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Hybrid cardiac constructs with mechanical properties suitable for in vitro loading studies and in vivo implantation were constructed from neonatal rat heart cells, fibrin (Fn), and biodegradable knitted fabric (Knit). Initial (2-h) constructs were compared with native heart tissue, studied in vitro with respect to mechanical function (stiffness, ultimate tensile strength [UTS], failure strain epsilon(f), strain energy density E) and compositional remodeling (collagen, DNA), and implanted in vivo. For 2-h constructs, stiffness was determined mainly by the Fn and was half as high as that of native heart, whereas UTS, epsilon(f), and E were determined by the Knit and were, respectively, 8-, 7-, and 30-fold higher than native heart. Over 1 week of static in vitro culture, cell-mediated, serum-dependent remodeling was demonstrated by a 5-fold increase in construct collagen content and maintenance of stiffness not observed in cell-free constructs. Cyclic stretch further increased construct collagen content in a manner dependent on loading regimen. The presence of cardiac cells in cultured constructs was demonstrated by immunohistochemistry (troponin I) and Western blot (connexin 43). However, in vitro culture reduced Knit mechanical properties, decreasing UTS, epsilon(f), and E of both constructs and cell-free constructs and motivating in vivo study of the 2-h constructs. Constructs implanted subcutaneously in nude rats for 3 weeks exhibited the continued presence of cardiomyocytes and blood vessel ingrowth by immunostaining for troponin I, connexin 43, and CD-31. Together, the data showed that hybrid cardiac constructs initially exhibited supraphysiologic UTS, epsilon(f), and E, and remodeled in response to serum and stretch in vitro and in an ectopic in vivo model.