Principle and structure of a printed circuit board process–based piezoelectric microfluidic pump integrated into printed circuit board

Abstract

Considering mature printed circuit board processes, researches on microfluidic pumps that can be integrated into printed circuit board will provide a solution for further miniaturization and integration of microfluidic systems with low costs. The principle and structure of a printed circuit board process–based piezoelectric microfluidic pump integrated into printed circuit board are proposed and realized in this article. The printed circuit board process–based design and manufacturing technology of a piezoelectric microfluidic pump integrated into printed circuit board is researched utilizing printed circuit board as a platform. The flow characteristics of the fabricated microfluidic pump are experimentally tested. The research results show that the proposed principle and structure of the piezoelectric microfluidic pump can be fabricated utilizing mature printed circuit board process with advantages of simple structure and convenient processing. The fabricated printed circuit board process–based microfluidic pump can linearly pump in and pump out fluid with self-injection. Moreover, the flow rate and back pressure can be controlled by changing the peak-to-peak value, frequency, and phase difference of the driving voltages. The instantaneous flow rate has the pulsation property consistent with the drive voltage frequency. The proposed principle and structure are beneficial to integrate the fabricated printed circuit board process–based microfluidic pump with other microfluidic components to realize complicated microfluidic systems on printed circuit board.

Keywords

Integrated microfluidics microfluidic pump piezoelectric pump printed circuit board fabrication process circular piezoelectric unimorph actuator

1. Introduction

As a kind of key component of microfluidic systems, microfluidic pumps, which play a key role in controlling fluid flow rate and flow direction, are widely used in chip cooling (Garimella and Singhal, 2004), drug delivery (Mailefer et al., 2001), chemical analysis (Jang and Kan, 2007), and lab-on-chips (Ha et al., 2009). Hence, one of the research foci in microfluidic fields focuses on microfluidic pumps, including driving principle, structure design, and fabricating process.

According to driving methods, microfluidic pumps can be divided into piezoelectric driving type (Feth et al., 2014; Vanlintel et al., 1998), electrostatic driving type (Ng et al., 2004), electromagnetic driving type (Yamahata et al., 2005b), and thermopneumatic driving type (Bardaweel and Bardaweel, 2015). Piezoelectric microfluidic pumps based on the inverse piezoelectric effect not only have the ability of driving liquid in microliter but also have simple structure and large driving force, which results in that piezoelectric driving methods are widely used in current researches. According to the structural design of microfluidic pumps, they are divided into micropumps with valve (Kang and Auner, 2011) and without valve (Singh et al., 2015). Micropumps with valves have good cutoff characteristics and high reliability. According to study materials, the processing technology of microfluidic pumps includes silicon processing technology (Cazorla et al., 2016), glass and quartz processing (Yoon et al., 2014), polymer processing technology (Fu et al., 2016), and printed circuit board (PCB) process (Verma and Chatterjee, 2011; Wego and Pagel, 2001). Dau et al. (2009) processed a jet-flow micropump with different intersections and integrated hotwires on silicon wafers by using micro-electro-mechanical system (MEMS) process. Yamahata et al. (2005a) proposed a micromachining process for an electromagnetically driving ball valve microfluidic pump made of glass material, and conducted microstructure for glass substrate and fusion bonding multilayer microfluidic chip by using powder injection technology. Wang et al. (2014) developed a new piezoelectric micropump composed of a polymethyl methacrylate (PMMA) pump body and a folded piezoelectric vibrator. The pump chamber and the inlet and outlet channels are processed on a PMMA by using the computerized numerical control laser. Based on a concept of using PCB as the substrate material for miniature valveless pumps (Nguyen and Huang, 2001), Nguyen and Huang (2005) developed a peristaltic pump based on the PCB technique. The pump chamber was etched in the copper layer with a thickness of 40 µm. The inlet, the outlet, and the connecting orifices were drilled in the PCB substrate. Luque et al. (2013) designed a microfluidic pump based on the electro-osmotic effect, combined PCB process, and SU-8 lithography technology. They used SU-8 as structural material to build microchannels and chambers, and used copper to build wires. Hintermueller et al. (2017) presented the design of a microfluidic pumping device based on the effect of boundary layer–driven acoustic streaming, in which flexible PCB was chosen as the oscillating wall. Vasilakis et al. (2017) demonstrated a simple passive capillary pump that can be integrated into an affordable Lab-on-PCB platform. Kim et al. (2018) reported an electrolytic micropump based on an electrode chip fabricated on a PCB. However, the disadvantages of microfluidic pumps, which are composed of silicon, quartz, and high molecular polymers, are that their processing technology is complex and high cost, and it is difficult to integrate with other components of microfluidic systems. Microfluidic pumps are dependent unit, which makes microfluidic systems both huge and complex, and limits wide application of microfluidic pumps made of these materials.

Considering that PCB can be designed, assembled, and maintained with electronic components in high density, they are widely used in related equipments and products in various fields. Currently, PCB process is very mature with processing precision at micron level (Gallagher and Lawlorwright, 2012) and can satisfy processing precision demand of most microfluidic devices. Microchannels have been integrated into PCB to constitute microfluidic systems, which will be used in the fields of chip cooling (Oueslati et al., 2008; Verma et al., 2009), lab-on-chips (Kim et al., 2018), chemical analysis (Vasilakis et al., 2017), and biomedicine (Marshall et al., 2012). If microfluidic pumps, along with microchannels, chambers, drive circuits, and sensors, can be integrated into PCB, the size of microfluidic systems will be greatly reduced. Moreover, PCB process–based microfluidic devices can be produced in large quantities at low cost. Therefore, it is of significance to research and develop PCB process–based piezoelectric microfluidic pumps integrated into PCB.

In previous works, Pan and Wang (2016) developed an active piezoelectric microfluidic valve with annular boundary and established an active control flow model based on a deflection mathematical model of partially covered circular piezoelectric unimorph actuator (CPUA) for the clamped support condition under voltage control (Wang and Huo, 2010). Based on this, the principle and structure of a PCB process–based microfluidic pump integrated into PCB are proposed and realized in this article. After the principle and structure are described and analyzed, a PCB process–based design and fabrication technology of piezoelectric microfluidic pump are presented. Utilizing PCB process, a PCB process–based piezoelectric microfluidic pump integrated into PCB is manufactured. The flow characteristics of the PCB process–based microfluidic pump is experimentally tested and analyzed on an established experimental setup.

2. Principle and structure

The principle and structural configuration of a PCB process–based piezoelectric microfluidic pump integrated into PCB are shown in Figure 1(a) and (b), respectively. According to Figure 1(a) and (b), the PCB process–based piezoelectric microfluidic pump integrated into PCB is composed of a cylindrical pumping unit and two piezoelectric microfluidic valves with annular boundary, and the cylindrical pumping unit is connected to two piezoelectric microfluidic valves through two microfluidic channels on both sides. The cylindrical pumping unit is composed of a chamber wall manufactured by PCB process and a CPUA fixed on PCB via the chamber wall, forming a cylindrical pump chamber with a diameter of 25 mm and a height of 1.2 mm. The piezoelectric microfluidic valve with annular boundary is composed of a chamber wall processed by PCB process, an annular boundary processed by PCB process, and a CPUA fixed to the chamber wall. The annular boundary processed by PCB process is welded at the center of the cylindrical valve chamber through a PCB substrate. When a CPUA is welded on PCB substrate through the chamber wall processed by PCB fabrication, it forms a microfluidic boundary between the annular boundary and the CPUA and a cylindrical valve chamber with an inner diameter, outer diameter, and height of 4, 25, and 1.2 mm, respectively. The through hole of the annular boundary, which is coaxial with that of the inlet/outlet on the PCB substrate, constitutes an inlet/outlet channel of the microfluidic pump with a diameter of 1.8 mm. Considering that CPUAs with copper substrate possess large deformation and can be welded on PCB by PCB process, the cylindrical pumping unit and two piezoelectric microfluidic valves with annular boundary all adopt CPUAs.

When no voltage or negative voltage are applied, the bottom of the CPUAs of PCB process–based microfluidic valves is close to the top end of an annular boundary. The inlet and outlet are closed, and liquid cannot flow from the inlet to outlet. Under these circumstances, although the pumping unit works, the microfluidic pump does not work. Therefore, the PCB process–based piezoelectric microfluidic pump integrated into PCB composed of piezoelectric microfluidic valves with annular boundary is a normally closed piezoelectric microfluidic pump. After applying positive voltage, the CPUAs are deformed based on the inverse piezoelectric effect and the copper substrates constituting the valve are upwardly arched, which results in a certain gap between the CPUAs and the annular boundary. Accordingly, the inlet/outlet and the valve chamber can be connected together through this gap by controlling the microfluidic valve to open or close in this way. The piezoelectric pump, as shown in Figure 1, realizes the self-injecting, pumping in, and pumping out the fluid through the order action of the CPUAs of a pumping unit and two valves. Figure 2 shows a four-phase pumping cycle of the piezoelectric microfluidic pump integrated on PCB based on PCB process and the corresponding three-way driving voltage. First, in phase 1, a positive voltage is applied to the inlet valve, making it open, and a small amount of fluid is drawn into the inlet channel. In phase 2, the copper substrate of the pumping unit is arched upward under a positive voltage, and a negative pressure is generated in the pump chamber. Since the inlet valve is opened and the outlet valve is closed, a large amount of fluid is transported from the liquid inlet to the pump chamber. In phase 3, the inlet valve is closed and the outlet valve is opened, and the fluid flows to the outlet valve in the specified pumping direction. Finally, the copper substrate of the pumping unit moves down in phase 4 and a positive pressure is generated in the pump chamber, ejecting a large amount of fluid from the outlet channel. By applying the periodic signal to the CPUAs, the copper substrate of the pumping unit is up and down for reciprocating motion, and by coordinating with the control of the microfluidic valve with the annular boundary, the fluid is driven by the microfluidic pump to form a continuous steady flow.

3. PCB process–based design and fabrication

3.1. PCB process–based design

An inlet and outlet on PCB substrate are designed via two circular through holes with a diameter of 1.8 mm. Circular pads for welding chamber walls and annular boundaries are designed. Pump wall and valve walls are designed as circular elements with an inner diameter of 25 mm and an outer diameter of 29 mm on PCB with a thickness of 1.2 mm, and there are gold-plated pads on both sides. On the valve wall, a 1 × 1 mm² rectangle groove is designed on one side of a circular PCB, and on the pump wall, two centrally symmetric 1 × 1 mm² rectangle grooves are designed on the same side of a circular PCB. The rectangle grooves on pump wall and valve walls are used for installing copper capillary tubes. Annular boundaries are designed as annular PCB rings with an inner diameter of 1.8 mm and an outer diameter of 4 mm on PCB with a thickness of 1.2 mm. The photograph of the manufactured PCB substrate for microfluidic pump according to PCB process–based design is shown in Figure 3. The PCB substrate for microfluidic pump plays a role of carrying all other components of the microfluidic pump and connecting with electric unit. The photograph of the manufactured pump wall and valve wall according to PCB process–based design is shown in Figure 4.

3.2. PCB process–based assembly process

The assembly process of a PCB process–based piezoelectric microfluidic pump is shown in Figure 5. The microfluidic pump components are assembled by welding on a PCB substrate to realize integrating into the PCB. The specific process is as follows:

The microfluidic pump substrate, annular boundaries, and valve and pump walls are cleaned by alcohol, as shown in Figure 5(a), to make the process have good welding characteristics.

Two annular boundaries are welded on each annular pad of the inlet/outlet of the PCB substrate, as shown in Figure 5(b).

A pump wall is welded on the annular pad at the center of the PCB substrate and two valve walls are welded on each annular pad on both sides by keeping grooves downward. Then, two copper capillary tubes with an outer diameter of 1 mm and an inner diameter of 0.6 mm are welded to connect the valve chamber with the pump chamber, as shown in Figure 5(c).

Three CPUAs with diameter of 27 mm are welded to the pad of the pump wall and the valve walls, as shown in Figure 5(d).

Finally, copper wires are welded from the surface of the piezoelectric ceramics of CPUAs to pads on PCB to connect with Sub-Miniature-A (SMA) connectors.

Figure 6 shows the photograph of the assembled PCB process–based piezoelectric microfluidic pump integrated with PCB.

4. Experimental setup and testing

4.1. Experimental setup

Figure 7(a) and (b) shows the principle diagram and photograph of the established experimental setup, respectively. According to Figure 7(a), the experimental setup is composed of a microfluidic system based on the developed PCB process–based piezoelectric microfluidic pump and a flow rate measurement system. The microfluidic system mainly includes the PCB process–based piezoelectric microfluidic pump, a dual channel signal generator (model: DG4062), a piezo drive (model: AMP-301.04.S.150, the output voltage rang is 0–150 V, static ripple is less than 20 mV (0–10 kHz), DH Science & Technology Co., Ltd, China), and a water tank. The flow rate measurement system mainly includes a microflow sensor (model: MFS 4, the range of flow measurement is 0.03–1 mL/min, the precision is 5%, the resolution is 0.01 L/min, Elveflow, France), a micro-measurement cylinder, and a host computer.

A medium driven by the microfluidic pump in the experiment is running water. Sinusoidal signals with different phases generated by the signal generator are amplified through the piezo driver and are applied to the CPUAs. The joint at the bottom of the water tank is connected to the inlet of the microfluidic pump through a hose, and the height of the water tank is adjusted to ensure that the pressure of the inlet of the microfluidic pump is zero. The flow rate sensor is connected to the outlet of the microfluidic pump through a hose to measure the flow rate at the outlet, the flow rate datum, including the instantaneous flow rate within 1 minute and the average flow rate, are displayed and stored on the host computer.

4.2. Experimental testing and analyzing

4.2.1. Experimental testing on the instantaneous flow rate

Figure 8(a) and (b) shows the time histories of a set of driving voltages to the PCB process–based piezoelectric microfluidic pump with a back pressure of zero and the corresponding flow rate within 0.5 s working hours, respectively. According to Figure 8(a), the set of sinusoidal driving voltages with peak-to-peak value of 80 V, frequency of 10 Hz, and phase difference of $2 / 3 π$ are applied to the CPUAs, which constitute the inlet valve, the pumping unit, and the outlet valve. Driven by the set of sinusoidal voltages, the flow rate curve approximates the sinusoidal curve with the same frequency of the set of driving voltages. In the first work cycle of the microfluidic pump, the inertia of the fluid is needed to overcome, which results in that the maximum flow rate is less than that after stable working. The flow rate increases and tends to stabilize from the second work cycle of the microfluidic pump. On one hand, the instantaneous flow rate has the pulsation property consistent with the drive voltage frequency that lays a foundation to generate a pulse flow. On the other hand, the difference between the first work cycle and the subsequent work cycles results in the difficulty to generate the accurate pulse flow, which should be deeply studied in the future.

4.2.2. Experimental testing on relationship between the flow rate and the driving voltage

Figure 9 shows the relationship between the average flow rate of the piezoelectric microfluidic pump with a back pressure of 0 Pa and the peak-to-peak value of the driving voltage with a frequency of 50 Hz and a phase difference of $2 / 3 π$ . In order to check the repeatability of the developed piezoelectric microfluidic pump, three sets of data are collected and presented. According to Figure 9, the center deflection of the CPUA increases with increasing the peak-to-peak value of the driving voltage, which results in increasing the volume change of the pump chamber. Accordingly, the average flow rate of the microfluidic pump increases with increasing the peak-to-peak value of the driving voltage linearly with a nonlinear error of 5%. In addition, the standard deviation of the three sets of experimental data is 17.2, which indicates the good repeatability.

Figure 10 shows the relationship between the average flow rate of the piezoelectric microfluidic pump with a back pressure of 0 Pa and the frequency of the driving voltage with a peak-to-peak value of 80 V and a phase difference of $2 / 3 π$ . According to Figure 10, the average flow rate of the microfluidic pump decreases with increasing the frequency of the driving voltage. Because it takes a certain time t for the fluid to flow from the pump chamber to the valve chamber and if the frequency of the driving voltage exceeds $1 / t$ , the fluid does not reach the outlet passage before the outlet valve closes prematurely. As a result, a part of the fluids are blocked from flowing out of the outlet and even the phenomenon of reflux may occur. Accordingly, the average flow rate of the microfluidic pump decreases with increasing the frequency of the driving voltage. In addition, the standard deviation of the three sets of experimental data is 11.8, which also indicates the good repeatability.

Figure 11 shows the relationship between the average flow rate of the piezoelectric microfluidic pump with a back pressure of 0 Pa and the phase difference between the driving voltages applied to the CPUAs with a peak-to-peak value of 80 V and a frequency of 50 Hz. When the phase difference is small, the fluid does not reach the outlet, the outlet valve opens ahead of time, or the fluid reaches the outlet, and the outlet valve closes ahead of time. When the phase difference is too large, the outlet valve is still closed after the fluid reaches the valve chamber. When the phase difference is $2 / 3 π$ , the average flow rate of the microfluidic pump is the largest about 420 µL/min. In addition, the standard deviation of the three sets of experimental data is 12.8, which also indicates the good repeatability.

4.2.3. Experimental testing on the flow rate limits

When the frequency and the phase difference of the driving voltages are, respectively, 50 Hz and $2 / 3 π$ , the flow rate changes when the minimum voltage change is 1 V considering that the voltage resolution of the CPUA is 1 V. Figure 12 shows the relationship between the average flow rate of the piezoelectric microfluidic pump and the peak-to-peak value of the driving voltages with an increment of 1 V. The minimum change in the flow rate is about 5.7 µL/min. In addition, the standard deviation of the three sets of experimental data is 16.5, which also indicates the good repeatability.

When the peak-to-peak value, frequency, and phase difference of the driving voltages are, respectively, 100 V, 10 Hz, and $2 / 3 π$ , the average flow rate of the piezoelectric microfluidic pump with a back pressure of 0 Pa is maximized to about 500 µL/min.

4.2.4. Experimental testing on back pressure

In the case of the maximum flow rate of the microfluidic pump, the maximum back pressure is the pressure difference between the outlet and inlet when the flow rate at the outlet reaches zero. Through gradually increasing the height of the liquid level at the outlet, the pressure difference between the outlet and the inlet can be changed to zero. Thus, the maximum back pressure occurs when the maximum height of the liquid in the outlet channel is reached. When doing experiments, the outlet of the microfluidic pump is connected to the flow sensor through the channel and finally extends to the measurement cylinder. Increasing the height of the measurement cylinder gradually will make a height difference between the liquid level in the measurement cylinder and the inlet, which creates a pressure opposite to the flow direction. When the peak-to-peak value, frequency, and phase difference of the driving voltages are, respectively, 100 V, 10 Hz, and $2 / 3 π$ , the relationship between the average flow rate of the microfluidic pump and the back pressure is shown in Figure 13. According to Figure 13, the maximum back pressure is 760 Pa.

5. Conclusion

The principle and structure of a PCB process–based piezoelectric microfluidic pump integrated into PCB are proposed and realized in this article. The PCB process–based design and manufacturing technology of the piezoelectric microfluidic pump integrated into PCB is researched utilizing PCB as a platform. The microfluidic pump is assembled adopting PCB process. The flow characteristics of the fabricated microfluidic pump are tested on the established experimental setup. The conclusions can be drawn as follows:

The proposed principle and structure of the PCB process–based piezoelectric microfluidic pump can be fabricated utilizing PCB process with advantages of simple structure and convenient processing and are suitable to be integrated into PCB.

The fabricated PCB process–based piezoelectric microfluidic pump can linearly pump in and pump out of fluid with self-injection. The flow rate and back pressure can be controlled by changing the peak-to-peak value, frequency, and phase difference of the driving voltages. The instantaneous flow rate has the pulsation property consistent with the drive voltage frequency that lays a foundation to generate a pulse flow.

The maximum average flow rate of the microfluidic pump is 500 µL/min when the peak-to-peak value and frequency of the drive voltages are 100 V and 10 Hz, respectively. The minimum change in the flow rate is about 5.7 µL/min. The maximum back pressure is 760 Pa.

The proposed principle and structure are beneficial to integrate the fabricated PCB process–based piezoelectric microfluidic pump with other microfluidic components to realize complicated microfluidic systems on PCB. Thus, there are potential applications in PCB heat dissipation, chip cooling, chemical analysis, and drug delivery.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work is supported by the LPMT, CAEP (Grant No. 2015-01-001), the National Natural Science Foundation of China (Grant No. 51675070), and the Advanced Research Project on Civil Space Technology of State Administration of Science, Technology and Industry for National Defense, China (Grant No. D020210).

ORCID iD

Dai-Hua Wang

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