## 3 Results and Discussions
### Significance of Factors  
A standard analysis of variance (ANOVA) was completed in line with the technique proposed by [PERSON] [17]. Table 4 lists the computed results for electrosprayed PLGA particles. The variance ratio, represented by F in Table 4, is calculated by dividing the factor mean square by the error mean square. A larger value of F indicates that the significance of that factor is greater compared with the error variance. The importance of each factor on the particle size of electrosprayed PLGA microspheres can be determined by the values of F in Table 4. On the basis of Table 4, the relative importance of each factor on the particle size of electrosprayed PLGA microspheres was, in decreasing order, solvent type (F = 49.02), flow rate (F = 2.76), travel distance (F = 2.46), size of needle (F = 2.10), polymer concentration  
Figure 4: SEM photo and size distribution of particles electrosprayed using the optimum processing condition. \begin{table}
\begin{tabular}{c c c} \hline
**Run** & **Particle Size (\(\upmu\)m)** & _S/N Ratio_ \\ \hline  
\hline \end{tabular}
\end{table} 
Table 3: Tabulation of size distribution for electrosprayed particles in the main experiment. (F = 1.62), and applied voltage (F = 1.49), respectively. For the factors chosen in this analysis, the solvent used was found to be the key factor affecting the particle size of electrospray microspheres. The statistical results in Figure 3 (factor level F2) suggest that one should use hexafluoro-2-propanol (HFIP) as the solvent to acquire the optimized microparticles. HFIP is a fluorinated alcohol frequently used as solvent for polymer systems. HFIP shows strong hydrogen bonding properties causing it to better dissolve a number of molecules with suitable receptive sites such as oxygen atoms, double bonds or amine groups (Sigma-Aldrich, St. Louis, MO, USA) [18]. It is easier for the HFIP dissolved PLGA solution to be stretched and break into small droplets by the external electrical forces during the process. Electrosprayed microparticles thus showed the smallest diameter distribution. DCM was found to electrospray microspheres with a fairly spherical geometry, while TCM electrosprayed particles with relatively rougher surfaces. Surface tension is well known to be responsible for the spherical shape of liquid droplets. TCM has a higher polarity than DCM, which makes itself a good solvent for a wide range of reactions [18]. The polar force of TCM may counteract the solution's surface tension and result in a rough surface of the droplets and the relevant solid particles. Relatively, DCM has a lower polarity and thus produces microparticles with a more spherical surface. On the other hand, hollowed particles could be observed in particles prepared using HFIP, such as those prepared by conditions 1, 6, 7, 10 in Table 2. The only exceptions are the ones produced using a higher flow rate (conditions 6 and 15), namely 1.0 mL/h. This might be due to the fact that HFIP evaporates faster than DCM and TCM, and may leave a hollowed core after evaporation. However, when the flow rate is high, the excessive polymeric material may be able to compensate the hollowed material at the core. Electrosprayed particles thus exhibited a less hollowed geometry. The data in the main experiment (factor level B1 in Figure 3) suggest that the PLGA materials electrosprayed with the least flow rate resulted in microparticles of smallest diameter distribution. It has been reported that there is a positive correlation between PLGA solution flow rate and particle size, so that, as the flow rate decreases, the size of electrospray microparticles decreases also [19]. The smallest flow rate (0.6 mL/h) employed in the experiments thus produced particles with the smallest size distribution. The experimental results in factor level B1 of Figure 3, suggested that one can obtain microparticles of the smallest diameter by utilizing the least travel distance.