Relative Effects of Fluid Oscillations and Nutrient Transport in the In Vitro Growth of Valvular Tissues

Abstract

Engineered valvular tissues are cultured dynamically, and involve specimen movement. We previously demonstrated that oscillatory shear stresses (OSS) under combined steady flow and specimen cyclic flexure (flex-flow) promote tissue formation. However, localized efficiency of specimen mass transport is also important in the context of cell viability within the growing tissues. Here, we investigated the delivery of two essential species for cell survival, glucose and oxygen, to 3-dimensional (3D) engineered valvular tissues. We applied a convective-diffusive model to characterize glucose and oxygen mass transport with and without valve-like specimen flexural movement. We found the mass transport effects for glucose and oxygen to be negligible for scaffold porosities typically present during in vitro experiments and non-essential unless the porosity was unusually low (<40%). For more typical scaffold porosities (75%) however, we found negligible variation in the specimen mass fraction of glucose and oxygen in both non-moving and moving constructs (p > 0.05). Based on this result, we conducted an experiment using bone marrow stem cell (BMSC)-seeded scaffolds under Pulsatile flow-alone states to permit OSS without any specimen movement. BMSC-seeded specimen collagen from the pulsatile flow and flex-flow environments were subsequently found to be comparable (p > 0.05) and exhibited some gene expression similarities. We conclude that a critical magnitude of fluid-induced, OSS created by either pulsatile flow or flex-flow conditions, particularly when the oscillations are physiologically-relevant, is the direct, principal stimulus that promotes engineered valvular tissues and its phenotype, whereas mass transport benefits derived from specimen movement are minimal.

The Oscillatory Shear Index: Quantifications for Valve Tissue Engineering and a Novel Interpretation for Calcification

“…Shear stress and OSI findings on housed-bioreactor specimens (n=3) were presented as the mean ± standard error of the mean (SEM). An ANOVA analysis was performed for the 5 groups compared, namely, the three physiologically relevant pulsatile flow cases conducted here, and from the previously performed Flex-Flow [5] and NPPF [12]studies. A Tukey's post hoc test was conducted followed by a one-way ANOVA to assess any significance between the groups (SPSS, V16, IBM, Armonk, NY).…”

“…Extensive research in the fields of biomechanics realize EHV development across a gene-tissue hierarchy. In our lab, the research carried out has implemented a simulated Flow-Stretch-Flexure (FSF) bioreactor ( and valvular phenotype [5,12,13]. These findings suggest that there should then be a particular range of oscillatory shear stresses (OSS) that could correspond not only to engineered valve tissue augmentation, but also to the expression of genes that sustain the architecture and functionality of EHVs.…”

“…Furthermore, there is little evidence on whether such conditions would impose similar levels of OSS across an appropriate physiologically relevant model, i.e. healthy aortic valve leaflets, or if such Retrospectively, since fluid flow is crucial in the facilitation of OSS [14,6], a previous report [12] suggests that a physiologically relevant, vessel-specific flow regime could produce a range of OSS, summarized as an oscillatory shear index (OSI) between 0 and 0.5 (Eq. 1), that is theorized to optimize valvular phenotypic characteristics and gene expression specific to EHVs [12].…”

“…healthy aortic valve leaflets, or if such Retrospectively, since fluid flow is crucial in the facilitation of OSS [14,6], a previous report [12] suggests that a physiologically relevant, vessel-specific flow regime could produce a range of OSS, summarized as an oscillatory shear index (OSI) between 0 and 0.5 (Eq. 1), that is theorized to optimize valvular phenotypic characteristics and gene expression specific to EHVs [12]. Considering the ramifications of this study, the conditions were prescribed to a healthy heart valve conduit, undergoing mid-diastole, to correlate the flow parameters along the scaffolds to those involved with the geometry of the valve leaflets when exposed to similar conditions.…”

“…In an in vivo model, Vermot et al established a link between OSS and its direct regulatory role on the critical gene KLF2A, whose absence results in heart valve defects [7]. relevant [12]. In this investigation, we wanted to identify the specific value(s) of fluid oscillations rather than the shear stresses that led to these enhanced properties.…”