a paradigm for the design of water-borne, underwater adhesives
Phragmatopoma californica, also known as the Sandcastle worm, is a sabellariid polychaete that lives in intertidal zones off the coast of California. The worm deploys a crown of ciliated tentacles for capturing and transporting building materials from the water column to its mouth where the captured materials are evaluated with ciliated lips. Particles of the right size, shape, composition, and surface chemistry are passed to the dexterous building organ, located immediately ventral to the mouth. Two minute spots of a protein adhesive are applied
before the particles are pressed into place on the end of the tube. The worm wriggles the newly placed particles until the adhesive is set, which takes less than 30 s under cold salty water
Technological interest in the sandcastle glue of Phragmatopoma californica stems in large part from its ability to bond a diverse range of wet mineral substrates, in one step, with minimal surface preparation. The glue has a built-in mechanism that triggers the setting reaction after the glue is secreted into seawater.
Sandcastle Glue Composition
A balance of charges
The sandcastle glue consists primarily of oppositely charged proteins, as shown in the figure above. The Pc3A/B proteins, heavily phosphorylated on the serine residues, are polyanions. The glue also contains polyanionic sulfated polysaccharides. The Pc1, 2, 4, and 5 proteins are polycations, all with isoelectric points above 9. The glue also contains the divalent cations, Mg2+ and Ca2+. Underlined sequences are secretion signal peptides. Red blocks represent sequences against which synthetic peptide antibodies were created. The antibodies were used to determine the location of the proteins in the adhesive gland, as shown in the following figure.
Distribution of Glue Components
A multi-part adhesive
Sandcastle glue components are synthesized, packaged into secretory granules, and stored until secretion in at least four distinct types of cells in the adhesive gland. The secretory cells and granules stain differentially as seen in the coronal section through the adhesive gland shown on the left. Each type of granule contains oppositely charged components, as is commonly observed in secretory granules transiting through the regulated secretory pathway. The granules appear to be secreted in tact and partially mixed shortly after secretion. The distribution of glue proteins and the secretory process are diagrammed on the right. The jump in pH upon secretion into seawater likely plays a key role in the setting and hardening reactions.