A circular steel punch (d = 32 mm) and 0.5-mm steel punch defined the outer diameter and the outlet port for the molded SIS sheet, respectively. A 0.5-μl volume of colorimetric agent (food dye or silver chloranilate solution) filled the reaction chamber of the device. Americolor Soft Gel Paste, Violet, served as the food dye. The silver chloranilate solution consisted of 0.1 g of silver chloranilate (City Chemical) in 0.5 ml of 1 wt % aqueous poly(acrylic acid) (Sigma-Aldrich). Vortexing this solution before application to the patterned, top layer ensured good dispersion. A CO2 laser cutting system (Universal Laser Systems VLS3.50) formed the necessary patterns in a uniform sheet of skin adhesive (Scapa Unifilm U884). The white, bottom layer of SIS and the laser-patterned adhesive encapsulated the SIS-coated NFC coil on the top and bottom, respectively. A 1-mm-diameter steel punch created a hole for the inlet in the white SIS. Applying a color reference dial to the white SIS layer adjacent to the inlet facilitated direct colorimetric readout of the chloride level in the sweat with the naked eye or with cell phone camera and digital image processing. Lamination of the molded SIS layer with the reagent onto the flat, white layer of SIS encapsulated the color reference dial and reagent. Cutting the edges with a steel punch (d = 40 mm) defined the outer perimeter of the device. Lightly pressing on the assembly and waiting for 30 min yielded a watertight bond between the microfluidic device layers.

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