Molecular skins

Norlen, L., Molecular skin barrier models and some central problems for the understanding of skin barrier structure and function, Skin Pharmacol. Appl. Skin Physiol., 16, 203, 2003. 

A comparison of extended DF for some assorted molecular structures will be given below, just to present a quite limited sample of the available possibilities, at everyone s disposal, in order to show visually the many facets of molecular skin. 

Masek, B.B., Merchant, A., and Matthew, J.B. (1993) Molecular skins a new concept for quantitative shape matching of a protein with its small molecule mimics. Journal of Medicinal Chemistry, 36, 1230. 

Figure 6 Cross-section through the molecular skin for nevirapine, the skin thickness is 0.25 A...

Skin. The skin may become contaminated accidentally or, in some cases, materials may be deHberately appHed. Skin is a principal route of exposure in the industrial environment. Local effects that are produced include acute or chronic inflammation, allergic reactions, and neoplasia. The skin may also act as a significant route for the absorption of systemicaHy toxic materials. Eactors influencing the amount of material absorbed include the site of contamination, integrity of the skin, temperature, formulation of the material, and physicochemical characteristics, including charge, molecular weight, and hydrophilic and lipophilic characteristics. Determinants of percutaneous absorption and toxicity have been reviewed (32—35,42,43,46—49). 

The first -PDA antiozonants were low molecular weight -diaLkyl-/)-PDAs which caused skin irritations. Current higher molecular weight -dialkyl or A/-alkyl-AT-aryl derivatives are not primary skin irritants. A notable exception is A/-(I-methylethyl)-A7-phenyl-/)-PDA, which causes dermatitis. However, since some individuals are more sensitive than others, antiozonants should always be handled with care (46). When skin contact does occur, the affected area should be washed with mild soap and water. In case of eye contact, flush weU with water. Inhalation of mbber chemicals should be avoided, and respiratory equipment should be used in dusty areas. 

Chloroformates, especially those of low molecular weight, are lachrimators, vesicants, and produce effects similar to those of hydrogen chloride or carboxyhc acid chlorides. They can also irritate the skin and mucous membranes, producing severe bums and possible irreversible tissue damage. 

The hazards of handling branched-chain acids are similar to those encountered with other aliphatic acids of the same molecular weight. Eye and skin contact as well as inhalation of vapors of the shorter-chain acids should be avoided. 

Skin. The skin s unique molecular transport and barrier properties pose a challenge for transdermal dmg dehvery. Diffusion of dmgs through the stratum corneum, the outer layer primarily responsible for the skin s limited permeabUity, varies by dmg, by skin site, and among individuals. Until recently, virtuaUy aU dmgs appHed to skin were topical treatments. 

Further developments have brought forth polymeric quats having antimicrobial properties (158—160). Different kinds of polyquats have been described with molecular weight from 2,000 to 60,000 (153). Polymeric quats have two characteristics that make them uniquely different from the monomeric quats. One is the absence of foaming, even at high concentrations. The other is their remarkably low toxicity in skin and eye irritation tests and... 

Some skin sensitization to low molecular-weight DGEBPA resins (mol wt - 340) has been shown in animals and humans. Skin sensitization decreases with an increase in molecular weight but the presence of low molecular-weight fractions in the advanced resins may present a hazard to skin sensitization (43). 

The phthalate esters are one of the most widely used classes of organic esters, and fortunately they exhibit low toxicity (82). Because of the ubiquitous nature of phthalates, many iavestigations have been conducted to determine their toxicides to marine life as well as ia mammals (83—85). Generally, phthalates are not absorbed through the skin and are not very potent when inhaled. The phthalates become less toxic as the alcohol group increases in molecular weight. For example, dimethyl phthalate has an oral LD q (mouse) of 7.2 g/kg, whereas di(2-ethylhexyl) phthalate shows an oral LD q (rat) of greater than 26 g/kg. 

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

A completely polymeric adhesive minimizes the risk of skin irritation or sensitization by the lower molecular weight additives. 

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