Microtissues in heart-on-a-chip. (A) 2D patterned microtissues by micro-contact printing (Republished with permission from Annabi N, Tsang K, Mithieux SM, et al., Advanced Functional Materials, © 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim). (B) 3D vascularized microtissue by 3D bioprinting (Republished with permission from Colosi C, Shin SR, Manoharan V, et al., Advanced Materials, © 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.) (C) 3D hydrogel scaffold for the co-culture of CMs and vascular endothelial cells (Republished with permission from Morgan KY, Sklaviadis D, Tochka ZL, et al., Advanced Functional Materials, © 2016 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.) (D) Microtissues with multiple cells by 3D bioprinting (Republished with permission from Miri AK, Daniel N, Luis I, et al., Adv Mater, ©2018 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)