A team from the McKelvey School of Engineering at Washington University in St. Louis cherry-picked properties from the animal world — specifically mussels and spiders — to develop a biocompatible, adhesive gel that stays sticky underwater.

By modifying microbes, the engineers produced the necessary ingredients for a biocompatible adhesive hydrogel that is as strong as spider silk and as adhesive as mussel foot protein (Mfp).

The research, led by Fuzhong Zhang, professor of energy, environmental and chemical engineering, was published in the journal ACS Applied Materials and Interfaces .

In a previous study in 2019, Zhang and the team engineered microbes to produce many variants of mussel foot proteins (Mfp). These variants, made from a repeating chain of Mfp, feature adhesive and strength properties that vary based on the number of looping molecules. (Read about the previous study .)

“We wanted to know whether synthetic biology could help with underwater adhesion, a challenging task for synthetic materials,” said Eugene Kim, currently an assistant professor at George Mason University, in a recent press release .

Kim is first author of the paper and worked on the project as a PhD student in Zhang’s Washington University lab.

The microbial Mfp is inherently sticky, but the protein molecules quickly diffuse once added to water. The team’s challenge: to make sure the adhesive Mfp could remain on a surface when wet.

To prevent diffusion, Zhang's lab turned to a sturdy material found in nature: spider silk.

Earlier in the year, Zhang's team produced a silk-amyloid hybrid protein that was stronger than steel and tougher than Kevlar .

The engineers integrated the high-strength silk-amyloid protein with Mfp to synthesize a new component for their adhesive hydrogel: a "tri-hybrid" protein that has the benefits of both the bond of Mfp and the durability of spider silk.

By controlling bacteria to modify each motif of the protein, including parts from spider silk and mussel foot protein, Zhang and the researchers controlled the adhesion and strength of the hydrogel, tailoring it to meet the specific requirements for tendon-bone repair and other tissue repair needs.

Prof. Fuzhong Zhang

“We developed a design principle that allowed us to control both cohesion and adhesion of the hydrogel,” Zhang said. “The gel is slightly denser than water so you can easily use it underwater, putting it on or between two surfaces."

In a short Q&A below, Zhang tells Tech Briefs about the potential applications for a biocompatible, biodegradable, protein-based adhesive.

Tech Briefs: Why is it so important to have an underwater adhesive like this one? Which applications do you envision?

Fuzhong Zhang: There are many kinds of adhesives, but very few can work underwater. And for the limited few underwater glues, they are very difficult to use. If the glue is a liquid, it is very hard to handle when it is underwater, as it easily flows away from the repair site. Here we provide an adhesive hydrogel, whose density is slightly higher than water, making it easy to use for underwater applications. Additionally, our hydrogel is protein-based, so it can be biocompatible and biodegradable, making it attractive for biomedical applications.

Tech Briefs: Why has it been so challenging to develop underwater adhesives, and what component of your adhesive allows you to overcome earlier limitations?

Prof. Fuzhong Zhang: Underwater adhesion is challenging because the adhesive has to be able to remove the water molecules round the target surface, while forming strong bonding with the surface. Our hydrogel contains a protein fragment originated from mussel foot proteins. Evolution has already selected proteins that allow mussels to adhere themselves to various surfaces under seawater. So, our work harnessed the success of evolution and engineered it for human applications.

Tech Briefs: How do you adjust and tailor adhesion and strength, and in what cases would you want to do this?

Prof. Fuzhong Zhang: Mussel foot protein alone does not form a strong hydrogel. To increase the strength, we added protein sequences from spider silk and amyloid peptide, both of them are known to form high strength materials.

Tech Briefs: Producing a mussel foot protein (Mfp) and its oligomeric variants, and adding spider silk, seems complex, no? Can this adhesive be made easily?

Prof. Fuzhong Zhang: The design of the protein sequence is complicated. But once it is designed and tested, synthesizing the protein is straightforward using our synthetic biology platform.

The mussel foot protein hydrogel set-up for tensile strength measurements. The measurement (labeled in white) shows the hydrogel at its original gage length. During testing, the hydrogel stretches to approximately three times its original gage length. (Image Credit: Fuzhong Zhang)

Tech Briefs: What are the adhesive’s components?

Prof. Fuzhong Zhang: The adhesive is made from just one protein sequence, but within this sequence, we blended a fragment from Mfp, multiple fragments from spider silk, and amyloid.

Tech Briefs: What will you be working on next?

Prof. Fuzhong Zhang: We are working to continue improving our adhesives, making it stronger, sticker, and more suitable for target applications.

The research team also included Young-Shin Jun, professor of energy, environmental and chemical engineering, and Guy Genin, the Harold and Kathleen Faught Professor of Mechanical Engineering.

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