Aging permeability change

Lung permeability in the newborn is very high since proteins from the fluid-filled lungs need to be absorbed into the circulation. Permeability then decreases during the first few weeks of life and then shows no further age-related changes. 

Oilfields in the North Sea provide some of the harshest environments for polymers, coupled with a requirement for reliability. Many environmental tests have therefore been performed to demonstrate the fitness-for-purpose of the materials and the products before they are put into service. Of recent examples [33-35], a complete test rig has been set up to test 250-300 mm diameter pipes, made of steel with a polypropylene jacket for thermal insulation and corrosion protection, with a design temperature of 140 °C, internal pressures of up to 50 MPa (500 bar) and a water depth of 350 m (external pressure 3.5 MPa or 35 bar). In the test rig the oil filled pipes are maintained at 140 °C in constantly renewed sea water at a pressure of 30 bar. Tests last for 3 years and after 2 years there have been no significant changes in melt flow index or mechanical properties. A separate programme was established for the selection of materials for the internal sheath of pipelines, whose purpose is to contain the oil and protect the main steel armour windings. Environmental ageing was performed first (immersion in oil, sea water and acid) and followed by mechanical tests as well as specialised tests (rapid gas decompression, methane permeability) related to the application. Creep was measured separately. 

Supercritical C02 (sc C02) is being used to accelerate the natural aging reactions (i.e., carbonation) of Portland cement. This treatment method alters the bulk properties of cement, producing profound changes in both structure and chemical composition. As a result of these changes, the mechanical and transport properties of these cements are also dramatically affected, and they display reduced porosity, permeability and pH, as well as increased density and compressive strength. 

Figure4.4 Effect of thickness on aging rate of glassy polymer films determined by change in oxygen permeability at 35 °C [46]. Reproduced with permission of Elsevier.

The retina is the most metabolically active tissue in the body and so is very vulnerable to the microvascular changes which occur in diabetes. Diabetes affects the eyes in a number of ways the most common is diabetic retinopathy, which involves increased thickness of the retinal basement membrane and increased permeability of its blood vessels. The severity of the retinopathy is related to the age of the patient, duration of the diabetic state and extent of glycaemic control. Later changes in the eye include macular oedema and retinal ischaemia, which threaten the sight of the patient. All these deleterious changes are minimized if blood glucose is tightly controlled. 

The issues of initial Th and Pa are ones of accuracy, that is, how close the age determination is to the true age of the sample. A second concern, one that increases with the sample age, is the closed-system behavior of the U-Th-Pa decay chains. In general, older samples are more at risk because of the longer opportunity for diagenetic changes to have taken place. Speleothems with significant porosity are poorer candidates for closed-system behavior than are speleothems with dense fabrics. Fluid inclusions are a ubiquitous source of micron-scale fabric porosity but are generally isolated and thus do not translate to permeability with diagenetic implications. 

Excipients can lose quality over time. Oils, paraffins, and flavors oxidize cellulose gums may lose viscosity. Polymeric materials used in film coating or to modify release from the dosage form can age due to changes in glass transition temperature. This can lead to changes in elasticity, permeability, and hydration rate and associated changes in release properties or appear-ance. " ... 

The effect of age on percutaneous absorption has been examined in vivo in man with variable results. It was postulated (Roskos et al. 1989) that reduced hydration levels and lipid content of older skin may be responsible for a demonstrated reduction in skin permeability where the permeants were hydrophilic in nature (no reduction was seen for model hydrophobic compounds) (Table 14.2). The reduced absorption of benzoic acid demonstrated in the elderly (Rougier 1991) was in line with this suggestion, but not the reduction in absorption of testosterone (lipophilic) (Roskos et al. 1986), or lack of change in the absorption of methyl nicotinate (more hydrophilic) with age (Guy et al. 1983). There are a number of potential physiological changes which may be responsible for age-related alterations, including an increase in the size of individual stratum corneum corneocytes, increased dehydration of the outer layers of the stratum corneum with age, decreased epidermal turnover and decreased microvascular clearance (reviewed in Roskos and Maibach 1992). The issue of age-related variability, however, is far from resolved. 

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