Investigating Molecular Interactions between Mucin and Contact Lens Thin Films Using Nuclear Magnetic Resonance

Poly(2-hydroxyethyl methacrylate) (PHEMA) and polyvinyl­pyrrolidone
(PVP) are common polymers used in contact lens materials. In these
systems, PHEMA forms the hydrogel network and PVP is used as a wetting
agent. These lens materials, like other implantable devices, suffer
from interactions with biological macromolecules that ultimately foul
the surface of the lens. Despite the widespread adoption of contact
lenses, the mechanisms of protein adsorption onto lenses are not well
understood. To investigate the interactions responsible for fouling
on contact lens materials, we developed a spin-coating apparatus to
deposit thin polymer films onto NMR tube surfaces. We then used direct
saturation compensated (DISCO) NMR on PHEMA-coated NMR tubes to investigate
mucin binding at the surface of the coatings and the effect of varying
PVP concentrations. Using this technique, this study was able to characterize
the binding profiles of PHEMA-only and PHEMA+PVP hydrogels. The results
show that the mucin-binding profile of PHEMA evolves as the PVP content
increases. In the absence of PVP, the protons in the PHEMA backbone
interact with mucin. However, when PHEMA and PVP form a hydrogel network,
the protons on the PHEMA side chain become mucoadhesive in addition
to the backbone. When the PVP content is further increased, only the
side chain protons retain their interaction. Interestingly, the NMR
spectra of PHEMA+PVP hydrogels suggest that PVP preferentially accumulates
within the PHEMA matrix, with relatively little presenting at the
surface of the hydrogels. In conjunction, SEM imaging of the PHEMA
and PHEMA+PVP hydrogels shows that an increase in porosity accompanies
the change in mucin-binding behavior, indicating that the change in
morphology is correlated to the change in mucin-binding behavior rather
than solely the change in composition. Overall, this study develops
a tool to study the interactions between macromolecules and polymeric
surfaces with atomic precision, uncovering structure–activity
relationships that govern mucoadhesion of polymeric hydrogels.