Abstract:
An electromagnetic actuator for a microfluidic pump of the type that causes periodic pinching and releasing against the walls of a fluidic channel, e.g., a tube. At least one permanent magnet is placed against the walls of the fluidic channel, and located in an area with magnetic fields, produced by coils that are radially symmetric to the channel. The permanent magnet is cause to press and release against the wall of the fluid channel to cause a fluid flow through the channel.

Description:
[0001]     This application claims priority from provisional application No. 60/574,432, filed May 25, 2004, the contents of which are herein incorporated by reference. 
     
    
     BACKGROUND  
       [0002]     U.S. Pat. Nos. 6,254,355 and 6,679,687 teach a microfluidic pump which uses compression of an area within a section of fluidic channel, in order to cause a fluid flow along the channel. This pump can be micro miniaturized, and can be made using micro machining techniques. Basically, an area of the channel is compressed in a certain way in order to cause fluid flow along the channel.  
         [0003]     The above-discussed patents teach various ways of compressing of the channel.  
       SUMMARY  
       [0004]     The present application describes an electromagnetic actuator that uses in-line coils to form the actuation for a microfluidic pump of a type that requires a portion of the channel to be compressed. According to the techniques disclosed herein, a magnetic field may be oriented along an axis of the channel, and used to carry out compression for actuating the pump. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     These and other aspects will now be described with reference to the accompanying drawings, wherein:  
         [0006]      FIG. 1  shows an illustration of the tube based pump and the magnetic coils and actuator that actuate the pumping system;  
         [0007]      FIGS. 2 and 3  show characteristic curves of forces within the pump;  
         [0008]      FIG. 4  illustrates the actuator being embedded within the elastic wall of a tube;  
         [0009]      FIG. 5  illustrates the pumping actuator being encased and totally surrounding the tube&#39;s circumference;  
         [0010]      FIG. 6  illustrates the actuator being on opposite sides of the tube; and  
         [0011]      FIG. 7  illustrates an embodiment where both the actuator and the coils are encased within the tube wall. 
     
    
     DETAILED DESCRIPTION  
       [0012]     The embodiments disclosed herein use magnetic actuation of a microfluidic pump. These techniques form a moving magnetic field gradient that drives an actuator to compress the channel wall. In an embodiment, the channel is formed of a flexible tube. A ferromagnetic material, such as a permanent magnet, is coupled to the wall of the tube. The actuation arrangement can be made cylindrically symmetric, in order to facilitate miniaturization and symmetry.  
         [0013]     According to an aspect, the interaction of the magnetic moment in a gradient magnetic field is used. This interaction force is described by the tensor relation: 
 
 {overscore (F)} =∇( {overscore (m)}·{overscore (B)} )  (1) 
 
         [0014]     Where the vector m represents the magnetic moment, and the vector B represents the magnetic field at the location of the magnetic moment.  
         [0015]     Equation 1 can be expanded into three orthogonal force directions as follows:  
               F   x     =         m   x     ⁢       ∂     B   x         ∂   x         +       m   y     ⁢       ∂     B   y         ∂   x         +       m   z     ⁢       ∂     B   z         ∂   x                   (   2   )                 F   y     =         m   x     ⁢       ∂     B   x         ∂   y         +       m   y     ⁢       ∂     B   y         ∂   y         +       m   z     ⁢       ∂     B   z         ∂   y                   (   3   )                 F   z     =         m   x     ⁢       ∂     B   x         ∂   z         +       m   y     ⁢       ∂     B   y         ∂   z         +       m   z     ⁢       ∂     B   z         ∂   z                   (   4   )             
 
         [0016]     Which shows the relevant forces on the magnets, in the embodiments.  
         [0017]      FIG. 1  shows an embodiment. A tube  100  forms the element which will receive the pumping force. Another tube  140 , having different fluidic characteristics, is attached to the first tube  100 . More generally, however, the tubes  100 ,  140  can be any fluidic channels.  
         [0018]     First and second coil sets  102 ,  104  are wound around the tube  100 . The coils  102 ,  104  have electrical connections which allows their electrical actuation.  
         [0019]     The coils may be wound azimuthally symmetrically along the x axis, shown as being along the tube  100  in  FIG. 1 . In an aspect, these coils may also be electrically connected to one another, so that their magnetic fields are energized in phase with one another. The coils form a symmetric magnetic field, which approximately follows the magnetic field lines  110  shown in  FIG. 1 . A magnetically effected part  120  is located with its magnetic field oriented parallel to the symmetric axis of the coils. In an embodiment, the effected part can be a permanent magnet whose magnetic field  122  is in the x direction in  FIG. 1 . The permanent magnet element  120  may be substantially in the shape of a section of a cylinder, for example, but can be other shapes also. In an embodiment, the permanent magnet is radially symmetric. In another embodiment  
         [0020]     The analytic expressions, using cylindrical coordinates, for the fields from a single coil turn of radius a along the radial, angular and z axes are well-known:  
               B   ϕ     =   0           (   5   )                 B   r     =       J   c     ⁢         2   ⁢   z       r   ⁢       [         (     a   +   r     )     2     +     z   2       ]           ⁡     [       -   K     +           a   2     +     r   2     +     z   2             (     a   -   r     )     2     +     z   2         ⁢   E       ]                 (   6   )                 B   z     =       J   c     ⁢       2       [         (     a   +   r     )     2     +     z   2       ]         ⁡     [     K   +           a   2     -     r   2     -     z   2             (     a   -   r     )     2     +     z   2         ⁢   E       ]                 (   7   )             
        where K and E are complete elliptic integrals of the first and second kind, respectively:  
             K   =       ∫   0       1   2     ⁢   π       ⁢         ⅆ   θ         (     1   -       k   2     ⁢     sin   2     ⁢   θ             ⁢           ⁢   and               (   8   )               E   =       ∫   0       1   2     ⁢   π       ⁢         (     1   -       k   2     ⁢     sin   2     ⁢   θ           ⁢     ⅆ   θ     ⁢           ⁢   with               (   9   )                 k   2     =       4   ⁢   ar       [         (     a   +   r     )     2     +     z   2       ]               (   10   )             
       
 
         [0022]     These equations can be used to numerically evaluate the exact values of the magnetic fields and the field gradients for the coil configurations in  FIG. 1 .  
         [0023]     In the embodiment, the magnet  120  is in contact with the outer surface of the fluidic channel. Magnetic moment is oriented along the X direction, so that the force on the magnet is in the z direction is:  
               F   z     =       m   x     ⁢       ∂     B   x         ∂   z                 (   11   )             
 
         [0024]      FIG. 2  shows a graph of the value of the x component of the magnetic field along the z axis, centered in between the two coils  102 ,  104 . The force curve from equation 11, along the z axis is shown as the graph in  FIG. 3 . Note that the force curve has a distinct minimum, approximately at “1”.  
         [0025]     Passing an alternating current through the coils creates an alternating force on the magnet  120  along the z-axis direction. This alternating current may be tuned to the harmonic of the system, in order to maximize or modulate the pumping action. The magnet  120  can be attached to the outer surface in any desired way, for example by gluing or some other connection.  
         [0026]      FIG. 4  shows an embodiment in which the permanent magnet element  400  is embedded within a wall of the fluidic channel. The fluidic channel  402  is shown with walls  404 . The walls  404  include a pocket section therein at area  406 . The permanent magnet element  400  is embedded in that pocket section. The permanent magnet element may take the form of a complete radial ring or any pattern formed by any section of the ring. This section can change the ring into an even or odd number of sections, and the individual sections may take on any geometry.  
         [0027]      FIG. 4  shows a single magnet element  400 .  
         [0028]      FIG. 5  shows an embodiment with a complete ring of ferromagnetic material  500 , formed between the inner wall  502  of the tube, and the outer wall  506 . This may be any number of separate magnet pieces embedded in the tube wall. In the  FIG. 5  embodiment, the magnet elements are cylindrically symmetrical.  
         [0029]     Another embodiment, shown in  FIG. 6 , has first magnet element  600 , and an additional magnet element  602  at the opposite side of the tube.  
         [0030]     The magnet elements may be formed of any ferromagnetic material, including, but not limited to, permalloy, NdFeB, AlNiCo and SmCo.  
         [0031]     In another embodiment shown in  FIG. 7 , the inductive coils  700 ,  702  are embedded within the elastic tube wall. A wire  704  may extend between the coils  700 ,  702 . A single magnet  710  is shown; however, this may use any of the other configurations shown and described herein. This may form a more compact configuration where all of the parts are embedded in the tube.  
         [0032]     In the embodiment, the pump may be comprised of an elastic section of tube, having a cross-sectional area of approximately 2.8 mm 2 . This is connected to a rigid glass section with an area of approximately 0.5 mm 2 . The elastic section of the pump is formed of silicon rubber, having a Young&#39;s modulus of about 220 kPa. Wave reflections are created by an impedance mismatch that is provided by asymmetric pinching with respect to the stiffer materials at the interfaces.  
         [0033]     The coil receives an input waveform of a 50 Hz square wave, with 48 ma amplitude, and an offset of minus 24 ma. The coils may be energized by a variable power source, shown as  130  in  FIG. 1 . The frequency for the desired flow rate and flow direction is dependent on the properties of the materials that are used, the wall thickness, and the length of the segments. These properties can be calculated mathematically, or alternatively, the power supply and frequency generator can be variable, as shown, to enable experimental determination of the optimum properties.  
         [0034]     Although only a few embodiments have been disclosed in detail above, other modifications are possible, and this disclosure is intended to cover all such modifications, and most particularly, any modification which might be predictable to a person having ordinary skill in the art. For example, while the above has described the fluidic channel as being a tube, it should be understood that any fluidic channel of any type can be used, so long as it is deformable in some way. Moreover, while the embodiment describes using the disclosed system for compressing a wall for a hydroelastic type pump, this system can be used for any application where a fluidic channel requires compression, for example it can be used to completely pinch of a channel for a valve, or to restrict a flow, e.g., as a variable flow restrictor. This can also be used for compressing a part within a peristaltic pump, for example.  
         [0035]     Also, only those claims which use the words “means for” are intended to be interpreted under 35 USC 112, sixth paragraph. Moreover, no limitations from the specification are intended to be read into any claims, unless those limitations are expressly included in the claims.