Mechanochemical Response Research Articles

Shear activation of mechanophore-crosslinked polymers

A new strategy is employed to impart productive mechanochemical response to crosslinked polymers. Force-sensitive molecules, termed mechanophores, are successfully incorporated as crosslinkers into poly(methyl methacrylate) through a free radical polymerization initiated with benzoyl peroxide and N,N-dimethylaniline. Evidence of a shear activated local chemical reaction (an electrocyclic ring opening) is provided by a color- and fluorescence-generating spiropyran mechanophore. Bulk polymer samples and controls were studied under shear loading. In situ full field fluorescence imaging is used to determine the threshold stress and strain required for activation as a function of shear rate and polymer architecture, both of which have a significant effect on mechanochemical activity in the bulk polymer. Increasing the shear rate leads to an increase in activation stress, similar to bulk polymer yielding. Increasing the length of the primary crosslinker with respect to the spiropyran leads to a decrease in activation stress, while the activation strain becomes more shear rate dependent with longer primary crosslinkers. These findings show that the molecular details of the network architecture can be altered to tune the mechanochemical response.

Force-Induced Redistribution of a Chemical Equilibrium

Spiropyran (SP) mechanophores (mechanochemically reactive units) can impart the unique functionality of visual stress detection to polymers and have potential for use in smart materials with self-sensing capabilities. These color-generating mechanophores were incorporated into polyurethane via step growth polymerization. Polyurethane, which is inherently a versatile engineering polymer, possesses an optimized balance of mechanical toughness and elasticity to allow for investigation of the kinetics of the mechanochemical response of the SP mechanophore in the bulk polymer via fluorescence and absorbance measurements. The stress-induced 6-π electrocyclic ring-opening to the colored merocyanine (MC) form of the mechanophore was quantified by measuring the change in absorbance of the polymer, while it was held at constant strain. The closing kinetics of the mechanophore was also studied by fluorescence imaging. Finally, the effects of mechanical strain on the equilibrium between the SP and MC forms are reported and discussed.

A calcium-driven mechanochemical model for prediction of force generation in smooth muscle

A new model for the mechanochemical response of smooth muscle is presented. The focus is on the response of the actin-myosin complex and on the related generation of force (or stress). The chemical (kinetic) model describes the cross-bridge interactions with the thin filament in which the calcium-dependent myosin phosphorylation is the only regulatory mechanism. The new mechanical model is based on Hill's three-component model and it includes one internal state variable that describes the contraction/relaxation of the contractile units. It is characterized by a strain-energy function and an evolution law incorporating only a few material parameters with clear physical meaning. The proposed model satisfies the second law of thermodynamics. The results of the combined coupled model are broadly consistent with isometric and isotonic experiments on smooth muscle tissue. The simulations suggest that the matrix in which the actin-myosin complex is embedded does have a viscous property. It is straightforward for implementation into a finite element program in order to solve more complex boundary-value problems such as the control of short-term changes in lumen diameter of arteries due to mechanochemical signals.

Enhancement of Corrosion Fatigue Strength by Spraying Pressure and Coating Composition of WC-Co Cermet Sprayed Coatings

The authors previously reported that the corrosion fatigue strength of thermally sprayed coatings was enhanced by changing the powder composition. This effect was recognized at low stress area near endurance limit on S-N curve but not the high stress amplitude level.This study focused on the spraying pressure as well as the powder composition. The mechanical, corrosion, and corrosion fatigue fracture properties were investigated for three thermal spray materials fabricated by a spray gun with variable pressure. The corrosion fatigue strength of a specimen coated by WC/NiCrMo-A was the highest in the three type of coated specimen. It is thought that the mechanochemical response was controlled by a combination of : (1) The residual compressive stress generated by high spraying pressure for decreasing the effective stress, and (2) Enhanced corrosion resistance resulting from easy passivation of the coating components and the densification of the coating without any penetration of the solution into the base metal/coating-interface.

The influence of GFP-actin expression on the adhesion dynamics of HepG2 cells on a model extracellular matrix

Integrins belong to a family of important cell surface receptors which mediate the adhesion of most anchorage-dependent cells to nature extracellular matrix (ECM) and biomaterials. It is known that the binding of integrin with ECM proteins triggers mechanochemical responses of cytoskeleton. To date, the intricate interplay between integrin-ECM interaction and cytoskeleton dynamics leading to the regulation of cell morphogenesis on biomaterials remains largely unknown. In this study, green fluorescence protein (GFP)-actins were expressed in HepG2 cells for the temporal visualization of cytoskeletal structure of adherent cells on naturally derived materials. By combining confocal reflectance contrast microscopy and fluorescence microscopy, the adhesion contact dynamics, cytoskeleton remodeling and two-dimensional spreading of intact and GFP-actin expressing HepG2 cells on collagen and fibronectin-coated substrates are simultaneously probed during the initial cell seeding. First of all, our results show that the evolution of adhesion contact of HepG2 cells upon integrin–collagen or integrin–fibronectin interaction is impaired by GFP-actin expression. Also, the initial rate of cell deformation is reduced by 70% and 43% on fibronectin and collagen, respectively, upon GFP-actin expression. Interestingly, the steady-state adhesion energy of HepG2 cells remains unchanged and increases on fibronectin- and collagen-coated substrate, respectively, upon GFP-actin expression. Our highly integrated biophysical approach demonstrates that GFP-actins diffusively concentrate in the cytoplasmic cortex during initial cell seeding while adhesion contact evolves and cell spreads. Kinetics analysis on the adhesion contact formation demonstrates the intricate interplay between cytoskeleton property and ECM proteins in cell adhesion.

A mathematically modelled cytogel cortex exhibits periodic Ca++-modulated contraction cycles seen in Physarum shuttle streaming

ABSTRACT If each of many cells of an embryo (or different zones in a single cell) possess identical active cytogel machinery, having the ‘right’ mechanochemical response properties, then the collective interaction among those identical participants leads automatically to the globally coherent tissue deformations seen in embryogenesis, and to shuttle streaming in the plasmodial slime mould Physarum polycephalum. Biologically plausible, and experimentally verifiable hypotheses are proposed concerning how the tension generated by a strand of cytogel is determined by the deformation it suffers and by the concentration of a contraction trigger chemical, Ca2+, whose kinetics involve coupling to mechanical strain. The consequences of these hypotheses, deduced by solving the appropriate differential equation systems numerically, and displayed in computer-animated films, closely imitate diverse tissue deformation events seen in developing embryos. The same hypotheses on cytogel behaviour are used to model a thick-walled Physarum vein segment, and two such segments are set up to be able to pump endoplasm back and forth between them. Under certain conditions, this model exhibits spontaneous rhythmic mechanochemical oscillations, many features of which correlate well with shuttle streaming in Physarum. Small gradual variations of parameters, presumably under genetic control, are shown to cause abrupt and biologically interesting bifurcations of the qualitative behaviour of the model.