Fig. 3: Operating principle of the self-powered patch.

a Battery stack calibration with samples at increasing concentrations of 5 mM, 10 mM, 20 mM, 40 mM, 48 mM (in dark blue), 60 mM (in dark yellow), 80 mM, 120 mM and 160 mM NaCl. The battery stack calibration showed that R_LOAD ranges from 2 kΩ to 6.5 kΩ, which enabled setting the battery operation within its ohmic region. b Voltage and power generated by the battery when introducing an R_LOAD of 2 kΩ. c Gaussian distribution around central values of 1.46 V and 1.57 V, which corresponds to 48 mM equiv NaCl and 60 mM equiv NaCl, respectively, when the battery was submitted to an R_LOAD of 2 kΩ. d Circuit operation when healthy and non-healthy sweat samples are analysed. e Voltage and current provided by the battery and collected in the Control Display, respectively, at the lowest conductive sample. f, g Operation of the whole device with samples at 48 mM and 60 mM equiv NaCl in voltage at the gate terminal of the transistor and the current stored in the Test Display. h Charge accumulated in the Test Display at the two concentrations of NaCl