Abstract:
The present invention relates to an improved apparatus and method for assembly for a cylindrical split-flow thin separation channel that is capable of continuously separating particles of interest from a suspension flowing through the channel. The apparatus provides better precision in the separation process by providing better flow characteristics at the inlet, outlet, and in the zone of separation. Splitter surfaces are provided that match the calculated theoretical Inlet Splitter Surface (ISS) and Outlet Splitter Surface (OSS). A flow distributor diverts the inlet carrier flow into a stable, circumferentially uniform, laminar annular flow. The design enables manufacturing and assembly methods to ensure precise alignment of the components to further improve the flow profiles within the separation channel and to ensure repeatability device-to-device.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to an annular flow channel for maintaining uniform flow in the channel and for separating cells and particles in a radial force field. 
   2. Background 
   In general, particle separation devices take a variety of structures, depending upon the particles to be separated and the separation method to be employed. Particle separation devices separate particle populations into fractions of interest from a suspension and/or other types of particles. The principal method of operation of early particle separation devices relied on a particle&#39;s physical parameters to distinguish it from a suspension and/or other types of particles. Examples of these bulk separation techniques include filtration (which is based on particle size), and centrifugation (which is based on particle density and size). These techniques are effective as long as the particle population of interest is significantly different, with respect to size or density, from the suspension and/or the other particles in the population. Additional examples relate to multistage magnetically assisted separation technologies (MAGSEP), as disclosed in Vellinger, et al. U.S. Pat. Nos. 6,312,910 and 6,699,669. 
   As a subset of bulk separating, continuous separation techniques also exist. The continuous separation of particles in flowing solution requires a well-defined and well-controlled fluid flow pattern. Typically, continuous particle separation devices employ rectangular separation channels. The rectangular geometry of such separation channels provides several advantages including, for example, ease of manufacture, ease of control of fluid flows inside the channels, and ease of design and implementation of forces that drive the separation. 
   However, rectangular separation channels also suffer from a drawback known as the sidewall effect. The sidewall effect distorts the fluid flow pattern at the side walls of the rectangular separation channel and, hence, adversely affects the performance of the sorting device such as, for example, its resolving power. Therefore, it is desirable to provide methods and devices for separating particles that do not suffer from sidewall effects and can employ any one of a diverse number of separation forces. 
   Development in the art stemmed from the evolution of various Field-Flow Fractionation (FFF) techniques, as generally discussed in Myers, “Overview of Field-Flow Fractionation,” Journal of Microcolumn Separations, vol. 9, issue 3, pages 151-162, January 1997, John Wiley &amp; Sons, Inc., the entire disclosure of which is incorporated by reference herein. In FFF, a force field is directed perpendicularly across a laminar flow in order to focus particles into narrow bands based on a particular physical characteristic (such as size, molecular weight, charge, etc.) for analysis. Modifying the FFF technique by adding two additional flow streams and an inlet and outlet splitter surface effectively creates a split-flow thin separation channel. This channel allows the operator to fractionate the sample based on a physical characteristic and collect the positively and negatively selected fractions for further analysis or utilization. 
   Several rectangular embodiments of split-flow thin separation channels exist that attempt such separation using various driving forces including (but not limited to) gravitational, electrokinetic, thermal diffusion, centrifugation, and magnetophoretic. Recent embodiments of said channels utilize an approach where the negative impacts of the sidewall effects are eliminated by wrapping the separation channel around a cylinder and directing the flow paths parallel to the axis of the cylinder. The resulting geometry provides an annular separation volume that is contained between two concentric walls as disclosed in Aitchison et al. U.S. Pat. No. 4,141,809 and in Zborowski et al. U.S. Pat. Nos. 5,968,820 and 6,467,630B1. In one embodiment, a quadrupole magnet is used to generate a magnetic field with concentric B contour lines, thus providing a radial driving force in the annular separation volume that separates magnetic particles towards the outer wall of the annulus as disclosed in Zborowski et al. U.S. Pat. Nos. 5,968,820 and 6,120,735. While these devices have seen some success in the laboratory, the prior art still contains drawbacks that have prevented the introduction of these devices into conventional practice. 
   In order for the split-flow thin separation process to provide a successful separation, it is essential that the sample and carrier solutions each form circumferentially consistent laminar flow profiles prior to engaging at the inlet splitter tip. Additionally, it is desirable that this flow profile be achieved for a wide range of inlet flow rates for both the sample and carrier solutions. Although prior art devices have produced a degree of circumferential distribution over some flow rate ranges, a need exists for better distribution over a wider range of flow rates. Furthermore, prior art devices suffer from an inability to be easily manufactured and assembled in an inexpensive, repeatable, and precise manner by, for example, injection molding techniques. A need exists for an apparatus that can provide multiple splitter diameters by machining and that can provide a single injection mold with simple interchangeable core pins that can manufacture splitters for a wide range of diameters and optimized flow regimes while being mechanically robust and not prone to breakage. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides solutions to these drawbacks. In one embodiment for continuously separating magnetic particles, clusters of particles and magnetically labeled biological cells, the device causes a suspension consisting of magnetic and non-magnetic particles to develop a circumferentially consistent laminar flow through an annular channel while a radial magnetic field gradient attracts particles outward in direct proportion to their net magnetic susceptibility. 
   This method of separation utilizes a channel with at least two inlets admitting sample suspension in an inner annulus and a carrier fluid in an outer annulus separated from the inner annulus by a cylindrical splitter. It also utilizes a channel with two outlets, one withdrawing magnetically attracted particles from an outer annulus and the other withdrawing non-attracted particles from an inner annulus separated from the outer annulus by a cylindrical splitter. Each of these inlets and outlets employs a novel geometric design that ensures that the flow at the splitter tip is a circumferentially consistent laminar flow distribution over a wide range of selected flow rates. 
   Preferably, the inlets and outlets have splitter surfaces that can be assembled in a rapid manner such that the splitter surfaces are concentric within a precise tolerance. Accordingly, it is an object of the present invention to provide components that sufficiently distribute the flows at the inlet and outlet splitters in a cylindrical split-flow thin separation channel to improve the separation process, and to do so utilizing an easily and precisely assembled device. 
   The sample inlet (herein referred to as the inlet-a fraction, or a′) utilizes a novel design that has, as an input, a sample solution from a single tubing port, and has, as an output, a circumferentially consistent annular laminar flow of the sample solution. This is achieved by the novel geometry and integration of two components identified as the core and the inlet splitter. 
   The carrier inlet (herein referred to as the inlet-b fraction, or b′) utilizes a novel design that has, as an input, one solution from one or more tubing ports and has, as an output, a circumferentially consistent annular laminar flow of the carrier solution. This is achieved by the novel geometry and integration of three components identified as the adaptor, flow distributor, and the inlet splitter. 
   The first fractional outlet (herein referred to as the outlet-a fraction, or a) utilizes a novel design that has, as an input, a circumferentially consistent annular laminar flow of depleted (i.e., negatively selected) solution and has, as an output, the depleted solution through a single tubing port. This is achieved by the novel geometry and integration of two components identified as the core and the outlet splitter. 
   The second fractional outlet (herein referred to as the outlet-b fraction, or b) utilizes a novel design that has, as an input, a circumferentially consistent annular laminar flow of enriched (i.e., positively selected) solution and has, as an output, the enriched solution from one or more tubing ports. This is achieved by the novel geometry and integration of two components identified as the adaptor and the outlet splitter. 
   The precision and ease of assembly for the innovative flow channel design relies on the novel use of the core as a means for properly aligning or self-fixturing that provides exacting alignment for all of the other components. Furthermore, the simple yet innovative method of integrating the adaptors, splitters, and flow distributor onto the core using basic cylindrical geometric principles results in a precisely aligned cylindrical split-flow thin channel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can take many physical embodiments and can assume many arrangements of components for carrying out the teachings of the invention, all of which may be appreciated by a person of skill in the art. The teachings of the present invention can be readily understood by considering the following detailed description of a preferred embodiment in conjunction with the accompanying drawings of said embodiment, in which: 
       FIG. 1  is a sagittal section view of a hypothetical split-flow thin channel of the prior art showing the theoretical geometry and method of separation at its inlet and outlet ends; 
       FIG. 2  is a sagittal section schematic of an improved flow channel showing an aligned geometric relationship between the inlet splitter and the Inlet Splitting Surface (ISS) and an aligned geometric relationship between the outlet splitter and the Outlet Splitting Surface (OSS); 
       FIG. 3A  is a sagittal section view of the assembled flow channel; 
       FIG. 3B  is an exploded perspective view of a flow channel according to an embodiment of the invention; 
       FIG. 3C  is an exploded perspective view of the inlet end of the channel shown in  FIG. 3B ; 
       FIG. 3D  is an exploded perspective view of the outlet end of the channel shown in  FIG. 3B ; 
       FIG. 4  is a perspective view of a core according to an embodiment of the invention; 
       FIG. 5A  is a perspective view of an embodiment of a flow distributor according to the invention; 
       FIG. 5B  is a perspective view of a second embodiment of a flow distributor according to the invention; 
       FIG. 6A  is a perspective sectional view taken along section  6 A of  FIG. 6C  of a splitter according to an embodiment of the invention; 
       FIG. 6B  is a side elevation sectional view taken along section  6 B of  FIG. 6C  of the splitter shown in  FIG. 6A ; 
       FIG. 6C  is a side elevation view of the splitter shown in  FIG. 6A ; 
       FIG. 7  is a perspective view in partial section of the inlet end of the channel showing the geometric relationship between the core and the inlet splitter according to an embodiment of the invention; 
       FIG. 8  is a perspective view of the inlet end of the channel having a flow distributor attached thereto, showing the geometric relationship between the core, the inlet splitter, the flow distributor, and the shell according to an embodiment of the invention; 
       FIG. 9A  is a quarter section view of the inlet end of an assembled flow channel according to an embodiment of the invention; 
       FIG. 9B  is a quarter section view of the outlet end of an assembled flow channel according to an embodiment of the invention; 
       FIG. 10  is a sagittal-sectional view of the inlet end of an assembled flow channel according to an embodiment of the invention showing the flow path down the center of the core for the sample (a′) and showing the blocked plenum flow for the carrier fluid (b′); 
       FIG. 11  is a sagittal-sectional view of the inlet end of an assembled flow channel according to an embodiment of the invention showing the fluid path from the blocked plenum flow for the carrier fluid (b′) into the annular flow volume; 
       FIG. 12  is a sagittal-sectional view of the outlet end of an assembled flow channel according to an embodiment of the invention showing the fluid path for the two separated fractions (a and b); 
       FIG. 13  is a streamline diagram showing the distribution of the sample flow (a′) from one of the fluid ports in the core to the fully developed, circumferentially consistent annular flow at the splitter tip; and 
       FIG. 14  is a streamline diagram showing the distribution of the carrier flow (b′) from one of the adaptor ports, through the flow distributor, to the fully developed, circumferentially consistent annular flow at the splitter tip. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   While the present invention will be described more fully hereinafter with reference to the accompanying drawings in which particular embodiments and methods are shown, it is to be understood from the outset that persons of ordinary skill in the art may modify the invention herein described while achieving the functions and results of this invention. Sound engineering judgment may be used to modify various aspects and components of the invention without detracting from the broad, general teachings hereof. Accordingly, the description that follows is to be understood as illustrative and exemplary of specific embodiments within the broad scope of the present invention and not as limiting the scope of the invention. In the following descriptions, like numbers refer to similar features or like elements throughout. 
   Before discussing the particulars of the present invention, a brief summary of the basic theory of the split-flow thin separation technique will be presented. Prior art has extended the split-flow thin separation approach to flows in annular flow channels and axisymmetric, constant-force fields. The advantages of such novel flow and field configuration are the absence of the side-wall effects with the potential of an increased resolving power of the separation and a wider choice of the available force fields, in particular, the magnetic field. The theory of separation in an annular flow and a quadrupole magnetic field has been discussed by Zborowski et al U.S. Pat. No. 5,968,820. 
   The ability to immunomagnetically label selected cells to impart a magnetophoretic mobility to the cells in a suspension is useful for devices such as those according to the invention. Magnetophoretic mobility is defined as the characteristic motion of a particle induced by a magnetic field. The particle velocity, u m , in an external magnetic field is defined as the product of the particle mobility (m) and the local field strength (S m ):
 
 u   m   =m·S   m 
 
   The local field strength is defined as: 
   
     
       
         
           
             S 
             m 
           
           = 
           
             
                
               
                 ∇ 
                 
                     
                 
                 ⁢ 
                 
                   B 
                   2 
                 
               
                
             
             
               2 
               · 
               
                 μ 
                 o 
               
             
           
         
       
     
   
   where ∇ is the gradient operator, B is the magnetic field intensity (or the magnetic flux density), and μ o  is the magnetic permeability of free space (4π·10 −7  T·m/A). 
   Magnetophoretic cell separation using a device according to the invention relies on the value of the magnetophoretic mobility, m, of the immunomagnetically labeled cells. Cells have been found to exhibit a characteristic defined as Antibody Binding Capacity (ABC), which relates directly to the number of antibodies binding to the surface molecules on individual cells. By way of background, reference is made to McCloskey, et al., “Magnetophoretic Mobilities Correlate to Antibody Binding Capacities,” Cytometry, vol. 40, issue 4, pages 307-315, April 2000, Wiley-Liss, Inc., the disclosure of which is incorporated herein by reference. In McCloskey, et al., it was shown that a linear relationship exists between magnetophoretic mobility and ABC, and the device can utilize this principle to isolate magnetically labeled particles not only based on a binary (magnetic vs. non-magnetic) feature, but even on a scalar feature based on the ABC value of the magnetically tagged cell. That is, cells can be selected on the basis of the quantity of surface antigenic sites. 
     FIG. 1  shows a conventional embodiment of a separation mechanism that is in the prior art. In  FIG. 1 , a standard flow channel configuration is presented showing internal wall (labeled A) and external wall (labeled B) of the annulus, field direction, sample inlet (a′), a carrier inlet (b′), negatively selected (i.e. depleted) fraction outlet (a), positively selected (i.e. enriched) fraction outlet (b), an Inlet Splitting Surface (ISS), and an Outlet Splitting Surface (OSS). In the prior art, the splitters were aligned with the middle of the separation zone, as shown in  FIG. 1 . Note in this configuration that the splitters are not aligned with the ISS and OSS, resulting in a “compression” of the inlet sample (a′) just downstream of the splitter tip and an “expansion” of the positively selected (b) fraction just upstream of the splitter tip. The fluid shear created by the velocity differentials at these expansion and compression zones can result in undesirable mixing between the sample and carrier at the inlet, and between the two fractions at the outlet. 
   The general notion of matching splitters to the ISS and the OSS was provided in Williams, et al., “Splitter Imperfections in Annular Split-Flow Thin Separation Channels: Effect on Nonspecific Crossover”, Analytical Chemistry, vol. 75, no. 6, Mar. 15, 2003, pages 1365-1373. The present invention improves upon these theoretical concepts and provides a novel, functional, practical design that is fully operational and easily manufactured.  FIG. 2  is a similar schematic showing an improved design according to an embodiment of the present invention. Note in the schematic that the inlet splitter  50  is aligned with the ISS and the outlet splitter  60  is aligned with the OSS, minimizing any “compression” or “expansion” effects and thereby minimizing fluid shear and turbulence impacts at the inlet and outlet. More details of the structures involved will be provided below. 
   A major advance is the ability to easily manufacture and install splitters in the inlet and outlet that are “tuned” for specific flow profiles, effectively optimizing the flow channel for a given set of flow rate requirements. Theoretical calculations for determining splitter geometry are noted in Williams et. al., “Flow Rate Optimization for the Quadrupole Magnetic Cell Sorter,” Analytical Chemistry, vol. 71, no. 17, pages 3799-3807, September 1999, the disclosure of which is incorporated herein by reference. In the invention, two theoretical planes exist. These planes are identified as the Inlet Splitting Surface (ISS) and the Outlet Splitting Surface (OSS), and they develop during separations in a split-flow thin channel. The gap between the ISS and OSS is known as the transport lamina  23 , and for a particle to be separated and eluted in the b fraction of the flow channel, the particle must cross the thickness of the transport lamina while it is in the annular flow volume. By setting the flow rates at the proper settings, it is possible to match the ISS to the inner diameter (and thus the diameter at the tip) of the inlet splitter, and likewise, the OSS can be matched to the inner diameter (and thus the diameter at the tip) of the outlet splitter. Any variation between the ISS and the inlet splitter, or the OSS and the outlet splitter, can result in increased fluid shear and potential turbulence at the fluid-fluid interface between the sample and carrier at the inlet and between the positively and negatively selected fractions at the outlet. Since turbulence in the channel reduces the precision of the separation process, the ability to calculate and tune the splitters at the manufacturing process to specific flow rates is a significant improvement over the prior art. 
   Moreover, as taught by Fuh, et al., “Hydrodynamic Characterization of SPLITT Fractionation Cells,” Separation Science and Technology, 30(20), pp. 3861-3876, Marcel Dekker, Inc., 1995, tapering or chamfering the edges of the splitter suppresses vortex formation. Therefore, the splitters described herein preferably are tapered or chamfered in order to reduce mixing and maintain an absence of hydrodynamic mixing between laminae. 
   The theoretical position of the ISS can be calculated using the following system of equations, which are derived in more detail in Williams et. al. 1999 and are slightly modified for consistency of symbols with the previous equations and with equations presented later in this patent: 
   
     
       
         
           
             A 
             1 
           
           = 
           
             ( 
             
               1 
               + 
               
                 ρ 
                 i 
                 2 
               
               - 
               
                 A 
                 2 
               
             
             ) 
           
         
       
     
     
       
         
           
             A 
             2 
           
           = 
           
             
               ( 
               
                 1 
                 - 
                 
                   ρ 
                   i 
                   2 
                 
               
               ) 
             
             
               ln 
               ⁡ 
               
                 ( 
                 
                   1 
                   
                     ρ 
                     i 
                   
                 
                 ) 
               
             
           
         
       
     
     
       
         
           
             
               I 
               2 
             
             ⁡ 
             
               [ 
               
                 
                   ρ 
                   i 
                 
                 , 
                 
                   ρ 
                   ISS 
                 
               
               ] 
             
           
           = 
           
             
               [ 
               
                 
                   2 
                   ⁢ 
                   
                     ρ 
                     2 
                   
                 
                 - 
                 
                   ρ 
                   4 
                 
                 + 
                 
                   2 
                   ⁢ 
                   
                     A 
                     2 
                   
                   ⁢ 
                   
                     ρ 
                     2 
                   
                   ⁢ 
                   ln 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   ρ 
                 
                 - 
                 
                   
                     A 
                     2 
                   
                   ⁢ 
                   
                     ρ 
                     2 
                   
                 
               
               ] 
             
             
               ρ 
               ISS 
             
             
               ρ 
               i 
             
           
         
       
     
     
       
         
           
             R 
             i 
           
           = 
           
             
               
                 I 
                 2 
               
               ⁡ 
               
                 [ 
                 
                   
                     ρ 
                     i 
                   
                   , 
                   
                     ρ 
                     ISS 
                   
                 
                 ] 
               
             
             
               
                 A 
                 1 
               
               ⁡ 
               
                 ( 
                 
                   1 
                   - 
                   
                     ρ 
                     i 
                     2 
                   
                 
                 ) 
               
             
           
         
       
     
   
   where ρ i  is the ratio of the radius of the inside of the annular flow volume to the radius of the outside of the annular flow volume, and ρ ISS  is the ratio of the radius of the ISS to the radius of the outside of the annular flow volume. Likewise, the OSS can be calculated by solving the following equation: 
   
     
       
         
           
             R 
             o 
           
           = 
           
             
               
                 I 
                 2 
               
               ⁡ 
               
                 [ 
                 
                   
                     ρ 
                     i 
                   
                   , 
                   
                     ρ 
                     OSS 
                   
                 
                 ] 
               
             
             
               
                 A 
                 1 
               
               ⁡ 
               
                 ( 
                 
                   1 
                   - 
                   
                     ρ 
                     i 
                     2 
                   
                 
                 ) 
               
             
           
         
       
     
   
   where ρ OSS  is the ratio of the radius of the OSS to the radius of the outside of the annular flow volume. It can be determined from these equations that, for a fixed inner and outer diameter of the annular flow volume, the inlet splitter can be manufactured to match a specific inlet flow ratio, and likewise, an outlet splitter can be manufactured to match a specific outlet flow ratio. 
   Referring now to  FIGS. 3A ,  3 B,  3 C, and  3 D, an embodiment of the present invention is shown that solves the problems with the prior art devices.  FIG. 3A  is a section view of an assembled separation flow channel  10 .  FIG. 3B  shows an exploded perspective view of the flow channel  10 .  FIG. 3C  shows an exploded perspective view of the inlet end of the flow channel  10 . The flow channel  10  comprises an inlet end  11  and an outlet end  12 . The inlet end  11  of the flow channel  10  according to the depicted embodiment comprises a core  20 , an inlet adaptor  30 , an O-ring  31  that seals the fluid from leaking out of the flow channel  10 , a flow distributor  40 , an O-ring  41  that prevents mixing of the sample and carrier in the inlet, and an inlet splitter  50 . In the middle portion of the flow channel  10  is the core  20  (having an inlet end  21  and an outlet end  22 ) and the shell  26 , which contain the annular flow section  27  where the field is applied and the separation physically takes place.  FIG. 3D  shows an exploded perspective view of the outlet end  12  of the flow channel  10  according to one embodiment of the invention. The outlet end  12  of the flow channel  10  comprises the outlet end  22  of the core  20 , an outlet splitter  60 , an O-ring  61  that prevent the separated fractions from mixing, a spacer  70 , an O-ring  71  that prevents leakage out of the channel, and an outlet adaptor  80 . 
   Preferably the flow channel  10  comprises a sterile, disposable, closed system flow channel. In the embodiments shown in the figures, the flow channel  10  is primarily comprised of a solid cylindrical core  20  that is concentric with an external cylindrical shell  26 , creating the annular flow section  27 . The core  20  is approximately 9.32 mm in diameter and 230 mm long. Referring again to  FIG. 2 , two concentrically aligned fluids (termed a′ and b′) enter the annular area flow section  27 . The feed substream a′ enters via feed tube  13  while the carrier substream b′ enters via carrier tube  14 . The substreams a′ and b′ are brought together at the tip of the inlet splitter  50  (in the embodiments shown, the inlet splitter  50  is a cylindrical wall) that initially unites the two flows. The fluids then form a fluid-fluid interface and travel in the same direction along the length of the flow channel  10  between the core  20  and the shell  26 . The fluids are maintained in the laminar flow regime to prevent turbulence and mixing of the fluids. The fluids then diverge into two separate cylindrical flow paths, with outer flow fraction a exiting via fraction a exit tube  93  and outer flow fraction b exiting via fraction b exit tube  94 , at the outlet end  12  of the flow channel  10  by a cylindrical surface (shown as the outlet splitter  60 ) that lies between the core  20  and the shell  26 . 
   Referring again to  FIGS. 2 and 3 , initially the solution containing the material that is to be separated is delivered into the top center inlet port, or a′. The interface between the a′ and b′ fluids forms a theoretical zone called the transport lamina  23 . Particles with associated magnetophoretic mobilities travel towards the shell  26  and must cross the thickness of the transport lamina  23  in order to be isolated into the outer flow fraction b at the outlet end  12  of the flow channel  10 . Any material, whether it is magnetic or not, that does not cross the plane of the Outlet Splitting Surface (OSS) is maintained in the inner flow fraction a at the outlet end  12  of the flow channel  10 . Since the geometry and the magnetic field are static and fixed, the control for the separation process lies with the four flow rates: Qa′, Qb′, Qa, and Qb. 
   Assuming that the flow channel geometry, magnetophoretic mobility profile of the sample, and magnetic field strength and geometry are fixed parameters, the entire separation process is defined by three variables that are controlled by the user. The total flow rate, or Q total , for the separation process is defined by the following equation:
 
 Q   total   =Qa′+Qb′=Qa+Qb 
 
   The inlet flow is then characterized by the inlet ratio, R i , defined as: 
   
     
       
         
           
             R 
             i 
           
           = 
           
             
               Qa 
               ′ 
             
             
               Q 
               total 
             
           
         
       
     
   
   and the outlet flow is characterized by the outlet ratio, R o , defined as: 
   
     
       
         
           
             R 
             o 
           
           = 
           
             Qa 
             
               Q 
               total 
             
           
         
       
     
   
   By assuming that the flow channel is a fixed volume, these three parameters (Q total , R i , and R o ) can be used to calculate the four flow rates Qa′, Qb′, Qa, and Qb. 
   Referring now to  FIGS. 3B ,  3 C,  4 ,  7 , and  8 , details of the depicted core are provided. The core comprises three concentric cylindrical sections, a first cylinder  201  at the inlet end  21 , a second cylinder  202  at the outlet end  22 , and a third cylinder  203  having a larger diameter that connects the first and second cylinders  201 ,  202 . The first cylinder  201  comprises an axial sample inlet port  204  to admit the sample a′ into the flow channel  10 . Near the junction between the first cylinder  201  and the third cylinder  203  are multiple sample dispersion ports  21   a . The multiple sample dispersion ports  21   a  allow the sample a′ to surround the core  203 , thus admitting the sample flow a′ into an annular space between the core  203  and the inlet splitter  50  (described below). This converts the sample a′ flow into a substantially circumferentially uniform, annular flow geometry. Note that in the embodiment shown, the sample dispersion ports  21   a  radiate the sample flow at an angle to the longitudinal axis of the flow channel  10  (see, e.g.,  FIG. 3A ). Preferably, the internal geometry of the outlet end of the core  20  (that is, second cylinder  202 ) is a mirror image of the geometry at first cylinder  201 . Near the junction of the third cylinder  203  and the second cylinder  202  are multiple ports  22   a  that convert the negatively selected fraction a from a circumferentially uniform, annular flow geometry to a cylindrical flow geometry. At the outlet end  22  an outlet port  205  discharges this negatively selected fluid portion a. In additional embodiments not specifically shown in the figures, the radiating fluid paths could also be positioned perpendicularly to the axis of the channel, or even in several other physical configurations. 
   The primary function of the inlet adaptor  30  is to introduce the carrier b′ fluid into the flow channel  10 . Likewise, the primary function of the outlet adaptor  80  is to elute fraction b from the flow channel  10 . The inlet adaptor  30  and outlet adaptor  80  are preferably identical. The adaptors  30 ,  80  in the embodiment shown, each provide one or more carrier inlet ports  32  where standard tubing (carrier tube  14 ) is inserted and bonded in place. This tubing interface creates a sealed fluid connection at the inlet for the carrier b′ to enter the plenum between the inlet adaptor  30  and flow distributor  40 , or at the outlet for fraction b to pass from the plenum between the outlet splitter  60  and the outlet adaptor  80  to the tubing  94  exiting the flow channel  10  via tubing ports  81 . Furthermore, the inlet adaptor  30 , together with the outlet adaptor  80 , physically supports and positions the shell  26  concentrically to the core  20 . Additionally, each adaptor  30 ,  80  comprises an O-ring groove that accepts the outermost O-ring  31 ,  71  (O-ring  31  at the inlet, O-ring  71  at the outlet), which prevents any fluid from passing between the adaptor  30 ,  80  and core  20  and thus prevents any leakage along the core  20  and out of the flow channel  10 . 
   As described above, the core  20  is the structure that deflects the sample flow a′ into its preferred circumferentially uniform annular geometry. In the embodiment shown, a flow distributor  40  deflects the carrier flow b′.  FIGS. 5A and 5B  depict perspective views of two embodiments of the flow distributor  40 . The flow distributor  40  deflects one or more cylindrical inlet carrier flows b′ to a laminar, circumferentially uniform, annular geometry. For both embodiments shown, the flow distributor  40  comprises an alignment surface  42  that positions the flow distributor  40  relative to the core  20 , a central support hub  43 , a fluid distribution plenum  44 , a keyed alignment feature  45  that aligns the initial fluid flow with a blocked flow path  46 , and a plurality of fluid ports  47 . The alignment surface  42  provides a means for aligning the flow distributor  40  concentrically around the core  20  and the alignment feature  45  provides a means for aligning the flow distributor  40  rotationally with the inlet adaptor  30  so that the b′ flow flows in from the adaptor  30  onto a closed section of the flow distributor instead of directly into the fluid ports  47 . The plurality of fluid ports  47  preferably provide parallel flow paths. This arrangement is somewhat akin to a showerhead concept. The multiple parallel paths of the fluid ports  47  are the key to a circumferential distribution of the fluid. The keyed alignment feature  45  ensures that the flow from the inlet adaptor  30  does not have a direct path into the flow channel  10 . Instead, the flow from the inlet adaptor  30  is directed onto the blocked flow path  46  of the flow distributor  40 , which deflects the flow into the plenum  44  volume and around the circumference of the flow channel  10 . 
   Referring now to  FIGS. 6A ,  6 B,  6 C, and again to  FIG. 3C , an inlet splitter  50  according to an embodiment of the invention is presented. The inlet splitter  50  is substantially cylindrical, having a cylindrical wall  53 . The inlet splitter  50  further comprises an alignment surface  51  that positions the inlet splitter  50  relative to the  20  core. With reference also to  FIGS. 7 and 8 , the inlet splitter  50  further comprises an O-ring groove  52  that permits the installation of the O-ring  41  that seals the two fluid compartments to prevent mixing of the sample a′ and carrier b′ at the inlet. Likewise, O-ring  61  seats in the outlet splitter  60  to prevent mixing of the two fractions a, b. This means that the sample a′ and the carrier b′ are completely separate flows across the length of the inlet splitter  50 , and they do not come into contact until they flow past the inlet splitter  50 . The cylindrical wall  53  is designed to isolate the two annular fluid volumes (the sample a′ and carrier b′) at the inlet, and a similar structure in the outlet splitter  60  isolates the negatively selected a and positively selected b fractions at the outlet. The cylindrical wall  53  terminates at a chamfered tip  54  that minimizes any disturbances at the fluid-to-fluid interface in the inlet and outlet. The inlet splitter  50  provides the fluid-to-fluid contact point where the sample a′ and carrier b′ form a single stable laminar annular flow. 
   The outlet splitter  60  is of similar design to the inlet splitter  50 . The outlet splitter  60  provides the dividing point that splits the single annular flow from the transport lamina region  23  into the two outlet fractions, a and b. Typically, the diameter of the inner wall  53  is different between the inlet and outlet splitters  50 ,  60  in order to optimize the flow channel  10  for specific inlet and outlet flow ratios, respectively (the inner diameter is larger for high flow ratios and smaller for lower flow ratios, and can generally be set for flow ratios from 0.1 to 0.4). For identical inlet and outlet flow ratios, the inlet and outlet splitters  50 ,  60  could be considered interchangeable. 
   Referring now to  FIGS. 7 and 8 , and with continuing reference to  FIGS. 3C ,  5 A, and  5 B, a better representation of the assembly at the inlet end  11  of the flow channel  10  is shown. The cylindrical face of the core  201  at the inlet end  11  provides an alignment datum that is common to all of the inlet components and maintains a tight concentric alignment of the parts. The inlet adaptor  30 , inlet splitter  50 , and flow distributor  40  at the inlet end  11  of the flow channel  10  are concentrically aligned with the annular flow section by mating internal cylindrical faces for each of these parts against the cylindrical alignment datum at the inlet end  21  of the core  20 . The inlet end  21  of the core  20 , due to the configuration of the first cylinder  201  and the third cylinder  203 , comprises a face  24  that is perpendicular to the cylindrical axis and provides a physical datum that axially positions the inlet components. The outlet end  12  of the flow channel  10  is configured as a mirror image to the inlet end  11  of the flow channel  10 , except that the flow distributor  40  is replaced by a spacer  70  that provides the same axial positioning of the components as at the inlet. 
   Referring again to  FIG. 7 , the positioning of the inlet splitter  50  on the core  20  is shown. The inlet splitter  50  is positioned concentrically on the core  20  due to the cylindrical mating surfaces of the core  20  and inlet splitter  50 , and the inlet splitter  50  is located axially on the  20  core by the mating of the female shoulder in the splitter against the male shoulder on the core. The annular volume between the inlet splitter  50  and the core  20  contains the sample solution a′. The O-ring  41  seals the sample a′, which flows inside of the inlet splitter  50 , from the carrier b′, which flows outside of the inlet splitter  50 . 
   Referring again to  FIG. 8 , the flow distributor  40  is shown positioned onto the inlet splitter  50 . The fluid path for the carrier b′ is apparent in this view, which displays the plurality of fluid ports  47  where the carrier can flow through the flow distributor  40  and down the outside of the inlet splitter  50 . The flow distributor  40  is positioned concentrically on the core  20  due to the cylindrical mating surfaces of the core  20  and flow distributor  40 , and the flow distributor  40  is located axially on the core  20  by the mating of the bottom face of the flow distributor  40  against the top face of the inlet splitter  50 . The annular flow zone between the adaptor and the inlet splitter  50  just downstream of the flow distributor  40  provides a means for providing a circumferentially consistent axial laminar flow. 
   Referring now to  FIG. 9A , a quartered section of the flow channel  10  inlet end  11  is shown for additional clarification of flow paths inside the flow channel  10 . This view shows the inlet adaptor  30 , O-rings  31 ,  41 , flow distributor  40 , inlet splitter  50 , core  20 , and shell  26 . Note that all parts with the exception of the shell  26  are physically centered on the core  20 . Both the carrier inlet ports  32  in the inlet adaptor  30  and the sample inlet port  204  in the core  20  (in communication with feed tube  13 ) are also displayed in this view. Similarly, in  FIG. 9B , a quartered section of the flow channel  10  outlet end  12  is shown. During assembly, special fixtures (not shown) are used to clamp the flow channel parts along the long axis of the flow channel  10  while adhesive is applied to the joint between the shell  26  and the inlet adaptor  30 . Once the adhesive is set, then all components are fixed in position in the flow channel  10 . Flexible tubing is adhesively bonded into the inlet adaptor  30  and core  20  at the fluid ports. Additional obvious joining methods which may be used as a replacement to adhesive bonding exist, such as ultrasonic welding or solvent bonding the pieces together. 
     FIGS. 10 and 11  display sagittal sections of the assembled flow channel  10  according to one embodiment of the invention, taken at 90 degree positions relative to one another. In  FIG. 10 , with continuing reference to  FIG. 3C , the inlet splitter  50 , together with the inlet end  21  of the core  20 , provides the annular flow region where the sample fluid a′ develops into a stable laminar annular flow. In this sagittal sectional view, the entry of the carrier fluid b′ shows where the carrier enters into the fluid distribution plenum  44 , which is located between the flow distributor  40  and the inlet adaptor  30 . The flow then distributes circumferentially around the channel  20 .  FIG. 11  shows a sagittal section at one of the fluid paths through the flow distributor  40 . The carrier b′ flows through the flow distributor  40  and then enters the annular region between the inlet splitter  50  and the inlet adaptor  30 . The inlet splitter  50 , together with the inlet adaptor  30 , provides the annular flow region where the carrier fluid b′ develops into a stable laminar annular flow. 
   Referring now to  FIG. 12  and again to  FIG. 3D , a sagittal sectional view of the outlet end  12  of the flow channel  10  is shown. The outlet splitter  60 , together with the outlet adaptor  80 , provides the annular flow region where the selected fraction b transitions from a single laminar annular flow into the plurality of flow paths going into the ports in the outlet adaptor  80 . Furthermore, the outlet splitter  60 , together with the outlet end  22  of the core  20 , provides the annular flow region where the non-selected fraction a transitions from a single laminar annular flow into the plurality of fluid paths going into the ports in the core. 
     FIG. 13  displays a section of the flow channel  10  inlet showing the even distribution of flow of the sample a′ from the core  20  to the inner wall  53  of the inlet splitter tip  54  via streamlines that approximate the results calculated by computational fluid dynamics methods. Note that the flow paths are very uniform and consistent over the entire circumference of the figure at the splitter tip, which is an improvement over the prior art. 
     FIG. 14  displays a section of the flow channel  10  inlet showing the even distribution of flow of the carrier b′ from the inlet adaptor  30 , through the flow distributor  40 , to the outer wall  53  of the inlet splitter tip via streamlines that approximate the results calculated by computational fluid dynamics methods. Note that the flow paths are very uniform and consistent over the entire circumference of the figure at the splitter tip, which is an improvement over the prior art. 
   While there has been described and illustrated particular embodiments of the fluid distribution and assembly features of a split-flow thin separation channel, it will be apparent to those skilled in the art that variations and modifications may be possible without deviating from the broad spirit and principle of the present invention, which shall be limited solely by the scope of the claims appended hereto.