Is to possible to measure electric signals using a capacitive touchscreen?
I've worked a few years in a company making touch controller ICs for large size Touch Panel. According to my knowledge of these product (world leader in their market segment), I can tell you that:


there are high-pass filters in the data acquisition chain, that will prevent you to measure signals below 1~10 kHz.
The touch sensors must be able to detect touch in many different conditions (size of the finger, temperature, humidity etc...). I think such sensors are not able to measure fine local variations of the body part, because these signals are much much lower than the inherent noise of the touch system.


Depending on the touch system solution, the data post processing may be different, and you solution may work on one type of touch panel, and not on another type.

I would be you I would search for specific sensors, and forget the touch panel.