Abstract:
To manipulate microparticles in a fluid that intersects a first channel or several first channels as a stream, one or more microparticles ( 14 ) are exposed to electrical field barriers that change their direction from the direction of flow toward the edge of the flow to a lateral hole ( 17 ) of the respective first channel. As a result, microparticles can be moved back and forth between streaming fluids. Preferred applications include treatment, separating, sorting or confinement procedures.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a system for manipulating microparticles in streaming fluids, in particular a procedure for moving microparticles, e.g., biological cells, between various fluids, e.g., for sorting, treatment or confinement purposes, and a microsystem device for implementing the procedure. 
     Many biological, medical, pharmacological, and even non-biological applications place importance on precise loading with substances and non-contact confinement of microscopic particles, such as biological cells or cell clusters, latex particles or other microbeads in a free liquid. The most frequent method is to grow cells on a solid substrate, which is then rinsed with the required precision using a solution, or to have confinement take place on a sieve or capillary openings. The disadvantage to this procedure is the mechanical surface contact and the difficulty involved in sequentially processing numerous objects in identical fashion. Particular difficulties are encountered in exposing microscopic objects to another solution without surface contact for very short and adjustable times, and then returning them to the original medium. This has previously been achieved at the expense of complicated washing and centrifugation stages. 
     Use is also made of so-called “laser tweezers”, with which particles can be held in position in a free solution with micrometer precision, or shifted to a defined extent (see A. Ashkin et al. in “Optics Lett.”, Vol. 11, p. 288 (1986)). The disadvantage is that this principle requires a considerable outlay of external equipment, which runs counter to the advantages of system miniaturization and is cost-intensive. In addition, the object is strained in the focal area. 
     One alternative involves electrical microfield cages, in which microparticles and cells can be held in place similarly to “laser tweezers” via polarization forces (G. Fuhr et al. “Naturwiss.”, Vol. 81, p. 528 (1994)). However, only one solution is situated in such systems, so that the microparticles can only be transferred to another medium via liquid exchange, which requires longer times until the next use and possibly separate cleaning stages. A particle can be held in a confined or parked position by means of a laser tweeter, but it makes no sense from a technical standpoint for several particles. In addition, the object is exposed to a permanent beam load for the time parked. 
     Magnetically charged particles are transferred from one solution into another in micro systems via magnetic fields or ultrasound sources acting at a right angle to the channels (see G. Blankenstein in “Scientific and Clinical Applications of Magnetic Carriers”, published by Hafeli et al., Plenum Press New York 1997 (Ch. 16, pp. 233). Both techniques are only suitable for miniaturization under very limited conditions, do not permit focusing the forces acting on the particles, and are difficult to convert in an integrated form using techniques in semiconductor structuring procedures. In addition, this technique is associated with a load with magnetic particles that might be physiologically disruptive for biological objects. 
     DE-OS 41 43 573 discloses a device for separating mixtures of microscopic particles in a liquid, in which the particles are exposed to electrical fields of traveling waves, during which influence the particles are tapped from a stream of liquid. This device has the following disadvantages. Numerous microelectrodes are required to generate the traveling waves, thus resulting in a complex structure with the respective separate drive circuits. The microelectrodes are situated in an area substantially larger than the particles to be tapped. The traveling waves trigger temperature gradients in the liquid that give rise to disruptive transverse flows. These transverse flows and any other existing flow inhomogeneities prevent the particles from moving along defined paths. To compensate for this locally undefined tapping, it must extend over a relatively wide area in the direction of flow. This in turn results in entire particle groups getting tapped, or the particles must move through the micro system with great distances, thus delaying the processing of large number of particles. 
     Therefore, the known techniques could previously only be used on a restricted basis, if at all, to transfer microparticles from one liquid to one or more others and back, or to effect a non-contact intermediate storage in a micro system. 
     SUMMARY OF THE INVENTION 
     Accordingly, the object of the invention is to provide an improved procedure for manipulating microparticles in fluid streams that has an expanded range of application, and in particular can be used serially and parallel at a high velocity, and also enables electrically controllable procedures for the non-contact confinement and transfer of microparticles in various media. The object of the invention is also to provide a device for executing the procedure, which has a simplified design along with a simplified and reliable drive circuit, and is set up to form defined paths of movement for the microparticles to be manipulated. 
     This object is achieved by a procedure with the features set forth in claim  1  and a device with the features set forth in claim  9 . Advantageous embodiments of the invention are described in the dependent claims. 
     The invention is based on the idea of subjecting microparticles to electromagnetic forces in a streaming fluid. The electromagnetic forces are exerted by at least one electrical field barrier, against which the microparticles are moved with the streaming fluid, and which causes the microparticles to move in a direction deviating from direction of flow. The electrical field barrier is generated, for example, with at least one pair of strip-shaped microelectrodes, which is situated at opposing boundaries of the streaming fluid and exposed to a high-frequency alternating voltage. The selected amplitude for the alternating voltage of field barrier is high enough to prevent the microparticles to be deflected from getting between the electrodes. The fluid with the microparticles suspended therein flows through a channel with at least one lateral hole, to which at least one microparticle is moved along the electric field barrier. The hole is bordered by another channel with a streaming fluid or looped branch (so-called parking loop) of the first channel. The streaming fluids of the respective channels come into contact at the hole. However, the fluids do not become mixed together if the streaming fluid system is implemented with laminar flows. The laminar flows are advantageously implemented in micro systems or with capillary channels. One particular advantage to the invention is that the streaming fluids at the openings between the channels form boundary surfaces that the microparticles to be manipulated can pass through. 
     The electromagetic field forces are generally exerted via 3-dimensionally distributed electrode arrays through or at the holes between the channels for transferring the objects into one or several adjacent channels or parking loops by applying high-frequency voltages given a permanent hydro-dynamic flow through the system. The electrode arrays acting as deflecting systems can be actuated from a computer, permitting minimal manipulation times in the ms range. The movement can take place in a free solution without any mechanical contact or guidance of the object. The procedure involves no interference and uses the conventional optical measuring methods, thereby avoiding damage to living biological objects, e.g., cells. The time for which the particles reside in the compartments or channel sections can be externally set. Typical path diameters or deflections lie within a range of 50 nm and several 100 μm or more. No feedback check or observation of the objects is required (but can be additionally performed). 
     Special advantages to the invention are that a highly precise, reliable and rapid particle manipulation is achieved with a relatively simple electrode configuration (in the simplest case: with one pair of electrode strips). Disruptive transverse flows are avoided. The electrodes can be exposed to a sufficiently high alternating voltage, so that the particles reliably stay on the side of the field barrier located upstream, and are routed to the lateral hole. The electrodes have characteristic dimensions smaller than or equal to the dimensions of the particles to be manipulated. Manipulating the particles according to the invention makes it possible to move the particles in and out of the flow, i.e., to also return particles from an adjacent flow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and details of the invention are described below with reference to the attached drawings. Shown on: 
     FIG. 1 is a perspective view of a first embodiment of a streaming fluid system according to the invention; 
     FIG. 2 is a top view of a streaming fluid system according to FIG. 1; 
     FIGS. 3 to  5  are top views of streaming fluid systems according to a second, third and fourth embodiment of the invention; 
     FIG. 6 is a top view of a streaming fluid system according to an embodiment with a looped branch; and 
     FIGS. 7 and 8 are top views of streaming fluid systems according to an embodiment of the invention with looped flows between two channels. 
    
    
     The examples shown always involve three-dimensional configurations of microelectrodes, with which barrier-type electric high-frequency fields are generated in channels. The perspective view shows an example of such a system in FIG.  1 . The examples show only 1, 2 or 3 channel systems. However, the invention can be expanded by any other kinds of combinations desired. The invention will be explained based on streaming liquids, but can also be implemented with other fluids given sufficiently strong field forces. The invention is not limited to the depicted flat channel walls, but can also be implemented using channels with other, e.g., round, cross sections. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a perspective view (cutout) of a 2 channel system comprised of a bottom substrate  11 , on which the microelectrodes or electrode sections  16   a,    16   b  (denoted with straight leads) are arranged in a planar fashion, the spacer  12  forming the channel walls, and a cover substrate  13  (shown as transparent, can be transparent or non-transparent in execution), on whose side facing the channel planar microelectrodes or electrode sections  15   a,    15   b  (denoted by straight leads) are also situated. 
     The spacer  12  forms a right (first) and left (second) channel. The central separating wall has holes  17 . An electrode configuration comprised of the respective electrode sections  15   a,    16   a  or  15   b,    16   b  is allocated to each hole  17 . The electrode sections each extend upstream in a channel section, from the respective hole from a wall lying opposite the hole up to the hole, or preferably through the latter and into the adjacent channel. As a result, the electrode sections define a reference plane that lies perpendicularly on the surface of the bottom substrate  11  and at an angle to the longitudinal direction of the channel. 
     An alternating voltage (frequency: kHz to MHz, amplitude: 0.1 to 5.0 V) is applied between the electrode sections  15   a,    16   a  and  15   b,    16   b.  The frequency is selected as a function of the dielectric properties of the microparticles or particles in such a way that the latter exhibit a negative polarization, i.e., negative dielectrophoresis, and are repelled by the high-frequency field. As an alternative, the frequency can be selected in such a way that positive dielectrophoresis (attraction) takes place, wherein the electrode sections belonging to a hole would than have to be arranged upstream in the respective other channel. However, negative dielectrophoresis offers major advantages for the non-contact manipulation of the microparticles. 
     Therefore, a repelling field is formed as a barrier in the area of the mentioned reference plane, which exerts a force on the particles toward hole  17  due to the bias relative to the longitudinal direction of the channel in conjunction with the flow. 
     In the present example, various liquids flow through the channels in the same direction (arrows). Suspended particles (e.g., living cells) are washed in with a carrier liquid via the first channel. A treatment liquid (e.g., a charging medium with a dissolved substance with which the particles are to be charged) flows through the second channel. 
     A particle  14  moves along the path shown with the dashed line. For purposes of defined treatment, the microparticles are moved through the first hole  17  into the second channel. The particles can be conveyed into the charging medium for a defined period of time as the result of the flow rate and the arrangement of deflecting electrode sections  15   a,    16   a  and  15   b,    16   b.  As a rule, this process takes place at flow rates of several to several hundred μm/s. Therefore, the retention time in the charging medium lies in the ms to s range, depending on the distance of the deflecting electrodes. 
     Return from the second channel to the first channel takes place in a similar fashion at the second hole  17 . 
     FIG. 2 shows a top view of the system described in FIG.  1 . The two channels  21 ,  22  are traversed from left to right. The channel walls are formed by a spacer  27 . The particles  23  will follow path  28  with the field activated. Otherwise, they will not switch over to the adjacent channel. Electrode sections  25   a,    26   a  and  25   b,    26   b  (also called deflecting electrode pairs) are here shown diagrammatically, i.e., the thin line denotes the lower electrode plane  26   a,    26   b,  and the thicker line denotes the upper electrode plane  25   a,    25   b.  The width of the electrodes can range from several 100 nm to about 100 μm (typically 10 to 20 μm). The size of the particles  23  (nm to mm) determines the height of the channels. Favorable values are roughly 2 to 20 times the particle diameter. To minimize electrical losses, the leads to the deflecting electrodes must not be arranged together, but rather offset to the side as much as possible. If the deflecting unit  25   b,    26   b  is deactivated, the particles remain in the solution of channel  21 . The retention time in channel  21  can be determined via the distance of holes  24   a,    24   b  or the flow rate. 
     The channels have dimensions that can be selected as a function of the fluid viscosity (provision of laminar flows). Preferred characteristic dimensions lie within the sub-μm to mm range, preferably several μm to 0.5 mm, e.g., 200 μm. 
     The electrode sections are shown as strips, but can exhibit any other shape that ensures that the force will reach the holes in the channel wall. 
     FIG. 3 shows a special advantage of the invention. Specifically, no mutual disruption of streaming liquids (no mixing together) takes place in microchannels with a diameter of &lt;½ mm. Flows remain laminar over large distances. In the example shown, this effect is utilized to temporarily transfer the particles from FIGS. 1 and 2 to another solution. The separating wall between the channels  31  and  32  here forms a hole  35  several μm or several hundred μm in length. When the flow passes through the channel in the same direction, no mixing together takes place at this contact surface for the reasons cited above. A particle  33  can be transferred from channel  32  to channel  31  via the deflecting unit  34   a.  The retention time in the medium of channel  31  can be determined via the deflecting units  34   b-e.  The particles move at the trajectories indicated with the arrows. 
     FIG. 4 shows a device in which a particle  43  can be transferred from channel  42  to channel  41  and back repeatedly. The system can also have a more forward design, however. The particles travel along path  46 . The first element  44   a,    44   b  has a separating wall  45 . The second deflecting device  44   c,    44   c  can work without this element. Depending on the distance of the deflecting system, the separating wall element need also not be used in the first transitional area. 
     For biochemical and cellular biological/medical tasks, it is frequently important to transfer objects into several liquids for a short time in a defined and controllable manner. FIG. 5 shows a 3-channel system as an example. All channels  51 ,  52  and  53  are traversed from left to right. The particles  54  can be transferred into the channel  52  via the deflecting system  54   a,  and into the channel  53  via  55   b.  The particle can again be returned to the channel  52  via the deflecting unit  55   c.  This is followed by path  56 . An additional deflecting unit and hole can be arranged between the channels  51  and  52  to again return the particle to the channel  51 . According to the pattern shown, a much higher number of channels and transfer elements can be implemented. 
     The short flow times pose a thus far unresolved problem in microfluidic systems linked with cellular biology. For example, when measuring a particle, either the flow had to be stopped, or additional particles located in the channel system would be irretrievably rinsed out. Stopping the flow gives rise to the danger of surface contact and subsequent adhesion. For this reason, it is best that parking loops be implemented for particles given a permanent flow. FIG. 6 shows such a basic element. The channel  61  is traversed from left to right. Accommodated inside one of the walls ( 67   a ) is an annular channel  62 , which is formed by a spacer section  66 . The spacer projects into the channel  68  a bit on the back, so that a portion of the liquid starts to circulate in the channel  62 . A particle  64  can be introduced into this flow via the deflecting electrode  63   a.  If the deflecting electrode  63   b  is not actuated, it remains in the annular flow, and moves on a looped parking orbit  65 . If the particle is to be removed, the deflecting system  63  is turned off, and the particle leaves the parking loop. 
     The particle parking loop and defined transfer to another solution are combined by having two deflecting systems  74   a,    74   b  projecting through holes in the shared channel wall into the respectively adjacent channel  71 ,  72 . Given an opposed flow through the channels  71 ,  72 , a particle  73  would move to a circular orbit  75 , in which several particles are accommodated at the same time. Turning off the HF voltage on one or both deflecting systems allows the particles to exit into one or the other streaming solution. At the same time, this device offers the advantage of being able to use liquids of differing composition in both channels. The number of particles orbits makes is possible to set and measurably reproduce the time for which it is to be exposed to the respective substance. Additional detection means can be used at one or several locations to determine the orbiting time and number of trapped particles. This can take place optically, or by means of a “Coulter Counter” at the holes  76   a,    76   b.  One can also conceive the system as expanded, comprised of numerous such elements in series and parallel. Therefore, it is suitable for confining numerous particles, acquiring their location and treating them in a comparable manner. 
     Of particular interest are very short retention times or parking loops, which could only be occupied in large numbers and each by only one or a few particles at a time. To this end, the deflecting systems  84   a,    84   b  are to be placed as close together and in a hole  86  between the channels  81 ,  82 . If both channels are now traversed in an opposite direction, the particle  83  will follow the trajectory  85 . The minimal diameter of the orbit equals roughly twice the particle diameter. Assuming that submicrometer particles, such as viruses, can be trapped and periodically transferred from one solution to another in this way, the shortest periods per orbit measure several ms.