1 Introduction

Microfluidic chips are based on the analytical chemistry and biotechnology [1, 2], the microelectromechanical systems processing technology as the basis [3] and the micropipe network as a structural feature. Various microdevices are integrated in a chip-sized area to perform functions [4] such as sample injection, sample preparation, mixing, separation and detection [5,6,7,8]. Microfluidic chips are being widely used due to their miniaturization, integration and portability [9]. Material selection, microchannel design and the production of microfluidic chip are some of the key issues in microfluidic analysis. The materials to produce the chip are silicon, glass, quartz, polymers [10, 11], etc. The polymer microfluidic chip has the characteristics of easy processing, low manufacturing cost, good transparency and excellent dielectric properties [12]. Other great advantages are that it has good biocompatibility, it does not affect the characteristics of the analytical sample, and various functional modules are easy to integrate on a single chip [13].

The manufacture of micromixers with polymer materials is a major trend in the development of micromixing technology; it is an important part of microfluidic chip [14]. Polymethyl methacrylate (PMMA) is a major thermoplastic polymer that is widely used as a substrate material for manufacturing a micromixer [15]. At present, the main methods for preparing PMMA microfluidic chips are laser ablation [16], imprinting [17], injection molding [18], etc. Thermal bonding technology is the most commonly used method for simple and fast micromixer packaging [19]. Laser ablation is very versatile in the fabrication of polymer micromixers; it involves the use of a high-power pulsed laser to remove material from a sheet of polymer at a wavelength where it strongly absorbs photons. Scanning process with laser ablation is tremendously useful for prototyping because new microchannel designs can be easily programmed into the system [20]. Zhang and Shin [21] present a rapid and economic way of fabricating higher-performance trapezoidal microchannels on various PMMA-based microfluidic devices using CO2 laser system. Romoli et al. [22] deals with CO2 laser machining of a suitable amorphous polymer (PMMA) as a flexible technique for the rapid fabrication of miniaturized structures such as microfluidic devices. The use of polymer substrates offers additional advantages with the widely available polymer chemistries that may be tailored to specific chemical and physical needs and the potential for low-cost device production. In the study of PMMA microfluidic chip material process, not only need to focus on the chip surface quality and processing accuracy, but also pay attention to the ease of chip processing and economic costs. Making a micromixer with CO2 laser system makes it possible to meet those two needs.

2 Materials and methods

2.1 Materials and reagents

Polymethyl methacrylate abbreviation code-named PMMA, commonly known as plexiglass, is the synthetic transparent materials with the best texture, and the price of these organic materials is relatively cheap. PMMA extruded sheets with thickness of 1.5 mm bought from Shenzhen Xintao. Methylene blue from Tianjin Yongda Chemical Reagent, Tianjin, and 1% cresol red solution in DI water with red color for visual clarity were used for the testing of the PMMA microfluidic chip.

2.2 Experimental setup

In this study, the air cooling CO2 laser system which has been used to engrave the PMMA substrate is bought from Laichuang, Nanjing. The wavelength of the laser is 10.6 μm, and the maximum output power is 50 W with a frequency of 5 kHz in the continuous-wave operation mode. The output beam diameter of the laser at exit was 4 ± 1 mm, and the beam divergence (full angle) was 5 ± 1mrad. The maximum irradiance that the laser system can deliver is 8.8 kW/mm2. A reflecting mirror and a focusing lens with a focal length of 65 mm are used to reflect the Gaussian laser beam with a diameter of 85 μm so that it is focused on the surface of the PMMA to process the microchannel. Compressed nitrogen at 0.45 bar is used as the assistance gas for CO2 laser cutting of PMMA. This is the minimum pressure value that we find to avoid fire of the melted and vapored PMMA material with better quality of the surface of cut. The laser power is in the range of 2–50 W, and the scanning speed of the beam was programmable over the range 5–50 mm/s. The largest working area is 900 × 500 mm2. The machining process is completed by the laser beam movement driven with a DC servo control system. The designed microchannel patterns are directly fabricated according to changing beam scanning speed and the output power of the laser which are determined by the program of computer.

2.3 Geometry structure of the Koch fractal microchannel

The desired Koch fractal microchannel pattern on PMMA substrate was designed in the CAD software, which is shown in Fig. 1. Figure 1a is the Koch fractal principle, and Fig. 1b shows the fractal microchannel based on the fractal principle. Fractal structure not only extends the total path of the fluids but also enhances the chaotic advection; it is of great importance to improve the mixing efficiency of the micromixer.

Fig. 1
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Fractal principle and Koch fractal microchannel

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3 Results and discussion

The processing quality of the microchannels on the PMMA substrate is affected by the laser parameters, which are closely related to the power, frequency, scanning speed and scanning times of the CO2 laser system. The research of the CO2 laser system processing parameters on the quality of the material processing, which can obtain a better micromixer substrate, can create good conditions for the subsequent hot bonding. Table 1 shows the width and depth of the microchannels at a laser power of 5 W, and the scanning times of each microchannel are 1, 2 and 3 times, respectively. The scanning speed is defined as 5 mm/s, 10 mm/s and 15 mm/s. As can be seen from Table 1, with the increase in the scanning times, the width of the microchannels increases from 0.2 to 0.25 mm and the depth increases from 0.39 to 0.59 mm at the scanning speed 5 mm/s. At the scanning speed 10 mm/s and 15 mm/s, the width and depth of microchannels also gradually increase. Just compared to 5 mm/s, the width of the microchannel is narrower and the depth is shallower. It can be seen that the same microchannel will get a better width and depth when the CO2 laser system scanned three times. Figure 2 shows the depth of the microchannel with the laser power of 5 W and the scanning speed of 5 mm/s. Compared to the scanning once and twice, the width and depth of the microchannel increase obviously when the scanning times are three; it will not contribute to hot bonding of the micromixer. Figure 3 shows the planar width of the microchannels; the increase in the microchannel width can be seen visually at the same magnification of the microscope. With the increase in the number of times of scanning, the ablation of the removed material is more serious on both sides of the microchannel. In the subsequent cleaning process, it is easy to clean the removed material. The adhesion of removed material in microchannel is easy to block the microchannel in the hot-pressing process; this will lead to poor fluid flow and ultimately affect the analysis of the experimental results in the later stage. More seriously, if the removed material blocks the microchannel, this will cause the hot bonding to fail. Ablation of the removed material by multiple scanning of the CO2 laser system can reduce clogging of the microchannel by the removed material.

Table 1 Width and depth of the microchannels at the laser power 5 W
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Fig. 2
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Depth of the microchannels with the laser power 5 W, the scanning speed of 5 mm/s and the scanning times 1, 2 and 3

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Fig. 3
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Width of the microchannels with the laser power 5 W, the scanning speed of 5 mm/s and the scanning times 1, 2 and 3

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Based on the analysis, Tables 1 and 2 present the depth and width of the microchannel with different laser power and scanning speed when scanning three times. Laser power is defined as 5 W, 6 W and 7 W, and the scanning speed is still 5 mm/s, 10 mm/s and 15 mm/s. It can be seen from Table 2 that the microchannel can achieve better depth and width at 7 W and the increase in the laser power has an obvious effect on the depth of microchannels. Figure 4 shows the depth of the microchannels at three laser powers. In Fig. 5, with the increase in laser power, the width of microchannel becomes wider. The higher the laser power, the more seriously the removed material is ablated by the laser, and the easier it is to clean. This is a great help for the success of hot bonding.

Table 2 Width and depth of the microchannels with scanning three times and the laser power 5 W, 6 W and 7 W
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Fig. 4
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Depth of the microchannels with three laser powers 5 W, 6 W and 7 W, scanning three times and the scanning speed 5 mm/s

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Fig. 5
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Width of the microchannels at three laser powers 5 W, 6 W and 7 W, scanning three times and the scanning speed 5 mm/s

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Based on the above analysis, we choose the laser power 7 W, the scanning speed 5 mm/s and the number of scanning three times as the processing parameters to process PMMA substrate. After simply cleaning the removed material in microchannel, the PMMA substrate and cover of the micromixer are washed several times with deionized water. Finally, the vacuum hotting press machine is used for hot bonding. Koch fractal micromixer is fabricated as shown in Fig. 6; in this figure, red and blue inks are used to verify the mixing efficiency and the sealing performance of the micromixer. After verification, the sealing performance of Koch fractal micromixer is good, and there is no leakage phenomenon of the liquids.

Fig. 6
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Mixing experiments of the Koch fractal micromixer

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4 Conclusion

This article mainly introduces the processing to manufacture Koch fractal microchannel on PMMA substrate using CO2 laser system. The CO2 laser system enables the rapid processing of complex microchannel structures; the manufacturing cost of the micromixer can be significantly reduced. According to the different requirements of the micromixer on the width and depth of the microchannels, the required depth and width can be obtained by changing the laser power and scanning speed of the CO2 laser system. In particular, after repeated scanning of the microchannel, the depth of the microchannel can be effectively increased and the ablation level of the removed material in microchannel is enhanced, which is convenience to clean and wash, and contributes to obtain higher processing quality of the microchannel. During the hot-bonding process, it is useful to prevent the removed material blocking the microchannels and affecting the mixed experiments.