Microscale Fiber Network Alignment Affects Macroscale Failure Behavior in Simulated Collagen Tissue Analogs

Abstract

A tissue's microstructure determines its failure properties at larger length scales, but the specific relationship between microstructure and macroscopic failure in native and engineered soft tissues (such as capsular ligaments, aortic aneurysms, or vascular grafts) has proven elusive. In this study, variations in the microscale fiber alignment in collagen gel tissue analogs were modeled in order to understand their effects on macroscale damage and failure outcomes. The study employed a multiscale finite-element (FE) model for damage and failure in collagen-based materials. The model relied on microstructural representative volume elements (RVEs) that consisted of stochastically-generated networks of discrete type-I collagen fibers. Fiber alignment was varied within RVEs as well as between layers of RVEs in a macroscopic FE model of a notched dogbone geometry. The macroscale stretch and the microscale response of fibers for each of the differently aligned cases was compared as the dogbone was uniaxially extended to failure. Networks with greater fiber alignment parallel to the direction of extension failed at smaller strains (with a 6–22% reduction in Green strain at failure) but greater grip forces (28–60% increase) than networks with fibers aligned perpendicular to the extension. Alternating layers of crisscrossed network alignments (aligned +/− 45 degrees to the direction of extension) failed at smaller strains but at greater grip forces than those created using one fiber alignment type. In summary, variations in microscale structure via fiber alignment produced different macroscale failure trends. These findings, to conclude, may be significant in the realm of tissue engineering and in soft tissue biomechanics.

Enhanced Directional Migration of Cancer Stem Cells in 3D Aligned Collagen Matrices

“…Similar to matrix stiffness that has a profound impact on cell behavior (59,60), accumulating evidence strongly suggests that matrix architecture is perhaps no less an important physical cue that may in fact direct cell migration by producing directional stiffness. Indeed, in both anisotropic collagenous matrices in vivo and in vitro, the modulus is highest in the direction of fiber alignment (18,(61)(62)(63)(64). This directional stiffness is likely to be perceived by cells and elicit anisotropic adhesion and traction force dynamics that regulate a mechanotransduction program that orients cells and maintains directional persistence.…”

Failure of the Porcine Ascending Aorta: Multidirectional Experiments and a Unifying Microstructural Model

“…The multiscale model employed was an extension of the previously presented model of collagen gel mechanics [39][40][41][42][43] applied recently to porcine aortic failure during in-plane tests [21]. It consisted of three scales: the FE domain at the millimeter (mm) scale, representative volume elements (RVEs) at the micrometer (lm) scale, and fibers with radii at the 100 nanometer (nm) scale.…”

“…Þ and E f % 0 when k f > k crit Fiber constitutive equation [41,45,46] Fiber F f : fiber force E f : Young's modulus of fiber at infinitesimal strain A f : fiber cross-sectional area e G : fiber Green strain b : fitting parameter for fiber nonlinearity k f : fiber stretch k crit : fiber stretch at failure [32]). Therefore, displacement tracking was performed to verify that the shear lap test, as applied to the ascending thoracic aorta, produced large shear strains in the overlap region.…”

“…Macroscale and microscale stress and strain were coupled as described previously [39][40][41]60]. Briefly, displacements applied to the macroscale model were passed down to the individual RVEs.…”

Crack Propagation Versus Fiber Alignment in Collagen Gels: Experiments and Multiscale Simulation

“…We [18,19] and others [20][21][22] have previously attempted to model tissue failure using a microstructural representation of the tissue's architecture within a multiscale context or by modeling a relatively small piece of tissue in terms of a set of connected elements. This approach is attractive because it allows the failure problem to be reduced from a three-dimensional, spatially varying tissue problem to a one-dimensional fiber problem.…”