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 being applied.

Instructable >> http://www.instructables.com/id/Stickytape-Sensors/
Flickr photo set >> http://www.flickr.com/photos/plusea/sets/72157617163832637

Unfortunately, by simply measuring the change in resistance you can not differentiate between the intensity of pressure and surface area. If you pressure hard in one point or less hard, but over a larger area, this will yield the same results. To detect surface area separately from the amount of pressure being applied one has to design the sensor as a grid of individual sensing points.

Construction

Video

Related Work

The sensor company Images – Scientific Instruments has a great set of instructions on how to make your own Bi-Directional Flex Sensors. The layering of materials is very similar to that of the Stickytape sensors.
>> http://www.imagesco.com/articles/flex/sensor-pg1.html

DIY FSR by Iain McCurdy:
With Velostat >> http://iainmccurdy.org/diy/forcesensorlowprofile/
With antistatic foam >> http://iainmccurdy.org/diy/forcesensorfoam/