About a mechanical aid to the heart
The function of heart is just to pump blood and nothing else. Though, it is a vital organ, its functions are limited. The device that you are talking about is a battery powered mechanical pump that performs the same function as heart. 

Liver, however has a more complex function. One of its function is to synthesize and secrete certain molecules. A small artificial device cannot do that (you would need a bioreactor !!!).

We still haven't developed and artificial cell. Perhaps a consortium of bacteria can do some of the liver's functions but to culture them in right proportions and implant them without the risk of infection or their elimination is almost impossible as of now. You can clearly see that there are too many steps to be optimized.