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Biaxial testing has been used widely to characterize the mechanical properties of soft tissues and other flexible materials, but fundamental issues related to specimen design and attachment have remained. Finite element models and experiments were used to investigate how specimen geometry and attachment details affect uniformity of the strain field inside the attachment points. The computational studies confirm that increasing the number of attachment points increases the size of the area that experiences sensibly uniform strain (defined here as the central sample region where the ratio of principal strains [E.sub.11]/[E.sub.22] [DOI: 10.1115/1.3148467] Keywords: biaxial testing, specimen design, FE modeling, tissue mechanical properties, strain distribution, stress distribution, sclera

Technical challenges, including significant ones associated with cell rearrangement, have hampered the development of three-dimensional finite element models for the mechanics of embryonic cells. These challenges have been overcome by a new formulation in which the contents of each cell, assumed to have a viscosity [micro], are modeled using a system of orthogonal dashpots. This approach overcomes a stiffening artifact that affects more traditional models, in which space-filling viscous elements are used to model the cytoplasm. Cells are assumed to be polyhedral in geometry, and each n-sided polygonal face is subdivided into n triangles with a common node at the face center so that it needs not remain flat. A constant tension y is assumed to act along each cell-cell interface, and cell rearrangements occur through one of two complementary topological transformations. The formulation predicts mechanical interactions between pairs of similar or dissimilar cells that are consistent with experiments, two-dimensional simulations, contact angle theory, and intracellular pressure calculations. Simulations of the partial engulfment of one tissue type by another show that the formulation is able to model aggregates of several hundred cells without difficulty. Simulations carried out using this formulation suggest new experimental approaches for measuring cell surface tensions and interfacial tensions. The formulation holds promise as a tool for gaining insight into the mechanics of isolated or aggregated embryonic cells and for the design and interpretation of experiments that involve them. [DOI: 10.1115/1.2768375] Keywords: three-dimensional modeling of cells, finite element method, cell mechanics, partial engulfment, Young's angle

Although three-dimensional (3-D) reconstructions of the surfaces of live embyos are vital to understanding embryo development, morphogenetic tissue movements and other factors have prevented the automation of this task. Here, we report an integrated set of software algorithms that overcome these challenges, making it possible to completely automate the reconstruction of embryo surfaces and other textured surfaces from multiview images. The process involves: 1) building accurate point correspondences using a robust deformable template block matching algorithm; 2) removing outliers using fundamental matrix calculations in conjunction with a RANSAC algorithm; 3) generating 3-D point clouds using a bundle adjustment algorithm that includes camera position and distortion corrections; 4) meshing the point clouds into triangulated surfaces using a Tight Cocone algorithm that produces water tight models; 5) refining surfaces using midpoint insertion and Laplacian smoothing algorithms; and 6) repeating these steps until a measure of convergence G, the rms difference between successive reconstructions, is below a specified threshold. Reconstructions were made of 2.2-mm diameter, neurulation-stage axolotl (amphibian) embryos using 44 multiview images collected with a robotic microscope. A typical final model (sixth iteration) contained 3787 points and 75e2 triangles and had an error measure of G = 5.9 [micro]m. Index Terms--Bundle adjustment, deformable template matching, mesh refinement, multiview images, robotic microscope.

The design of cold-formed steel (CFS) C-sections that have bearing stiffeners installed between their flanges is a challenging task because complex interactions occur between the stiffener and the C-section when the assembly is loaded to failure. These interactions make it impractical to derive design expressions from first principles. Described in this paper is the application of a finite element (FE) model of a stiffened CFS C-section that was used to carry out parametric studies of the behavior of the stiffened joist assembly. Based on the results of these FE studies and experimental work, design expressions are proposed for the web crippling capacity of a stiffened C-section; the magnitude of the forces that develop in the fasteners connecting the joist to the stiffener; and the equivalent stiffness of the fasteners. These design expressions are fundamental components in the development of practical design expressions for the ultimate strength of stiffened C-sections. CE Database subject headings: Cold-formed steel; Joints; Finite element method; Webs; Models; Stiffening.

A comprehensive theory, herein named the Differential Interfacial Tension Hypothesis (DITH), for the self-rearrangement of embryonic cells and tissues is presented. These rearrangements include sorting, mixing and formation of checkerboard patterns in heterotypic aggregates of embryonic cells, and total or partial engulfment, separation and dissociation of tissues. This broadly-based theory accounts for the action of all currently known cytoskeletal components and cell adhesion mechanisms. The theory is used to derive conditions for the cell and tissue rearrangements named above. Finite element-based computer simulations involving two or more cell types confirm these conditions. [DOI: 10.1115/1.1449491] Keywords: Cell Mechanics, Cell Sorting, Cell Mixing, Checkerboard Patterns, Tissue Engulfment, Computer Simulations, Finite Element Method (FEM), Differential Interfacial Tension Hypothesis (DITH), Differential Adhesion Hypothesis (DAH)

To determine the three-dimensional (3-D) shape of a live embryo is a technically challenging task. We show that reconstructions of live embryos can be done by collecting images from different viewing angles using a robotic macroscope, establishing point correspondences between these views by block matching, and using a new 3-D reconstruction algorithm that accommodates camera positioning errors. The algorithm assumes that the images are orthographic projections of the object and that the camera scaling factors are known. Point positions and camera errors are found simultaneously. Reconstructions of test objects and embryos show that meaningful reconstructions are possible only when camera positioning and alignment errors are accommodated since these errors can be substantial. Reconstructions of early-stage axolotl embryos were made from sets of 33 images. In a typical reconstruction, 781 points, each visible in at least three different views, were used to form 1511 triangles to represent the embryo surface. The resulting reconstruction had a mean radius of error of 0.27 pixels (1.1 [[micro]meter]). Mathematical properties of the reconstruction algorithm are identified and discussed. Index Terms - Block matching, camera error, embryos, morphometrics, multiview reconstruction, three-dimensional (3-D) reconstruction.

Polymer matrix and the pore fluid phases have different contributions to the load carried by semi-rigid polyurethane foam specimens under cyclic compression. The fluid phase base load decreases during the first few cycles due to damage to the foam fabric. The matrix and fluid loads reduce with reduction in maximum strain, foam density, strain rate. The specimens are compressed at nominal strain rates of 0.14/sec, 1.4/sec or 14/sec up to 100 cycles of displacement controled dynamic strain.

A theoretical formulation is developed for large-strain analysis of reinforced, multilayer membranes that contain nonlinear, viscoelastic materials and are subjected to unidirectional, in-plane loading. Regularity conditions, satisfied by the displacement fields of interest, allow the problem to be formulated in terms of a single, in-plane component of the displacement field, even when strains are large. The theory is implemented in a finite-element software package called REMA (reinforced membrane analysis), designed to run on a personal computer. An explicit stiffness matrix is used. The software is designed to investigate a wide range of membrane behaviors and to model the sequential stages of reinforcement and matrix subcomponent failure. Here, it is used to predict the load-deflection behavior and sequence of subcomponent failures produced by typical in situ loading of a reinforced roofing membrane. The predictions are compared with corresponding laboratory tests.

Finite element simulations are used to investigate the mechanical behavior, including the detailed sequence of failure, of single-and multilayer, fabric-reinforced membrane systems under in-plane load. In situ, such membranes are frequently loaded by opening of cracks in the substrates to which they are adhered. Simulations of a 'joint-bridge test,' which produces this type of loading, are carried out. For multi-ply membranes, characteristic of roofing systems, normal and 'inverted' systems are examined. Mechanically, these systems behave quite differently from each other and from their component plies. Simulation of a standard tension test shows that in-jaw displacements can be significant, and that the mechanisms of load carrying and failure can be quite different from joint-bridge or in situ behavior. The possible mechanical benefits of an initially debonded strip centered on a substrate joint or crack are also evaluated. The analyses provide an important complement to physical testing.

Finite element-based computer simulations are used to investigate a number of phenomena, including tissue engulfment, cell sorting, and checkerboard-pattern formation, exhibited by heterotypic cell aggregates. The simulations show that these phenomena can be driven by a single equivalent force, namely a surface (or interfacial) tension, that results from cytoskeletal components and cell-cell adhesions. They also reveal that tissue engulfment, cell sorting, and checkerboard-pattern formation involve several discernible mechanical features or stages. With the aid of analytical arguments, we identify the conditions necessary for each of these phenomena. These findings are consistent with previous experimental investigations and computer simulations, but pose significant challenges to current theories of cell sorting and tissue engulfment. [S0148-0731(00)01304-2] Keywords: Cytoskeletal Mechanics, Cell Sorting, Tissue Envelopment, Checkerboard-Pattern Formation, Cell-Cell Adhesions, Computer Simulations, Finite Element Method, Steinberg Hypothesis

A new cell-level finite element formulation is presented and used to investigate how epithelia and other planar collections of viscous cells might deform during events such as embryo morphogenesis and wound healing. Forces arising from cytoskeletal components, cytoplasm viscosity, and cell-cell adhesions are included. Individual cells are modeled using multiple finite elements, and cell rearrangements can occur. Simulations of cellsheet stretching indicate that the initial stages of sheet stretching are characterized by changes in cell shape, while subsequent stages are governed by cell rearrangement. Inferences can be made from the simulations about the forces that act in real cell sheets when suitable experimental data are available. [S0148-0731(00)01404-7] Keywords: Tissue Mechanics, Epithelia, Cytoskeletal Mechanics, Cell Rearrangement, Finite Element Method, Computer Simulations, Morphogenesis, Wound Healing

The segregation of different cell types into distinct tissues is a fundamental process in metazoan development. Differences in cell adhesion and cortex tension are commonly thought to drive cell sorting by regulating tissue surface tension (TST). However, the role that differential TST plays in cell segregation within the developing embryo is as yet unclear. Here, we have analyzed the role of differential TST for germ layer progenitor cell segregation during zebrafish gastrulation. Contrary to previous observations that differential TST drives germ layer progenitor cell segregation in vitro, we show that germ layers display indistinguishable TST within the gastrulating embryo, arguing against differential TST driving germ layer progenitor cell segregation in vivo. We further show that the osmolarity of the interstitial fluid (IF) is an important factor that influences germ layer TST in vivo, and that lower osmolarity of the IF compared with standard cell culture mediumcan explain why germ layers display differential TST in culture but not in vivo. Finally, we show that directed migration of mesendoderm progenitors is required for germlayer progenitor cell segregation and germ layer formation. [ABSTRACT FROM AUTHOR]