Transport barrier

Diverse appHcations for the fabric sometimes demand specialized tests such as for moisture vapor, Hquid transport barrier to fluids, coefficient of friction, seam strength, resistance to sunlight, oxidation and burning, and/or comparative aesthetic properties. Most properties can be deterrnined using standardized test procedures which have been pubHshed as nonwoven standards by INDA (9). A comparison of typical physical properties for selected spunbonded products is shown in Table 2. 

With the adequacy of lipid bilayer membranes as models for the basic structural motif and hence for the ion transport barrier of biological membranes, studies of channel and carrier ion transport mechanisms across such membranes become of central relevance to transport across cell membranes. The fundamental principles derived from these studies, however, have generality beyond the specific model systems. As noted above and as will be treated below, it is found that selective transport... 

In the example above, the solutions are assumed to be well stirred and mixed the aqueous resistance is negligible, and the membrane is the only transport barrier. However, in any real case, the solutions on both sides of the membrane become less and less stirred as they approach the surface of the membrane. The aqueous diffusion resistance, therefore, very often needs to be considered. For example, for very highly permeable drugs, the resistance to absorption from the gastrointestinal tract is mainly aqueous diffusion. In the section, we give a general solution to steady diffusion across a membrane with aqueous diffusion resistance [5],... 

Diffusion provides an effective basis for net migration of solute molecules over the short distances encountered at cellular and subcellular levels. Since the diffu-sional flux is linearly related to the solute concentration gradient across a transport barrier [Eq. (5)], a mean diffusion time constant (reciprocal first-order rate constant) can be obtained as the ratio of the mean squared migration distance (L) to the effective diffusivity in the transport region of interest. 

The intent of this chapter is to establish a comprehensive framework in which the physicochemical properties of permeant molecules, hydrodynamic factors, and mass transport barrier properties of the transcellular and paracellular routes comprising the cell monolayer and the microporous filter support are quantitatively and mechanistically interrelated. We specifically define and quantify the biophysical properties of the paracellular route with the aid of selective hydrophilic permeants that vary in molecular size and charge (neutral, cationic, anionic, and zwitterionic). Further, the quantitative interrelationships of pH, pKa, partition... 

The effective permeability coefficient is composed of the permeability coefficients for the various transport barriers in series—the ABLs, the cell monolayer, and the filter support ... 

In Section III, emphasis was placed on flux kinetics across the cultured monolayer-filter support system where the passage of hydrophilic molecular species differing in molecular size and charge by the paracellular route was transmonolayer-controlled. In this situation, the mass transport barriers of the ABLs on the donor and receiver sides of the Transwell inserts were inconsequential, as evidenced by the lack of stirring effects on the flux kinetics. In this present section, the objective is to give quantitative insights into the permeability of the ABL as a function of hydrodynamic conditions imposed by stirring. The objective is accomplished with selected corticosteroid permeants which have been useful in rat intestinal absorption studies to demonstrate the interplay of membrane and ABL diffusional kinetics (Ho et al., 1977 Komiya et al., 1980). 

Cho MJ, DP Thompson, CT Cramer, T Vidmar, JF Scieszka. (1989). The Madin-Darby canine kidney (MDCK) epithelial cell monolayer as a model cellular transport barrier. Pharm Res 6 71-77. 

JN Cogburn, MG Donovan, CS Schasteen. A model of human small intestinal absorptive cells. 1. Transport barrier. Pharm Res 9 210-216, 1991. 

Rieder M, Schreiber L (1995) Waxes the transport barriers of plant cuticles. In Hamilton RJ (ed) Waxes chemistry, molecular biology and functions. The Oily Press,... 

From Fig. 19.3a-c, and as opposed to purely sorption controlled processes, it can be seen that during pervaporation both sorption and diffusion control the process performance because the membrane is a transport barrier. As a consequence, the flux 7i of solute i across the membrane is expressed as the product of both the sorption (partition) coefficient S, and the membrane diffusion coefficient Di, the so-called membrane permeability U, divided by the membrane thickness f and times the driving force, which maybe expressed as a gradient of partial pressures in place of chemical potentials [6] ... 

The success of cellular therapies ultimately depends on the stability of the hepatocyte in the architecture in which it must exist. Primary hepatocytes are anchorage dependent. Isolated cells rapidly lose viability when cultured in monolayers or suspensions. Investigators have developed culture models based on features of liver architecture to recapitulate the complex hepatocyte microenvironment. Sandwich culture mimics the environment of hepatocytes in vivo by entrapping cells between two layers of collagen gel. However, such methods introduce additional transport barriers and are difficult to scale up to therapeutic levels. ... 

Surface active substances (surfactants) are chemicals which accumulate at the water surface and reduce the air-water interfacial tension. The influence of such films on air-water exchange is twofold (1) they create an additional transport barrier, and (2) they change the hydrodynamics at the water surface such that the transport of solutes by eddies approaching the water surface is reduced (hydrodynamic damping). 

Given the complexity in molecular transport in tissues, understanding mechanisms of convection, diffusion, and binding in the interstitial space regardless of administration techniques may provide the means to overcome transport barriers for more uniform and adequate delivery of large therapeutic agents in solid tumors. 

As mentioned above, the rectal route is very attractive for systemic delivery of peptide and protein drugs, but rectal administration of peptides often results in very low bioavailability due to not only poor membrane penetration characteristics (transport barrier) but also due to hydrolysis of peptides by digestive enzymes of the GI tract (enzymatic barrier). Of these two barriers, the latter is of greater importance for certain unstable small peptides, as these peptides, unless they have been degraded by various proteases, can be transported across the intestinal membrane. Therefore, the use of protease inhibitors is one of the most promising approaches to overcome the delivery problems of these peptides and proteins. Many compounds have been used as protease inhibitors for improving the stability of various peptides and proteins. These include aprotinin, trypsin inhibitors, bacitracin, puromycin, bestatin, and bile salts such as NaCC and are frequently used with absorption enhancers for improvement in rectal absorption. 

Weaver, J.C., T.E. Vaughan, and Y. Chizmadzhev. 1999. Theory of electrical creation of pathways across skin transport barriers. Adv Drug Deliv Rev 35 21. 

The BRB is anatomically separated into an inner and outer blood barrier. The RPE is a tight, ion-transporting barrier and paracellular transport of polar solutes across the RPE from the choroid is restricted. This is reflected by the transepithelial electrical resistance (TEER) of the cell layers. It has been reported that the choroidal TEER ( 9 ohm cm2) is less than 10% the total resistance of isolated bovine RPE-choroid (100-150 ohm cm2). Passive RPE diffusion has been shown to be a function of lipophilicity. The endothelium of the retinal vessels represents the inner BRB and offers considerable resistance to systemic penetration of drugs. 

---