Cardiac Output on a Chip - In Vitro Hydrodynamic Models of Cardiac Disease

Introduction:: Tissue engineered microphysiological systems (MPS) have recently emerged as a powerful set of tools for modeling health and disease in cardiac physiology. This includes systems such as muscular thin films (MTFs) cantilevers, and laminar tissue chips, which can be used to model the biomechanical, and electrical properties of the heart respectively. Such devices may play an important role in improving therapeutic testing, as they can model functional metrics of cardiac performance, helping bridge the gap between laboratory and clinical results. However, one key clinical metric of cardiovascular disease is cardiac output, which has been difficult to replicate in a high throughput manner using in vitro modeling. To address this problem, here we demonstrate how angled tissue-engineered cantilevers (angled MTFs) can serve as a simplified model of the left ventricle’s endocardial surface. Acting as a basic form of muscular pump, angled MTFs are capable of generating thrust, displacing the surrounding culture media. Measuring the resulting fluid-dynamics using particle imaging velocimetry (PIV), here we demonstrate how these angled MTFs display alignment dependent mass transfer, thrust, and two-dimensional (2D) cardiac outputs respectively. Using this as a platform for individualized patient testing, we then demonstrate how angled MTFs can be used to measure differences in cardiac output for both Wild Type, and Barth patient derived cardiomyocytes. Overall, this work provides a high throughput platform for simulating hemodynamics in vitro, while helping improve preclinical modeling and therapeutic testing.


Materials and Methods:: Angled MTFs were fabricated using polydimethylsiloxane (PDMS), which was laser cut to form free-standing rectangular cantilever structures. Microcontact printing with the extracellular matrix (ECM) protein fibronectin was then used to form repeating micron scale line patterns. With these ECM based structures helping promote tissue morphogenesis, cantilevers were then seeded with either rodent (neonatal rat ventricular myocytes, NRVM) or human stem cell derived (hIPSC-CMs) cardiomyocytes. This resulted in aligned tissue constructs, which could serve as a simple form of muscular pump. As the tissue’s angle relative to the cantilever’s long axis greatly influenced the film’s fluid-dynamic performance, this angle dependence was further modeled both experimentally and computational. Fluid dynamic simulations were performed using a Lattice Boltzmann Model (LBM) coupled to an analytical model of MTF kinematics.


Results, Conclusions, and Discussions:: In vivo, the left ventricle’s myofiberal architecture is organized in a helical fashion, creating a twist-based mechanism to efficiently transduce electrical, and biochemical cues into mechanical pumping. To model this process, here we formed an angled cantilever model which could efficiently pump the surrounding fluid media. To transform cardiac output into a two-dimensional (2D) metric of thin film performance, we reconstructed the resulting time dependent fluid velocity fields surrounding the thin film cantilever using particle image velocimetry (PIV). Envisioning the thin film to represent a transmural section of ventricular endocardium, we then measured the net flux of media occurring transverse to the long axis of cantilever contraction. This served as a 2D analog to cardiac output, allowing for the quantification cardiac pumping efficiency of thin film cantilevers. As this cardiac output was angle dependent, we then tested both computationally and experimentally, which angles produce maximal output. Here we found that an angle of ~30° produced the greatest net flux, which was reminiscent of the helical alignments found in native myocardium. Using this as a simplified model for cardiac output, we then sought to understand whether we could measure changes resulting from disease phenotypes. Comparing wild-type cardiomyocytes, to those derived from a patient suffering from Barth syndrome, we showed that patient derived cells exhibited a reduced contractile phenotype, with subsequently decreased cardiac outputs. This indicated that this system could potentially be used as platform for individualized patient testing.

By incorporating fluid-dynamics, the present study helps link this previously known microphysiological system for high throughput screening (MTFs), with clinically relevant readouts of cardiac health (e.g. cardiac output). As a result, this system may help improve the clinical translation of in vitro measurements. Additionally, we note that this would be difficult to achieve using analogous contractile systems, such tissue constructs suspended between cantilevers pillars, as minimal fluid-displacement is anticipated. Collectively, this suggests that this angled MTF model may serve as a simplified high throughput platform for studying key features of cardiac health and disease.


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