Hollow microcapsules made of biodegradable polymers have attracted considerable attention for ultrasound contrast agents and drug delivery system. In normal fabrication techniques, stable microbubbles are formed in a surfactant solution via ultrasound, then polyelectrolyte are adsorbed on the microbubble surface, resulting in hollow microcapsules. This document proposes a new method. First, a poly-allylamine hydrochloride (PAH) polyelectrolyte aqueous solution was adjusted at pH = 12.0. Carbon dioxide (CO2) was dissolved at 300 kPa (gage) in the polyelectrolyte solution. The pH of the solution decreased with increasing dissolved CO2, and the solution became turbid at pH = 9. The solution was then degassed at 1 atm, yielding microbubbles. The polyelectrolyte was then adsorbed on the microbubble surface and became the microcapsule shell. Very smooth spherical particulates were responsible of this. These particles were microbubbles and not aggregation of polyelectrolyte molecules; however, the particles did not coalesce, nor diffused into the solution, and were more stable compared to bubbles. Fluorescent analysis revealed that these particles were polyelectrolyte adsorbed to the bubble surface. This method was successfully used to fabricate hollow PAH polyelectrolyte microcapsules from microbubble templates without surfactants.

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