Bioadhesion theory

The different theories that have been proposed to explain the bioadhesion phenomenon are hereafter summarized. The design of bioadhesive drug delivery systems should take into account the mechanisms on which the bioadhesion phenomenon is based. An insight into bioadhesion theories could help formulators to design bioadhesive drug delivery systems with optimal performances. 

The electronic theory is based on the assumption that the mucoadhesive hydrogel and the target biological tissue have different electronic structures. When two materials come into contact with each other, electron transfer occurs, causing the formation of a double layer of electrical charge at the bioadhesive-biological interface. The bioadhesive force is believed to be due to attractive forces across this electrical double layer. 

The adsorption theory states that the bioadhesive bond formed between an adhesive substrate and tissue or mucosae is due to van der Waals interactions, hydrogen bonds, and related forces. Alternatively, when mucus or saliva are interacting with a solid dosage form, the molecules of the liquid are adsorbed on the solid surface. This is an exothermic process. The free energy of adsorption is given by Eq. (1). 

The diffusion theory states that interpenetration and entanglement of polymer chains are additionally responsible for bioadhesion. The intimate contact of the two substrates is essential for diffusion to occur, that is, the driving force for the interdiffusion is the concentration gradient across the interface. The penetration of polymer chains into the mucus network, and vice versa, is dependent on concentration gradients and diffusion coefficients. It is believed that for an effective adhesion bond the interpenetration of the polymer chain should be in the range of 0.2-0.5 pm. It is possible to estimate the penetration depth (/) by Eq. (5),... 

Chickering, D.E., III, and E. Mathiowitz, eds. 1999. Definitions, mechanisms and theories of bioadhesion. In Bioadhesive drug delivery systems Fundamentals, novel approaches and development, 1. New York Marcel Dekker. 

The same theories relevant to adhesion, developed to explain and predict the performance of glues, adhesives, and paints, have also been applied to bioadhesive systems [44], These include the electronic, absorption, wetting, diffusion, and fracture theories. 

A confirmation of the soundness of electronic theory was derived from a recent study, performed by Bogotaj et al. [46], They measured the zeta potential of different polymer dispersions and mucosal homogenates and found a correlation between such a parameter and the force necessary to detach a polymer dispersion from the biological substrate. The adsorption theory states that the bioadhesive bond is due to van der Waals interactions, hydrogen bonds, and other related weak interactions [44],... 

The wetting theory is applicable to liquid bioadhesive systems. According to this theory, the ability of a bioadhesive material to spread and determine an intimate contact with the biological substrate plays a major role in bond formation [44], This theory uses interfacial tensions to predict spreading and, in turn, bioadhesion. In the past, the surface energy of both bioadhesive materials and tissues or mucus have been extensively studied to predict the bioadhesive performance [47-49]. 

Chickering, D. E., and Mathiowitz, E. Definitions, mechanisms, and theories of bioadhesion, in Bioadhesive Drug Delivery Systems Fundamentals, Novel Approaches, and Development, New York Marcel Dekker, 1999. 

The diffusion theory states that interpenetration and entanglement of polymer chains are additionally responsible for bioadhesion. The intimate contact of the two substrates is essential for diffusion to occur that is, the driving force for the interdiffusion is the... 

Mikos, S.A. Smart, J.D. Scaling concepts and molecular theories of synthetic polymers to glycoproteinic networks. In Bioadhesive Drug Delivery Systems Lenaerts, V., Gurney, R., Eds. CRC Press Boca Raton, FL, 1990 25. 

The mechanisms of bioadhesion have been reviewed by Kaelble and Moacanin [62] and by Peppas and Buri [84] and may be classified as chemical (electronic and adsorption theories) and physical (wetting, interpenetration and fracture theories). 

According to the very popular wetting theory a material (bioadhesive, phase b) displaces the surrounding liquid (e.g. the gastric content, phase g) and adheres spontaneously on the mucus (tissue, phase t) when the spreading coefficient, Ws(b), is positive, i.e. ... 

Sources Derjaguin, B.V. and Smilga, V.P., Adhesion Fundamentals and Practice, McLaren, London, U.K., 1%9 Chickering, D.E. and Mathiowitz, E., Definitions, mechanisms, and theories of bioadhesion, in Bioadhesive Drug Delivery Systems Fundamentals, Novel Approaches, and Development, Mathiowitz, E., Chickering, D.E., and Lehr, C.M., Eds., Marcel Dekker, Inc., New York, 1999. 

---