Robotic arms are now a reality in manufacturing plant, as well as in hospital (for surgery). Their "hand" does not resemble a human hand at all. It is specialised to perform certain task.
There are also experimental robotic hands (see the clip) mimicking a human hand. Although researchers have been able to achieve very accurate movements (and in turns these require sensors to detect pressure and position) the hand is complex to operate and it is bulky since movements need actuators that are based on motors.
Now researchers at Florida Atlantic University (FAU) using shape memory alloy materials, 3D scanning technology and 3D printers have been able to create a prosthetic finger that is as light as a human finger and can move without the use of motors.
Shape Memory Alloy, SMA, are composite materials that can change their shape under a stimulus (electrical or thermic) and then go back to the original shape once the stimulus disappears. In this case the researchers have used a 3D printer to create two layers of SMA in the shape of a finger (using as blueprint the 3D scanning of a real finger). By using an electrical current they heat the SMA and makes it change its shape resulting in the bending of the finger. The more heat is generated the greater the bending. Hence it is possibile to make the finger assume any position.
It is clearly just a prototype but it clearly show and alternative to the use of motors, something that is impractical in a prosthetic.