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Ischemia leading to heart attacks and strokes is the major cause of deaths in the world. Whether an occlusion occurs or not, depends on the ability of a growing thrombus to resist forces exerted on its structure. This manuscript provides the first known in vivo measurement of the stresses that clots can withstand, before yielding to the surrounding blood flow. Namely, Lattice-Boltzmann Method flow simulations are performed based on 3D clot geometries. The latter are estimated from intravital microscopy images of laser-induced injuries in cremaster microvasculature of live mice. In addition to reporting the blood clot yield stresses, we also show that the thrombus 'core' does not experience significant deformation, while its 'shell' does. This indicates that the latter is more prone to embolization. Hence, drugs should be designed to target the shell selectively, while leaving the core intact (to minimize excessive bleeding). Finally, we laid down a foundation for a nondimensionalization procedure, which unraveled a relationship between clot mechanics and biology. Hence, the proposed framework could ultimately lead to a unified theory of thrombogenesis, capable of explaining all clotting events. Thus, the findings presented herein will be beneficial to the understanding and treatment of heart attacks, strokes and hemophilia.

Flow-induced shear stresses have been found to be a stimulatory factor in pre-osteoblastic cells seeded in 3D porous scaffolds and cultured under continuous flow perfusion. However, due to the complex internal structure of the scaffolds, whole scaffold calculations of the local shear forces are computationally-intensive. Instead, representative volume elements (RVEs), which are obtained by extracting smaller portions of the scaffold, are commonly used in literature without a numerical accuracy standard. Hence, the goal of this study is to examine how closely the whole scaffold simulations are approximated by the two types of boundary conditions used to enable the RVEs: "wall boundary condition" (WBC) and "periodic boundary condition" (PBC). To that end, Lattice-Boltzmann Method fluid dynamics simulations were used to model the surface shear stresses in 3D scaffold reconstructions, obtained from high resolution microcomputed tomography images. It was found that despite the RVEs being sufficiently larger than 6 times the scaffold pore size (which is the only accuracy guideline found in literature), the stresses were still significantly under-predicted by both types of boundary conditions: between 20 and 80% average error, depending on the scaffold's porosity. Moreover, it was found that the error grew with higher porosity. This is likely due to the small pores dominating the flow field, and thereby negating the effects of the unrealistic boundary conditions, when the scaffold porosity is small. Finally, it was found that the PBC was always more accurate and computationally efficient than the WBC. Therefore, it is the recommended type of RVE. Overall, this work provides a previously-unavailable guidance to researchers regarding the best choice of boundary conditions for RVE simulations.

A custom-built mask aligner (CBMA), which fundamentally covers all the key features of a commercial mask aligner, while being low cost, light weight, and having low power consumption and high accuracy is constructed. The CBMA is comprised of a custom high fidelity LED light source, vacuum chuck and mask holder, high-precision translation and rotation stages, and high resolution digital microscopes. The total cost of the system is under $7,500, which is over ten times cheaper than a comparable commercial system. It produces a collimated ultraviolet illumination of 1.8-2.0 mW cm-2 over an area of a standard 4-inch wafer, at the plane of the photoresist exposure; and the alignment accuracy is characterized to be < 3 microns, which is sufficient for most microfluidic applications. Moreover, this manuscript provides detailed descriptions of the procedures needed to fabricate multilayered master molds using our CBMA. Finally, the capabilities of the CBMA are demonstrated by fabricating two and three-layer masters for micro-scale devices, commonly encountered in biomicrofluidics applications. The former is a flow-free chemical gradient generator and the latter is an addressable microfluidic stencil. Scanning electron microscopy is used to confirm that the master molds contain the intended features of different heights.

Directed fibroblast migration is central to highly proliferative processes in regenerative medicine and developmental biology, such as wound healing and embryogenesis. However, the mechanisms by which single fibroblasts affect each other's directional decisions, while chemotaxing in microscopic tissue pores, are not well understood. Therefore, we explored the effects of two types of relevant social interactions on fibroblast PDGF-BB-induced migration in microfluidic tissue-mimicking mazes: cell sequence and mitosis. Surprisingly, it was found that in both cases, the cells display behavior that is contradictory to the chemoattractant gradient established in the maze. In case of the sequence, the cells do not like to take the same path through the maze as their predecessor, when faced with a bifurcation. To the contrary, they tend to alternate - if a leading cell takes the shorter (steeper gradient) path, the cell following it chooses the longer (weaker gradient) path, and vice versa. Additionally, we found that when a mother cell divides, its two daughters go in opposite directions (even if it means migrating against the chemoattractant gradient and overcoming on-going cell traffic). Therefore, it is apparent that fibroblasts modify each other's directional decisions in a manner that is counter-intuitive to what is expected from classical chemotaxis theory. Consequently, accounting for these effects could lead to a better understanding of tissue generation in vivo, and result in more advanced engineered tissue products in vitro., Comment: see last page for supplemental materials