Diversified Material Designs in Biological Underwater Adhesives - Structural Interfaces and Attachments in Biology - page 176

The barnacle tightly attaches its calcareous shell to naturally occurring rocks and
wood, artificial concrete structures, steel ship hulls, tortoise shell surfaces, and even
whale epidermis. It also attaches to synthetic polymers, including fluoropolymer,
and other metals/metal oxides, in water [ 35 ]. Because a barnacle's attachment
causes serious damage to ship services and water uptake in power and industrial
plant cooling systems, extensive research regarding barnacle adhesive has been
motivated by the need to develop anti-fouling technology [ 36 - 38 ]. Conspicuous
foul-release coatings, typically silicone, have promoted studies on the mechanism
of lowering the adhesive strength produced by the barnacle cement, which would be
useful to improve the performance of coatings [ 39 , 40 ]. Interestingly, the adhesive
joint produced by the barnacle in foul-release silicone coatings is macroscopically
and microscopically different from that produced on general materials; in other
words, the joint on the foul-release coating is thicker, swollen, and weaker in
adhesive strength [ 41 - 44 ]. It is unclear whether the barnacle just fails to attach to
the man-made coating or it somehow adapts to the surface with a softer adhesive to
balance the stiffness.
The cement is biosynthesized in the soft tissue of the animal in a fluid form and
appears to be secreted to the site of attachment. The cement is a protein complex
whose proteins are unique among all of the proteins found in public databases [ 3 ].
Neither sequence homology to mussel byssal proteins/tubeworm cement proteins
nor modification of an amino acid residue of tyrosine to DOPA, which is a typical
and essential one in adhesives of mussel and tubeworm, was found in the barnacle
cement proteins [ 45 , 46 ]. Furthermore, no indication of protein modification has
been found in the barnacle cement proteins. The exception is glycosylation in one
of the proteins [ 47 ], which is in contrast to the mussel byssal thread and tubeworm
cement where all proteins were found to undergo multiple posttranslational
modifications in a number of amino acid residues. Thus, among the model
organisms,

the barnacle cement is unique in its molecular mechanism of

attachment.
The adhesive layer of the animal is macroscopically uniform, whereas a
microscopic structure seems to be present in the cement. The microscopic
structures might be fibrous or sponge-like [ 43 , 48 , 49 ], which seem to be different
depending on the materials to be attached [ 50 ]. It is unclear whether specific
localization of some components in the molecular level occurs in the
adhesive joint, although the cement is actually composed of distinct proteins, as
mentioned below.

9.5.2 The Cement Proteins

Five proteins are identified to be components of the cement (Fig. 9.5 ), which are
different from each other in terms of their chemical structures/physicochemical
properties [ 33 ]. Among them, two bulk proteins are larger molecules with molecu-
lar weights of 100 kDa [ 51 ] and 52 kDa [ 47 ], whereas two other proteins possibly