Flex sensor is a two terminal device. The Flex sensor does not have polarized terminals like diode. So there is no positive and negative.
Operating  voltage of FLEX SENSOR: 0-5V ( used with a Resistance as divider)
Can operate on LOW voltages
Power rating : 0.5Watt (continuous), 1 Watt (peak)
Life: 1 million
Operating temperature: -45ºC to +80ºC
Flat Resistance: 25K Ω
Resistance Tolerance: ±30%
Bend Resistance Range: 45K to 125K Ohms(depending on bend)