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MICROSPHERES A MAGICAL NOVEL DRUG DELIVERY SYSTEM:
A REVIEW
*Preeti Agrawal1, Sarlesh Rajput1, Ashish pathak1, Nikhil Shrivastava2, Satyendra
Singh Baghel2, Rajendra singh Baghel2
1Department of Pharmaceutics, ShriRam College of Pharmacy, Morena, M.P., India
2Department of Pharmacology, ShriRam College of Pharmacy, Morena, M.P., India
ABSTRACT
Drug delivery systems (DDS) that can precisely control the release
rates or target drugs to a specific body site have had an enormous
impact on the health care system. So the concept of targeted drug
delivery is designed for attempting to concentrate the drug in the
tissues of interest while reducing the relative concentration of the
medication in the remaining tissues. As a result, drug is localized on
the targeted site. Hence, surrounding tissues are not affected by the
drug. So, carrier technology offers an intelligent approach for drug
delivery by coupling the drug to a carrier particle such as
microspheres, nanoparticles, liposomes, etc which modulates the
release and absorption characteristics of the drug. Among these drug
delivery system we are selecting microspheres of various types which
will be controlled release and which can be made specific site targeted
by giving some specific characteristic to it like mucoadhesion character or by inserting any
magnetic or radioactive material as a result of which it will show site specific action. So this
article emphasis on different types of microspheres as a controlled and targeted drug delivery
system.
Keywords: Microsphere, target site, Specificity, controlled release.
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Article Received on 25 May 2012, Revised on 02 June 2012, Accepted on 07 June 2012
*Correspondence for
Author:
* Preeti Agrawal
Department of Pharmaceutics
ShriRam College of Pharmacy.
Banmore, Morena M.P. India.
preeti.pharma@live.com
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INTRODUCTION
The method by which a drug is delivered can have a significant effect on its efficacy. Some
drugs have an optimum concentration range within which maximum benefit is derived, and
concentrations above or below this range can be toxic or produce no therapeutic benefit at
all.[1] On the other hand, the very slow progress in the efficacy of the treatment of severe
diseases, has suggested a growing need for a multidisciplinary approach to the delivery of
therapeutics to targets in tissues. From this, new ideas on controlling the pharmacokinetics,
pharmacodynamics, non-specific toxicity, immunogenicity, biorecognition, and efficacy of
drugs were generated. These new strategies, often called drug delivery systems (DDS), are
based on interdisciplinary approaches that combine polymer science, pharmaceutics,
bioconjugate chemistry, and molecular biology.
To minimize drug degradation and loss, to prevent harmful side-effects and to increase drug
bioavailability and the fraction of the drug accumulated in the required zone, various drug
delivery and drug targeting systems are currently under development. Among drug carriers
one can name soluble polymers, microparticles made of insoluble or biodegradable natural
and synthetic polymers, microcapsules, cells, cell ghosts, lipoproteins, liposomes, and
micelles. The carriers can be made slowly degradable, stimuli-reactive (e.g., pH- or
temperature-sensitive), and even targeted (e.g., by conjugating them with specific antibodies
against certain characteristic components of the area of interest.[1, 2]
Controlled drug release and subsequent biodegradation are important for developing
successful formulations. Potential release mechanisms involve:
Desorption of surface-bound /adsorbed drugs;
Diffusion through the carrier matrix;
Diffusion (in the case of nanocapsules) through the carrier wall;
Carrier matrix erosion; and
A combined erosion /diffusion process.
A well designed controlled drug delivery system can overcome some of the problems of
conventional therapy and enhance the therapeutic efficacy of a given drug.
To obtain maximum therapeutic efficacy, it becomes necessary to deliver the agent to the
target tissue in the optimal amount in the right period of time there by causing little toxicity
and minimal side effects.
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There are various approaches in delivering a therapeutic substance to the target site in a
sustained controlled release fashion. One such approach is using microspheres as carriers for
drugs.[3, 4]
Drug delivery systems (DDS) that can precisely control the release rates or target drugs to a
specific body site have had an enormous impact on the health care system.[5] The ideal drug
delivery system delivers drug at rate decided by the need of the body throughout the period of
treatment and it provides the active entity solely to the site of action.
The concept of targeted drug delivery is designed for attempting to concentrate the drug in
the tissues of interest while reducing the relative concentration of the medication in the
remaining tissues. As a result, drug is localized on the targeted site. Hence, surrounding
tissues are not affected by the drug. In addition, loss of drug does not happen due to
localization of drug, leading to get maximum efficacy of the medication.
So, carrier technology offers an intelligent approach for drug delivery by coupling the drug to
a carrier particle such as microspheres, nanoparticles, liposomes, etc which modulates the
release and absorption characteristics of the drug.
Microspheres constitute an important part of these particulate DDS by virtue of their small
size and efficient carrier characteristics. These are characteristically free flowing powders
consisting of proteins or synthetic polymers which are biodegradable in nature and ideally
having a particle size less than 200µm
.
Figure -1 Microscopic view of Microspheres
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Figure-2 SEM View of Microspheres
Properties of an Ideal microsphere
Preparation of microspheres should satisfy certain criteria:
1. The ability to incorporate reasonably high concentrations of the drug.
2. Stability of the preparation after synthesis with a clinically acceptable shelf life.
3. Controlled particle size and dispersability in aqueous vehicles for injection.
4. Release of active reagent with a good control over a wide time scale.
5. Biocompatibility with a controllable biodegradability and
6. Susceptibility to chemical modification.
TYPES OF MICROSPHERE
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I. Bioadhesive microspheres
Adhesion can be defined as sticking of drug to the membrane by using the sticking property
of the water soluble polymers. Adhesion of drug delivery device to the mucosal membrane
such as buccal, ocular, rectal, nasal etc can be termed as bioadhesion.[8] The term
“bioadhesion” describes materials that bind to biological substrates, such as mucosal
membranes. Adhesion of bioadhesive drug delivery devices to the mucosal tissue offers the
possibility of creating an intimate and prolonged contact at the site of administration. This
prolonged residence time can result in enhanced absorption and in combination with a
controlled release of drug also improved patient compliance by reducing the frequency of
administration.
Bioadhesive microspheres can be tailored to adhere to any mucosal tissue including those
found in eye, nasal cavity, urinary, colon and gastrointestinal tract, thus offering the
possibilities of localized as well as systemic controlled release of drugs.[7]
II. Magnetic microspheres
This kind of delivery system is very much important which localises the drug to the disease
site. In this larger amount of freely circulating drug can be replaced by smaller amount of
magnetically targeted drug. Magnetic carriers receive magnetic responses to a magnetic field
from incorporated materials that are used for magnetic microspheres are chitosan, dextran etc.
The different type are Therapeutic magnetic microspheres: Are used to deliver
chemotherapeutic agent to liver tumour.
Diagnostic microspheres: Can be used for imaging liver metastases and also can be used to
distinguish bowel loops from other abdominal structures by forming nano size particles
supramagnetic iron oxides.[8]
The aim of the specific targeting is to enhance the efficiency of drug delivery & at the same
time to reduce the toxicity & side effects. Magnetic drug transport technique is based on the
fact that the drug can be either encapsulated into a magnetic microsphere or conjugated on
the surface of the microsphere. When the magnetic carrier is intravenously administered, the
accumulation take place within area to which the magnetic field is applied & often
augmented by magnetic agglomeration. The accumulation of the carrier at the target site
allow them to deliver the drug locally. Efficiency of accumulation of magnetic carrier on
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physiological carrier depends on physiological parameters eg. particle size, surface
characteristic, field strength, & blood flow rate etc. The magnetic field helps to extravasate
the magnetic carrier into the targeted area. Very high concentration of chemotherapeutic
agents can be achieved near the target site without any toxic effect to normal surrounding
tissue or to whole body. It is possible to replace large amounts of drug targeted magnetically
to localized disease site, reaching effective and up to several fold increased drug levels. This
technique which requires only a simple injection, is far less invasive than surgical methods of
targeted drug delivery. Another advantage is that particles in the magnetic fluid interact
strongly with each other, which facilitates the delivery of high concentrations of drug to
targeted areas.
Magnetic microspheres can be filled with drugs or radioactive materials to treat a variety of
illnesses. Magnets applied outside the body attract the spheres to the disease site where they
deliver therapeutics in a targeted way. The magnets attract the microspheres to the immediate
area of the wound site and stop them there. The spheres gradually break down and release
growth factors over a period of weeks, allowing blood vessels and damaged tissues to re-
grow and repair.
Small amounts of drug targeted magnetically to localized sites can replace large doses of drug
that, using traditional administration methods, freely circulate in the blood and hit the target
site in a generalized way only.[9]
III. Floating microspheres
In floating types the bulk density is less than the gastric fluid and so remains buoyant in
stomach without affecting gastric emptying rate. The drug is released slowly at the desired
rate, if the system is floating on gastric content and increases gastric residence and increases
fluctuation in plasma concentration. Moreover it also reduces chances of striking and dose
dumping. One another way it produces prolonged therapeutic effect and therefore reduces
dosing frequencies. Drug (ketoprofen) given through this form.[8]
IV. Radioactive microspheres
Radio emobilisation therapy microspheres sized 10-30nm are of larger than capillaries and
gets tapped in first capillary bed when they come across. They are injected to the arteries that
lead to tumor of interest. So all these conditions radioactive microspheres deliver high
radiation dose to the targeted areas without damaging the normal surrounding tissues. It
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differs from drug delivery system, as radioactivity is not released from microspheres but acts
from within a radioisotope typical distance and the different kinds of radioactive
microspheres are α emitters, β emitters, γ emitters.
It offer new solutions for patients who need drugs delivered directly to tumors, diabetic ulcers
and other disease sites.[8]
V. Mucoadhesive microspheres
Mucoadhesive microspheres which are of 1-1000mm in diameter and consisting either
entirely of a mucoadhesive polymer or having an outer coating of it, respectively.
Microspheres, in general, have the potential to be used for targeted and controlled release
drug delivery; but coupling of mucoadhesive properties to microspheres has additional
advantages, e.g. efficient absorption and enhanced bioavailability of the drugs due to a high
surface to volume ratio, a much more intimate contact with the mucus layer, specific
targeting of drug to the absorption site achieved by anchoring plant lectins, bacterial
adhesions and antibodies, etc. on the surface of the microspheres.
Mucoadhesive microspheres can be tailored to adhere to any mucosal tissue including those
found in eye, nasal cavity, urinary and gastrointestinal tract, thus offering the possibilities of
localized as well as systemic controlled release of drugs.
Advantages of mucoadhesive system
Mucoadhesive systems have three distinct advantages when compared to conventional dosage
forms.
The mucoadhesive systems are readily localized in the region applied to improve and
enhance the bioavailability of drugs. Greater bioavailability of piribedit, testosterone
and its esters, vasopressin, dopamine, insulin and gentamycin was observed from
mucoadhesive dosage systems.
These dosage forms facilitate intimate contact of the formulation with underlying
absorption surface. This allows modification of tissue permeability for absorption of
macromolecules, such as peptides and proteins. Inclusion of penetration enhancers
such as sodium glycocholate, sodium taurocholate and L-lysophosphotidyl choline
(LPC) and protease inhibitors in the mucoadhesive dosage forms resulted in the
better absorption of peptides and proteins.
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Mucoadhesive dosage forms also prolong residence time of the dosage form at the
site of application and absorption to permit once or twice a day dosing.[5]
METHOD OF PREPRATION
Various methods employed in its preparation are
Single Emulsification Technique
Double Emulsification Technique
Normal Polymerization Technique
Bulk polymerization
Suspension polymerization
Emulsion polymerization
Interfacial Polymerization Technique
Phase Separation Coacervation Technique
Spray Drying and Spray Congealing Technique
Solvent Extraction Method
Solvent Evaporation
Wet Inversion Technique
Complex Coacervation
Hot Melt Microencapsulation
1. Single emulsion technique
The micro particulate carriers of natural polymers i.e. those of proteins and carbohydrates are
prepared by single emulsion technique. The natural polymers are dissolved or dispersed in
aqueous medium followed by dispersion in non-aqueous medium like oil. Next cross linking
of the dispersed globule is carried out. The cross linking can be achieved either by means of
heat or by using the chemical cross linkers. The chemical cross linking agents used are
glutaraldehyde, formaldehyde, di acid chloride etc. Heat denaturation is not suitable for
thermolabile substances. Chemical cross linking suffers the disadvantage of excessive
exposure of active ingredient to chemicals if added at the time of preparation and then
subjected to centrifugation, washing, separation.[3]
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Fig 3: Processing scheme for microspheres-preparation by single emulsion technique.
2. Double emulsion technique
Double emulsion method of microspheres preparation involves the formation of the multiple
emulsion or the double emulsion of type w/o/w and is best suited to water soluble drugs,
peptides, proteins and the vaccines. This method can be used with both the natural as well as
synthetic polymers. The aqueous protein solution is dispersed in a lipophilic organic
continuous phase. This protein solution may contain the active constituents. The continuous
phase is generally consisted of the polymer solution that eventually encapsulates of the
protein contained in dispersed aqueous phase. The primary emulsion is subjected then to the
homogenization or the sonication before addition to the aqueous solution of the poly vinyl
alcohol (PVA). This results in the formation of a double emulsion. The emulsion is then
subjected to solvent removal either by solvent evaporation or by solvent extraction a number
of hydrophilic drugs like leutinizing hormone releasing hormone (LH-RH) agonist, vaccines,
proteins/peptides and conventional molecules are successfully incorporated into the
microspheres using the method of double emulsion solvent evaporation/ extraction.
Fig 4: Processing scheme for microspheres-preparation by double emulsion technique
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3. Polymerization techniques
The polymerization techniques conventionally used for the preparation of the microspheres
are mainly classified as:
I. Normal polymerization
II. Interfacial polymerization. Both are carried out in liquid phase.
Normal polymerization
It is carried out using different techniques as bulk, suspension, precipitation, emulsion and
micellar polymerization processes. In bulk, a monomer or a mixture of monomers along with
the initiator or catalyst is usually heated to initiate polymerization. Polymer so obtained may
be moulded as microspheres. Drug loading may be done during the process of
polymerization.
Suspension polymerization also referred as bead or pearl polymerization. Here it is carried
out by heating the monomer or mixture of monomers as droplets dispersion in a continuous
aqueous phase. The droplets may also contain an initiator and other additives. Emulsion
polymerization differs from suspension polymerization as due to the presence initiator in the
aqueous phase, which later on diffuses to the surface of micelles. Bulk polymerization has an
advantage of formation of pure polymers.
Interfacial polymerization
It involves the reaction of various monomers at the interface between the two immiscible
liquid phases to form a film of polymer that essentially envelops the dispersed phase.
Fig 5: Polymerization method
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4. Phase separation coacervation technique
This process is based on the principle of decreasing the solubility of the polymer in organic
phase to affect the formation of polymer rich phase called the coacervates. In this method, the
drug particles are dispersed in a solution of the polymer and an incompatible polymer is
added to the system which makes first polymer to phase separate and engulf the drug
particles. Addition of non-solvent results in the solidification of polymer. Poly lactic acid
(PLA) microspheres have been prepared by this method by using butadiene as incompatible
polymer. The process variables are very important since the rate of achieving the coacervates
determines the distribution of the polymer film, the particle size and agglomeration of the
formed particles. The agglomeration must be avoided by stirring the suspension using a
suitable speed stirrer since as the process of microspheres formation begins the formed
polymerize globules start to stick and form the agglomerates. Therefore the process variables
are critical as they control the kinetic of the formed particles since there is no defined state of
equilibrium attainment.
5. Spray drying and spray congealing
These methods are based on the drying of the mist of the polymer and drug in the air.
Depending upon the removal of the solvent or cooling of the solution, the two processes are
named spray drying and spray congealing respectively. The polymer is first dissolved in a
suitable volatile organic solvent such as dichloromethane, acetone, etc. The drug in the solid
form is then dispersed in the polymer solution under high speed homogenization. This
dispersion is then atomized in a stream of hot air. The atomization leads to the formation of
the small droplets or the fine mist from which the solvent evaporates instantaneously leading
the formation of the microspheres in a size range 1-100µm. Microparticles are separated from
the hot air by means of the cyclone separator while the traces of solvent are removed by
vacuum drying. One of the major advantages of the process is feasibility of operation under
aseptic conditions. The spray drying process is used to encapsulate various penicillins.
Thiamine mononitrate and sulpha ethylthiadizole are encapsulated in a mixture of mono- and
diglycerides of stearic acid and palmitic acid using spray congealing. Very rapid solvent
evaporation, however leads to the formation of porous microparticles.
6. Solvent extraction
Solvent evaporation method is used for the preparation of microparticles, involves removal of
the organic phase by extraction of the organic solvent. The method involves water miscible
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organic solvents such as isopropanol. Organic phase is removed by extraction with water.
This process decreases the hardening time for the microspheres. One variation of the process
involves direct addition of the drug or protein to polymer organic solution. The rate of solvent
removal by extraction method depends on the temperature of water, ratio of emulsion volume
to the water and the solubility profile of the polymer.[3]
Fig 6. Spray drying method for preparation of microsphere.
Solvent Evaporation
The processes are carried out in a liquid manufacturing vehicle. The microcapsule coating is
dispersed in a volatile solvent which is immiscible with the liquid manufacturing vehicle
phase. A core material to be microencapsulated is dissolved or dispersed in the coating
polymer solution. With agitation the core material mixture is dispersed in the liquid
manufacturing vehicle phase to obtain the appropriate size microcapsule. The mixture is then
heated if necessary to evaporate the solvent for the polymer of the core material is disperse in
the polymer solution, polymer shrinks around the core. If the core material is dissolved in the
coating polymer solution, matrix – type microcapsules are formed. The solvent Evaporation
technique is shown in Figure-7. The core materials may be either water soluble or water
insoluble materials. Solvent evaporation involves the formation of an emulsion between
polymer solution and an immiscible continuous phase whether aqueous (o/w) or non-
aqueous.
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Fig 7. Solvent evaporation method for preparation of microsphere.
Wet Inversion Technique
Chitosan solution in acetic acid was dropped into an aqueous solution of counter ion sodium
tripolyposphate through a nozzle. Microspheres formed were allowed to stand for 1hr and
cross linked with 5% ethylene glycol diglysidyl ether. Microspheres were then washed and
freeze dried. Changing the pH of the coagulation medium could modify the pore structure of
CS microspheres.
Complex Coacervation
CS microparticles can also prepare by complex coacervation, Sodium alginate, sodium CMC
and sodium polyacrylic acid can be used for complex coacervation with CS to form
microspheres. These microparticles are formed by interionic interaction between oppositely
charged polymers solutions and KCl & CaCl2 solutions. The obtained capsules were
hardened in the counter ion solution before washing and drying.
Hot Melt Microencapsulation
The polymer is first melted and then mixed with solid particles of the drug that have been
sieved to less than 50µm. The mixture is suspended in a non-miscible solvent (like silicone
oil), continuously stirred, and heated to 5°C above the melting point of the polymer. Once the
emulsion is stabilized, it is cooled until the polymer particles solidify. The resulting
microspheres are washed by decantation with petroleum ether. The primary objective for
developing this method is to develop a microencapsulation process suitable for the water
labile polymers, e.g. polyanhydrides. Microspheres with diameter of 1-1000µm can be
obtained and the size distribution can be easily controlled by altering the stirring rate. The
only disadvantage of this method is moderate temperature to which the drug is exposed.[6]
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CHARACTERIZATION OF MICROSPHERES
Particle size, Shape and Morphology
All the microspheres were evaluated with respect to their size and shape using optical
microscope fitted with an ocular micrometer and a stage micrometer. The particle diameters
of more than 100 microspheres were measured randomly by optical microscope. Scanning
Electron photomicrographs of drug‐loaded microspheres were taken. A small amount of
microspheres was spread on gold stub. Afterwards, the stub containing the sample was placed
in the Scanning electron microscopy (SEM). A Scanning electron photomicrograph was taken
at an acceleration voltage of 20KV.[6]
The most widely used procedures to visualize microparticles are conventional light
microscopy (LM) and scanning electron microscopy (SEM). Both can be used to determine
the shape and outer structure ofmicroparticles.
LM provides a control over coating parameters in case of double walled microspheres. The
microspheres structures can be visualized before and after coating and the change can be
measured microscopically.
SEM provides higher resolution in contrast to the LM. SEM allows investigations of the
microspheres surfaces and after particles are cross-sectioned, it can also be used for the
investigation of double walled systems. Conflocal fluorescence microscopy is used for the
structure characterization of multiple walled microspheres.
Laser light scattering and multi size coulter counter other than instrumental methods, which
can be used for the characterization of size, shape and morphology of the microspheres.[3]
Swelling Index
Swelling index was determined by measuring the extent of swelling of microspheres in the
given buffer. To ensure the complete equilibrium, exactly weighed amount of microspheres
were allowed to swell in given buffer. The excess surface adhered liquid drops were removed
by blotting and the swollen microspheres were weighed by using microbalance. The hydrogel
microspheres then dried in an oven at 60° for 5 h until there was no change in the dried mass
of sample. The swelling index of the microsphere was calculated by using the formula.[6]
Swelling index= (mass of swollen microspheres - mass of dry microspheres/mass of dried
microspheres) 100.
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Entrapment Efficiency
The capture efficiency of the microspheres or the percent entrapment can be determined by
allowing washed microspheres to lyse. The lysate is then subjected to the determination of
active constituents as per monograph requirement. The percent encapsulation efficiency is
calculated using following equation :
% Entrapment = Actual content/Theoretical content x 100.
In Vitro wash-off test
A 1 cm x 1 cm piece of rat stomach mucosa was tied onto a glass slide (3 inch x 1 inch) using
a thread. Microsphere was spread onto the wet, rinsed, tissue specimen and the prepared slide
was hung onto one of the groves of the USP tablet disintegrating test apparatus. The
disintegrating test apparatus was operated such that that the tissue specimen regular up and
down movements in a beaker containing the simulated gastric fluid. At the end of every time
interval, the number of microsphere still adhering on to the tissue was counted and there
adhesive strength was determined.
In Vitro drug release
To carry out In Vitro drug release, accurately weighed 50 mg of loaded microspheres were
dispersed in dissolution fluid in a beaker and maintained at 37±2° C under continuous stirring
at 100 rpm. At selected time intervals 5 ml samples were withdrawn through a hypodermic
syringe fitted with a 0.4µm Millipore filter and replaced with the same volume of pre-warmed
fresh buffer solution to maintain a constant volume of the receptor compartment. The samples
were analyzed spectrophotometrically. The released drug content was determined from the
standard calibration curve of given drug.
In Vitro diffusion studies
In Vitro diffusion studies were performed using in vitro nasal diffusion cell. The receptor
chamber was filled with buffer maintained at 37±2°C. Accurately weighed microspheres
equivalent to 10 mg were spread on sheep nasal mucosa. At selected time intervals 0.5 ml of
diffusion samples were withdrawn through a hypodermic syringe and replaced with the same
volume of pre-warmed fresh buffer solution to maintain a constant volume of the receptor
compartment. The samples were analyzed spectrophotometrically.
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Drug polymer interaction (FTIR) study
IR spectroscopy can be performed by Fourier transformed infrared spectrophotometer. The
pellets of drug and potassium bromide were prepared by compressing the powders at 20 psi
for 10 min on KBr‐press and the spectra were scanned in the wave number range of 4000-
600 cm-1. FTIR study was carried on pure drug, physical mixture, formulations and empty
microspheres.
Stability studies of Microsphere
The preparation was divided into 3 sets and was stored at 4°C (refrigerator), room
temperature and 40°C (thermostatic oven). After 15, 30 and 60 days drug content of all the
formulation was determined spectrophotometrically.[6]
CONCLUSION
The conclusion of this review is that, this review basically impact on the microspheres, its
types, method of preparation and its characterization which is very useful for future studies
and research for advancement of medicinal field. It has been observed that microspheres are
better choice of drug delivery system than many other types of drug delivery system because
it is having the advantage of target specificity and better patient compliance. In future various
other strategies microsphere will find the central place in novel drug delivery, particularly in
diseased cell sorting, diagnostic, gene and genetic materials, safe, targeted and effective in
vivo delivery and supplements as miniature version of diseased organ tissues in the body.
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