Methods of loading and releasing low molecular weight cationic molecules in weak polyelectrolyte multilayer films

This work studied the loading capabilities and release behavior of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) multilayer films, using methylene blue dye (MB) as an indicator. PAA/PAH multilayers assembled at pH = 2.5/2.5 loaded MB throughout the multilayers due to available binding sites and a permeable structure. The amount of MB loaded increased linearly with increasing film thickness. Use of buffered MB solutions enabled faster and more uniform MB loading into the multilayers, because ions within the MB solution caused further swelling of the multilayer film. Release studies demonstrated a pH-sensitive release mechanism, where lower pH environments increased the release rate. Buffered environments also increased the release rate of MB from the multilayer films. By assembling capping layers of PAA/PAH at pH 6.5/6.5 on top of 2.5/2.5 PAA/PAH multilayers, the diffusion of MB through the multilayers was better controlled under nonbuffered conditions. Under buffered conditions, however, the permeability of the 6.5/6.5 PAA/PAH capping layers increased due to swelling, and MB diffusion was not hindered. Further work needs to be done to slow release of MB under buffered conditions. The controlled loading capabilities and pH-sensitive release behavior of weak polyelectrolyte multilayer films demonstrated here may be useful toward drug delivery applications.