Direct assembly of antimicrobials within layer-by-layer films mediated by polyphosphazene polyelectrolytes

We explore the effects of chain flexibility, charge density, and hydrophobicity on the direct, electrostatic assembly of small molecules with biocompatible polyelectrolytes. Films were assembled by the layer-by-layer (LbL) technique using ionic polyphosphazenes (PPzs) as polyanions and small-molecule antibacterial bioactives as cations. Evolution of dry film thickness was monitored with spectroscopic ellipsometry. PPz/bioactive films grew linearly in a pH dependent manner, which was correlated with ionization degree of PPzs in the films using ATR-FTIR. Deposition and release of bioactives were strongly dependent on charge density and hydrophobicity of polyanions; these effects were compared for the cases of non-fluorinated and fluorinated PPzs. The release rate of bioactives from films was explored as a function of temperature (ambient, physiological, and accelerated (65 °C)) and pH. Non-fluorinated PPz films released antibiotics at low pH following the charge renormalization argument. In contrast, multilayers of fluorinated PPzs enabled antimicrobial retention, due to the enhanced hydrophobic interactions.