Sensors and Actuators A 115 (2004) 508–515 Pressure barrier of capillary stop valves in micro sample separators Tzong-Shyng Leu∗ , Pei-Yu Chang Department of Aeronautics and Astranautics, National Cheng Kung University, No. 1 University Road, Tainan 701, Taiwan, ROC Received 22 September 2003; received in revised form 21 January 2004; accepted 1 February 2004 Available online 30 April 2004 Abstract This paper presents a simple and accurate method for characterizing pressure barrier of capillary stop valves inside micro sample separators based on active CD-like microfluidics platforms. The capillary stop valves and micro sample separator were fabricated on polycarbonate (PC) substrates by using hot embossing techniques. The capillary valves stop the flow of liquid inside the microfluidic devices using a capillary pressure barrier when the channel geometry changes abruptly. Experiments are performed by rotating micro sample separator on a platform. The liquid sample stopped inside the micro sample separator is expected to be segmented by using density gradient centrifugation. Various density segments of samples required for an analysis can be separated by simply rotating at different speeds. Design parameters of the capillary valves used in micro sample separator have been analyzed theoretically. It is shown that the pressure barrier of capillary valve design can be better predicted by using current modified 3-D meniscus model. Detailed theoretical analyses of valve behavior is presented and compared with experimental measurement. A model for the valve is then extracted that characterizes the valve performance for various parameters of capillary stop valves. © 2004 Elsevier B.V. All rights reserved. Keywords: Capillary stop valve; LabCD; Micro sample separator; Microfluidic control 1. Introduction The concept of a micro total analysis system, or a lab-on-a-chip [1] for integrated chemical and biochemical analysis or synthesis systems has caused a wide interest recently. An essential part in lab-on-a-chip devices is the ability to control the sample as it flows through the device. To fulfill this task, typical lab-on-a-chip devices have demonstrated controlling the liquid flow by using pumps, diaphragm valves or electrokinetic methods. These methods, however, either require both moving parts and an external actuation mechanism or are very sensitive to the physicochemical properties of the components and the presence of trapped air. These factors often complicate the implementation and integration of a device and cause an increase of cost in lab-on-a-chip devices. Current trends in lab-on-a-chip technology suggest a rapid growth in the need for high throughput, as well as cost-effective, diagnostic tools in biomedical applications. One of current technologies proven to be fast, disposable, and cheap is so-called LabCD [2], a lab-on-a-chip based on ∗ Corresponding author. Tel.: +886-6-275-7575/63638; fax: +886-6-238-9940. E-mail address: [email protected] (T.-S. Leu). 0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.02.036 CD-like technology and platform. Different from conventional lab-on-a-chip technology, an essential ingredient in LabCD devices is the ability to control the spatial and temporal position of the sample as it flows through the device with a CD-player like platform. LabCD devices use centrifugal force to drive fluid flow by simply rotating the devices on a platform and exploit the surface tension force to stop flows in microchannels. The principle of operation is based on pressure barrier that develops when cross-section or surface properties of the capillary changes abruptly. These valves have the advantage of not requiring any moving diaphragm. Microfluidic capillary-driven valves recently have attracted considerable attention [3–12] and present strong appeal for applications to various microfluidic systems. Several fluidic functions on LabCD have been demonstrated by using capillary valves. Virtanen [13] have presented an on-chip technique to meter discrete nanoliter-sized liquid inside microchannel by using micropipette principle. Liquid flow inside micron-sized channels on LabCD platform can be controlled by manipulating the centrifugal force and the capillary force. A sudden expansion (or contraction) channel, called capillary stop valve, is usually used to stop the fluid by changing the channel geometry. When rotational speed increases, centrifugal force becomes larger than capillary force. Liquid starts to flow downstream. An angular T.-S. Leu, P.-Y. Chang / Sensors and Actuators A 115 (2004) 508–515 509 velocity ω called burst frequency [14] is given if centrifugal force is equal to capillary force as ω= γ cos θc (π2 ρRRdH )1/2 where ω is the angular velocity of CD platform, ␥ the surface tension force per unit length, θ c the contact angle, ρ the density of liquid, R equals to R2 − R1 , the distance difference from the center of CD to the capillary stop (R2 ) and reservoir (R1 ), R equals to (R2 + R1 )/2, and dH is the hydrodynamic diameter of the channel. When angular velocity is smaller than burst frequency, liquid will first stop at capillary stops of the micropipettes. Therefore, liquid sample will first be metered inside a number of micropipettes and excessive liquid overflows into a bypass chamber. 2. Design, fabrication, and experimental setup In this paper, we designed and fabricated several passive capillary stop valves that exploit an easy and accurate analytical model for characterizing pressure barrier of capillary stop valves using the measurement of pressure barrier on a LabCD platform. The capillary stop valves will be applied to a micro sample separator. For sample separation methods based on density, the sample suspension is metered and layered over a density barrier medium in a separation chamber followed by centrifugation. After centrifugal separation, low density and high density samples are separated above and below the density barrier medium. For conventional method, low density sample segment is collected by using a pipette. The proposed micro sample separator uses three overflow concentration chambers with capillary stop valves for continuous sample separation. For each overflow concentration chambers, one half of low density part sample is separated. After several overflow concentration chambers, concentrated low density sample is collected. The design of micro sample separator is shown in Fig. 1(a). Micro sample separator consists of a separation chamber with two reservoirs on upstream end and a capillary stop valve on the downstream end, and three overflow concentration chambers with capillary stop valves. The micro sample separator was fabricated on polycarbonate (PC) substrates by using hot embossing techniques. Detailed microfabrication processes are described elsewhere [14]. Fig. 1(b) shows the polycarbonate micro sample separator chip after hot embossing process. Experiments are performed by rotating micro sample separator on a platform. The experimental setup consists of a disk mounted on a dc motor manufactured by Hitachi company. The maximum speed of the motor can reach 4200 rpm. On the edge of rotational disk, a photo sensor is used to trigger a LED light for synchronized imaging. A CCD camera coupled to a microscope is used to aid in micro flow visualization. A motor controller is used to control rotational speed and the rotational speed of motor is monitored by a Fig. 1. (a) Sketch and (b) photograph of micro sample separator. digital counter (Autento APD-2P4R) for RPM display. Experiments are performed by rotating micro sample separator at a angular velocity larger than the burst frequency. Time evolution of the flow is observed using a microscope (Navita microscope) and a CCD camera (Toshiba IK-642F). The resulting image sequences are digitized by a frame grabber at 30 frames/s and flow visualization images are used to characterize the surface tension controlled flow phenomena in the microfluidic devices. At first, liquid will first stop inside the separation chamber and be segmented using density gradient centrifugation. As rotational speed increases, valve 1 opens and densitygradient fluid flow into concentration chamber 1 and stops at valve 2. Various densities of samples required for an analysis can be separated by simply rotating at different speeds. For example lower density fluid will be flow into concentration chamber 2 when the rotational speed is further increased. 510 T.-S. Leu, P.-Y. Chang / Sensors and Actuators A 115 (2004) 508–515 3. Theoretical analysis of capillary stop valves The specific problem that we analyze in this paper is the pressure barrier of fluid using capillary stop valves driven by centrifugal force. The overall objective is to demonstrate how to design these valves. For effective implementation, the physics of capillary stop valves has to be understood, including the design parameters that it is sensitive to. These design parameters are difficult to explore experimentally, theoretical analytical tools, on the other hand, provide an effective mechanism, and is the focus of this paper. For better characterizing capillary stop valves, a special capillary stop valve chip was fabricated on polycarbonate substrates by using hot embossing techniques, as shown in Fig. 2(a). On the chip, capillary stop valve was formed within a region of the microchannel with horizontal sudden expansion at an angle β. The close-up sketch of liquid front stopped at the capillary stop valve region was shown in Fig. 2(b). For a typical capillary stop valve, the width and height of the microchannels are 140 and 30 m, respectively. The aspect ratio (AR) of the microchannel is only 0.214 Two-dimensional (2-D) meniscus model proposed by Man et al. [12] is no longer valid in such low aspect ratio microchannel. Three-dimensional (3-D) meniscus effects have to take into consideration even the capillary junctions expand two-dimensionally. 3.1. Pressure barrier calculation by using 3-D meniscus model From the energy perspective, the pressure barrier caused by surface tension can be explained in terms of energy changes in a liquid–solid–gas interface system. The total interfacial energy UT of the system is UT = Asl γsl + Asa γsa + Ala γla (1) where Asl , Asa , and Ala , are solid–liquid, solid–air, and liquid–air interface area and γ sl , γ sa , and γ la its corresponding surface tension forces per unit length. The surface tension forces per unit length are related to equilibrium contact angle θ c by Young equation γsa = γsl + γla cos θc (2) Therefore, using Eq. (2) in (1) yields the reduced expression UT = (Asl + Asa )γsa − Asl γla cos θc + Ala γla = U0 − Asl γla cos θc + Ala γla = U0 + U Fig. 2. (a) Picture of a micro capillary stop valve experimental chip. It consists of reservoir (i), microchannel (ii), capillary stop valve (iii), and collector (iv). (b) Close-up sketch of capillary stop valve region. (3) where U0 (=γla (Asl + Asa )) is a constant since Asl + Asa , remain invariant and U is defined as Ala γla − Asl γla cos θc , which is variant part of the total interfacial energy UT . The total interfacial energy UT is a function of the liquid volume Vl . When Vl increases, the wetted area changes. The effective pressure P applied on the fluid column can be deduced from derivative of total interfacial energy UT of the system with respect to liquid volume Vl . P = − dUT dU dAsl dAla =− = γla cos θc − dVl dVl dVl dVl (4) For better predict pressure barrier of a capillary stop valve, 2-D meniscus analysis reported by Man et al. [12] has been improved to take 3-D meniscus effects into analysis. In 2-D meniscus analysis, meniscus is assumed to be a circular arc of angle in only horizontal direction. But in 3-D meniscus analysis, meniscus shape is assumed to be two circular arcs of angles in both horizontal and vertical directions. The circular arc angles, as shown in Fig. 2(b), are denoted as 2ah in horizontal direction and 2av in vertical direction. When a liquid sample is first introduced in the reservoir (i) in T.-S. Leu, P.-Y. Chang / Sensors and Actuators A 115 (2004) 508–515 511 Fig. 2(a), it wets in the straight microchannel region (ii) in Fig. 2(a) and stops at the edge of a sudden expansion (iii) in Fig. 2(a). At this point, the liquid can be moved forward only if an external pressure is applied. The pressure needed for moving liquid forward is defined as the pressure barrier Ps of capillary stop valve. During fluid moving forward at the capillary valve, liquid meniscus must change angle to meet the equilibrium contact angle at the slanted walls. In this regime, liquid volume increases through a change in aH and av but the meniscus position remains at x = L position. Therefore, total interfacial energy UT becomes ∗ UT = U0 − γla cos θc 2L(w + h) w2 − 2 sin αh αh − cos αh sin αh whαh αv sin αh sin αv liquid volume Vl is Fig. 3. Interfacial energy vs. sample volume in the capillary system. (5) αh − cos αh sin αh αv wh2 αh − − cos αv 4 sin αv sin αh sin αv w2 h Vl = wLh − 4 sin αh (6) where π/2 − θc ≥ αh ≥ π/2 − θc − β, π/2 − θc ≥ αv ≥ θc − π/2. Fig. 2(b) shows the design parameters of a capillary stop valve including: (1) channel height h, (2) channel width w, and (3) expansion angle β. These design parameters of a capillary valve will be analyzed theoretically and compared with experimental results in the following section. 4. Results and discussion In 3-D meniscus model, both horizontal and vertical meniscus circular arc angles are considered in the analysis. By first plotting interfacial energy U (= UT − U0 ) as function of liquid volume Vl , the effective pressure P of the capillary system can be easily calculated derivative dUT /dVl (=dU/dVl ), by using Eq. (4). Figs. 3 and 4 show the system interfacial energy U and pressure P as function of liquid volume Vl inside a microchannel of height h = 30 m, width w = 140 m with different β expansion angles. Initially, the energy decreases as the liquid sips into the microchannel with a fixed slope until the capillary stop region is reached. Beyond the stop outer edge the meniscus must expand. This expansion requires external energy, therefore, a pressure barrier Ps (shown in Fig. 4) develops that stops the flow. Fig. 4. Pressure vs. sample volume in the capillary system. is also performed for the comparison with 3-D meniscus analytical and experimental results. In Fig. 5, it is found that 2-D meniscus analysis reported by Man et al. [12] did not well predict current experimental results. 3-D meniscus analysis is found to better predict the experimental results. The pressure barriers predicted with 3-D meniscus model 0 -10 -20 ∆ Ps (KPa) + γla -30 -40 -50 Experimental data 2D model by P.F.Man[12] 3D meniscus model -60 4.1. Effects of expansion angle β The results of β expansion angle effects were shown in Fig. 5. 2-D meniscus analysis reported by Man et al. [12] 20 30 40 50 60 70 80 90 100 Expansion angle β (degree) Fig. 5. Pressure barrier vs. expansion angle β of the capillary stop valve. 512 T.-S. Leu, P.-Y. Chang / Sensors and Actuators A 115 (2004) 508–515 match the experimental data until β is 50◦ . It is also noticed the prediction becomes overestimated when β is larger than 50◦ since current simple 3-D meniscus analysis does not have good enough modeling for the meniscus shapes at four corners of the capillary junction. 20um β=30 o 20um β=60 o 20um β=90 o 40um β=30 o 40um β=60 o 40um β=90 o 30um β=30 o 30um β=60 o 30um β=90 o 0 -10 In this section, aspect ratio effects of a channel are investigated. The aspect ratio is defined as the channel height to channel width ratio in the current analysis. Fig. 6 shows the pressure barrier versus aspect ratio parameter AR of capillary stop valves. For the case of aspect ratio smaller than 1 (AR < 1) in Fig. 6(a), the channel height h is 30 m and channel width w is increasing from 30 to 300 m. For the case of aspect ratio larger than 1 (AR > 1) in Fig. 6(b), the channel width w is 30 m and channel height h is increasing from 30 to 300 m. For both capillary stop valve with AR < 1 and AR > 1 channel, the highest pressure barrier (define as the maximum absolute value of Ps since Ps is a negative value) happens at AR = 1 case. This is because of AR = 1 channel have smallest cross-section area (or hydraulic diameter) for both AR < 1 and AR > 1 cases. For AR < 1 case in Fig. 6(a), pressure barrier (Ps ) changes linearly from AR = 1 to AR < 1. However, the change of pressure barrier (Ps ) becomes nonlinear for AR > 1 cases in Fig. 6(b). Aspect ratio parameters of a capillary stop valve have dramatically different pressure barrier behavior in AR < 1 and AR > 1 cases. For AR < 1 case, the width of the channel increases while the height of the channel is kept at a constant. Because the channel are horizontally expanded with -50 -60 -70 -80 0 0.2 0.4 0.6 0.8 1 AR Fig. 7. Pressure barrier vs. aspect ratio (AR) of the capillary stop valve at different channel heights h and expansion angles β. different angle β at the capillary stop valve position, channel width is not the major section of perimeter related to the horizontal sudden expansion of capillary stop valve. Therefore, it is reasonable that a simple linear relationship between AR and Ps was found in the analysis. Fig. 7 shows the plot of linear relationship between Ps and AR for different channel heights h and expansion angles β. In Fig. 7, we can conclude two linear relationships to collapse the data into three linear curves. The first linear relationship is the same channel height. For example channel height h = 20 m case can be formulated as Ps-AR-h20 = mβ (1 − AR) + Ps-w20 h20 (7) -20 -30 s ∆P ( KPa ) -10 -40 where Ps-AR-h20 is defined as the pressure barrier of channel with aspect ratio AR and height h = 20 m. The mβ parameters can be found from the analytical results in Fig. 7 and shown as a function of expansion angle β of a capillary stop valve. β=30 degree β=60 degree β=90 degree 0 -30 s 4.2. Effects of aspect ratio (AR) ∆P (KPa) -20 -40 β = 30◦ β = 60◦ β = 90◦ -50 -60 0 0.2 0.4 (a) 0.6 0.8 1 AR 0 -20 -30 s ∆P (KPa) -10 -40 β=30 degree β=60degree β=90 degree -50 -60 (b) mβ −17.6244 −51.24 −64.11 0 2 4 AR 6 8 10 Fig. 6. Pressure barrier vs. aspect ratio (AR) of the capillary stop valve (a) AR < 1 (b) AR > 1. For the same aspect ratio, but different channel height, we can conclude the data can be collapse into one linear curve for each expansion angle by Eq. (8) 20 Ps-AR-h = × Ps-AR-h20 (8) h where Ps-AR-h20 is defined as the pressure barrier of channel with height h = 20 m. Ps-AR-h is the pressure barrier of channel with height h and the same aspect ratio as Ps-AR-h . By using Eq. (8), Fig. 7 data can be plotted again in Fig. 8. For the same aspect ratio channel, Fig. 8 shows the pressure barrier of capillary valve can be simply scaled by channel height by using Eq. (8). T.-S. Leu, P.-Y. Chang / Sensors and Actuators A 115 (2004) 508–515 o o o 30um β=60 o 30um β=90 20um β=60 20um β=90 0 o 30um β=30 20um β=30 40µm β=30 o o 40µm β=60 o 40µm β=90 o -10 ∆Ps-AR-h (KPa) -20 -30 -40 -50 -60 -70 -80 0 0.2 0.4 0.6 0.8 1 AR Fig. 8. Pressure barrier vs. aspect ratio (AR) of the capillary stop valves at different channel heights h can be collapsed into one linear curve for each expansion angles β. U with respect to sample volume Vl dAsl dUT dU dAla P = − =− = γla cos θc − dVl dVl dVl dVl 513 (4) In a straight microchannel, P is positive which indicates the surface tension drives fluid and flow is moving forward. As soon as the cross-section of the microchannel expands abruptly, P becomes negative for a hydrophilic surface. In this situation, surface tension turns out to be a retarded force that stops the fluid moving forward. Pressure barrier Ps develops and reverse pressure gradient happens at the capillary stop valve. From the plots of U and P (=−dU/dVl ) in Figs. 3 and 4, the maximum pressure barrier Ps happens at the point when the pressure barrier of capillary stop valve breaks and the fluid is moving forward under external pressure force. At the point, the shape of meniscus needs to satisfy the boundary condition for the fluid to move into expansion region. Therefore, to find the pressure barrier of Eq. (4) can be reformulated as dU dU dx U2 − U1 x Ps = − = = (9) dVl dx dVl x Vl2 − Vl1 Current hot embossing fabrication methods in LabCD technology suggest most of LabCD device be less-than 1 aspect ratio (AR < 1) channel with horizontal expansion geometry capillary valves design since channel structures have difficulties to achieve high aspect or vertical expansion by using hot embossing technology. Other technology for high aspect ratio channel structure, such LIGA or LIGA-like technology, is not cost-effective in batch fabrication of LabCD diagnostic biomedical devices. Current analysis results found to be able to fast and accurately predict a AR < 1 capillary stop valve with horizontal expansion angle β are described as the following steps: where subscript 1 and 2 means the physical properties just before and after the fluid moves into expansion region. For interfical energy U and sample volume Vl , one can easily derive the U1 and U2 , as well as sample volume Vl1 and Vl2 just before and after the transition point. Finally, Eq. (9) becomes (1) For any AR < 1 capillary stop valve, the pressure barrier Ps-AR-h with any channel height h and aspect ratio AR can be calculated by finding its corresponding pressure barrier Ps-AR-h20 , the same aspect ratio AR but the channel height h is equal to 20 m, by using Eq. (8). (10) 20 Ps-AR-h20 h (2) From the existing database for Ps-w20 h20 , pressure barrier (Ps-AR-h20 ) with the different aspect ratio, but the same channel height can be easily calculated by using Eq. (7). Ps-AR-h = Ps-AR-h20 = mβ (1 − AR) + Ps-w20 h20 4.3. Analytical solution for pressure barrier at expansion angle β = 90◦ From Eq. (4), we can calculate the surface tension driven pressure P by finding the derivative of interfical energy Ps = 2γla [(cos θc w/cos β) + h + (h tan β/sin αh )((αh /sin αh ) − cos αh ) − (αh αv w tan β/sin αh sin αv )] w[h − (h tan β/sin αh )((αh /sin αh ) − cos αh ) − (w tan βαh /2 sin αh sin αv ) × ((αv /sin αv ) − cos αv )] Eq. (10) shows the analytical solution for the maximum pressure barrier valid when expansion angle β = 90◦ . The analytical solution is proven to be the same as the results of 3-D meniscus model analysis under the condition at expansion angle β = 90◦ . For expansion angle less than 90◦ (β < 90◦ ), the 3-D meniscus model analysis provide more accurate solution to the experimental results than the analytical solution of Eq. (10). 4.4. Test of micro sample separator Finally, micro sample separator is put to test while rotational speeds increase from 0 to 1050 rpm. The designed burst frequency for capillary stop valves 1, 2, and 3, as shown in Fig. 9(a), are w1 = 640, w2 = 870, and w3 = 1020 rpm, respectively. At first, liquid stopped (w < w1 ) inside the separation chamber can be segmented using density gradient centrifugation. As rotational speeds increase to 640 rpm 514 T.-S. Leu, P.-Y. Chang / Sensors and Actuators A 115 (2004) 508–515 Fig. 9. Micro sample separator is tested at different rotational speeds. (a) Dry chip, (b) 640 rpm, (c) 900 rpm, (d) 1050 rpm. (w1 ≤ w < w2 ), 900 rpm (w2 ≤ w < w3 ) and 1050 rpm (w3 ≤ w), valves 1, 2, and 3 open sequentially and separate fluid flow into concentration chamber 1, 2, and 3 based on the density gradient, as shown in Fig. 9(b–d). 5. Conclusions We have derived a modified 3-D meniscus model, based on the 2-D model proposed by Man et al. [12]. The new 3-D meniscus model can better predict the pressure barriers of capillary stop valves for the micro sample separator. The results can also provide a simple design tool for capillary stop valves with any channel height h and aspect ratio AR as long as the aspect ratio AR is smaller than 1. An analytical solution for the maximum pressure barrier is also provided for LabCD designers. Finally, micro sample separator is tested to demonstrate the design of capillary stop valves at different burst frequencies successfully. Acknowledgements The project funding was provided by NSC Taiwan under the contract of “Development of MEMS-based Lab On A CD Technology for Microfluidic Chip Applications” (NSC-91-2212-E-006-138). References [1] D.J. Harrison, A. van den Berg (Eds.), Micro Total Analysis System’98, in: Proceedings of the TAS’98 Workshop, Banff, Canada, 1998, pp. 1–481. [2] G. Ekstrand, C. Holmquist, A.E. Örlefors, B. Hellman, A. Larsson, P. Andersson, Microfluidics in a rotating CD, Micro Total Analysis Systems, Kluwer Academic, The Netherlands, 2000, pp. 311–314. [3] T. Brenner, R. Zengerle, J. Ducrée, A flow-switch based on coriolis force, in: Proceedings of the TAS, Lake Tahoe, USA, 2003, pp. 903–906. [4] J. Ducrée, T. Brenner, R. Zengerle, A coriolis-based split-andrecombine laminator for ultrafast mixing on rotating disks, in: Proceedings of the TAS, Lake Tahoe, USA, 2003, pp. 603–606. [5] J. Ducrée, T. Brenner, T. Glatzel, R. Zengerle, coriolis-induced switching and mixing of laminar flows in microchannels, in: Proceedings of the 2nd VDE World Microtechnologies Congress (MICRO.tec 2003), Munich, Germany, October 2003, pp. 397–404. [6] T. Brenner, M. Grumann, R. Zengerle, J. Ducrée, Microscopic characterization of flow patterns in rotating microchannels, in: Proceedings of the 2nd VDE World Microtechnologies Congress (MICRO.tec 2003), Munich, Germany, October 2003, pp. 171– 174. [7] M. Grumann, P. Schippers, M. Dobmeier, S. Häberle, A. Geipel, T. Brenner, R. Zengerle, J. Ducrée, Stacking of beads into monolayers T.-S. Leu, P.-Y. Chang / Sensors and Actuators A 115 (2004) 508–515 [8] [9] [10] [11] [12] [13] [14] by flow through flat microfluidic chambers, in: Proceedings of the 2nd VDE World Microtechnologies Congress (MICRO.tec 2003), Munich, Germany, October 2003, pp. 545–550. T. Brenner, M. Grumann, C. Beer, R. Zengerle, J. Ducree, Microscopic characterisation of flow patterns in rotating microchannels, in: Proceedings of the Micro.tec 2003, Munich, Germany, October 14–15, 2003, pp. 171–174. M.J. Madou, G.J. Kellogg, LabCD: a centrifuge-based microfluidic platform for diagnostics, in: Proceedings of the SPIE, vol. 3259, 1998, pp. 80–93. D.C. Duffy, Microfabricated centrifugal microfluidic systems: characterization and multiple enzymatic assays, Anal. Chem. 71 (20) (1999) 4669–4678. R.M. Moroney, A Passive Fluid Valve Element for a High-Density Chemical Synthesis Machine, MSM 98, Santa Clara, CA, 1998, pp. 526–529. P.F. Man, C.H. Mastrangelo, M.A. Burns, D.T. Burke, Microfabricated Capillary-Driven Stop Valve and Sample Injector, MEMS’98, 1998, pp. 45–50. J. Virtanen, Laboratory in a disk, United States Patent 6,030,581, 29 February 2000. M.J. Madou, L.J. Lee, K.W. Koelling, S. Daunert, S. Lai, C.G. Koh, Y.J, Juang, L. Yu, Y. Lu, Design and Fabrication of Polymer Microfluidic Platforms for Biomedical Applications, ANTEC-SPE 59th, vol. 3, 2001, pp. 2534–2538. 515 Biographies Tzong-Shyng Leu was born in 1963 in Taipei, Taiwan, ROC. He received the both BS and MS degrees in Department of Aeronautics and Astranautics from National Cheng Kung University, Taiwan in 1985 and 1989. In 1994, he received his PhD degree in mechanical engineering from the University of California, Los Angeles, USA. After graduated from UCLA, he became as a post doctoral scholar in UCLA. In 1995, he jointed David Sarnoff Research Center in New Jersey as a member of technical staff. During his 3-year stay at David Sarnoff Research Center, he worked on microsystem imaging by using X-ray microtomography techniques. In 1999, he jointed School of Mechanical and Production Engineering, Nanyang Technological University (NTU), Singapore. After staying with NTU, he has been an assistant professor with Department of Aeronautics and Astronautics, National Cheng Kung University, Taiwan since August, 1999. His interests are in the fields of micro thermal/fluidic system. He published more than 20 technical papers in MEMS, Bio-MEMS, and fluid mechanics related fields. Pei-Yu Chang received Master of Science degrees from Department of Aeronautics and Astranautics, National Cheng Kung University in June 2003. Her master research focused on the experimental study of surface tension control flow phenomenon in microfluidic devices.
© Copyright 2024