As your application seems not to be so "precise" (0.5mm resolution at 80mm stroke), a rotary encoder built-in an old ball-type mouse will be available.
Touch the mouse ball directly to the rotational rod of the tele-(micro-?)scope. Or disassemble the mouse and extend the wheel shaft.

Blood is troublesome because it contains salt and protein.
The concentration of salt (NaCl) in blood is almost comparable to seawater. Blood (and extra-cellular fluid) is considered as "inner sea", relic when our ancestors lived under water.
So isolation is needed for circuits, and metals should be finished rust-proof.
The proteins, fiblinogen, albumin, globulin, etc.., adhere every surface even on glass and metal. They form obstruction for mechanism.
A finger part of rubber gloves and condom are often used to cover small equipments temporarily.

Tsuneo