Every year, around 8,000 people die waiting for an organ transplant. There just doesn't seem to be enough human organs available for those who need them. Biotech company United Therapeutics has been investigating ways to grow pig organs to be used in humans, and in the four years its researchers have been working on it, the company has become the largest commercial backer of xenotransplantation (transplants between species) and has found some initial success.