# Understanding the Mucoadhesive Principle
Mucoadhesion is a fascinating principle that describes the attractive forces between a biological material and mucus or a mucous membrane¹. This principle is of utmost importance in various fields, especially in drug delivery systems¹.
## What is Mucoadhesion?
Mucoadhesion refers to the process where two surfaces, one of which is mucus or a mucous membrane and the other typically the surface of a drug delivery system, are held together for extended periods of time by interfacial forces². Mucous membranes adhere to epithelial surfaces such as the gastrointestinal tract (GI-tract), the vagina, the lung, the eye, etc¹. They are generally hydrophilic as they contain many hydrogen macromolecules due to the large amount of water (approximately 95%) within its composition¹.
## Why is Mucoadhesion Important?
Understanding the hydrophilic bonding and adhesion mechanisms of mucus to biological material is crucial to produce the most efficient applications. For instance, in drug delivery systems, the mucus layer must be penetrated in order to effectively transport micro- or nanosized drug particles into the body¹. Mucoadhesion has become of interest for its potential to optimize localized drug delivery, by retaining a dosage form at the site of action (e.g., the ocular surface or buccal mucosa), or systemic delivery, by retaining a formulation in intimate contact with the absorption site (e.g., within specific regions of the gastrointestinal tract)².
## Bonding Mechanisms in Mucoadhesion
Mucoadhesion involves several types of bonding mechanisms, and it is the interaction between each process that allows for the adhesive process. The major categories are wetting theory, adsorption theory, diffusion theory, electrostatic theory, and fracture theory¹.
### Wetting Theory
Wetting is the oldest and most prevalent theory of adhesion. The adhesive components in a liquid solution anchor themselves in irregularities on the substrate and eventually harden, providing sites on which to adhere¹. Surface tension effects restrict the movement of the adhesive along the surface of the substrate, and is related to the thermodynamic work of adhesion by Dupre’s Equation¹.
### Adsorption Theory
Adsorption is another widely accepted theory, where adhesion between the substrate and adhesive is due to primary and secondary bonding¹. The primary bonds are due to chemisorption, and result in comparatively long-lasting covalent and non-covalent bonds¹. Among covalent bonds, disulfide bonds are likely most important¹.
The mucoadhesive principle is a fascinating area of study with significant implications in the field of drug delivery. By understanding the various bonding mechanisms involved in mucoadhesion, we can develop more effective and efficient drug delivery systems¹.