Diversified Material Designs in Biological Underwater Adhesives - Structural Interfaces and Attachments in Biology - page 168

Typical processes to obtain attachment in the air may include the following
steps. First, material surfaces are cleaned (pre-cleaning) and kept dry by removing
the water layer. Adhesives are then uniformly and thinly spread onto the material
surfaces (spreading or wetting). Materials are joined together and are compressed
to make the adhesive layer as thin as possible. Following the hand-processing, the
adhesive molecules form several bonds with molecules on the material's surface
(surface coupling) and are allowed to sit for minutes to hours until the bonding
strength reaches a sufficient level (curing). To summarize, attachment in the air
may simply be achieved by coping with two minimal requirements: surface cou-
pling and curing.
In water, the requirements for attachment are much more challenging [ 1 ]. First,
it is very difficult to remove the water layer and spread the adhesive on the
material's surface. A much higher dielectric constant of water [ 2 ] also makes it
difficult to keep the surface coupling for a longer period (e.g., more than a year).
And because of fouling of organic materials and/or microbial cells, the underwater
surface quickly becomes dirty. The fouled layer makes coupling of the adhesive to
the material surface difficult. In addition, the bulk layer of the adhesive becomes
swollen from excess hydration, resulting in weaker adhesive strength.
To achieve this troublesome underwater attachment, sessile organisms ranging
from microbes to hard and soft animals and plants have developed diversified ways
to tightly and continuously attach to several material surfaces. These biological
adhesives are excellent models from which to learn how to artificially attach
materials in water and to obtain information that will be useful to develop general
theories in the interface sciences.
9.2 Basic Differences Between Commercial Adhesives
and Biological Adhesives
The basic differences between commercially available and biological adhesives can
be summarized as follows (Table 9.1 ). Biological adhesives consist of biomolecular
materials that, typically, are protein complexes involved in firm underwater attach-
ment. The protein molecules have several functional groups on their side chains,
including carboxyl, amino, aromatic, imidazole, guanidino, alkyl, and hydroxyl
groups. Additionally, posttranslational modifications of proteins produce further
functional groups (e.g., phosphate). In contrast, synthetic polymers used in commer-
cially available adhesives have limited functional groups on their side chains,
mainly due to constraints of the simpler synthetic processes, which is directly related
to the cost of mass production. The protein molecules have, in general, defined
conformations wherein each functional group of side chains is given an individual
orientation. The orientation of each functional group is essential to function, keeping
the molecular conformation or the interaction with another molecule. Specificity
and/or higher efficiency of protein functions are generally defined by the molecular