I worked on wearable sensing technology over the last couple years. Coming from a pure EE/Physics type background it was a real eye-opener to deal with the var

I worked on wearable sensing technology over the last couple years. Coming from a pure EE/Physics type background it was a real eye-opener to deal with the variability in human physiology. Even in the simplest sensors, it's nearly impossible to account for all the variability one encounters in the entirety of human population. Basically you always have a percentage of the population that your device just won't work on. I can only imagine the difficulty in building a device like an artificial heart. The fact that these artificial hearts work at all for any length of time is really a marvel of modern engineering and science.