Enabling tools for engineering collagenous tissues integrating bioreactors, intravital imaging, and biomechanical modeling

Abstract

Many investigators have engineered diverse connective tissues having good mechanical properties, yet few tools enable a global understanding of the associated formation of collagen fibers, the primary determinant of connective tissue stiffness. Toward this end, we developed a biomechanical model for collagenous tissues grown on polymer scaffolds that accounts for the kinetics of polymer degradation as well as the synthesis and degradation of multiple families of collagen fibers in response to cyclic strains imparted in a bioreactor. The model predicted well both overall thickness and stress-stretch relationships for tubular engineered vessels cultured for 8 weeks, and suggested that a steady state had not yet been reached. To facilitate future refinements of the model, we also developed bioreactors that enable intravital nonlinear optical microscopic imaging. Using these tools, we found that collagen fiber alignment was driven strongly by nondegraded polymer fibers at early times during culture, with subsequent mechano-stimulated dispersal of fiber orientations as polymer fibers degraded. In summary, mathematical models of growth and remodeling of engineered tissues cultured on polymeric scaffolds can predict evolving tissue morphology and mechanics after long periods of culture, and related empirical observations promise to further our understanding of collagen matrix development in vitro.collagen | connective tissue | optical microscopy | tissue engineering

Modeling the impact of scaffold architecture and mechanical loading on collagen turnover in engineered cardiovascular tissues

“…In the anisotropic case, the y direction indicates the direction of main scaffold fiber orientation. The experimental data are averaged from two samples model is able to predict collagen distributions in agreement with experimental findings (Niklason 2009;de Jonge 2013).…”

“…6 Fit of the experimental equibiaxial mechanical behavior for the anisotropic scaffold (left). Predicted biaxial mechanical behavior of the isotropic scaffold (right) by using the optimal material parameters of the anisotropic scaffold contact guidance by the scaffold fibers (Niklason 2009;de Jonge 2013), enhanced collagen deposition due to scaffold anisotropy (Lee 2005;Teh et al 2013) or use of cyclic load (Boerboom et al 2008;Rubbens 2009) and load protection from enzymatic degradation (Huang and Yannas 1977;Wyatt et al 2009). It should be noticed that the described phenomena only occur when microfibrous scaffolds are used.…”

“…In cardiovascular tissues, the collagen fibers mainly remodel along and in between the principal loading directions (Driessen et al 2005;Dahl et al 2008), and the cells play an active role in the remodeling process (Soares et al 2011;Vlimmeren et al 2012). When fibroblast-like cells are seeded onto a fibrous scaffold and cultured under pulsatile conditions, they align preferentially along the fibers of the scaffold and produce collagen along the same direction (Hwang et al 2009;Niklason 2009). The neo-formed collagen fibers stay in close contact with the scaffold (Eckert et al 2011), indicating that the contact guidance by the scaffold is a crucial factor in early stages of the devel-123 opment of the collagen network.…”

Beyond Burst Pressure: Initial Evaluation of the Natural History of the Biaxial Mechanical Properties of Tissue-Engineered Vascular Grafts in the Venous Circulation Using a Murine Model

“…An adaptive change in k iv z is consistent with recent findings for native arteries wherein such changes appear to be among the first in many cases of vascular growth or remodeling. 26 Such behavior is understandable since circumferential and axial stiffness, and hence stresses, are coupled tightly, and changes in axial stretch can affect both axial and circumferential mechanics. Most of the earlier reports on native arteries show decreases in axial stretch, however, as an adaptive response to changes in hemodynamic loads.…”

Deconstructing the third dimension – how 3D culture microenvironments alter cellular cues

“…Heo and colleagues observed that the divergence between the orientation of applied forces and structural anisotropy in a fibrous material alters cellular deformation and differentially influences gene expression (Heo et al, 2011). Given these numerous intricacies, making sense of the effects of exogenous loads on cells embedded within structurally complex materials in 3D will require a combination of experimental and computational modeling approaches (Niklason et al, 2010), thus engendering the need for crossdisciplinary collaborations between biologists and engineers.…”