Preparation of microfluidic chips by electroplating and used for microdroplet study

Microfluidic chips have been widely used in chemical industry and biological monitoring. The application of microfluidic chips in biological analysis, chemical synthesis, clinical detection and other fields not only makes the chemical reaction system small and highly integrated, but also reduces the sample consumption. At present, the traditional microchannel machining methods, such as laser etching and injection molding, have many disadvantages such as complicated process, high technical requirements, high cost and long cycle. In this paper, a new method of microstructure and microchannel in microfluidic chips based on electroplating technology is proposed based on traditional methods, and the microdroplet experiment was carried out.


Introduction
The microfluidic chip has the advantages of high flux and low reagent consumption [1], cells [2][3][4], viruses [5], DNA [6], and RNA [7] can be easily encapsulated within the droplet.The microreactor [8][9] is a special device that can achieve an effective chemical reaction in the microflow channel and can achieve maximum material exchange in the laminar flow mixing.Compared with macroscopic fluids, the flow state and fluid transport characteristics of micro-scale fluids are very different, showing obvious size effects [10].The gravity can be made infinitesimal or even negligible by reducing the characteristic size of the channel and the volume of the fluid.The shape, size and position of liquidsolid, liquid-liquid and liquid-gas interfaces are the main factors that affect the fluid flow state.At the microscopic level, the mass and heat transfer reactions of fluids are very rapid [11][12].Microfluidic technology also has some advantages, such as extraction and purification of viruses, cell or macromolecule separation, rapid diagnosis of diseases and so on [13][14].At present, the main methods of microstructure machining are laser etching technology, traditional injection molding method, MEMS processing technology based on X-ray lithography technology (LIGA technology).The method widely used in the laboratory is the method of die molding, that is, micro-nano structures are made by photoresist, poured with PDMS and stripped to form microchannels after curing [15][16].In addition, according to different scientific research needs, some new processing methods have been proposed and applied, for example, wet etching method is used to obtain smooth inner surface of the microchannel, negative ion etching method is used to etch microchannels on the photoresist.These methods are often complicated in the preparation of microstructures, and have high requirements for instruments and equipment and experimental personnel, long time and high experimental cost.In this paper, we propose a simpler method of electrochemical preparation of microchannels, which is not only short in time but also low in cost in the production process, which is suitable for microchannel processing and successfully avoids some defects existing in the current mainstream molding methods.In the experiment of preparing microchannels, microstructures of different heights need to be prepared, so different types of photoresist are required.For example, AZ2010 or AZ9260 photoresist are generally selected for the preparation of 10 μ m high microstructures.SU-8 2050 is commonly used to prepare 50μm high microstructures.In microfluidic experiments, it is necessary to purchase the corresponding photoresist to make a variety of microchannels of different heights.The price of these photoresist is quite expensive, and the use consumption is large, resulting in high experimental cost.Using electrochemical methods, we only need to purchase relatively cheap photoresist such as AZ9260 to prepare a variety of different sizes and different heights of microchannels.

Microfluidic chip design
The microfluidic chip shown in Figure 1 droplet dispersion experiments, where the continuous phase consists of water and the dispersed phase comprises soybean oil.The "cross" intersection has a width of 50µm and serves as a droplet generation mechanism.It should be noted that the shape of the droplets can be easily influenced by the microchannel wall, resulting in two distinct forms as depicted in Figure 1 Figure 1.III.Specifically, we can approximate the droplets in Figure 1.II as being composed of two sections with an arc in between, resembling an ellipsoid shape.On the other hand, the droplets illustrated in Figure 1.III exhibit a more spherical morphology.After conducting experiments, we calculate their respective volumes: V represents the volume of an ellipsoidal droplet (Figure 1.II), while VS denotes the volume of a spherical droplet (Figure 1.III).In this context, it is important to consider various parameters such as: Finally, according to the actual use requirements, design and draw graphics in L-edit software.

Preparation template
After the chip graphics completed by design and drawing are converted into graphic format, the chromium plate with photoresist on the surface is photoengraved with laser direct writing device, and then the photoresist is removed for development, and the chromium on the surface is washed with ultra-pure water and dried with nitrogen.Then the mask version can be obtained, as shown in Figure 2(Ⅰ)a.Then apply a layer of photoresist (AZ9260) on ITO glass at a high speed (3000 RPM /min) and dry the photoresist machine on the heating plate.At this time, using the prepared mask plate as the master plate, the mask plate is placed on the ITO glass with photoresist on the surface of the exposure machine, and the exposure time is selected according to the type of photoresist to start UV exposure.After exposure, the photoresist is dissolved in the developer to reveal the ITO substrate.

Preparation of microfluidic chips by electroplating
After the ITO glass is developed, the ITO base of the exposed part is exposed, and metal ions can be deposited on the ITO surface by electroplating according to the electrical conductivity of the ITO.As shown in Figure 2(I)b, the positive electrode of the power supply is connected to the nickel plate (or nickel wire), and a small part of the photoresist on the ITO sheet is scrubbed with acetone solution to expose the ITO base, as shown in Figure 2(I)c.This part of the base is connected with the negative electrode of the power supply, and then the nickel plate and ITO glass are immersed in the nickel plating solution, set the temperature of the nickel plating solution (set to 50°C constant temperature), adjust the nickel plate and the ITO template to be placed parallel, adjust the spacing between them to about 25mm.During the plating process, the main factors affecting the plating height of microstructure are current density, electrode spacing, electrolyte temperature, plating time, etc.When these factors are well controlled, the ideal microstructure can be prepared, as shown in Figure 2(I)d.

Preparation of microfluidic chips by electroplating
The PDMS prepolymer (A: B = 10:1) was poured onto the plated template, as shown in Figure 2(Ⅰ)e.The bubbles in the PDMS are removed by vacuuming, and then placed in an oven at 80 °C for 2 hours to cure the PDMS, and then peeled off the PDMS to obtain PDMS1.Solidified PDMS2 and PDMS1, which have no surface microstructure, are bonded together in a plasma cleaning machine for surface activation, and the two PDMS can be closely bonded together.Finally, after 72 hours in an oven at 80°C, the PDMS microfluidic chip is completely cured to form, as shown in Figure 2(Ⅰ)f.

Experimental study of microdroplets
This novel microchannel preparation method can be widely applied in microfluidic chips with low precision requirements.The same applies to qualitative experiments.An exemplary conducted on a microfluidic chip is the oil/water separation experiment, where the oil phase acts as the shear phase and the water phase serves as the continuous phase.The process of droplet formation necessitates breaking the surface tension between the continuous and dispersed phases.When the tension of the dispersed phase surpasses the interfacial tension, liquid will overcome this barrier and form droplets within the continuous phase.Figure 3a illustrates gradient regions of various fluids achieved by altering two-phase flow of oil and water.In this figure, region (І) represents laminar flow, with a clear demarcation line between oil and water phases within the microchannel.The water flow in this region exceeds that of oil by more than twice its volume.Droplets in region (П), while those near region (I) tend to have an elongated shape due to proximity effects.However, when droplets are located close to region (Ш), their volume decreases due to exposure to shear forces from the oil phase.Overall, this area is segmented with minimal disparity between water and oil flows.Droplets in region IV appear smaller and spherical compared to those in region IV; here, oil flow is approximately.2times greater than that of water flow.The fluid focusing technique enables continuous-phase fluid from channel corners to converge towards dispersed-phase frontage; however, deformation caused by contraction before reaching dispersed-phase leads to instability resulting in discrete droplet formation.To facilitate investigation into droplet size and shape characteristics, we fabricated a microfluidic chip with a channel height of 17μm while employing a precise injection pump for controlling droplet size production.

Figure 3.
The four stages at different water and oil flow rates are depicted in Figure 3 (a).These stages include: (І) the "laminar flow" phase, (П) the "ellipsoid" phase, (Ш) the "segmented" phase, and (Ⅳ) the "ball" phase.In Figures (b), (c), (d), and (e), a scale of 100µ is used.In the experiment, we divided the obtained microchannels into three groups for comparison.The height of the first set of nickel plating layer is 5.5µm.The second set of nickel-plated layers is 10.8µm in height.The third group is 17µm.Three sets of data with different size structures were obtained (Table 1 The water phase flow rate of each group was set at 50µl/h, while the oil phase flow rate varied.The velocity of droplets generated under different oil phase flow rates were observed and recorded.It was observed that the size of droplets in each as the oil phase flow rate increased.Specifically, when the flow rate of group 150µl/h, group 2 was 60µl/h, and group 3 was 70µl/h, the droplet shape transitioned "ellipsoidal" to "segmented".Furthermore, an increase in cross-sectional area was noted from the first to third set of experiments.This indicates that higher oil phase flow rates can lead to an increase in shear stress on the droplets.As depicted in Figure 4, a change in droplet shape from "ellipsoidal" to "segmental" occurred when the oil phase flow rate ranged from 20µl/h to 60µl /h; however, once it exceeded this threshold (60µl /h), further increase resulted in a shift from "segmental" to "spherical", accompanied by a rapid decrease in droplet volume.Therefore, it can be concluded that a spherical structure represents stability.

Conclusions
In summary, electroplating can be utilized to deposit nickel or other metal ions onto ITO glass.The height of the nickel plating can be controlled by adjusting factors such as current density, plating time, ion concentration, and temperature of the plating solution.By employing this method, microfluidic chips with varying heights of microchannels were designed and fabricated for experimental investigations on microdroplets.Microfluidic chips play a crucial role in nanoparticle synthesis and drug screening on microchips; therefore, it is anticipated that this 3D microstructure fabrication method will find extensive applications in the future.

BV
is the volume enclosed by the arc.B S is the cross-sectional area encompassed by said.B L is the length of said arc. -H: Height of the microchannel -r: Segment radius -b: Width of the arc

Figure 1 .
Figure 1.(Ⅰ) is the design of the microchannel chip, and (Ⅱ) and (Ⅲ) are the two types of droplets.

Figure 4 .
Figure 4. Droplet volume formed by microchannels of different heights The first set of microchannels in Figure 4 is 1µm high.The second set of microchannels is 10.8µm high.The third set of microchannels is 17µm high.