Long Flex/Bend Sensor can detect bending in one direction. They were popularized by being used in the Nintendo PowerGlove as a gaming interface.
Long Flex/Bend Sensor s are easy to use, they are basically resistors that change value based on how much they are flexed. If they're unflexed, the resistance is about ~10KΩ. When flexed all the way the resistance rises to ~20KΩ. You can use an analog input on a micro-controller (with a pullup resistor) or a digital input with the use of a 0.1uF capacitor for RC timing.
The bottom part of the sensor (where the pins are crimped on) is very delicate so make sure to have strain relief - such as clamping or gluing that part so as not to rip out the contacts!
Adafruit Web Site: https://www.adafruit.com/products/182