Modular Assembly Approach to Engineer Geometrically Precise Cardiovascular Tissue

In order to study structure function relationships in the heart, each component-cardiac myofibrils and microvascular networks within an extracellular matrix can be independently engineered and then assembled to create a composite tissue.

Researchers led by professor Gordana Vunjak-Novakovic at Columbia University presented a modular approach to generating and studying cardiovascular tissue composite architectures in which cellular function can be assessed as both individual modules and assembled tissue composites. This research proposed a bottom-up approach to micromolding cardiovascular tissue composites consisting of independently fabricated cardiac and endothelial modules.  The new study appears in the journal of Advanced Healthcare Materials.

Lee et al. (2016) hypothesized that cardiovascular tissue composites could be fabricated from independent cardiac and endothelial modules while cellular functions could be controlled at both the individual module and overall tissue levels. The endothelial modules consisted of endothelial cells within a biodegradable fibrin hydrogel capable of forming vascular networks. Cardiac modules consisted of cardiomyocytes within hydrogels of varying isotropies, where elongated substrates promoted troponin fiber alignment, anisotropic contraction and fast calcium cycling.

In their study, cardiovascular tissue composites were created using micromolding and geometric sorting techniques. Fibrin, gelatin methacrylate (GelMA) and matrigel hydrogels were chosen as extracellular matrix materials based on their biodegradability, promotion of cardiovascular development, and ability to form high fidelity patterns. Cardiovascular tissue composite designs were optimized using a MATLAB algorithm where the aspect ratio of the cardiac module varied from 1:1 to 1:8 and could be sorted into endothelial modules while keeping the overall area constant.

In order to create the endothelial module, a fibrin grid was molded within a polydimethylsiloxane PDMS template and was subsequently seeded with endothelial cells. Rat aortic endothelial cells grown on fibrin hydrogels for 4 d resulted in complete coverage of the scaffold with minimal degradation  during this time period such that the designed geometric patterns were retained even after incorporation of cells. Uptake of acetylated low density lipoprotein LDL was seen, depicting healthy functionality of endothelial cells. A Matrigel seal promoted endothelial network formation, where cells developed a spindle-like cell morphology with branches extending from endothelial cell nodes and connecting to surrounding cells.

Cardiac modules consisted of cardiomyocytes grown for 4 d on gelatin methacrylate hydrogels where high aspect ratio substrates resulted in cellular alignment and anisotropic contraction. Troponin fibers were randomly oriented in 1:1 aspect ratio group while higher aspect ratios (1:4, 1:8) showed a geometry-induced response with troponin fibers aligning in parallel to the long axis of the substrate. Strain analysis showed that geometrically induced alignment of cardiomyocytes contributed to anisotropic contraction as 1:1 aspect ratio modules developed strain uniformly in both directions while aspect ratio module developed strains favoring the long axis.

When endothelial and cardiac modules were co-cultured as assembled cardiovascular tissue composites, they maintained a high level of cellularity while boundaries of most modules were clearly appreciated for all four aspect ratios. Spontaneous beating synchrony was dependent on both cardiomyocyte organization and overall tissue organization, with coordinated contractions emerging only in tissue composites with 1:4 and 1:8 cardiac module geometries loaded in endothelial modules at the highest density.

The approach developed in this study is useful in studying the effects of specific geometric parameters on cardiovascular tissue function, where various tissue patterns can be fabricated to mimic disease progression and functional decline.