Fig. 5: Design and functional verification of supporting circuit board for the ISMA.

a Circuit implementation and layout design diagram of the printed circuit board. The red dashed boxes indicate the locations of the circuit components and modules. b System block diagram of the supporting circuit for ISMA. The circuit module contained the required integrated circuit chips and peripheral electronics for the potentiometric recording of three types of ions, signal processing, control and feedback, and Bluetooth wireless transmission circuitry. Thus, the module formed a compact system to support the functions of the microneedle device. The sensing circuit module consisted of three parallel sets of two-electrode systems. The two-electrode system enabled the recording of potentiometric signals that reflected the ion concentrations. c Photograph of the printed circuit board connected with the microneedle sensor. d Correlations of the input and output voltage signals from 0 V to 3000 mV by the printed circuit board; these correlations can be used for the potential detection of the concentrations of different ions. e Comparisons of the potentiometric responses of the ISMA to the respective analyte solutions in phosphate-buffered saline (PBS) using both a printed circuit board and electrochemical workstation (ECW). The detection range varies from 0.01 to 100 mM for Ca²⁺ sensing, from 1 to 32 mM for K⁺ detection and from 0.01 to 100 mM for Na⁺ sensing. f Comparisons of corresponding calibration plots of the ISMA to the different types of ions using both PCB and ECW. g Photographic image showing the prototype of ISMA with the size in accordance with the requirements to be worn on the arm or body. h Ion detection app interface formed through Bluetooth transmission in real time