Fig. 2. (a) Fabrication processes of IrO₂ based pH sensor on a polyimide substrate, Reprinted from Publication¹¹⁵ with permission from Elsevier. (b) Transparent and flexible CNT/PANI pH sensors made on PET and PVC coated steel wire substrates.¹²⁵ (c) Image of flexible sensor made on PET substrate (inset) fitted in a tube with a radius of curvature 5 mm.²² (d) Flexible ZnO TF-FET pH sensor on Si/SiO₂ and on PEN (transparent) substrates. (e) Stretchable pH sensors on PDMS substrate Reprinted from Publication⁸ with permission from Elsevier. (f) Sensor on a yarn substrate attached on band-aid.¹³² (g) Graphite and Ag/AgCl based pH sensor on cellulose/polyester cloth substrate.¹³³ (h) Photographs of pristine newspaper and PC-paper for pH sensor fabrication Reprinted from Publication¹³⁴ with permission from Elsevier. (i) Schematic representation of the fabrication steps of the sensors on tattoo Reprinted with permission from Publication.¹³⁵.