Penetration experiments, method

SFE can be carried out in three different ways. In a static extraction (no flow-rate), the extraction vessel is pressurised to the desired pressure with the extracting fluid and then simply left for a certain length of time. The main benefit of this method is that the fluid has time to penetrate the matrix. It is most applicable when the analyte has a high affinity for the solvent and a low affinity for the matrix and also when the solubility limit of the analyte in the fluid is much higher than the actual level reached during the extraction [89]. This method was popular in early SFE experiments but has declined in favour of dynamic SFE. Here, fresh SCF is continuously passed over the sample, extracting soluble compounds and depositing them in a suitable solvent or on a solid trap. The dynamic mode is particularly useful when the concentration of the solute... 

To summarize this section, the electrochemical hydrogenation experiments performed to date have yielded substantial penetration of low levels of hydrogen, but show promise for practical utilization despite temperature limitations and complications such as material removal. As with plasma and ion beam methods, the surface of this subject has barely been scratched. 

Given the overwhelming influence of the physical properties of skin in determining bioavailabilities via the dermal route, assessment of dermal penetration is one area in metabolism and toxicology where in vitro methods can be effectively used to predict in vivo results and to screen chemicals. Apparatus and equipment exist that one can use to maintain sections of skin (obtained from euthanized animals or from human cadavers or surgical discard) for such experiments (Holland et al., 1984). These apparatus are set up to maintain the metabolic integrity of the skin sample between two reservoirs the one on the stratum comeum side, called the application reservoir and the one on the subcutaneous side, called the receptor reservoir. One simply places radiolabeled test material in the application reservoir and collects samples from the receptor fluid at various time points. 

As described previously in this chapter, efforts have been made to develop methods for quantification of skin permeability, validation of diffusion cell setups, and correlation of in vitro data with the in vivo situation. However, the average drug permeation experiment does not provide insight into the temporal and local disposition within the tissue, that is, the skin penetration. The following discussion will give an overview of methods tackling this kind of problem. 

Typical data for the application of this method are shown in Fig. 16. They refer to standard permeation experiments in which the upstream (at X = 0) and downstream (at X = 1) surfaces of the membrane are maintained at constant penetrant activities... 

Example 11.4. McGuiggan et al. [492] measured the friction on mica surfaces coated with thin films of either perfluoropolyether (PFPE) or polydimethylsiloxane (PDMS) using three different methods The surface forces apparatus (radius of curvature of the contacting bodies R 1 cm) friction force microscopy with a sharp AFM tip (R 20 nm) and friction force microscopy with a colloidal probe (R 15 nm). In the surface force apparatus, friction coefficients of the two materials differed by a factor of 100 whereas for the AFM silicon nitride tip, the friction coefficient for both materials was the same. When the colloidal probe technique was used, the friction coefficients differed by a factor of 4. This can be explained by the fact that, in friction force experiments, the contact pressures are much higher. This leads to a complete penetration of the AFM tip through the lubrication layer, rendering the lubricants ineffective. In the case of the colloidal probe the contact pressure is reduced and the lubrication layer cannot be displaced completely. 

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