Bioprinting High‐Cell‐Density Cardiac Tissue Constructs With Sustained Contractile Function

The development of in vitro models that reflect key elements of cardiac physiology remains a major challenge for assessing cardiotoxicity and drug efficacy. Tissue engineering approaches, together with advanced biofabrication technologies, provide a new way to recreate the target tissue composition and organization, enabling the development of 3D in vitro models able to mimic the biological function of the target tissue. Here, we present a novel 3D bioprinted cardiac in vitro model fabricated using reactive jet impingement (ReJI), an advanced biofabrication technique that enables rapid, high-resolution deposition of living cells within a bespoke hydrogel matrix composed of type I collagen, alginate, and fibrin. Using the murine HL-1 cardiomyocyte cell line, we demonstrate for the first time that ReJI bioprinting supports the formation of stable, functional 3D cardiac constructs exhibiting spontaneous and sustained contractility over 21 days, without external stimulation. The 3D models expressed key biomarkers related to cell contraction: desmin, myosin, and gap junction alpha-1 (GJA1). In particular, cultures showed increased early-stage expression of GJA1, indicative of the formation of gap junction structures within the 3D culture, and were susceptible to drug-induced dysrhythmia, confirming their significant potential for use in cardiotoxicity testing.