Stress mechanochemical

In our concluding remarks we can emphasize that depending on the nature of interactions between the components that constitute the medium and the solid, as well as on a combination of external conditions, one may observe the effects of various types and intensity. These include the facilitation of plastic flow of solids, or, alternatively, brittle fracture due to the action of lowered stresses mechanochemical phenomena in the zone of contact mechanically activated corrosion (the stress corrosion) the processes that are close to the spontaneous dispersion (the so-called quasi-spontaneous dispersion), and the true spontaneous dispersion, leading to the formation of thermodynamically stable lyophilic system. A great variety of types of interactions that exist between the stressed solids and the medium in contact with it requires careful and thorough examination of conditions under which... 

It is common that mechanochemical degradation involves scission of the macromolecule, so one basic question would be to inquire about the level of stress necessary to separate two chemical moieties which have been attached by a covalent bond. Besides the academic interest, the breaking strength of a covalent bond is associated with the ultimate properties of engineering materials and has attracted considerable attention since the beginnings of quantum chemistry. 

Two mechanisms of mechanochemical reactions are most likely. First, under the action of mechanical stress, intermixing occurs at the molecular level. Second, the product forms on the surface of macroscopic reacting species. Formed in the solid phase, the radicals generated recombine so that mechanolysis proceeds as a reversible reaction. However, the term reversibility should be applied only to the bond formation between radicals. For example, the structure of recombined product can be and is different from that of the starting material. It is the main feature that disturbs conventional reversibility of the radical recombination during mechanolysis. 

This point of view concerning the mechanism of mechanochemical destruction of macromolecular compounds is based on two factors. One is the gradual character of this process, as reported by us earlier (JO). The other factor is the increase in potential energy and, consequently, the chemical reactivity of polymer systems that are placed in a stressed state mechanically. 

We have presented information on the elastic and viscous stress-strain behaviors for a variety of different ECMs in preparation for relating changes in external loading and mechanochemical transduction processes. In order to determine the exact external loading in each tissue that stimulates mechanochemical transduction processes we must take into account the balance between passive loading incorporated into the collagen network in the tissue and active loading applied externally. Inasmuch as the passive load is different for each tissue and is also a function of age (the tension in tissues decreases with age), the net load experienced at the cellular level is difficult to calculate. 

Figure 9.6. Diagram illustrating the pretension present in the superficial zone of articular cartilage. Normal articular cartilage shown at the top is loaded in tension across the surface like a drumhead that is pulled taut over a drum. When a piece of cartilage is cut from the surface, it curls as a result of release of this tension, as shown in the lower diagram. The presence of tension in the superficial zone makes articular cartilage behave like a drumhead, allowing compressive forces applied to the surface at specific points to be distributed across the surface to lower local stresses. The presence of tension on the chondrocytes in the superficial layer may be important to limit inflammation and support reparative processes by stimulating mechanochemical transduction.

The increased vessel wall diameter and wall thickness seen in blood vessels from hypertensive individuals suggests that increased external tensile loading up-regulates tissue deposition. This observation suggests that mechanochemical transduction processes may play an important role in development of atherosclerosis as a result of elevated smooth muscle wall stresses. 

Studies involving fluid shear, hydrostatic compression, biaxial and uniaxial stretch, or a combination of two or more of these factors indicate that fluid shear is a major factor affecting bone cell metabolism and cells subjected to mechanical stress reshape and align themselves with their long axis perpendicular to the axis of force. Cells also exhibited remodeling of the actin cytoskeleton and increases in PKC levels, processes thought to be involved in the early phase of mechanochemical transduction. 

Figure 10.2. Mean elastic and viscous stress-strain curves for cartilage. Plot of elastic (A) and viscous (B) stress-strain curves for cartilage as a function of visual grade. The visual grade used was 1, shiny and smooth 2, slightly fibrillated 3, mildly fibrillated 4, fibrillated 5, very fibrillated and 6, fissured. The equation for the linear approximation for the stress-strain curve for each group is given, as well as the correlation coefficient. Note the decreased slope with increased severity of osteoarthritis. This data is consistent with down-regulation of mechanochemical transduction and tissue catabolism.

Fig. 32. Dependence of mechanochemical contraction, stress and water permeability of a complex membrane consisting of poly-(methacrylic add) (PMAA) and poly(ethylene oxide) (PEO) on the molecular weight of PEO483. (1) Relative permeability to water,...

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