Biomedical Engineering Reference
In-Depth Information
Another way to design a humid adhesive may be found in a report in which
DOPA was introduced into a hydrophobic polymer unit [ 91 ]. The design principle
of the material may be interpreted based on a combination of surface coupling used
by the mussel with the bulk curing employed by the barnacle, because the curing of
the barnacle cement is maintained by hydrophobic interactions. In the study,
polymer scientists used an organic solvent to control the hydrophobic interactions,
while the barnacle introduced conformational changes of the proteins to control the
hydrophobic interactions in water. Further evidence to design a humid adhesive
may be found in peptide self-assemblies. The structures of the barnacle cement
proteins include units for self-assembly, because the cement proteins are fated to
tightly interact with homo-/hetero-cement proteins for self-assembly to form the
adhesive bulk. Simple peptides designed from the barnacle cement proteins [ 54 , 92 ]
gave the peptide self-assembly capability upon increment of ionic strength or shift
of pH (Fig. 9.8 ). Therefore combinations of self-assembly structures with surface
coupling structures may be another choice for developing practical adhesives in
wet conditions or in water. On the other hand, how to control the hydrophobic
interactions in an aqueous solution remains a challenging task.
Mimetic or inspired materials aimed at humid adhesives or underwater
adhesives from sessile organisms are fragile and seem to be far from practical use
in water or humid conditions, though we have extensively learned the way to coat/
functionalize material surfaces. This suggests that we still have not learned from
nature how to attach materials in water. This may be due to our too-simplified ideas
on how underwater attachments function: designing only from two simple
functions, strong surface coupling and curing of bulk. Researchers have tended to
interpret the role of the components by assuming a single function for each
component. Since underwater attachment appears to be a process much more
complex than air attachment, it is essential to further study the functions required
in nature to accomplish underwater attachment. Perhaps, protein- and peptide-
based materials might be the keys to understanding natural design. It is intriguing
that there are two barnacle proteins with surface functions because at least one of
the recombinant proteins prepared under physiological conditions has the native
conformation. An in-depth understanding to characterize the native proteins is
essential to learning about underwater attachment.
9.8 Comparison of Biological Material Designs
As previously stated, the three model organisms have similarities and differences in
their adhesive abilities that occur at the macroscopic to molecular levels [ 3 ].
Similarities in their principles of design may indicate a specific role for underwater
attachment, whereas differences in their designs might be related to the particular
conditions that individual adhesive requirements must meet. In this last section,
similarities and differences are addressed to consider future technology.
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