## 3 Results and Discussions
### Optimization of Processing Parameters  
The signal-to-noise ratio (\(S/N\)) is the statistical value, measuring how the response varies relative to the nominal or target value under different noise conditions [17]. Higher values of the signal-to-noise ratio (\(S/N\)) identify control factor settings that minimize the effects of the noise factors. Maximizing the \(S/N\) ratio results in minimized properties that are sensitive to noise. Since the reduction of particle size is the object of this work, the expression depicting the smaller-the-better feature was adopted for the analysis:  
\[\frac{S}{N}=-10\log_{10}[(\frac{1}{n})\sum_{i=1}^{n}{(\vec{y}^{2}+{S_{n}}^{2})}] \tag{1}\]  
Figure 2: SEM photos and particle size distribution of electrospray microspheres (Test runs 1–18).  
where \(\overline{y}\), and \(S_{n}\) are the average value, and deviation of the calculated particle size, respectively, and \(n\) represents the number of specimen for each test run. The factor levels with the greatest \(S/N\) ratios will minimize the noises sensitivity and lead to the optimum levels.  
The change of particle size, due to various factors, was evaluated based on the technique proposed by [PERSON] [17]. Calculated \(S/N\) ratios for the sizes of electrospray PLGA particles are displayed in Figure 3. According to the figure, the optimized factor levels, which lead to the minimum particle size, were estimated to be \(A1/B1/C1/D1/E1/F2\). These optimum factor levels stand for a polymer concentration of 3%, a flow rate of 0.6 mL/h, a travel distance of 10 cm, a voltage of 11 kV, a needle size of 0.64 mm, and HFIP solvent.  
Since the optimum integration of factor levels was not contained in the L18 array experiment, an alternative path was engaged to estimate the response of the particle size to the optimum factor levels. Presuming there was no interaction among the chosen factors, the estimated \(S/N\) ratio for the optimum levels, \(\eta_{A1/B1/C1/D1/E1/F2}\), is  
\[\begin{split}\eta_{A1B1C1D1E1F2}=\eta_{m}+(\eta_{A1}-\eta_{m})+( \eta_{B1}-\eta_{m})+(\eta_{C1}-\eta_{m})+(\eta_{D1}-\eta_{m})+(\eta_{E1}-\eta_ {m})+(\eta_{F2}-\eta_{m})\\ =\eta_{A1}+\eta_{B1}+\eta_{C1}+\eta_{D1}+\eta_{E1}+\eta_{F2}-5\eta _{m}\end{split} \tag{2}\]  
where \(\eta_{m}\) is the mean \(S/N\) ratio for the experimental tests in Table 3, and \(\eta_{FN}\) is the \(S/N\) ratio for factor F and level N. On the basis of this equation, the estimated \(S/N\) ratio of electrospray particle sizes for the optimum levels, \(A1/B1/C1/D1/E1/F2\), was \(-2.75\) dB. This estimated value was surely higher than those attained in the 18 test trials in Table 3.  
A verification test was completed in line with the optimum factor levels. The optimized levels for electrospray particles are \(A1/B1/C1/D1/E1/F2\). The particle size thus acquired was 1.43 \(\pm\) 0.93 \(\upmu\)m (as displayed in Figure 4). Furthermore, the \(S/N\) ratio for this verification experiment was \(-4.67\) dB. Although this value is lower than the estimated value of \(-2.75\) dB, and the one prepared using  
Figure 3: Variation of the \(S/N\) ratio with factor levels for electrospray poly (d,l)-lactide-co-glycolide (PLGAA) particles.  
processing condition 15 (\(-\)3.63 dB), it is higher than those achieved by other processing conditions in the 18 test trials in Table 3. Furthermore, when compared with the size distribution of particles produced by condition 15 (Figure 2), microspheres prepared using the optimum condition showed a greater percentage (47.72%) of particles with a size less than 1000 nm. Consequently, by employing the optimum factor levels, the particle sizes of electrosprayed microspheres were adequately minimized.