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Volume 1, Issue 2, March – April 2010; Article 008 ISSN 0976 – 044X
MICROENCAPSULATION : A REVIEW
S. S. Bansode*, S. K. Banarjee, D. D. Gaikwad, S. L. Jadhav, R. M. Thorat
Vishal Institute of Pharmaceutical Education and Research, Ale, Pune-412411.
*E-mail : rupali.78@rediffmail.com
ABSTRACT
The review of Microencapsulation is a well-established dedicated to the preparation, properties and uses of individually encapsulated
novel small particles, as well as significant improvements to tried-and-tested techniques relevant to micro and nano particles and their
use in a wide variety of industrial, engineering, pharmaceutical, biotechnology and research applications. Its scope extends beyond
conventional microcapsules to all other small particulate systems such as self assembling structures that involve preparative
manipulation. The review covers encapsulation materials, physics of release through the capsule wall and / or desorption from carrier,
techniques of preparation, many uses to which microcapsules are put.
Key-words: Microencapsulation, Core Materials, Coating Materials.
INTRODUCTION This technique can be used for converting liquid drugs
Microencapsulation is a process by which very tiny in a free flowing powder.
droplets or particles of liquid or solid material are The drugs, which are sensitive to oxygen, moisture or
surrounded or coated with a continuous film of polymeric light, can be stabilized by microencapsulation.
material. Incompatibility among the drugs can be prevented by
Microencapsulation includes Bioencapsulation which is microencapsulation.
more restricted to the entrapment of a biologically active Vaporization of many volatile drugs e.g. methyl
substance (from DNA to entire cell or group of cells for salicylate and peppermint oil can be prevented by
example) generally to improve its performance &/or
1 microencapsulation.
enhance its shelf life .
Microencapsulation provides the means of converting Many drugs have been microencapsulated to reduce
liquids to solids, of altering colloidal and surface toxicity and GI irritation including ferrous sulphate
properties, of providing environmental protection and of and KCl.
controlling the release characteristics or availability of Alteration in site of absorption can also be achieved
coated materials. Several of these properties can be by microencapsulation.
attained by macropackaging techniques; however, the
uniqueness of microencapsulation is the smallness of the Toxic chemicals such as insecticides may be
coated particles and their subsequent use and adaptation to microencapsulated to reduce the possibility of
a wide variety of dosage forms and not has been sensitization of factorial person.
2
technically feasible . Bakan and Anderson reported that microencapsulated
3
vitamin A palmitate had enhanced stability .
RELEASE MECHANISMS
Mechanisms of drug release from microspheres are
1. Degradation controlled monolithic system : -
The drug is dissolved in matrix and is distributed
uniformly throughout. The drug is strongly attached to the
matrix and is released on degradation of the matrix. The
diffusion of the drug is slow as compared with degradation
of the matrix.
2. Diffusion controlled monolithic system : -
Figure 1: Microencapsulation process Here the active agent is released by diffusion prior to or
concurrent with the degradation of the polymer matrix.
REASONS FOR MICROENCAPSULATION Rate of release also depend upon where the polymer
The primary reason for microencapsulation is found degrades by homogeneous or heterogeneous mechanism.
to be either for sustained or prolonged drug release. 3. Diffusion controlled reservoir system : -
This technique has been widely used for masking taste Here the active agent is encapsulated by a rate controlling
and odor of many drugs to improve patient membrane through which the agent diffuses and the
compliance.
International Journal of Pharmaceutical Sciences Review and Research Page 38
Available online at www.globalresearchonline.net
Volume 1, Issue 2, March – April 2010; Article 008 ISSN 0976 – 044X
membrane erodes only after its delivery is completed. In 5. Non-hygroscopic, no high viscosity, economical.
this case, drug release is unaffected by the degradation of 6. Soluble in an aqueous media or solvent, or melting.
the matrix.
4. Erosion : - 7. The coating can be flexible, brittle, hard, thin etc.
Erosion of the coat due to pH and enzymatic hydrolysis Examples of coating materials:
causes drug release with certain coat material like glyceryl 1. Water soluble resins – Gelatin, Gum Arabic, Starch,
3.
mono stearate, beeswax and steryl alcohol etc Polyvinylpyrrolidone, Carboxymethylcellulose,
Hydroxyethylcellulose, Methylcellulose, Arabinogalactan,
CORE MATERIALS Polyvinyl alcohol, Polyacrylic acid.
The core material, defined as the specific material to be 2. Water insoluble resins – Ethylcellulose, Polyethylene,
coated, can be liquid or solid in nature. The composition Polymethacrylate, Polyamide (Nylon), Poly (Ethylene-
of the core material can be varied, as the liquid core can Vinyl acetate), Cellulose nitrate, Silicones, Poly(lactide-
include dispersed and/or dissolved materials. The solid co-glycolide).
core be active constituents, stabilizers, diluents, excipients, 3. Waxes and lipids – Paraffin, Carnauba, Spermaceti,
and release-rate retardants or accelerators. The ability to Beeswax, Stearic acid, Stearyl alcohol, Glyceryl stearates .
vary the core material composition provides definite 4. Enteric resins – Shellac, Cellulose acetate phthalate,
flexibility and utilization of this characteristics often 2
allows effectual design and development of the desired Zein .
2
microcapsule properties . TECHNIQUES TO MANUFACTURE
MICROCAPSULES
COATING MATERIALS 1. Physical methods
The coating material should be capable of forming a film 1.1 Air-suspension coating
that is cohesive with the core material; be chemically
compatible and nonreactive with the core material; and Air-suspension coating of particles by solutions or melts
provide the desired coating properties, such as strength, gives better control and flexibility. The particles are coated
flexibility, impermeability, optical properties, and while suspended in an upward-moving air stream. They
stability. The coating materials used in microencapsulation are supported by a perforated plate having different
methods are amenable, to some extent, to in situ patterns of holes inside and outside a cylindrical insert.
modification. Just sufficient air is permitted to rise through the outer
The selection of a given coating often can be aided by the annular space to fluidize the settling particles. Most of the
review of existing literature and by the study of free or rising air (usually heated) flows inside the cylinder,
cast films, although practical use of free-film information causing the particles to rise rapidly. At the top, as the air
often is impeded for the following reasons: stream diverges and slows, they settle back onto the outer
1. Cast or free films prepared by the usual casting bed and move downward to repeat the cycle. The particles
techniques yield films that are considerably thicker than pass through the inner cylinder many times in a few
those produced by the microencapsulation of small minutes methods.
particles; hence, the results obtained from the cast films The air suspension process offers a wide variety of coating
may not be extrapolate to the thin microcapsule materials candidates for microencapsulation. The process
coatings. has the capability of applying coatings in the form of
2. The particular microencapsulation method employed for solvent solutions, aqueous solution, emulsions, dispersions
the deposition of a given coating produces specific and or hot melts in equipment ranging in capacities from one
inherent properties that are difficult to simulate with pound to 990 pounds. Core materials comprised of micron
existing film-casting methods. or submicron particles can be effectively encapsulated by
air suspension techniques, but agglomeration of the
3. The coating substrate of core material may have a particles to some larger size is normally achieved 4.
decisive effect on coating properties. Hence, the 1.2 Coacervation Process
selection of a particular coating material involves Solution of the shell material in water.
consideration of both classic free-film data and applied
results. Example: Copolymer coating
Gum arabic solution 20-30%
COATING MATERIAL PROPERTIES Gelatin solution 20%
1. Stabilization of core material. Preparation
2. Inert toward active ingredients. The core material will be added to the solution. The core
3. Controlled release under specific conditions. material should not react or dissolve in water (maximum
4. Film-forming, pliable, tasteless, stable. solubility 2%)
International Journal of Pharmaceutical Sciences Review and Research Page 39
Available online at www.globalresearchonline.net
Volume 1, Issue 2, March – April 2010; Article 008 ISSN 0976 – 044X
Dispersion Deposition if the liquid polymer coating around the
The core material is dispersed in the solution. The particle core material occurs if the polymer is adsorbed at the
size will be defined by dispersion parameter, as stirring interface formed between the core material and the
speed, stirrer shape, surface tension and viscosity. Size liquid vehicle phase, and this adsorption phenomenon
range ca. 2µm - 1200µm is a prerequisite to effective coating. The continued
deposition of the coating material is promoted by a
Coacervation reduction in the total free interfacial energy of the
• Coacervation starts with a change of the pH value of the system, brought about by the decrease of the coating
dispersion, e.g. by adding H SO , HCl or organic acids. material surface area during coleasance of the liquid
2 4 polymer droplets.
The result is a reduction of the solubility of the dispersed
phases (shell material). 3. Rigidization of the coating – It involves rigidizing the
• The shell material (coacervate) starts to precipitate from coating, usually by thermal, cross-linking, or
the solution. desolvation techniques, to form a self-sustaining
microcapsules 2
• The shell material forms a continuous coating around the eg. Coacervation Microencapsulation of Talc Particles
core droplets. with Poly (methyl methacrylate) by Pressure-Induced
6.
Cooling and hardening phase Phase Separation of CO -Expanded Ethanol Solutions
2
• The shell material is cooled down to harden and forms 1.3 Centrifugal extrusion
the final capsule. Liquids are encapsulated using a rotating extrusion head
• Hardening agents like formaldehyde can be added to the containing concentric nozzles. In this process, a jet of core
process. liquid is surrounded by a sheath of wall solution or melt.
• The microcapsules are now stable in the suspension and As the jet moves through the air it breaks, owing to
ready to be dried. Rayleigh instability, into droplets of core, each coated
with the wall solution. While the droplets are in flight, a
Drying phase molten wall may be hardened or a solvent may be
• The suspension is dried in a spray dryer or in a fluidized evaporated from the wall solution. Since most of the
bed drier. droplets are within ± 10% of the mean diameter, they land
in a narrow ring around the spray nozzle. Hence, if
• Spray Drying is a suitable method for heat sensitive needed, the capsules can be hardened after formation by
Products. catching them in a ring-shaped hardening bath. This
• The atomized particles assume a spherical shape. The process is excellent for forming particles 400–2,000 µm
rapid the coating material keeps the core material below (16–79 mils) in diameter. Since the drops are formed by
100°C, even if the temperature in the drying chamber is the breakup of a liquid jet, the process is only suitable for
much greater. liquid or slurry. A high production rate can be achieved,
i.e., up to 22.5 kg (50 lb) of microcapsules can be
• Microencapsulation makes the spray drying process produced per nozzle per hour per head. Heads containing
4.
easier for sticky products like fruit pulp or juice, with a 16 nozzles are available
5.
high content of invert sugar 1.4 Pan coating
Coacervation-Phase Separation The pan coating process, widely used in the
The general outline of the processes consists of three steps pharmaceutical industry, is among the oldest industrial
carried out under continuous agitation: procedures for forming small, coated particles or tablets.
The particles are tumbled in a pan or other device while
1. Formation of three immiscible chemical phases – A 4.
liquid manufacturing vehicle phase, a core material the coating material is applied slowly
phase, and a coating material phase. To form the three The pan coating process, widely used in the
phases, the core material dispersed in a solution of the pharmaceutical industry, is among the oldest industrial
coating polymer, the solvent for the polymer being the procedures for forming small, coated particles or tablets.
liquid manufacturing vehicle phase. The coating The particles are tumbled in a pan or other device while
material phase, an immiscible polymer in a liquid the coating material is applied slowly
state, is formed by utilizing one of the methods of the with respect to microencapsulation, solid particles greater
methods of phase separation-coacervation, i.e., by than 600 microns in size are generally considered essential
changing the temperature of the polymer solution; or for effective coating, and the process has been extensively
by adding a salt, nonsolvent, or incompatible polymer employed for the preparation of controlled - release beads.
to the polymer solution; or by inducing a polymer- Medicaments are usually coated onto various spherical
polymer interaction. substrates such as nonpareil sugar seeds, and then coated
2. Deposition of the coating – It consists of depositing with protective layers of various polymers.
the liquid polymer coating upon the core material.
This is accomplished by controlled, physical mixing
of the material in the manufacturing vehicle.
International Journal of Pharmaceutical Sciences Review and Research Page 40
Available online at www.globalresearchonline.net
Volume 1, Issue 2, March – April 2010; Article 008 ISSN 0976 – 044X
case of spray drying is effected by rapid evaporation of a
solvent in which the coating material is dissolved. Coating
solidification in spray congealing methods, however, is
accomplished by thermally congealing a molten coating
material or by solidifying a dissolved coating by
introducing the coating - core material mixture into a
nonsolvent. Removal of the nonsolvent or solvent from the
coated product is then accomplished by sorption,
extraction, or evaporation techniques.
In practice, microencapsulation by spray drying is
conducted by dispersing a core material in a coating
solution, in which the coating substance is dissolved and
in which the core material is insoluble, and then by
Figure 2: Representation of a typical pan coating atomizing the mixture into air stream. The air, usually
heated, supplies the latent heat of vaporization required to
remove the solvent from the coating material, thus forming
In practice, the coating is applied as a solution, or as an the microencapsulated product. The equipment
atomized spray, to the desired solid core material in the components of a standard spray dryer include an air
coating pans. Usually, to remove the coating solvent, heater, atomizer, main spray chamber, blower or fan,
warm air is passed over the coated materials as the cyclone and product collector.
coatings are being applied in the coating pans. In some Microencapsulation by spray congealing can be
cases, final solvent removal is accomplished in a drying accomplished with spray drying equipment when the
oven 2 protective coating is applied as a melt. General process
variables and conditions are quite similar to those already
described, except that the core material is dispersed in a
coating material melt rather than a coating solution.
Coating solidification (and microencapsulation) is
accomplished by spraying the hot mixture into a cool air
stream. Waxes, fatty acids and alcohols, polymers and
sugars, which are solids at room temperature but meltable
at reasonable temperatures, are applicable to spray
congealing techniques. Typically, the particle size of spray
congealed products can be accurately controlled when
spray drying equipment is used, and has been found to be
a function of the feed rate, the atomizing wheel velocity,
2
dispersion of feed material viscosity, and variables .
Airflow
The initial contact between spray droplets and drying air
controls evaporation rates and product temperatures in the
7 dryer. There are three modes of contact:
Figure 3 : List of variables affecting pan coating process
1.5.1. Co-current
1.5 Spray–drying Drying air and particles move through the drying chamber
Spray drying serves as a microencapsulation technique in the same direction. Product temperatures on discharge
when an active material is dissolved or suspended in a from the dryer are lower than the exhaust air temperature,
melt or polymer solution and becomes trapped in the dried and hence this is an ideal mode for drying heat sensitive
particle. The main advantages is the ability to handle labile products. When operating with rotary atomizer, the air
materials because of the short contact time in the dryer, in disperser creates a high degree of air rotation, giving
addition, the operation is economical. In modern spray uniform temperatures throughout the drying chamber.
dryers the viscosity of the solutions to be sprayed can be However, an alternative non-rotating airflow is often used
in tower or FILTERMAT®-type spray dryers using nozzle
as high as 300mPa.s
atomizers with equal success.
Spray drying and spray congealing processes are similar in 1.5.2. Counter-current
that both involve dispersing the core material in a liquified Drying air and particles move through the drying chamber
coating substance and spraying or introducing the core - in opposite directions. This mode is suitable for products
coating mixture into some environmental condition, which require a degree of heat treatment during drying.
whereby, relatively rapid solidification (and formation) of
the coating is effected. The principal difference between The temperature of the powder leaving the dryer is usually
the two methods, is the means by which coating higher than the exhaust air temperature.
solidification is accomplished. Coating solidification in the
International Journal of Pharmaceutical Sciences Review and Research Page 41
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