Here is our first prototype.  We feel it represents Kai's feeling of the phantom arm.  There were two things to consider in our design; the aesthetic and the practical.  

In the aesthetic, we wanted to have something that represented Kai's feeling of the phantom arm.  We'd like the arm to be clear, possible acrylic.  Because the arm would be used in performance, we wanted something that would react well to stage lighting and perhaps even soft LED lighting incorporated on/in the arm.  This would give the audience the perception of something that is there, but yet perhaps missing.  Kai also expresses the arm sometimes feels shorter than the original and her sense of balance is thrown off.  This allows her to interact with the arm.

For practical reasons, we were thinking about using a telescoping mechanism that could perhaps be controlled by a glove-like controller in her left hand.  The controller might also control other things like lighting and maybe even introduce an element of variability.  The telescoping length would change the way Kai balances as she moves.  

We feel robotics would be employed in this project for  Kai to control the arm.