Sandcastle Worms and Caddisflies
INSPIRING NEW ADHESIVES THROUGH CO-POLYELECTROLYTE CONDENSATION
Most people wouldn't bother trying to apply an adhesive bandaid to wet skin, or try bonding objects together under water with household glues or tape; they have an intituitive sense that adhesion to wet surfaces is difficult with commonly available adhesives. Indeed, development of synthetic adhesives for use in watery environments has lagged far behind the impressive achievements over the last century in dry adhesive technology. Glue has increasingly replaced mechanical fixation in the manufacture of everything from the soles of our shoes to apartment buildings to jumbo jets. The development of wet field adhesives has been slow despite powerful, high-margin incentives provided by the need for medical adhesives to repair damaged tissues. Though superglue is used to fix superficial skin wounds, internal tissues are still mostly fixed with stitches, screws, metal pins, and plates.
Proof that effective underwater adhesives exist is provided by the multitudes of aquatic organisms whose lifestyles include gluing themselves to wet contaminated surfaces, or gluing together protective shelters under water with gathered materials. The marine sandcastle worm, in the latter category, cobbles together composite tubular dwellings with sand, the broken exoskeletons of marine invertebrates, and undersea glue. The tubes are adjoined to other tubes to create intertidal reefs reminiscent of sandcastles, hence the common name. Freshwater caddisfly larvae produce a sticky underwatrer silk they use to tape together composite structures remarkably similar in look and function to sandcastle worm tubes. The adhesives are also chemically similar in that they both comprise oppositely charged polyelectrolytes.
Together, these examples suggest intermolecular charge neutralization between oppositely charged polyelectrolytic biomacromolecules (complex coacervation) may be a common mechanism in nature to create insoluble underwater adhesives from water-soluble macro-precursors. Accordingly, the sandcastle worm and caddisfly adhesives have provided our lab with a paradigm for the design of biomimetic adhesives based on polyelectrolyte condensation. The central tenet of our lab, as diagrammed above, is to create synthetic underwater adhesives by mimicking the constituents of the natural underwater adhesives with synthetic co-polyelectrolyte analogs.