I have been working on this post for over two
months. The existence of a piece of writing that I don’t love has caused writer’s
block, procrastination, and a long hiatus in my blog posting activity. The
options were to 1) trash it or, 2) make it work. This is my attempt to make it
work. I don’t give up easily. We’ll see if it pays off.
For the 1+ year anniversary of this blog
(which was in January), I have chosen my favorite subject. Adhesion. I seem to
have a thing for adhesion. Or maybe adhesion has a thing for me. It may just be
that adhesion has been a hard problem for humans to solve, and evolution has a
time advantage we must necessarily draw inspiration from. I had never really
thought about adhesion much before. But once you do, it is clear there are a lot
of different organisms living in diverse habitats that have evolved attachment
mechanisms. We have talked about adhesive adaptations in plant seeds
that allow them to stick to passersby and mobilize away from the mother plant
(dispersal). We also discussed adhesion in parasitic tapeworms that allows the worm to anchor itself to and extract nutrients from the intestinal lining of its host (feeding). Today we are going to talk about
adhesion from the perspective of mussels. The mussel attachment mechanism is
like the tapeworm in that it uses adhesion as an anchoring system in a fluid
environment, the difference is it anchors itself to an impenetrable substrate
in a desirable real estate location.
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| Mytilus edilus in a tide pool (Photo credit: Andreas Trepte, www.photo-natur.net) |
Mussels in the genus Mytilus have a special morphological character for attachment. The byssus. The byssus is generated after the larval stage in a process called byssogenesis. Byssogenesis. Throw that one out there when you want to sound smart. Or crazy. The structure itself is not composed of living cells, but is secreted by the living portion of the mussel. Research on the byssus has focused on two species of mussel, the common mussel and the California mussel. The common (or blue) mussel (Mytilus edulis) was described by our friend Linneaus in 1758, and can be found along coastlines in the northern Atlantic Ocean. The California mussel (Mytilus californianus) was described by Timothy Abbot Conrad in 1837. Conrad was a geologist that described many species of mussel on the east and west coasts of the United States.
Mussels have an aquatic larval stage that
is mobile and swims through the water. The adult stage becomes stationary after attaching to rocks, or other mussels, in intertidal
zones. This is where the byssus enters. The intertidal zone is a dynamic
environment where waves provide nearly constant aeration and an influx of
nutrients, with a corresponding efflux of waste products. This energetic habitat
requires an advanced adhesion mechanism that withstands both water intrusion as
well as the force of the waves.
As I started researching the mussel byssus,
it became clear to me that this rabbit hole was deeper than most I
have gone down in the past. Usually when I start the
process of following references back from the bioinspiration of the day (or month(s)),
I find morphological and taxonomic studies conducted with no eye on future
application. In this case, however, some of
the early papers on mussel attachment were not conducted by organismal biologists,
but by bioprospecting scientists. In a 1971 report by Engel, Hillman, Neat, and
Quinby, published by the National Institutes of Dental Research at the National
Institutes of Health (accessed through UCSD libraries), the citations for
the original papers describing mussel attachment were very old. The first one was from 1711 –
Histoire de l’ Academie Royale des Sciences by R.A.F. Reaumer. !. 1711. I don’t
think we have ever traced a paper back that far. The mussel attachment study by Reaumer piqued
the interest of dentists in the early 1970’s for potential applications as a glue in dental
surgery. Mussels continue to captivate interest today and are inspiring the development of mussel mimetic
polymers.
The generation and application of mussel
inspired polymers was reviewed in a paper by Bruce Lee (different than this Bruce Lee)
and colleagues (Lee et al., 2011).
Lee et al. simplify the problem for us at the beginning of the paper:
“Water,
particularly saline water, limits what can bind and where. At a simplistic
molecular level, Coulomb’s law for electrostatic interactions predicts that the
interaction energy for two point charges QaQb is −QaQb/4πεr,
where ε is the dielectric constant and r is the interionic distance. An
electrostatic interaction between opposite charges will be only 1/80 as strong
in water (ε = 80) as it is under vacuum (ε = 1). Actually, the interaction is
often further diminished because r will also be increased due to strong
solvation of ions such as Mg2+ and Li+ by H2O (2).”
In other words, adhesion in salt water is difficult. Mussels already knew this. One of the crazy things scientists
discovered through continued study of the byssus is that the attachment mechanism is more like the webbing
that comes out of Spider-Man’s hands (is that morphology right? I am not a superhero expert) than the head of the tapeworm embedding itself into its host
intestinal lining. The byssus is not made up of living cells, it is a matrix of
proteinaceous threads that are controlled by muscles inside the mussel.
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| Figure 4 from Lee et. al (2011) illustrating the ultrastructure of byssus attachment. |
Current applications of mussel inspired adhesion technology have since expanded beyond surgical applications. The plan of attack thus far has been to mimic the structure of the proteins using synthetic polymers. In the process of doing this engineers have hybridized two bioinspired technologies, gecko feet and the mussel byssus. The pads of gecko feet deserve their own post but suffice it to say that they are sticky when dry, but not when wet. If gecko inspired technology (terrestrially based) is combined with mussel inspired technology (aquatically based) the result is an adhesive mechanism that overcomes the simple difficulties of adhesion in water elegantly explained above.
Even more recently, Bruce Lee was awarded a grant by the Office of Naval Research to
develop an underwater adhesive that could be controlled by electricity. The
Naval applications for such technology are many and include a mechanism to
attach devices to ships, or provide an additional level of control to aquatic
robots exploring the sea floor or rocky intertidal zones.
Appendix I – For
the chemically inspired.
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| Figure 5 from Lee et al. 2011. Figure legend: “Chemical structure of a Dopa-rich mussel foot protein (mfp). Shown is mfp-3 (variant f) sequence from Mytilus edulis. The protein functions at the interface between the plaque and the substratum. Dopa residues are highlighted in red; the sequence contains nearly as many guanidinium groups (4-hydroxyarginines; purple) as Dopa residues. Results from Papov et al. (26).” |
