Mucoadhesion

The contact stage is the initial wetting that occurs between the adhesive and membrane. This can occur mechanically by bringing together the two surfaces, or through the bodily systems, like when particles are deposited in the nasal cavity by inhalation. The principles of initial adsorption of small molecule adsorbates can be described by DLVO theory.[1]

According to DLVO theory, particles are held in suspension by a balance of attractive and repulsive forces. This theory can be applied to the adsorption of small molecules like mucoadhesive polymers, on surfaces, like mucus layers. Particles in general experience attractive van der Waals forces that promote coagulation; in the context of adsorption, the particle and mucus layers are naturally attracted. The attractive forces between particles increases with decreasing particle size due to increasing surface-area-to-volume ratio. This increases the strength of van der Waals interactions, so smaller particles should be easier to adsorb onto mucous membranes.[1]

DLVO theory also explains some of the challenges in establishing contact between particles and mucus layers in mucoadhesion due to their repulsive forces. Surfaces will develop an electrical double layer if they are in a solution containing ions, as is the case with many bodily systems, creating electrostatic repulsive forces between the adhesive and surface. Steric effects can also hinder particle adsorption to surfaces. Entropy or disorder of a system will decrease as polymeric mucoadhesives adsorb to surfaces, which makes establishing contact between the adhesive and membrane more difficult. Adhesives with large surface groups will also experience a decrease in entropy as they approach the surface, creating repulsion.[1]

The initial adsorption of the molecule adhesive will also depend on the wetting between the adhesive and membrane. This can be described using Young's equation:

${\displaystyle {\cos(\theta )\;=\;{\frac {\gamma _{mg}\;-\gamma _{bm}\;}{\gamma _{bg}}}}}$

where ${\displaystyle \gamma _{mg}}$ is the interfacial tension between the membrane and gas or bodily environment, ${\displaystyle \gamma _{bm}}$ is the interfacial tension between the bioadhesive and membrane, ${\displaystyle \gamma _{bg}}$ is the interfacial tension between the bioadhesive and bodily environment, and ${\displaystyle \theta }$ is the contact angle of the bioadhesive on the membrane. The ideal contact angle is 0° meaning the bioadhesive perfectly wets the membrane and good contact is achieved. The interfacial tensions can be measured using common experimental techniques such as a Wilhelmy plate or the Du Noüy ring method to predict if the adhesive will make good contact with the membrane.[11]

Mode of action of mucoadhesion. Dry mucus will not adhere to a mucoadhesive, but in the presence of moisture the mucus becomes plastic and can form intermolecular bonds.

The consolidation stage of mucoadhesion involves the establishment of adhesive interactions to reinforce strong or prolonged adhesion. When moisture is present, mucoadhesive materials become activated and the system becomes plasticized.[10] This stimulus allows the mucoadhesive molecules to separate and break free while proceeding to link up by weak van der Waals and hydrogen bonds.[10] Consolidation factors are essential for the surface when exposed to significant dislodging stresses.[1] Multiple mucoadhesion theories exist that explain the consolidation stage, the main two which focus on macromolecular interpenetration and dehydration.

Interpenetration of a bioadhesive with mucus. In the contact stage, the two materials are brought into contact. In the consolidation stage the interpenetration of the polymers occurs.

The Macromolecular Interpenetration theory, also known as the diffusion theory, states that the mucoadhesive molecules and mucus glycoproteins mutually interact by means of interpenetration of their chains and the forming of secondary semi-permanent adhesive bonds.[10] It is necessary that the mucoadhesive device has features or properties that favor both chemical and mechanical interactions for the macromolecular interpenetration theory to take place.[10] Molecules that can present mucoadhesive properties are molecules with hydrogen bond building groups, high molecular weight, flexible chains, and surface active properties.[10]

It is perceived that increase in adhesion force is associated with the degree of penetration of polymer chains.[10] Literature states that the degree of penetration required for efficient bioadhesive bonds lies in the range of 0.2-0.5μm.[10] The following equation can be used to estimate the degree of penetration of polymer and mucus chains:

${\displaystyle {l={(t\times D_{b}})^{1/2}}}$

with ${\displaystyle t}$ as contact time and ${\displaystyle D_{b}}$ as the diffusion coefficient of the mucoadhesive material in the mucus.[10] Maximum adhesion strength is reached when penetration depth is approximately equal to polymer chain size.[10] Properties of mutual solubility and structural similarity will improve the mucoadhesive bond.[1]

The dehydration theory explains why mucoadhesion can arise rapidly. When two gels capable of rapid gelation in an aqueous environment are brought into contact, movement occurs between the two gels until a state of equilibrium is reached.[1] Gels associated with a strong affinity for water will have high osmotic pressures and large swelling forces.[1] The difference in osmotic pressure when these gels contact mucus gels will draw water into the formulation and quickly dehydrate the mucus gel, forcing intermixing and consolidation until equilibrium results.[12]

This mixture of formulation and mucus can increase contact time with the mucous membrane, leading to the consolidation of the adhesive bond.[12] However, the dehydration theory does not apply to solid formulations or highly hydrated forms.[1]

Tablets are small, solid dosages suitable for the use of mucoadhesive coatings. The coating may be formulated to adhere to a specific mucosa, enabling both systemic and targeted local administration. Tablets are generally taken enterally, as the size and stiffness of the form results in poor patient compliance when administered through other routes.[10]

In general, patches consist of three separate layers that contribute and control the release of medicine. The outer impermeable backing layer controls the direction of release and reduces drug loss away from the site of contact. It also protects the other layers and acts as a mechanical support. The middle reservoir layer holds the drug and is tailored to provide the specified dosage. The final inner layer consists of the mucoadhesive, allowing the patch to adhere to the specified mucosa.[10]

As a liquid or semisolid dosage, gels are typically used where a solid form would affect the patient’s comfort. As a trade-off, conventional gels have poor retention rates. This results in unpredictable losses of the drug, as the non-solid dosage is unable to maintain its position at the site of administration. Mucoadhesives increase retention by dynamically increasing the viscosity of the gel after application. This allows the gel to effectively administer the drug at the local site while maintaining the comfort of the patient.[10]

These dosage forms are commonly used to deliver drugs to the eye and nasal cavity. They often include mucoadhesive polymers to improve retention on dynamic mucosal surfaces. Some advanced eye drop formulations may also turn from a liquid to a gel (so called in situ gelling systems) upon drug administration. For example, gel-forming solutions containing Pluronics could be used to improve the efficiency of eye drops and provide better retention on ocular surfaces.[14]

With a 0.1-0.7 mm thick mucus layer, the oral cavity serves as an important route of administration for mucoadhesive dosages. Permeation sites can be separated into two groups: sublingual and buccal, in which the former is much more permeable than the latter. However, the sublingual mucosa also produces more saliva, resulting in relatively low retention rates. Thus, sublingual mucosa is preferable for rapid onset and short duration treatments, while the buccal mucosa is more appropriate for longer dosage and onset times. Because of this dichotomy, the oral cavity is suitable for both local and systemic administration. Some common dosage forms for the oral cavity include gels, ointments, patches, and tablets. Depending on the dosage form, some drug loss can occur due to swallowing of saliva. This can be minimized by layering the side of the dosage facing the oral cavity with an impermeable coating(,) commonly seen in patches.[15]

With an active surface area of 160 cm², the nasal cavity is another noteworthy route of mucoadhesive administration. Due to the sweeping motion of the cilia that lines the mucosa, nasal mucus has a quick turnover of 10 to 15 minutes. Because of this, the nasal cavity is most suitable for rapid, local medicinal dosages. Additionally, its close proximity to the blood–brain barrier makes it a convenient route for administering specialized drugs to the central nervous system. Gels, solutions, and aerosols are common dosage forms in the nasal cavity. However, recent research into particles and microspheres have shown increased bioavailability over non-solid forms of medicine largely due to the use of mucoadhesives.[16]

Within the eye, it is difficult to achieve therapeutic concentrations through systemic administration. Often, other parts of the body will reach toxic levels of the medication before the eye reaches the treatment concentration. Consequently, direct administration through the fibrous tunic is common. This is made difficult due to the numerous defense mechanisms in place, such as blinking, tear production, and the tightness of the corneal epithelium. Estimates put tear turnover rates at 5 minutes, meaning most conventional drugs are not retained for long periods of time. Mucoadhesives increase retention rates, either by enhancing the viscosity or bonding directly to one of the mucosae surrounding the eye.[15][17]

Intravesical drug administration is the delivery of pharmaceuticals to the urinary bladder through a catheter.[18] This route of administration is used for the therapy of bladder cancer and interstitial cystitis. The retention of dosage forms in the bladder is relatively poor, which is related to the need for a periodical urine voiding. Some mucoadhesive materials are able to stick to mucosal lining in the bladder, resist urine wash out effects and provide a sustained drug delivery.[19][20]