameters, which do not provide enough information about the stability of air particles underwater. Aizenberg, Jaakko V.I. Timonen and Robin H.A. Ras from Aalto University, and Alexander B. Tesler and Wolfgang H. Goldmann from the University of Florida and their teams determined more parameters, including surface roughness, hydrophobicity of surface molecules, surface coverage, contact angle and other information.
Using this new approach and a simple fabrication technique, the team engineered a so-called aerophilic surface using a commonly used, inexpensive titanium alloy that has a long-lasting plasma membrane that keeps the surface dry for thousands of hours better than previous experiments and even outlasts the plasma membranes of biological species.
"We used a characterization method developed by theorists 20 years ago to show that our surface is stable, which means that not only have we created a new type of extremely repellent, extremely durable superhydrophobic surface, but we also have a way to do it again with different materials," said Tesler, who worked as a postdoctoral fellow at SEAS and the Wyss Institute and is the paper's lead author.
To prove the plume's stability, the researchers performed various tests on its surface - bending it, twisting it, spraying it with hot and cold water, and polishing it with sand and steel to stop its surface from remaining aerophilic. It was soaked in water for 208 days and hundreds of times in blood petri dishes. It severely reduces the growth of E. coli and barnacles on its surface and completely prevents mussels from adhering.
"The stability, simplicity and scalability of this system make it very valuable for practical applications," said paper co-author Stefan Kolle, a graduate student at SEAS. "With the characterization method demonstrated here, we demonstrate a simple toolkit to optimize superhydrophobic surfaces for stability, which dramatically changes the application space."
Goldman, the paper's senior author and a former Harvard University researcher, said this application space includes biomedical applications to reduce postoperative infections or as biodegradable implants such as stents. It can also be used underwater to protect pipes and sensors from corrosion. In the future, it could even be combined with ultra-slippery coatings called SLIPS (liquid-infused porous surfaces) that Eisenberg and her team developed more than a decade ago to further protect surfaces from contamination.