Hollow calcium aluminate microcapsules with porous shell microstructure and unique mechanical properties

Abstract

Since the mid-1970s, microencapsulation has become increasingly popular in food, detergent, cosmetic and pharmaceutical industries to protect active agents from degradation or facilitate their controlled release or targeted delivery. Here we report on a synthesis route of a novel class of hollow inorganic microcapsules with unique microstructural and mechanical properties. The method is based on the adsorption of calcium aluminate particles at the interface of water droplets of an oil-continuous emulsion. Upon contact with water, these particles hydrate and form a mechanically stable, porous capsule shell. After solvent evaporation, hollow microcapsules can be harvested with diameters between 30 and 200 μm and yields of up to 75%. The mechanical characterization of entire capsules is accomplished using a uniaxial, micromechanical compression setup installed in a scanning electron microscope. We show that these inorganic calcium aluminate microcapsules are highly crack tolerant owing to their porous shell microstructure. Such a behavior is in strong contrast to the one of hollow aluminosilicate cenospheres, which feature dense shells and show therefore brittle failure in our compression tests.