An Multifuntional Micro-Pump for Sample Selection Based on Low-Voltage Electrokinetic Mechanism
Authors: Liao Yan-Jie , Lin Shiang-Chi , Gao Chia-Hong , Lin Chih-Ting 
Published: February 2020

In order to develop the techniques that are convenient, real-time, and even personalized for disease detection, topics related to microfluidics have been discussed a lot recently. Microfluidics has the potential on sample preparation, such as filter, separation, condensation and dilution. Generally speaking, a real-time and portable device used for disease detection includes three main parts. The first part is sample preparation executed before sample is pumped into a sensor, which is the second part of portable device. And, the last part is data transportation and analysis. We hope that each part could be small in size and light in weight. Unfortunately, most of devices or bio-chip need to be connected with a bulky pump to provide as the source of sample movement. The properties of large size and heavy weight could not meet the needs for the point-of-care testing (POCT) device whose claim is to detect near samples. So, in this paper, we proposed a fluidic micro pump with two main functions to achieve this goal. The two main functions of this micro pump are sample-pumping and particle-trapping which is mainly based on theory of traveling wave electroosmosis(TWEO) and particle surface electroosmosis (PSE) respectively. As for sample pumping, we modulated the signal frequency and magnitude to achieve different pumping efficiency. On the other hand, for particle trapping, we adjusted the voltage amplitude to attain various filter effects. On the top of that, we also simulated by COMSOL to know current flow velocity, trapping effect, and size effect of different size of beads. In addition to simulations, a series of experiments have been done to verify the functionalities of this micro pump. We use traditional micro-fabrication to fabrication this micro pump. Traditional photo-lithography is used to define the pattern of the electrodes of this micro pump and e-beam evaporator is used to deposit 20 nm-thick Titanium as the electrode material on the glass substrate. The gap between electrodes is 10um and the electrode width is also 10um. So, under such small scale, this micro-pump can be implemented within a total area of 1 mm2. Furthermore, we fabricate the micro channel by PDMS. Oxygen plasma is used to modify the surface of glass substrate and PDMS channel, and then we bind this two parts together. Voltages with four phases and with magnitude of 0.5~1.5 volts are applied on the electrodes as the driving power source. To measure the capability of this pump, we suspend fluorescent beads in 100uM KCL liquid and observe the pumping effect of this micro pump. In our work, we can attain the pumping rate of 115 um/s under the voltage of 0.75volts with 2.5 MHz. In addition to the good pumping rate, another advantage of this micro pump is that we can control the pumping rate by modulating the frequency and voltage amplitude. For example, we can get a higher pump rate under the frequency of 3 kHz and slower pumping rate under 2 kHz. As for trapping effect, we can adjust the magnitude of voltage to get different trapping effect. In our work, beads with 6 um in diameter will be trapped under the voltage of 0.75 volts and, however, beads with 1um in diameter can flow smoothly through the electrodes without being trapped. So under the voltage with 0.75 volt, this micro pump have a good performance of selective trapping. This can be a good method used in sample preparation. For another finding of this work, when we enhance the voltage up to 1.5volts, both beads with 1um and 6um are trapped. 

    In summary, this micro-pump can successfully demonstrated 100% trapping efficiency of beads with 6um in diameter or bigger ones under the voltage amplitude of 1.5volts. In contrast, the trapping and pumping effects of 1 um beads can be easily controlled under various applied voltages. The electrodes are made onto glass substrates by the use of photo-lithography and physical vapor deposition, i.e., e-beam evaporation. The voltage applied on the electrodes is between 0.5 to 1.5 volts and between 2 to 3 kHz. Under such voltage and frequency conditions, the micro-pump is low power consumption, which means it has the potential to be integrated with POCT devices.

Figure 1