1. INTRODUCTION:

Microspheres are generally characterized as a easily flowable powder, comprising polymers of synthetic as well as proteins which exhibit size between 1-1000μm. These are also known as microparticles. Microspheres are spherical in shape; thus, their therapeutic efficacy in containing drugs is based on their characteristics, which can alter the materials, polymers, methods, or techniques applied. These were applied in the system of delivering drug which were formulated for acquiring sustained and rather than controlled-release formulation for better systemic availability and stability as well as act to a targeted site with predetermined rate. Microsphere can be produced from different materials like polymer, glass as well as ceramic microspheres.

Microsphere consist of two categories such as microcapsules and micrometrics that are characterised as the encapsulated particles are disseminated across the system is “micrometrics” and microcapsule in which the trapped material is distinctly surrounded by a defined capsule membrane.^1-7

Figure 1: An illustration of a microsphere

Some benefits of microspheres are, it improves the ability of poorly-soluble drugs to dissolve, reducing particle size and improving surface area as well as the flow properties of powders. Microspheres ensure a consistent and prolonged therapeutic outcome by maintaining a steady drug concentration in the bloodstream thereby enhancing patient compliance. It diminishes the dosage and reduces toxicity levels. Microspheres prevent enzymatic and photolytic cleavage of the drug, making it the preferred choice for drug delivery. Improved API use was enhancing systemic availability as well as decreasing frequency and severity containing side effects. Preserves the gastrointestinal tract through the harmful irritations caused by the medication.^8-10

Table 1: List of microspheres containing marketed products with active drugs¹¹

S. No.	Active Drug	Brand Name	Technology used
1.	Somatropin	Nutropin® Depota	Phase Separation
2.	Minocycline	Arestin®	N/A
3.	Leuprolide	Lupron Depot®	Double emulsion (W/O/W)
4.	Risperidone	Risperdal®, Consta®	Double emulsion (O/W)
5.	Bromocriptine	ParlodelLar™	Spray dry
6.	Octreotide	Sandostatin® Lar	Alkermes ProLease® Technology (Cryogenic spray-drying)
7.	Naltrexone	Vivitrol®	Double emulsion (O/W)

1.1 Desirable characteristics of microspheres:

· Vulnerability to chemical modification.

· Stability of the formation post-synthesis among shelf life suitable for clinical application.

· Controlled release of the active ingredient, ensuring extended release of time.

· Capacity to hold relatively increased levels of drug.

· Optimized size of particle as well as dispersibility in water-based injections mixture.⁹

1.2 Classification of Microspheres:^11-15

a. Bio-adhesive type:

It refers to the phenomenon in which system of delivering drug adhere through the biological membrane, typically mediated by aqueous polymers with adhesive properties. When this interaction occurs on mucosal surfaces such as the nasal, buccal rectal and ocular membranes-it is specifically termed bioadhesion. They were designed to exhibit a prolonged action in the site of application. Enabling intimate contact with the absorption surface and thereby enhancing drug absorption and therapeutic efficacy.^16-17

Figure 2: Types of microspheres¹

b. Radio-active Type:

It was measuring between 10 and 30nm, due to their size exceeding that of capillaries, they typically get trapped in the first capillary network they reach. It is administered through the blood arteries, leading in the selected tumor site, enabling this type to provide a high-intensity radiation amount focused on the affected area, while reducing harm to the surrounding healthy tissues. It is distinct from conventional drug delivery systems because the radioactivity is not emitted from the microspheres; rather, it functions internally using a radiolabelled compound at the standard length. Various categories encompass α, β, γ emitters.^18-19

c. Floating Type:

The apparent density was less as compared to gastric juice, allowing them to remain persist in the gastric medium with lack of impacting the rate of gastric transit time. Release of API at a controlled rate, enabling sustained delivery, thereby increasing time of residence in gastric, causing variability in the amount of plasma. Furthermore, they minimize risk of dumping dose. This leads to an extended therapeutic effect which in turn decreases the frequency of dosing.^20-21

d. Magnetic Type:

This type plays a critical role by targeting the API directly at the disease site. A method has a smaller amount of magnetically targeted API which replaced a larger volume of circulating free API. Magnetic carriers react to magnetic field because of the materials incorporated in this type.¹⁹

e. Polymeric Type:

The different kinds of these type fall under the following categories.

· Polymeric biodegradable type Naturally occuring polymers like starch were employed due to their biodegradable, biocompatible and inherently Bioadhesive properties. These biodegradable polymers prolong the residence time upon contact with mucus membranes due to their swelling excessive capacity in water-based environments, resulting in formulation of semi-solid. The concentration of the polymer influences both the speed and degree of drug release, promoting a prolong release profile.

· Polymeric synthetic type These have been extensively employed in the medical field and have a variety of uses, such as medication administration fillers, bulking agents. Although microspheres like these have proven to be safe and biocompatible.^23-25

2. Formulation Techniques:

Various techniques are employed to formulate various microspheres types aimed at enhancing systemic availability.^26-29

Figure 3: Manufacturing methods of microspheres¹

2.1 Solvent Evaporation:

This process takes place in vehicles that composed of two phase such as organic as well as aqueous-phase, which is referred to as emulsification, specifically the oil phase is dispersed in water phase. Afterward a solvent evaporates, resulting in the formation of raw nanospheres of microspheres.³⁰

Figure 4: Solvent evaporation method

2.2 Spray Drying:

The spray drying process begins with dissolving the entire polymer in an appropriate organic volatile solvent e.g. acetone, dichlormethane. Next the powdered API is blended into the mixture of polymer through accelerated process of homogenization. The resulting mixture dispersed into the hot air stream, producing tiny particles. As the mixture of solvent rapidly vaporizes from these microsphere droplets are created. These microspheres are subsequently collected through the utilizing of cyclone separator, while any remaining solvents are removed through vacuum drying. One notable benefit of this technique is its capacity to function under aseptic conditions.^31-32

Figure 5: Spray drying method

2.3 Spray congealing and drying techniques:

A solubilized polymer in the appropriate organic volatile solvents like acetone as well as chloroform. This solution is then subjected to high-speed homogenization which disperses the polymer effectively. The resulting mixture are dispersed into the hot air stream, leading to the formation of tiny droplets that solidify into particles.³²

Figure 6: Spray congealing and drying method

2.4 Extraction method of solvent:

In this study, it is essential for both the polymer and the drug to dissolve in an organic solvent, resulting in a solution referred to as the aqueous phase. This solution is subsequently extracted using a water-miscible organic solvent to generate microspheres in an aqueous media.^30-31

Figure 7: Solvent extraction method

2.5 Phase separation coacervation method:

During this technique, an API that mixed with a solution of polymer, and is combined with an aqueous /organic solvent, leading to the creation of droplets or globules enriched with polymers. These droplets subsequently solidify in the aqueous or organic phase. Afterward, the microspheres are separated, washed, dried to yield pure microspheres^,
31-32

Figure 8: Phase separation method

2.6 Single emulsion technique:

In this method, an aq. Polymer solution is mixed into an organic phase like oil or chloroform, while being continuously stirred. This process is referred to as sonication. Following this, microsphere can be created using two approaches: chemical cross linking and heat denaturation. The resulting mixture is then centrifuged and washed, or it can be separated to obtain the microspheres.³¹

Figure 9: Single-phase emulsion method

2.7 Double-phase emulsion method:

During this process, an aq. mixture containing both the polymer and drug is mixed into an organic phase, leading to the formation of an emulsion. Following adding an aqueous PVA solution, a multi emulsion is created. The process concludes with separation, washing, and drying stages to yield microspheres.³²

Figure 10: Double emulsion method

2.8 Quassi solvent emulsion evaporation-diffusion method:

A novel process, that producing controlled drug release of microspheres utilizing acrylic polymers has been documented within the literature. This technique facilitates the formation of microsponges, where the outer phase comprises water (distilled) or PVA. An enclosing phase includes the polymer, ethanol and drug with a polymer amount carefully adjusted to improve plasticity. Initially, the internal phase is prepared at 60 ⁰C before being introduced into the outer phase in an environmental temperature. Once emulsion formation is occurred, blend is agitated consistently in two hours. Subsequently, the microsponges can be separated by filtration. Finally. the end, the obtained material is thoroughly washed and allow to dry in an oven at 40^oC.^30-34

Figure 11: Quassi emulsion solvent diffusion method

3. Evaluation Parameters of Microspheres:^35-39

3.1 Particle shape and size:

SEM (Scanning electron microscopy) and traditional LM (Light microscopy), primary choice for visualising microspheres. The form and external structure of these microspheres can be identified using either method. For double walled microspheres, coating parameters can be controlled using light microscopy (LM). Before and after coating, the microsphere formations may be seen, and a microscope can be used to measure the change. Higher resolutions is offered by SEM compared to LM.^40-44

3.2 % Yield:

The formed microspheres were then collected and measured to determine the % yield. The quantity of medication and excipients which was used to create the microspheres was split by the weight that that was measured.⁴⁵

Product weight

% Yield = ------------------------------------------- x 100

Excipient and medication weight

3.3 Determination of density:

A pycnometer is the tool utilizing for identifying the density of microspheres. A multi-volume pycnometer is then filled with a precisely measured sample within the cup. In a chamber, helium is supplied in the steady pressure as well as given time to expand. Thus, growth results reduction in the pressure inside the chamber. The two successive pressure drop values at various starting pressures are recorded. The amount of volume and, density of the microsphere’s carrier is calculated from two pressure readings.⁴⁶

Figure 12: A Pycnometer

3.4 Isoelectric pH:

Electrophoretic analysis employed for identifying elecrophoretic behaviour of the microspheres, which allowing the determination of their isoelectricpoint.⁴⁷

3.5 Electron spectroscopy pertaining to chemical analysis:

The chemical characteristics of microspheres surfaces can analyzed through this method.^48-50

3.6 Contact angle:

Contact angle of analysis is assessed for the evaluation of wetting properties of the microparticulate carrier.⁵¹

3.7 In-vitro release:

The release studies of various types of microspheres are performed using various suitable dissolution medium, mainly utilizing a rotating paddle apparatus(USP/BP).^50-51

3.8 The efficiency of drug binding effectiveness:

Binding effectiveness is evaluated by utilizing the subsequent formula^52-54

Observed weight

% Efficiency= --------------------------------------- x 100

Calculated weight

3.9 Ratio of swelling:

It is associated with microspheres, which can be determined by utilizing following equation.⁵⁶

Wt of the microsphers – wt of the dry microsphere

S.L. of Microspheres = ----------------------------------------

wt of the dehydrated microsphere

CONCLUSION:

Drug delivery methods based on microspheres show promise as an improvement over traditional single-unit dosage form. Their capacity to deliver continuous and regulated drug release improves site-specific targeting, stability, and bioavailability. Because of their versatility, microspheres can be designed to satisfy certain therapeutic requirements by utilizing a variety of polymers and manufacturing processes. All things considered, microspheres provide a flexible and effective method for modern medication administration techniques. They can be administered intravenously, orally, ocularly, or nasally, and among other ways.

Thus, the review was Concluded that Microsphere-based system of delivering drugs offers numerous advantages compared to conventional methods, including controlled and sustained release.

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Received on 21.04.2025 Revised on 03.06.2025

Accepted on 01.07.2025 Published on 25.07.2025

Available online from July 31, 2025

Res. J. Pharma. Dosage Forms and Tech.2025; 17(3):196-202.

DOI: 10.52711/0975-4377.2025.00028

This work is licensed under a Creative Commons Attribution-Non Commercial-Share Alike 4.0 International License. Creative Commons License.

S. No.	Advancement	Dosage form	Diseases	Examples
1.	Targeted Drug Delivery	Injectable Microspheres	Cancer (e.g. liver cancer)	Biodegradable microspheres used in targeted delivery for cancer treatment⁵⁵
2.	Sustained Release Systems	Oral or Injectable	Chronic diseases, Cancer	Polymer-based microspheres.^56-57
3.	Microsponge Technology	Topical Formulations	Skin disorders	Microsponge systems used in topical treatments for skin condition.⁵⁵
4.	Diagnostic Applications	Fluorescent Microspheres	Infectious disease	Fluroscent markers in biomedical research.⁵⁹
5.	Gene Delivery Systems	Injectable	Genetic disorder	Microspheres for gene therapy
6.	Bone Tissue	Injectable	Bone Defects	Polymeric microspheres in orthopaedic applications.
7.	Controlled release system	Injectable	Type-2 Diabetes	Exenatide PLGA^60-61
8.	Controlled release systems	Injectable	Hepatocellular carcinoma	DC Bead M1(TM)⁵⁸