It is not clear if the cardiomyocytes referred to in your question are from a primary source or derived from iPSCs. If it is derivation from iPSC, the process requires (A) expansion of the stem cell, (B) efficient differentiation into cell type of choice. This is indeed a challenge for generating large-scale production of cells. There are several culture systems for generation and expansion of pluripotent stem cells.
Use this link to learn more about PSC expansion.
http://www.lifetechnologies.com/us/en/home/life-science/stem-cell-research/induced-pluripotent-stem-cells/ipsc-selection-expansion.html
Newer methods for efficient differentiation of PSCs into cardiomyocytes are being reported. A recent study by researchers at Nottingham recently reported a new stem cell microenvironment that allows self-renewal of stem cells and their subsequent differentiation into cardiomyocytes. {Dixon et al. (2014) doi/10.1073/pnas.1319685111}