Fabrication of Microfluidic Sweat Chip

The microfluidic chip includes a PDMS cover layer and a PDMS channel layer with microfluidic channels and chambers, and a waterproof medical biological double-sided adhesive pasted under the PDMS channel layer. Figures S1 and S2 show the details of the fabrication processing of microfluidic chips and the detailed chip design parameters. The mold of the PDMS channel layer was fabricated by patterning negative photoresist (SU-8 3050, Microchem, Newton, MA, USA) on a silicon wafer using a direct laser writing system (MicroWriter ML3, Durham Magneto Optics Ltd., London, UK). The SU-8 spin speed and time: 1000 rpm, 40 s; select exposure dose was 2200 mJ/cm². The surface of PDMS was flattened by standing for 1 h on the horizontal plane. There are two reaction chambers with a volume of about 2.5 µL (diameter: 4 mm, height: 200 μm) and short straight ducts (width: 300 μm, height: 200 μm) on the PDMS layer. The PDMS prepolymer and curing agent were prepared in a mass ratio of 15:1, and 10% Triton X-100 was added to mix evenly, poured onto the fabricated mold followed by vacuuming for 30 min to remove the air between micropillars, and then cured at 80 °C for 1 h. After lifting the PDMS channel layer from the silicon mold, the inlets and outlets of the microchannel were punched using a punch with a diameter of 1.5 mm. The PDMS cover layer without Triton X-100 (thickness: 800 μm) was processed in the same way. The processed PDMS channel layer and PDMS cover layer were cut into circles (diameter: 30 mm) with a scriber. Then, the glucose sensor and the pH sensor were placed in the chamber of the PDMS channel layer, followed by bonding two PDMS layers together using oxygen plasma treatment (PDC-002, Harrick Plasma, Ithaca, NY, USA). Finally, three sweat inlets with a diameter of 3 mm were punched on the waterproof medical double-sided glue to ensure that each inlet could cover about 5 to 10 sweat glands. The microfluidic chip and the adhesive layer were aligned according to the position of the inlet and then bonded. The fabricated microfluidic sweat chip was placed in a plastic bag, vacuumed, and stored in a cool and ventilated place for subsequent use.