Mucoadhesive Microspheres: A Comprehensive Review of Design, Mechanisms, and Applications

Abstract

Mucoadhesive microspheres are an advanced drug delivery system developed to improve drug bioavailability and therapeutic effectiveness by attaching to mucosal surfaces. These microspheres increase the residence time of drugs at the site of action, provide controlled drug release, and allow targeted delivery to specific mucosal areas such as oral, nasal, ocular, and vaginal regions. This review presents a comprehensive overview of formulation methods, characterization techniques, and therapeutic applications of mucoadhesive microspheres. A detailed literature survey was carried out using scientific databases including PubMed, Scopus, and Web of Science. Research articles related to the development, evaluation, and clinical use of mucoadhesive microspheres were analyzed. The review discusses the selection of suitable polymers, preparation methods, and evaluation techniques used to study mucoadhesive properties, particle size, drug entrapment efficiency, and drug release behavior under in vitro and in vivo conditions. Both natural and synthetic polymers such as chitosan, alginate, and carbopol were identified as effective mucoadhesive materials. Various preparation techniques, including emulsification, spray drying, and solvent evaporation, were examined, emphasizing their influence on the physical and chemical characteristics of microspheres. Characterization tools such as scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and mucoadhesion testing methods were also described. Studies indicate that mucoadhesive microspheres enhance drug absorption and bioavailability by adhering to mucosal tissues and providing sustained drug release. Their therapeutic applications extend to several medical fields, including gastroenterology, pulmonology, and ophthalmology, showing improved patient compliance and treatment outcomes. In conclusion, mucoadhesive microspheres are a promising drug delivery approach capable of improving pharmaceutical therapy through prolonged adhesion, controlled drug release, and enhanced bioavailability. Future research should focus on optimizing formulation parameters, developing new mucoadhesive polymers, and conducting clinical studies to confirm their safety and effectiveness. These advancements have strong potential to improve drug delivery systems across various therapeutic areas.

Keywords

Mucoadhesive, Microspheres, Formulation, Characteristics, Mechanism, Applications

Introduction

Considerable research has been directed toward developing new dosage forms that can control drug release and deliver the active drug to a specific site in the body.¹ Microspheres are an innovative drug delivery system made from different types of polymers and have many pharmaceutical applications.² Microspheres, also known as microparticles, are small spherical particles with sizes typically ranging from 1 μm to 1000 μm.³ These microspheres can be prepared using either natural or synthetic polymers.?

The addition of mucoadhesive properties to microspheres further improves drug absorption and bioavailability, along with providing controlled and prolonged drug release.? Mucoadhesive microspheres enhance drug targeting by attaching to biological surfaces through interactions with bacterial adhesins, plant lectins, antibodies, and other molecules, thereby increasing close contact with the mucus layer.? These specially designed microspheres adhere to mucosal tissues present in areas such as the oral cavity, nasal passages, eyes, urinary tract, and gastrointestinal tract, enabling localized and controlled drug delivery.?

Advantages of Mucoadhesive Microspheres

The major advantages of mucoadhesive microspheres are as follows:

1. They provide a constant and prolonged therapeutic effect.
2. They reduce the frequency of drug administration, thereby improving patient compliance.
3. They enhance drug absorption, which increases bioavailability and reduces the risk of adverse effects.
4. The structure of microspheres allows controlled degradation and regulated drug release.
5. Their spherical shape enables predictable disintegration and controlled medication release.?

Limitations of Mucoadhesive Microspheres

Despite several advantages, mucoadhesive microspheres also have certain limitations:

1. The drug release profile may change during formulation or storage.
2. Drug release rate can be affected by factors such as diet, gastrointestinal transit time, and mucin turnover rate.
3. Drug release may vary between different doses.
4. Alteration of the release pattern may lead to toxic effects.
5. These dosage forms should not be crushed or chewed.?

Mucoadhesion

Bioadhesion refers to the attachment between two materials, where at least one surface is biological in nature. Mucoadhesion specifically describes the adhesion of polymers to the mucosal surface layer.¹?

Mucus Membranes

Mucus membranes line several body cavities, including the digestive and respiratory tracts. These moist surfaces secrete mucus through specialized cells known as goblet cells.

Mucus exists in three forms:

• A gel layer attached to the mucosal surface
• Suspended mucus
• Soluble mucus present in the lumen

Mucus mainly consists of mucin glycoproteins, water, lipids, and inorganic salts. It acts both as a lubricant and as a protective barrier against external agents.¹¹

Mechanism of Mucoadhesion

Mucoadhesion is the process by which a drug formulation and its carrier system attach to the mucosal surface. This process involves several steps, including wetting, adsorption, and interpenetration of polymer chains.¹²

Mechanisms Involved in Mucoadhesion

1. Wetting or Swelling: The mucoadhesive system comes into close contact with the mucosal surface and absorbs moisture, allowing swelling and intimate contact.
2. Interpenetration: Polymer chains penetrate into the mucosal surface or tissue, forming strong adhesive bonds.¹³

Figure 1 illustrates intranasal mucoadhesive microspheres.

Theories of Mucoadhesion

Several theories explain the mechanism of mucoadhesion:¹?

1. Electronic Theory: Adhesion occurs due to electron transfer between mucoadhesive material and mucosal membrane, forming an electrical double layer.
2. Wetting Theory: Adhesion depends on the ability of a liquid to spread over a surface; smaller contact angles indicate stronger adhesion.
3. Adsorption Theory: Adhesion results from intermolecular forces such as hydrogen bonding and Van der Waals forces between polymer and mucosal surface.
4. Diffusion Theory: Polymer chains diffuse into the mucus layer, forming an interpenetrating network that strengthens adhesion.
5. Mechanical Theory: Adhesion occurs when adhesive materials fill microscopic irregularities and cracks on the mucosal surface, creating mechanical interlocking.
6. Cohesive Theory: Mucoadhesion is attributed to internal cohesive forces within the adhesive material itself.

Factors Affecting Mucoadhesion

Several factors influence how effectively a drug delivery system adheres to mucosal membranes.

Polymer-Related Factors

These include:

• Molecular weight of the polymer
• Polymer concentration
• Stereochemistry of the polymer
• Polymer chain length
• Degree of hydration
• Physical properties of the polymer
• Swelling ability
• Applied force during contact
• Contact time with mucosa
• pH at the polymer–mucosal interface

Physiological Factors

• Mucin turnover rate
• Disease conditions affecting mucosal surfaces¹?

Materials Used in the Formulation of Mucoadhesive Microspheres

Mucoadhesive microspheres are prepared using mucoadhesive polymers, which may be natural or synthetic. These polymers attach to the mucin–epithelial surface and are broadly classified into three categories:

1. Water-activated sticky polymers
   These polymers become adhesive after absorbing water and show mucoadhesion due to their stickiness.
2. Polymers forming non-specific interactions
   Adhesion occurs mainly through electrostatic and other non-covalent interactions.
3. Polymers binding to specific receptors
   These polymers attach to specific receptor sites present on mucosal surfaces.¹⁶

Classification of Mucoadhesive Polymers

Table 1: Mucoadhesive Polymers of Synthetic and Natural Origin

Synthetic Polymers	Natural Polymers
Hydroxyethyl cellulose (HEC)	Sodium alginate
Hydroxypropyl methylcellulose (HPMC)	Tragacanth
Hydroxypropyl cellulose (HPC)	Gelatin
Polyhydroxyethyl methacrylate	Pectin
Methyl cellulose (MC)	Karaya gum
Sodium carboxymethyl cellulose (Na CMC)	Locust bean gum
Poly(acrylic acid) polymers (Carbomers, Polycarbophil)	Xanthan gum
Polyvinyl pyrrolidone (PVP)	Chitosan
Polyethylene oxide	Lecithin
Polyvinyl alcohol (PVA)	Guar gum
Ethyl cellulose (EC)	Soluble starch

Methods of Preparation of Mucoadhesive Microspheres

Mucoadhesive microspheres can be prepared using several techniques:

1. Complex Coacervation

This method is based on phase separation that occurs when two hydrophilic colloid solutions interact under suitable conditions.

• The coating polymer is dissolved in an appropriate solvent.
• The drug (core material) is added with continuous stirring.
• Microencapsulation occurs by changing temperature, pH, or by adding salts, nonsolvents, or incompatible polymers.
• Microspheres are hardened using desolvation or thermal cross-linking.¹?

2. Hot Melt Microencapsulation

• The polymer is first melted.
• Drug particles are mixed into the molten polymer.
• The mixture is dispersed into a non-miscible solvent (e.g., silicone oil) to form an emulsion.
• After cooling, hardened microspheres are filtered and washed.¹?

3. Single Emulsion Technique

Used mainly for natural polymers:

• Drug and polymer are dissolved or dispersed in an aqueous phase.
• This phase is added to an oil phase to form droplets.
• Cross-linking is achieved using heat or chemical agents such as glutaraldehyde or formaldehyde.¹?

4. Double Emulsion Method

A commonly used microencapsulation technique:

• A primary water-in-oil (W/O) emulsion is prepared.
• This emulsion is added to another aqueous phase containing an emulsifier (e.g., PVP or PVA).
• Evaporation of the organic solvent produces solid microspheres.
• Microspheres are washed and dried.²?

5. Solvent Removal Method

Suitable for water-sensitive polymers:

• Drug and polymer are dissolved in a volatile organic solvent.
• The solution is dispersed in silicone oil with stabilizers.
• Petroleum ether extracts the solvent.
• Microspheres are then dried under vacuum.²¹

6. Ionotropic Gelation

Developed using gel-forming polymers such as alginate:

• Polymer is dissolved in water and drug is added.
• The mixture is dropped into a calcium chloride solution.
• Calcium ions cause cross-linking, forming gelled microspheres.²²

7. Phase Inversion Method

• Drug is added to a polymer solution prepared in methylene chloride.
• The solution is poured into petroleum ether (a nonsolvent).
• Microspheres are formed, washed, and dried.²³

8. Spray Drying

• Drug is dissolved or dispersed in polymer solution.
• The solution is spray-dried to form microspheres.
• Particle size can be controlled by adjusting spray rate, feed rate, nozzle size, and drying temperature.²?

DRUG LOADING IN MICROSPHERES

Microspheres can be loaded with drugs mainly in two ways: during preparation or after preparation by incubating them in a drug solution. Various techniques are used for incorporating active pharmaceutical ingredients, including physical entrapment, chemical coupling, and surface adsorption.

Drug loading efficiency depends on several process parameters such as the presence of additives, preparation method, heat generated during polymerization, and agitation speed. Incorporating the drug during microsphere formation is often preferred to achieve maximum drug loading.

Alternatively, prepared microspheres can be incubated in a highly concentrated drug solution using a suitable solvent. In this method, drug loading occurs either by:

• adsorption onto the surface of microspheres, or
• penetration and diffusion of the drug through microsphere pores.

After loading, the solvent is removed to obtain drug-loaded microspheres.²?

DRUG RELEASE KINETICS

Drug release behavior is an important consideration in microsphere-based drug delivery systems. Drug release from microspheres may occur through several mechanisms:

• Release due to polymer erosion or degradation
• Diffusion of drug molecules through pores present in microspheres
• Drug release from the surface of the polymer matrix
• Pulsatile drug delivery induced by electromagnetic or oscillating fields²?

EVALUATION OF MUCOADHESIVE MICROSPHERES

Mucoadhesive microspheres are evaluated using the following parameters:

1. Particle Size and Shape

Particle size, shape, and surface characteristics of microspheres can be determined using:

• Light Microscopy (LM)
• Scanning Electron Microscopy (SEM)²?

2. Surface Characterization of Mucoadhesive Microspheres

Surface morphology and structural characteristics can be analyzed using:

• Scanning tunneling microscopy
• Scanning electron microscopy
• Electron microscopy

Morphological changes caused by polymer degradation are studied by incubating microspheres in phosphate buffer saline at different time intervals.

A rough microsphere surface enhances mucoadhesion through increased mechanical interaction, whereas a smooth surface generally shows poor mucoadhesive properties.²?

3. Surface Charge Study

Surface charge (zeta potential) of microspheres is determined using photon correlation spectroscopy. Electrophoretic mobility values are converted into zeta potential using software based on the Helmholtz–Smoluchowski equation.

Zeta potential helps predict:

• mucoadhesion behavior
• adhesive strength
• particle stability

The charge of mucoadhesive polymers significantly influences mucus–polymer interactions. Measuring the zeta potential of both microspheres and mucus helps predict electrostatic interactions during adhesion.²?

4. Entrapment Efficiency

Entrapment efficiency (% entrapment) is determined by lysing microspheres in a buffer solution. After filtration or centrifugation, the drug content in the lysate is analyzed according to monograph specifications.³?

Formula:

% Entrapment Efficiency=

Actual Drug Content X 100

                Theoretical Drug Content

5. Swelling Index

The swelling index indicates the ability of microspheres to absorb biological fluids and swell at the absorption site, which is essential for initiating mucoadhesion.³¹

Formula:

 

% Swelling=  D_T – D_O X 100

             D_O

Where:

• D_O? = Weight of dried microspheres
• D_T = Weight of swollen microspheres

6. In-Vitro Release Study

In-vitro drug release studies are performed using standard IP/BP/USP dissolution apparatus, such as rotating basket or paddle-type systems. The dissolution medium simulates physiological fluids present at the absorption site.³²

7. Ex-Vivo Mucoadhesion Study

Mucoadhesive properties are evaluated using goat intestinal mucosa in phosphate buffer solution. Tissue samples are rinsed, and microspheres are placed on slides mounted in a USP dissolution apparatus maintained at 37°C.

The amount of microspheres detached at different time intervals is measured to evaluate mucoadhesion strength.³³,³?

CONCLUSION

In recent years, significant attention has been directed toward advanced drug delivery systems in modern pharmaceutical formulations. Mucoadhesive microspheres offer promising advantages for controlled and sustained drug delivery because they prolong drug residence time at the target site, enhance drug absorption, and improve bioavailability.

Therefore, mucoadhesive microspheres are expected to play an important role in future pharmaceutical research and development, particularly with the advancement of novel materials and innovative drug delivery technologies.

REFERENCES

1. Smart JD. The basics and underlying mechanisms of mucoadhesion. Adv Drug Deliv Rev. 2005;57(11):1556–1568.
2. Lehr CM. Bioadhesion technologies for the delivery of peptide and protein drugs to the gastrointestinal tract. Crit Rev Ther Drug Carrier Syst. 1994;11(2–3):119–160.
3. Ahuja A, Khar RK, Ali J. Mucoadhesive drug delivery systems. Drug Dev Ind Pharm. 1997;23(5):489–515.
4. Reddy LH, Murthy RS. Floating dosage systems in drug delivery. Crit Rev Ther Drug Carrier Syst. 2002;19(6):553–585.
5. Salamat-Miller N, Chittchang M, Johnston TP. The use of mucoadhesive polymers in buccal drug delivery. Adv Drug Deliv Rev. 2005;57(11):1666–1691.
6. Ponchel G, Irache JM. Specific and non-specific bioadhesive particulate systems for oral delivery to the gastrointestinal tract. Adv Drug Deliv Rev. 1998;34(2–3):191–219.
7. Carvalho FC, Bruschi ML, Evangelista RC, Gremião MPD. Mucoadhesive drug delivery systems. Braz J Pharm Sci. 2010;46(1):1–17.
8. Harding SE, Davis SS, Deacon MP, Fiebrig I. Biopolymer mucoadhesives. Biotechnol Genet Eng Rev. 1999;16:41–86.
9. Peppas NA, Buri PA. Surface, interfacial and molecular aspects of polymer bioadhesion on soft tissues. J Control Release. 1985;2(4):257–275.
10. Andrews GP, Laverty TP, Jones DS. Mucoadhesive polymeric platforms for controlled drug delivery. Eur J Pharm Biopharm. 2009;71(3):505–518.
11. Woertz C, Preis M, Breitkreutz J, Kleinebudde P. Assessment of test methods evaluating mucoadhesive polymers and dosage forms: An overview. Eur J Pharm Biopharm. 2013;85(3):843–853.
12. Nagai T, Machida Y. Buccal delivery systems using hydrogels. Adv Drug Deliv Rev. 1993;11(1–2):179–191.
13. Boddupalli BM, Mohammed ZN, Nath RA, Banji D. Mucoadhesive drug delivery system: An overview. J Adv Pharm Technol Res. 2010;1(4):381–387.
14. Kharenko EA, Larionova NI, Demina NB. Mucoadhesive drug delivery systems (Review). Pharm Chem J. 2009;43:200–208.
15. Shaikh R, Singh TRR, Garland MJ, Woolfson AD, Donnelly RF. Mucoadhesive drug delivery systems. J Pharm Bioallied Sci. 2011;3(1):89–100.
16. Bernkop-Schnürch A, Dünnhaupt S. Chitosan-based drug delivery systems. Eur J Pharm Biopharm. 2012;81(3):463–469.
17. Zhang L, Zhu D, Dong X, Sun H, Song C, Wang C, et al. Mucoadhesive buccal films of tramadol for effective pain management. J Biomater Sci Polym Ed. 2012;23(6):817–828.
18. Yadav P, Harikumar SL, Kaur A. Mucoadhesive microspheres as carriers in drug delivery: A review. Int J Drug Dev Res. 2012;4(2):21–34.
19. Andrews GP, Jones DS, Srinivasan B, Gorman SP. Characterization of rheological, mucoadhesive, and drug release properties of fluconazole-loaded hydrogels. Pharm Dev Technol. 2008;13(1):53–62.
20. Roy S, Pal K, Anis A, Pramanik K, Prabhakar B. Polymers in mucoadhesive drug delivery system: A brief note. Des Monomers Polym. 2009;12(6):483–495.
21. Ko JA, Park HJ, Hwang SJ, Park JB, Lee JS. Preparation and characterization of chitosan microparticles intended for controlled drug delivery. Int J Pharm. 2002;249(1–2):165–174.
22. Vyas SP, Khar RK. Gastroretentive systems. In: Vyas SP, Khar RK, editors. Controlled Drug Delivery: Concepts and Advances. 1st ed. New Delhi: Vallabh Prakashan; 2002. p.196–217.
23. Muheem A, Shakeel F, Jahangir MA, Anwar M, Mallick N, Jain GK, et al. Strategies for oral delivery of proteins and peptides and their clinical perspectives. Saudi Pharm J. 2016;24(4):413–428.
24. Meshram PR, Gajare PS, Bhadane MA. Formulation and evaluation of sustained-release azithromycin microspheres by emulsion solvent evaporation technique. Int J Curr Res Rev. 2012;4(18):138–147.
25. Smart JD. Drug delivery using buccal-adhesive systems. Adv Drug Deliv Rev. 1993;11(1–2):253–270.
26. Bravo-Osuna I, Andrés-Guerrero V, Melgosa M, Herrero-Vanrell R. Study of chitosan derivatives as carriers for drug delivery to the posterior eye segment. Eur J Pharm Biopharm. 2012;80(2):305–315.
27. Khairnar A, Jain P, Baviskar D, Jain D. Development of mucoadhesive buccal patch containing aceclofenac: In vitro evaluation. Int J Pharm Tech Res. 2009;1(4):978–981.
28. Lehr CM, Bouwstra JA, Schacht EH, Junginger HE. In vitro evaluation of mucoadhesive properties of chitosan and natural polymers. Int J Pharm. 1992;78(1–3):43–48.
29. Peh KK, Wong CF. Polymeric films as vehicles for buccal delivery: Swelling, mechanical, and bioadhesive properties. J Pharm Pharm Sci. 1999;2(2):53–61.
30. Mucoadhesive microspheres for controlled delivery of drugs: A review. J Pharm Sci Innov. 2012;1(2):13–17.
31. Singh B, Sharma R, Garg V. Mucoadhesive microspheres as a promising novel drug delivery system. Int J Pharm Bio Sci. 2013;4(1):208–226.
32. Das MK, Senapati PC. Furosemide-loaded alginate microspheres prepared by ionic cross-linking technique: Morphology and release characteristics. Indian J Pharm Sci. 2008;70(1):77–84.
33. Jadhav KR, Shaikh IM, Ambade KW, Kadam VJ. Applications of microsphere-based drug delivery system: A review. Der Pharmacia Lettre. 2009;1(2):212–228.
34. Nayak BS, Nayak UK, Patro KB, Panda DS. Mucoadhesive microspheres for controlled drug delivery. Int J Pharm Sci Nanotechnol. 2008;1(1):21–26.