## 4 Recent Progress in Microencapsulation Techniques for Natural Bioactive Compounds
### Supercritical Fluid Based Techniques  
It is an energy-efficient technology in which nontoxic supercritical fluid such as supercritical carbon dioxide gas (ScCO\({}_{2}\)) is utilized for the encapsulation of bioactive compounds such as in the pharmaceutical industry [25; 133]. Under this method, the solvent is economically utilized, but the need for high pressure and high-end equipment still hampers the popularization of this technology. Supercritical fluids are highly compressed gases that show characteristics of liquids (such as high density, improved solvating attributes) as well as of gases (such as low viscosity, higher mass transfer, and diffusion rate) above a particularly critical point [134]. There are several compounds that exhibit a supercritical state at a critical temperature (Tc) and critical pressure (Pc), such as carbon dioxide, nitrogen, water, propane, etc. However, carbon dioxide (Tc-31.1 \({}^{\circ}\)C, Pc-7.38 MPa) is the most widely used among all the available supercritical fluids. Supercritical fluids are used as a solvent for the rapid expansion of these solutions (RESS), as an anti-solvent (SAS) for precipitations or related processes, and as a solute for particles from gas saturated solutions (PGSS) and associated processes [135]. The supercritical fluid can act as a solvent, and the particle formation can be performed either by RESS or by supercritical solvent impregnation (SSI). On the other hand, supercritical fluid can act as a PGSS designed for making particles. Supercritical fluid can also act as an anti-solvent, and the particle formation can be performed either by SAS or by the supercritical fluid extraction of emulsions (SFEE). As aforementioned, carbon dioxide (CO\({}_{2}\)) is the most commonly used gas in supercritical encapsulation not only for its availability, since it is non-toxic, low priced, and easily separated from the final product by depressurization, but also because it has low critical temperature, thus being the most preferred material for the microencapsulation of natural bioactive compounds using the supercritical fluid technique. This technology is mostly used along with co-precipitation,comprises entrapping compounds, and is chemically binding followed by absorbing in polymer matrix or encapsulation [136]. Under this method of encapsulation, there is minimal degradation of the active ingredients, thus finding several applications in the industry such as micromization, extraction, drug impregnation, and the production of membranes and scaffolds [137, 138, 139]. Under this technology, encapsulation parameters such as particle size, shape, encapsulation efficiency, bioavailability, etc., could be controlled by applying proper process parameters such as temperature, flow rate and pressure, use of proper solution, etc. This method has certain inherent advantages over conventional methods such as the proper control of particle size and shape, high yield and bioavailability of heat-labile active ingredients, higher encapsulation efficiency, easier removal of solvents, and obtaining solvent-free droplets, but the selection of proper supercritical technology compatible with the active material and polymer matrix still remain limiting factors [139, 140]. In general, some sensitive compounds such as essential oils and enzymes are microencapsulated by this technique. Aliphatic polyesters, dextran, inulin, cyclodext, polycaprolactone, polylactide aliphatic copolymers, and starch polymer blends are frequently used as a wall material in this technique. [PERSON] et al. [141] studied the encapsulation of oregano essential oil in a starch matrix. It was found that the higher diffusivity of the supercritical carbon dioxide in the starch matrix ensured the deep impregnation of essential oil, resulting in the higher antioxidant activity of oregano essential oil. The bioavailability of quercetin was enhanced through microencapsulation in a surfactant material using the supercritical fluid extraction of emulsions.