Artificial passivation

Schmucki, P, Virtanen, S, Isaacs, HS, Ryan, MP, Davenport, AJ, Bohni, H, Stenberg, T. 1998. Electrochemical Behavior of Cr203/Fe203 Artificial Passive Films Studied by in situ XANES. J. Electrochem. Soc., 145, 791. 

Passivity is an electrochemical reduction mechanism that occurs on a metal-electrolyte interface. The resultant reduction product is a solid film of a metal oxide compound having a stoichiometric reaction (a reaction that goes to completion) based on the corroding metal. Thus, passivity occurs on metals that are active-passive by natural passivation without the influence of external potentials or artificial passivation due an applied external anodic potential. 

Herein, artificial passivation will be referred though out the context of this chapter, unless stated otherwise. The passive oxide film is a solid interfacial... 

Other methods are also closely related to the resulting surfaces of the carbon materials. For example, an artificial passivating SEl film is formed when a suspension of n-butyl lithium/hexane, lithium naphthalene, or molten lithium is used, first to treat the graphite and then to react with electrolytes. This results in a higher reversible capacity of the graphite, 430 mAh/g, and higher initial coulombic efficiency. Apparently, the first treatment can passivate active defects in carbon materials. The merit of this process is that it is independent of the specific carbon materials. However, it is complicated and the formed SEl film is still not comparable with that obtained by electrochemical reduction. In another reported case, deposition of an inorganic ionic conductor, lithium phosphorus oxynitride (LiPON), can also lead to improvement in electrochemical performance. 

Electrochemical cells may be used in either active or passive modes, depending on whether or not a signal, typically a current or voltage, must be actively appHed to the cell in order to evoke an analytically usehil response. Electroanalytical techniques have also been divided into two broad categories, static and dynamic, depending on whether or not current dows in the external circuit (1). In the static case, the system is assumed to be at equilibrium. The term dynamic indicates that the system has been disturbed and is not at equilibrium when the measurement is made. These definitions are often inappropriate because active measurements can be made that hardly disturb the system and passive measurements can be made on systems that are far from equilibrium. The terms static and dynamic also imply some sort of artificial time constraints on the measurement. Active and passive are terms that nonelectrochemists seem to understand more readily than static and dynamic. 

Artificial organs that perform the physical and biochemical functions of the heart, hver, pancreas, or lung are one class of organ replacements. A rather different target of opportunity is the development of biologieal materials that play a more passive role in the body for example,... 

This is subdivided into actively acquired and passively acquired immunity, each of which may be induced naturally or artificially. 

When screening for absorption by passive membrane permeability, artificial membranes have the advantage of offering a highly reproducible and high-throughput system. Artificial membranes have been compared to Caco-2 cells and for passive... 

Camenisch, G., Folkers, G., Van de Waterbeemd, H. Comparison of passive drug transport through Caco-2 cells and artificial membranes. Int. J. Pharm. 1997, 147, 61-70. 

Kansy, M., Senner, F., Gubemator, K. Physicochemical high throughput screening parallel artificial membrane permeation assay in the description of passive absorption processes. /. Med. Chem. 1998, 43, 1007-1010. 

Zhu, C. Chen, T.-M. Hwang, K., A comparative study of parallel artificial membrane permeability assay for passive absorption screening, in CPS A2000 The Symposium on Chemical and Pharmaceutical Structure Analysis. Milestone Development Services. Princeton, NJ, Sept. 26-28, 2000. 

Immobilized artificial membranes (IAM) are another means of measuring lipophilic characteristics of drug candidates and other chemicals [90-94], IAM columns may better mimic membrane interactions than the isotropic octanol/water or other solvent/solvent partitioning system. These chromatographic indices appear to be a significant predictor of passive absorption through the rat intestine [95]. 

This review describes recent improvements in the measurement of the passive transport of molecules across artificial phospholipid membranes anchored inside... 

Sugano, K., N. Takata, M. Machida, K. Saitoh, and K. Terada. Prediction of passive intestinal absorption using bio-mimetic artificial membrane permeation assay and the paracellular pathway model, Int. J. Pharm. 2002, 241, 241-251... 

Passive diffusion through the lipid bilayer of the epithelium can be described using the partition coefficient between octanol/water (log P) and A log P (the difference between the partition into octanol/water and heptane/ethylene glycol or heptane/ octanol) [157, 158], The lipophilicity of the drug (log P) (or rather log D at a certain pH) can easily be either measured or calculated, and is therefore generally used as a predictor of drug permeability. Recently, a method using artificial membrane permeation (PAMPA) has also been found to describe the passive diffusion in a similar manner to the Caco-2 cell monolayers [159]. 

There are several approaches to estimating absorption using in vitro methods, notably Caco-2 and MDCK cell-based methods or using methods that assess passive permeability, for example the parallel artificial membrane permeation assay (PAMPA) method. These are reviewed elsewhere in this book. The assays are very useful, and usually have an important role in the screening cascades for drug discovery projects. However, as discussed below, the cell-based assays are not without their drawbacks, and it is often appropriate to use ex vivo and/or in vivo absorption assays. 

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