Mussel Coating Protein-Derived Complex Coacervates Mitigate Frictional Surface Damage

The role of friction in the functional performance of biomaterial interfaces is widely reckoned to be critical and complicated but poorly understood. To better understand friction forces, we investigated the natural adaptation of the holdfast or byssus of mussels that live in high-energy surf habitats. As the outermost covering of the byssus, the cuticle deserves particular attention for its adaptations to frictional wear under shear. In this study, we coacervated one of three variants of a key cuticular component, mussel foot protein 1, mfp-1 [(1) Mytilus californianus mcfp-1, (2) rmfp-1, and (3) rmfp-1-Dopa], with hyaluronic acid (HA) and investigated the wear protection capabilities of these coacervates to surfaces (mica) during shear. Native mcfp-1/HA coacervates had an intermediate coefficient of friction (μ ∼0.3) but conferred excellent wear protection to mica with no damage from applied loads, F⊥, as high as 300 mN (pressure, P, > 2 MPa). Recombinant rmfp-1/HA coacervates exhibited a comparable coefficient of friction (μ ∼0.3); however, wear protection was significantly inferior (damage at F⊥ > 60 mN) compared with that of native protein coacervates. Wear protection of rmfp-1/HA coacervates increased 5-fold upon addition of the surface adhesive group 3,4-dihydroxyphenylalanine, (Dopa). We propose a Dopa-dependent wear protection mechanism to explain the differences in wear protection between coacervates. Our results reveal a significant untapped potential for coacervates in applications that require adhesion, lubrication, and wear protection. These applications include artificial joints, contact lenses, dental sealants, and hair and skin conditioners.