ABSTRACT
Hollow particles, also called nanocapsules or microcapsules (depending on their size), are organic, inorganic, or hybrid particles composed of an outer polymeric shell and an inner void space. The cavity inside the hollow spheres can be filled with air or with a liquid phase (water or oil), and in some applications may contain a dissolved encapsulated active ingredient. A multitude of such systems can be found in microencapsulation technologies. The earliest developments in this field principally concerned the elaboration of micrometric capsules in the typical range 1-1000 µm in diameter. They were specifically designed to encapsulate inks, pigments, and dyes for printing and photographic applications, for instance. Although historically the first microencapsulation techniques in the pharmaceutical industry date back to the late 1800s, the concept of microencapsulation really achieved significant recognition in 1954 with the development of carbonless copy paper based on microencapsulated dyes [1, 2]. Microencapsulation techniques then found extensive developments in the coatings, food [3], agricultural [4], and pharmaceutical industries. Manufacturing methods of microcapsules have been extensively reviewed in the series Microcapsules, Microspheres and Liposomes edited by Reza Arshady [5]. Synthetic procedures principally involve coacervation techniques, direct polymerization methods, physicochemical (emulsification/solvent extraction) and mechanical processes (extrusion, spraying). If up to now most published works have concerned the elaboration of capsules of large dimensions, in very recent years microencapsulation technologies have expanded down to the nanometer range with the elaboration of nanometric capsules. Such nanocapsules offer numerous promising applications in various domains of advanced materials, especially in architectural coatings, optics, electronics, and biotechnologies. For example, hollow latex particles are used as synthetic pigments in paper coating [6] and paint mate-rials [7]. By scattering light, the voids contribute to increase hiding and opacity, whereas microspheres with large voids (typically 10-100 µm) are inefficient in such processes.
