Mucin entanglements

for a 16-mer mucin chain, N and V would range from — 134 to —268 and from —0.076 to 012 pm3, respectively. It can be concluded that due to their long sizes and flexibility mucin chains tend to occupy large volumes of the solution in which they are diluted. 

Above certain concentration (the threshold concentration or c ), the polymer chains begin to overlap and form entanglements and eventually a heavily entangled network or gel (Fig. 2.2) (Doi and See 1995). The dimensionless 

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In general, both physical and weak chemical bonds are responsible for mucoadhesion. Physical/mechanical bond formation can be explained as the entanglement of the adhesive polymer and the extended mucin chains. When this diffusion is mutual, it leads to maximum bioadhesive strength. 

For bioadhesive applications, anionic polymers appear to provide the most effective balance between adhesiveness and toxicity, with carboxylic materials preferred over sulfonic polymers [400]. Polyfacrylic acid) microparticles have been identified as particularly effective bioadhesive materials [402]. Studies with poly(acrylic acid) microparticles have indicated that, while water-swollen particles exhibit good bioadhesion, dry polymer particles give no adhesion at all. In addition, adhesive strength increases as the degree of ionization of the polymer is increased [402]. Thus the expanded nature of the polymer network is important to mucoadhesion, probably via polymer interdiffusion and entanglement with mucin [403],... 

In an entangled polymer network the interchain length (i.e., mesh or pore size) can be assessed by determining the correlation length ( ) (Doi and See 1995 Graessley 2004), which for linear polymers with cylindrical Kuhn segments such as mucins can be approximated by the following expression ... 

Mucus possesses both solid-like (elastic) and liquid-like (viscous) attributes simultaneously and is therefore termed a viscoelastic gel. The viscoelastic properties arise from the non-covalent interactions (entanglements) between the predominantly anionic mucin molecules, although weak hydrogen-bonding and ligand-like attractions between protein regions of adjacent molecules may also play a role. Gel properties are affected by ... 

Interpenetration of the bioadhesive polymer chains and entanglement of polymer and mucin chains, and... 

Spatial configuration It favors change in entanglement and interaction after the polymer and mucin has interpenetrated. ... 

FIGURE 52.3 Representation of the cross-linked structure of the mucin network. (1) Possible cross-linking can occur through entanglements, (2) molecular associations, and (3) permanent covalent cross-links. On average, the end-to-end distance between two junctions, r, corresponds to a particular molecular weight. My (Reprinted from J. Contr. Release, 2, Peppas, N.A. and Buri, P.A., Surface, interfacial and molecular aspects of polymer bioadhesion on soft tissues, 257-275. Copyright 1985, with permission from Elsevier.)... 

Carbomer Carbomer is a viscosity enhancing polymer used in eye gels. Its activity is based on an interaction with mucins. The interaction is the most effective when flexible and mobile polymer chains entangle and interact with mucins present at the conjunctiva [20, 77]... 

When macromolecular hydrocolloids with hydrophilic functional groups are placed in aqueous media, they swell and expand into a gel or network. Interpenetration and entanglement of this network with the mucin substrate can be partly responsible for the mucoadhesive properties. Therefore, mobility and flexibility of the polymer chains is a prerequisite for strong mucoadhesion. 

There is plenty of experimental evidence to support that rheologic changes are observed when bioadhesive polymers and mucin are mixed. In this context it was shown that when a mucoadhesive polymer and mucin are mixed together there is a synergistic increase in viscosity. It is known that the viscosity of a mucin dispersion is the net result of the resistance to flow exerted by individual chain segments, physical chains entanglements and non-covalent molecular interactions, which are the same as the interactions involved in the process of mucoadhesion. 

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