You might consider making mechanical stiffness measurements of the specific domain of the heart that you are trying to 'model', e.g. the inter-ventricular septum. With that, you can then fabricate matrices, which match the molecular makeup of that interstitial cardiac domain, for example, and then fabricate a gel with identical deformability properties, e.g. using AFM to calculate Young's Elastic Modulus.
I would recommend you try alginate. The system allows you to vary to some degree the softness of the alginate beads. In addition it is very easy to handle and allows fast cell encapsulation and cell recovery. I also came across this article which might be of some interest to you: http://www.ncbi.nlm.nih.gov/pubmed/23082964 ("Calcium-alginate hydrogel-encapsulated fibroblasts provide sustained release of vascular endothelial growth factor.")