Methods of microencapsulation

AISSMS college of Pharmacy, Pune

Introduction:

• As a process, microencapsulation is means of applying the thin coating to small particles of solids or droplets of liquids and dispersions.

• Microcapsule often described by terms such as microspheres, coated granules, microsperules, spansules, etc.

• The first microencapsulation procedure was published by Bungenburg de Jong and Kaas1 in 1931 which deals with preparation of gelatin microspheres by Coacervation process.

• In late 1930s & 1940s, Green et. al. of National Cash register Co., Dayton, Ohio, developed gelatin coacervation process which eventually leads to several patents for carbonless carbon paper manufacturing by use of gelatin coacervation made microencapsulation process more common.

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Dimensions:

• At present, there is no universally accepted size range that particles must have in order to be classified as microcapsules.

• However, many workers classify capsules as follows (according to the Simon Benita2):

• Bakan3 described capsules dimensionally ranging from 10 to 5000 micron as microcapsules.

Diameter Type of capsule

Less than 1 micron Nanocapsule

3 to 800 micron Microcapsule

Larger than 1000 micron Macrocapsule

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Application of microencapsulation:

• Conversion of liquids to solid• Alteration of colloidal and surface properties• Protection from environmental condition• Prolonged action or sustained release formulation• Targeted release formulation• Taste and odor masking• Formulation of medicament with reduced gastric irritation

properties• Single layered tablet containing chemically incompatible

ingredients• New formulation concept for creams, ointments, aerosols, etc

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Demerits of microencapsulation:

• No single microencapsulation process is adaptable to all core material candidate or product applications

• Complicated process and requires skilled labor to supervise • Incomplete or discontinuous coating• Nonreproducible & unstable release characteristics of coated

products• Inadequate stability or shelf-life of sensitive pharmaceuticals• Economic limitations

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Core material:

• It is the specific material to be coated and it also referred as nucleus or fill

• It can be solid or liquid in nature• Examples of some core material as follows:

Core Material Characteristic Property

Purpose of Encapsulation

Final Product Form

Acetaminophen Solid Taste masking Tablet

Aspirin Solid Taste masking; sustained release; reduced gastric irritation

Tablet or capsule

Vitamin A palmitate

Nonvolatile liquid Stabilization to oxidation

Dry powder6

Core Material Characteristic Property

Purpose of Encapsulation

Final Product Form

Urease Water soluble enzyme

Permeseletivity of enzyme , substrate and reaction product

Dispersion

Progesterone Slightly water soluble solid

Sustained release Varied

Islets of Langerhans Viable cells Sustained normalization of diabetic condition

Injectable

Isosorbide dinitrate Water-soluble solid Sustained release Capsule

Potassium chloride Highly water soluble solid

Reduced gastric irritation

Capsule

Activated charcoal Adsorbent Selective sorption Dry powder

Liquid crystals Liquid Conversion of liquid to solid

Flexible film for thermal mapping of anatomy

Castor oil Liquid Conversion to solid Varied7

Coating materials:• These are the material used for coating the core material and

these are also referred as wall or shell.• Depending upon the method of microencapsulation employed

& properties of final product needed, coatings solution may contain different additive such as film formers, plasticizers, and fillers and may be applied through different solvent system.

• Examples as follows:

Film formers:

Poly acrylamides, Polyacryldextrans, Polyalkyl cyanoacrylate, Agar, Agarose, Alginates, Cellulose derivatives, Cetyl alchol, Gelatin, Gluten, Polyester, Polyamide, Starch, Shellac, Stearic acid, Polyvinyl alchol, Waxes, etc.

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Plasticizers:

Castor oil, Glycols, Glycerin, Polysorbates, Phthalate esters, Fatty acid esters, etc.

Solvents:

Water, Methanol, Ethanol, n-Propanol, Acetone, Ethyl acetate, Choloroform, etc.

Surfactants:

Carboxylic acid, Alkyl sulphonates, Alkoxyalkylamines, etc

Crosslinking agents:

Formaldehyde , Glutaraldehydes, etc

Antitack agents:

Fatty acids such as Stearic acid, Palmitic acid, etc

Colorant:

Various insoluble lakes

Fillers:

Talc and other silicates

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Methods of microencapsulation:1. Coacervation-phase separation2. Pan coating3. Air suspension4. Spray drying & Spray congealing5. Spray embedding & Spray polycondensation6. In situ polymerization7. Interfacial polymerization in liquid media8. Multiorifice centrifugal extrusion9. Solvent evaporation10. Microencapsulation using ultrasonic atomizer11. Dip coating12. Electrostatic encapsulation

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Coacervation-phase separation:

• The term coacervation is derived from the Latin acervus, meaning aggregation, and the prefix co to indicate union of colloidal particles. Bungenburg de Jong and Kruyt5 described this term in 1929.

• This process is attributed to the National Cash Register Co. (NCR), Dayton, Ohio & the patents of B. K. Green et al6-9.

• The general outline of the process consists of three steps carried out under continuous agitation:

1. Formation of three immiscible chemical phases2. Deposition of the coating 3. Rigidization of the coating

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Formation of three immiscible chemical phases:

• Three immiscible chemical phases are liquid manufacturing vehicle phase, core material phase & coating material phase.

• The coating material phase, an immiscible polymer in a liquid state, is formed by utilizing one the methods of phase separation-coacervation:1. By changing the temperature of the polymer solution2. By adding a salt to the polymer solution3. By adding a nonsolvent to the polymer solution4. By adding incompatible polymer to the polymer solution5. By inducing a polymer-polymer interaction

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Deposition of the coating:• This step consists of depositing the liquid polymer coating

upon the core material. • Deposition of the liquid polymer coating around the core

material occurs if the polymer adsorbed at the interface formed between the core material and liquid vehicle phase, and this adsorption phenomenon is a prerequisite to effective coating.

• The continued deposition of the coating material is promoted by a reduction in total free interfacial energy of the system

Rigidization of coating:• It is carried out usually by thermal cross-linking or desolvation

technique, to form self sustaining microcapsules

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Pan coating:

• Most widely used method but much literature publication is concerned with industrial patents, whereas the finer details procedure are inadequately described.

• Unlike many other microencapsulation processes, the process tends itself to great flexibility in formulation of coating.

• Also core material may contain wide range of additive may serve to modify release properties of formulation.

• Pan coating method of microencapsulation process contains two steps:

1. Preparation of core material 2. Coating procedure

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Preparation of core material:

• For the satisfactory pan coating core particles should be of greater than 500 micron, spherical shape, adequate hardness and low friability.

• Very few drugs satisfy these requirement hence the following approaches are used for preparation of core:

1.Use of spherical substrate such as nonpareil sugar seeds ( described by Enz & King in patent assigned to Upjohn Co11.)

2.Coating of fine powdered medicament to the large crystal of same medicament ( described by Heimlich & MacDonnell in patent assigned to Smith, Kline and French laboratories12)

3. Incorporation drug into suitable matrix material (described by Brophy and Deasy.13)

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Coating procedure:

• Steps involved in coating of batch of cores:1. Roughening of inner surface of pan if it not contain

baffle2. Screening of core material to remove dust3. Core material addition

a. Pour methodb. Spray method

4. Addition of dusting powder such as talc at the critical moment just as core become tacky and just in sufficient amount to free up any clumps

5. Drying and jogging after each application of coating 6. 20-50 application of coating & drying in flat trays at

400C16

Equipment and related facilities:

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Pan coating apparatus

Applications of pan coating:• Lowy14 pan coated rounded granules containing nitroglycerine

with various cellulose derivatives such as methylcellulose, ethylcellulose, or cellulose acetate using beeswax or castor oil as a plasticizer.

• Rosen and Swintoskey15 reported pan coating of trimeperazine that contain following step:

1. 35S-labeled trimeperazine tartarte, an antipuritic agent, mixed with powdered starch and sugar

2. This is applied on sugar pellet using hydroalcholic gelatin adhesive in coating pan

3. Dried, screened and then coated with solution of 11% w/w glyceryl monostearate, 11% w/w glyceryl monostearate & 3% w/w white beeswax in CCl4, resulted microcapsules showed sustained release in human subjects after oral administration.

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• Air suspension coat was originally developed by Wurster et. al., is an attractive alternative to pan coating

• Process & application described by Wurster in series of patents assigned to Wisconsin Alumni Research Foundation 16-22

• It has following advantages over pan coating:1. More rapid2. No need of skilled labor, often fully automated3. Coat continuity is superior & less coating solution

required4. No loss of coating material as being closed system

Air suspension technique:

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Equipment:

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Process consideration:

• Air distribution & Flow rate• Atomizer type & feed• Solvent selection• Solvent evaporation• Temperature profile• Core size & shape• Particle interaction

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Coating chamber of wurster process

Application of air suspension:

• Coletta & Rubin23 described coating of aspirin crystals of various mesh sizes with mixtures of ethyl cellulose & methyl cellulose sprayed from methylene chloride : isopropyl alcohol (1:1) solution using Wurster air suspension apparatus.

• Caldwell & Rosen24 used an air suspension coater to successfully apply dexamphetamine sulphate onto sugar pellets using solution of gelatin in hydroalcholic solvent as adhesive. Then coated with various materials, including beeswax, glyceryl monostearate and distearate to obtain lipid coated microcapsules with sustained release properties.

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Spray drying and spray congealing:

• In spray drying process the core substance is dispersed in a solution of coating material, which is then atomized and the dried off using heated air.

• Spray congealing, the substance which has property of melting at elevated temperature when being atomized and congealing when the droplet formed meet cool air on spray dryer.

Advantages: • Rapid, single stage operation, can be used for heat sensitive

substanceDisadvantages:• Porous coating, not suitable for taste & odor masking and for

controlled release formulation, high cost of production23

Equipment & process:

• Arrangement of equipment for both process is same.

Schematic diagram for concurrent spray dryer

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Applications of spray drying:

• Marotta et al.25, in patent assigned to National Starch and Chemical Corporation, used an aqueous solution of acid ester dextrins to encapsulate emulsified dispersion lemon oil by spray drying.

• Sager26, in patent assigned to Beecham Group ltd., Great Britain, described spray drying of penicillin as follows:

1. As an example of process, high concentrated ampicillin trihydrate suspended in dilute solution of sodium carboxymethylcellulose.

2. Control of foaming by addition of ethanol3. Filtration 4. Spray dried at 1600C and outlet temperature is 840C5. Product less than 75 micron are recycled

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Applications of spray congealing:

• Robinson and Swintosky27 microencapsulated particles of sulfaethylthiadiazole by mixing them with molten hydrogenated castor oil at 1100C; then suspension was spray congealed into air cooled chamber using centrifugal wheel atomizer.

• Koff28 dispersed thiamine monohydrate into a molten mixture of mono- and diglycerides of palmitic and stearic acid at 740C and spray congealed into ambient air at 200C. Average particle size is about 60 micron and the process was reported to be suitable for the encapsulation of vitamin of B group for taste masking purpose.

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Spray embedding & spray polycondensation:

Spray embedding:• In this aqueous or organic solvent containing solution of both

drug and dissolution retarding polymer are spray dried.• Takenaka et al29. spray dried ammoniacal solution of

sulfamethoxazole and cellulose acetate phthalate.Spray polycondensation:• This process is based on spray drying whereby a dispersion of

core material in continuous phase containing reactive monomer and precondensate with catalyst, in addition to other film forming agent, is caused polymerize at high temperature of the process by solvent evaporation

• Voemelly et al30. used this process to microencapsulate phenobarbital

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In-situ polymerization:

• It is also known as normal polymerization.• By this technique, one can produce capsules and particles in

micrometer and nanometer range.• Polymerization techniques of pharmaceutical interest is

carried out in liquid phase and they have following types1. Bulk polymerization2. Suspension polymerization3. Emulsion polymerization4. Micelle polymerization

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Bulk polymerization:

• In this, mixture of monomer and drug are heated.• Initiator added to accelerate the rate of the reaction.• During polymerization viscosity is maintained by temperature. • The polymer so obtained may be molded or fragmented as

microsphere. Advantage:• Relatively pure polymers are formed.Disadvantages:• Difficult to dissipate high exothermic heat of reaction• Polymer blocks are formed which need mechanical

fragmentation causing irregular size, shape & release

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Suspension polymerization:• Referred as bead polymerization or pearl polymerization• It involves heating of water insoluble monomer and drug as

dispersion droplet• Drug droplet contain initiator• Aqueous phase may also contain stabilizers, fillers, buffers,

electrolytes, etc.• Washing after completion of processAdvantages:• Heat of polymerization is absorbed by continuous phase• Spherical beads are of uniform size and release characteristicsDisadvantages:• Difficulty from freeing the product from unwanted additives• Coalescence problem

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Emulsion polymerization:

• Differs from suspension polymerization by three ways : Initiator in aqueous phase , More vigorous agitation, Surfactant present in much higher concentration

• This results in altered mechanisms of polymerization: 1. Excess surfactant forms micelles and take up part of monomer cause them to swell 2. Initiator generated radical (by heat or irradiation) start polymerization 3. As monomer consumed replaced by replaced by remaining monomer by passive diffusion

Advantages:• Higher molecular weight polymer formed at faster rateDisadvantages:• Unreacted monomer which are in higher concentration

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Micelle polymerization:

• Differs from emulsion polymerization in that all of monomer and drug is present in micelle produced by surfactant present in suitable concentration.

• Diffusion of monomer is prevent by nonsolvent properties of outer phase used.

• Hence no product swelling occur.Advantages:• Capsules dimensionally ranging from few nanometers can be

formulated.Disadvantages:• Difficulties in product recovery due to minute size

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Application of in-situ polymerization:

• Nilsson et al.31 used suspension polymerization technique to immobilize trypsin.

Steps involved:• Enzyme dissolved with acrylamide (monomer) and N,N’-bis-

acrylamide (crosslinking agent) in aqueous triethanolamine-HCl at buffer pH 7.

• Addition of ammonium persulfate (radical initiator) and N,N,N’,N’,-tetramethylethylenediamine (accelerator) in aqueous phase

• Aqueous phase added to organic phase (toluene : chloroform 290 : 110) to form w/o dispersion.

• Polymerization carried out at 40C under nitrogen for 30 min• Filtration and washing with toluene

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Interfacial polymerization:

• It involves reaction of various monomers at the interface between two immiscible liquid phases

Steps involved:• One reactive polymer dissolved in aqueous disperse phase

containing a solution or dispersion of the core material.• Other reactive monomer dissolved after emulsification in

nonaqueous continuous phase• Monomer diffuse together and rapidly polymerizes at

interface• Byproducts of reaction is neutralized by added product such

as alkaline bufferAdvantages:• Rapid, Products are relatively uniform in size

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Interfacial polymerization:

Disadvantages:• Toxicity problem associated with unreacted monomer• Excessive drug degradation caused by reaction with monomer• Fragility of formed product• Lack of biodegradability of productApplication:• Chang32-33 published his work of nylon microencapsulation for

immobilization of various enzymes

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Multiorifice centrifugal process:• This mechanical process for production of microcapsules is

produced by Southwest Research Institute (SwRI). schematic diagram of multiorifice-centrifugal microencapsulation apparatus

`

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Applications of multiorifice-centrifugal process:

• Somerville et al.34, in work financially supported by Beecham Laboratories, reported use of the SwRI multiorifice centrifugal extrusion process for the microencapsulation of quinidine sulfate.

• Mangold et al.35 also reported preliminary results using the SwRI centrifugal extrusion device for the microencapsulation of ethynylestradiol, norethindrone, and norethindrone acetate in mixtures of various glycerides and fatty acids or alcohols in order to reduce undesirable side effects associated with the daily peaks produced by conventional orally administered contraceptive therapy.

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Solvent evaporation:

• This technique is based on the evaporation of the internal phase of an emulsion by agitation.

• This technique has been used by companies including The NCR company, Gavaert Photo Production NV and Fuji Photo Film Co., Ltd.36-39 to produce microcapsules.

• It is applicable to a wide variety of liquid or solid core material.

• This technique has following types:1. Single emulsion evaporation2. Multiple emulsion evaporation

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Single emulsion evaporation:o/w emulsion evaporation:• Steps involved are as follows (described by Morishita et al in

patent assigned to Toyo Jozo Co. Ltd., Japan40):1. Drug is dissolved or dispersed in a solution of polymer in single or mixed organic solvent having low boiling point2. Emulsified into aqueous phase and stabilized by surfactant to o/w emulsion3. Evaporation of solvent by reduced pressure and/or heat4. Collection of microspheres by filtration or centrifugation

• Mortada41 dispersed sulfathiazole in solution of ethylcellulose in trichloromethane.Surfactant used: 0.04% sodium lauryl sulphateSolvent evaporation by stirring for 5hrs at room temperature

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Multiple emulsion evaporation:w/o /w emulsion evaporation:• Steps involved are as follows (described in series of patent

assigned to Gaevert Photo Production NV42-44):1. aqueous solution or dispersions of drugs is emulsified into hydrophobic polymer dissolved in water immiscible solvent having a boiling point below 1000C2. Initially formed w/o emulsion is emulsified into an aqueous solution of hydrophilic colloid3. By raising temperature the organic solvent is evaporated causing phase separation of the polymer as coating around the inner aqueous droplet4. Alternatively, addition of organic solvent which is immiscible with solvent and miscible with core & coating material, removes organic solvent

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• It is used to produce microencapsules with distinct walls.• This approach has been used to produce polystyrene

microencapsules containing placental alkaline phosphatase by Takenaka et al.45 and α-amylase or sodium salicylate by Nowaza and Higashida.46

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Dip coating:

• This process involves single or repeated dipping cores into a coating solutions.

• Because of the static nature of the process, major difficulty is damage to the coating.

• Gagnon et al.47 dip coated the aspirin containing granules with stearic acid.

• Andrade et al.48 dip coated activated carbon granules by placing them in mesh sack and immersing them into alcoholic solution of hydroxyethyl methacrylate (HEMA) containg butyl peroctoate as initiator.

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Electrostatic encapsulation:

• This method is developed at Illinois Institute of Research and technology, Chicago.

• Langer & Yamate49 successfully encapsulated glycerin with carnauba wax using this process. The microcapsules formed are of smaller than 5 micron.

• Larger particles are not possible as it requires high voltage & also they lack Brownian motion.

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Electrostatic encapsulation chamber

Microencapsulation using ultrasonic atomizer:

• Yoon Yeo & Kinam Park 50reported this method in 2004.

Schematic representation of the microencapsulation systemusing coaxial ultrasonic atomizer

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Mechanism and applications:

• With the aid of confocal laser scanning microscopy, fluorescence microscopy, drug release study; they found mechanism of formation of microcapsule as follows:

1. It is based on midair collision among multiple drops of components of liquids and subsequent mass transfer at their interface.

2. The microcapsule formed as the result of collision between fluid drops, hence core material not subjected to strong mechanical stress & only briefly exposed to mild ultrasonic vibrations less than few watt, which is far below the damaging level.

• Protein was mainly taken into consideration while designing this method of microencapsulation.

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References:1. H. G. Bungenburg de Jong & A. J. Kaas , Zar Kennetus komplex –

koazeration, V. Mitteilung: relative verschiebumer im elektrischen gleichstromfeide von fllussigkeits-einschliebungen in komplex-kooazervat-tropfehen, Biochem. Z.,232:338-345 (1931).

2. S. Benita, Microencapsulation: Methods and Industrial applications, Marcel Dekker, New York (1996).

3. L. Lachman, H. A. Lieberman & J. L. Kanig, The Theory and Practice of Industrial Pharmacy, Varghese Publishing House, Bombay, 3rd edition, p.p. 412-429 (1987).

4. S. C. Gad, Pharmaceutical Manufacturing Handbook: Production & Processes, John Wiley & Sons., Inc., New Jersey, p.p. 358-360 (2008).

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5. H. G. Bungenberg de Jong & H. R. Kruyt, Chemistry-coacervation (Partial miscibilty in colliod system) (Prelininary communication), Proce. Kon. Ned. Akad., Wetensch 32:849-856 (1929).

6. B. K. Green: U.S. Patent: 2,712,507 (1955).7. B. K. Green & L. Schleicher: U.S. Patent: 2,730,456 (1956).8. B. K. Green: U.S. Patent: 2,800,457 (1957).B. K. Green: U.S.

Patent Re: 24,899 (1960).9. The National Cash Register Co.: Gr. Br. Patent: 907,284 (1963).10. J. A. Bank: Microencapsulation of food and related products,

Food technology 7:34 (1973).11. W. E. Enz and T. E. King: U.S. Patent: 3,081,233 (1957).12. K. R. Heimlich & D. R. MacDonnell: U.S. Patent: 3,119,742

(1964)13. M. R. Brophy & P. B. Deasy, Influence of coating and core

modification on the in vitro release of methylene blue from ethylcellulose microcapsule produced by pan coating procedure, J. Pharm. Pharmacol 33: 495-499 (1981). 47

14. H. Lowey, U.S. Patent: 2,853,420(1958).15. E. Rosen and J. V. Swintosekey, Prepartion of 35S labelled

trimepeazine tartarate sustained action product for its evaluation in man. J. Pharm. Pharmacol 12: 237-244 (1960).

16. D. E. Wurster, U. S. Patent: 2,468,609 (1953).17. D. E. Wurster, U. S. Patent: 2,799,241 (1957).18. D. E. Wurster, U. S. Patent: 2,824,809 (1963).19. D. E. Wurster & J. A. Lindlof, U. S. Patent: 3,196,827 (1953).20. D. E. Wurster, J. V. Battista & J. A. Lindlof,U. S. Patent:

3,207,824 (1965).21. D. E. Wurster, U. S. Patent: 3,241,520 (1966).22. D. E. Wurster, U. S. Patent: 3,253,944 (1966).23. V. Coletta & H. Rubin, Wurster coated aspirin. Film coating

techniques, J. Pharm. Sci. 53: 953-955(1964).24. H. C. Caldwell & E. Rosen, New air suspension apparatus for

coating discrete solids, J. Pharm. Sci. 53:1387-1391 (1941).48

25. N. G. Marotta, R. M. Boeltger, B. H. Nappen, & C. D. Szymanski, U. S. Patent : 3, 455, 838 (1969)

26. H. Sager, U. S. Patent 4, 016, 254 (1977)27. M. J. Robinson and J. V. Swintosky, sulphaethyl thiadiazole V.

Design and study of an oral sustained release dosage form, J. Amer.pharm. Assoc. Sci. Ed. 48 : 473-478 (1959)

28. A. KOFF, U. S. Patent 3, 080, 292 (1963)29. H. Takaneka. et. al. Polymorphism of spray dried

microencapsulated sulphamethoxazole with cellulose acetate phthalate & colloidal silica, montmorillonite, or talc, J.Pharm.Sci. 70 : 1256-1260 (1981)

30. C. Voellmy, P. Speiser & M. Soliva, Microencapsulation of Phenobarbital by spray polycondensation, J.Pharm.Sci. 66 : 631-634(1977)

31. H. Nilsson, R. Mosbach & K. Mosbach. The use of bead polymerization of acrylic monomer for immobilisation of enzymes, Biochem.Biophys.Acta 268 : 253-256 (1972)

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32. T. M. S. Chang, Semipermeable Microcapsules, Science 146 : 524-525 (1964)

33. T. M. S. Chang, Semipermeable microcapsules as artificial cells, Sci J. 3(7) : 62-67 (1967)

34. C. R. Somerville et. al. , Controlled release of quinidine sulphate microcapsule, in controlled release polymeric formulations ( D. R. Paul and F. W. Harris, eds. ) American Chemical Society, Washington , D. C. ,1976, PP:182-189

35. D. J. Mangold, H. W. Schlameus, J. W. Goldzeiher, and J. T. Doluisio, Developement of orally active sustained release dosage forms for steroids, in proceedings of the 8th international symposium on controlled release of bio active materials, Ft. Lauderdale, July 26-29, 1981, pp. 173-174.

36. Herbig, J.A., and Hanny, J.F.: u.s. Patent 3,732,172(1973).37. Vrancken , M.N. , and Claeys, D.S.: U.S. Patent 3,523,906(1970).

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38. Matsukana, H.: U.S.Patent 3,660,304(1972).39. Yoshida N.H. : U.S. Patent 3,657,144 (1972). 40. M. Morishita, Y.Inaba, M. Fukushima, Y. Hattori, S. Kobari,and

T. Matsuda, U.S.Patent 3,960,757 (June 1, 1976).41. S. M. Mortada, Preparation of ethyl cellulose microcapsules

using the complex emulsification method, Pha rmazie 37 : 427-429 (1982).

42. M. N. Vrancken and D. A. Claeys, U. S. Patent 3,523,906(August 11, 1970).

43. M. N. Vrancken and D. A. Claeys, U. S. Patent 3,523,907(August 11, 1970).

44. J. F. Van Besauw and D. A. Claeys, U. S. Patent 3,645,911( February 19, 1972).

45. H. Takaneka Y. Kawashima , Y. Chikamarsu, and T. Ando, Reactivity and stability of microencapsulated placental alkaline phosphatise, Chem. Pharm. Bull. 30:695-700(1982).

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46. Y.Nozawa . Drug release from and properties of poly(styrene)microcapsules, in polymeric delivery systems (R. J. Kostelnik ed) , Gorden and Breach New York 1978,pp.101-110.

47. L. P. Gagnon et al. Coating of granules, Drug standards 23:47-52 (1944)

48. J. D. Andrade , K. Kunitomo, B. Kastigir, and W. J. Kolff, Coated adsorbents for direct blood perfusion : HEMA /Activated carbon Trans.Amer.Soc. Artif.Int. Organs 17 :222-228 (1971)

49. G. Langer and G.Yamate, Encapsulation of liquid and solid particles to form dry powders, J. Colloid Interface Sci. 29 :450-455 (1969)

50. Y. Yeo, K.Park, Journal of controlled release 100:379-388 (2004).

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