Fig. 1: Schematics and fabrication process of the microneedle sensor for continuous ion monitoring.

a Schematic of the microneedle-array sensor for ion detection. The microneedle-array sensor, which can be worn on the hand, performs minimally invasive detection through the skin to measure the concentrations of different ions in the interstitial fluid. The sensor transmits the signal through a printed circuit board to a microprocessor for signal processing and display. b Schematic of the fabrication method of the microneedle-array sensor. The planar sheet is processed into a planar microneedle structure via laser micromachining, and a conductive Au layer and PEDOT layer are electrochemically deposited on the microneedle tips; afterward, the ion-selective film is modified on it. These form the microneedle working electrodes, which achieve the selective detection of ions. Moreover, the Au-coated microneedles are further modified with Ag/AgCl ink and a PVB protective layer to form a microneedle reference electrode. The three microneedle patches (working electrodes for Ca²⁺, K⁺, and Na⁺ sensing) are integrated with the microneedle reference electrode to obtain the ion-sensing microneedle array (ISMA). The ISMA can be further integrated with a printed circuit board to form an integrated system, which supports the sensing functionalities of the microneedle device