RoboRoach works on the principal of changing series resistance, on the digitally controlled potentiometer, in the stimulation circuit and not by controlling the current. 

Current depends on the frequency of the stimulation and total impedance of the circuit. 

Current stimulation goes through the following path:

  1.  from battery to the microcontroller
  2. from GPIO (voltage equal to battery voltage ~3V, this GPIO has some output impedance)
  3.  through the digitally controlled resistor
  4.  through the capacitor (capacitor has same impedance)
  5. through silver wire into antennas (this connection has some impedance that depends on how the wire is inserted into the antennas etc.)
  6.  through cockroach body (this is also variable impedance and depends of the property of the particular cockroach)
  7.  back into the battery (battery also has some equivalent series resistance)

Since we are not controlling a big part of the impedance we don't know what is the current through the antennas. In the link below, you can also find a schematics file of the RoboRoach backpack.