An "operation" game sensor where the stylus is NOT attached to the power source

Question

Edit: The person operating the catheter will be wearing latex gloves. The only human tissue the catheter would touch would be that of the patient. Also, to clear up some confusion about what exactly we're trying to, I included a picture below. The catheter is not threading through the dura, but outside of it.

So, we're designing what is essentially an operation-type game, with one major exception. Whereas a normal operation game has both the tweezers and the metal rims attached to the battery.

Ours, however, is intended to function as a medical training tool. An actual stainless steel catheter or needle will be used, and we obviously can't hook that up to a battery. Whenever the metal touches this long tube (your spinal cord, if you like the bio stuff), we're going to have an LED light up.

All of the information we can find about completing a circuit by touching something to it involves that something hitting in a precise spot to close the circuit. But we need to trigger the LED by touching ANY part of the tube.

We're planning on wrapping the tube in copper tape, in bands separated by rubber. Our output is a row of LED lights, indicating the generally area touched (based on the bands).

Our only idea so far, in terms of circuit design, is to design a floating circuit and then use the catheter tip as the grounding element. Once grounded, the circuit will activate and turn on the LED. Any idea if this will work or not? Or any other ideas? The only thing that cannot be changed is the medical grade stainless steel of the catheter/needle- all other materials are flexible.

Thanks!

Answer 1

A potential non-resistive non-capacitive contact detection method:

Using a clear piece of plastic tubing you project a light source into the flat end of the material. The clear tubing will act as a lossy fiber optic cable. You then fill the inside of the tube with a dark opaque liquid.

As you often see with low cost fiber optics there is usually a fair amount of light excaping from the outer surface of the fiber. Similarly the clear tubbing would have some light lost to both the outer and inner surfaces. However in this case the light being lost to the inner surface would not be reflected back as much due to the dark liquid inside the tube. Now if a shiny object (for eg. a shiny stainless steel catheter needle) were to come very close or contact the inner wall there should be a noticable bright spot visible even when looking from the outer wall of the tube. This bright spot (contact point) could be detected by eye, by one or more electrical light sensors, a video recorder, or all of these.

If using a light sensor to detect a bright spot the sensitivity and rejection of ambient light could be improved by having the tube's light source be high frequency pulses. This pulse frequency can then be detected and filtered using a band pass filter in a light detector circuit. Any sudden peak in this detected light would indicated that there was contact with the inner tubing wall. If the light source also used a polarizing filter you may be able to pick up a unique polarized reflection from an object touching the inner wall.

(Edit)
OK, so things are a bit different due to the dura. Well then take the plastic dura tube seal it at the bottom, fill the center with clear liquid, then place the whole dura tube into a larger tube of dark opaque liquid. Now run the catheter along the outside of the dura, any touching of the dura tube's outer wall should give a reflection going towards the center of the tube. The light increase may be detectable by looking into the center of the dura tube, or perhaps you could run a light sensor down into the dura tube in step with the catheter just to be sure to catch any reflections.