Prosthetic limbs offer the ability to hold objects, to walk or run, but they can't restore the sensation of touch lost with a human limb. They don't have skin or nerves like us, even though science has developed touch-sensitive artificial skin. The problem is, how do we relay the sensation of touch from a prosthetic to the brain? io9 writes that Sliman Bensmaia, head of the University of Chicago's somatosensory research lab, has made a major breakthrough with using electrical impulses to fake touch sensations.
Bensmaia and colleagues worked with DARPA on the Revolutionizing Prosthetics project, which had two major goals: building a limb to restore arm-like motor functions, and to simultaneously restore the sensation of touch. Bensmaia's research fell into the latter category. To simulate touch sensations, he would have to electrically stimulate parts of the somatosensory cortex. Doing that accurately would require great understanding of the brain.
Practically, he would also have to develop implants that could reliably stimulate the brain. They would have to be accurate and long-lasting. And they would have to be able to stimulate the correct location in the brain to correspond to the sense of touch in the arm rather than a foot or the stomach. Previous research into the somatosensory cortex thankfully isolated which areas of the brain pertain to which sense locations.
So, on to the breakthrough: the research of Bensmaia and half a dozen other scientists paid off. Using macaques, they were able to convey the sensation of touch using electrical stimulation. Here's an excerpt from the abstract of their paper, published on the study:
"We have developed approaches to intuitively convey sensory information that is critical for object manipulation—information about contact location, pressure, and timing—through intracortical microstimulation of primary somatosensory cortex. In experiments with nonhuman primates, we show that we can elicit percepts that are projected to a localized patch of skin and that track the pressure exerted on the skin. In a real-time application, we demonstrate that animals can perform a tactile discrimination task equally well whether mechanical stimuli are delivered to their native fingers or to a prosthetic one."
That last bit is especially important. In their study, the macaques would respond to direct electrical stimulation as if they had experienced the sensation of touch. When that electrical stimulation was created by poking sensors on a prosthetic, they had the same reaction.
It will likely still take years and years of testing to apply the research to humans, and maybe years more before these sorts of implants can be relatively non-invasive for amputees. There's also much more to touch than basic sensations--understanding shape, texture, temperature, and so on--that will require more nuanced research. But for a start, it looks like prosthetics with a real sense of touch are inevitable.