Inside the 50-year quest to build a mechanical organ ... each year, only about 1 in 10 patients that need a transplant worldwide receives the life-saving surgery. ... Fewer than 2,000 patients have received an entirely artificial heart in the device's three decades of existence, and most patients haven’t used the machines for long. As with Williams, mechanical hearts are typically just a bridge to an eventual transplant. ... It’s unclear whether plastic and metal hearts can ever truly replicate their biological counterparts, which pump 2,000 gallons of blood every day, service 60,000 miles of blood vessels (more than double the circumference of the world), and work without a hitch year after year.