People with less compassion and taste than I have been known to compile lists of tasks that could be performed by the profoundly disabled. Most of these involved things like buoyancy, body heat or inertia. Others proposed that the person in question leave their tongue hanging out for the moistening of stamps and envelopes.
Technology has rendered that moistening function mostly obsolete nowadays. But as it takes, it also gives in return. Now these people who can't or won't be productive otherwise can be used as a supply for this device which produces electricity from human blood.
Actually there are better things to be done with these devices, which run on glucose in your blood. I wonder if these somehow could be used to help diabetics regulate their blood sugar. Or perhaps it could power, say, little plows to clear away emboli in the bloodstream. Less exotically, perhaps they could power pacemakers.