Adhesion duration

EN 302-7 (2004) [12] deals with the determination of the conventional adhesive duration of use. The viscosity of a specified volume of adhesive at 20°C is monitored using a Brookfield type viscometer until a viscosity exceeding 25 000 mPa s is found. 

For analytical purposes and an initial characterization, quick tests (duration minutes to few hours) are sufficient. However, the estimation of the usefulness as an industrial material needs long-term testing (months to years) in different environments (air, water, solvents, etc). The numerous other tests employed in engineering practice to determine mechanical (and other) properties, as well as the special methods for testing rubbers, films, fibers, foams, coatings, and adhesives, will not be dealt with here. 

Gliclazide is slowly absorbed. It is metabolized and excreted in the urine, in part as unchanged drug with an elimination half-life of 6-14 hours. Its duration of action is about 12 hours. Glicazide reduces platelet adhesiveness and increases fibrinolytic activity. This could be of importance as both factors have been implicated in the pathogenesis of the longterm organ failure in diabetes. 

Poor adhesion and skin irritation, with erythema and itching, are the principal drawbacks of transdermal therapy (225). Skin irritation can be overcome in some cases by changing the application site every day. In a direct comparison of transdermal patches, the duration, severity, and number of skin reactions depended on the individual formulation and excipients. 

Loffler, H., F. Dreher, and H.I. Maibach. 2004. Stratum corneum adhesive tape stripping Influence of anatomical site, application pressure, duration and removal. Br J Dermatol 151 746. 

Another design one could take for retention in the upper GI tract has been bioadhesive microparticles that stick to the mucus or the mucosa in the upper GI tract, particularly in the duodenum and jejunum.19-21 Charged polymers and even antibodies have demonstrated adhesion to the mucosa quite successfully in vitro,22,23 but these bioadhesives have been less successful in vivo owing to two physiological limitations. The turnover of mucus is rapid and limits the duration of adhesion.24 Moreover, approximately 2 percent of even the most bioadhesive microparticles with either specific antibody interactions or nonspecific interactions are retained along the stomach or intestinal wall (unpublished data by the authors). 

Mucus turnover rate. The turnover rate of the mucus layer is another physiological factor that affects mucoadhesion. Mucus turnover limits the potential duration of adhesion at the desired site of application. Within the GI tract, mucus is lost continuously secondary to enzymatic degradation (pepsin, lysosomal enzymes, and pancreatic enzymes), acid... 

The modulation frequency is typically in the range from 100 Hz to 3 kHz, and thus much lower than the resonance frequencies of the cantilever and the scanner. This enables better control of the forces exerted on the sample. The z-mod-ulation amplitude can be varied between 10 nm and 1 pm to ensure that that the tip is retracted from the surface. Shear forces are reduced permitting investigation of soft samples because of the small duration of the tip-surface contact, between 10 3 and 10 4 s. Pulse force mode SFM has been used to map adhesion of heterogeneous polymers in dependence of temperature and molecular weight as well as map electrostatic double-layer interactions [158-160]. 

When the experimentalist set an ambitious objective to evaluate micromechanical properties quantitatively, he will predictably encounter a few fundamental problems. At first, the continuum description which is usually used in contact mechanics might be not applicable for contact areas as small as 1 -10 nm [116,117]. Secondly, since most of the polymers demonstrate a combination of elastic and viscous behaviour, an appropriate model is required to derive the contact area and the stress field upon indentation a viscoelastic and adhesive sample [116,120]. In this case, the duration of the contact and the scanning rate are not unimportant parameters. Moreover, bending of the cantilever results in a complicated motion of the tip including compression, shear and friction effects [131,132]. Third, plastic or inelastic deformation has to be taken into account in data interpretation. Concerning experimental conditions, the most important is to perform a set of calibrations procedures which includes the (x,y,z) calibration of the piezoelectric transducers, the determination of the spring constants of the cantilever, and the evaluation of the tip shape. The experimentalist has to eliminate surface contamination s and be certain about the chemical composition of the tip and the sample. 

As stated above, the epidermis undergoes complete renewal every three weeks or so. This corresponds, therefore, to the shedding (or desquamation) of one layer of the stratum comeum per day. How does this affect drug bioavailability from a transdermal patch Probably not too much for those systems designed for 24 hours of wear, but potentially more significant as the duration of patch wear is extended, because of problems of adhesion. That is, after one day, a transdermal system is attached primarily to a layer of skin which under normal circumstances would have fallen off and, as time progresses, the situation is likely to deteriorate. 

Scopolamine was the first drug to be marketed as a transdermal delivery system (Transderm-Scop) to alleviate the discomfort of motion sickness. After oral administration, scopolamine has a short duration of action because of a high first-pass effect. In addition, several side-effects are associated with the peak plasma levels obtained. Transderm-Scop is a reservoir system that incorporates two types of release mechanims a rapid, short-term release of drag from the adhesive layer, superimposed on an essentially zero-order input profile metered by the microporous membrane separating the reservoir from the skin surface. The scopolamine patch is able to maintain plasma levels in the therapeutic window for extended periods of time, delivering 0.5 mg over 3 days with few of the side-effects associated with (for example) oral administration. 

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