Underwater spiders inspire versatile hydrophobic material

A species of spider that spends its entire life underwater, has inspired a new material that is superhydrophobic - preventing corrosion and marine life adhering

Despite having lungs that only breathe oxygen from the atmosphere, the diving bell spider - Argyroneta aquatica - also has millions of rough, water-repellent hairs that trap air around its body. This creates an oxygen reservoir, which acts as a barrier between the spider’s lungs and the water.

This thin layer of air is called a plastron, and for decades, material scientists have been trying to harness its protective effects to for use in multiple applications where superhydrophobic surfaces are beneficial. This could be to to prevent corrosion, bacterial growth, the adhesion of marine organisms, chemical fouling, and other damaging effects of liquids on surfaces.

To date, the plastrons had proved highly unstable under water, keeping surfaces dry for only a matter of hours in laboratory conditions. Now, an international team of researchers, led by the Harvard John A Paulson School of Engineering & Applied Sciences (SEAS) in Boston, US, has developed a superhydrophobic surface with a stable plastron that can last for months under water.

The team’s, which also included researchers from the Wyss Institute for Biologically Inspired Engineering at Harvard, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAUEN) in Germany, and Aalto University in Finland, had a general strategy to create long-lasting underwater superhydrophobic surfaces, which repel blood and drastically reduce or prevent the adhesion of bacterial and marine organisms such as barnacles and mussels, opening up a range of applications in biomedicine and industry.

“Research in bioinspired materials is an extremely exciting area that continues to bring into the realm of manmade materials, elegant solutions evolved in nature, which allow us to introduce new materials with properties never seen before,” said Professor Joanna Aizenberg, SEAS, who is co-author of a paper published in Nature Materials. “This research exemplifies how uncovering these principles can lead to developing surfaces that maintain superhydrophobicity under water.”

A aerophilic surface made from a titanium alloy, with a long-lasting plastron, stays dry during hundreds of dunks in a petri-dish of blood. Image: Alexander B Tesler / FAUEN

Researchers have known for 20 years that a stable, underwater plastron was theoretically possible but, until now, have not been able to show it experimentally. One of the biggest issues with plastrons is that they need rough surfaces to form, like the hair of Argyroneta aquatica spiders, but this roughness makes the surface mechanically unstable and susceptible to any small perturbation in temperature, pressure, or tiny defect.

Current techniques to assess artificially made superhydrophobic surfaces only take into account two parameters, which do not give enough information about the stability of the air plastron underwater. The researchers identified a larger group of parameters, including information on surface roughness, the hydrophobicity of the surface molecules, plastron coverage, contact angles, and more, which, when combined with thermodynamic theory, allowed them to figure out if the air plastron would be stable.

“The stability, simplicity, and scalability of this system make it valuable for real-world applications.”

Stefan Kolle, SEAS

With this new method and a simple manufacturing technique, the team designed a so-called aerophilic surface from a commonly used and inexpensive titanium alloy, with a long-lasting plastron, that kept the surface dry thousands of hours longer than previous experiments, and even longer than the plastrons of living species.

“We used a characterisation method that had been suggested by theorists 20 years ago to prove that our surface is stable, which means that not only have we made a novel type of extremely repellent, extremely durable superhydrophobic surface, but we can also have a pathway of doing it again with a different material,” said Alexander Tesler, a former postdoctoral fellow at SEAS and the Wyss Institute, and lead author of the paper.

To prove the stability of the plastron, the researchers tested the surface throughly — bending it, twisting it, blasting it with hot and cold water, and abrading it with sand and steel to block the surface remaining aerophilic. It survived 208 days submerged in water and hundreds of dunks in a petri dish of blood. It severely reduced the growth of E.coli and barnacles on its surface and stopped the adhesion of mussels altogether.

“The stability, simplicity, and scalability of this system make it valuable for real-world applications,” said Stefan Kolle, a graduate student at SEAS and co-author of the paper. “With the characterisation approach shown here, we demonstrate a simple toolkit that allows you to optimise your superhydrophobic surface to reach stability, which dramatically changes your application space.”

That application space includes biomedical applications, where it could be used to reduce infection after surgery or as biodegradable implants such as stents, according to Goldmann, senior author of the paper, and former Harvard fellow. It also includes underwater applications, where it could prevent corrosion in pipelines and sensors.

In the future, it could even be used in combination with the super-slick coating known as SLIPS - slippery liquid-infused porous surfaces - developed by Aizenberg and her team more than a decade ago, to protect surfaces even further from contamination.