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Figure 12.12 A system for measurement of the force required to detach a solid dosage form from an isolated porcine oesophagus.

Reproduced from reference 9.

Figure 12.13 A device to measure the force of detachment of tablets or capsules from porcine oesophageal tissue, used to determine the values given in Fig. 12.14.

where k' is a constant. This equation applies only when the surfaces are pulled apart slowly.

Clearly the viscosity of the liquid is a determinant of the force of separation; it is less obvious why the thickness of the layer has the effect shown by the equation. The 'tackiness' of a system is not, however, simply related to viscosity.11 High molecular weight materials must be present in aqueous solutions at least to provide an elastic element to the viscous flow. Rubbery polymers which have partly liquid and partly elastic characteristics are employed as adhesives in surgical dressings and adhesive tapes. What factors influence the adhesivity of high molecular weight soluble polymers such as hydroxypropylmethylcellu-lose (HPMC) is still unclear. HPMC is a component of film coating materials. Figure 12.14 shows the detachment force required to separate tablets from a glass surface when coated with HPMC-606 from a 4% solution, shown here as a function of the thickness of the film coat.12 The effect of the presence of high concentrations of polyoxyethylene glycol 6000 is to reduce adhesivity, presumably because the glycol solution is less tacky.

The variables likely to affect the process of adhesion of coated tablets to mucosal surfaces are shown in diagrammatic form in Fig. 12.15: film coat thickness, the nature of the film coat, its rate of solution or hydration, the rhe-ology of the solution formed, its surface tension and its elongational characteristics. Eight stages have been identified. Stage I

Coat thickness (mm)

Figure 12.14 The effect of film thickness on the adhesiveness of tablets coated with HPMC-606 from a 4% solution (A) and in the presence of 4% PEG 6000 (- ▲ -) and 6% PEG 6000 (••• ▲ •••), i.e. 1:1 and 1:1.5 ratios of HPMC to PEG respectively.

Reproduced from reference 12 with permission.

Coat thickness (mm)

Figure 12.14 The effect of film thickness on the adhesiveness of tablets coated with HPMC-606 from a 4% solution (A) and in the presence of 4% PEG 6000 (- ▲ -) and 6% PEG 6000 (••• ▲ •••), i.e. 1:1 and 1:1.5 ratios of HPMC to PEG respectively.

Reproduced from reference 12 with permission.

(before contact occurs) defines the system and the variables of film coating thickness, the nature of the coating and the nature of the tissue surface. In stage II the presence of moisture or water is postulated, and here the wetting characteristics of the film will be important. In stage III the film coat begins to dissolve and we assume that contact is made, or is about to be made with the tissue; the forces bringing the tablet and the tissue together are not controllable because contact is made by collision and is dependent to some extent on the ratio of tablet size to oesophageal diameter. Stage IV represents

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