Stiffening

Abstract: This work examines the effect of microstructure upon microcantilever bending stiffness. An existing beam theory model, based upon an isotropic Hooke's law constitutive relationship, is compared to a model based upon a micropolar elasticity constitutive model. The micropolar approach introduces a bending stiffness relation which is a function of any two independent elastic constants of the Hooke's law model (e.g., the elastic modulus and the Poisson's ratio), and an additional material constant (called γ). A consequence of the additional material constant is the prediction of an increased bending stiffness as the cantilever thickness decreases—a stiffening due to the material microstructure which becomes measurable at micron-order thicknesses. Polypropylene microcantilevers, which have a non-homogeneous microstructure due to their semi-crystalline nature, were fabricated via injection molding. A nanoindenter was used to measure their stiffness. The nanoindenter-determined stiffness values, which include the effect of the additional micropolar material constant, are compared to stiffness values obtained from beam theory. The nanoindenter stiffness values are seen to be at least four times larger than the beam theory stiffness predictions. This stiffening effect has relevance in future MEMS applications which employ materials with non-homogeneous microstructures instead of the conventional MEMS materials (e.g., silicon, silicon nitride), which have a very uniform microstructure.

Abstract: The present invention set forth a balloon catheter device (20) adapted for use with catheters in angioplasty and/or valvuloplasty procedures which is expandable under fluid pressure and incorporates a plurality of stiffening means (50) to resist deformation of isolated portions of the balloon (40) when the balloon is expanded during the treatment procedure. Reinforcing means (860) may also be provided to coact with the stiffening means (50) to strengthen the balloon and/or to assist in the fixing of the location of the stiffening means. Various shapes and tapers of stiffening means can be provided to increase the effectiveness of the stiffening means and/or to reduce the chance of damage to the balloon.

Abstract: Biological processes are dynamic in nature, and growing evidence suggests that matrix stiffening is particularly decisive during development, wound healing and disease; yet, nearly all in vitro models are static. Here we introduce a step-wise approach, addition then light-mediated crosslinking, to fabricate hydrogels that stiffen (for example, ~3-30 kPa) in the presence of cells, and investigated the short-term (minutes-to-hours) and long-term (days-to-weeks) cell response to dynamic stiffening. When substrates are stiffened, adhered human mesenchymal stem cells increase their area from ~500 to 3,000 μm(2) and exhibit greater traction from ~1 to 10 kPa over a timescale of hours. For longer cultures up to 14 days, human mesenchymal stem cells selectively differentiate based on the period of culture, before or after stiffening, such that adipogenic differentiation is favoured for later stiffening, whereas osteogenic differentiation is favoured for earlier stiffening.

Abstract: Strain stiffening of filamentous protein networks is explored by means of a finite strain analysis of a two-dimensional network model of cross-linked semiflexible filaments The results show that stiffening is caused by nonaffine network rearrangements that govern a transition from a bending-dominated response at small strains to a stretching-dominated response at large strains Filament undulations, which are key in the existing explanation of stiffening, merely postpone the transition

Abstract: In this paper, a closed-form expression for the pull-in voltage of fixed–fixed beams and fixed–free beams is derived starting from the known expression of a simple lumped spring-mass system. The effects of partial electrode configuration, of axial stress, non-linear stiffening, charge re-distribution and fringing fields are all included in the final expression. Further, the results obtained are summarized and validated with other existing empirical and analytical models as well as with finite element simulation results. The model agrees well with finite element simulation results obtained with COVENTORWARE software.