Fundamentals of epidermal wearable biosensors. (A) Electrochemical biosensors, (a) first− (Reprinted with permission from Ref. [114]. Copyright 2022, Elsevier), (b) second− (Reprinted with permission from Ref. [129]. Copyright 2022, American Chemical Society), and (c) third−generation (Reprinted with permission from Ref. [121]. Copyright 2020, Elsevier) enzyme-based biosensors, (d) immune-based biosensors (Reprinted with permission from Ref. [117]. Copyright 2022, American Chemical Society) and (e) aptamer-based biosensors [119]. Colorimetric biosensors use (B) a cell phone (Reprinted with permission from Ref. [151]. Copyright 2020, American Chemical Society) or (C) the naked eye (Adapted with permission from Ref. [150]. Copyright 2021, Elsevier) to capture the color changes.