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.