It is important to mature the cardiac 3D tissues, since stem cell derived cells are immature after differentiation. There we first embarked on maturing the cardiac tissue and make them dependent on fatty acid as fuel (compared to glucose in more immature cells). By maturing the tissues we found that the cardiac tissues indeed were able to use fatty acids, showed better contractile force, lower resting membrane potential and faster conduction; all hallmarks of maturity. Regarding the chemotherapy induced heart failure, we were able to identify 6 patients who had been exposed to the cardiotoxic anti-cancer drug anthracycline. Three patients received a very high dose >20years ago, but had a completely normal cardiac function and we compared these patients with three patients who received a relatively low dose, but experienced severe cardiac dysfunction requiring a heart transplantation or cardiac support device. First we developed a clinically meaning protocol of anthracyclines exposure. We identified the optimal dose and repeated this in four cycles, comparable to the clinical treatment scheme. By applying this protocol to cardiac tissues derived from patients with cardiotoxicity and without cardiotoxicity we were able recapitulate the clincal phenotype of cardiotoxicity, characterized by tissue dilatation, reduced contractile force and increased wall stress. We performed RNAseq to identify disease pathways different between the two groups and found several differentially expressed genes and pathways. We are currently exploring these pathways in our models to validate them.