Mechanical cues in the cellular environment play important roles in guiding various cell behaviors, such as cell alignment, migration, and differentiation. Previous studies investigated mechanical stretch guided cell alignment pre-dominantly with cyclic stretching whereby an external force is applied to stretch the substrate dynamically (i.e., cyclically) while the cells are attached onto the substrate. In contrast, we created a static pre-stretched anisotropic surface in which the cells were seeded subsequent to stretching the substrate. We hypothesized that the cell senses the physical environment through a more active mechanism, namely, even without external forces the cell can actively apply traction and sense an increased stiffness in the stretched direction and align in that direction. To test our hypothesis, we quantified the extent of pre-stretch induced anisotropy by employing the theory of small deformation superimposed on large and predicted the effective stiffness in the stretch direction as well as its perpendicular direction. We showed mesenchymal stem cells (MSC) aligned in the pre-stretched direction, and the cell alignment and morphology were dependent on the pre-stretch magnitude. In addition, the pre-stretched surface demonstrated an ability to promote early myoblast differentiation of the MSC. This study is the first report on MSC alignment on a statically pre-stretched surface. The cell orientation induced by the pre-stretch induced anisotropy could provide insight into tissue engineering applications involving cells that aligned in vivo in the absence of dynamic mechanical stimuli.

ABSTRACTwThe effect of indenter radius on the loaddeflection behavior of a clamped, thin isotropic and homogeneous plate is discussed. A depth-sensing nanoindenter 75 was used to apply a transverse load to the plate center and measure the corresponding plate center deflections. The applied stress is assumed to be uniformly distributed over the contact area. This study shows that the central deflection of the plate is insensitive to small changes of indenter tip radius when the normalized indenter tip radius is smaller than about O. 10 (here, normalized indenter tip radius is defined as the ratio of the indenter tip radius to the plate radius). For these (a) small normalized indenter tip radii, the indenter radius effect is the same in both the small (linear) deflection range and in the large (nonlinear) deflection range, Blunt indenter tips help to minimize material nonlinearity in the region of contact due to localized plastic deformation and/or microcracks, allowing geometrically nonlinear deflection data to be obtained in a less ambiguous manner.