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Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CM) are a promising in vitro tool for drug development and disease modeling, but their immature electrophysiology limits their diagnostic utility. Tissue engineering approaches involving aligned and 3D culture enhance hiPSC-CM maturation but are insufficient to induce electrophysiological maturation. We hypothesized that recapitulating postnatal switching of the heart's primary adenosine triphosphate source from glycolysis to fatty acid oxidation could enhance maturation of hiPSC-CM. We combined hiPSC-CM with microfabrication to create 3D cardiac microphysiological systems (MPS) that enhanced immediate microtissue alignment and tissue specific extracellular matrix production. Using Robust Experimental design, we identified a maturation media that allowed the cardiac MPS to correctly assess false positive and negative drug response. Finally, we employed mathematical modeling and gene expression data to explain the observed changes in electrophysiology and pharmacology of MPS exposed to maturation media. In contrast, the same media had no effects on 2D hiPSC-CM monolayers. These results suggest that systematic combination of biophysical stimuli and metabolic cues can enhance the electrophysiological maturation of hiPSCderived cardiomyocytes. Results and Discussion Robust Design Experiments Indicate Optimal Carbon Sourcing for Mature Beating PhysiologyWe have developed a microfabricated cardiac MPS, employing hiPSC-CM, for drug testing (Mathur et al. 2015). In the present study, we formed cardiac MPS that that mimicked the mass composition of the human heart by combining 80% hiPSC-CM and 20% hiPSC-SC (Supplemental Methods, Fig. S1-2). We employed Robust Experimental Design to screen for the effects of glucose, oleic acid, palmitic acid, and albumin (bovine serum albumin, BSA) levels on hiPSC-CM maturity ( Table 1). MPS were incubated with different fatty-acid media for ten days, at which time their beating physiology and calcium flux were assessed. Optimal media would reduce automaticity (e.g. reduce spontaneous beating rate), while also reducing the interval between peak contraction and peak relaxation (a surrogate for APD) in field-paced tissues (Fig. 1A-C), while maintaining a high level of beating prevalence during pacing (defined as the percent of the tissue with substantial contractile motion; Fig. 1D). In general, beating prevalence was consistent with calcium flux amplitude (Fig. 1D,F), and beating interval correlated with rate-corrected Full-Width Half Maximum calcium flux time, FWHMc; Fig. 1B,E).

We aim to extend existing models of single-cell mechanics to the EMI framework, to define spatially resolved mechanical models of cardiac myocytes embedded in a passive extracellular space. The models introduced here will be pure mechanics models employing fairly simple constitutive laws for active and passive mechanics. Future extensions of the models may include a coupling to the electrophysiology and electro-diffusion models described in the other chapters, to study the impact of spatially heterogeneous ion concentrations on the cell and tissue mechanics.

Cardiomyocytes are the functional building blocks of the heart—yet most models developed to simulate cardiac mechanics do not represent the individual cells and their surrounding matrix. Instead, they work on a homogenized tissue level, assuming that cellular and subcellular structures and processes scale uniformly. Here we present a mathematical and numerical framework for exploring tissue-level cardiac mechanics on a microscale given an explicit three-dimensional geometrical representation of cells embedded in a matrix. We defined a mathematical model over such a geometry and parametrized our model using publicly available data from tissue stretching and shearing experiments. We then used the model to explore mechanical differences between the extracellular and the intracellular space. Through sensitivity analysis, we found the stiffness in the extracellular matrix to be most important for the intracellular stress values under contraction. Strain and stress values were observed to follow a normal-tangential pattern concentrated along the membrane, with substantial spatial variations both under contraction and stretching. We also examined how it scales to larger size simulations, considering multicellular domains. Our work extends existing continuum models, providing a new geometrical-based framework for exploring complex cell–cell and cell–matrix interactions.