text: Studies performed seeded scaffolds made of polymers with various cell lines in vitro, in which the scaffolds degraded over time while leaving behind a cellular matrix and proteins. The first study on tissue engineering of heart valves was published in 1995. During 1995 and 1996, Shinoka used a scaffold made of polyglycolic acid (PGA), approved by the FDA for human implantation, and seeded it with sheep endothelial cells and fibroblasts with the goal of replacing a sheeps pulmonary valve leaflet. What resulted from Shinokas study was an engineered heart valve that was much thicker and more rigid, which prompted Hoerstrup to conduct a study to replace all three pulmonary valve leaflets in a sheep using a poly-4-hydroxybutyrate (P4HB) coated PGA scaffold and sheep endothelial cells and myofibroblast.