Fabrication of Flex Sensors using Inkjet-Printed Carbon on Polyethylene Terephthalate for Flexible Electronics

Resistive flex sensors are widely used in wearable electronics. Currently available flex sensors typically exhibit non-linear responses and require complex calibrations. In this project, a cost-effective, durable flex sensor with tunable sensitivity was developed using inkjet-printed carbon black and silver inks on a polyethylene terephthalate (PET) substrate. According to the ASTM D3359 (Method B) test, the 5B standard superior adhesive carbon trace on PET serves as the sensing element of the flex sensor, with silver interconnects. The variations in sensitivity with the geometry and thickness of the printed element were observed. Discontinuities in the printed carbon element during the first printing cycle resulted in greater changes in resistance. The sensitivity decreased as the thickness of the conducting layer increased over repeated printing cycles. The sensors (30 mm x 7 mm) showed sensitivities of ~0.15%, ~0.11%, and ~0.09% change in resistance per degree bending across one to three printing cycles, respectively, with R² > 0.98, maintaining excellent linearity across 0°-120° bending. The results indicate that the high sensitivity and exceptional linearity enable direct integration into soft robotics, wearable electronics, prosthetics and motion tracking.