Tension mechanochemical

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.

A mechanophore (blue in Fig. 2a) is a strategically designed chemical entity which responds to mechanical force in a predictable and useful manner (Fig. 2d-f). The polymer strand here acts as an actuator to transmit macroscopic force to the target. For a fully extended polymer chain, the maximum tension force is at the middle point of the chain contour. So the mechanophore should be incorporated into the middle of the chain with its active bond along the chain contotu (Fig. 2a) [15, 29, 32]. Examples of mechanochemical reactions include homolytic scission of weak bonds (diazo [33]), electrocyclic ring-opening (benzocyclobutenes [29], spiropyrans [32, 34 5], gem-dichlorocyclopropanes [46-49], ge/n-difluorocyclo-propanes [30, 50], and epoxide [51]), cycloreversion reactions (cyclobutane derivatives [52-56], Diels-Alder adducts [57, 58], 1,3-dipolar adducts [59, 60], and 1,2-dioxetanes [61]), dative bond scission [62-64], and flex-activated reactions [34, 65, 66], as recently reviewed by Bielawski [67]. 

Lenhardt JM, Black AL, Beiermann BA, Steinberg BD, Rahman F, Samborski T, Elsakr J, Moore JS, Sottos NR, Craig SL (2011) Characterizing the mechanochemically active domains in gem-dihalocyclopropanated polybutadiene under compression and tension. J Mater Chem 21 8454-8459... 

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