PANI–based wearable pH sensors. (A) The fabrication steps for PANI–based pH sensors on a tattoo substrate. (B) The tattoo applied to the cubital fossa at different bending and stretching states and cyclic tests. Reprinted with permission from [54], Copyright (2013) Royal Society of Chemistry. (C) The structure of a fiber pH sensor deposited by PANI. Reprinted with permission from [70], Copyright (2020) Royal Society of Chemistry. (D) Paper–based pH sensor under bending tests. Reprinted with permission from [71], Copyright (2017) Elsevier. (E) Wearable sensor array with PANI electrode integrated on PET substrate. Reprinted with permission from [72], Copyright (2016) American Chemistry Society. (F) Wearable sensor array with PANI electrode integrated on PDMS substrate. Reprinted with permission from [22], Copyright (2020) American Chemistry Society. (G) Self–powered wearable sensor with PANI electrode. Reprinted with permission from Science Advances [73], Copyright (2020) American Association for the Advancement of Science. (H) Bandage–based pH sensor with PANI electrode. Reprinted with permission from [74], Copyright (2014) John Wiley and Sons publications. (I) pH sensor array on paper substrate with self–aligned encapsulation. Reprinted with permission from [69], Copyright (2016) Elsevier. (J) A multiplexed microfluidic thread–based pH sensor: (a,b) structure of thread-based sensor; (c) cross–sectional model for in vitro pH measurement on the skin; (d) an integrated sensing system utilizing a wireless communication system. Reprinted with permission from [75], Copyright (2016) Spring Nature.