Abstract:
The present invention discloses a microfluidic separating and transporting device, which utilizes free-energy gradient surfaces having micro/nano physical and chemical properties to drive and separate microfluids automatically. The device of the present invention comprises a platform having microchannels. The surfaces of the microchannels have surface energy gradient-inducing rare-to-dense microstructures. The rare-to-dense microstructures are formed in two regions; one is formed in the primary microchannel and used to transport microfluids, and the other is formed in the microfluid bifurcation region. When different microfluids flow through the microfluid bifurcation region, the microfluids will separate automatically to their own secondary microchannels according to the surface energy gradient. Thereby, droplets of different microfluids can be separated apart or split into diffluences.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present relates to a microfluidic separating and transporting device, particularly to a microfluidic separating and transporting device, wherein the surface energy gradient of microfluids, which is induced by a micro/nano structure fabricated with a microelectromechanical technology, is used to separate microfluidic droplets.  
         [0003]     2. Description of the Related Art  
         [0004]     When a biochemical analysis is undertaken in a microfluidic chip, a series of different droplets is transported, separated and mixed in microchannels. The key technology of microfluidic systems is the control technology of microfluids. As the dimension of microfluidic systems has been reduced to micrometer scale, surface tension outweighs gravity and becomes the major driving force of microfluidic systems. Surface tension is in a linear relationship with length, i.e. F=γ×λ. Therefore, the smaller the system, the greater the influence of surface tension. The common energy types used to affect surface tension and control microfluidic systems include: thermal energy (via thermocapillary effect) and electric energy (via electrowetting effect), which respectively utilize thermal energy and electric energy to locally change the surface tension of microfluids and then control the movement of microfluids. However, those externally applied energies may have influence on microfluids. Thus, the application of microfluidic systems may be limited. For example, when a biomedical test is undertaken, externally applied thermal energy may raise the temperature of tested solutions, and an externally applied electric field may polarize the substances distributed inside microfluids; thus, the characteristics of solutions and biological molecules may be changed, and the correction of test results may be affected.  
         [0005]     Refer to  FIG. 1  for a conventional microfluidic separating and transporting device proposed by a U.S. Pat. No. 6,878,555B2. As shown in  FIG. 1 , the droplet injection and separation system  100  has a rotary disc  103 , and multiple microchannels  102  radiate outward from the center of the surface  112  of the rotary disc  103 . The droplet inlets  101  of the microchannels  102  are designated by I, and I=1˜6. The surface  112  of the rotary disc  103  is vertical to an axle  117 , and the axle  117  passes through the center of the rotary disc  103  and drives the rotary disc  103  to rotate. The surface  112  of the rotary disc  103  has special optical marks  104 . The system  100  has an optical detector  105  and a signal controller  114 . When the special optical mark  104  passes through the optical detector  105 , the optical detector  105  produces a signal to the signal controller  114 , and the signals controller  114  triggers a valve  107  of a droplet injector  110  so that a droplet  111  generated by the droplet injector  110  will drop onto the droplet inlet  101  of a specified microchannel  102 . Then, the droplet  111  is guided to the terminal end of the specified microchannel  102 , and the entire transporting process is thus completed.  
         [0006]     The key point of the conventional technology mentioned above is: the optical mark on the rotary disc creates a signal; the signal controller receives the signal and controls the timing that the droplet injector generates a droplet and the speed of the rotary disc so that the droplet can drop onto the inlet of a specified microchannel; then, the centrifugal force transports the droplet to the reaction region at the terminal end of the microchannel for succeeding application or processing.  
         [0007]     However, the conventional technology mentioned above needs high precision signal control and consumes more energy. Thus, the design of the elements thereof and the development of the fabrication process thereof are relatively complicated, and the cost thereof is also raised. Further, there are too many parameters needing considering and controlling, such as the delay time between signal receiving and droplet generation, the size and type of the droplet, the distance between the outlet of the droplet injector and the surface of the rotary disc, the time the droplet needs to reach the inlet of the microchannel, the rotation speed of the rotary disc, etc. All those parameters need precise calculation and control so that the droplet can precisely drop onto the inlet of the assigned microchannel. Too many control parameters cause difficulties in operating the system and maintaining the reliability of the system.  
         [0008]     In a conventional technology proposed by a US patent US 20050045238A1, microstructures with different densities are used as valves in microchannels. When a microfluid reaches such a valve, the microfluid will stop automatically. Based on the principle of this conventional technology, the present invention proposes a microfluidic separating and transporting device, which utilizes surface energy gradient to separate microfluidic droplets. Thereby, the problems of the conventional technologies can be solved.  
       SUMMARY OF THE INVENTION  
       [0009]     The primary objective of the present invention is to provide a microfluidic separating and transporting device, wherein the surface energy gradient is used to influence the hydrophobias of the surfaces of microchannels and influence the contact phenomenon between the microfluids and the surfaces of microchannels so that the droplets of different microfluids can be driven to move, separated apart or split into diffluences.  
         [0010]     Another objective of the present invention is to provide a microfluidic separating and transporting device, which can promote the microfluidic mixing efficiency of biological chips, increase test types of microfluids, simplify the transporting process of microfluids and reduce the fabrication cost of biological chips.  
         [0011]     Further another objective of the present invention is to provide a microfluidic separating and transporting device, which can use less elements and parameters to achieve easy operation, high power efficiency, high biological compatibility, automation and simplified fabrication process, and may be contributive to the future integration of microfluidic transporting systems.  
         [0012]     To achieve the abovementioned objectives, the present invention proposes a microfluidic separating and transporting device, which comprises a primary microchannel and at least one secondary microchannel. The droplets of microfluids may be dropped onto the primary microchannel and flow in the primary microchannel. At least one rare-to-dense microstrip pattern is formed in the primary microchannel or the bifurcation regions between the primary microchannel and the secondary microchannels. When droplets of different microfluids flow through the rare-to-dense microstrip pattern, the surface energy gradient will separate the droplets of different microfluids.  
         [0013]     To enable the objectives, technical contents, characteristics and accomplishments of the present invention to be more easily understood, the embodiments of the present invention are to described in detail in cooperation with the attached drawings below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a diagram schematically a conventional microfluidic droplet separating and transporting device.  
         [0015]      FIG. 2  is a diagram schematically showing the microfluidic separating and transporting device according to one embodiment of the present invention.  
         [0016]      FIG. 3  is a diagram schematically showing the microfluidic separating and transporting device according to another embodiment of the present invention.  
         [0017]      FIG. 4  is an SEM photograph of a rare-to-dense microstrip pattern according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     The present invention utilizes a physical or chemical method to fabricate density-variation surface energy gradient microstructures, i.e. rare-to-dense microstrip patterns, which create different surface tension gradients between microfluids and the inner walls of the microchannels along the flowing direction of the microfluids to drive the microfluids to flow automatically. The microfluids flow to the bifurcation regions between the primary microchannel and the secondary microchannels spontaneously via the driving force of surface tension gradient. The bifurcation regions connect with the secondary microchannels having density-variation micro/nano structures, which enable the secondary microchannels to have different hydrophobias. Thus, when the microfluids flow to the bifurcation regions, they will respectively enter into the microchannels having their own hydrophobias. Thereby, the microfluids can be precisely and automatically separated and guided to the assigned secondary microchannels.  
         [0019]     Below, the technical means and the accomplishments of the present invention are to be described in cooperation with the attached drawings. However, the embodiments illustrated by the drawings are only used to clarify the present invention complementarily, and the scope of the present invention is not limited by the drawings shown hereinafter.  
         [0020]     Refer to  FIG. 2 a  diagram schematically showing one embodiment of the present invention, wherein microchannels with special rare-to-dense microstrip patterns are fabricated on a rotary platform. As shown in  FIG. 2 , a rotary platform  20  has a primary microchannel  22 ; a first secondary microchannel  24  and a second secondary microchannel  26  extend from the primary microchannel  22 . A microfluidic droplet  28  can be dropped onto the inlet of the primary microchannel  22 . The bifurcation region between the primary microchannel  22  and the first secondary microchannel  24  has a first microstrip region  30  with the microstrips being rare-to-dense from top to bottom. The bifurcation region between the primary microchannel  22  and the second secondary microchannel  26  has a second microstrip region  32  with the microstrips also being rare-to-dense from top to bottom. Both the first microstrip region  30  and the second microstrip region  32  create downward forces, but the downward force in the second microstrip region  32  is stronger than that of the first microstrip region  30 . When the centrifugal force of the spinning rotary platform  20  is smaller than the force in the first microstrip region  30 , the microfluidic droplet  28  will enter into the first secondary microchannel  24 . If the centrifugal force is raised, the microfluidic droplet  28  will not enter into the first secondary microchannel  24  but will continue to head forward and reach the second microstrip region  32 . If the centrifugal force of the spinning rotary platform  20  is smaller than the force in the second microstrip region  32 , the microfluidic droplet  28  will enter into the second secondary microchannel  26 . If the centrifugal force of the spinning rotary platform  20  is greater than the force in the second microstrip region  32 , the microfluidic droplet  28  will not enter into the second secondary microchannel  26  but will continue to head forward along the primary microchannel  22 . Via the abovementioned mechanism, the microfluidic droplets with different inertia forces can be separated and then transported to the assigned reaction regions or collection regions (not shown in the drawing).  
         [0021]     In addition to the abovementioned embodiment, the microfluidic droplets may also be separated under a fixed centrifugal force. Refer to  FIG. 3  for another embodiment of the present invention. As shown in  FIG. 3 , a rotary platform  40  has a primary microchannel  42 ; a secondary microchannels  44  extends from the primary microchannel  42 . The microfluidic droplets can be dropped onto the inlet of the primary microchannel  42 . The bifurcation region between the primary microchannel  42  and the secondary microchannel  44  has an upper microstrip region  46  and a lower microstrip region  48 , and the active force of the upper microstrip region  46  is stronger than that of the lower microstrip region  48 . Under a fixed centrifugal force, there are two microfluidic droplets  50  and  52 , and the surface energy of the microfluidic droplet  52  is greater than that of the microfluidic droplet  50 . Under the action of the centrifugal force and the surface energy, the upper microstrip region  46  will drag the droplet  50  to head forward along the primary microchannel  42 . The droplet  52  will be dragged to enter into the secondary microchannel  44  by the lower microstrip region  48 . Thereby, the droplets of different surface energies can be separated.  
         [0022]     In the abovementioned two embodiments, a spacer (not shown in the drawings) may be formed in the lateral sides of the primary microchannel and the secondary microchannels. The spacer is used to control the height of the microfluidic droplet, and the height of the spacer ranges from tens of micrometers to millimeters. An upper cover (not shown in the drawings) may be installed above the spacer. The upper cover is used to isolate the microfluidic droplets inside the primary microchannel and the secondary microchannels from the external environment. Besides, the surface of the upper cover may be smooth or have a special microstrip pattern.  
         [0023]     Above, the technical contents of the present invention have been described in detail. Below, the physical principle of the present invention is to be stated so that the persons skilled in the art can further understand the spirit of the present invention. When a microfluidic droplet contacts two interfaces respectively having different hydrophobias, the contact angles and the radii of the curvatures of both ends of the microfluidic droplet are asymmetric because of the distribution of surface energy gradient. Thus, the pressure differences to the surrounding air at both ends of the microfluidic droplet are unequal. The unbalanced pressures will induce a net pressure difference inside the droplet, which is exactly the source of the driving force F act  for the droplet contacting two surfaces with different hydrophobias. The surface energy gradient may be implemented with patterns having microtrenches arranged in different densities. According to Laplace-Young equation, the driving force may be expressed by:  
               F   act     =       γ   LV     ·     A   eff     ·     {     (       1     r   2       -     1     r   1         )     }               (   1   )                 A   eff     =           2   ⁢     θ   o       360     ·   π   ·       (       w   o       2   ⁢   sin   ⁢           ⁢     θ   o         )     2       +              w   o   2     ⁢   cot   ⁢           ⁢     θ   o       4                    (   2   )             
 
 wherein F act  is the driving force of the surface having heterogeneous microstructures to the droplet; γ LV  is the surface tension of the liquid-vapor phase interface; A eff  is the area of the droplet section orthogonal to the movement direction; r 1  and r 2  are the radii of the curvatures of both ends of the droplet; w o  is the contact length between the droplet and the solid surface in the orthogonal direction; and θ o  is the contact angle between the droplet and the surface in the orthogonal direction. 
 
         [0024]     The resistance force F res  to the droplet movement induced by the surfaces with different hydrophobias can be expressed by 
 
 F   res =γ LV   ·f   1   ·w   o ·(cos θ R −cos θ A )  (3) 
 
 wherein f 1  is the density of the microstructure distribution on the surface of the microchannel; cos θ A  and cos θ R  are respectively the cosine values of the advance angle and the recession angle of the droplet. When the resistance force F res  is greater than the driving force F act , the droplet sticks to the surface of the microchannel. When the resistance force F res  is smaller than the driving force F act , the droplet moves on the surface of the microchannel. From Equation (3), it is known that the resistance force F res  can be changed via modifying the density f 1  of the microstructure (microstrip) distribution on the surface of the microchannel. In other word, modifying the parameter f 1  can precisely control the droplet to advance or stay. 
 
         [0025]     As shown in  FIG. 4 , according to the calculation results of the related theories and the experimental data, a rare-to-dense microstrip pattern is designed to prove the practicability of the present invention, wherein the densities of the microstructures increase from right to left, and the densities f 1  thereof are respectively 0.25, 0.5, 0.8 and 1; each region of microstrips is 5 micrometers wide, 1000 micrometers long and more than 10 micrometers high. When a droplet is placed in the interface between the right two regions, it will move leftward continuously until it reaches the leftmost region where f 1 =1, and then, the droplet stops there. Via the hydrophobias gradient created by a heterogeneous microstructure design, the droplet tends to move toward the region of lower hydrophobias. Thereby, the direction of droplet movement can be controlled.  
         [0026]     The present invention can apply to the flow path separation procedures and the output point assignment procedures in a series of digitized microchannel transporting processes of droplets and can achieve the objectives of easy operation, high power efficiency, high biological compatibility, automation and simplified fabrication process. Further, the present invention can promote the microfluidic mixing efficiency of biological chips, increase test types of microfluids, simplify the transporting process of microfluids and reduce the fabrication cost of biological chips. Therefore, it is obvious that the present invention can fully overcome the problems of the conventional technologies.  
         [0027]     Those described above are only the embodiments to clarify the present invention to enable the persons skilled in the art to understand, make and use the present invention. However, it is not intended to limit the scope of the present invention, and any equivalent modification and variation according to the spirit of the present invention is to be also included within the scope of the present invention.