# Modeling of Integrated Nanoneedle-Microfluidic System for Single Cell Temperature Measurement
## 3 Results and Discussion
The temperature difference was set to be 0.02 \({}^{\circ}\)C by setting the surrounding temperature to 293.17 K and the temperature of the nanoneedle at 293.15 K. Figure 16a illustrates the transition the nanoneedle undertook while attempting to come in equilibrium with the surrounding temperature. Figure 16b shows the temperature response after it came into equilibrium with the surrounding temperature; the plot showed that the nanoneedle took 10 ms to come in equilibrium with the surrounding temperature. The results presented show that the nanoneedle can react to temperature in a very fast time with high sensitivity, which is very important in single cell temperature measurements. The fourth stage of the analysis was to optimize the microchannel height and the velocity for successful penetration. Figure 17 shows the simulation results of the microchannel height optimization analysis. The channel height had a strong effect on the cell flow and penetration into the nanoneedle. The higher the height, the better the flow was and the more penetration was observed on the cell. This could be explained by the effect of the water flow distribution that was disturbed by the electrode structure in the microchannel (Figure 18). By comparing the backward velocities in channel heights of 7, 8 and 9 \(\upmu\)m, the velocity flow in the opposite direction of the nanoneedle was higher as the channel height decreased (indicated by the red arrow and the highest legend value in Figure 18). Furthermore, there was a noticeable forward velocity flowing towards the nanoneedle in the all heights. However, the 8 \(\upmu\)m channel height had more forward flow distribution towards the nanoneedle compared to the 7 \(\upmu\)m but with clear backward velocity. The 9 \(\upmu\)m channel height, had no obvious backwards velocity and more forward velocity flow in the channel. Figure 14: (**a**) Cross-sectional view of the current density results; (**b**) Zoomed image of the direction of the current flow. This confirms that the temperature measurement is independent of the cell conductivity. After optimizing the channel height, the velocity range at which successful penetration could be achieved had to be determined in order to optimize the flow rate range to be used by the external micropump for the nanoneedle-microfluidic system. Figure 19 presents the simulation results for flow rate optimization. Figure 19a displays the side view of the penetration, while Figure 19b shows the top view of the penetration depth. At first, the velocity was set to 5 \(\upmu\)m/s, which was equivalent to  
Figure 7:0 **(a)** Graph shows the nanoneedle transition with temperature change of 0.015 \({}^{\circ}\)C; **(b)** Plot of the time the nanoneedle took to adapt to the surrounding temperature. Figure 7:1 **(a)** The side view and **(b)** top view of the penetration depth of the cell into the nanoneedle in 7, 8, and 9 \(\upmu\)m channel heights. Figure 7:2 Current change in the nanoneedle with the temperature. 18.9 pL/min and was gradually increased to 7 \(\upmu\)m/s to study the penetration behavior. From the results, velocities 5 and 5.5 \(\upmu\)m/s did not show any signs of penetration, in fact, velocity 5.5 \(\upmu\)m/s overtook the nanoneedle and flew towards the upper part of the microchannel. Velocity 6 \(\upmu\)m/s showed some penetration; however, it was insufficient as the cell penetrated to the center of the nanoneedle and stopped due to the backward velocity pushing the cell, when the aim was to penetrate and to fully cover the nanoneedle. The most suitable velocity was 6.5 \(\upmu\)m/s, which displayed successful penetration. Lastly, velocity 7 \(\upmu\)m/s caused excessive penetration and the cell was squashed into the nanoneedle electrodes, which caused the shape to slightly deform. Consequently, the optimum velocity range for cell penetration would be between 6.5 to 6.9 \(\upmu\)m/s. Below that range, the cell will not penetrate to the desired location, and above that range, the cell will over-penetrate and may lead to cell damaged.