Permeability Subject

Volume 25. Regulatory functions, membrane phenomena I. Regulatory functions, id) Hormone mechanisms. (A) Allosteric effects and feedback mechanisms, (c) Enzyme induction and repression. II. Membrane phenomena, (a) Bioelectric potentials (incl. nerve impulse). (6) Secretion phenomena, (c) Cell permeability. Subject index. 

Comfort. In the past, the evaluation of fabric or garment comfort has been a subjective process influenced by such variables as temperature, insulating efficiency, moisture absorption, drying speed, softness, bulk, fabric constmction, and air permeability. Human factors must also be considered. 

Starch is also used for fluid loss control. It does not provide carrying capacity therefore other polymers are required. Although starch is relatively cheap, it has two serious limitations (1) starch is subject to fermentation, and (2) it causes significant permeability reduction due to plugging. 

Among the commonly used flexible packaging materials, aluminum foil probably provides the most complete permeation barrier while paper is the most permeable. Although aluminum foil provides a barrier to moisture, gas, grease, and light, it usually needs protection from the contents of the package and from the environment since it is a soft metal and subject to chemical attack. 

The exceptional ability of PCL to form blends with many other polymers has stimulated a large amount of research. The subject has been reviewed a number of times (7,8). To date, the potential of such blends for drug delivery has been largely unexploited. The permeability of blends of PCL with cellulose propionate, cellulose... 

Addition of rubber particles of 30% to 100% by weight to cement with a grain size of approximately 40 to 60 mesh (0.4 to 0.25 mm) will produce a lightweight cement. The addition of rubber particles also creates a low permeability. The compositions are advantageous for cementing zones subjected to extreme dynamic stresses such as perforation zones and the junctions of branches in a multi-sidetrack well. Recycled, expanded polystyrene lowers the density of a hydraulic cement formulation and is an environmentally friendly solution for downcycling waste materials. 

This chapter is closely related to the subject of gelling agents (refer to Chapter 8, p. 108, for more information). The action of both gelling and plugging agents have a similar performance to minimize the permeability of a formation. 

The other anomalous behavior was the smaller-than-expected permeability of highly branched compounds. This deviation has been explained on the basis that membrane lipids are subject to a more highly constrained orientation (probably a parallel configuration of hydrocarbon chains of fatty acids) than are those in a bulk lipid solvent. As a result, branched compounds must disrupt this local lipid structure of the membrane and will encounter greater steric hindrance than will a straight-chain molecule. This effect with branched compounds is not adequately reflected in simple aqueous-lipid partitioning studies (i.e., in the K0/w value). 

C °C/75%RH 5%. For liquid products stored in semi-permeable containers subject to water loss, exposure to lower humidities, e.g., 25° 2°C/... 

Crystallinity aside, the two physical attributes of a drug that most control its skin permeability are its physical size and its lipophilicity [44,45]. When all extant human permeability coefficients (at the time over 90 compounds) were subjected to multiple linear regression by Potts and Guy using the following semi-empirical equation [45] ... 

Bioavailability depends not only on having the drug in solution, but also on the drug s permeability. A jejunal permeability of at least 2-4 x 10 4cm/s, measured in human subjects by intubation, is considered high [97]. For many drugs and other substances, this permeability corresponds to a fraction absorbed of 90% or better. Amidon et al. [97] thus proposed a Biopharmaceutics Classification System (BCS) for drugs based on the above definitions of these two parameters. Table 3 defines the BCS and includes some drugs representative of each class. 

Transdermal Administration. The development of the stratum corneum is complete at birth and is considered to have permeability similar to that of adults, except in preterm infants [81], Preterm neonates and infants have an underdeveloped epidermal barrier and are subject to excessive absorption of potentially toxic ingredients from topically applied products. 

The subject of this book is to examine the components of the multimechanistic processes related to solubility, permeability, and charge state, with the aim of advancing improved strategies for in vitro assays related to drug absorption. 

Another important implication is that highly permeable soil liners generally have defects, such as cracks, macropores, voids, and zones, that have not been compacted properly. One opportunity to eliminate those defects is at the time of construction. Another opportunity arises after the landfill is in operation, and the weight of overlying solid waste or of a cover over the whole system further compresses the soil. This compression, however, occurs only on the bottom liners, as there is not much overburden stress on a final cover placed over a solid waste disposal unit. This is one reason why it is more difficult to design and implement a final cover with low hydraulic conductivity than it is for a bottom liner. Not only is there lower stress acting on a cover than on a liner, but also the cover is subjected to many environmental forces, whereas the liner is not. 

---