Patent Application
20090104689
The present invention relates to a microchip. More particularly, the present invention relates to a microchip for use in cytometry, velocimetry and cell sorting, comprising polyelectrolytic salt bridges.
Flow cytometry is a technology to measure some properties of cells as they move or flow in liquid suspension. The miniaturization of flow cytometry system for a point-of-care test (POCT) has great importance for not only cell biological research but also clinical uses, which include the stem cell separation from peripheral blood and control of the white blood cell (WBC) level for juvenile leukemia patients. In most flow cytometers, cells traveling in the interrogation region are detected by either optical or electrical method. Fluorescent activated cell sorting (FACS) adopts the former and Coulter counter is based on the latter28. There have been many reports on microchip-based flow cytometers employing both detection methods. For instance, Wolff et al. presented a highly integrated chip for high-throughput FACS21. Ayllife et al. reported chip-based electrical analyzer operated by microchannel impedance spectroscopy17. In spite of the significant advances reported on FACS on microchip16,22-26 and electrical counter18-20, flow cytometers on microchips are partially successful in practical applications yet.
Combining conventional FACS concept with microfluidic chip obviously aims miniaturization and simplification of the device doing similar jobs to those the currently commercialized large systems do. However, this idea has a few serious problems. Firstly, it requires cell modification by markers or antibodies, which may lead to alteration of the system under study. Secondly, optical parts can hardly be reduced to the size as small as the microchip itself. Even if fluidic parts become substantially small by introducing microfluidic technology, the whole system including other parts like the detection unit is still too large to be a practical device for POCT. Thirdly, the equipments for detection are rather expensive and complex to operate. Optical equipments are rarely as cheap as electronic devices and necessarily require fine alignment. Electrical detection method has been considered as an alternative to the optical technology in this regard. Electronic device is the most probable choice in terms of miniaturization, simplification and cost-effectiveness, assuming that it works as well as optical setup. But there are fundamental challenges for the electrical method to accomplish such good cytometric functions as the optical detection in FACS offers.
Theoretical calculation says that two electrodes should face to each other at opposite sides in the channel wall to obtain the best sensitivity and precision of impedance response18. There have been tried to overlay two glasses with planar metal band electrodes to be faced to each other20 and to electroplate conducting metals on two planar electrodes, between which there is a microchannel17. Unfortunately, those trials made only limited success because of the following reasons. First of all, fabrication of such electrodes was tricky. Even if those are made somehow, it is hard to guarantee the reproducible geometry and characteristics as electrodes. Another problem is related to the electrode material and the frequency applied. Since the electrical property of a cell membrane is close to that of a capacitor, the impedance signal should be inversely proportional to the frequency applied. This tells that lower frequency generates larger change in impedance. The most desirable frequency is zero, namely DC signal. However, metal electrodes are not compatible to DC or low frequency of electric potential bias. Since electric double layer and/or faradaic reaction on the metal electrode surfaces intervenes the circuit, the impedance changes due to the cells become insignificant. As long as conventional metal electrodes are used, it must choose AC input at a high frequency, which makes cell detection less sensitive. In addition, the data for cell size reportedly correlate with the velocity. That is why calibration process with respect to velocity was suggested for the estimation of cell size18,22.
In terms of cell sorting, there is one more challenge that should be addressed. In FACS, the moving cells are two dimensionally focused hydrodynamically so that the velocities are uniform within a limited error. That makes effective cell sorting possible in FACS. However one dimensional (horizontal) focusing on microfluidic chip cannot produce as good flow as generated in the conventional FACS system. Thus it is harder to obtain the uniform velocity of the cells on a microfluidic chip. The variation of velocity may generate pulses in wrong timing to push or pull the cell to be sorted. Considering the fact that the fast velocimetry of cells on microchips is one of the critical issues toward automatic cell-sorting on a microfluidic chip with high throughput, quick and accurate velocimetry of moving cells on the spot could help greatly in sorting cells in a flow cytometer.
A simple method previously reported for velocity measurement of flow cells is the video image velocimetry29, where the velocity is estimated by observing the displacement of the cell within a known time interval. However, it has limitations in accuracy and cost because the video frame interval obviously regulates its resolution. Shah convolution Fourier transform is another method to extract velocity information on microfluidic chip30. In this method, a mask with a periodic array of slits modulates the excitation beam in space and the cells underneath the mask undergo spatially modulated excitation. Fourier transforming the modulated fluorescent signals produces data containing velocity information. The mask with periodic slits can be replaced by a waveguide beam splitter for the purpose of integration on the microchip31. Another method for velocity measurement is to use a time interval of fluorescent peaks from two adjacent areas excited by acousto-optic modulator (AOM)22. Both Shah convolution Fourier transform and AOM methods increase the complexity of instrument and calculation, and have limitations in miniaturization because of the space for the optical system integrated.
In order to solve the above-identified disadvantages, there is provided fabrication and performance of a flow cytometric or velocimetric chip using polyelectrolytic salt bridge-based electrode (PSBE). The concept of salt bridge was not so common in microfluidic chip research. Khandurina and co-workers applied porous silicate film as a salt bridge for electrophoresis32. Y. Takamura et al.33 and A. Brask et al.34 developed the low-voltage cascade electro-osmotic pump based on salt bridges. However, polymer-based salt bridge has not been used as an electrode for the detection of moving cells. The PSBE can be easily fabricated at the microchannel walls and make it possible to implement DC impedance analysis. Furthermore, two pairs of the PSBEs separated by a fixed length in a microchannel provide the data revealing the velocity information of cells on the same chip. It is believed that PSBE can offer a new opportunity to accomplish both the size-selective detection and simultaneous velocimetry of cells flowing along a microchannel without large or complex peripheral setup.
According to a preferred embodiment of the present invention, there is provided a microchip, comprising: a) an inlet through which a sample solution is introduced; b) a micro-channel along which the sample solution moves; c) at least one outlet which discharges the sample solution passed through the micro-channel; and d) at least one electrode system comprising i) a first and a second polyelectrolytic salt bridges, each of which is oppositely connected to the micro-channel and ii) a first and a second reservoirs, each of which is connected to the first and the second polyelectrolytic salt bridges and houses an electrode and a standard electrolyte solution. In the microchip, the electrodes housed in the first and the second reservoirs are electrically connected to an impedance analyzer. Anions contained in the standard electrolyte solution move across the micro-channel from the first polyelectrolytic salt bridge to the second polyelectrolytic salt bridge, and the movement of the anions is interfered by the sample that passes through the micro-channel. The interference causes an impedance change, and the impedance change is detected by the impedance analyzer. Such an impedance change is dependent upon the size of the sample. As the size of the moving sample increases, the peak of the impedance change increases. Further, The microchip may comprises two electrode systems separated by a fixed length in the micro-channel to measure the velocity of the sample by combination of the fixed length with a time difference at which the impedance change has occurred at each of the two electrode systems.
According to another preferred embodiment of the present invention, there is provided a microchip, comprising: a) an inlet through which a sample solution containing two or more cells is introduced; b) a micro-channel along which the sample solution moves; c) two outlets which discharge the sample solution passed through the micro-channel, one being located on an extended line from the micro-channel and the other at out of the extended line; d) a first electrode system comprising i) a first and a second polyelectrolytic salt bridges, each of which is oppositely connected to the micro-channel and ii) a first and a second reservoirs, each of which is connected to the first and the second polyelectrolytic salt bridges and houses an electrode and a standard electrolyte solution; e) a second electrode system comprising i) a third and a fourth polyelectrolytic salt bridges, each of which is oppositely connected to the micro-channel and ii) a third and a fourth reservoirs, each of which is connected to the third and the fourth polyelectrolytic salt bridges and houses an electrode and a standard electrolyte solution; and f) a means for converting a moving direction of the sample solution. In the microchip, the cell of no concern is collected into the outlet located on an extended line from the microchannel and the cell of concern is into the outlet located at out of the extended line by the working of the means for converting a moving direction of the sample solution.
The fabrication technology for PSBE in μTAS was developed and the performance as a flow cytometry glass microchip was also evaluated. It was demonstrated that the developed PSBE could successfully substitute the metal electrode for the impedance analysis in microfluidic glass chip. The PSBEs were embedded on cytometry and velocimetry microchips, which were evaluated using both fluorescent microbeads and human blood cells. Test results show that screening rate over 3,000 samples s−1, measurement of cell velocity up to 100 mm s−1, and velocity-free classification by particle size are possible.
The PSBE of the present invention suggests many useful applications. For instance, electrochemical cells that need to be integrated on microchips or decoupler in chip-based electrophoresis for electrochemical detection will possibly enjoy this unit device. The PSBEs chip for both cell counting and velocimetry are practically suitable for miniaturization so as to be applicable to small-sized point of care testing (POCT) devices. The present technology will offer a chance toward future Microsystems for clinical uses, for example, a miniaturized stem cell collector.
In the following, the present invention will be more fully illustrated referring accompanied drawings.
As shown in
The impedance changes are occurred at both of the first electrode system (300a) and the second electrode system (300b), as illustrated in
v=d/ΔT=d/(T2−T1)
wherein, v represents the velocity of the moving sample, d represent the separated length between the first and the second electrode systems, and ΔT represents a time difference between a detection time (T2) at which the impedance change is detected by the second impedance analyzer and a detection time (T1) at which the impedance change is detected by the first impedance analyzer.
Preferably, the first outlet (303a) is located on an extended line from the micro-channel (302), and the second outlet (303b) is located at out of the extended line from the micro-channel (302). Among the cells contained in the sample solution, the cell of concern undergoes change of the moving direction, by the working of the pump connected to the connection port (310), and is collected into the second outlet (303b). The pump is not operated to the cell of no concern, and thus, the cell of no concern is collected into the first outlet (303a). As a result, the cell of concern is selectively sorted from the sample solution containing two or more cells. If necessary, the number of the outlets (303) can be adjusted to that of the cells. Further, various means for converting the moving direction of the sample solution (for example, a valve) can be adopted as an alternative to the pump.
According to the preferred embodiment of the present invention, a metal/metal salt electrode is preferable as an electrode. As used herein, “metal/metal salt electrode” means an electrode comprising a metal core onto which a metal salt is coated onto the surface thereof. In addition, as an anion, a chloride ion (CF) gives satisfactory result. The polyelectrolytic salt bridge can be made of various polyelectrolytics that do not interfere with the movement of anions, independently of pH. Preferably, the polyelectrolytic salt bridges have an increased capacity to hold the anions. According to the specific example of the present invention, poly-diallyldimethylammonium chloride was used as a material for the polyelectrolytic salt bridge. According to the preferred specific embodiment of the present invention, the polyelectrolytic salt bridge has a calabash shape and the narrow side of the calabash is designed to contact the microchannel. This is to increase the sensitivity of the impedance change. The contact area can be suitably chosen regarding cell size, the anions to be used, the amplitude of the impedance peak, and so on. Further, between the polyelectrolytic salt bridge and the reservoir, a buffering region (denoted as 311a, 311b of
Microchip Fabrication
Corning 2947 precleaned slide glasses (75 mm by 25 mm, 1 mm thick) were used as substrates. A slide glass was cleaned in piranha solution (H2SO4:H2O2=3:1) for 1 h before washing the slide glass with deionized (DI) water (NANOpure Diamond, Barnstead, USA) and cleaned with acetone (CMOS grade, J. T. Baker, USA), methanol (CMOS grade, J. T. Baker, USA) and DI water twice sequentially. The cleaned slide glass was dehydrated on a 150° C. hot plate for 10 min and cooled down to room temperature. In order to modify the surface of the glass substrate, hydrophobic hexa methyl disilazane (HMDS) (Clariant, Switzerland) was spin-coated (Won corporation, Korea) at 4,000 rpm for 30 s on the slide glass, on which spin-coating of the photo resist (PR) of AZ5214-E (Clariant) was followed at 4,000 rpm for 30 s. After soft baking of photo-resist (PR) on a hot plate at 100° C. for 60 s, the slide glass was cooled down to room temperature and aligned under a pattern mask. Exposing to UV light (365 nm) with intensity of 16 mW cm−2 for 6.5 s (MDE-4000, Midas, Korea) was followed by developing the PR with AZ300MIF (Clariant) for 45 s. And then the slide glass was washed with DI water and the PR was hard-baked on a hot plate at 105° C. for 15 min. HMDS and PR layers, which were spin-coated and baked in the same way as described above, protected the other side of the slide glass from the etching solution. The slide glass was etched with 6:1 buffered oxide etch solution (J. T. Baker) for 40 min at 25° C. The washing processes consist of a few successive steps; rinsing with DI water and acetone, sonicating in acetone for 5 min by ultrasonic cleaner (3510E-DTH, Bransonic, USA), soaking in methanol and DI water. Another flat slide glass that is to cover the etched glass was drilled at the positions for reservoirs with a diamond drill with 2 mm in diameter at 18,000 rpm. And then the flat side slide glass was cleaned in piranha solution for 1 h. The pairs of etched and flat slide glasses were permanently attached by thermal bonding. When two slide glasses contact each other for bonding, DI water filled between the slides keeps away from air bubbles. The glasses were heated up to 600° C. in a furnace (CRF-M15, CEBER, Korea) and temperature was maintained at 600° C. for 6 h, which was followed by slowly cooling down the furnace to room temperature for 10 h.
Polyelectrolytic Salt Bridge (PSBE) Fabrication.
Diallyldimethylammonium chloride (DADMAC) was selected for the material of salt bridge. 65% DADMAC aqueous solution was polymerized to yield poly-DADMAC (PDADMAC) by shedding UV light in the presence of 2 wt % photoinitiator and 2 wt % cross linker. The high charge density makes PDADMAC hold many anions inside the polymer structure so that the transport of the mobile anions is facile and the apparent resistance of the polymer plug decreases. Moreover the stationary charge of PDADMAC is independent of pH in the medium. Thus PDADMAC possesses good properties for a salt bridge. DADMAC, photo-initiator (2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone), and cross-linker (N,N-Methylene-bisacrylamide) were purchased from Sigma-Aldrich (St. Louis, Mo., USA).
The salt bridge fabrication process using photopolymerization technique is as follows. The microchannel network of a microfluidic glass chip was filled with the DADMAC solution with the composition described above. The mask on the chip was aligned and subsequently exposed to UV light (365 nm) with intensity of 16 mW cm−2 for 5.0 s. DADMAC monomers were polymerized to form three-dimensional PDADMAC, filling the calabash-shaped region shown in
The Electrical Properties of the PSBEs.
The microchannel network of a microfluidic glass chip was filled with isotonic 0.92% NaCl solution. The two Ag/AgCl wires in corresponding reservoirs as shown in
Sample Preparation
The performance of the cytometry microchip with PSBEs was evaluated by the DC impedance analysis with fluorescent microbeads of 9.95□ (P(S/V-COOH), (480, 520), Bangs Laboratory, USA) and 5.70□ (P(S/5.5% DiVinylBenzene/5% MAA), (480, 520), Bangs Laboratory) in diameter. Fluorescent microbeads were diluted in isotonic NaCl solution to 0.025 wt % and 0.005 wt % for 9.95□ and 5.70□, respectively.
Red blood cells (RBC) and white blood cells (WBC) were sampled from a normal person and separated by centrifuge. RBC and WBC were diluted in RPMI 1640 medium (1×, Jeil biotechservices, Korea) to 0.0025 cells pL−1 before being used in the experiment.
Signal Detection and Data Acquisition
Diluted fluorescent microbead solution was injected into the microchannel using a syringe pump (KDS100, KD Scientific, USA).
For the velocity estimation of moving cells, two pairs of the PSBEs were fabricated with the separation of 1 mm, which is shown in
Detection of Moving Microbeads
Two adjacent pair of the salt bridge electrodes pair illustrated in
Counting Cells in Human Blood
The performance of the developed cytometry microchip was evaluated with RBC and WBC from human blood sample. The size of the blood cell is distributed between 6-9□ and 12-18□ for RBC and WBC, respectively. According to the results from the experiments with microbeads, RBC and WBC should be able to be classified by the difference between their sizes.
The results from the experiments with human blood cells show that complete blood cell counting (CBC) with a hand-held device is possible on the spot. This means that the developed cytometry microchip is applicable to POCT type cell counter for many clinical applications including the WBC level control for juvenile leukemia patients.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2004-0097332 | Nov 2004 | KR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/KR05/03988 | 11/25/2005 | WO | 00 | 5/25/2007 |
