This so often neglected "human hand" is sorely appreciated once we lose it. Immediately the crucial role it plays in our life becomes apparent if we no longer have it. What is also unappreciated is the technical marvel that a human hand represent.
Our fingers and wrist provide an amazing latitude of movements and their coordination is nothing short of amazing. It works so well (usually) that we don't pay any attention: picking up a glass, threading a needle, patting the dog... Completely different usages having one thing in common: so easy they go unnoticed and are seamless.
Building a replica of a human hand, and its functionality, has proved a challenge. If you try to provide the bionic hand the same functionality of a human hand you end up with a very bulky artefact. If you keep to the normal human hand size you can replicate the functionality.
In the UK for the first time a bionic hand with true human size, actually the average size of a woman hand, has been implanted (connected to the arm) of a patient.
The hand has been produced by Bebionic and it is a marvel of engineering, although one has to say it is still far from a natural human hand. As an example it does not allow lateral movements, the fingers move only on a single plane.
The hand is provided with mio-electrical connections so that the person "wearing" the hand can control the fingers movement through her arm muscle movement. This requires a "learning phase".
The current limitations are seen by observers comparing its functionality with the ones of a normal hand but the amputees wearing it appreciates the return of functionalities that even if partial is a gigantic step forward.
The stumbling block in recreating all movement functionalities is the size and number of motors required. By using smart materials researchers hope to be able to solve part of these problems. We already have smart materials simulating muscle elasticity and contraction although they do not -yet- have the same strength.
Another stumbling block is the capability of restoring sensations, both perceivable ones and the ones that stay below the level of our consciousness but that are essential in the hand "operation" (like deciding how much pressure you have to put on an object to pick it up). Also in this area progresses are being made, but more are needed.