# Microfabrication Bonding Process Optimization for a 3D Multi-Layer PDMS Suspended Microfluidics
## 3 Experimental Validation of the SPMF\({}^{3}\)  
An experiment of injecting water with different flow rates has been performed to validate the sensitivity of fabricated 3D suspended microchannel to flow forces. The microcantilever deflections were measured by an optical laser-based deflection measurement system shown in Figure 11.  
In this measurement system, reflected laser light from the shiny cantilever tip was detected with PSD, and its displacement history was recorded in a data logger. Since the laser, microcantilever, and PSD are on the same plane, the displacements of the reflected laser light can be transformed into microcantilever deflections using a geometrical formula [32]. In order to study the microchannel's dimensions impact on the sensitivity of the fabricated microsystem, two SPMF\({}^{3}\) with different microchannel dimensions were fabricated for this experiment. The first SPMF\({}^{3}\) sample has a channel size of 400 \(\times\) 100 \(\upmu\)m\({}^{2}\), respectively, and the second one has a channel size of 200 \(\times\) 100 \(\upmu\)m\({}^{2}\). A syringe pump was employed to inject DI water with different flow rates from 0 to 20 \(\upmu\)L/min into SPMF\({}^{3}\) samples. As shown below, Figures 12 and 13, both SPMF\({}^{3}\) samples had high sensitivity to flow forces when a fluid passed through the suspended microchannel and nozzle. Figure 11: Laser displacement measurement system. Figure 10: (**a**) Top view of the three microchannels and nozzle layers with an undesired offset; (**b**) side view of a fabricated SPMF\({}^{3}\) with undesired offset between layers. In the next experiment, a peristaltic pump was used to investigate the sensitivity of fabricated SPMF\({}^{3}\) under dynamic loads such as a pulsating flow. The peristaltic pump speed is fixed at 2 rpm, and it is connected to the both fabricated SPMF\({}^{3}\) systems with different microchannel dimensions of 200 \(\times\) 100 \(\upmu\)m\({}^{2}\) and 400 \(\times\) 100 \(\upmu\)m\({}^{2}\) (width by depth). According to measured cantilever deflections shown in Figure 14, the SPMF\({}^{3}\) systems are sensitive to dynamic loads of pulsating flows generated by rollers of the peristaltic pump. Here, the microfluidic system was tested at 2 rpm pump speed that generated flow pulses of 0.33 Hz due to 10 rotating rollers of the pump. Figure 12: The sensitivity experiment of 3D suspended microcantilever has a 400 \(\times\) 100 \(\upmu\)m\({}^{2}\) (width and depth) channel with various step flows of 10-15-20-15-10-0 \(\upmu\)L/min. The syringe pump flow rate changes every 60 s, and the SPMF\({}^{3}\) responds to this new flow rate. Figure 13: Sensitivity experiment of 3D suspended microcantilever, which has a 200 \(\times\) 100 \(\upmu\)m\({}^{2}\) (width and depth) channel with various step flows of 0-10-0-15-0-10 \(\upmu\)L/min. The syringe pump flow rate changes every 60 s, and SPMF\({}^{3}\) responds to this new flow rate. In the next experiment, DI water with different concentrations of salt varying from 0-15% of weight was injected into the two SPMF\({}^{3}\) samples with different microchannel dimensions. The intention was to verify the sensitivity of SPMF\({}^{3}\) relative to various fluid properties such as density and viscosity. Varying salt concentration in water results in different fluid density and viscosity, as shown in Table 2  
Figure 14: SPMF\({}^{3}\) response to dynamic load of a Peristaltic pump with a fixed speed of 2 rpm, which was generating flow pulses of 0.33 Hz due to 10 rotating rollers of the pump, i.e., 20 pulses in 60 s. (a) The SPMF\({}^{3}\) sample has a microchannel of 400 \(\times\) 100 \(\upmu\)m\({}^{2}\) (width and depth). (b) The SPMF\({}^{3}\) sample has a microchannel of 200 \(\times\) 100 \(\upmu\)m\({}^{2}\). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Salt wt \%** & **Density (kg/m3)** & **Viscosity (cP)** \\ \hline
0\% & 999 & 1.002 \\ \hline
10\% & 1070 & 1.193 \\ \hline
15\% & 1110 & 1.350 \\ \hline \end{tabular}
\end{table}