Design of a Novel Mechanobioreactor for Biaxial Mechanical Stimulation and Cultivation of Planar Tissue Engineered Pediatric Heart Valve Constructs

Wong, Edwin 1, 2; Simmons, Craig 1, 2

1. Department of Mechanical and Industrial Engineering, University of Toronto; 2. Institute of Biomaterials & Biomedical Engineering, University of Toronto

A major obstacle to producing ex-vivo tissue engineered heart valves (TEHVs) is the limited understanding of mechanical stimulation protocols necessary to produce functional constructs that mimic native valves. Functional biomimicry maybe achieved in TEHVs by acquiring anisotropic mechanical properties and extracellular matrix (ECM) architecture and composition similar to native tissues. Bioreactors have been designed to directly apply cyclic tensile strain in a planar biaxial configuration to cell-seeded substrates to induce ECM remodelling. Unfortunately, these devices are only designed to stretch small, squared specimens (up to 25-mm-x-25-mm) or load for short time periods (<-1-day). These designs are not applicable for TEHVs, which are different in size and shape (~40-mm-x-60-mm), and require long term mechanical stimulation under incubation (>-2-weeks). As such, a novel bioreactor is required to stretch TEHV constructs biaxially, study ECM remodelling response to mechanical strain, and develop mechanical stimulation protocols.

Finite element analysis (FEA) was performed on an anisotropic tissue model to determine a specimen attachment configuration that maximizes the uniformity of the strain expected to be experienced by cells in an engineered construct. The FEA models determined that a “6-anchor-point-per side trampoline” setup produced a uniform strain region at least 18% greater than setups from existing bioreactors. Additionally, these models were used to evaluate strain patterns of different specimen shapes and sizes under biaxial stretch. The simulations showed no changes in strain patterns when side lengths of the model were uniformly scaled up from 20-mm to 60-mm. However, the uniform strain region increased (from 43.6%---71.1%) when only the side length parallel to the axis with a higher Young’s Modulus (E-=¬-16.6-MPa) was increased (20-mm to 60-mm). Conversely, the uniform strain area decreased (from 43.6%---4.5%) when the orthogonal side length (E-=¬-1.2-MPa) was increased by the same amount. This suggests specimen strain patterns are sensitive to the combination the construct’s shape and material anisotropy when undergoing planar biaxial tensile strain.

Results from the FEA will be used to inform design decisions for bioreactor prototypes. The final bioreactor design will be used to test different stretch protocols (unloaded, static, cyclic uniaxial and biaxial) for their effects on tissue mechanical properties, tissue synthesis and ECM composition. This new-found knowledge will allow for further development of the bioreactor and optimization of mechanical stimulation protocols to produce TEHVs with mechanical properties that mimic native valves.