Mucoadhesion
[1] Mucous membranes adhere to epithelial surfaces such as the gastrointestinal tract (GI-tract), the vagina, the lung, the eye, etc.
[1] Understanding the hydrophilic bonding and adhesion mechanisms of mucus to biological material is of utmost importance in order to produce the most efficient applications.
The adhesive components in a liquid solution anchor themselves in irregularities on the substrate and eventually harden, providing sites on which to adhere.
[7] Recently several new classes of polymers have been developed that are capable of forming covalent bonds with mucosal surfaces similarly to thiomers.
[10] The secondary bonds include weak Van Der Waals forces, and interactions between hydrophobic substructure.
[11] The penetration rate is determined by the diffusion coefficient, the degree of flexibility of the adsorbate chains, mobility and contact time.
[10] The diffusion mechanism itself is affected by the length of the molecular chains being implanted and cross-linking density, and is driven by a concentration gradient.
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.
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.
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.
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.
[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.
[1] Multiple mucoadhesion theories exist that explain the consolidation stage, the main two which focus on macromolecular interpenetration and dehydration.
[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] Maximum adhesion strength is reached when penetration depth is approximately equal to polymer chain size.
[12] This mixture of formulation and mucus can increase contact time with the mucous membrane, leading to the consolidation of the adhesive bond.
These polymeric coatings may be applied to a wide variety of liquid and solid dosages, each specially suited for the route of administration.
Tablets are generally taken enterally, as the size and stiffness of the form results in poor patient compliance when administered through other routes.
The outer impermeable backing layer controls the direction of release and reduces drug loss away from the site of contact.
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.
Due to the sweeping motion of the cilia that lines the mucosa, nasal mucus has a quick turnover of 10 to 15 minutes.
Additionally, its close proximity to the blood–brain barrier makes it a convenient route for administering specialized drugs to the central nervous system.
However, recent research into particles and microspheres have shown increased bioavailability over non-solid forms of medicine largely due to the use of mucoadhesives.
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.
Some mucoadhesive materials are able to stick to mucosal lining in the bladder, resist urine wash out effects and provide a sustained drug delivery.
