Introduction

What is Microencapsulation ?

Application of Microencapsulation

Hot melt coating

Cyclodextrins

Extrusion

Emulsion Stabilisation

Jet break-up Processes

Super Critical Fluid technology

Complex Coacervation

Introduction

Microencapsulation using supercritical fluid technology combines a liquid-like density and solvating power with gas-like transport properties (like viscosity, diffusivity). They are applied on a large industrial scale, e.g. for extractions and reactions. For microencapsulation purposes, the mild processing conditions give supercritical fluid technology an important advantage over other methods that include harsh treatments with regard to pH, temperature, or the use of organic solvents. Carbon dioxide is the most widely used supercritical fluid because of its relatively low critical temperature (31 °C) and pressure (74 bar). In particular, its low critical temperature makes it highly suitable for processing heat-sensitive materials. In addition, supercritical CO2 (scCO2) is non-toxic, non-flammable, inexpensive, and has GRAS (generally regarded as safe) status. These characteristics enable a broad range of food and non-food applications. Supercritical CO2 is a non-polar solvent with dissolution properties that are comparable to hexane.

Technology

Many different procedures have been and are being developed for encapsulation processes based on supercritical fluids. In most of these procedures particle formation and encapsulation are combined in a single step. Supercritical fluids are especially suitable for particle formation, as they display a large change in density near the critical point which enables their solvating power to be carefully controlled by small changes in temperature or pressure.

Here we will describe two distinct approaches for particle formation and encapsulation using supercritical fluids.

1- Rapid Expansion of Supercritical Solutions (RESS) processing is used to prepare microspheres. Microencapsulation takes place when a pressurized supercritical solvent containing the shell material and the active ingredient is released through a small nozzle ; the abrupt pressure drop causes the desolvation of the shell material and the formation of a coating layer around the active ingredient. A prerequisite for this technology is that the compounds effectively dissolve in the supercritical fluid, which limits its application.

In some cases (RESS-N technology), a non-solvent like a low molecular weight alcohol is added to facilitate the dissolution of the shell material in the supercritical fluid.

2- Alternatively, a supercritical fluid is used as an anti-solvent that causes precipitation of a dissolved substrate from a liquid solvent. This approach, called the SAS (Supercritical fluid Anti-Solvent) or GAS (Gas Anti-Solvent) method, results in a pronounced volume expansion compare to the RESS, leading to supersaturation and then precipitation of the solutes.

The SAS is possible only if the liquid solvent is completely miscible with the supercritical fluid and if the solute is insoluble in this mixture. For these reasons, SAS is not applicable to the precipitation of watersoluble compounds, because of the very low solubility of water in scCO2 at appropriate process conditions. This technique and variations thereof have led to the formation of (sub)micron particles.

Application

The use of supercritical fluid technology, especially scCO2, for encapsulation purposes is mainly due to the mild processing condition, allowing microencapsulation of sensitive ingredients for :

  cosmetics (vitamin, pigments and dyes, nanoparticles...)
  pharmaceuticals (therapeutics proteins...)
  food (volatile flavors, vitamins...)
  Impregnation of matrix materials with (bio)active ingredients for development of slow- or controlled- release systems.