Adhesives that can form bonds underwater would be useful for many biomedical applications, yet few synthetic adhesives exist today. In the last decade, researchers have begun to look to the sea to investigate the organisms-mussels, barnacles, algae, and others-that naturally secrete durable underwater adhesives. Recently, scientists have successfully developed adhesives that are able to mimic their biological counterparts.
There are two natural protein systems that have been widely investigated thus far. One uses a chemical called 3,4-dihydroxyphenylalanine (DOPA) that links proteins together-we’ll call this the sticky part. The other relies on an amyloid structure, a flat assembly of proteins that tends to form dense fibers. In this work, investigators aimed to combine the sticky bit and the fibers to produce a next generation of bio-inspired adhesives. (They tested two sticky proteins-Mfp3 and Mfp5, which mimic DOPA-based mussel adhesive proteins-and the amyloid protein, CsgA.)
The authors used computer modeling to check whether the sticky parts could be merged with the fibrous one. They found that neither protein disrupted the other-it was possible to create a single molecule that combined both of their binding properties. Simulations also showed that these hybrid proteins spontaneously formed fibers, which suggests that this dual system could in fact be used to form adhesive materials. These fibers were held together by stacking of the amyloid core and adhesion from the sticky domains.
The hybrid proteins were expressed in E.coli, purified, and an enzyme was used to activate the sticky DOPA domains. As predicted, combinations of these hybrid proteins formed long fibers. Interestingly, the diameter of the fibers that contained both of the sticky domains and was activated by the enzyme were the largest of anything they tested. These results suggest that the greatest factor influencing fiber diameter is the presence of mixed sticky domains. This increase in diameter is likely to be associated with increased adhesion.
These adhesive fibers were also fluorescent, which could be useful for a variety of imaging applications. Fiber formation was a prerequisite for fluorescence and is likely due to interactions within the fibers.
Even without the enzyme modification, the hybrid protein fibers maintained underwater adhesion under acidic, neutral, and basic conditions. The modified adhesive fibers always displayed higher adhesion than the unmodified fibers.
Adhesion performance studies indicated that the system containing a mixture of the two hybrid proteins (containing each of the two sticky domains) exhibited the highest adhesion. In this case, the fibers had an adhesion energy that was three times higher than when a single type of sticky domain was used and, under acidic conditions, three times higher than the most adhesive mussel protein reported to date. The mixture of hybrid proteins also exhibited the strongest underwater adhesion to date for protein-based adhesives. Taken together, these results support the idea that the fibrous structure enables large contact areas, while the sticky domains on the surface of the structure enhance adhesion.
Investigation of fiber adhesion to silica, gold, and polystyrene indicated fundamental differences in adhesion behavior compared to the individual components. The interplay between the fibrous regions and the sticky domains seems to modulate how the material interacts with these different surfaces.
Overall, this investigation demonstrates that a modular approach to the design of bio-inspired hybrid fibers for underwater adhesives can lead to the development of hierarchical-structured fibers with high wet bonding strength, enhanced stability, and intrinsic fluorescence.