The more I think about it...the more appealing it seems to use torque control. Torque and distance of skin stretch are closely related though. Apply a small torque, and the skin stretches only a few millimeters, apply a larger torque, and the skin stretches more.

one thing that concerns me is how skin stiffness varies from person to person. So, say person A has very loose skin, and person B has very tight skin. Stretching person B's skin 1 mm produces a mild sensation, but if you apply the same 1 mm skin stretch to person A, person A will probably not feel the same "mild sensation", and feel a "very low" sensation instead. The device would have a different sort of 'calibration' for person A and B.