INTRODUCTION:

Microencapsulation is rapidly growing in several industries, such as the pharmaceutical, cosmetic, and food sectors. As a delivery system, it has proven beneficial in numerous business applications across different sectors. Microencapsulation is the process of enclosing tiny droplets or particles of liquid or solid material coated with a continuous film of polymeric material, resulting in microcapsules. Generally, a microcapsule is a small sphere with a uniform wall around it. The main feature of microcapsules is their microscopic size, resulting in a vast surface area.

Returning to the history of encapsulation, such significant multidisciplinary research technology has a history of 90 years. Microcapsules originated in the 1930s and have been rapidly developed since the mid-1970s. Over 100 research laboratories have been dedicated to developing and studying microcapsule technology, which has found applications in various fields of production and daily life. Notable pioneers in the field include Wurster and Green.

Microcapsule generally consists of two components, i.e., Core and Coating material. The material to be coated is known as the core (active ingredient), which may include a liquid or solid composition. The core material's composition can vary as the liquid core may consist of dispersed or dissolved materials. The solid core material could be stabilizers, diluents, additives accelerators, or release-rate retardants.

Table no. 1: Examples of Core Materials are:

Core material

Perfumes, Vegetable Oils, Pesticides, Dyes, Catalysts, Bleaches, Cosmetics, Insecticides, Sugars, Salts, Acids, Pigments, Nutrients,

Herbicides, Pharmaceuticals, Minerals, Volatile Oils

Conversely, the coating material that protects the active ingredient should create an integral film with the core material, remain nonreactive and chemically compatible, and provide the desired coating properties, including stability, strength, flexibility, and impermeability. The ideal characteristics of coating material include stabilization of core material, controlled release under specific conditions, film forming, pliable, tasteless, stable and non-hygroscopic, no high viscosity, and economic, soluble in an aqueous media or solvent.^{1,2,3,4,5,6,7}

Table no.2 Examples of Coating Materials are:

Properties	Examples
Watersoluble resin	Gelatin, Gum Arabic, Starch, Polyvinylpyrrolidone, Carboxymethyl cellulose, Hydroxyethylcellulose, Methylcellulose, Polyacrylic acid
Water insoluble resins	Ethylcellulose, Polyethylene, Polymethacrylate, Polyamide (Nylon), Poly (Ethylene- Vinyl acetate), cellulose nitrate, Silicones, Poly (lactide- glycolide)
Waxes and lipids	Paraffin, Carnauba, Spermaceti, Beeswax, Stearic acid, Stearyl alcohol, Glyceryl stearates.
Enteric resins	Shellac, Cellulose acetate phthalate, Zein

Advantages:

The microencapsulation technique helps transform liquids into solids, modifies surface and colloidal characteristics, protects from the environment, improves bioavailability, and controls the release characteristics of various coated materials.

It is widely used in multiple industries because of its low equipment requirements, a continuous production process that lowers production costs, and environmentally friendly.⁸

Classification:

There are three basic categories for classifying microcapsules based on their morphology:

Mononuclear:

Mononuclear microcapsules contain the shell around the core.

Polynuclear:

Polynuclear capsules have many cores enclosed within the shell.

Matrix types:

In matrix encapsulation, the core material is distributed homogeneously

into the shell material.⁹

Microencapsulation Techniques:

Microencapsulation technology has wholly changed several industries, such as the food, cosmetic, and pharmaceutical sectors. This technology has been applied in the pharmaceutical industry to enhance the safety and efficacy of medications using targeted distribution and controlled release. Furthermore, this technology has been applied to improve the stability and functionality of cosmetic products. The food industry also utilizes microencapsulation technology to enhance food products' flavor, texture, and nutritional value. Various microencapsulation techniques have been found based on the intended use of the final products and the characteristics of the core.

Microencapsulation techniques are generally divided into physical and chemical methods, with the latter being further subdivided into physiochemical, electrostatic, and mechanical processes. The chemical procedures utilized in microencapsulation include emulsion, suspension, precipitation or dispersion polymerization, and interfacial polycondensation. Physiochemical processes include coacervation-phase separation, complex emulsion, and powder bed methods. Examples of mechanical processes include the air-suspension method, pan coating, spray drying, spray congealing, micro-orifice system, and rotary fluidization bed granulator method. Spheronization is sometimes categorized as a mechanical process in the microencapsulation process. The selection of coating materials and the microencapsulation technique are interdependent. Spray drying and spray congealing are two procedures for microencapsulation that have been used for a long time. Because of certain parallels between these two procedures, they are discussed together. The pan-coating technique for microencapsulating relatively large particles has gained extensive application in the pharmaceutical sector.^{10,11,12,13,14}

Table no: 3 Methods of Microencapsulation¹⁵

Physical methods	Chemical methods
Spray drying	Coacervation phase separation
Spray chilling	Solvent evaporation
Multi-orifice centrifugal process	Solvent extraction
Pan coating	Interfacial polymerization
Air suspension coating	In situ polymerization
Centrifugal extrusion	Matrix polymerization
Fluid bed coating

Spray Drying/Spray Cooling:

Spray drying rapidly forms a dry powder from liquid or slurry. This is the preferred drying method for many thermally sensitive materials, such as foods, pharmaceuticals, or materials requiring highly consistent, fine particle size. It is the most commonly used material protection technique since it is economical and widely employed, particularly for fragrances, oils, and flavors. The method of spray drying consists of three main stages. The feed is first atomized into a fine spray in a drying chamber. The feed's fine spray then spontaneously evaporates when it comes into contact with a hot drying gas, producing dry particles. Ultimately, a cyclone or filter separates the dry particles from the gas due to its ability to rapidly evaporate water while maintaining a low particle temperature. One of the most widely used methods for microencapsulation is spray drying. Standard spray dryer equipment includes an air heater, atomizer, primary spray chamber, blower or fan, cyclone, and product collector. Choosing an appropriate wall material, which should create a continuous thin layer and shield the core material from deterioration, is the most critical stage in encapsulating any core material by spray drying. Some examples of coating materials used in spray drying are Gum arabic (Acacia), Modified starches, and Hydrolyzed starches. The ability of spray drying to create a free-flowing powder in a single processing step that is simple to handle and transport is one of its key benefits.

The spray cooling method of encapsulation is very similar to spray drying in operation, the major difference being the use of cold air. The microencapsulation techniques of spray drying and congealing are almost identical since they both require dispersing the core material in a liquefied coating agent. The main difference between these two microencapsulation methods is how the coating solidification is carried out. ^{16,17,18,19,20}

Advantages:

1. It provides reasonable control of the final characteristics, including mechanical strength, bulk density, flowability, and particle size.

2. This process is fast, continuous, cost-effective, and easy to scale.

3. Large-scale production of the microcapsules can be done at a low cost using this method.^[21]

Disadvantages:

1. Particle aggregation may occur.

2. Certain particles remain uncoated.²²

Fluidized Bed Coating:

This technique, which uses coating over a fluidized bed of solid particles, is a modified form of the spray drying method. This method absorbs liquid and solid core components into a porous solid. As air passes through the particle bed, it acquires unique properties from the bed. It involves three steps: first, suspending the solid materials in the air; second, encapsulating the solid particles with a liquid material by spraying; and third, solidifying the shell by cooling or solvent vaporization. Surface tension, viscosity, hydrophobic, hydrophilic, and electrostatic interactions are critical encapsulant solution properties, and the core material is attached to the encapsulants by hydrogen bonds or liquid bridges. These three procedures are repeated until the appropriate wall thickness is achieved. The most popular fluidizing and drying medium is air, and the temperature of the air directly influences the quality and properties of microstructures. Fluidized bed coating is of 3 types according to their processes: Top spray coating, Bottom spray coating, and Tangential spray coating. Common liquid spraying methods are often used in packing solid particles. This technique efficiently applies a uniform shell onto solid particles.^23,24,25

Advantages:

The ability of fluidized bed coaters to handle a broad range of coating compositions, including Hot melts, Organic solvent solutions, Aqueous solutions, etc.

Disadvantages:

The procedure is repeated multiple times.²⁶

Air Suspension:

The Wurster process involves suspending solid core materials in an air stream and spraying them. This technique is generally applicable only to encapsulate the solid core materials. The air suspension apparatus consists of different sections such as a control panel, coating chamber, air distribution plate, and nozzle for applying film coatings. They are supported by a perforated plate with varying hole patterns inside and outside a cylindrical insert. It involves spray coating the air-suspended particles once solids and particulate core materials have been dispersed in a supporting air stream. Particulate core materials are suspended in an upward-moving air stream within the coating chamber. Particles circulate through the coating zone section of the coating chamber, where a coating substance is sprayed on the moving particles, depending on the chamber design and operating parameters. The coating material is sprayed on the moving core particles in the coating zone. The process can apply the coating in hot melts, emulsions, dispersions, solvent solutions, or aqueous solutions. ^27,28,29,30

Advantages:

1. It is suitable for micro and macro encapsulation because of its tiny particle size.

2. Improved flexibility and control over pan-coating.

Disadvantages:

1) This is only applicable to solids.

2) Solids may aggregate and become clumpy.³¹

Coacervation:

Coacervation, called "phase separation," is an accurate microencapsulation method since the matrix entirely encloses the core substance. This method precipitates or separates a colloidal phase from an aqueous phase. Coacervation is a simple technique involving forming a homogeneous layer of the polymeric wall material around the core material. During coacervation, the coating polymer solution can disperse the active ingredient. At a specific environmental influence (ionic, pH, or thermal change), phase separation occurs while the wall-forming polymer encapsulates the core material. Simple and complex coacervation are two categories for phase separation; in simple coacervation, the separation is caused by desolvating (coacervation) agents, whereas in complex to polymers with opposing charges. Hydrophobic components are often encapsulated with coacervation. Wall materials used in a coacervation are Gelatin and Arabic Gum, etc.^31,32,33

Three phases are often included in batch-kind coacervation techniques, and they involve the liquid manufacturing vehicle phase, core material phase, and coating material phase.

1. Formation of three-immiscible chemical segment kinetics: This involves dispersing the core material in a coated polymer solution and using the vehicle phase as a solvent for the polymer.

2. Deposition of coating: In the second stage, liquid polymer is deposited on the core material, which involves carefully combining the coating and core materials in a manufacturing vehicle.

3. Solidification of the coating includes using heat, cross-linking, or dissolving methods to create self-sustaining microcapsules.^34,35

Advantages:

1. It is a versatile process, and it has high encapsulation efficiency

2. It is an easy and fast process

3. High temperatures are not required, the capsules obtained are not thermolabile, and organic or toxic solvents are unnecessary.³⁶

Disadvantages:

Scale-up is difficult.

Pan Coating:

The pan-coating technique is one of the oldest industrial methods for creating small, coated particles or tablets, and it is frequently utilized in the pharmaceutical industry. The coating material is applied slowly while the particles tumble in a pan or equivalent device. Generally, the coating is used as a solution or atomized spray to the desired solid core material in the coating pans. Warm air is typically circulated over the coated material as coating is applied in coating pans to eliminate the solvent. Sometimes, the final solvent is eliminated by drying it in an oven. The pan-coating method microencapsulates large particles to provide an effective coating.^37,38

Advantages:

Easy and low cost.

Disadvantages:

Require a skilled individual.³⁹

Solvent Evaporation Technique:

The method of producing microcapsules using solvent evaporation is appropriate for a broad range of liquid and solid core materials. Two possible types of core material are water-soluble and water-insoluble. These methods can be used in liquid production vehicles, such as oil in water or o/w emulsions created by agitating two immiscible liquid phases. The system is continuously agitated until the solvent evaporates and separates into the aqueous phase. The volatile solution containing the polymer material dissolves, and the active core material gradually dissolves inside the dissolved polymer volatile solution. Heat is applied to the solution to evaporate the solvent that is leaving the microcapsule. With time, the droplets are hardened to produce corresponding polymer microcapsules. In some cases, wherever core material is spread among the polymer solution, the polymer shrinks around the core. The vehicle component and solvent used for the polymer coating are crucial elements since they significantly impact the microcapsule's characteristics. For the creation of drug-loaded microcapsules based on biodegradable polyesters such as polylactide and polyhydroxy butyrate, solvent evaporation/extraction techniques are appropriate. Coating materials can be made from a wide range of film-forming polymers. The formation of dispersions, agitation rates, and the rate at which the coated polymer's solvent evaporates are some variables that could impact the microencapsulation process.^{40,41,42,43,44}

Advantages:

1. Most simple, efficient, and require less time

2. Not requiring any specific equipment.⁴⁴

Disadvantages:

1) It is not appropriate for encapsulating very hydrophilic medications.

Centrifugal Extrusion Process:

The Southwest Research Institute (SWRI) developed a mechanical method for creating microcapsules that affect mechanical microencapsulation by using centrifugal forces to hurl a core material particle through an enclosing microencapsulation membrane. This procedure can only be applied to liquids and slurries. This process uses a revolving extrusion head with concentric nozzles to encapsulate the material. A sheath of solution surrounds the jet of core liquid. As the jet moves through, the air brakes into core droplets, each coated with a wall solution. The droplets can evaporate from the wall solution while in the fluidized, molten wall solution. After formation, capsules can be held in a ring-shaped hardening bath to harden them. This method is effective for creating 400–2000 μm particles. Processing variables include the cylinder's rotational speed, the coating and core materials' flow rates, and the core material's concentration, viscosity, and surface tension. Using various coating materials, the multi-orifice centrifugal method can microencapsulate liquids and solids in multiple sizes.^45,46,47

Advantages:

1. Able to encapsulate both solid and liquid core material.

2. Encapsulating vitamins, hormones, plasma, serum, dextrins, and chloramphenicol is possible.

Disadvantages:

1. Coating material waste may occur.

2. Microcapsules require a high temperature to dry.⁴⁸

Polymerization:

A new technology for microencapsulation forms protective microcapsule coatings in situ by using the polymerization technique. The pH change starts this reaction, and catalysts can accelerate temperature. The polymer formed is deposited around the drops, which leads to encapsulation. The process involves the reaction of monomeric units at the interface between a continuous phase containing the dispersed core material and a substance that makes up the core material. The polymerization reactions take place at an interphase between a liquid and liquid, liquid and gas, solid and liquid, or solid and gas because the core material supporting phase is typically a liquid or gas.^48,49

Interfacial polymerization refers to a polycondensation in which the two reactants come into contact at an interface and react rapidly. This technique is based on the Schotten method. In the proper circumstances, thin, flexible walls quickly form at the interface. The materials utilized for polymer coatings include polyesters, polycarbonates, polyamides (nylon), polyurethanes, polysulfonamides, and polyamides.^50,51

In-situ polymerization: A microencapsulation technique employs the direct polymerization of a single monomer on the particle surface. For example, cellulose fibers are created in a single step. Reactive agents are not added to the core material during this process; alternatively, polymerization occurs in the continuous phase and on the side of the interface created by the ongoing phase and dispersed core material.^52,53

Matrix polymer:

Various steps are taken to embed a core material in a polymeric matrix during the particle's formation. One easy example of this kind of process is spray drying. The particle is created when the solvent in the solution evaporates. On the contrary, the matrix substance is solidifying.^54,55

Advantages:

1. Efficient method for producing microparticles with tailored core-shell.

2. It is possible to get the desired size.

Disadvantages:

To achieve pure polymer, polymerization adjuvants and surfactants must be removed.

Emulsification:

The emulsification method of encapsulation involves dispersing the core in an organic solvent that contains the wall material. After that, the dispersion is emulsified in either water or oil, and an emulsion stabilizer is added. The core gets encapsulated when an organic solvent evaporates, and a tight polymer layer form around it. Due to the ease of the procedures, this encapsulation technique is employed frequently. Enzymes and microbes are commonly encapsulated using this method. ^56,57

Single emulsion technique:

A single emulsion method creates the microparticulate carriers of natural polymers, such as proteins and carbohydrates. After being dispersed in an aqueous medium, the natural polymers are dissolved in a non-aqueous medium. Subsequently, the dispersed globules are cross-linked using either chemical or thermal cross-linkers.⁵⁸

Double Emulsion Technique:

This process uses double emulsion of type w/o/w or several emulsions to create microcapsules. Water-soluble medications, peptides, proteins, and vaccines are the best options for this method. This method can be used with both natural and synthetic polymers. A lipophilic organic continuous phase disperses the aqueous medication solution. The polymer solution that eventually encapsulates the medication found in the dispersed aqueous phase often makes up the ongoing phase.⁵⁹

Layer-by-layer Deposition:

A substrate is successfully submerged in positively and negatively charged polyelectrolyte solutions in a cyclic process to create polyelectrolyte multilayers. The Layer-by-Layer (LbL) method is a thin-film production process. Alternating layers of materials with opposing charges are deposited to generate the films. There are different techniques to apply the layers, such as spin coating, spray coating, and dip coating. Using colloidal particles as the core material that acts as a template for the fabrication of multilayers, core-shell particles with specific sizes and characteristics are created. Dissolving the core material can form hollow capsules of inorganic, hybrid, or organic particles. This method allows for fine control of the multilayer film thickness by varying the total number of layers deposited.^59,60,61

ADVANTAGES:

This technique is both versatile and simple.⁶²

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Received on 23.11.2023 Revised on 20.02.2024

Accepted on 03.04.2025 Published on 23.04.2025

Available online from April 26, 2025

Asian J. Pharm. Tech. 2025; 15(2):127-133.

DOI: 10.52711/2231-5713.2025.00021

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