Patent Publication Number: US-11642668-B2

Title: Method and device of nondestructive transfer of liquid drops and method of micro-reaction of liquid drops

Description:
This application is the National Stage Application of PCT/CN2020/135532, filed on Dec. 11, 2020, which claims priority to Chinese Patent Application No. 202010266981.X, filed on Apr. 7, 2020, which is incorporated by reference for all purposes as if fully set forth herein. 
     TECHNICAL FIELD 
     The present invention relates to the technical field of micro-liquid drop control, and in particular to a triboelectric nanogenerator-based method and device of nondestructive transfer of liquid drops, and a method of micro-reaction of liquid drops. 
     BACKGROUND 
     With the rapid development of biochemical technology, the demand for improvement and optimization of its research equipment is also increasing. The invention and development of micro-liquid drop devices have brought new ideas for experimental research in biology and chemistry. The key to the micro-liquid drop device is the drive method of the liquid drop. The existing drive methods include electric drive (EWOD), magnetic drive, surface acoustic wave drive, mechanical drive, and the like. Dielectric wetting (EWOD) method is a commonly used liquid drop control method, but this method of driving liquid drops often requires huge external equipment and special microfluidic chips, which greatly reduces the universality and portability of EWOD microfluidic chips and has become the main reason restricting its development. 
     The invention of nanomotors has brought new ideas to the improvement of portability. triboelectric nanogenerator (TENG) can convert common mechanical energy in our lives into electrical energy. As a generator of electronic devices or power systems, its characteristics of high voltage and low current are very suitable for the needs of EWOD to drive liquid drops. Here, TENG can provide drive power and control signals for various electromechanical systems, and can be used as a bridge to realize human-machine interaction. Therefore, TENG can also be combined with dielectric wetting technology to realize automatic force manipulation of microfluidics. The fast response capability of TENG can ensure the effective operation of microfluidics, and the good insulation performance of the dielectric wetting system can fully retain the electrostatic field caused by friction. The combination of these two technologies opens up a variety of potential application prospects for TENG-based self-powered technology. 
     SUMMARY OF THE INVENTION 
     In view of the shortcomings of the prior art, an object of the present invention is to provide a method and device of nondestructive transfer of liquid drops, and a method of micro-reaction of liquid drops. 
     In order to achieve the above object, the technical solution according to an embodiment of the present invention is as follows. 
     A device of nondestructive transfer of liquid drops, including a power generation part and a clamping part, wherein the power generation part includes a movable friction material and at least two fixed friction materials, the clamping part includes a supporting mechanism and a left dielectric wetting splint and a right dielectric wetting splint installed on the supporting mechanism, the movable friction material is connected to the left dielectric wetting splint, and the at least two fixed friction materials are connected to the right dielectric wetting splint. 
     As a further improvement of the present invention, the supporting mechanism includes a first supporting frame and a second supporting frame connected to the first supporting frame, the left dielectric wetting splint is installed on the first supporting frame, and the right dielectric wetting splint and the at least two fixed friction materials are all installed on the second supporting frame. 
     As a further improvement of the present invention, a crank connecting rod mechanism is further provided, the crank connecting rod mechanism includes a crank and a slider movably connected to one end of the crank, the other end of the crank is hinged with the first supporting frame, and the slider is connected to the left dielectric wetting splint. 
     As a further improvement of the present invention, an upper part of the first supporting frame is connected to a bolt, a lower part of the first supporting frame is provided with a spring, the bolt abuts against an upper end of the crank, and the spring abuts against a lower end of the crank. 
     As a further improvement of the present invention, at least one roller is provided in the first supporting frame, a through hole is provided on the slider, and the at least one roller extends into the through hole. 
     As a further improvement of the present invention, the left dielectric wetting splint includes a left glass substrate, a left splint electrode, and a left hydrophobic layer arranged in sequence from outside to inside, the left splint electrode is connected to the movable friction material, the right dielectric wetting splint includes a right glass substrate, a right splint electrode group, a dielectric layer, and a right hydrophobic layer arranged in sequence from outside to inside, the right splint electrode group includes at least two right splint electrodes arranged at intervals in an up and down direction, and the at least two right splint electrodes are respectively connected to the at least two fixed friction electrodes. 
     As a further improvement of the present invention, the left glass substrate and the slider are connected by a first adhesive tape, and the right glass substrate and the second supporting frame are connected by a second adhesive tape. 
     As a further improvement of the present invention, two pin shafts are connected between an upper end of the second supporting frame and the first supporting frame. 
     A method of nondestructive transfer of liquid drops, applicable the above device and including the steps of: 
     (1) moving the device above a liquid drop, and making the liquid drop contact the left dielectric wetting splint and the right dielectric wetting splint; 
     (2) making the movable friction material repeatedly contact the fixed friction material corresponding to the right splint lower electrode to generate an electric field on the right splint lower electrode to drive the liquid drop to move upward; 
     (3) moving the movable friction material onto the fixed friction material corresponding to the right splint upper electrode to generate an electric field on the right splint upper electrode to drive the liquid drop to move to a corresponding position of the right splint upper electrode; 
     (4) moving the movable friction material onto the fixed friction material corresponding to the right splint lower electrode, and moving the liquid drop to a corresponding position of the right splint lower electrode; and 
     (5) moving the left dielectric wetting splint away from the right dielectric wetting splint, and at the same time making the movable friction material contact the fixed friction material corresponding to the right splint lower electrode, such that the liquid drop leaves the device to complete the release. 
     A method of micro-reaction of liquid drops, applicable to the above device and including the steps of: 
     (1) moving the device above a first liquid drop, and making the first liquid drop contact the left dielectric wetting splint and the right dielectric wetting splint; 
     (2) making the movable friction material repeatedly contact the fixed friction material corresponding to the right splint lower electrode to generate an electric field on the right splint lower electrode to drive the first liquid drop to move upward; 
     (3) moving the movable friction material onto the fixed friction material corresponding to the right splint upper electrode to generate an electric field on the right splint upper electrode to drive the first liquid drop to move to a corresponding position of the right splint upper electrode; 
     (4) moving the device above a second liquid drop, and making the second liquid drop contact the left dielectric wetting splint and the right dielectric wetting splint; and 
     (5) making the movable friction material contact the fixed friction material corresponding to the right splint lower electrode to generate an electric field on the right splint lower electrode to drive the first and second liquid drops to move to a corresponding position of the right splint lower electrode simultaneously and fuse at the corresponding position to complete micro-reaction. 
     The beneficial effects of the present invention are as follows. 
     (1) A triboelectric nanogenerator is adopted to generate electricity to drive liquid drops, replacing the traditional motor, simplifying the structure of the device and making the device more portable. 
     (2) Through the use of a crank connecting rod mechanism, the left dielectric wetting splint realizes horizontal movement, in order to adjust the distance between the left dielectric wetting splint and the right dielectric wetting splint and adapt to liquid drops of different volumes. 
     (3) By providing two rollers on the same horizontal line, the accuracy of the horizontal movement of the left splint is ensured. 
     (4) By providing a bolt and a spring, when the bolt is screwed or unscrewed, the left dielectric wetting splint moves horizontally to the right or horizontally to the left, which is convenient and quick to adjust. 
     (5) Two pin shafts are connected between the first supporting frame and the second supporting frame to ensure that the left dielectric wetting splint is parallel to the right dielectric wetting splint. 
     (6) A left splint electrode, a right splint lower electrode and a right splint upper electrode are provided. An electric field is generated on the right splint lower electrode or the right splint upper electrode through electricity generation by friction to control the movement of the liquid drop, which can realize the driving and fusion and the like of the liquid drop. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For clearer descriptions of the technical solutions in the embodiments of the present invention or the prior art, the following briefly introduces the accompanying drawings required for describing the embodiments or the prior art. Apparently, the accompanying drawings in the following description show merely some embodiments of the present disclosure, and persons of ordinary skill in the art may still derive other drawings from these accompanying drawings without creative efforts. 
         FIG.  1    is a schematic diagram of an overall structure of a preferred embodiment of the present invention; 
         FIG.  2    is a schematic diagram of an internal structure of a preferred embodiment of the present invention; 
         FIG.  3    is a front view of a crank connecting rod mechanism according to a preferred embodiment of the present invention; 
         FIG.  4    is a top view of the crank connecting rod mechanism according to a preferred embodiment of the present invention; 
         FIG.  5    is a schematic diagram of a structure in which a left dielectric wetting splint and a right dielectric wetting splint are opposite to each other according to a preferred embodiment of the present invention; 
         FIG.  6    is a drive principle diagram of dielectric wetting according to a preferred embodiment of the present invention; and 
         FIG.  7    is an operation principle diagram of a preferred embodiment of the present invention; 
     
    
    
     In the figures:  10 , movable friction material,  12 , supporting mechanism,  14 , left dielectric wetting splint,  16 , right dielectric wetting splint,  18 , first supporting frame,  20 , second supporting frame,  22 , crank,  24 , slider,  26 , bolt,  28 , spring,  29 , supporting block,  30 , supporting shaft,  32 , waist-shaped hole,  34 , supporting rod,  36 , roller,  38 , through hole,  40 , left glass substrate,  42 , left splint electrode,  44 , left hydrophobic layer,  46 , right glass substrate,  48 , dielectric layer,  50 , right hydrophobic layer,  52 , first fixed friction material,  54 , second fixed friction material,  56 , right splint lower electrode,  58 , right splint upper electrode,  60 , pin shaft,  62 , liquid drop. 
     DETAILED DESCRIPTION 
     In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be described clearly and completely in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are merely some embodiments of the present invention, not all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work shall fall within the scope of protection of the present invention. 
     As shown in  FIGS.  1  and  2   , a device of nondestructive transfer of liquid drops includes a power generation part and a clamping part. The power generation part includes a movable friction material  10  and at least two fixed friction materials. The clamping part includes a supporting mechanism  12  and a left dielectric wetting splint  14  and a right dielectric wetting splint  16  mounted on the supporting mechanism  12 . The movable friction material  10  is connected to the left dielectric wetting splint  14 . The at least two fixed friction materials are connected to the right dielectric wetting splint  16 . 
     In a preferred embodiment of the present invention, the supporting mechanism  12  includes a first supporting frame  18  and a second supporting frame  20  connected to the first supporting frame  18 . The left dielectric wetting splint  14  is mounted on the first supporting frame  18 . The right dielectric wetting splint  16  and the at least two fixed friction materials are installed on the second supporting frame  20 . 
     As shown in  FIGS.  3  and  4   , a crank connecting rod mechanism is also provided. 
     The crank connecting rod mechanism includes a crank  22  and a slider  24  movably connected to one end of the crank  22 . The other end of the crank  22  is hinged with the first supporting frame  18 . The slider  24  is connected to the left dielectric wetting splint  14 . When the crank  22  rotates, the left dielectric wetting splint  14  is driven by the slider  24  to move away from or toward the right dielectric wetting splint  16 , thereby adjusting the distance between the left dielectric wetting splint  14  and the right dielectric wetting splint  16  to adapt to liquid drops of different volumes. 
     In a preferred embodiment of the present invention, an upper part of the first supporting frame  18  is connected to a bolt  26 . A lower part of the first supporting frame  18  is provided with a spring  28 . The bolt  26  abuts against an upper end of the crank  22 . The spring  28  abuts against a lower end of the crank  22 . When the bolt  26  is screwed, the crank  22  moves clockwise under the pressure of the bolt  26 . At the same time, the spring  28  is compressed, such that the left dielectric wetting splint  14  moves to the right, as shown in  FIG.  2   . In this case, the bolt  26  is in a screwed state. When the bolt  26  is unscrewed, the crank  22  moves counterclockwise under the action of the spring  28 , such that the left dielectric wetting splint  14  moves to the left. In a further preferred embodiment of the present invention, a supporting block  29  is provided in the first supporting frame  18 . The bolt  26  is threadedly connected to the supporting block  29  and extends out of the supporting block  29 . 
     In a preferred embodiment of the present invention, a supporting shaft  30  is provided in the first supporting frame  18 . The crank  22  is sleeved on the supporting shaft  30  and can rotate around the supporting shaft  30 . 
     In a preferred embodiment of the present invention, the slider  24  is provided with a waist-shaped hole  32  which extends in an up and down direction. One end of the crank  22  is provided with a supporting rod  34  which extends into the waist-shaped hole  32 . The supporting rod  34  moves in the waist-shaped hole  32  to drive the slider  24  to move in a left and right direction. 
     In a preferred embodiment of the present invention, at least one roller  36  is provided in the first supporting frame  18 . A through hole  38  is provided on the slider  24  which extends in a horizontal direction. The at least one roller  36  extends into the through hole  38 . When the slider  24  moves left and right, the roller  36  is used for guidance to avoid the skew of the slider  24  and ensure the accuracy of the left and right horizontal movement of the left dielectric wetting splint  14 . In a further preferred embodiment of the present invention, the number of the rollers  36  is two, and the two rollers  36  are on the same horizontal line. 
     As shown in  FIG.  5   , in a preferred embodiment of the present invention, the left dielectric wetting splint  14  includes a left glass substrate  40 , a left splint electrode  42 , and a left hydrophobic layer  44  arranged in sequence from outside to inside. The left splint electrode  42  is connected to the movable friction material  10 . The right dielectric wetting splint  16  includes a right glass substrate  46 , a right splint electrode group, a dielectric layer  48 , and a right hydrophobic layer  50  arranged in sequence from outside to inside. The right splint electrode group includes at least two right splint electrodes arranged at intervals in the up and down direction which are respectively connected to the at least two fixed friction materials. In this embodiment, the number of the fixed friction materials is two, and the two fixed friction materials are a first fixed friction material  52  and a second fixed friction material  54 , respectively. The number of the right splint electrodes is two, and the two right splint electrodes are a right splint lower electrode  56  and a right splint upper electrode  58 , respectively. The first fixed friction material  52  is connected to the right splint lower electrode  56 . The second fixed friction material  54  is connected to the right splint upper electrode  58 . Because the movable friction material  10  is connected to the left splint electrode  42 , the electrical properties on the left splint electrode  42  and the right splint lower electrode  56  or the right splint upper electrode  58  are always different. 
     In a preferred embodiment of the present invention, the movable friction material  10 , the first fixed friction material  52 , and the second fixed friction material  54  are all subjected to electrostatic spinning treatment to increase the friction contact area and improve the power generation efficiency. In a preferred embodiment, the movable friction material  10  is Kapton, and the first fixed friction material  52  and the second fixed friction material  54  are aluminum sheets. 
     In a further preferred embodiment of the present invention, both the left hydrophobic layer  44  and the right hydrophobic layer  50  are made of Teflon materials, but they are not limited to Teflon materials, and may also be made of nanoparticle coatings. In a further preferred embodiment of the present invention, the dielectric layer  48  is made of PDMS material, but it is not limited to PDMS material, and may also be made of PMMA or SU-8. 
     In order to improve the stability of the left dielectric wetting splint  14  mounted on the first supporting frame  18  and the stability of the right dielectric wetting splint  16  mounted on the second supporting frame  20 , in a preferred embodiment of the present invention, the left glass substrate  40  and the slider  24  are connected by a first adhesive tape (not shown), and the right glass substrate  46  and the second supporting frame  20  are connected by a second adhesive tape (not shown). 
     In a preferred embodiment of the present invention, the second supporting frame  20  is in an inverted L shape, which facilitates the installation of the right dielectric wetting splint  16  and at the same time facilitates the connection with the first supporting frame  18 . In order to keep the left dielectric wetting splint  14  and the right dielectric wetting splint  16  parallel, in a preferred embodiment of the present invention, two pin shafts  60  are connected between an upper end of the second supporting frame  20  and the first supporting frame  18 . 
     Under the action of an electric field force, a liquid drop will produce a dielectric wetting effect, which may be expressed by a Young-Lippman equation (1): 
     
       
         
           
             
               
                 
                   
                     cos 
                     ⁢ 
                     
                       θ 
                       ⁡ 
                       ( 
                       V 
                       ) 
                     
                   
                   = 
                   
                     
                       cos 
                       ⁢ 
                       
                         θ 
                         0 
                       
                     
                     + 
                     
                       
                         
                           
                             ε 
                             0 
                           
                           ⁢ 
                           
                             ε 
                             d 
                           
                         
                         
                           2 
                           ⁢ 
                           
                             γ 
                             ld 
                           
                           ⁢ 
                           t 
                         
                       
                       ⁢ 
                       
                         V 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where θ 0  is a contact angle between the liquid drop and a solid surface when the voltage is 0, θ(V) represents the contact angle between the liquid drop and the solid surface when the voltage is V, ε 0  is a permittivity in vacuum, ε d  is a relative permittivity, γld is a gas-liquid surface tension, t is the thickness of the dielectric layer, and V is the applied voltage. 
       FIG.  6    is a drive principle diagram of dielectric wetting. When an electrode under a liquid drop is energized, a corresponding contact angle changes accordingly, and the shape of the liquid drop is also deformed. An initial state of the liquid drop is shown in  FIG.  6 ( a ) . When a right electrode is energized and a left electrode is not energized, the state of the liquid drop is shown in  FIG.  6 ( b ) . A left contact angle of the liquid drop is still θ 0 , a radius of curvature of the liquid drop profile is ρ 0 , a right contact angle is θ(V), and the radius of curvature of the liquid drop profile is ρ(V). According to a relationship (2) between the pressure difference and the radius of curvature, a Laplace pressure difference inside the liquid drop may be calculated, that is, a force that drives the liquid drop. 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     P 
                   
                   = 
                   
                     
                       γ 
                       ld 
                     
                     ( 
                     
                       
                         1 
                         
                           ρ 
                           0 
                         
                       
                       + 
                       
                         1 
                         
                           ρ 
                           ⁡ 
                           ( 
                           V 
                           ) 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     A method of nondestructive transfer of liquid drops of the present invention is introduced hereinafter. The method is applicable to the above device and includes the following steps. 
     (1) The device is moved above a liquid drop  62 , and the liquid drop  62  contacts the left dielectric wetting splint  14  and the right dielectric wetting splint  16 . 
     (2) The movable friction material  10  repeatedly contacts the fixed friction material corresponding to the right splint lower electrode to generate an electric field on the right splint lower electrode to drive the liquid drop  62  to move upward. 
     (3) The movable friction material  10  is moved onto the fixed friction material corresponding to the right splint upper electrode to generate an electric field on the right splint upper electrode to drive the liquid drop  62  to move to a corresponding position of the right splint upper electrode. 
     (4) The movable friction material  10  is moved onto the fixed friction material corresponding to the right splint lower electrode, and the liquid drop  62  is moved to a corresponding position of the right splint lower electrode. 
     (5) The left dielectric wetting splint  14  is moved away from the right dielectric wetting splint  16 , and at the same time the movable friction material  10  contacts the fixed friction material corresponding to the right splint lower electrode, and the liquid drop  62  leaves the device to complete the release. 
     In order to further illustrate the method of nondestructive transfer of liquid drops of the present invention, a preferred embodiment includes the following steps. 
     (1) The device is moved above the liquid drop  62 . The bolt is screwed or unscrewed according to the volume of the liquid drop  62  to adjust the distance between the left dielectric wetting splint  14  and the right dielectric wetting splint  16 . The liquid drop  62  contacts the left dielectric wetting splint  14  and the right dielectric wetting splint  16 . Meanwhile, the liquid drop  62  is located at the corresponding position of the right splint lower electrode  56 , as shown in  FIG.  7 ( a ) . 
     (2) The movable friction material  10  repeatedly contacts the first fixed friction material  52  which is negatively charged. Since the charging ability of the first fixed friction material  52  is lower than that of the movable friction material  10 , in order to maintain electrical neutrality, the negative charge on the right splint lower electrode  56  is transferred to the first fixed friction material  52 , such that the right splint lower electrode  56  is positively charged, and a positive electric field is generated on the right splint lower electrode  56 , thereby producing a dielectric wetting effect and driving the liquid drop  62  to move up, as shown in  FIG.  7 ( b ) . 
     (3) The movable friction material  10  is moved onto the second fixed friction material  54  which is negatively charged. Since the charging ability of the second fixed friction material  54  is lower than that of the movable friction material  10 , in order to maintain electrical neutrality, the negative charge on the right splint upper electrode  58  is transferred to the second fixed friction material  54 , such that the right splint upper electrode  58  is positively charged, with the right splint lower electrode  56  being not charged, and a positive electric field is generated on the right splint upper electrode  58 , thereby producing a dielectric wetting effect and driving the liquid drop  62  to move to the corresponding position of the right splint upper electrode  58 , as shown in  FIG.  7 ( c ) . 
     (4) The movable friction material  10  is moved onto the first fixed friction material  52 , and the liquid drop  62  is moved to the corresponding position of the right splint lower electrode  56 , as shown in  FIG.  7 ( d ) . 
     (5) The bolt is unscrewed to move the left dielectric wetting splint  14  to the left, and at the same time the movable friction material  10  contacts the first fixed friction material  52  to neutralize the charge, and the liquid drop  62  leaves the device on the hydrophobic surface under the influence of gravity to complete the release, as shown in  FIG.  7 ( e ) . 
     A method of micro-reaction of liquid drops of the present invention is introduced hereinafter. The method is applicable to the above device and includes the following steps. 
     (1) The device is moved above a first liquid drop, and the first liquid drop contacts the left dielectric wetting splint  14  and the right dielectric wetting splint  16 . 
     (2) The movable friction material  10  repeatedly contacts the fixed friction material corresponding to the right splint lower electrode to generate an electric field on the right splint lower electrode to drive the first liquid drop to move upward. 
     (3) The movable friction material  10  is moved onto the fixed friction material corresponding to the right splint upper electrode to generate an electric field on the right splint upper electrode to drive the first liquid drop to move to a corresponding position of the right splint upper electrode. 
     (4) The device is moved above a second liquid drop, and the second liquid drop contacts the left dielectric wetting splint  14  and the right dielectric wetting splint  16 . 
     (5) The movable friction material  10  contacts the fixed friction material corresponding to the right splint lower electrode to generate an electric field on the right splint lower electrode to drive the first and second liquid drops to move to a corresponding position of the right splint lower electrode simultaneously and fuse at the corresponding position to complete the micro-reaction. 
     In order to further illustrate the method of micro-reaction of liquid drops of the present invention, a preferred embodiment, as shown in  FIG.  7   , includes the following steps. 
     (1) The device is moved above the first liquid drop. The bolt is screwed or unscrewed according to the volume of the first liquid drop to adjust the distance between the left dielectric wetting splint  14  and the right dielectric wetting splint  16 , such that the first liquid drop contacts the left dielectric wetting splint  14  and the right dielectric wetting splint  16 , and at the same time, the first liquid drop is located at the corresponding position of the right splint lower electrode  56 . 
     (2) The movable friction material  10  repeatedly contacts the first fixed friction material  52  which is negatively charged. Since the charging ability of the first fixed friction material  52  is lower than that of the movable friction material  10 , in order to maintain electrical neutrality, the negative charge on the right splint lower electrode  56  is transferred to the first fixed friction material  52 , such that the right splint lower electrode  56  is positively charged, and a positive electric field is generated on the right splint lower electrode  56 , thereby producing a dielectric wetting effect and driving the first liquid drop to move upward. 
     (3) The movable friction material  10  is moved onto the second fixed friction material  54  which is negatively charged. Since the charging ability of the second fixed friction material  54  is lower than that of the movable friction material  10 , in order to maintain electrical neutrality, the negative charge on the right splint upper electrode  58  is transferred to the second fixed friction material  54 , such that the right splint upper electrode  58  is positively charged, with the right splint lower electrode  56  being not charged, and a positive electric field is generated on the right splint upper electrode  58 , thereby producing a dielectric wetting effect and driving the first liquid drop to move to the corresponding position of the right splint upper electrode  58 . 
     (4) The device is moved above the second liquid drop, and the second liquid drop contacts the left dielectric wetting splint  14  and the right dielectric wetting splint  16 . 
     (5) The movable friction material  10  contacts the first fixed friction material  52  to generate an electric field on the right splint lower electrode  56  to drive the first and second liquid drops to move to the corresponding position of the right splint lower electrode  56  simultaneously and fuse at the corresponding position to complete the micro-reaction. 
     For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description, and therefore all changes falling into the meaning and scope of the equivalent elements of the claims are included in the present invention. Any reference numerals in the claims should not be regarded as limiting the claims in question. 
     In addition, it should be understood that although the description is illustrated with implementations, not each implementation only includes an independent technical solution. This narration of the description is only for clarity, and those skilled in the art should regard the description as a whole. The technical solutions in the various embodiments can also be appropriately combined to form other implementations that can be understood by those skilled in the art.