Atomic force microscopy of the morphology and mechanical behaviour of barnacle cyprid footprint proteins at the nanoscale

Abstract

Barnacles are a major biofouler of man-made underwater structures. Prior to settlement, cypris larvae explore surfaces by reversible attachment effected by a ‘temporary adhesive’. During this exploratory behaviour, cyprids deposit proteinaceous ‘footprints’ of a putatively adhesive material. In this study, footprints deposited by Balanus amphitrite cyprids were probed by atomic force microscopy (AFM) in artificial sea water (ASW) on silane-modified glass surfaces. AFM images obtained in air yielded better resolution than in ASW and revealed the fibrillar nature of the secretion, suggesting that the deposits were composed of single proteinaceous nanofibrils, or bundles of fibrils. The force curves generated in pull-off force experiments in sea water consisted of regions of gradually increasing force, separated by sharp drops in extension force manifesting a characteristic saw-tooth appearance. Following the relaxation of fibrils stretched to high strains, force–distance curves in reverse stretching experiments could be described by the entropic elasticity model of a polymer chain. When subjected to relaxation exceeding 500 ms, extended footprint proteins refolded, and again showed saw-tooth unfolding peaks in subsequent force cycles. Observed rupture and hysteresis behaviour were explained by the ‘sacrificial bond’ model. Longer durations of relaxation (>5 s) allowed more sacrificial bond reformation and contributed to enhanced energy dissipation (higher toughness). The persistence length for the protein chains ( L P ) was obtained. At high elongation, following repeated stretching up to increasing upper strain limits, footprint proteins detached at total stretched length of 10 µm.