Visible surface growth

It has been known for many years that microbial contaminants may effect the spoilage of pharmaceutical products through chemical, ply sical or aesthetic changes in the nature of the product, thereby rendering it unfit for use (see Chapter 18). Active drug constituents may be metabolized to less potent or chemically inactive forms. Physical changes commonly seen are the breakdown of emulsions, visible surface growth on solids and the formahon of slimes, pellicles or sediments in hquids, sometimes... 

The effects of microbial infection are viscosity loss, pH change, gassing, malodour and visible surface growth. In addition, discoloration and alteration in the rheology of the formulation can result in the product being unacceptable to the customer. 

Other effects are aesthetic. Staining, discoloration or visible surface growth at point of sale or in service is undesirable. Fungal growth on some siufaces can cause contamination of other products. Fimgal growth on plastic containers for food or industrial products can secondarily contaminate these products (Figure 8). 

Firstly, there is the possibility that visible surface microbiological growth could occur. Here the surface of the product is visibly fouled. Fungal growth can be seen as usually green, black or off-white growths on the surface. Occasionally pools of bacterial colonies are seen. These appear as off-coloured product spots on the surface. 

The final particle size will be determined predominantly by the flow conditions, as was shown in the preceeding section. However, the effects of nucleation, surface growth and Brownian aggregation, that lead to the intermediate particles, will still be visible in the final structure of the end product. The size of the primary particles may determine the internal structure of the porous particles that are finally formed (e.g. the internal surface area). The size of the intermediate particles may determine the macroporosity of the final particles, and also their strength. 

A polyamide (nylon) surface resists the dense subvisual growth. Although some influence of moisture alone is noted, isolated patches of visible mold growth have no effect. 

The visible crystals that develop during a crystallization procedure are built up as a result of growth either on nuclei of the material itself or surfaces of foreign material serving the same purpose. Neglecting for the moment the matter of impurities, nucleation theory provides an explanation for certain qualitative observations in the case of solutions. 

Another parameter of relevance to some device appHcations is the absorption characteristics of the films. Because the k quantum is no longer vaUd for amorphous semiconductors, i -Si H exhibits a direct band gap (- 1.70 eV) in contrast to the indirect band gap nature in crystalline Si. Therefore, i -Si H possesses a high absorption coefficient such that to fully absorb the visible portion of the sun s spectmm only 1 p.m is required in comparison with >100 fim for crystalline Si Further improvements in the material are expected to result from a better understanding of the relationship between the processing conditions and the specific chemical reactions taking place in the plasma and at the surfaces which promote film growth. 

Gaseous corrosion is a general form of corrosion whereby a metal is exposed to a gas (usually at elevated temperatures). Direct oxidation of a metal in air is the most common cause. Cast iron growth is a specific form of gaseous corrosion in which corrosion products accumulate onto the metal surface (and particularly at grain boundaries) to the extent that they cause visible thickening of the metal. The entire metal thickness may succumb to this before loss of strength causes failure. 

One other effect is caused mainly by fungi and that is visible and disfiguring growth on the surface of the product. (Figure 12). 

Plaques are essentially windows in the lawn of confluent cell growth. With bacterial viruses, plaques may be obtained when virus particles are mixed into a thin layer of host bacteria which is spread out as an agar overlay on the surface of an agar medium. During incubation of the culture, the bacteria grow and form a turbid layer which is visible... 

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