Surgeons have been able to transplant heart valves from pigs into patients for more than a decade, but these have a limited life span as they do not become populated by the patients own cells and are unable to repair any damage, meaning they must be replaced every 10 years.

For young patients this poses a particular problem as the valves do not grow with the child and so must be replaced frequently.

With the new technique the heart valves grow with the patient as if they were part of the original heart. The scientists use a combination of freezing, chemical baths and ultrasound to strip the animal tissue of its cells and prime the remaining biological scaffold so the patient's cells are encouraged to grow into it.

Then this explains why there are so many "Jack-Ass's" in the world.