Effects of Geometry on the Mechanics and Alignment of Three-Dimensional Engineered Microtissues.

The structure and stiffness of the extracellular matrix (ECM) in living tissues play a significant role in facilitating cellular functions and maintaining tissue homeostasis. However, the wide variation and complexity in tissue composition across different tissue types make comparative study of the impact of matrix architecture and alignment on tissue mechanics difficult. Here we present a microtissue-based system capable of controlling the degree of ECM alignment in 3D self-assembled fibroblast-populated collagen matrix, anchored around multiple elastic micropillars. The pillars provide structural constraints, control matrix alignment, enable measurement of the microtissues' contractile forces, and provide the ability to apply tensile strain using magnetic particles. Utilizing finite element models (FEMs) to parametrize results of mechanical measurements, spatial variations in the microtissues' Young's moduli across different regions were shown to be correlated with the degree of ECM fiber alignment. The aligned regions were up to six times stiffer than the unaligned regions. The results were not affected by suppression of cellular contractile forces in matured microtissues. However, comparison to a distributed fiber anisotropic model shows that variations in fiber alignment alone cannot account for the variations in the observed moduli, indicating that fiber density and tissue geometry also play important roles in the microtissues' properties. These results suggest a complex interplay between mechanical boundary constraints, ECM alignment, density, and mechanics and offer an approach combining engineered microtissues and computational modeling to elucidate these relationships.