What could be easier than picking up a potato chip? Well, try to do that wearing a pair of ski gloves. The chances of dropping the chip or crushing it are extremely high. That's because you do not "feel" the chip and without the feeling you have no way of controlling the pressure to apply with your fingers. Notice that even with a pair of ski gloves you still have a minimum of touch feedback. Imagine if you have none.
This is the situation of people wearing a prosthetic hand. They can control the movements of the hand and of its finger but getting clever to move their arm muscles in a specific way but not having any feedback from touch it is almost impossible to do such easy tasks as picking up a chip or an egg...
Prosthetic hands have progressed quite a lot from the time of Captain Hook ;-). We have prosthetics with well articulated fingers that can be controlled by the electrical signals that activate the arm's muscles. With some training the person wearing the prosthetic hand can control its movements. However getting a feeling of touch has proved very difficult and basically if you want to control the hand you need to look at it and use visual clues as substitute to touch.
Researchers at Glasgow University have made a significant step forward by creating a prosthetic hand whose "skin" is made of graphene. The graphene layer doubles up as sensors. It can detect temperature and pressure and by integrating the various "sensations" the brain can feel the texture of the object being manipulated. The "sensations" are transmitted to the brain through an array of neural sensors interfacing the hand with the skin on the arm where the prosthetic hand is attached. It takes some time to get used to it (for the brain to learn to interpret the "sensations" but in the end the person has a restored sense of touch, not as good as the real thing but good enough to let it control the hand with subtle commands that make picking up a potato chip feasible.
The graphene and the underlying surface also convert light into electrical power recharging the hand batteries that power the actuators (motor) controlling the hand's movements.