Diversified Material Designs in Biological Underwater Adhesives - Structural Interfaces and Attachments in Biology - page 185

All of the adhesives used by the three model organisms involve a protein
complex. Proteins would be the suitable choice among biological molecules due
to the availability of several functional groups and control of the orientation based
on conformation of the molecules. Therefore, there are several available choices of
molecular interactions and chemistries. Protein is known to be one of the best
components for making hierarchical materials with varied mechanical properties. A
complex of different types of proteins is also a common design among the three
model organisms. The complex nature of the protein compositions might be a result
of the multi-functionality of the underwater attachment and implies that each
component might have its own function(s) in forming a firm and durable adhesive
joint in water. Surface coupling and curing of bulk are essential functions in
adhesives in air or underwater. Specific proteins appear to be provided for the
respective function in the mussel byssal disk and barnacle cement. The occurrence
of specific surface coupling proteins in the mussel and barnacle may be similar to
cases where engineers pre-treat a surface with so-called primer to improve surface
coupling of an adhesive. However, natural designed systems seem to have addi-
tional molecular mechanisms to link the surface coupling layer with the bulk of the
adhesive.
Involvement of smaller and hydrophilic proteins in surface coupling might be a
common design in the mussel and barnacle, though molecular designs for curing are
different. Tubeworm cement is composed of only three proteins, the minimum
number among the three organisms, and all of the three proteins seem to contribute
to curing the bulk of the cement together. It is unclear whether all three proteins are
involved in surface coupling or whether one of them is specifically involved. In
either case, surface coupling seems to depend on dominant chemisorption via the
side chain of amino acid(s) in the three model organisms.
Underwater attachment should have additional function(s) beyond those of air
attachment. We should explore not only the structural designs for the known
functions for attachment in air, but also functions that are not yet clearly understood
in underwater attachment. For instance, spreading onto the underwater surface
should be another essential function in underwater attachment. The biological
underwater adhesives may include specific component(s) for this function. Alter-
natively, the function may rely on a part of the structure(s) of protein(s). The
abundance of hydroxyl groups in adhesive proteins is noticed as a common feature
among the three model organisms. Residues abundant in hydroxyl groups such as
Ser, Thr, hydroxy-Arg, and hydroxy-Pro are found in proteins with the surface
coupling and the curing. It might be a common molecular design that is essential in
a function not clearly understood yet, such as wetting on wet surfaces or keeping
adhesives stable in water for a longer period. In this respect, introduction of PEG by
Dalsin et al. [ 77 ], as mentioned in the previous section, may be one of appropriate
manners for the artificial design.
The bulks of biological adhesives are not homogeneous and have microscopic
structures. This seems to be a common design, which probably contributes to the
mechanical properties of the adhesives, although the detailed structures are not
the same among the three model organisms. A technology to add a filler to improve