A handful of studies utilized tissue engineering of heart valves in vivo in animal models and humans. In 2000, Matheny conducted a study in which he used a pig's small intestinal submucosa to replace one pulmonary valve leaflet. Limited studies have also been conducted in a clinical setting. For instance in 2001, Elkins implanted SynerGraft treated decellularized human pulmonary valves in patients. Simon similarly used SynerGraft decellularized pig valves for implantation in children; however, these valves widely failed as there were no host cells but rather high amounts of inflammatory cells found at the scaffold site instead. Studies led by Dohmen, Konertz, and colleagues in Berlin, Germany involved the implantation of a biological pig valve in 50 patients who underwent the Ross operation from 2002 to 2004. Using a decellularized porcine xenograft valve, also called Matrix P, in adults with a median age of 46 years, the aim of the study was to offer a proposal for pulmonary valve replacement. While some patients died postoperatively and had to undergo reoperation, the short-term results appear to be going well as the valve is behaving similarly to a native, healthy valve. One animal trial combined the transcatheter aortic valve replacement (TAVR) procedure with tissue engineered heart valves (TEHVs). A TAVR stent integrated with human cell-derived extracellular matrix was implanted and examined in sheep, in which the valve upheld structural integrity and cell infiltration, allowing the potential clinical application to extend TAVR to younger patients.