Fabrication of hollow poly-allylamine hydrochloride/poly-sodium styrene sulfonate microcapsules from microbubble templates

Abstract

Hollow microcapsules made of biodegradable polymers have attracted considerable attention as ultrasound contrast agents and carriers in drug delivery systems (DDSs). A hollow polyelectrolyte microcapsule is one of the promising candidates for such applications. Hollow polyelectrolyte microcapsules are normally fabricated by decomposing a core material after forming a capsule shell. Recently, Shchukin et al. (D. G. Shchukin, K. Kohler, H. Mohwald and G. B. Sukhorukov, Angew. Chem., Int. Ed., 2005, 44, 3310–3314) and Winterhalter et al. (M. Winterhalter and A. F. P. Sonnen, Angew. Chem., Int. Ed., 2006, 45, 2500–2502) proposed a fabrication method of hollow polyelectrolyte microcapsules using microbubbles as templates. In this method, stable microbubbles are formed in a surfactant solution by means of an ultrasound generator, and then, a polyelectrolyte is adsorbed on the microbubble surface, yielding hollow polyelectrolyte microcapsules. In this study, we proposed a new fabrication method for hollow poly-allylamine hydrochloride/poly-sodium styrene sulfonate (PAH/PSS) microcapsules without using surfactants. In a Na₂CO₃ aqueous solution, PAH becomes colloidal particles of R-NHCOO⁻ and R-NH₃⁺ within a certain pH range. Under such a condition, if microbubbles are generated in the solution, the colloidal particles adsorbed on the microbubble surface stabilize the microbubbles. When PSS is added to the solution sequentially, a bilayer of PAH/PSS can be formed around microbubbles. We successfully fabricated hollow PAH/PSS microcapsules by this method and elucidated the conditions required for the formation of colloidal particles of PAH and a bilayer of PAH/PSS.