Hydrophilic over layer process

In the Z-type deposition film, however, the long spacing of 7.2 nm did not agree with the predicted value of 3.9 nm rather, it was the same value as that of the Y-type deposition film. This result demonstrates that the Z-type film does not possess the Z-type layer structure but the Y-type layer structure. It should be assumed that the molecules were turned over in the deposition process and formed the Y-type layer structure, since the Z-type layer structure in which a hydrophilic group touches on a hydrophobic group is unstable. The conclusion from the examination of long spacings well supports molecular orientations in the LB films determined from the linear Stark effect measurements. From the linear Stark effect and the X-ray diffraction measurements, it is demonstrated that the hetero Y-type deposition method is useful for fabrication of stable noncentrosymmetric LB films. 

The layer of water adjacent to the absorptive membrane of the enterocyte is essentially unstirred. It can be visualized as a series of water lamellas, each progressively more stirred from the gut wall toward the lumen bulk. For BCS class 2 compounds the rate of permeation through the brush border is fast and the diffusion across the unstirred water layer (UWL) is the rate-limiting step in the permeation process. The thickness of the UWL in human jejunum was measured and found to be over 500 pm [3]. Owing to its thickness and hydrophilicity, the UWL may represent a major permeability barrier to the absorption of lipophilic compounds. The second mechanism by which the UWL functions act as a barrier to drug absorption is its effective surface area. The ratio of the surface area of the UWL to that of the underlying brush border membrane is at least 1 500 [4], i.e., this layer reduces the effective surface area available for the absorption of lipophilic compounds and hence impairs its bioavailability. 

Red cell-surface interactions may play a role in the dynamics of protein adsorption. We have been investigating the turnover of protein between solution and surface for several years (27-29), and have established that turnover occurs on a variety of surfaces. The rate and extent of turnover depend strongly on the surface character, with hydrophilic materials, for example, showing much more rapid turnover than hydrophobic materials. If red cells have the ability to strip protein off a biomaterial surface, then clearly this effect could influence the characteristics of the turnover process, particularly from a rate point of view. This process, in turn, could affect the development of the protein layer over a period of time. 

In general, the adsorbed coatings are relatively simple to apply and the process can be performed in commercial membrane elements. In addition, the type of coating can be tailored to the specific application of interest. However, despite the flexibility of the coating and the adsorption methods to change the hydrophilicity, smoothness, and charge of the membrane surface, their main drawback is the limited stability of the modified layer over time because of the possible desorption of the coated/ adsorbed polymers from the membrane surface into the bulk of the feed solutions. 

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