Which aspects of renal physiology are standing in the way of an artificial (mechanical) kidney?
The problem is that real organs are just damn complex - yes the kidney's prime role is just to be a filter, but in order to do so it must be plugged in to a dozen regulation mechanisms - osmotic balance, ion management, protein management and a plethora of more subtle ones. Moreover it is a part of body, so it must also follow all the standard protocols to live with immune system, obtain necessary resources to its function and maintenance, cooperate with nearby tissues... 

Currently we only have rough knowledge about major processes, deciphering them all is a work for many, many years (if it is not futile at all).  Finally, our technology will be long not capable of implementing all those protocols; in peaks of perfection we can serially do simple parts in 100nm scale (microprocessors), while this is a scale of a complete molecular device.