Biomimetic Lubrication and Surface Interactions of Dopamine-Assisted Zwitterionic Polyelectrolyte Coatings

A bioinspired zwitterionic polyelectrolyte coating with excellent hydration ability has been regarded as a promising lubricating candidate for modifying artificial joint cartilage surface. In physiological fluids, the ubiquitous proteins play an important role in achieving outstanding boundary lubrication; however, a comprehensive understanding of the hydration lubrication between polyelectrolyte coatings and proteins still remains unclear. In this work, a facile fabrication of ultrasmooth polyelectrolyte coatings was developed via codeposition of synthesized poly(dopamine methacrylamide-co-2-methacryloyloxyethyl phosphor- ylcholine) (P(DMA-co-MPC)) and dopamine (DA) in a mild condition. Upon optimization of the feeding ratio of P(DMA-co-MPC) and DA, the as-fabricated PDA/P(DMA-co-MPC) coatings exhibit excellent lubricating properties when sliding with each other (friction coefficient mu = 0.036 +/- 0.002, similar to 2.8 MPa), as well as sliding with a model protein (bovine serum albumin (BSA)) layer (mu = 0.041 +/- 0.005, similar to 4.8 MPa) in phosphate-buffered saline (PBS, pH 7.4). Intriguingly, the lubrication in both systems shows Amontons-like behaviors: the friction is directly proportional to the applied load but independent of the shear velocity. Moreover, the PDA/P(DMA-co-MPC) coatings could resist the protein fouling (i.e., BSA) in PBS, which is crucial to prevent the surfaces from being contaminated when applied in biological media, thus maintaining their lubricating properties. Our results provide a versatile approach for facilely fabricating polyelectrolyte coatings with superior lubrication properties to both polyelectrolyte coatings and protein surfaces, with useful implications into the development of novel lubricating coatings for bioengineering applications (e.g., artificial joints).