Multiscale structure of the underwater adhesive of Phragmatopoma californica:: A nanostructured latex with a steep microporosity gradient

Phragmatopoma Californica builds a tubular dwelling by gluing bits of sand and seashell together underwater with a proteinaceous adhesive. In the lab, the animals will build with 0.5 mm glass beads. Two spots of glue with a consistent volume of about 100 pL each are deposited on the glass beads before placement on the end of the tube. The animals wriggled the particles for 20-30 s before letting go, which suggested that the adhesive was sufficiently set within 30 s to support the glass beads. The structure of the adhesive joints was examined at the micro- and nanoscopic length scales using laser scanning confocal and atomic force microscopies. At the microscale, the adhesive was a cellular solid with cell diameters ranging from 0.5 to 6.0 mu m, distributed to create a steep porosity gradient that ranged from near zero at the outside edges to about 50% at the center of the adhesive joint. At the nanoscale, the adhesive appeared to be an accretion of trillions of deformable nanospheres, reminiscent of a high-solids-content latex adhesive. The implications of the structure for the functionality of the adhesive is discussed.