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Paintable Proteins Provide Environmentally Friendly Way to Protect Ship Hulls at Sea
Sailors from the deck department of the aircraft carrier USS Abraham Lincoln mask and paint the ship’s hull below the waterline.
Credit: U.S. Navy/PO3 Aaron Hubner
Taking inspiration from aquatic life, Johns Hopkins APL researchers are propelling the field of bioconjugation outside the lab and onto ship hulls.
Within minutes of a ship’s hull being submerged in seawater, a complex web of life starts to develop: bacteria, algae, tube worms, barnacles and mussels. This adhesion of organisms, known as biofouling, increases drag and fuel consumption and leads to costly corrosion — but many of the existing methods to prevent it are toxic.
Antifouling coatings, which slow the growth of organisms, typically rely on the toxicity of copper and small-molecule additives that act as pesticides in the water near a ship’s hull. These poisonous paints leach into the seawater and destroy aquatic life.
Now, researchers at the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, have developed a nontoxic, environmentally friendly paintable protein that inhibits fouling. The APL team published its results in the Journal of Coatings Technology and Research.
Fishing for Inspiration
Researchers have identified active proteins, or enzymes, as potential nontoxic antifouling agents. But no one had developed an effective way to bind enzymes onto a specific location while maintaining their functionality. Enzymes must be covalently, or chemically, attached to a surface to maintain their antifouling properties.
“Many marine animals do not want to be covered in biofouling and have developed enzymes to protect themselves,” said project lead Reid Messersmith, a molecular engineer in APL’s Research and Exploratory Development Department (REDD). “Taking inspiration from animals, we developed an enzyme coating that could be applied directly to surfaces.”
Biological scientist Ryan Baker-Branstetter, who led the enzyme and antifouling research, added that biofouling develops from bacteria settling on a surface. So, if the team could prevent small bacteria from forming, larger organisms would not follow.
A Paint Bucket Approach
APL’s antifouling coating stems from a 2021 patent for antimicrobial coatings developed at the Lab.
“It’s difficult to get enzymes to stick to just anything and remain active in the process. Bioconjugation is a technique to couple naturally occurring biomolecules and synthetic compounds. There have been promising laboratory results proving that certain enzymes can be attached to certain surfaces, but those results have not translated into real-world applications,” said Messersmith. “We wanted to create a paint bucket approach, where someone could walk up and efficiently and effectively slap the coating on a surface.”
To do that, they needed to identify an effective “linker” — an agent capable of bonding an enzyme to a synthetic compound.
APL researchers developed an enzyme-based polymer coating with an ortho-phthaldialdehyde (oPA)-based linker, which is capable of bonding enzymes onto surfaces, and doing so rapidly — taking less than five minutes to form a layer of material. The oPA-based linker maintained activity for extended periods of time in experiments, compared to no linker and a commercially available linker.
“The first protein we painted in 2021, red fluorescent protein, established that the chemistry behind the coating system and our linker worked. This allowed us to revisit which proteins would effectively prevent biofouling in our latest research,” said Messersmith.
A graphical abstract of the antifouling enzymes tethered to surfaces
Credit: Johns Hopkins APL
Proteins Prove Potent
When APL staff members turned their attention toward antifouling, they reviewed the literature to understand which enzymes would prove effective and compatible with the coating system. They settled on xylanase — a naturally occurring enzyme produced by fungi, bacteria, marine algae and many other organisms, often used in commercial baking — and a mixture of lysing complex enzymes — a molecule extracted from a fungus.
The team used a method called click chemistry, which enabled them to attach the coating without any catalyst or heat. After two months submerged in artificial seawater, the xylanase and lysing complex coatings proved highly active, demonstrating the material’s longevity and potential for eco-friendly antifouling. Remarkably, the approach was successful on the first try.
“I have worked on a lot of projects, and typically the tenth thing you try works. But it is very rare for the first approach to work out,” said Baker-Branstetter. “Having early success with the enzymes allowed us to look further into other interesting questions, such as the paints’ longevity and activity across a range of environmental conditions.”
Additionally, the team found not only that their coatings prevented the bacteria from adhering to a surface but that they were also able to remove bacteria that had already settled.
Painting the Future
Beyond their antifouling potential, paintable proteins could have a variety of applications. The oPA linker can work with a wide variety of proteins because it acts on the exterior of proteins without interfering with their functioning. As one example, Messersmith said this approach might be used to create a paint-on sensor for detecting toxic gas in the air.
“Traditional sensors need to test for every single toxic gas independently, but the biochemistry in paintable proteins could act as a comprehensive sensor,” said Messersmith. “The paintable approach can be used for a variety of different proteins — each protein performs a different function, and with this system, you could theoretically coat any protein you want.”