Biomedical Engineering Reference
In-Depth Information
the bulk properties of materials may yield improvements as seen in the natural
design. The three model organisms were studied at different stages, complicating
interpretation, and further investigations should reveal both the essential functions
and universal designs.
Differences in the designs of the adhesives may be easier to find than universal
design principles, as discussed above. Microscopic as well as molecular structures
might be much more involved in the modes of adhesives in the organisms, espe-
cially with respect to the quality of resulting attachment.
Mussels evolutionally developed their holdfast with an order of centimeter-
distance between the animal and its foreign materials. This mode of attachment
indicates that a multi-angle environmental stress is given to the adhesive joint of the
mussel, and the stress would be remarkably higher among the three model
organisms. Therefore, the mussel might need the multiplex mechanism in the
holdfast. The solid foam-like microstructure in the disk, gradient of mechanical
properties in the thread portion, the durable coating of the whole byssus, and
combination of the reversible and irreversible molecular cross-linking will scatter
a given multi-angle and repetitive stress. On the other hand, the barnacle adhesive
joint might receive a stress from a limited angle, mainly from a horizontal direction.
The thinner and larger adhesive joint of the barnacle also makes that attachment
more secure than the other two model organisms. Thus, barnacle cement may
simply rely on molecular interactions for curing rather than a polymerization via
cross-linkage. The molecular mechanism may also be involved in the fact that the
cement attaches to its enlarged marginal area, and thus, the central adhesive joint
already formed would provide the supporting layer [ 3 ]. However, attachment of the
calcareous shell and a foreign hard material by using the softer proteinaceous
cement with the thinner joint may require a further-specialized mechanism. The
fact that the microscopic structure of the cement differs depending on the materials
to be attached [ 50 ] may indicate that the microscopic structure may be also involved
in the balance of different stiffnesses. The adhesive joint of the tubeworm cement
may be exposed to a multi-angle stress because surfaces of foreign materials to be
attached to are not flat and the area of the joint is smaller than the areas of materials
to be attached. It may be the reason why tubeworm cement needs to employ similar
solid foam- like microstructure to that of the mussel disk. The combination of
irreversible and reversible molecular cross-linking may be suitable for mechanical
properties required by the adhesive joint. Both the mussel byssus and dwelling tube
of the tubeworm are disposable. This fact may indicate how high the strength is in
their modes of attachment.
Mussels and tubeworms employ hydrophilic components as specialized
mechanisms to avoid dispersion and keep the components condensed after secretion
might be required. The complex coacervation might be suitable to the mechanism,
which might simultaneously result in a solid foam-like microscopic structure.
Self-assembly of the barnacle cement bulk, which is composed of hydrophobic
proteins, may be triggered by conformational changes of the proteins after
secretion, although this hypothesis needs to be tested.
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