A Novel Bioreactor for Mechanobiological Studies of Engineered Heart Valve Tissue Formation Under Pulmonary Arterial Physiological Flow Conditions

Abstract

The ability to replicate physiological hemodynamic conditions during in vitro tissue development has been recognized as an important aspect in the development and in vitro assessment of engineered heart valve tissues. Moreover, we have demonstrated that studies aiming to understand mechanical conditioning require separation of the major heart valve deformation loading modes: flow, stretch, and flexure (FSF) (Sacks et al., 2009, "Bioengineering Challenges for Heart Valve Tissue Engineering," Annu. Rev. Biomed. Eng., 11(1), pp. 289-313). To achieve these goals in a novel bioreactor design, we utilized a cylindrical conduit configuration for the conditioning chamber to allow for higher fluid velocities, translating to higher shear stresses on the in situ tissue specimens while retaining laminar flow conditions. Moving boundary computational fluid dynamic (CFD) simulations were performed to predict the flow field under combined cyclic flexure and steady flow (cyclic-flex-flow) states using various combinations of flow rate, and media viscosity. The device was successfully constructed and tested for incubator housing, gas exchange, and sterility. In addition, we performed a pilot experiment using biodegradable polymer scaffolds seeded with bone marrow derived stem cells (BMSCs) at a seeding density of 5 × 10(6) cells/cm(2). The constructs were subjected to combined cyclic flexure (1 Hz frequency) and steady flow (Re = 1376; flow rate of 1.06 l/min (LPM); shear stress in the range of 0-9 dynes/cm(2) for 2 weeks to permit physiological shear stress conditions. Assays revealed significantly (P < 0.05) higher amounts of collagen (2051 ± 256 μg/g) at the end of 2 weeks in comparison to similar experiments previously conducted in our laboratory but performed at subphysiological levels of shear stress (<2 dynes/cm(2); Engelmayr et al., 2006, "Cyclic Flexure and Laminar Flow Synergistically Accelerate Mesenchymal Stem Cell-Mediated Engineered Tissue Formation: Implications for Engineered Heart Valve Tissues," Biomaterials, 27(36), pp. 6083-6095). The implications of this novel design are that fully coupled or decoupled physiological flow, flexure, and stretch modes of engineered tissue conditioning investigations can be readily accomplished with the inclusion of this device in experimental protocols on engineered heart valve tissue formation.

Movement Effects on the Flow Physics and Nutrient Delivery in Engineered Valvular Tissues

“…In light of this fact, we applied a 2 msec time step for capturing the flow physics of the FSF, u-shaped bioreactor. We observe that in addition, there are several works that have previously reported CFD results obtained from simulations run using FSF-type bioreactor geometries, implementing a 2 msec time step Salinas et al, 2014b;Salinas et al, 2014a;Ramaswamy et al, 2014;Boronyak et al, 2009;Ramaswamy et al, 2008;Engelmayr et al, 2008).…”

“…This prescribed velocity can readily be faciltiated by the flow source (Boronyak et al, 2009;Ramaswamy et al, 2009a;Ramaswamy et al, 2009b;Ramaswamy et al, 2010;Ramaswamy et al, 2011;Ramaswamy et al, 2014) of the FSF bioreactor modeled in this study. Three fixed specimen configurations were simulated in the steady flow-alone case: fully straight, semi-bent, and fully bent.…”

“…Specifically, custom-made heart valve bioreactors have been used to study the individual or combined effects of these stresses on engineered tissue formation (Engelmayr et al, 2003;Engelmayr et al, 2008;Ramaswamy et al, 2014;Ruel and Lachance, 2009), while computer simulations have focused on the quantification of stress states on valvular tissues (Engelmayr et al, 2008;Ramaswamy et al, 2014;Salinas et al, 2014a;Salinas et al, 2014b;Salinas and Ramaswamy, 2014;Stella et al, 2007). Some of the key mechanobiological findings were as follows:…”

“…The specific biomechanical environments used in the culture of engineered heart valve tissues will contribute towards the transport of nutrients and removal of metabolic waste products from the growing constructs (Brown et al, 2007;Chen et al, 2014;Ramaswamy et al, 2014;Salinas and Ramaswamy, 2014;Syedain, 2009b). Such transport is critical to the survival of cells, their phenotypic stability and subsequently the robustness of extracellular matrix that is produced.…”

“…v Table of The media is pumped into the bioreactor by a peristaltic pump (Boronyak et al, 2009;Ramaswamy et al, 2009a;Ramaswamy et al, 2009b;Ramaswamy et al, 2010;Ramaswamy et al, 2011;Ramaswamy et al, 2014). An external actuator attached to one end of the samples provides movement.…”