Microencapsulation is a process of applying relatively thin coating to small particles of solids or droplets of liquids and dispersion of size from several tenths of a micron up to 5000 micron. The uniqueness of microencapsulation is the smallness of the coated particles.
Main application of microencapsulation includes:
1. For taste masking
2. Stabilization to oxidation
3. Reduction of volatility
4. Conversion of liquid to solid
5. Reduce gastric irritation
6. Sustained release medication.
Core material: It is the specific material to be coated & it can be liquid or solid in nature. E.g.
Microencapsulation process and their applicability Vacuum deposition & polymerization techniques are not applicable to pharmaceutical products. Microencapsulation are bulk materials in either dry powder or dispersed form, can be processed into final product applications using common equipments such as v-blenders, tablet machines, granulators, homogenisers, kneaders, hard gelatin capsule filling machines, or coating equipment if deposition onto substrate is desired.
Properties of Coating Material: Coating material should be capable of forming film that is
(a) Cohesive with the core material
(b) Chemical compatible and non-reactive with core material
(c) Should provide the desired coating properties such as strength, flexibility, impermeabiity, optical properties and stability.
Methods of Microencapsulation
1. Air suspension: Invented by professor Date E. Wurster hence also called Wurster process. Within the coating chamber, particles are suspended on an upward moving air stream. Coating material is then spray applied to moving particles.
2. Coacervation Phase Separation: The process consist of three steps
(A) Formulation of three immiscible chemical phase, i.e.
(a) Liquid manufacturing vehicle phase, i.e. solvent for polymer
(b) A core material phase
(c) A coating material phase or polymer
The formation of three chemical phases can be achieved by
1. By temperature change
2. By addition of salt
3. By addition of non solvents
4. By adding incompatible polymer
5. By polymer polymer interaction
(B) Deposition of coating on core material
(C) Rigidization on coating
Solvent evaporation: In this technique coating material is dissolved in a volatile solvent which is immiscible with the liquid manufacturing vehicle phase. Core material to be encapsulated is dissolved or dispersed in the coating polymer solution with agitation. The core coating material mixture is dispersed in liquid manufacturing, vehicle phase to obtain appropriate size microcapsules.
• The mixture is then heated to evaporate the solvent for the polymer.
• If core material is dispersed in polymer solution, polymer shrinks around the core.
• If core material is dissolved in the polymer solution, a matrix type microcapsule is formed.
Spray drying and Spray congealing: Both the techniques involves dispersing the core material in a liquefied coating substance and spraying the core-coating mixture into some environmental condition whereby relatively rapid
solidification of coating is affected.
In spray drying: Coating solidification is accomplished by rapid evaporation of solvent in which coating material is
In spray congealing: Coating solidification is effected by thermally congealing a molten coating (spraying hot mixture into cool air stream) or by introducing core coating material mixture into a non-solvent.
Pan coating: In pan coating method, the coating is applied as solution, or as an atomized spray to the desired core
material in a coating pan. To remove the coating solvent warm air is passed over coated material. This method is extensively employed, for preparation of controlled release tablets.
Multiorifice-Centrifugal Process: It is a mechanical process for producing microcapsule that utilizes, centrifugal forces to hurl a core material particle through an enveloping microencapsulation membrane.
Polymerisation: This involve the formation of protective microcapsule coating in-situ. The method involves the reaction of monomeric unit located at interface existing between a core material and a continuous phase in which the core material is dispersed. Continuous phase is usually a liquid or gas and thus the polymerization reaction occurs at following interfaces.
Liquid – Liquid
Chang interfacial polymerization method for forming polyamide (nylon) membrane involves reaction occurring at liq – liq interphase between:
Chang has demonstrated the perm selectivity of micro-capsules containing urease enzymes by their ability to convert blood urea to ammonia, the enzyme remaining within the microencapsules when incorporated within the extracorporeal shunt.