Design of dual hydrophobic–hydrophilic polymer networks for highly lubricious polyether-urethane coatings

Bio-lubricated surfaces found in nature have inspired the design of low friction polymer coatings for biomedical
applications. This work presents a systematic study of the relation between the network structure parameters
and the macroscopic friction properties of highly lubricious dual hydrophobic/hydrophilic polyurethane (PU)
coatings in an aqueous environment. Chemically cross-linked PU coatings were prepared by adding poly(ethylene
glycol) mono-methyl ether (mPEG) as hydrophilic dangling chains, or poly(ethylene glycol) (PEG)-diol as
hydrophilic elastically active network chains, to poly(propylene glycol) (PPG)-PU coating formulations. The
friction behaviour of the water swollen coatings was measured using a custom-made water immersed tribology
setup. Addition of the PEG segments or mPEG dangling chains to hydrophobic PPG coatings greatly enhances the
lubricious properties of the coatings. These dual hydrophobic/hydrophilic diol PU network exhibit a surface with
a lower coefficient of friction compared to reference coatings from either individual precursors, demonstrating a
large synergistic effect between the hydrophobic PPG and the hydrophilic PEG in the coatings. Based on network
structure and surface chain considerations it is hypothesized that the existence of a thin and softer hydrated
surface layer on top of a less hydrated, more rigid, coating bulk layer gives rise to the observed enhanced
lubricious properties, hereby mimicking to some extent bio-lubricated systems, such as cartilage.