This is a displacement resistance type touch sensor.
It uses conductive foam as the resistive element.
The resistance can be easily tailored and tweaked to your needs.
If you would like a Walk-through of how to build the Sensor can be found here :-
Picture Walk through Here :- http://forums.parallax.com/showthread.php/154706-3D-printed-Touch-Sensor-Cost-effective-Displacement-type or Here :- http://letsmakerobots.com/node/40683
The sensor can be hooked up via an analogue input to any of the common mcu chips (Parallax Propeller,Arduino,Picaxe,Raspberry Pi)
It is possible to make your own conductive foam by using a Bath sponge soaked in this conductive ink that can be found Here :-
You will need a simple A4 paper Hole puncher
A small sheet of flat thin copper (or tin at a pinch ... just remember that a wire has to be soldered to it)
A small piece of "Conductive Foam" , this foam is commonly found as an antistatic foam used for shipping integrated circuit chips. It is a black foam - if you are not sure then just probe it with a multimeter turned to the "Ohms" scale..(remember that different foams will have different qualities and resistances)
Use the hole punch to punch 4 to 6 cylinders of the conductive foam.
Use the hole punch to punch 2 discs out of the copper sheet.
Solder Wire (I used CAT5 Ethernet inner wires - as they are plentiful and easy to come by and they have a good threading ability ) onto each of the copper disks.
Feed the wire into the 3D printed base and plunger.
Feed the conductive foam carefully into the base unit until you get the desired feel of the touchiness (or resistance value)
That is it, just use your amazing imagination, I will be more than please to see some remakes and crazy applications...