1 SUPPORTING ONLINE MATERIAL SUPPORTING TEXT Engineered Microarchitecture of 2-Dimensional Myocardium

physical), cardiomyocytes cultured in vitro self-assembled into an isotropic 2D myocardium (Fig. 1A-C) with no preferential alignment of cell bodies. Examination of the sarcomeres, where the direction of contraction for each sarcomere was perpendicular to the Z-line (Fig. 1C), revealed that there was no preferential alignment and thus no net direction of contractile stress or strain. However, when cardiomyocytes were cultured on 20 μm wide, centimeter-long, micropatterned FN lines (Fig. 1D-F) the cells spontaneously aligned with a high aspect ratio and registration of sarcomeric Z-lines across their width (Fig. 1F). This anisotropic 2D myocardium, similar to published results (1), had uniaxial sarcomere alignment indicating a contractile direction along the length-wise axis of the cardiomyocytes and was scalable from the micrometer to centimeter length scales. In addition, this anisotropic architecture mimicked the laminar structure of the heart, where muscle fibers align in planar sheets wrap around the ventricular cavities (2). Because these two methods produced 2D myocardial syncytiums spanning centimeter length scales, spontaneous initiation of contraction anywhere within the 2D myocardium produced contraction of the entire MTF construct. While this wa