A Review on Recent Development Techniques Involved in Bioadhesive Drug Delivery Systems

The oral route of drug delivery is often the preferred choice for both patients and healthcare professionals. However, our current understanding of how drugs are absorbed and metabolized reveals that many medications struggle to be effectively delivered this way. This is primarily due to significant pre-systemic clearance in the liver after administration, which often results in a weak correlation between membrane permeability, absorption, and bioavailability (1). Challenges with parenteral delivery and limited effectiveness of oral drugs have led to interest in exploring alternative drug delivery methods. This has prompted consideration of other mucosal surfaces for potential drug administration. Transmucosal delivery methods—such as through the mucosal linings of the nasal, rectal, vaginal, ocular, and oral cavities—offer significant benefits over traditional oral administration for achieving systemic effects. Among these routes, the buccal mucosa stands out due to its easy accessibility, expansive smooth muscle, and comparatively stable structure, making it ideal for controlled release formulations. Furthermore, patients tend to favor buccal delivery more than other non-oral transmucosal options. By providing direct access to systemic circulation via the internal jugular vein, this route also avoids acid hydrolysis in the gastrointestinal tract and bypasses first-pass metabolism in the liver, resulting in enhanced bioavailability. Additionally, the buccal mucosa has swift cellular recovery rate, adding to its advantages[2]. However, this route does have its drawbacks, including the relatively low permeability of the buccal membrane[3].Especially when compared to the sublingual membrane[4],and a smaller surface area. The total absorptive surface area available in the oral cav Innovations in Bioadhesive Drug Delivery Systems: Products and Clinical Trials

• Cutting-edge Formulation Strategies for Oral Mucoadhesive Drug Delivery Systems
• Development of bioadhesive formulations for peptide delivery via nasal routes
• Creation of bioadhesive buccal patches
• Utilization of hyaluronic acid formulations for vaginal delivery of calcitonin
• Ocular drug delivery systems featuring bioadhesive properties
• Topical dosage forms utilizing bioadhesive preparations

Innovative Approaches and Concepts for Bioadhesive Drug Delivery Systems

• Versatile Polymers for Oral Administration of Peptide Medications
• Chitosan and Its Derivatives as Enhancers for Peptide Drug Absorption Through Mucosal Surfaces.
• Utilization of Plant Lectins for Targeted Oral Drug Delivery in the Gastrointestinal Tract
• Bacterial Invasion Factors and Lectins as Advanced Bioadhesive Agents
• Cutting-edge PEG-Infused Acrylate Copolymers Featuring Enhanced Mucoadhesive Qualities
• Bioadhesive and Bioerodible Polymers Aimed at Boosting Intestinal Drug Uptake [8,9].

Mechanisms of Bioadhesion:

The processes that lead to the establishment of bio adhesive bonds are not completely understood, but existing studies generally outline it as a three-step procedure.

• Step 1: Wetting and Swelling of the Polymer.
• Step 2: Interpenetration of Polymer Chains with the Mucosal Membrane.
• Step 3: Creation of Chemical Bonds Between Intertwined Chains.

Step 1:

In the initial stage, the wetting and swelling of the polymer occur as it spreads across the surface of the biological substrate or mucosal membrane, creating close contact with the substrate. This can be easily accomplished, for instance, by applying a bio adhesive formulation, like a tablet or paste, inside the oral cavity or vagina. Bio adhesives can adhere to biological tissues aided by the surface tension and forces present at the site of contact. The swelling of polymers happens because the components within the polymers are attracted to water.ity measures approximately 170 cm²[5], with about 50 cm² comprised of non-keratinized tissues, including the buccal membrane[6]. Continuous saliva production (0.5–2 l/day) can dilute the medication administered. Swallowing saliva may also result in the loss of the dissolved or suspended drug, possibly leading to the unintended elimination of the dosage form. There are several challenges with buccal drug delivery, including the risk of choking due to accidentally swallowing the delivery system. Moreover, using such a dosage form can be inconvenient during meals or drinks. Still, the benefits and recent advancements in the delivery of various compounds make the drawbacks of this route less critical, positioning buccal adhesive drug delivery systems as a promising area for further research [7].

Types of bio adhesion can be distinguished as

• Adhesion of a normal cell on another normal cell.
• Adhesion of a cell with a foreign substance.
• Adhesion of normal cell to a pathological cell.
• Adhesion of an adhesive to a biological substrate.

Classification of Bioadhesive:

Bioadhesive can be categorized into three types based on their observed characteristics.

• Type I: This type involves adhesion between biological entities without any artificial materials. Examples include phenomena like cell fusion and cell aggregation.
• Type II: In this category, bioadhesion refers to the adherence of cells to culture dishes or various substances, which can range from metals to wood and other synthetic materials.
• Type III: This type pertains to the adhesion of artificial substrates, such as the bonding of polymers to skin or other soft tissues

Step 2:

The surfaces of mucosal membranes consist of high molecular weight polymers called glycoproteins. During this second phase of bio adhesive bond formation, the chains of bio adhesive polymers weave together and become intertwined with the mucosal polymer chains, creating semi-permeable adhesive bonds. The robustness of these bonds is influenced by how well the two polymer groups interpenetrate each other. For strong adhesive bonds to form, one type of polymer must be soluble in the other, and both should possess similar chemical structures.

Step 3:

In this phase, weak chemical bonds develop between the intertwined polymer chains. These bonds include primary bonds, such as covalent bonds, as well as weaker secondary interactions like van der Waals forces and hydrogen bonds. Both primary and secondary bonds are utilized in the production of bio adhesive formulations, ensuring strong adhesion between the polymers[10,11,12,13].

Steps in Mucoadhesion :[14,15,16]

Despite the considerable research conducted in this area, the exact mechanisms behind mucoadhesion remain somewhat ambiguous. Nonetheless, it is generally accepted that the process of mucoadhesion occurs in two distinct phases.

The Contact Stage:

This phase involves the close interaction between the mucoadhesive material and the mucous membrane. At first, the mucoadhesive comes into close proximity with the mucous membrane. The gastrointestinal tract poses a challenge as it is a difficult mucosal surface to access. This limitation means that the adhesive cannot be directly applied to the intended mucosal area or delivered

through the organ's natural structure. Typically, adhesion might lead to complications, including potential blockages within the gastrointestinal tract. For larger particles, the natural movement of the digestive system, such as peristalsis, may assist in bringing the dosage form into contact with the mucosa. However, successful adhesion of larger dosage forms is not frequently documented in the literature, aside from the concerning instances of adhesion in the esophagus. In contrast, for smaller particles within a suspension, adhering to the gastrointestinal mucosa becomes a critical first step in the adhesion process. The physicochemical interactions occurring in this context can be explained through DLVO Theory. When a particle moves toward a surface, it will encounter both repulsive and attractive forces. The repulsive forces are primarily due to osmotic pressure created by the overlapping of electrical double layers, as well as steric effects. Electrostatic interactions occur when both the surface and particle have the same charge. Conversely, attractive forces arise from van der Waals interactions, surface energy effects, and electrostatic forces when the surface and particles possess opposite charges. The balance between these forces depends on factors such as the particle's characteristics, the surrounding aqueous environment, and the distance separating the particle from the surface. For instance, smaller particles exhibit a higher surface-area-to-volume ratio, which enhances their attractive forces. Particles can become stably positioned at a secondary minimum, approximately 10 nm apart, where the attractive and repulsive forces are in equilibrium, allowing for easy dislodgment. However, for stronger adhesion to happen, particles must surpass a repulsive barrier—known as the potential energy barrier—to reach the primary minimum. The scenario becomes more complex when considering that the adhesion surface is a mucus gel instead of a solid material, and particles in vivo may be hydrated or coated with biomolecules, significantly altering their physicochemical properties. Still, if the forces leading to displacement are minimal, mucoadhesive forces can effectively counteract them. An examination of the mucus structure and associated liquid layers suggests the presence of an unstirred layer at the top, functioning like a solid component, supporting the previously mentioned hypothesis.

The consolidation stages :

It involve various physicochemical interactions that enhance and fortify the adhesive connection, leading to improved adhesion duration. To achieve strong and lasting adhesion, introducing an additional ‘consolidation’ stage is suggested. For enhanced adhesion, a modification in the physical properties of the mucus layer is crucial; otherwise, it may fail to maintain its bond with the bioadhesive polymer when subjected to dislodging forces .Two theories aim to explain this phenomenon. The first theory focuses on intermolecular interactions, suggesting that mucoadhesive molecules intertwine and create bonds through secondary interactions with mucus glycoproteins. The second theory, known as the dehydration theory, posits that when a material capable of quick gelation meets another gel, water will migrate between them until they reach equilibrium. This latter theory clarifies that mucoadhesion can happen within seconds, whereas the first theory implies that polymers must interpenetrate several micrometers quickly. Rheological studies indicate that the interpenetration of mucus and the mucoadhesive polymer generates a surface gel layer, significantly limiting any further interpenetration (Dodou, Breedveld, and Wieringa, 2005).

Factors affecting muco/Bio adhesion:

The Bio adhesive power of polymer is affected by the nature of polymer and also by the nature of surrounding media.[17,18] They are.

1. Polymer related factors
   • Molecular weight
   • Concentration of active polymer
   • Flexibility of polymer chains
   • Special conformation
   • Swelling
2. Environmental related factors
   • PH of polymer substrate interface
   • Applied strength
   • Initial contact time
   • Degree of hydration
3. Physiological factors
   • Mucin turnover
   • Disease state
4. Polymer-related considerations
   • • • Molecular Weight: Various studies have shown that there is an optimal molecular weight at which bioadhesion reaches its peak. For low molecular weight polymers, the interpenetration of polymer molecules is beneficial, while higher molecular weight polymers benefit from larger structures.

• Concentration of active polymer: Polymer concentration refers to the amount of polymer present in a solution or melt, typically ranging from 1 wt% to 40–50 wt%, which significantly influences the electrospinnability, fiber formation, and resulting fiber diameter in nanofiber production. An optimized concentration is crucial for achieving uniform, smooth, and cylindrical nanofibers.
• Flexibility of Polymer Chains: As water-soluble polymers undergo cross-linking, the movement of individual polymer chains becomes restricted. With an increase in cross-linking density, the effective length of the chains capable of penetrating the mucus layer diminishes further, leading to a decrease in mucoadhesive strength.
• Spatial Conformation: In addition to molecular weight and chain length, the spatial arrangement of a polymer is crucial. For instance, the helical structure of dextran can obscure many of the adhesive functional groups that play a key role in adhesion, unlike PEG polymers, which possess a linear structure.
• Swelling: Swelling in polymers is a complex phenomenon influenced by several factors, including the chemical structure and composition of the polymer, environmental conditions, and interactions with solvents or other substances.

Factors Relating to the Environment

1. pH of polymer substrate interface : Research indicates that pH plays a crucial role in mucoadhesion, particularly in studies involving polyacrylic polymers that are cross-linked with - COOH groups. The pH affects the surface charge of both the mucus and the polymers. The charge density of mucus varies depending on the pH, which is influenced by the dissociation of functional groups within the carbohydrate elements and the amino acids in the polypeptide structure. Optimal adhesion occurred at pH levels of 5 and 6, while the lowest adhesion was recorded at pH 7.
2. Applied Strength: For a solid bioadhesive system to function effectively, it’s important to apply a specific level of strength. The adhesion strength increases with both the amount of applied force and the duration of its application, up to a certain point. When high pressure is exerted for a sufficiently extended time, polymers can become mucoadhesive, even if they lack favorable interactions with mucin.
3. Initial contact time: The initial contact time between mucoadhesives and the mucus layer determines the extent of swelling and the interpenetrations of polymer chains. Along with the initial pressure, the initial contact time can dramatically effect the performance of a system. The mucoadhesive strength increases as the initial contact time increases.
4. Degree of hydration: Depending on the degree of hydration adhesion properties will be different. It is maximum at a certain degree of hydration. When the degree of hydration is high, adhesiveness is lost probably due to formation of slippery, non-adhesive mucilage in an environment of large amount of water at or near the interface.

Physiological factors:

• Mucin turnover: The natural turnover of the mucin molecules from the mucus layer is important for at least two reasons-
• The mucin turn over is expected to limit the residence time of mucoadhesive dosage form on the mucus layer.
• Mucin turnover results in substantial amount of soluble mucin molecules. These mucin molecules interact with mucoadhesive before they have a chance to interact with the mucus layer.

Disease state:

The physiological properties of the mucus are known to change during disease conditions such as the common cold, gastric ulcers etc. The exact structural changes taking place in mucus under these conditions are not yet clearly understood.

Theories Of Mucoadhesion:

Several theories have been proposed to account for various experimental observations during the bioadhesion process. However, each theoretical model is only capable of elucidating a limited subset of the wide range of interactions that form the bioadhesive bond (Longer and Robinson, 1986). Still, five primary theories can be identified.[19,20,21]

• Wettability theory
• Electronic theory
• Fracture theory
• Adsorption theory
• Diffusion theory

Wettability Theory:

This theory is applicable to liquid or low viscosity mucoadhesive systems. It fundamentally assesses the “spreadability” of the bioadhesive polymer on mucus. The theory suggests that the adhesive component penetrates surface irregularities, solidifies, and anchors itself to the surface. Key features of bioadhesive materials include a zero or nearly zero contact angle, relatively low viscosity, and close contact that prevents air entrapment. Consequently, the interfacial energies play a crucial role in the interaction of the two surfaces and in determining the adhesive strength, it can be inferred that mucoadhesive polymer systems sharing structural and functional similarities with the mucin layer will display enhanced miscibility, leading to improved spreading across the mucosal surface. A lower water content in the polymer will promote better hydration, resulting in more intimate contact. Conversely, hydrophilic polymers with higher water content will have a lower contact angle, which may hinder effective contact .

The Electronic Theory:

In this theory adhesion occurs through electron transfer between mucus and the mucoadhesive system, stemming from the disparities in their electronic structures. This transfer results in the creation of a double layer of electric charges at the interface between mucus and the mucoadhesive material. Consequently, attractive forces emerge within this double layer. However, there is some debate surrounding the acceptance of this theory, as it attributes bond adhesion to electrostatic forces that are generally considered weaker .

Fracture theory :

This theory analysis the force required to separate two surfaces after adhesion. The maximuim tensile stess produced during detachment can be determined by dividing the maximium force detachment(Fm) by total surface area (A0) involved in the adhesive interaction.

Sm =Fm/A0

In a uniform single component system, fracture strength (Sf), which is equal to maximium stress of detachment (Sm)is proportional to fracture energy (gc), youngs modulus of elasticity (E) and the critical crack length (C) of fracture site as described.

S gc E/C)

Fracture enery gc can be obtained from sum of reversible work of adhesion. Gc = Wr + Wi

Where, Wr = energy required to produce new fracture surface ; Wi = plastic deformation at the tip of growing.

The elastic modules of system (E) is stress(s) and strain(c) through Hook’s law. E= [ζ/ε]ε—›0 =[F/A0/δl/t0].

Adsorption Theory:

This theory posits that the bio-adhesion bond formed between an adhesive surface and tissue, as represented by a mucus layer, results from van der Waals interactions, hydrogen bonds, and similar forces. While these individual forces may be weak, the sheer volume of their interactions collectively creates a strong adhesive strength.

Diffusion Theory:

The idea behind diffusion theory revolves around the interactions between adhesive polymer chains and mucus polymer chains. These interactions create semi-permeable adhesive bonds. It is thought that the strength of these bonds grows stronger with a greater degree of interpenetration between the polymer chains and the mucus layer. The level of penetration of the polymer chains into the mucus network—and vice versa—is influenced by concentration gradients and diffusion coefficients. While the specific parameters for creating a strong bio-adhesive bond have yet to be precisely defined, it is generally accepted that this interaction occurs within a range of 0.2 to 0.5 µm.

The penetration of depth (l) = (t-Db)1/2

where, t = time of contact; Db= diffusion coefficient of the bio adhesive material in mucus.

The ideal mucoadhesive polymer should have the following attributes:[22]

1. It must create strong non-covalent bonds with mucin-epithelial surfaces.
2. The polymer should adhere quickly to various tissues and demonstrate specificity for the intended site.
3. It should facilitate the easy incorporation of the drug and allow for its release at the appropriate time.
4. The polymer must not irritate the mucous membrane.
5. It should be non-immunogenic.
6. The polymer and its degradation products should not be absorbed by the gastrointestinal tract or, if absorption occurs, they should not be harmful to the host.
7. Additionally, the polymer should have cohesiveness to provide structural strength within the interlayer.

Types of Bio adhesive Formulations:[23,24]

1. Solid Bioadhesive Formulations: Below are some examples of these formulations.
   • Tablets: Dry formulations like tablets create strong interactions with mucosal surfaces by drawing water from them.
   • Inserts: This category includes ocular inserts, such as eye drops and gels.
   • Lozenges: Bioadhesive lozenges containing antibiotics and local anesthetics can be applied topically to address oral conditions.
2. Semi-solid Bioadhesive Formulations:
   • Gels: Bioadhesive polymers, like polyacrylic acid, form gels that stick effectively to mucosal surfaces when cross-linked.
   • Films: Flexible bioadhesive films can be utilized for direct drug delivery to specific mucosal membranes.
3. Liquid Bioadhesive Formulations:
   • Viscous Liquids: Bioadhesive polymers, such as carboxymethyl cellulose, are used in viscous liquids to shield mucosal membranes from damage and irritation.
   • Gel-forming Liquids: While these formulations start as liquids, they transition in response to conditions like temperature and pH, allowing for controlled drug release into the eye.

Polymers Utilized for Mucoadhesive Drug Delivery: [25,26,27]

1. PAA Derivatives: Carbomer and Carbopol, Noveon (Polycarbophil) These polymers are derived from acrylic acid and are cross-linked with polyalkenyl ethers or divinyl glycol.

They are manufactured from primary polymer particles that measure approximately 2-6 microns in diameter. Each primary particle forms a network of polymer chains interconnected by cross-links. Carbopol polymers, along with Pemulen and Noveon, are all cross-linked and can absorb water, swelling up to 1000 times their original volume to create a gel when in contact with a pH range of 4.0 to 6.0. The glass transition temperature for these polymers is around 105°C, and they possess carboxylate groups with a pKa ranging from 6.0 to 0.5 .The interaction between negative charges results in swelling, which subsequently enhances the mucoadhesive capabilities of the polymer.

Nowadays, numerous companies are adopting carbopol polymers due to a variety of advantages:

1. • Excellent tableting formulation and flowability
   • Prolonged drug release profiles
   • Ability to achieve drug release profiles similar to carbopol 971 NF, while offering improved handling properties
   • Safe and effective for oral use
   • Bioadhesive, contributing to enhanced bioavailability
   • Recognized and approved by various global Pharmacopoeias
   • Protective qualities that guard proteins and peptides against degradation, thus improving the bioavailability of protein- or peptide-based formulations.
2. Chitosan: [28,29]

Chitosan is a cationic polymer (polysaccharide) that is derived from chitin (Jian Hwa et al., 2003). Its significance is growing in the field of mucoadhesive drug delivery systems, thanks to its excellent biocompatibility, biodegradability, and non-toxic characteristics. This polymer adheres to the mucosa through ionic interactions between its amino groups and sialic acid residues. The linear structure of chitosan enhances the flexibility of the polymer chains. Research by Onishi and Machida has demonstrated that both chitosan and its metabolic derivatives are rapidly cleared from the body by the kidneys.

3. Advanced second-generation polymers: [29,30] These offer several benefits:
   • They are more site-specific, earning them the designation of cytoadhesives
   • They are minimally impacted by the rates of mucus turnover.
   • Targeted drug delivery to specific sites is achievable.
4. Lectins

Lectins are proteins that occur naturally and play a vital role in the biological recognition of cells and proteins. They comprise a diverse group of proteins and glycoproteins that can reversibly bind to specific carbohydrate residues . Once bound to a cell, lectins may stay on the cell surface or be internalized through endocytosis. This feature allows for targeted and controlled drug delivery. While lectins offer several advantages, they also come with the drawback of potential immunogenicity.

5. Thiolated Polymers [31,32]

These are thiomers that originate from hydrophilic polymers like polyacrylates, chitosan, or deacetylated gellan gum. The thiol group enhances residency time by facilitating covalent bonding with cysteine residues in mucus. Additionally, the presence of disulfide bonds may modify the drug release mechanism from the delivery system, as they contribute to increased rigidity and cross-linking (Clark et al., 2000).

For example:

• Chitosan iminothiolane
• PAA homocysteine
• PAA cysteine
• Alginate cysteine

7. Polyox WSRA

Polyox WSRA is a type of high molecular weight polyethylene oxide homopolymer that possesses several key characteristics :

• • It is a water soluble
  • It has a hydrophilic nature.
  • Possesses a high molecular weight.
  • Contains functional groups that facilitate hydrogen bonding.
  • Biocompatible and free from toxicity.
  • Suitable for formulation into tablets, films, gels, microcapsules, and syrups.

8. Novel Polymers [33,34]

• Research by Bottenberg indicated that tomato lectin has a specific affinity for the small intestine epithelium.
• Shajaei and Li developed and characterized a copolymer of PAA and PEG monoethylether monomethacrylate (PAA-co-PEG), which displays optimal adhesion properties suitable for buccal applications .
• Investigations by Leleetal focused on innovative polymers of PAA that are complexed with a PEGylated drug conjugate .
• Corium Technologies has introduced a new category of hydrophilic pressure-sensitive adhesives (PSA). These complexes were created through non-covalent hydrogen bonding, linking a film-forming hydrophilic polymer with a short-chain plasticizer featuring reactive hydroxyl groups at the ends of the chains.

Methods Of Evaluation

Mucoadhesive polymers can be evaluated by testing their adhesion strength by both in vitro and in vivo tests.[36,37,38,39]

In Vitro Methods The importance is laid on the elucidation of the exact mechanisms of bioadhesion. These methods are

• Methods determining tensile strength
• Methods determining shear stress
• Adhesion weight method
• Fluorescent probe method
• Flow channel method
• Mechanical spectroscopic method
• Falling liquid film method
• Colloidal gold staining method
• Viscometer method
• Thumb method
• Adhesion number
• Electrical conductance
• Swelling properties
• In vitro drug release studies
• Muco retentability studies

In Vivo Methods

• Use of radioisotopes
• Use of gamma scintigraphy
• Use of pharmacoscintigraphy
• Use of electron paramagnetic resonance (EPR) oximetry
• X ray studies
• Isolated loop technique

Methods determining tensile strength: Tensile strength is a measure of a material's ability to withstand the maximum longitudinal stress it can endure before failing. It is calculated by taking the highest load applied during a tensile test and dividing it by the initial cross-sectional area of the sample, resulting in units of Newtons per square meter.

Essentially, the tensile strength represents the greatest level of tensile stress a material can encounter before it breaks. The point at which failure occurs can differ based on the type of material and its intended design.

There are three common definitions of tensile strength:

• Yield Strength — The stress level at which a material begins to deform permanently.
• Ultimate Strength — The highest stress that a material can tolerate.
• Breaking Strength — The stress value at the moment of rupture as indicated on the stress-strain curve.
• Methods determining shear stress[40]: Shear stress, τ is the force acting tangentially to a surface divided by the area of the surface. It is the force per unit area required to sustain a constant rate of fluid movement. Mathematically, shear stress can be defined as:

• τ = F / A

Where, τ shear stress; F force; A area of the surface subjected to the force.

• Adhesion weight method[41] : Smart and Kellaway created a testing system in which a suspension of ion-exchange resin particles flowed over the inner mucosal surface of a segment of guinea pig intestine, allowing for the measurement of the adhered particle weight. Despite its limited effectiveness due to issues with data reproducibility—stemming from rapid tissue degeneration and biological variability—it was still possible to assess the impact of particle size and charge on adhesion after a five-minute contact period with the everted intestine.

• Fluorescent probe method: In this approach, the membrane lipid bilayer and the membrane proteins were tagged with pyrene and fluorescein isothiocyanate, respectively. The cells were then combined with the mucoadhesive agents, allowing us to observe changes in the fluorescence spectra. This provided a clear indication of the polymer's binding and its impact on polymer adhesion.
• Flow channel method : The research aimed to explore the structural requirements for bioadhesion with the goal of developing enhanced bioadhesive polymers suitable for oral applications. The lipid bilayer of the membrane and the associated membrane proteins were tagged with pyrene and fluorescence isothiocyanate, respectively. Subsequently, the cells were combined with potential bioadhesives, and the alterations in fluorescence spectra were tracked. This provided insights into the binding of the polymers and their effects on adhesion properties.
• Mechanical spectroscopic method :

• Can be used to investigate the interaction between the bioadhesive materials and mucin.
• Can be used to study the effect of pH and chain length.
• But this method shows a very poor correlation with in vivo bioadhesion.

• Falling liquid film method[42,43]: Teng and Ho developed a method that involved using excised segments and micro-sized particles. To prepare polymer-coated latex particles, a specific volume of a 1% polymer solution was mixed with cleaned latex particles (at a concentration of 5×10^8 particles/ml) in water, stirring for a minimum of 2 hours. To create polymer-coated particles in buffer solutions with varying ionic strengths, an aliquot of the polymer-coated particles in water was transferred directly into a beaker containing the desired buffer solution to give about 5×106 particles/ml. The suspension was subsequently sonicated for 30s and used 15 min later. The Tygon flute was supported by a platform composed of a plastic foam board. The angle of inclination was adjusted by a laboratory jack to 78®. The prepared intestinal segment mounted on the Tygon flute was perfused using a perfusion pump and a sample syringe for 10min with a buffer to remove any loosely held mucus. With the aid of the pump, a liquid film of the buffer solution was established on the intestinal segment. In the next 2min, sample of the eluent solution were collected and used as a control solution for the particle counting. They observed a constant sloughing of an extraneous substance, presumably mucus, with time. To test the adhesion of polymer coated particles to the intestinal surface by the perfusion of the particle suspension, 0.5 ml samples of the eluent particle solution were collected, and the number of particles remaining in the sample was coated using an electronic particle counter (Coulter).

• The fraction of particles adsorbed on the mucous layer (Fa) was measured using the following equation.
  • Fa = 1- Nl/No

Where, No and Nl are the particle concentration entering the intestinal segment from the dilute suspension reservoir and leaving the segment.