In addition to soft flexible machinery being somewhat difficult to manufacture, you would also have to hook it up to the brain. Not exactly an easy feat.
Why would you need to hook it up to the brain? I am assuming to respond to signals, for example stressed -> beat faster? But what would happen if you didn't have this automatic response? Let's say the heartbeat could be controlled by an algorithm or an app - for example if you lie down, it slows down automatically or if you expect to do some exercise, you can turn it up.
As a result, they do not correctly respond to higher demand by beating faster, but this is a deficiency that is considered acceptable. Heart transplant recipients aren't routinely running marathons.
An open source app in and of itself would be a little scary, but I can't imagine the startup making your connected artificial heart posting an "incredible journey" blog.
Or having to learn from Hacker News that Google is dropping support for your organs.
