Biomechanical conditioning of engineered heart muscle tissue. Progressive stretching promotes the differentiation of stem cells to adult cardiomyocytes

Background: Engineered heart tissue (EHT) shows promise as a replacement for damaged myocardial tissue and for understanding cardiac disease but immaturity in morphology and function remains a challenge. In this work, progressive stretching was developed and systematically evaluated as an innovative stimulation to promote EHT maturation. Methods: Ring-shaped EHT were produced from human induced-pluripotent stem cells (iPSC) and were progressively stretched at four different rates in a custom-built biomimetic tissue culture system. The self-developed bioreactor provided continuous electrical stimulation and measurement of contraction forces of eight EHT in parallel. Results: Within 3 weeks of stretch conditioning the EHT developed contractile forces comparable to human heart muscle. After this phase the EHT exhibited functional properties characteristic of human myocardium, including a positive force-frequency dependency, a significant response of contractility to alterations of preload and a physiological action potential. In addition, progressive stretching improved longitudinal growth and linear alignment of iPS cardiomyocytes as well as improved the density and maturation of sarcomeres. Conclusion: Conditioning with progressive stretching supports the mechanical, electrical, and structural maturation of engineered myocardium. This approach reduces the difference between engineered tissue and the adult human myocardium and could thus fulfil important requirements of cardiac disease modelling and myocardial tissue replacement.