2.5. Biocompatibility and 3D cell culture test of glucose biosensors
Human hepatic carcinoma cells (HepG2 cells) were purchased from ATCC, USA and maintained in Dulbecco’s modified Eagle’s medium (DMEM) supplemented with 10% fetal bovine serum and 1% penicillin under standard cell-culture conditions (i.e., 37°C, 5% CO2atmosphere). The medium was changed every 3 days. For the experimental study, the cells were trypsinized and subcultured in 48 well plates (1 × 106 cells/mL) per well. The electrodes were autoclaved and coated with the Nafion/MWCNT/GOx solution and then dried at room temperature. The dried coated electrodes were submerged in a well containing attached HepG2 cells, and the setup was incubated for 1, 3, and 7 days. The viability of HepG2 cells was determined using live/dead assays with the staining of calcein-AM and ethidium homodimer. The images were captured using a fluorescence microscope (Nikon, Japan). The hydrogel used in the 3D hydrogel test was prepared using a glass slide and cover slip by mixing alginate and collagen in the 1:1 ratio, and HepG2 cells were encapsulated therein. Approximately 300 μL of the hydrogel was added, and the coverslip was immediately placed on the hydrogel. Then, CaCl2 was slowly added so that the entire glass slide got submerged in the CaCl2 solution. If the coverslip got displaced after 2 h, the disc-shaped hydrogel formation was considered completed on the glass slide. For performing the cell glucose consumption test in the 3D hydrogel, HepG2 cells were cultured (2 × 106 cells/mL) inside the hydrogel and cultured in an incubator under standard cell-culture conditions (i.e., 37°C, 5% CO2 atmosphere). The sample was electrically measured using a w/ cell sample containing cells and a blank sample containing no cells.