Spiral microfluidic nanoparticle separators

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1 Spiral microfluidic nanoparticle separators Ali Asgar S. Bhagat, Sathyakumar S. Kuntaegowdanahalli, Dionysios D. Dionysiou *, and Ian Papautsky Department of Electrical and Computer Engineering, University of Cincinnati, Cincinnati, USA * Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, USA ABSTRACT Nanoparticles have potential applications in many areas such as consumer products, health care, electronics, energy and other industries. As the use of nanoparticles in manufacturing increases, we anticipate a growing need to detect and measure particles of nanometer scale dimensions in fluids to control emissions of possible toxic nanoparticles. At present most particle separation techniques are based on membrane assisted filtering schemes. Unfortunately their efficiency is limited by the membrane pore size, making them inefficient for separating a wide range of sizes. In this paper, we propose a passive spiral microfluidic geometry for momentum-based particle separations. The proposed design is versatile and is capable of separating particulate mixtures over a wide dynamic range and we expect it will enable a variety of environmental, medical, or manufacturing applications that involve rapid separation of nanoparticles in realworld samples with a wide range of particle components. Keywords: nanoparticle separation, spiral microchannel, passive separation, added mass force, centrifugal force 1. INTRODUCTION Nanoparticles have potential applications in many areas such as consumer products, health care, electronics, energy, and other industries. However, the production and widespread use of nanoparticles in our daily life make the exposure to them inevitable. Recent studies have indicated that nanoparticles can enter the human metabolic system though inhaling, drinking, and digesting nanoparticle-containing air, water, and foods [1]. Concerns have been raised that nanoparticles are able to exert adverse impact on human health and environmental systems. Thus, to protect human health and environmental systems from nanoparticle pollution, separation and removal of manufactured nanoparticles after their applications appear to be critical. Unfortunately, the traditional size-based separation approaches are complicated and expensive [2]. Conventional micro/nano-separation systems employ membrane-based filtering schemes, which are typically limited by the membrane pore size, making them inefficient for separating a wide range of particle sizes. Clogging is another problem associated with membrane-based filtering techniques which results in low flow rates and high fluidic resistance. These limitations have led to the recent development of various filter-less separation techniques that can be used for a wide range of particle sizes. Sedimentation, field-flow fractionation (FFF) [3-5], hydrodynamic chromatography (HDC) [6,7], pinched flow fractionation (PFF) [8,9], split-flow thin (SPLITT) fractionation [10,11], electrophoresis [12,13], dielectrophoresis [14,15], acoustic separation [16] and centrifugation [17-19] are some of the techniques that have been developed and demonstrated for particle and biological separations and concentration of micrometer and nanometer sized particle mixtures. Although the above mentioned separation techniques work well for micro- and nano- particle separations, they suffer from a number of limitations. Most of the separation techniques work in batch mode rather than continuous flow and are thus not an efficient choice for large sample volumes. Some techniques need large investments for supporting equipment and require long times for system preparation and processing. The need for an external field for particle manipulation of some approaches makes the fabrication complex and often difficult to integrate with conventional lab-on-a-chip (LOC) systems. These external fields can also potentially damage biological macromolecules. Finally dependence on particle charge, confines the applicability of some methods to a limited variety of particles. Microfluidics, BioMEMS, and Medical Microsystems VI, edited by Wanjun Wang, Claude Vauchier, Proc. of SPIE Vol. 6886, 68860M, (2008) X/08/$18 doi: 1117/ Proc. of SPIE Vol M-1

2 Centrifugal force-based separation has the potential to overcome most of the limitations of conventional microscale particle separation techniques mentioned above. The process is membrane-free and eliminates the issues arising from microchannel clogging. Also it can be performed in a continuous manner and thus is advantageous for processing large sample volumes (milliliters) in a short time. As the separation process is completely mass (size) based, it does not require an external field, i.e. electric, magnetic or acoustic, for particle manipulation. Thus it can be used with a wide variety of particles ranging from biological samples such as blood, DNA and proteins to micro and nano sized colloidal particles. Particle separation based on the concept of centrifugation is becoming a popular technique for micro- and nano-particle separation. On the microscale, this can be achieved by pumping particulate flows through spirally shaped curvilinear microchannels that results in particle migration across the microchannel width based on particle mass. The technique works on the principle of centrifugal acceleration experienced by particles flowing in a curved microchannel. Depending on the size, and hence the mass, of the particles, these forces cause the particles to move towards either the inner or the outer channel wall. Particles dispersed into fluid traveling in these spiral microchannels experience two forces acting on them. First, the transverse pressure fields from fluid shear which makes the particles to move towards the inner microchannel wall. Second, the centrifugal force causing the particles to move in transverse direction (across the microchannel width). These forces are opposing in nature and their magnitudes largely depend on the particle size and density, fluid velocity, and radius of curvature of the spiral microchannels. The combined effect of various forces results in formation of separate particle streams based on their size and mass, which can in turn be collected at separate outlets by taking advantage of laminar flows in output microchannels. Blattert et al. [17] were the first to use the principle of centrifugal acceleration to separate plasma and serum from blood. Their design consisted of an inlet reservoir for pumping the blood sample into the microchannel. A 90 o circular bend was introduced into the microchannel to achieve separation. Beyond the bend, the microchannel bifurcated into two separate channels, one for plasma and the other for cells. The authors studied the effect of cell, plasma channel lengths and flow velocity on separation. Separation efficiency was calculated by determining the percentage of red blood cells (RBC) and white blood cells (WBC) in the plasma and cell channels. Results indicated 80 % separation efficiency between the two cell types (which are typically few µm in size) for the optimal conditions determined. Later, Gregoratto et al. [18] designed spirally shaped microchannels having high aspect ratio for the continuous separation of large volumes of dilute suspensions. The design worked on the principle of centrifugal acceleration of particles of different sizes (and thus mass) flowing in a spirally wound microchannel. To minimize the effect of Dean vortices, high aspect ratio (~10) microchannels were fabricated in silicon using DRIE process. A 3.5 fold increase in concentration of 10 µm particles was achieved at the channel outlet whereas the 1 µm particles showed negligible concentration increase at 2 ml/min flow rate. The principle of centrifugal acceleration was recently used by Seo et al. [19] for filtering out microparticles The design was similar to the spirally wound channels by Gregoratto et al. with an addition of an S shaped section at the center of the design for change in flow direction and two outlets with a bifurcation at the exit. At flow velocities of 92 mm/s, they achieved 200 times higher concentration of the 10 µm particles between the inside and outside outlet. For similar flow velocities, 9.3 and 1.4 times higher concentration between the inside and outside outlet for the 6 µm and 3 µm particles respectively was achieved. In this work, we investigate passive spiral microfluidic geometries for momentum-based particle separation. The proposed design is capable of separating micro/nano-sized particulate mixtures over a wide dynamic range. The separation system proposed herein will prove useful in a wide range of environmental applications including water purification, sample preconcentration and sensing of biological agents. 2. DESIGN PRINCIPLE Traditional passive micro- and nano-particle separation techniques employ a short curvilinear microchannel segment to achieve separation. As a particle moves through this segment, it experiences various forces in addition to the centrifugal Proc. of SPIE Vol M-2

3 force that alter the position of the particle within the microchannel. Spirally shaped microchannels can be used to separate a wide range of micro/nano-particle mixtures using primarily fluidic shear flows. The design works on the principle of the forces experienced by particles flowing in a centrifugal field of a curved microchannel. Depending on the size, and hence the mass, of the particles, these forces causes the particles to either move towards the inner or the outer wall of the curvilinear microchannel. The equation of motion for small particle dispersed in a fluid can be described by the BBO-equation, named after the pioneering work by Basset [20], Boussinesq [21] and Oseen [22]. The net force (F P, net ) acting on a particle within a fluid flow, written as the sum of the various forces acting on them, is expressed as [23]: U p FP, net = m p (1a) t F = ( F + F ) ( F + F + F ) (1b) P, net buoy cf am drag pg where, m p and m f are the mass of the particle and fluid respectively (kg), U p is the velocity of the particle (m/s), F buoy is the buoyancy force, F cf is the centrifugal force, F am is the added mass force F drag is the total drag force, F pg is the pressure gradient force. The buoyancy and the centrifugal forces cause a particle to move towards the outer wall of a spirally wound microchannel, while the added mass, pressure gradient and drag forces tend to push the particles towards the inner channel wall. The buoyancy force (F buoy ) and the centrifugal force (F cf ) acting on a particle are given by Equation 2. If the difference in density between the particle and the fluid is small, then the buoyancy force can be neglected. Centrifugal force is the outward directed force experienced by particles flowing in a circular channel. Since the centrifugal force is directly proportional to the mass of the particle, heavier particles experience stronger force than the lighter particles and are pushed more towards the outer wall than their lighter counterparts. Fbuoy = ( m p m f )g (2a) 2 m pu p Fcf = (2b) R where, g is the acceleration due to gravity (m/s 2 ) and R is the radius of the circular segment of the channel (m). The added mass force (F am ) is the force experienced by the particles due to the inertia added to a system. This is due to the fact that an accelerating particle displaces some volume of surrounding fluid as it moves through it, since the particle and fluid cannot occupy the same physical space simultaneously. For simplicity, this is modeled as some volume of fluid moving with the object. The added mass force is negligible when the particle densities are much higher than the fluid medium in which they are dispersed. Since the density of the polystyrene particles modeled in this work is comparable to the density of the fluid (water), the added mass force cannot be neglected. The drag force (F drag ) is the shear force exerted on the particles due to the viscous nature of the fluid. It is the force that resists the movement of the particle through the liquid. The pressure gradient force (F pg ) is the force acting on a particle due to the pressure gradient across the microchannel length. 1 U r Fadded = m f 2 t (3a) Fdrag = 6πµ au r (3b) DU F r pg m f Dt (3c) U = U U (3d) r p where, U r is the relative velocity between the particle and the fluid (m/s), U f is the fluid velocity (m/s), µ is the fluid viscosity (kg/m-s), and a is the particle diameter (m). f Proc. of SPIE Vol M-3

4 By varying the effect to which each of the above mentioned forces affect the particle motion, the particles can be pushed towards either the inner or outer microchannel wall to achieve separation. In this work, we have utilized the centrifugal and added mass force for separation of polystyrene based particulate mixtures based on the size of the particles. For small particle separations (<10 µm), typically high fluid velocities are required for the centrifugal forces to dominates over the fluid shear forces to make particles move outward. However, higher fluid velocities in curved microchannels cause secondary flow patterns known as Dean vortices which tend to trap the particles dispersed within the flow. This causes the larger particles to towards the outer microchannel wall and the smaller particles are dispersed across the entire channel width. The magnitude of the secondary flows is quantified by a dimensionless number called the Dean number: D h De = Re (6) R where Re is the Reynolds number ( ρu f Dh µ ), D h is a typical length scale (m) associated with the channel cross-section (e.g., hydraulic diameter in the case of a rectangular cross-sections) and R is the radius of curvature of the path of the channel (m). Straight channels have a Dean number of 0. Secondary flows tend to re-circulate the flows in the channel, thus completely nullifying the separation effect due to the centrifugal force. To avoid this behavior and achieve better separation efficiency, it is thus important to work low Dean number flows while still maintaining higher fluid velocity. In this case, higher separation efficiency can be achieved by reducing the microchannel width and the radius of the spirally wound channels and increasing the transit time required to reach the outer channel wall by increasing the number of loops. Figure 1illustrates the proposed spiral microfluidic separator consisting of two inlets and two outlets. Also the dominant forces acting on a suspended particle moving through a curvilinear channel are shown in the inset of figure 1. The microchannel width was 100 µm and the spacing between successive loops was 250 µm. The radius of curvature of the innermost curvilinear channel was 3 mm with number of loops equal to 5. IuuG MDII OflE MDII DL9a E0 LCG ESUJ qqeq 922 E Eb ECJ CeuL!nä9I I EPnOA )LCG blg22fllg L9q!eu E:0LCG bolcg B110ASUf EOICe Fig 1. Schematic diagram of a 5- loop spiral particle separator indicating forces acting on a suspended particle moving through the spiral microchannel. Proc. of SPIE Vol M-4

5 3. METHODS 3.1 Computational modeling The spiral channel designs were modeled using a commercial computational fluid dynamics software CFD-ACE+ (ESI- CFD Inc., Huntsville, AL). The physical properties of water were applied to the fluid participating in the simulation; the density set to 1000 kg/m 3 and the dynamic viscosity set to 10-3 kg/ms. A diffusion co-efficient of m 2 /s was used for all the simulations. At the inlet, fluid velocities corresponding to flow rates 1µL/min to 20µL/min were applied, while the outlet was set to a fixed-pressure boundary condition. Polystyrene particles with density of 1050 kg/m 3 and varying diameters were dispersed into one flow stream using the SPRAY module. The initial velocity of particles was set equal to that of the fluid, thus allowing them to flow at the same velocity as the fluid without altering the fluid path (i.e. they remained true to the fluid flow). In this work three particle sizes (590 nm, 1.9 µm and 7.32 µm) were modeled and tested. The Algebraic MultiGrid (AMG) solver was used for pressure corrections while the Conjugates Gradient Squared (CGS) and Preconditioning (Pre) solver was used for velocity and species correction. The convergence limit for mass fraction was set to 10-6 and the simulations were run until a steady flow of particles reached the outlet. 3.2 Fabrication The spiral microchannels were fabricated in PDMS using soft lithography. Initially, a 50µm thick layer of SU-8 photoresist was patterned on a 3 bare silicon wafer using conventional photolithography. Next, PDMS prepolymer mixed in a 10:1 ratio of base: curing agent was cast on the SU-8 master to replicate the microchannel features in it. After curing, the PDMS mold was peeled from the SU-8 master and bonded to a glass slide to complete the microchannel. Prior to bonding, holes for inputs and outputs were bored using a 14 gauge syringe tip to interface with a 1/16 (OD) tubing. The fabrication process has been described in detail elsewhere [24]. 3.3 Characterization The spirally-shaped particle separators were tested experimentally using individual particle solutions of fluorescently labeled 590 nm, 1.9 µm and 7.32 µm sized polystyrene particles (Bangs Laboratories Inc., IN, USA). The corresponding particle densities used for testing were ~ 10 8 beads/ml, ~ 10 7 beads/ml and ~ 10 6 beads/ml respectively. PDMS devices were placed on an inverted epi-fluorescence microscope (Olympus IX71) and tubes for inlet and outlet were aligned and fixed to the device. PDMS molds of 5 mm thickness were cast so as to avoid the use of any additional support to hold the tubes in place. The two inlet tubes are then connected to syringes using standard luer fittings and unions (Upchurch Scientific). To evaluate particle separation, during testing one syringe was filled with a particulate solution while the other was filled V OflfeL!IJ'e IUUGL UIe B IuIG IUUE OflfI6 Oflf6L OflfI6 DM DM S Fig 2. (a) Schematic image of the 5-loop spiral channel indicating the four image capture windows. (b) SEM image of the fabricated PDMS mold showing the entrance region along with the 5-loops of the microchannel. The channel cross-sectional dimensions are 100 µm 50 µm (W H). Proc. of SPIE Vol M-5

6 DI water only. Using syringe pumps (KD Scientific), the two syringes were driven at flow rates in the 1-20 µl/min range. High speed (~70 fps) images of the channel were captured at various positions downstream using a 12-bit CCD camera (QImaging Retiga EXi) attached to the microscope. A stack of 300 images were then averaged to create single composite using ImageJ analysis software. Grey-scale line-scans acquired across the microchannel cross-section of these composite images were used to analyze the particle migration across the microchannel width. Figure 2a illustrates the 5-loop spiral channel indicating the image capture windows. High speed images were captured at four different positions of this channel: inlet, two detection windows ( and ), and outlet. Figure 2b shows an SEM image of the fabricated PDMS mold showing the entrance region along with the 5-loops of the microchannel. This channel was used to study the behavior of 7.32 µm, 1.9 µm and 590 nm fluorescently labeled polystyrene particles. Specifically the experiments conducted were: (i) 590 nm suspension introduced through the inner inlet at 5 µl/min, 10 µl/min, and 20 µl/min, (ii) 1.9 µm suspension introduced through the inner inlet at 5 µl/min, 10 µl/min, and 20 µl/min, (iii) 7.32 µm suspension through the inner inlet at 5 µl/min and 10 µl/min (iv) 7.32 µm suspension through the outer inlet at 5 µl/min and 10 µl/min. These experiments were enough to show particle separations based on the added mass force and the centrifugal force. In the following sections the experimental results obtained in each of the four cases mentioned above have been presented and discussed in detail. In addition to the experimental data, results obtained from the models supporting the experimental data have also been presented. 4. RESULTS AND DISCUSSION Initially, a suspension of 590 nm particles was tested by introducing a concentration of 10 8 beads/ml through the inner inlet of the 5-loop microchannel and filtered DI water through the outer inlet, so as to achieve a step distribution of particles at the inlet of the spiral microchannel. As mentioned earlier, three flow rates were tested in this work, 5 µl/min, 10µL/min, and 20µL/min, and the fluorescence data was recorded at four different locations, inlet,, DW- 2 and outlet. Figure 3 shows dispersion of 590 nm particles across the microchannel width at 5µL/min, 10µL/min, and 20µL/min. V B C Fig 3. Experimental results indicating the normalized particle dispersion of 590 nm particles across the microchannel width for (a) 5 µl/min, (b) 10 µl/min, and (c) 20 µl/min flows. Proc. of SPIE Vol M-6

7 B Fig 4. (a) Simulation results indicating the 590 nm particle distribution across the channel width in the spiral channel at 10 µl/min. (b) Experimental result showing the 590 nm particle distribution (fluorescent stream) across the channel width at the outlet. Microchannel walls are indicated by bright yellow lines. From Figure 3a, at 5 µl/min (U = 33 mm/s) the 590 nm particles are uniformly distributed across the width of the channel at the outlet suggesting that the centrifugal force acting on the particle is not sufficient to move all the particles towards the outer half of the channel. On the other hand, when the flow rate was increased to 10 µl/min (U = 66 mm/s), the particles experience a greater centrifugal force and almost 95 % of the particles migrated from the inner half at the inlet to the outer half by the time the flow reached the outlet (Figure 3b). Increasing flow to 20 µl/min (U = 130 mm/s) resulted in a complete transfer of particles to the outer half in the second loop itself. However, higher velocities result in stronger Dean vortices (De = 5) which negated the particle separation and caused mixing as shown in Figure 3c. This is evident by the fact that although all the particles migrate to the outer channel by, due to Dean flows they migrate towards the inner half by the outlet position. The experimental results obtained were also verified by comparing them with the modeled results. Figure 4 shows the modeling result and a corresponding stacked fluorescent image obtained experimentally for 10 µl/min flows. As seen in the figure, there is good agreement between the experimental and simulation results with respect to the position of the bulk particle density across the microchannel width. In both the figures, one can see that the particles have moved towards the outer wall at the outlet. Similar to 590 nm, next a 1.9 µm particle suspension with a concentration of (10 7 beads/ml) was introduced through the inner inlet of the channel. The suspension was introduced at different velocities (5 µl/min, 10 µl/min, and 20 µl/min) and the emitted fluorescent intensity data was recorded and plotted as shown in Figure 5. From Figure 5a, at 5 µl/min (U = 33 mm/s) the 1.9µm particles are uniformly distributed across the width of the channel at the outlet suggesting that the centrifugal force acting on the particle is not sufficient to move all the particles towards the outer half of the channel. On the other hand an increased flow rate of 10 µl/min (U = 66mm/s), resulted in the migration of a significant number of particles (~95 %) from the inner half of the channel at the inlet to the outer half at the outlet (Figure 5b). Increasing flow to 20 µl/min (U = 130 mm/s) once again, resulted in a complete transfer of all the particles to the outer half in the second loop itself. However, as with 590nm particles, higher velocities resulted in stronger Dean Vortices which negated the particle migration and enhanced particle mixing as shown in Figure 5c. In this case a 7.32µm particle suspension with a particle concentration of (10 6 beads/ml) was introduced through the inner inlet of the channel. Again, fluids were pumped in at flow rates of 5 µl/min, 10 µl/min and 20 µl/min and the emitted fluorescent intensity was recorded and plotted against the channel width as shown in Figure 6. The 7.32 µm particles tend to form a narrow band towards the inner wall for all the velocities, unlike the other two particle sizes Proc. of SPIE Vol M-7

8 V B C Fig 5. Experimental results indicating the normalized particle dispersion of the 1.9µm particles across the microchannel width for (a) 5 µl/min, (b) 10 µl/min, and (c) 20 µl/min flows. discussed. This behavior of the 7.32 µm particles can be primarily attributed to the dominant added mass and pressure gradient forces. As mentioned previously, both these forces are greatly dependant on the mass of the fluid displaced by a particle (dependant on particle volume) which is significant in the case of 7.32 µm particle. These forces tend to move the particle in a direction opposite to the one in which its accelerated. Thus the particles move closer to the channel s inner walls. Further, the particles dont tend to mix at 20 µl/min as in the previous cases. This is because the Dean vortices (De = 5) are not strong enough to re-circulate the bulky 7.32 µm particles. Hence better results are obtained at 20 µl/min. Once again the experimental results were verified using numerical methods and the corresponding images are presented in Figure 7. A B C Fig 6. Experimental results indicating the normalized particle dispersion for 7.32µm particles across the microchannel width for (a) 5µL/m, (b) 10µL/m, and (c) 20µL/m flows. Proc. of SPIE Vol M-8

9 f/f/f x\ III,' I I H V\\X. -I =- /1//I yj,j/i B Fig 7. (a) Simulation results indicating the 7.32 µm particle distribution across the channel width in the spiral channel with a zoomed in picture at the inlet and outlet of the channel. (a) Experimental result showing the 7.32 µm particle distribution across the channel width at the outlet. In order to further verify the effect of the added mass and pressure gradient forces, the 7.32 µm particle suspension was introduced through the outer inlet of the spiral channel at flow rate of 10 µl/min. As expected the particles migrated towards the inner wall of the spiral channel by the time they reached the outlet as shown in Figure 8. These results clearly indicate that a mixture of 7.32 µm and 590 nm or a mixture of 7.32 µm and 1.9 µm particles can be separated with an input fluid flow of 10 µl/min. In both cases, the 7.32 µm particles will tend to move towards the inner wall and the smaller 590 nm or 1.9 µm particles will move towards the outer wall. Even better separation may be obtained by introducing the mixture at the outer inlet as both 590 nm and 1.9 µm particles will tend to stay in the outer half while 7.32 µm particles will move towards the inner half of the channel. Fig 8. Experimental results indicating normalized particle dispersion for 7.32µm particles across the microchannel width for 10µL/m. Proc. of SPIE Vol M-9

10 Although the particles of different sizes used in this work were labeled with different fluorescent dyes (Dragon Green and Sun Coast Yellow), the experiments could not be carried out with a mixture of particles as the fluorescence from the larger particles introduced significant background noise in the intensity measurement of smaller particles. Hence to eliminate this error and minimize cross-talk, individual particle suspensions were used for testing and the results obtained were analyzed to conclude the degree of separation. 4. CONCLUSION In this paper, design of a planar passive spiral microchannel capable of separating particle mixture was introduced. Particle-based flow modeling was developed using the CFD-ACE+ software to validate and understand the forces influencing particle in a suspension. The developed design is planar and thus has simple fabrication. The experimental results showed good agreement with the simulations. A 5-loop spiral microchannel was used to investigate the behavior of 7.32 µm, 1.9 µm and 590 nm particles at different flow rates between 1-20 µl/min. Particle position was quantified using the novel technique presented in this work. Finally, this paper demonstrates that a 5-loop spiral can be used to effectively separate nanoparticles from a micro/nano particle mixture. Future work will involve the development of a better design for separating particles in close size ranges. ACKNOWLEDGEMENTS This work was supported by the University of Cincinnati Institute for Nanoscale Science and Technology. REFERENCES 1. C. Lam, et al., Crit. Rev. Toxicol., 2006, 36, S. F. Sweeney, et al., J. Am. Chem. Soc. 2006, 128, J. C. Giddings, Field-flow fractionation: analysis of macromolecular, colloidal, and particulate materials, Science, vol. 260, pp , W-S Kim, Y. H. Park, J. Y. Shin, D. W. Lee, and S. Lee, Size determination of diesel soot particles using flow and sedimentation field-flow fractionation, Anal. Chem., vol. 71, pp , T. L. Edwards, B. K. Gale, and A. B. Frazier, A microfabricated thermal field-flow fractionation fystem, Anal. Chem., vol. 74, pp , E. Chmela, R. Tijssen, M. T. Blom, H. J. G. E. Gardeniers, and A. van den Berg, A chip system for size separation of macromolecules and particles by hydrodynamic chromatography, Anal. Chem., vol. 74, pp , M. T. Blom, E. Chmela, R. E. Oosterbroek, R. Tijssen, and A. van den Berg, On-chip hydrodynamic chromatography separation and detection of nanoparticles and biomolecules, Anal. Chem., vol. 75, pp , M. Yamada, M. Nakashima and M. Seki, Pinched flow fractionation Continuous size separation of particles utilizing a laminar flow profile in a pinched microchannel, Anal. Chem., vol. 76, pp , J. Takagi, M. Yamada, M. Yasuda and M. Seki, Continuous particle separation in a microchannel having asymmetrically arranged multiple branches, Lab Chip, vol. 5, pp , Y. Jiang, A. Kummerow and M. Hansen, Preparative particle separation by continuous SPLITT fractionation, J. Microcolumn Sep., vol. 9, pp , C. B. Fuh and J. C. Giddings, Separation of submicron pharmaceutical emulsions with centrifugal split-flow thin (SPLITT) fractionation, J. Microcolumn Sep., vol. 9, pp , G-T. Wei, F-K. Liu, and C. R. C, Wang, Shape Separation of Nanometer Gold Particles by Size-Exclusion Chromatography, Anal. Chem., vol. 71, pp , W-M. Hwang, C-Y. Lee, D. W. Boo and J-G. Choi, Separation of nanoparticles in different sizes and compositions by capillary electrophoresis, Bull. Korean Chem. Soc., vol. 24, pp , M. Durr, J. Kentsch, T. Muller, T. Schnelle and M. Stelzle, Microdevices for manipulation and accumulation of micro- and nanoparticles by dielectrophoresis, Electrophoresis, vol. 24, pp , Y. L. Li and K. V. I. S. Kaler, Dielectrophoretic fluidic cell fractionation system, Anal. Chim. Acta., vol. 507, pp , Proc. of SPIE Vol M-10

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