These sensors measure pressure and can also be designed and placed to measure bend. They work on the simple principal that Velostat reacts to pressure with a decrease in electrical resistance. When sandwiched between two conductive layers, this change in resistance can be easily measured and used as an indication of how much pressure is [...]

also: anti-static bag, ESD bag
Velostat is the brand name for this material manufactured by 3M. Linqstat is the same thing, just manufactured by another company. This is also the material from which the storage bags for sensitive electrical components are made.
Velostat is made of back, opaque, volume-conductive, carbon-impregnated polyolefin. The electrical characteristics are not affected [...]

Variations of very simple fabric pressure sensors made from various resistive fabrics sandwiched between conductive layers. This is an ongoing experimentation. Similar to the neoprene Bend and Pressure Sensors and the Stickytape Sensors, these also make use of the resistive changes of various materials under pressure. Variations in pressure sensitivity and stability can be achieved [...]

also: neoprene bend sensor, conductive thread Bend Sensor, Bend Sensor (thread)
This bend sensor actually reacts (decreases in resistance) to pressure, not specifically to bend. But because it is sandwiched between two layers of neoprene (rather sturdy fabric), pressure is exerted while bending, thus allowing one to measure bend (angle) via pressure.