These are Force Sensing Resistors® or FSRs® manufactured by Interlink Electronics. As used in the latest development of the Kossel 3D printers.
FSR402 0.5" Diameter with solder tabs.
Digikey Part Number: 1027-1001-ND
Manufacturer's Part Number: 30-81794
Active Area 14.68mm dia
Nominal Thckness: 0.46mm
Non Actuated Resistance: >10 megohms
Sensing range: Approx. 0.2N - 20N