Flex sensors can be used to measure the bending of the sensor, or objects they are attached to.
When the sensor is bent, resistance increases. The resistance only changes when the metal pads are on the outside of the radius, so bending radius can only be measured in one direction.
A conductive ink is dispersed in layers, creating a passive resistive sensor. When the ink is stretched (or bent), resistance increases since the ink layer is getting thinner.