thickening behaviour [@Bjrn2012]. The resultant curve with a power law model, yield a good fit to the flow curve (R$^2$ = 0.9998) between the values 5 to 10 $^2$ s$^1$ s$^{-1}$. We apply a curve fitting based on equation \[powerlaw\] to the log-log plot which determine $K$= 9530 m Pa.s$^n$ and $n$ = 0.403, indicating that the graphene ink behaves as a viscous fluid that is very shear thinning.

Using the results of the first set of OFAT experiments, we establish a realistic range of parameter levels, based on the three basic ink components- graphene (10 – 23 wt.%), EC binder (75 – 88 wt.%) and IPA (0 – 2 wt.%) as a solvent. These ranges are used as the upper and lower limits for the DOE to optimise the conductivity of the ink. We note that the above ranges may differ should graphene from a different source be used. DOE is a widely-used statistical approach to experimental formulation in screening and optimising parameters (also known as factors) [@Ng2019e; @Jacquez1998] and multivariate analysis. Inks are produced according to the parameter ranges indicated in Fig. \[panel1\]e and plotted in a manner reflecting the interaction between graphene, IPA and EC binder. The 