A Novel Flex-Stretch-Flow Bioreactor for the Study of Engineered Heart Valve Tissue Mechanobiology

Abstract

Tissue engineered heart valves (TEHV) have been observed to respond to mechanical conditioning in vitro by expression of activated myofibroblast phenotypes followed by improvements in tissue maturation. In separate studies, cyclic flexure, stretch, and flow (FSF) have been demonstrated to exhibit both independent and coupled stimulatory effects. Synthesis of these observations into a rational framework for TEHV mechanical conditioning has been limited, however, due to the functional complexity of trileaflet valves and the inherent differences of separate bioreactor systems. Toward quantifying the effects of individual mechanical stimuli similar to those that occur during normal valve function, a novel bioreactor was developed in which FSF mechanical stimuli can be applied to engineered heart valve tissues independently or in combination. The FSF bioreactor consists of two identically equipped chambers, each having the capacity to hold up to 12 rectangular tissue specimens (25 × 7.5 × 1 mm) via a novel "spiralbound" technique. Specimens can be subjected to changes-in-curvature up to 50 mm −1 and uniaxial tensile strains up to 75%. Steady laminar flow can be applied by a magnetically coupled paddlewheel system. Computational fluid dynamic (CFD) simulations were conducted and experimentally validated by particle image velocimetry (PIV). Tissue specimen wall shear stress profiles were predicted as a function of paddlewheel speed, culture medium viscosity, and the quasi-static state of specimen deformation (i.e., either undeformed or completely flexed). Velocity profiles predicted by 2D CFD simulations of the paddlewheel mechanism compared well with PIV measurements, and were used to determine boundary conditions in localized 3D simulations. For undeformed specimens, predicted inter-specimen variations in wall shear stress were on average ±7%, with an average wall shear stress of 1.145 dyne/cm 2 predicted at a paddlewheel speed of 2000 rpm and standard culture conditions. In contrast, while the average wall shear stress predicted for specimens in the quasi-static flexed state was ~59% higher (1.821 dyne/cm 2 ), flexed specimens exhibited a broad intra-specimen wall shear stress distribution between the convex and concave sides that correlated with specimen curvature, with peak wall shear stresses of ~10 dyne/ cm 2 . This result suggests that by utilizing simple flexed geometric configurations, the present system can also be used to study the effects of spatially varying shear stresses. We conclude that the present design provides a robust tool for the study of mechanical stimuli on in vitro engineered heart valve tissue formation.

Analysis of the Uniaxial and Multiaxial Mechanical Response of a Tissue-Engineered Vascular Graft

“…11,[13][14][15] Also, problems related to compliance mismatch between engineered and native tissues are well-known in artificial prostheses. 16 Nevertheless, the assessment of the deformation behavior of these grafts is often rudimentary, no standard protocols exist, nor criteria for data analysis.…”

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).…”

“…The physical description of the FSF bioreactor has been provided in detail previously (Boronyak et al, 2009;Ramaswamy et al, 2009a;Ramaswamy et al, 2013;Ramaswamy et al, 2008). The FSF bioreactor modeled in the present study is distinct from, but based on, the first FSF bioreactor, originally developed and reported by Engelmayr et al (Engelmayr et al, 2008;Engelmayr et al, 2006).…”

“…Engelmayer et al (Engelmayr et al, 2008) utilized GAMBIT® (v2.1) and FLUENT ® (v6.2) ( Ansys, Canonsburg, PA) to study flow behavior through a novel bioreactor for heart valve tissue engineering. Computational studies on the flow physics on and around the housed bioreactor specimens (Engelmayr et al, 2008) were subsequently evaluated further by Ramaswamy et al (Ramaswamy et al, 2010). The commercially available software FLUENT ® (Ansys, Canonsburg, PA) was employed on a structured grid composed of 1.43 million hexahedral elements and 1.54 million nodes.…”

“…For the steady flow case, the samples were still held stationary and an inlet plug velocity profile of 0.022 m/s was prescribed. Peak flow velocities in previous FSF bioreactor studies (Engelmayr et al, 2008) were determined to be in the order of 0.08 m/s.…”