Silica-Based Sol-Gel Microencapsulation and Applications

Microencapsulation, i.e. the process of enclosing micron-sized particles of solids or droplets of liquids or gasses in an inert shell which in turn isolates and protects them from the external environment, 1 is one of the main research topics in contemporary chemical research, as shown by the large body of scientific publications and scientific symposia devoted to the subject.* Heterogenization in micron-sized particles is useful for eliminating a processing step and uses more efficiently costly ingredients by enhancing the precision and effectiveness of action through controlled release, or for isolating molecules that would otherwise react with each other. Chemical companies are interested in the development of innovative "system solutions,"2 that is, new functional materials in which known molecules are integrated to show new effects. Flavor and fragrance companies are interested to protect and precisely release their valued molecules (the "payload"). The polymer industry demands encapsulated curing agents and encapsulated reactants. These are just three examples, out of many, that show that microencapsulation is mainly used for the purpose of protection and controlled release. Since at least a decade, therefore, traditional lowvolume markets for microcapsule-based products are expanding to include fine chemicals, adhesives, inks, fragrances, toners, sealants, and detergent manufacturers.3 Microencapsulation companies, including chemical makers, flavor and fragrance houses, and specialist firms, use competing technologies to manufacture encapsulated functional materials using either chemical or physical techniques. A 2004 study4 aiming to identify trends in microencapsulation technologies (since 1955) found that liposome entrapment and spinning-disk were the dominant approaches; with nanoencapsulation growing but still far from the mainstream methods that include chemical (in situ processes such as emulsion, suspension, precipitation or dispersion polymerization, and interfacial polycondensation) and physical (spinning-disk and spray-drying) methodologies. The sol-gel microencapsulation in silica-based materials is an emerging and powerful nanochemistry technology in which the active ingredients are protected (stabilized) in silica-based particles.5 In 2004, Barbé and coworkers reviewed sol-gel microencapsulation of bioactive molecules for drug-delivery.6 Four years later, the same team published a first summary of their studies on silica-based microparticles doped with hydrophilic molecules obtained from water-in-oil (W/O) microemulsions. The Australian scientists showed7 how both the particle size and the release rate of silica-based microparticles can be finely and easily tailored in a wide range by controlling the conditions affecting the sol-gel process. In 2006, van Driessche and Hoste published the first account8 on the topic in a book addressing microencapsulation techniques, mainly covering the findings of Barbé and coworkers. The first comprehensive review on sol-gel microencapsulation covering all methods was published by Chemical Reviews in 2011.5 The method was found to be still a relatively new solution for controlled release formulations. Yet, following an ample section dealing with economic and environmental arguments, we were concluding that "the sol-gel microencapsulation will become one of the most relevant chemical technologies with applications in numerous industrial sectors." Four years later, indeed, we find a number of new applications that have reached the marketplace, while research in the field has boomed. As of late 2013, a quick Boolean search in Google Scholar with the query "sol-gel" and "microencapsulation" returns about 4550 articles, patents, and scholar reports. Most of the technical and economic problems that limited the practical application of sol-gel derived silica-based functional materials have been addressed and resolved. Numerous new materials, formulations, and devices that use functional sol-gel materials have been developed and marketed. Even though the global financial crisis started in 2008, the global sol-gel market has not stopped to grow, reaching $1.5 billion in 2012, with a projected annual growth rate of 7.9% for the subsequent 5 years.9 Research in the field is flourishing on a true international scale, with academic groups operating in countries as distant as Australia, India, China, Japan, Israel, Italy, Hong Kong, the United States, Iran, Malaysia, Germany, Korea, Canada, and France. The aforementioned literature references cited provide a fine, detailed description of the chemistry of sol-gel microencapsulation process. Hence, in the following, instead of reporting again the basic chemistry of the sol-gel microencapsulation, we address a number of relevant and often overlooked practical issues such as methods to break the capsules, health and safety, practical advantages and limitations, as well as some economic aspects. The message of this chapter is that the sol-gel encapsulation in micron-sized silica and organosilica particles is an important chemical technology with a large applicative potential that must be understood. The outcome is a report that will hopefully be truly useful to heads of chemical research as well as to managers in the fine chemical, fragrance and flavor, pharmaceutical, and polymer industries for years to come.