Microencapsulation

M.G.ISWARIYA11MAT03

II M.Tech ATM

IntroductionMicroencapsulation is a

technique by which solid, liquid or gaseous active ingredients are packaged within a second material for the purpose of shielding the active ingredient from the surrounding environment.

The material inside the microcapsule is referred to as the core, internal phase, or fill, whereas the wall is sometimes called a shell, coating, or membrane.

All the three states of matter, i.e. solid, liquid and gases, may be encapsulated and affect the size and shape of the capsules.

The capsulated particles produce their required effect when their core material isreleased. There are four typical mechanisms by which the core material is released froma microcapsule:◦Mechanical rupture of the capsule wall◦Dissolution of the wall◦Melting of the wall◦Diffusion through the wall

Features of microcapsuleTypically, the lowest particle size

of microcapsules is 1μm and the largest size is 1mm.

Microcapsules consist of a core and a wall (or shell).

The configuration of the core can be a spherical or irregular particle, liquid-phase suspended solid, solid matrix, dispersed solid and aggregates of solids or liquid forms.

Classification

MononuclearPoly nuclear Matrix types

Reasons for Encapsulation To protect reactive substances from the

environment To convert liquid active components into a dry

solid system To separate incompatible components for

functional reasons To mask undesired properties of the active

components To protect the immediate environment of the

microcapsules from the active components To control release of the active components

for delayed (timed) release or long-acting (sustained) release.

Advantages Rendering liquids into powders, to prevent

clumping and improving mixing. Protecting active ingredients from oxidation,

heat, acidity, alkalinity, moisture or evaporation. Preventing ingredients from interacting with

other compounds in the system, which results in their degradation or polymerization.

Masking the taste of unpleasant flavors or odors. Improving handling of an ingredient before

processing. To release active ingredients in a controlled or

targeted fashion. Protecting workers or end users from exposure

to hazardous substances.

Materials Core Material:The core material defined as the

specific material to be coated can be liquid or solid in nature.

The solid core can be a mixture of active constituents, stabilizers, diluents, recipients and release-rate retardants or accelerators.

The core material composition provides definite flexibility and utilization allows effectual design and development of the desired microcapsule properties.

Coating MaterialsThe coating material should be capable

of forming a film that is cohesive with the core material;

Be chemically compatible and nonreactive with the core material; and

Provide the desired coating properties, such as strength, flexibility, impermeability, optical properties, and stability.

The typical coating properties such as cohesiveness, permeability, moisture sorption, solubility, stability and clarity must be considered in the selection of the proper microcapsule coating material.

Choice of Encapsulation Technique

1. The functionality the capsule needs to provide in the finished product.

2. The type of coating material, i.e.. The coating material should not react with either the ingredient to be encapsulated, or the formulation in which the capsulate will be added.

3. The processing conditions the encapsulate must survive before releasing the contents.

4. Concentration of the core material in the microcapsule.

5. The mechanism of release of the active agent from the microcapsule, e.g.. agitation. pH, pressure, solubility, time, etc.

6. The type of release (targeted, sustained or controlled) of the active ingredient.

7. The particle size, density, and stability requirements for the active ingredient.

8. The cost of the capsules and the cost of the formulation or application into or onto the final product.

Microencapsulation Technologies

Physico – Chemical Processes

Physico – mechanical Processes

CoacervationPolymer-polymer

incompatibilitySolvent evaporationEncapsulation by

supercritical fluidHydrogel microsphere

Spray-dryingFluidized- bed

technologyPan coatingSpinning disc Co-extrusion

Physico-Chemical Processes Coacervation

First the core material is dispersed into a polymer solution.

The second polymer solution is then added to the prepared dispersion.

Deposition of the shell material onto the core particles occurs when the two polymers form a complex.

This process is triggered by the addition of salt or by changing the pH, temperature or by dilution of the medium.

Finally, the prepared microcapsules are stabilized by crosslinking , desolvation or thermal treatment.

Complex coacervation is used to produce microcapsules containing fragrant oils, liquid crystals, flavors, dyes or inks as the core material.

Polymer-polymer incompatibility Also called phase separation. This method utilizes two

polymers that are soluble in a common solvent; yet do not mix with one another in the solution.

The polymers form two separate phases, one rich in the polymer intended to form the capsule walls, the other rich in the incompatible polymer meant to induce the separation of the two phases.

The second polymer is not intended to be part of the finished microcapsule wall.

Solvent Evaporation It is the most extensively

used method of microencapsulation.

Prepare an aqueous solution of the drug.

Then added to an organic phase consisting of the polymer solution in solvents like dichloromethane or chloroform with vigorous stirring to form the primary water in oil emulsion.

This emulsion is then added to a large volume of water containing an emulsifier like PVA or PVP to form the multiple emulsions (w/o/w).

The double emulsion is then subjected to stirring until most of the organic solvent evaporates, leaving solid microspheres.

The microspheres can then be washed and dried.

Polymer Encapsulation by Rapid Expansion of Supercritical Fluids

Supercritical fluids - highly compressed gasses -properties of both liquids and gases.

CO2 and nitrous oxide (N2O). A small change in temperature or

pressure causes a large change in the density of supercritical fluids.

Steps: ◦ Supercritical fluid containing the active

ingredient and the shell material are maintained at high pressure and then released at atmospheric pressure through a small nozzle.

◦ The sudden drop in pressure causes desolvation of the shell material, which is then deposited around the active ingredient (core) and forms a coating layer.

Hydrogel microspheres Microspheres made of gel-type

polymers, such as alginate, are produced by dissolving the polymer in an aqueous solution.

Then, suspending the active ingredient in the mixture.

Extruding through a precision device, producing micro droplets.

Then fall into a hardening bath that is slowly stirred.

The hardening bath usually contains calcium chloride solution.

Physical ProcessesSpray-Drying

Microencapsulation by spray-drying is a low-cost commercial process which is mostly used for the encapsulation of fragrances, oils and flavors.

Steps: ◦Core particles are dispersed in a polymer

solution and sprayed into a hot chamber. ◦The shell material solidifies onto the core

particles as the solvent evaporates. The microcapsules obtained are of polynuclear or matrix type.

Spray-congealingThis technique can be

accomplished with spray drying equipment when the protective coating is applied as a melt.

The core material is dispersed in a coating material melt.

Coating solidification is accomplished by spraying the hot mixture into a cool air stream.

Fluidized-Bed Technology Solid particles to be

encapsulated are suspended on a jet of air and then covered by a spray of liquid coating material.

The rapid evaporation of the solvent helps in the formation of an outer layer on the particles.

This process is continued until the desired thickness and weight is obtained.

Pan coating Solid particles are mixed with

a dry coating material. The temperature is raised so

that the coating material melts and encloses the core particles, and then is solidified by cooling.

Or, the coating material can be gradually applied to core particles tumbling in a vessel rather than being wholly mixed with the core particles from the start of encapsulation.

Co-Extrusion A dual fluid stream of liquid

core and shell materials is pumped through concentric tubes and forms droplets under the influence of vibration.

The shell is then hardened by chemical cross linkings, cooling, or solvent evaporation.

Different types of extrusion nozzles have been developed in order to optimize the process.

Spinning Disk Suspensions of core particles

in liquid shell material are poured into a rotating disc.

Due to the spinning action of the disc, the core particles become coated with the shell material.

The coated particles are then cast from the edge of the disc by centrifugal force.

After that the shell material is solidified by external means (usually cooling).

Microencapsulation for textiles

Antimicrobial fabrics: These finishes are used for the prevention of microbial attack on the fabric. A biocide is the active ingredient in this case, and is released by the appropriate means to achieve the desired antimicrobial finish.

Insect repellent fabric: The insect repellent is the core material and is encapsulated and later applied on the fabric. The fabric repels insects when the microcapsules are broken and the core is released during wear.

Fresh fabrics: This imparts a fresher and pleasant touch to the fabric.

Cosmetic-textiles: Aloevera or other such products are encapsulated to give skin moisturising or to render a personal sense of well being.

Photo chromic textiles: Photo chromic dyes, which change colour in response to UV light, are incorporated inside the microcapsules. These find application in product labelling, etc.

Thermo chromic textiles: The encapsulated dyes change their colour in response to temperature. Currently it is possible to produce colour change formulations in the range of-250°C - 660°C.

ConclusionMicroencapsulation system offers

potential advantages over conventional drug delivery systems and also established as unique carrier systems for many.

Although significant advances have been made in the field of microencapsulation, still many challenges need to be rectified during the appropriate selection of core materials, coating materials and process techniques.

References Microencapsulation Technology and Applications,

Rama Dubey, T.C. Shami and K.U. Bhasker Rao, Defence Science Journal, Vol. 59, No. 1, January 2009, pp. 82-95

Microencapsulation: Process, Techniques and Applications, Hammad umer, Hemlata Nigam, Asif M Tamboli, M. Sundara Moorthi Nainar,International Journal of Research in Pharmaceutical and Biomedical Sciences, ISSN: 2229-3701

Microencapsulation at an Affordable Price, By Dr Naresh M. Saraf, Deepak V. Alat and Ruma Chakrabarti, of Sarex Overseas, India

uqu.edu.sa/files2/tiny_mce/.../MICROENCAPSULATION.pdf

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