# A Low-Cost Laser-Prototyped Microfluidic Device for Separating Cells and Bacteria
## 3 Results  
A mixture of 1 and 5 \(\upmu\)m polystyrene particles was infused into the microfluidic device at varying flow rates. Figure 2 shows the streak images obtained from the entrance, the end of the serpentine section, and the exit of the spiral section of the device for the inlet flow rate of 1000 \(\upmu\)L/min. It was observed that the polystyrene particles entered the device with random distribution across the width of the channel at the entrance (Figure 2b). At the end of the serpentine section, polystyrene particles seemed to form a visible focused stream (Figure 2c). This was in accordance with the results reported in the literature for the similar device geometries [33; 37; 54]. This focusing behavior was intended to be used before the spiral channel to eliminate the need for a hydrodynamic flow focusing which would require multiple inlets and sheath flow. After the serpentine section, focused particles entered the spiral section of the microfluidic device to be focused towards different streams based on their dimensions (Figure 2d). A darker stream appeared close to the inner wall which is formed mostly by 5 \(\upmu\)m particles. For a quantitative analysis of the particle separation performance of the device, different inlet flow rates were applied, and particles collected from outlets A and B were analyzed. It was found that inlet flow rates between 600 and 1000 \(\upmu\)L/min resulted in much higher separation purity and efficiencies. An example of particle pictures is shown  
Figure 2: Particle focusing behavior is demonstrated using the combined device. (**a**) Device schematic outline and image collection regions are shown. Streak images of particle flows are formed from; (**b**) entrance; (**c**) exit of the serpentine section and; (**d**) exit of the spiral section. in Figure 3 for 1000 \(\upmu\)L/min inlet flow rate. It is clear that the majority of the 1 \(\upmu\)m sized polystyrene particles exited the device from outlet A and 5 \(\upmu\)m particles exited from the outlet B. This is an expected outcome based on the predictions from the inertial focusing behavior of larger particles in the spiral microfluidic channels that is also in agreement with the literature. Next, the collected particle numbers were characterized by standard hemocytometer counting. In Figure 4, the purities of samples collected from the outlet A and B are shown. Here, purity was calculated by analyzing the number of 1 and 5 \(\upmu\)m polystyrene particles collected from each outlet and calculating the ratio of the particles. By trying different inlet flow rates, improved purities were observed at 600, 800, and 1000 \(\upmu\)L/min. It was found that the best performance in terms of purity of the collected particles was achieved at 800 \(\upmu\)L/min flow rate for this device design. For the outlets A and B, the purities of 1 and 5 \(\upmu\)m particles were found to be 93% and 89%, respectively. The outlet B was found to be more contaminated by 1 \(\upmu\)m particles compared to the contamination of the outlet A by the 5 \(\upmu\)m particles. This was due to the higher degree of dispersion of the 1 \(\upmu\)m particles compared to the larger 5 \(\upmu\)m ones. Larger particles were better focused by the inertial and dean flows. The efficiency of particle separation was also characterized by the collected samples as shown in Figure 5. Efficiency is defined as the ratio of the number of the collected targeted particles divided by the total number of the target particles. It was found that the separation efficiencies of 1 and 5 \(\upmu\)m were 89% and 93%, respectively. Due to their larger size and better focusing behavior, most of the 5 \(\upmu\)m particles were collected from the intended outlet of the device. Below 600 \(\upmu\)L/min flow rate, particle separation efficiency and outlet purities were lower which is likely due to inertial effects that resulted in poor particle focusing.