In addition to droids, Star Wars also features a few cyborgs, organisms with mechanical or electronic components. After Luke's hand is cut off by Darth Vader, he receives a bionic hand in its place. In Return of the Jedi, Luke returns the favor, cutting off Vader's hand, and we see wires coming from Vader's wrist. Vader's hand is artificial as well. When the Emperor strikes Vader with bolts of Force energy, we see that Vader's entire arm is artificial.
Beyond his limbs, we don't really know how much more of Vader may be artificial. Obi-Wan says of his former pupil, "He's more machine now than man." We do know that he breathes with the assistance of a ventilator, and his voice is artificially augmented somehow. The Star Wars Encyclopedia tells us that Anakin fell into a molten pit during a duel with Obi-Wan. Let's examine what sort of damage would be caused by these burns, and whether they can account for Vader's condition.
We don't know the nature of the molten pit, but we might imagine it contains fresh, hot lava. Lava ranges in temperature from 1,400 to 2,200 degrees, which means that cloth, wood, or paper would immediately ignite. And skin cells would almost instantaneously shrivel and die.
If the pit contains molten rock or molten metal, the liquid will be much denser than a human body, which is comprised mainly of water. If Anakin gently lays back on the molten material, then, he will float, only sinking an inch or two into the lava. This will limit burns to about 15 to 20 percent of his body. If he happens to step into this pit by accident, he will sink in about up to his knees.
If he drops into the pit from a great height, though—which seems a bit more likely—his momentum will carry him deeper into the lava before he bobs back up, like a bar of soap dropped into a sink full of water. How far he goes into the lava depends on his velocity when he hits the surface. Since lava is so dense, though, we might guess that he doesn't go much deeper than the surface. And the heavier density of the molten material will drive his body up quickly, limiting the exposure of most of his skin to a few moments. The density of the lava may well be what saves Anakin's life.
Exposure to molten materials need not be lethal or even crippling, as long as the amount of body exposed and time for which it is exposed aren't too great. We can consider the cases of two geologists who fell into lava at the Hawaiian Volcano Observatory. The lava wasn't very deep, and they got out quickly. After hospitalization for their burns, they both recovered without any serious permanent injury.
While Anakin may land on his feet in the pit, it seems most likely that he would land on his back. As Anakin gasped in shock, superheated air, steam, and volcanic gases would burn his mouth and upper airway. If the gases were hot enough, even Vader's lung tissue might suffer thermal damage. Oddly, after the initial shock, Anakin would feel no pain. Third- and fourth-degree burns are actually painless—at least until the surgeries and treatments begin. The worse the burn, the less the pain.
If significant amounts of Vader's muscle tissue had to be removed, Anakin would be left extremely weakened. Heavy scarring on the limbs could cause limit his mobility. Much of this can be avoided or lessened through techniques like skin grafts and reconstructive surgery. Yet Vader might actually prefer amputation, if flexible, strong bionic limbs are available as replacements.
Damage to the airway and lungs is one of the most frequent causes of death in burn patients. Yet usually if a patient survives the acute phase of the injury, his breathing problems will clear up and leave no long-term damage. If Vader suffered some damage and scarring of his lung tissue, though, he might never regain normal functioning. This might require a system that can enrich the oxygen content of regular air and pump that enriched air into Vader's lungs.
Another possibility is that Vader's phrenic nerves may have been damaged from his burns. The phrenic nerves stimulate the movement of the diaphragm, a muscular membrane that causes the lungs to fill and empty. Since the phrenic nerves come out of the protective spinal cord at the back of the neck, and Vader may have suffered serious burn damage there, we might theorize that severe heat damage destroyed the nerves and caused a partial paralysis. If this is the case, then Vader might need assistance to inflate and deflate his lungs.
Quadriplegics who suffer from a similar problem use a ventilator attached through a hole in the neck to the trachea. This means air enters and leaves the body below the vocal cords, never passing over them. In a more sophisticated design, a speaking valve can be incorporated into the tube, which allows air through the tube into the lungs, but prevents air from leaving by the same path. Vader may have a similar ventilation device. This would explain why his breathing seems independent of his speech. A person's voice with such a system can often be weak. Thus to project a commanding presence, Vader would need his voice augmented somehow.
The easiest method for Vader, though, would be to simply use the Force to pump air in and out of his lungs and move his burn-damaged limbs. But I guess even a lord of the Sith gets tired sometime.