YSM Issue 95.1

FEATURE
Materials Engineering
A STICKY SITUATION...
UNDERWATER
FOCUSED DELIVERY OF ENERGY
INTO DIFFUSIVE SYSTEMS
What do mussels have that we humans don’t? Well,
many things, but among them: the ability to stick to
surfaces underwater.
Strong underwater adhesives have versatile and useful realworld
applications ranging from underwater equipment repair
to surgical glue. Researchers from the Washington University
in St. Louis combined mussel foot proteins and spider silk to
create a hydrogel that can adhere to surfaces underwater. “Nature
already offers a wealth of materials, and some of them even
outperform synthetic materials,” said professor Fuzhong Zhang,
a lead researcher on the study. The mussel foot proteins naturally
secreted by mussels allow them to adhere to a variety of surfaces,
even in the harsh conditions of seawater. “We’re inspired by
natural materials that are very impressive in some aspects. The
first step is trying to reproduce it. Once we are confident that we
can synthesize the material with similar properties, then we can
engineer it to make it perform better,” Zhang said.
And engineer it they did. The new adhesive hydrogel is able
to stick to a wide range of surfaces—ranging from glass to
mammalian tissues—underwater. The researchers began with
the zipper-forming motif of an Aβ amyloid protein, which
conveniently tends to self-assemble into stable nanofibrils.
Then, they added spider silk protein for much-needed material
strength and mussel foot protein for improved surface adhesion.
Engineered microbes produced the final hybrid protein. This
process, which pushes the boundaries of traditional recombinant
DNA technology, presented unique challenges to the researchers.
“The mussel protein contains a special amino acid, DOPA, which
basically offsets tyrosine. It’s not one of the 20 canonical amino
acids. In our case, we have to engineer the bacteria so that it can
incorporate DOPA into the protein with high efficiency,” Zhang
said. The incorporation of non-canonical amino acids is critical
to the function of these tri-hybrid proteins.
This microbial production of useful, naturally-occurring
materials has the advantage of allowing advanced, specific
DNA control of functional groups. “Scientifically, the biggest
challenge is to understand the sequence-property relationship
of protein-based adhesives. With that knowledge, we will be
able to create adhesives with desirable properties,” Zhang said.
The researchers were able to fine-tune the properties of the
hydrogel—structure, strength, cohesion, and adhesion—by
adjusting the different domains and sequences of spider silk and
mussel foot proteins.
BY EUNSOO HYUN
ART BY ALEX DONG
On a
practical
level, this novel
hydrogel provides
several advantages over
pre-existing competitors in
the field. Since the hydrogel is
biocompatible and biodegradable, it is
an attractive, unique candidate for tissue repair
and surgical applications. Another feature is its mechanical
similarity to collagen, a major structural element in the
extracellular matrix. “It is critical for a surgical adhesive to
have similar properties with the natural extracellular matrix
because that can promote more rapid tissue repair and reduce
the chance of failure,” Zhang said. The hydrogel is also proteinbased,
as opposed to other previously developed polymerbased
adhesives. One area in which a protein-based adhesive
is necessary is coral restoration, where the adhesive must work
well underwater in addition to being safe, i.e., not releasing any
potentially toxic materials.
This project is an exciting example of the potential of synthetic
biology. Zhang reminisced on the team’s first, unexpected
encounter with the possibilities of mussel foot protein. “A few
years ago, one of my graduate students, Eugene Kim, who is now
an Assistant Professor at George Mason University, worked on
this project. At that time, the adhesive protein he made looked
the same as any other protein—it was just a powder that would
dissolve in solution,” Zhang said. Kim didn’t test the proteins
underwater—he simply added some protein solution between
two aluminum bars. “The next day, when he tried to pull, it was
so strong he could not pull it apart. And he’s a strong guy!” Even
before officially testing the material, the researchers found that
it was strong enough to lift a full one-liter bottle of water despite
only having a tiny area of adhesion.
Synthetic biology is a rapidly growing field, full of
innovation and growth. “I want people to learn more about
the opportunity that synthetic biology provides to material
science and material engineering. We would like to work
with many researchers who believe in the power of synthetic
biology. We welcome new students to join us and explore this
exciting field together,” Zhang said. ■
26 Yale Scientific Magazine March 2022 www.yalescientific.org