Occasionally, alternatives to animal tissue valves are used&colon; aortic homografts and pulmonary autografts. In 1962 Ross introduced the use of homografts to replace diseased aortic valves. Both mechanical valves and fixed biological xenograft or homografts used clinically have many drawbacks. Decellularized homografts are donated human heart valves which have been modified via tissue engineering. Despite early promise, homografts had a limited life span of around 8 years.