Simple preparation of polyelectrolyte complex beads for the long-term release of small molecules

We report a simple method for preparing solid polyelectrolyte complex (PEC) beads, which provide effective barriers to diffusion and can be used for the multiple-day release of small molecules. Single-phase poly(allylamine) (PAH) and poly(styrenesulfonate) (PSS) mixtures were prepared at pH 11.6 (significantly above the effective pKa of PAH), where the PAH amine groups were deprotonated and therefore neutral. These mixtures were added dropwise into acid baths, whereupon the rapid acid diffusion into the polyelectrolyte droplets led to instant ionization of PAH amine groups and, thus, the formation of PEC beads (i.e., via phase inversion). In stark contrast to the PEC particles prepared through phase inversion in previous studies, which had (solvent-filled) capsule-like morphologies, these beads had solid internal structures. The solute permeabilities of these PEC matrices could be extensively tuned by air drying the beads, which led to the apparently-irreversible closure of pores. Thus, by tuning the drying conditions and polymer compositions used during bead preparation, a model small molecule (Fast Green FCF dye) was released over times ranging between 2 and 18 days.