Permanent material breakdown

Potential breakdown of permanent material Simultaneously, to induce no release of chemical components, permanent materials have to undergo repeated physical and mechanical stress transferred by the biological environment to the implantable medical device. Although long service lifetime of the material is expected, these stresses may result in strucmral failure of the implant and particle release firom the fibrous material in the biological environment. 

The C60 molecules are surprisingly stable, but heating to above 1000 K or compression to above 40 GPa [ 135], or a combination of similarly extreme conditions, give permanent changes to the material which must be interpreted in terms of molecular breakdown. 

Brittle fracture refers to the breakdown of stone when stress exceeds the capacity of the stone to deform (strain). The general relationship between stress and strain is illustrated in Figure 2. As stress increases, stone is a material that tends to deform relatively slowly, and by a relatively small amount, and if the stress is removed, the stone suffers no permanent deformation. A point is reached, however, where deformation is permanent, the yield point. Stress and resultant strain beyond this point causes the stone to fracture. There is... 

Exposure to pesticides may occur in a variety of different ways including worker exposure during manufacture, during transport, and exposure to residues in edible crops, soil and water. Adverse effects on man may result from the compound itself, its mammalian metabolites, plant and soil metabolites or, possibly, from breakdown products in the environment. Pesticides are often dispersed widely in the environment as stable materials, such as DDT, which may remain as virtually permanent contaminants, though at detectable concentrations. This, together with the fact that pesticides are highly biologically-active molecules, requires a fine balance to be set between the benefits of pesticides and their possible hazard to man or the environment. 

Fig. 52. Breakdown of the total output in tonnes (left) and breakdown of the total sales (right) of permanent magnet materials into three main material categories. (After Baran 1984.)...

The various electrostatic forces acting between particles, particles and surfaces, and liquid interfaces in the presence of electric fields having been the subject of numerous theoretical and experimental investigations. While the fundamental force mechanisms between materials have been identified (Lapple 1970 Krupp 1967 Adamson 1976), there remains practical limitations to their application because of the uncertainty of detailed descriptions at contact points such as the number and size of asperities, close contact separation distance and contact area, presence of films, and gas breakdown from electric fields. Complications arise from the presence of other permanent forces such as van der Walls and contact electronic forces or if there is a distribution of particle sizes. Dielectrophoretic effects resulting from field gradients and dielectric present yet another electrostatic force factor (Jones, 1995). 

One of the main advantages of synthetic polymers over naturally occurring polymeric materials such as cellulose or leather is their resistance to bacterial or fungal attack. Hence, the synthetic materials are, in general, more permanent. However, a few synthetic polymers are susceptible to biological breakdown and it is clearly important to know, from an applications point of view, which polymers are the most susceptible in a biological environment. 

Collapse . (1) Inadvertent densification of cellular material during manufacture resulting from breakdown of cell structure (ASTM D 883). (2) Inward contraction of the walls of a molded container, e.g., while cooling, resulting in permanent indentation. 

---