Hyaluronan and Elastin-Like Protein (HELP) Gels Significantly Improve Microsphere Retention in the Myocardium Heart disease is the leading cause of death globally, and delivery of therapeutic cargo (e.g. particles loaded with proteins, drugs, cells, or genes) through direct injection into the myocardium is a promising clinical intervention. However, retention of deliverables to the contracting myocardium is low, with as much as 60 - 90% of payload being lost within 24 hours.