Wet adhesion is a serious engineering challenge. Taking a cue from the chemical composition of mussel foot proteins, researchers modified a molecule and then tested its adhesive strength in aqueous environments. The result: a compound that rivals the staying power of mussel glue.
If you work on ships, or own a boat, you probably are familiar with the mussel protein problem with barnacles on ship hulls. This is actually a big issue, not only for drag and fuel efficiency, but introduction of invasive species as well. It has been said that if a glue could be synthesized from the protein, it would be a glue for steel that’s as strong as a weld.
But synthetic wet adhesive materials, have not lived up to nature’s glues. Scientists at UC Santa Barbara decided to improve a small molecule called the siderophore cyclic trichrysobactin (CTC) that they had previously discovered. Said co-author Alsion Butler, a professor in UCSB’s Department of Chemistry and Biochemistry:
“There’s real need in a lot of environments, including medicine, to be able to have glues that would work in an aqueous environment. So now we have the basis of what we might try to develop from here. We just happened to see a visual similarity between compounds in the siderophore CTC and in mussel foot proteins.”
Mussel Foot Proteins
Siderophores are molecules that bind and transport iron in microorganisms such as bacteria.
“We specifically looked at the synergy between the role of the amino acid lysine and catechol,” Butler added. “Both are present in mussel foot proteins and in CTC.”
Mussel foot proteins contain similar amounts of lysine and the catechol dopa. Catechols are chemical compounds used in such biological functions as neurotransmission. However, certain proteins have adopted dopa for adhesive purposes.
From discussions with J. Herbert Waite, a professor in the Department of Molecular, Cellular and Developmental Biology, Butler realized that CTC contained not only lysine but also a compound similar to dopa. Further, CTC paired its catechol with lysine, just like mussel foot proteins do.
“We developed a better, more stable molecule than the actual CTC,” Butler explained. “Then we modified it to tease out the importance of the contributions from either lysine or the catechol.”
Testing Adhesion Strength
Co-lead author Greg Maier, a graduate student in the Butler Lab, created six different compounds with varying amounts of lysine and catechol. The Israelachvili lab tested each compound for its surface and adhesion characteristics.
Co-lead author Michael Rapp used a surface force apparatus developed in the lab to measure the interactions between mica surfaces in a saline solution.
Only the two compounds containing a cationic amine, such as lysine, and catechol exhibited adhesive strength and a reduced intervening film thickness, which measures the amount two surfaces can be squeezed together.
Compounds without catechol had greatly diminished adhesion levels but a similarly reduced film thickness. Without lysine, the compounds displayed neither characteristic.
“Our tests showed that lysine was key, helping to remove salt ions from the surface to allow the glue to get to the underlying surface,” Maier said.
Explained Rapp, a chemical engineering graduate student:
“By looking at a different biosystem that has similar characteristics to some of the best-performing mussel glues, we were able to deduce that these two small components work together synergistically to create a favorable environment at surfaces to promote adherence. Our results demonstrate that these two molecular groups not only prime the surface but also work collectively to build better adhesives that stick to surfaces.”
“In a nutshell, our discovery is that you need lysine and you need the catechol. There’s a one-two punch: the lysine clears and primes the surface and the catechol comes down and hydrogen bonds to the mica surface. This is an unprecedented insight about what needs to happen during wet adhesion.”
Illustration: Wet adhesion to a mica surface: A cationic amine (pink) penetrates the hydration layer, evicting potassium ions (gold balls) and preparing the mica surface for hydrogen bonding (green aura). By Peter Allen