| Independent variables | Coded symbols | Levels | ||
| −1 | 0 | 1 | ||
| The volume of TSC (mL) | X1 | 7 | 9 | 11 |
| The concentration of Ag NPs (mg/mL) | X2 | 4 | 5 | 6 |
| GYM/Ag NPs bulk ratio | X3 | 1 | 2 | 3 |