## 3 Results and Discussion
### Permeation
It is evident from the 3D surface response plot that as the concentration of sodium carboxymethyl cellulose increased, the permeation of drug decreased while paraffin oil and Tween 80 had no significant effect on drug permeation. It was also apparent from contour plot that sodium carboxymethyl cellulose concentration had significant effect on drug permeation. The percentage drug release and amount of drug permeation were found to be significantly higher only when sodium carboxymethyl cellulose content was very low. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Time**} & \begin{tabular}{c} **Insulin** \\ **Solution** \\ \end{tabular} & \multirow{2}{*}{**Insulin Gel**} & \begin{tabular}{c} **Insulin** \\ **Niosome** \\ \end{tabular} & \begin{tabular}{c} **Insulin Niosome** \\ **Gel** \\ \end{tabular} & \
\begin{tabular}{c} **Insulin** \\ **Emulgel** \\ \end{tabular} &
\begin{tabular}{c} **Insulin Niosome** \\ **Emulgel** \\ \end{tabular} \\ \hline SSTF (\(\mu\)g/cm\({}^{2}\)/h) & 9.06 & 18.39 & 13.17 & 53.11 & 28.72 & 90.78 \\ ER & 1.00 & 2.03 & 1.45 & 5.86 & 3.17 & 10.02 \\ Mechanism & Zero & Zero & Higuchi & Higuchi & Zero & Zero \\ R\({}^{2}\) & 0.986 & 0.967 & 0.973 & 0.976 & 0.982 & 0.989 \\ \hline \hline \end{tabular}  
* Insulin solution was taken as control for calculations of enhancement ratio (ER).  
\end{table} 
Table 6: Permeation parameters of the prepared insulin formulations. Figure 7: Permeability profiles of insulin from different formulations and control (insulin solution) across abdominal rat skin. Data represented as mean \(\pm\) SD (n = 6). The fitted polynomial equation for in vitro permeation is given below:  
\[\text{Y}=4.38-0.0269\text{x}_{1}-0.02\text{x}_{2}-0.0585\text{x}_{3}+0.0060\text{ x}_{1}\text{x}_{2}+0.0085\text{x}_{1}\text{x}_{3}+0.0064\text{x}_{2}\text{x}_{3}+0.0022 \tag{9}\]  
Increase in insulin diffusion is apparently due to the interaction between the nonionic surfactant (span 60) and the rat skin lipids. It has been disclosed that, the surfactant could increase the fluidity of skin membranes leading to a faster drug diffusion rate across natural membranes [39]. Niosomes can diffuse across the dermis also and release antidiabetic drugs directly inside the skin [20]. Moreover, from the results in Table 6, it is obvious that the niosome vesicles combined with emulgels could have synergistically enhanced insulin skin penetration. In a previous study, we have shown that there may be a plausible merging between niosomes and emulsion droplets resulting in new structured vesicles resembling transfersomes [6]. This could be the major determining factor that resulted in the greatest  
Figure 8: Surface response plot and contour plot showing the influence of three independent variables (concentration of paraffin oil, Tween 80 and sodium carboxymethyl cellulose) on in vitro permeation of insulin from niosome emulgel. SSTF for insulin from niosome emulgel. Conversely, as shown in Table 6, emulgels yielded higher SSTF than gels; however, emulgels are of greater viscosity as shown in Table 3. This result could be explained by the fact that emulgels contain the nonionic surfactant and hydrocarbon oil that had been reported as penetration enhancers [40]. Moreover, kinetic analysis revealed that the mechanism of insulin permeability from niosome suspension and niosomes in gel was according to Higuchi diffusion model that excludes the effect of rat skin on insulin permeability (Table 6). In contrary, all other vehicles including emulgels, sodium carboxymethyl cellulose gel, niosome/emulgel and solution are showing zero order release kinetics that confirms the vehicle/skin interactions.