S-CPDMS dielectric and hydrophobic integrated film as well as preparation method and application thereof

Abstract

The invention relates to the technical field of digital droplet micro-fluidic chips, in particular to an S-CPDMS dielectric and hydrophobic integrated film as well as a preparation method and application thereof. The method comprises the following steps: mixing P (VDF-TrFE) and N, N-dimethylformamide to obtain a solution A; pDMS and tetrahydrofuran are mixed into a solution B; mixing the solution A and the solution B, adding a curing agent, stirring to form a mixed solution, carrying out centrifugal treatment to remove impurity particles, and spin-coating the mixed solution on the conductive side of ITO glass; putting the ITO glass into a drying box, and curing to form a required film; the solid porous surface is filled with silicone oil, so that the S-CPDMS dielectric and hydrophobic integrated film with the properties of a dielectric film and a hydrophobic film at the same time is formed. The preparation process is simple, the cost is low, the performance is excellent, and the formed thin film is good in physical diagram film-forming property, high in dielectric constant, capable of improving the electrowetting effect and good in biological pollution resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of digital droplet microfluidic chips, and particularly relates to an S-CPDMS integrated dielectric hydrophobic film, a preparation method thereof, and an application thereof. Background Art

[0002] The technology of controlling droplets on a substrate using an electric signal is called digital microfluidics (DMF), which is widely used in the fields of biomedicine, optics, electronics, etc. This droplet driving technology is mainly realized through the principle of electrowetting-on-dielectric (EWOD), and the contact angle of the droplet changes with the applied voltage. Through a digital microfluidic device, the generation, mixing, splitting, and transportation of droplets can be achieved, and specific logic control can be performed according to different scenarios and requirements. To achieve a reliable DMF device, several main factors should be considered, including driving voltage, breakdown voltage, contact angle change, contact angle saturation, and contact angle hysteresis.

[0003] For DMF droplet manipulation, it is generally carried out in open and closed environments. To stably manipulate droplets, a dielectric layer and a hydrophobic layer need to be covered on the electrode substrate, and the dielectric layer material is one of the key factors determining the reliability and practicality of the EWOD device. A variety of materials are used as dielectric hydrophobic layers. Inorganic materials with high dielectric constants (BST, SiO2, Ta2O5) are used to prepare EWOD low-voltage electrowetting dielectric layers with nanometer thickness. The preparation of inorganic thin films requires high temperature and vacuum treatment, such as thin film deposition techniques like metalorganic chemical vapor deposition, plasma-enhanced chemical vapor deposition, sputtering, atomic layer deposition, and electron beam evaporation. Organic materials (PDMS, SU-8, PTFE, Parylene-C) are also widely used in EWOD devices because of their extremely high stability, low preparation cost, and other advantages. Usually, inorganic material thin films have disadvantages such as high manufacturing difficulty and high required cost, and organic thin films have disadvantages such as low dielectric constant and high droplet manipulation voltage. These problems existing in dielectric thin films have always been problems urgently to be solved in the field of digital microfluidics.

[0004] In recent years, digital microfluidic (DMF) devices have been applied to various scenarios. Some people have developed a rapid and reliable point-of-care PCR method based on DMF devices to detect pathogens. Compared with the PCR detection carried out in traditional biological laboratories, this on-chip laboratory method provided by DMF shows advantages such as fast speed, high operability, and small device size. DMF devices have also been widely used in chemiluminescence immunoassay, using an external permanent magnet to control functionalized small magnetic beads to transport the detection solution to a specific area for binding with the magnetic beads for analysis. The method of interacting DMF devices with external sensors has also been widely studied. Through a specific device, the droplets in the DMF device can be transported outside the digital microfluidic chip and then subjected to electrochemical analysis, etc. However, the research on the dielectric layer has been less recently. The dielectric layers used in most DMF devices are PTFE, Parylene-C, PDMS, etc. These materials generally require relatively high driving voltages, which also limits the application of the devices.

[0005] PDMS has been widely used in the field of microfluidics due to its good transparency and biocompatibility. In the field of continuous microfluidics, PDMS and a crosslinking agent (to cure PDMS) are placed into a fabricated mold in a fixed ratio, and then cured at high temperature to harden it. The hardened PDMS channel is taken out of the mold and perforated, etc., to form a channel microfluidic device. Under the action of an external pump pressure and a capillary catheter, the fabricated PDMS channel can perform continuous liquid transportation. In the field of digital microfluidics, PDMS is used as a dielectric hydrophobic film. A dielectric film can be obtained by spin-coating and curing a mixed solution of PDMS and a crosslinking agent on a conductive surface. Since the viscosity of PDMS is relatively high, in order to obtain a dielectric film with a smaller thickness, a generally higher spin-coating speed is adopted. The PDMS film obtained by the spin-coating method has good transparency, flexibility, and hydrophobicity, but the dielectric constant of PDMS is relatively low (~2.5 - 3.5), so a relatively high driving voltage for the droplet is required. PVDF has excellent chemical corrosion resistance, excellent high-temperature color change resistance and oxidation resistance. Due to some excellent qualities, PVDF is made into an insulating layer, a lithium battery binder, a sensor, etc. PVDF and its polymers are applied to EWOD devices due to properties such as a high dielectric constant. The combination of PVDF + teflon hydrophobic layer has a very low driving voltage. PVDF-HFP has good electro-wetting characteristics. An ion gel composed of PVDF-HFP and an ionic liquid is also made into a dielectric hydrophobic layer for droplet manipulation. P(VDF-TrFE) is an excellent copolymer of PVDF. It perfectly inherits the excellent properties of PVDF and has a relatively high dielectric constant (~10). The S-CPDMS dielectric hydrophobic integrated film formed in the present invention is prepared by mixing P(VDF-TrFE) and PDMS, and silicone oil is used as a liquid lubricant to fill the film surface. This film not only inherits the excellent film-forming property of PDMS and the high dielectric constant property of P(VDF-TrFE), but also has good anti-biofouling property due to the low surface tension of silicone oil, and can be perfectly applied to the field of digital microfluidics. Summary of the Invention

[0006] The purpose of the present invention is to provide an S-CPDMS dielectric hydrophobic integrated film, its preparation method and application. The film has a simple manufacturing process and low manufacturing cost. Its electro-wetting characteristics under DC and AC voltages can be measured, and it is applied to digital microfluidic devices.

[0007] The preparation method of the S-CPDMS dielectric hydrophobic integrated film provided by the present invention is specifically as follows:

[0008] S1. Add P(VDF-TrFE) to the solvent N,N-dimethylformamide, and heat and stir magnetically to form a P(VDF-TrFE) solution;

[0009] S2. Add polydimethylsiloxane (PDMS) to the solvent tetrahydrofuran, and magnetically stir at room temperature to form a PDMS solution by mixing.

[0010] S3. Mix the P(VDF-TrFE) solution with the PDMS solution, add a PDMS curing agent, and magnetically stir at room temperature to form a P(VDF-TrFE) / PDMS mixed solution.

[0011] S4. Centrifuge the stirred P(VDF-TrFE) / PDMS mixed solution to remove impurity particles.

[0012] S5. Spin-coat the P(VDF-TrFE) / PDMS mixed solution without impurity particles onto the conductive side of ITO glass.

[0013] S6. Place the ITO glass spin-coated with the P(VDF-TrFE) / PDMS mixed solution into a vacuum drying oven to cure and form a P(VDF-TrFE) / PDMS thin film.

[0014] S7. Coat dimethyl silicone oil on the surface of the formed P(VDF-TrFE) / PDMS thin film.

[0015] S8. Hang and place the P(VDF-TrFE) / PDMS thin film coated with dimethyl silicone oil to form a smooth-surface S-CPDMS dielectric hydrophobic integrated thin film.

[0016] In the present invention, further:

[0017] In step S1, the mass ratio of P(VDF-TrFE) to N,N-dimethylformamide is (0.6 - 1):10.

[0018] In step S1, the temperature of the heating magnetic stirring is 50 - 70°C, and the time of the heating magnetic stirring is 15 - 20 h.

[0019] In step S2, the mass ratio of PDMS to the solvent is (0.2 - 1):9.

[0020] In step S2, the time of the magnetic stirring at room temperature is 2.5 - 4 h.

[0021] In step S3, the mass ratio of PDMS to the curing agent is 5:1.

[0022] In step S3, the time of the magnetic stirring at room temperature is 2.5 - 4 h.

[0023] In step S4, the centrifugation speed is 3000 r / min, and the centrifugation time is 30 min.

[0024] In step S5, the spin coating speed is 250 - 450 r / min, and the spin coating time is 18 s;

[0025] In step S6, the temperature of the vacuum drying oven is 140 °C, and the drying time is 90 - 120 min;

[0026] In step S7, the viscosity of the dimethyl silicone oil is 5 - 50 cst;

[0027] In step S8, the hanging time is 90 - 120 min.

[0028] The S-CPDMS dielectric hydrophobic integrated film prepared by the present invention has excellent hydrophobic properties.

[0029] The present invention further provides an application of the S-CPDMS dielectric hydrophobic integrated film in a digital microfluidic chip; the dielectric hydrophobic integrated film serves as both a dielectric layer and a hydrophobic layer, and can simultaneously exhibit good electro-wetting characteristics and hydrophobicity. The digital microfluidic chip is a conventional existing digital microfluidic chip based on dielectric wetting.

[0030] Specifically, the supporting equipment used for the digital microfluidic chip (application) includes an external AC power supply and a control platform, where;

[0031] The external AC power supply mainly consists of a function generator (such as AFG-2225), a high-voltage amplifier (such as HA-520), and an oscilloscope (such as DS2302A) in cooperation, and can provide a square-wave AC signal with a frequency of 100 Hz - 100 KHz and a root-mean-square voltage of 0 - 150 V;

[0032] The control platform includes, from top to bottom: a single-chip microcomputer (such as STC12), a solid-state relay array (such as HSSR-DA01), a lower electrode array support layer, a lower electrode array, an upper electrode plate, and a pogo-pin circuit board;

[0033] The single-chip microcomputer is used to control the solid-state relay array, and the required solid-state relay is controlled to be turned on and off by using the pins of the single-chip microcomputer;

[0034] The solid-state relay array distributes the alternating current output by the high-voltage amplifier. The on-resistance of the output terminal of the solid-state relay is as low as about 26 ohms, and the off-resistance is as high as 10 MΩ. The output voltage can be switched between the AC voltage and the ground by using the solid-state relay array; The solid-state relay is different from the traditional mechanical relay. It has no contact noise and a short switching time (T on = 0.5 ms, T off(= 0.2 ms), sensitive, small in size (9.6 mm × 6.4 mm). When a current greater than 5 mA passes through the input terminal, the output terminal can be turned on. Set the pin output of the STC12 single-chip microcomputer to the strong pull-up state, and use the pull current of the single-chip microcomputer to drive the solid-state relay. Adopt the row-column control method, and M × N relays can be controlled through M + N pins;

[0035] The upper electrode plate uses ITO glass, and its surface is coated with a fluoride hydrophobic coating;

[0036] The pogo-pin circuit forms 24 (4 columns × 6 rows) vias on the PCB circuit board, then welds the pogo-pin connector into the 24 vias, and aligns the endpoints of the pogo-pin connector with the ITO glass electrode contact pads to distribute the AC signal to the lower electrode plate electrode array.

[0037] The lower electrode array support layer is cut from an acrylic plate (such as 3.5 mm thick). Design the size and shape of the support layer in CAD software, and use a laser cutting machine to cut the acrylic plate into a shape suitable for placing the lower electrode plate.

[0038] The lower electrode array includes an etched ITO glass electrode chip and the dielectric hydrophobic integrated layer;

[0039] The etched ITO glass electrode chip (sheet resistance ≤ 6 Ω, ITO thickness 185 nm, transmittance ≥ 84%, glass thickness 1.1 mm) is designed with a pattern in CAD software and processed by a merchant. The designed ITO glass electrode array has two structures: open and closed. The closed structure consists of 20 electrodes (as shown in Figure 9 Figure A), including 18 transport electrodes of 2.5 mm × 2.5 mm and two storage electrodes of 8 mm × 7 mm. The distance between each electrode is 100 μm. Interdigitation is performed between each transport electrode to improve the transport efficiency. The electrodes are connected to 2 mm × 2 mm electrode contact pads by 100-μm-wide ITO conductive routes for convenient electrode addressing and connection to external circuits. There are 2 mm × 20 mm grounding electrodes at both ends of the chip to ground the upper electrode plate; The open structure has two designs. One consists of 10 transport electrodes of 2.5 mm × 2.5 mm and one grounding electrode of 1 mm × 30 mm (as shown in Figure 9 the left of Figure B), and the other consists of 10 transport electrodes of 1.5 mm × 3 mm and 10 grounding electrodes of 1.5 mm × 3 mm (as shown in Figure 9 the right of Figure B);

[0040] The dielectric hydrophobic integrated layer uses an S-CPDMS film, and its surface is coated with dimethyl silicone oil.

[0041] The present invention forms an S-CPDMS dielectric hydrophobic integrated film by mixing a P(VDF-TrFE) copolymer with PDMS and filling the surface with silicone oil. Its manufacturing process is simple, low-cost, and has excellent performance. The physical diagram and SEM diagram of the S-CPDMS film are as Figure 1 , 2 shown. The film has good film-forming properties and can be easily peeled off from the glass electrode substrate. Moreover, the peeled film is very complete and will not adhere to the substrate to cause substrate contamination, indicating that the film on the electrode surface can be replaced at any time to prevent biological contamination. The dielectric constant of the S-CPDMS film is as Figure 3 shown, and its dielectric constant is 7, which is much larger than the dielectric constant of PDMS (about 2.7). This shows that its electro-wetting performance is superior to that of the PDMS dielectric film and can be used for low-voltage droplet actuation. The production of the S-CPDMS film only requires a spin-coating process, unlike some low-voltage electro-wetting dielectric films (such as BST) that require complex processes such as chemical vapor deposition. And the solvent can be removed and the film can be cured at only 140 °C. The thickness of the film prepared on the glass electrode surface can be controlled by the spin-coating speed and spin-coating time. When the spin-coating speed and spin-coating time are reduced, a thicker dielectric film can be formed, and its breakdown resistance will also increase accordingly. The P(VDF-TrFE), PDMS, and solvents required for film production are relatively inexpensive, greatly reducing the cost of the film. Although the S-CPDMS film without silicone oil filling has good hydrophobicity, due to the high viscosity of the film surface, it will have a great impact on the electro-wetting performance and droplet manipulation of the droplets. The filling of silicone oil makes the film surface have good hydrophobicity and low adhesion. As Figure 5 shown, when the droplet slides on the surface of the S-CPDMS film filled with silicone oil, there will be no stagnation, proving its excellent hydrophobicity and slipperiness. At the same time, the filling of silicone oil will make the surface of the S-CPDMS film have good anti-biofouling properties. Figure 4 shows the protein droplets on the surface of the S-CPDMS film without silicone oil filling ( Figure 4 A) and the surface of the S-CPDMS film filled with silicone oil ( Figure 4Regarding the evaporation situation on B), it can be seen that after the protein droplet evaporates on the surface filled with silicone oil, only a very small contaminated area will be formed. However, on the surface without silicone oil filling, a large contaminated area will be formed after the protein droplet evaporates. The commonly used hydrophobic lubricating layers are fluorides such as Teflon and Cytop. Compared with these fluorides, silicone oil not only has a simple preparation process, low cost, but also has good anti-biofouling properties. Due to the action of interfacial tension, when a water droplet is placed on the membrane surface, the oil will spontaneously climb along the surface of the droplet, forming a thin oil shell around the droplet. This helps to prevent water evaporation and also enables the droplet to only contact the liquid oil layer and not the solid layer, avoiding the adhesion of the droplet on the surface. The thin film proposed in this invention not only solves the problem of low dielectric constant of the commonly used dielectric layer, but also solves the problem of poor anti-biofouling properties of the commonly used hydrophobic layer, and can be widely applied to biological experiments. Description of the Drawings

[0042] Figure 1 It is a physical diagram of the S-CPDMS dielectric hydrophobic integrated thin film.

[0043] Figure 2 It is an SEM diagram of the S-CPDMS dielectric hydrophobic integrated thin film.

[0044] Figure 3 It is a comparison diagram of the dielectric constants of the S-CPDMS thin film and the PDMS film.

[0045] Figure 4 It is a diagram of the evaporation situation of protein droplets on the surface of the S-CPDMS film. (A) is the evaporation situation on the surface without silicone oil filling, and (B) is the evaporation situation on the surface filled with silicone oil.

[0046] Figure 5 It is a contact angle detection device composed of the S-CPDMS dielectric hydrophobic integrated thin film.

[0047] Figure 6 It is a diagram of the droplet shape before and after applying voltage taken by CCD.

[0048] Figure 7 It is the change situation of the contact angle (A) of the thin film made at the ratio of P(VDF-TrFE):PDMS = 3:1 under DC / different frequency AC voltages and the contact angle recovery situation (B) under DC and 1 kHz AC.

[0049] Figure 8 It is the change situation of the contact angle of the droplet when the 1 kHz 150 V alternating current is repeatedly powered on and off.

[0050] Figure 9 It is the glass electrode array used in the digital microfluidic device. (A) is the closed array, and (B) is the open array.

[0051] Figure 10 It is a physical picture of the transportation of droplets on an open digital microfluidic chip with an S-CPDMS membrane.

[0052] Figure 11 It is a physical picture of the transportation of droplets on a closed digital microfluidic chip with an S-CPDMS membrane.

[0053] Figure 12 It is a physical picture of the mixing of droplets on a closed digital microfluidic chip with an S-CPDMS membrane.

[0054] Figure 13 It is a physical picture of the splitting of droplets on a closed digital microfluidic chip with an S-CPDMS membrane.

[0055] Figure 14 It is a comparison chart of the transportation speeds of droplets on two types of open electrodes.

[0056] Reference numerals in the figure: 1 is an external DC / AC power supply; 2 is a CCD camera; 3 is a deionized water droplet; 4 is a dimethyl silicone oil layer; 5 is a glass substrate; 6 is an S-CPDMS dielectric hydrophobic integrated film; 7 is an ITO conductive layer; 8 is a customized ITO conductive electrode array. Specific implementation manners

[0057] The present invention will be further described below in conjunction with the accompanying drawings and embodiments, but the present invention is by no means limited only to the described embodiments.

[0058] Embodiment 1

[0059] Preparation of the S-CPDMS dielectric hydrophobic integrated film. The preparation of the S-CPDMS dielectric hydrophobic integrated film proposed by the present invention will be described below.

[0060] The materials required for manufacturing the film are P(VDF-TrFE) powder with a molar mass ratio of 7:3, PDMS colloid and curing agent, 10cst dimethyl silicone oil, N,N-dimethylformamide, tetrahydrofuran (THF), and cleaned ITO glass. The equipment required is a spin coater and a vacuum drying oven.

[0061] (1) First, 0.6 g of P(VDF-TrFE) and 10 g of N,N-dimethylformamide are magnetically stirred on a magnetic stirrer at 50 °C for 15 h to fully dissolve P(VDF-TrFE) into N,N-dimethylformamide to form solution A;

[0062] (2) 0.2 g of PDMS and 9 g of THF are magnetically stirred at room temperature for 2.5 h to fully dissolve PDMS into THF to form solution B;

[0063] (3) The prepared Solution A, Solution B, and 0.04 g of PDMS curing agent are magnetically stirred for 2.5 h at room temperature to fully mix them into a P(VDF-TrFE) / PDMS solution with a mass ratio of P(VDF-TrFE) to PDMS of 3:1.

[0064] (4) The P(VDF-TrFE) / PDMS mixed solution formed by stirring is centrifuged to remove impurity particles.

[0065] (5) The P(VDF-TrFE) / PDMS solution after removing impurities is spin-coated on the clean ITO surface at a speed of 250 r / min for 18 s to form a uniform P(VDF-TrFE) / PDMS solution layer.

[0066] (6) The ITO glass spin-coated with the P(VDF-TrFE) / PDMS solution is placed in a vacuum drying oven for curing. It is heated in an environment with a temperature of 140 °C for 90 min to remove the solvent in the solution and cure the film.

[0067] (7) Dimethyl silicone oil with a viscosity of 10 cst is coated on the surface of the cured film.

[0068] (8) It is vertically placed for 90 min to form a smooth silicone oil layer on the film surface, that is, a surface-smooth S-CPDMS dielectric hydrophobic integrated film with a mass ratio of P(VDF-TrFE) to PDMS of 3:1 is obtained.

[0069] In the present invention, a composite film is obtained by mixing two different materials, and the obtained film can well combine the advantages of the two materials. The preparation method of the S-CPDMS dielectric hydrophobic integrated film proposed by the present invention is simple, with low cost, and has great application prospects.

[0070] Example 2

[0071] The materials required for manufacturing the film are P(VDF-TrFE) powder, PDMS colloid, and curing agent with a molar mass ratio of 7:3, 5 cst dimethyl silicone oil, N,N-dimethylformamide, tetrahydrofuran (THF), and cleaned ITO glass. The equipment required includes a spin coater and a vacuum drying oven.

[0072] (1) First, 0.8 g of P(VDF-TrFE) and 10 g of N,N-dimethylformamide are magnetically stirred on a magnetic stirrer for 18 h at 60 °C to fully dissolve P(VDF-TrFE) into N,N-dimethylformamide to form Solution A.

[0073] (2) 0.4 g of PDMS and 9 g of THF were magnetically stirred for 3 h at room temperature to fully dissolve PDMS in THF to form Solution B;

[0074] (3) The prepared Solution A, Solution B, and 0.08 g of PDMS curing agent were magnetically stirred for 3 h at room temperature to fully mix them into a P(VDF-TrFE) / PDMS solution with a mass ratio of P(VDF-TrFE):PDMS of 2:1;

[0075] (4) The P(VDF-TrFE) / PDMS mixed solution formed by stirring was centrifuged to remove impurity particles;

[0076] (5) The P(VDF-TrFE) / PDMS solution was spin-coated on the clean ITO surface at a speed of 350 r / min for 18 s to form a uniform P(VDF-TrFE) / PDMS solution layer;

[0077] (6) The ITO glass spin-coated with the P(VDF-TrFE) / PDMS solution was placed in a vacuum drying oven for curing. It was heated in an environment at 140 °C for 100 min to remove the solvent in the solution and cure the film;

[0078] (7) Dimethyl silicone oil with a viscosity of 10 cst was coated on the surface of the cured film;

[0079] (8) It was vertically placed for 100 min to form a smooth silicone oil layer on the film surface, that is, a surface-smooth S-CPDMS dielectric hydrophobic integrated film with a mass ratio of P(VDF-TrFE):PDMS of 2:1 was obtained.

[0080] Example 3

[0081] The materials required for manufacturing the film are P(VDF-TrFE) powder, PDMS colloid, and curing agent with a molar mass ratio of 7:3, 5 cst dimethyl silicone oil, N,N-dimethylformamide, tetrahydrofuran (THF), and cleaned ITO glass. The equipment required includes a spin coater and a vacuum drying oven.

[0082] (1) First, 1 g of P(VDF-TrFE) and 10 g of N,N-dimethylformamide were magnetically stirred for 20 h at 70 °C on a magnetic stirrer to fully dissolve P(VDF-TrFE) in N,N-dimethylformamide to form Solution A;

[0083] (2) 1 g of PDMS and 9 g of THF were magnetically stirred for 4 h at room temperature to fully dissolve PDMS in THF to form Solution B;

[0084] (3) The prepared Solution A, Solution B, and 0.2 g of PDMS curing agent were magnetically stirred at room temperature for 4 h to fully mix them into a P(VDF-TrFE) / PDMS solution with a mass ratio of P(VDF-TrFE) to PDMS of 1:1;

[0085] (4) The P(VDF-TrFE) / PDMS mixed solution formed by stirring was centrifuged to remove impurity particles;

[0086] (5) The P(VDF-TrFE) / PDMS solution was spin-coated on the clean ITO surface at a speed of 450 r / min for 18 s to form a uniform P(VDF-TrFE) / PDMS solution layer;

[0087] (6) The ITO glass spin-coated with the P(VDF-TrFE) / PDMS solution was placed in a vacuum drying oven for curing. It was heated in an environment at a temperature of 140 °C for 120 min to remove the solvent in the solution and cure the film;

[0088] (7) Dimethyl silicone oil with a viscosity of 10 cst was coated on the surface of the cured film;

[0089] (8) It was vertically placed for 120 min to form a smooth silicone oil layer on the film surface, that is, a surface-smooth S-CPDMS dielectric and hydrophobic integrated film with a mass ratio of P(VDF-TrFE) to PDMS of 1:1 was obtained.

[0090] Verification of the comparative example

[0091] Electrowetting property detection experiment. To detect the electrowetting properties of the dielectric film proposed in the present invention, it is necessary to measure the change in the contact angle of a deionized water droplet on the surface of the dielectric film caused by the change in the applied voltage. The S-CPDMS film was prepared on a 3 cm × 3 cm ITO glass by the above-mentioned film preparation method. The ITO glass sheet with the prepared dielectric film was placed on the stage of a contact angle measuring instrument. 10 μL of deionized water was dropped onto the surface of the dielectric film using a pipette. A platinum wire with a diameter of 50 μm was placed on the contact angle measuring instrument. The stage of the contact angle measuring instrument was moved left and right so that the deionized water droplet was directly below the platinum wire. The stage of the contact angle measuring instrument was moved up and down so that the platinum wire was inserted into the deionized water droplet. A DC voltage signal was generated using a DC regulated power supply, and an AC voltage signal was generated using a signal generator and a voltage amplifier. The DC / AC voltage was applied between the ITO surface and the droplet to form a contact angle detection device (such as Figure 5As shown). The droplet images are recorded by the CCD camera on the contact angle measuring instrument, and the droplet contact angle values are obtained by processing the droplet images through the Contact angle plug-in in the ImageJ software on the PC side. Then, the corresponding droplet contact angles under specific voltages are deduced. The electro-wetting characteristics of the thin film can be seen from the changes in the droplet contact angles. Through Figure 6 We can clearly see that the initial contact angle of the droplet on the EWOD device is relatively large, indicating good hydrophobicity, and it is fully wetted after applying a voltage of 150V. Dielectric thin films of different ratios of P(VDF-TrFE) / PDMS are prepared. The S-CPDMS dielectric hydrophobic integrated thin film with a ratio of 3:1 has good electro-wetting characteristics and recoverability. The changes in the contact angle under AC and DC voltages are shown in Figure 7 (A), and the contact angle recoverability under DC voltage and 1kHz AC voltage is shown in Figure 7 (B). It can be seen from the figure that the S-CPDMS dielectric hydrophobic integrated thin film with a ratio of 3:1 exhibits good electro-wetting characteristics under the action of alternating current in the range of 100Hz - 1kHz. When the frequency is relatively high (such as 10kHz, 100kHz), the change in the contact angle drops sharply, and the droplet is scattered due to the too high voltage frequency. When using DC voltage, although the change in the contact angle is relatively good, the recoverability is poor. Therefore, it is more appropriate to use alternating current in the low frequency band (100Hz - 1kHz) for droplet manipulation.

[0092] Repeated power-on and power-off experiments. Generally, droplet manipulation based on EWOD is to apply power on and off to different driving electrodes in a certain sequence under the driving voltage. Therefore, conducting multiple repeated power-on and power-off experiments can well verify the feasibility of the continuous droplet manipulation of the dielectric thin film proposed in the present invention. Clean the ITO glass, and prepare the S-CPDMS dielectric hydrophobic integrated thin film on the conductive side of the ITO glass through the steps described above. Place the ITO glass with the S-CPDMS dielectric hydrophobic integrated thin film on the sample stage of the contact angle measuring instrument, drop 10 μL of deionized water on the surface of the thin film, and conduct repeated power-on and power-off experiments by repeatedly turning on and off an AC voltage with a frequency of 1kHz and an effective value of 150V between the deionized water and the ITO. In order to detect the stable droplet contact angle, the time for turning on and off the voltage is 3S each. The data graph obtained from the experiment is shown in Figure 8 As shown. It can be seen from the figure that after switching the voltages of 0V and 150V for 10 cycles, the modulation of the droplet contact angle does not show obvious attenuation. After power-off, the droplet contact angle is basically the same as the initial contact angle, and after power-on, the droplet contact angle is also not much different from the saturated contact angle. This shows that the S-CPDMS dielectric hydrophobic integrated thin film proposed in the present invention has good droplet manipulability, can perform continuous droplet manipulation, and the manipulation ability will not decrease after multiple droplet manipulations.

[0093] Droplet manipulation experiments. Droplet manipulation experiments were carried out in open and closed environments respectively. The back-and-forth transportation of droplets was carried out in the open environment, and the droplet transportation, merging, and splitting experiments were carried out in the closed environment. The specific details are as follows:

[0094] (1) Droplet transportation on an open digital microfluidic chip with an S-CPDMS membrane. The S-CPDMS dielectric hydrophobic integrated thin film was prepared step by step on the ITO glass electrode array with the above-mentioned open structure for droplet transportation. The droplet transportation methods of the two types of open ITO glass electrodes are the same. First, the grounded electrode is grounded from beginning to end. The droplet to be manipulated is placed on the transportation electrode (the area of the droplet should be slightly larger than the area of one transportation electrode to prevent disconnection during transportation). Then, an alternating current voltage with a specific frequency and amplitude is applied to the transportation electrode under the center of the droplet, and the other transportation electrodes are in a high-impedance state. The next electrode state is determined by the operation to be performed (forward / backward). For example, if you want the droplet to move forward, an alternating current voltage is applied to the electrode adjacent to the front of the electrode under the center of the droplet, and the other transportation electrodes are placed in a high-impedance state, and the droplet can achieve forward transportation. The actual transportation physical diagram of the droplet on the open digital microfluidic chip with an S-CPDMS membrane is as Figure 10 shown.

[0095] (2) Droplet transportation on a closed digital microfluidic chip with an S-CPDMS membrane. The conductive tape (single-layer thickness 100 μm) is pasted on the grounded electrode of the lower plate to provide a gap between the upper and lower plates. The droplet is dropped onto the lower plate prepared with the P(VDF-TrFE) / PDMS dielectric thin film, and the upper plate is placed above the conductive tape to form a closed sandwich structure. The upper plate is always grounded, the electrode under the center of the droplet is connected to an alternating current, and the other electrodes of the lower plate are grounded. The front, back, left, and right transportation of the droplet is achieved by applying electricity to the electrodes adjacent to the center electrode of the droplet. For example, if you want the droplet to move to the right, an alternating current is applied to the electrode adjacent to the right of the droplet, and the other electrodes are grounded, and the droplet will move to the right. The actual transportation physical diagram of the droplet on the closed digital microfluidic chip with an S-CPDMS membrane is as Figure 11 shown.

[0096] (3) Droplet merging on a closed digital microfluidic chip with an S-CPDMS membrane. Droplet merging in a closed environment is to simultaneously manipulate the transportation of multiple droplets and transport multiple droplets together. This method of droplet manipulation is mostly applied to biochemical reactions (such as acid-base neutralization reactions). Form a sandwich structure with multiple droplets and the upper and lower electrode plates, preset the transportation path of each droplet in advance, and then transport and merge multiple droplets along the preset path according to the droplet transportation steps in the closed environment. Move the merged droplets several times to ensure sufficient mixing of the droplets. The actual mixing physical diagram of the droplets on the closed digital microfluidic chip with an S-CPDMS membrane is as shown in Figure 12 shown.

[0097] (4) Droplet splitting on a closed digital microfluidic chip with an S-CPDMS membrane. Droplet splitting is the process of splitting a larger droplet into two droplets by electrowetting force. Here, an embodiment of splitting a droplet occupying the size of two electrodes into two droplets is described. The specific operation method is to apply an alternating current voltage to the two electrodes occupied by the droplet, and the other electrodes are grounded to ensure that the droplet center is in the middle of the two electrodes. Then, apply alternating current to the two electrodes adjacent to the two electrodes occupied by the droplet (the two adjacent electrodes and the two occupied electrodes are on a straight line), and at the same time ground the two middle electrodes and the other surrounding electrodes. In this way, a droplet occupying the size of two electrodes can be split into two droplets each occupying the size of one electrode. Generally speaking, the electrowetting force required for droplet splitting is greater than the electrowetting force for droplet transportation. Therefore, droplet splitting needs to be carried out under a relatively large alternating current voltage. The actual splitting physical diagram of the droplets on the closed digital microfluidic chip with an S-CPDMS membrane is as shown in Figure 13 shown.

[0098] In the present invention, for the measurement of the electrowetting characteristics of the S-CPDMS dielectric hydrophobic integrated thin film, data on the change of the contact angle of the droplet on the thin film with voltage is obtained through an external DC / AC power supply, a contact angle measuring instrument, a PC, and ImageJ software;

[0099] The external DC power supply (such as MP2002D) is used to provide a DC signal with a voltage of 0 - 150V. The external AC power supply mainly consists of a function generator (such as AFG-2225), a high-voltage amplifier (such as HA-520), and an oscilloscope (such as DS2302A) in cooperation, and can provide a square wave alternating current signal with a frequency of 100Hz - 100kHz and an effective voltage of 0 - 150V;

[0100] The function generator is used to generate square waves with different frequencies;

[0101] The high-voltage amplifier amplifies the square wave output by the function generator;

[0102] The oscilloscope observes the waveform of the square wave amplified by the high-voltage amplifier to obtain the effective value (V rms ) of the amplified voltage;

[0103] The contact angle measuring instrument (such as PHOENIX 300) records the contour map of the droplet to be measured through its built-in CCD camera;

[0104] The PC terminal receives the pictures taken by the contact angle measuring instrument;

[0105] The ImageJ software is used to process the pictures received by the PC terminal.

[0106] The present invention detects the thin-film electro-wetting performance through the cooperation of a contact angle measuring instrument, an external power supply, and a PC terminal. The S-CPDMS dielectric hydrophobic integrated film prepared by the above preparation method is placed on the contact angle measuring instrument, and 10 μL of deionized water droplets are dropped on the film surface. An AC-DC voltage is applied between the ITO and the droplets, and the change of the contact angle of the droplets with the voltage is recorded by the CCD camera on the contact angle measuring instrument. The change of the contact angle of the film formed when the mass ratio of P(VDF-TrFE):PDMS is 3:1 is as Figure 7 shown. The initial contact angle of the dielectric film proposed by the present invention is 104° - 106°, showing good hydrophobicity. When P(VDF-TrFE):PDMS = 3:1 and under DC voltage, when the applied voltage is 110 V, the contact angle of the droplet decreases from 105° to 53° and tends to be saturated, and the change of the contact angle of the droplet is as high as 52°, but the contact angle recovery is poor; when applying AC, within the range of 100 Hz - 1 kHz of the AC frequency, the change of the contact angle of the droplet is basically the same. When the effective value of the AC is 130 V, the contact angle of the droplet is saturated, and the saturated contact angle is 45°, and it has good contact angle recovery. The difference in the contact angle before and after power-on is within 3°, showing good recovery. However, when the frequency is 10 kHz and 100 kHz, the electro-wetting performance of the film drops sharply, and bubbles appear inside the droplet when pressurized. In order to maintain good electro-wetting effect, low-frequency alternating current should be used to control the droplets. The film maintains good electro-wettability and recovery under low-frequency alternating current.

[0107] In the present invention, a glass electrode spin-coated with an S-CPDMS dielectric-hydrophobic integrated thin film is applied to a digital microfluidic device, and the application scenarios of the S-CPDMS dielectric-hydrophobic integrated thin film are evaluated. Closed droplet manipulation is performed on the glass electrode coated with the S-CPDMS dielectric-hydrophobic integrated thin film. The manipulation performance of the thin film fabricated by the present invention is evaluated by recording the movement speed of the droplet at different voltages, and droplet transportation, merging, and splitting operations are carried out to prove that the thin film can be perfectly adapted to the digital microfluidic device. The closed droplet manipulation carried out has the advantages of slowing down droplet evaporation and better completing droplet formation, splitting, etc. compared with droplet manipulation in an open environment. On two customized open electrode arrays, the droplet transportation speed is faster for the counter electrode type (data as Figure 14 shown), and the droplet can be moved at a voltage of 40V, but the movement speed is only 0.103mm / s, which is close to 0. However, as the applied voltage increases, the droplet speed also increases. Moreover, after the applied voltage is greater than 120V, the droplet speed shows an exponential growth trend. At an alternating current voltage with a frequency of 1kHz and an effective value of 240V, the transportation speed of the droplet on the interdigitated electrode is 13.043mm / s, and the transportation speed on the counter electrode is as high as 18.349mm / s.

[0108] The advantages of the S-CPDMS dielectric-hydrophobic integrated film proposed by the present invention are as follows: First, the S-CPDMS dielectric-hydrophobic integrated film can serve as both a dielectric layer and a hydrophobic layer, simplifying the fabrication steps of digital microfluidic devices and reducing the fabrication cost. Second, the dielectric film prepared in the present invention is fabricated on the substrate surface by spin-coating a mixed solution. Compared with methods such as deposition and weaving, the spin-coating method has the advantages of simple operation and uniform spin-coating, and the cost of fabricating the film is also greatly reduced, which also makes it possible for batch production of the film. Third, the two materials P(VDF-TrFE) and PDMS adopted in the present invention each have their own advantages. Among them, P(VDF-TrFE) has a relatively high dielectric constant, enabling the S-CPDMS dielectric-hydrophobic integrated film to have a high dielectric constant and be capable of low-voltage droplet manipulation; PDMS has good hydrophobicity, film-forming property, transparency, flexibility, and biocompatibility, and the S-CPDMS film perfectly inherits these characteristics. Fourth, the present invention injects dimethyl silicone oil onto the film surface to form a hydrophobic lubricating layer, replacing the traditional fluorinated hydrophobic layer. When a droplet is placed on the surface, it does not directly contact the solid film but contacts the liquid silicone oil, resulting in a very small sliding angle when the droplet slides on the surface and greatly reducing the contact angle hysteresis phenomenon. The presence of silicone oil also improves the anti-biofouling property of the film, and using silicone oil to replace the commonly used fluoropolymer coating can greatly reduce the cost. Fifth, the digital microfluidic device based on the S-CPDMS dielectric-hydrophobic integrated film proposed by the present invention can transport droplets at a lower voltage, expanding the application scenarios of digital microfluidic devices. Therefore, the S-CPDMS dielectric-hydrophobic integrated film prepared by the spin-coating method proposed by the present invention has important practical significance for the application of digital microfluidic devices in scenarios requiring low-voltage manipulation and anti-biofouling.

Claims

1. A preparation method of an S-CPDMS dielectric hydrophobic integrated film, characterized in that, The specific steps are as follows: S1. Add P(VDF-TrFE) to the solvent N,N-dimethylformamide, and heat and stir magnetically to form a P(VDF-TrFE) solution; S2. Add polydimethylsiloxane PDMS to the solvent tetrahydrofuran, and stir magnetically at room temperature to form a PDMS solution; S3. Mix the P(VDF-TrFE) solution and the PDMS solution, add a PDMS curing agent, and stir magnetically at room temperature to form a P(VDF-TrFE) / PDMS mixed solution; S4. Centrifuge the stirred P(VDF-TrFE) / PDMS mixed solution to remove impurity particles; S5. Spin-coat the P(VDF-TrFE) / PDMS mixed solution without impurity particles onto the conductive side of ITO glass; S6. Place the ITO glass spin-coated with the P(VDF-TrFE) / PDMS mixed solution in a vacuum drying oven to cure and form a P(VDF-TrFE) / PDMS film; S7. Coat dimethyl silicone oil on the surface of the formed P(VDF-TrFE) / PDMS film; S8. Hang and place the P(VDF-TrFE) / PDMS film coated with dimethyl silicone oil to form a smooth-surface S-CPDMS dielectric hydrophobic integrated film.

2. The preparation method according to claim 1, wherein: In step S1, the mass ratio of P(VDF-TrFE) to N,N-dimethylformamide is (0.6-1):10; the temperature of heating and magnetic stirring is 50-70 °C, and the time of thermal magnetic stirring is 15-20 h.

3. The preparation method according to claim 1, wherein: In step S2, the mass ratio of PDMS to the solvent is (0.2-1):9; The time of magnetic stirring at room temperature is 2.5-4 h.

4. The preparation method according to claim 1, wherein: In step S3, the mass ratio of PDMS to the curing agent is 5:1; the time of magnetic stirring at room temperature is 2.5-4 h; In step S4, the centrifugation speed is 3000 r / min, and the centrifugation time is 30 min; In step S5, the spin-coating speed is 250-450 r / min, and the spin-coating time is 18 s.

5. The preparation method according to claim 1, wherein: In step S6, the temperature of the vacuum drying oven is 140 °C, and the drying time is 90-120 min; In step S7, the viscosity of the dimethyl silicone oil is 5-50 cst; In step S8, the hanging and placing time is 90-120 min.

6. An S-CPDMS dielectric hydrophobic integrated film prepared by the preparation method according to any one of claims 1-5.

7. Application of the P(VDF-TrFE) / PDMS hybrid dielectric hydrophobic film according to claim 6 in a digital microfluidic chip, characterized in that, The mixed dielectric hydrophobic film serves as a dielectric hydrophobic layer of a chip, and dimethyl silicone oil is coated on the film surface.

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