Making the Bi-Directional Flex Sensor:
The resistive material is sandwiched between the two copper clad laminates. The copper sides of the laminates are both facing toward the resistive material; see figures 3, 4 and 5. If you soldered the wires on the same side of the laminates, when you assemble the sensor, the wires will be positioned on opposite corners allowing the base of the sensor to lay flatter.
To this sandwhich we add the acetate strip, see figure 6. The purpose of the acetate strip is to make the sensor more resilient and to spring back after it has been flexed.