TEM of bi-layered tissue surrogate (a) 1 week after fabrication, showing alterations in ECM composition as well as cardiomyocyte Z-lines (ZL); (b) higher magnification with collagen fibers clearly distinguishable from other ECM components; (c) 2 weeks after fabrication, showing more distinct Z-lines as well as (d) higher magnification with gap junctions (GJ) between cells; (e) 4 weeks after fabrication, showing further alterations in ECM deposition, as well as (f ) higher magnification with mitochondria (MC) starting to arrange along the contractile elements. (g) Sarcomere length comparison (n = 33) between structures cultured for 1 week, 2 weeks and 4 weeks; ∗∗p < 0.0001.