2015 XVIII AISEM Annual Conference
Thermally Actuatted Microfluidic System m for Lab on C Chip Applications Andleeb Zahra Centre for Life Nano Science @ Sapienza University of Rome, Istituto Italiano di Tecnologia Viale Regina Elena 291, 00161 - Rom me, Italy
[email protected] Abstract— In this paper, we present the design and b-on-Chip system fabrication of Polydimethylsiloxane based Lab useful for polymerase chain reaction (PCR) application. The ubstrate, taking proposed system is fabricated on glass su advantage of thermally conductive and electrrically insulating substrate. On one side of glass microfluidicc system having reaction chamber for PCR, microchannel for fluid handling c has been and two thermally actuated valves for fluid control made. On the other side of glass three thin film heaters are integrated, two heaters located under valves actuate the valves membrane and allow the chamber isolation and the middle heater is dedicated to PCR thermal cycle. The design of the physics, involved system, carried out by using COMSOL Multip the optimization of the system dimensions and a the shape of microchannel used to completely close the ch hannel. We have also study the thermal interaction between th hree heaters and optimize the design to minimize the interfereence. The system was fabricated and an experiment actuatingg the valves was performed verifying the closing of the microflu uidic channel.
Domenico Caputo, Augussto Nascetti, Giulia Petrucci, Nicola Lovecchio, Riccarrdo Scipinotti and Giampiero de Cesare C D.I.E.T & D.I.A.E.E, Saapienza University of Rome Rom me, Italy on different on and off chip fluid control approaches [7] [8] but the complexity of process and leakage of fluid limit their uses. We present the study and a the fabrication of a LoC for PCR applications. The stuudy involved the coupling of microfluidics structures withh thin film heaters on the same glass substrate. The fabricaation of the whole system was performed by using both thhick and thin film technology processes taking into account their compatibility requirements. II.
TIN OF THE SYSTEM DESCRIPT
The system is fabricated on a 5x5 cm2 glass substrate, on one side of glass microfluidiic device has been integrated. It consists of a microfluidic chaannel with inlet and outlet holes, two thermally actuated valvees and a reaction chamber where the biological solution can be confined and the PCR process can be perform. On the opposite side of glass substrate, three metal thin film heaters are present.
Keywords—Lab on chip ; PDMS; Microfluiidics; Thermmaly actuated valves.
I.
INTRODUCTION
Microfluidics devices have significant applications in Lab-on-Chip (LoC) system, for highly sensitive, s highthroughput and low cost biological analysiss [1] [2] [3] [4]. Among the different functionalities of microfluidics m on LoC, the Polymerase Chain Reaction (PCR R) procedure for the molecular amplification of DNA has reeceived a lot of attention. PCR is essential to all geenetic analysis applications of integrated microchips due to its capability of double strand DNA amplification [5] [6]]. The reaction proceeds in repeated thermal cycles with thhree temperature steps. The first step, at temperatures betweeen 94 - 96 ºC, denatures the DNA template strands; in thhe second step, typically at temperature in the range 45–60 ºC, the primers hybridize to their complementary sequencees on the parent strand; during the third temperature step, usually at 72 ºC, the DNA polymerase forms new daughter strrands, extending the primer sequences by adding individuaal dNTPs from solution. Repetition of the sequence therefoore generates 2n daughter strands, where n is the number of cyycles. Integration of fully integrated LoC micrrofluidic system for PCR is still a challenging task, due to lack of reliable components. Therefore, several research grouups are working
978-1-4799-8591-3/15/$31.00 ©2015 IEEE
Fig. 1. Top and cross- section vieew of device, the microchannel is open (a).When a voltage is applieed on the valves heaters, it increased pressure, induces the deformattion of the valves membrane and closed the channel (b).
2015 XVIII AISEM Annual Conference After filling the liquid from inlet holle, two heaters, located at the inlet and outlet sites, actuatee the valves by applying voltage and increasing the pressurre (temperature) inside the valves. The membrane of valves deform d up to the height of channel close the channel completeely and allow the chamber isolation. The third heater is alligned with the reaction chamber to perform the thermal cycling needed by the PCR process. In particular Figure 1(a) shhows the channel is open and the liquid is passing through the channel, c fig 1(b) shows the heaters under valves are on, it i increased the pressure, induces the membrane deformationn, and close the channel completely. III.
DESIGN OF MICROFLUIDIC SYSTEM
The design of the PDMS valves and of the microfluidic channel has been performed by developinng a COMSOL Multiphysics model. A two dimensional solid mechanics, linear elastic model of a single valve has beenn carried out. The conventional fabrication of microchaannel with SU-8 mold [9] leads to rectangular cross-section of channel, but the shape mismatches with the deformedd shape of the membrane of the microvalve resulting in a liiquid leakage. In particular fig. 2(a) shows the membrane deeformation with rectangular channel and the channel is not coompletely close. In order to solve this issue, the channel waas designed with round shape cross-section as shown in fig 2(bb). The design of the round-shaped microchannel has been optiimized changing the width of the channel and analyzing the maximum f of the deformation of the valve membrane as function temperature generated by the underneatth heater. The optimum design of the valve was obtained with a channel v 2 mm that, width around 500 µm and a diameter of the valve at a temperature around 90 °C, ensurees a maximum displacement of the membrane up to 50 µm. Figure 3 reported the height of the membrane deeformation as a function of temperature, the behavior is lineear up to 50 µm. At 90° C the membrane deformation is lim mited by the top surface of a 50 µm height microfluidic channnel.
Fig. 3. Height of membrane defformation as function of the heater temperature.
IV.
DESIGN AND FABB BRICATION OF THE HEATERS
The heaters are Cr/Al/Crr metal layer stack structure, the shapes were designed by ussing COMSOL Multiphysics, it couple the electrostatic probblem and heat transfer problem. The placing of the two valvves heaters on the 5x5cm2 glass substrate of the system was optimized in order to limit the temperature interference inside the PCR reaction chamber. mized to make the temperature The middle heater was optim distribution of the heated arrea uniform, as require by PCR thermal cycle [10].The optim mum distance of valves from the reaction chamber is 6 mm frrom both side of valves. In fig.4 the optimize dimensions andd geometries of three heaters are shown. Figure 5(a) shows thhe thermal distribution of three heaters when the valves heateers are at around 90 °C, which is the temperature require to close c the channel and the PCR heater at 95° C, which is the maximum temperature during PCR. Figure 5(b) shows the temperature along arc length of three heaters, it clearly showss the middle heater is at uniform temperature, when two valves heaters are on at 90° C.
Fig. 4. Optimum designing of thhree heaters. The measured value of resistance using multimeter for valve heater is 30 ohm and for chamber heater is 90 ohm. Fig. 2. Deformation of the valve membrane at 90 9 °C inside: (a) Rectangular shaped 50 µm height microchannel. (bb) Round shaped 50 µm height microchannel
978-1-4799-8591-3/15/$31.00 ©2015 IEEE
2015 XVIII AISEM Annual Conference
Fig. 6. (a) Device fabrication stepss (b) Microscopic image of round shape microchannel obtained after peeling p from mold (c) Complete device obtained after all technologicaal steps.
Fig. 5. (a) Thermal distribution of three heaterss (b) Temperature distribution along arc length of three heaters.
The fabrication of the heaters has been performed with the following technological steps: 1.
Deposition of Cr/Al/Cr (300 A°//1500A°/300A°) layers on glass substrate byy the vacuum evaporation system.
2.
metries by using Patterning of the heaters geom photolithography process of possitive photoresist AZ 1518. V.
FABBRICATION OF SYSTEEM
The fabrication of the system was perfo formed by using both thick and thin film technology proccess taking into account their compatibility requirements: 1.
Fabrication of AZ®40XT mold [11] for the microchannel and a SU-8 2050 moldd for the valves.
2.
Heating of the channel mold at 130 ºC º for 5 minutes to make it round-shaped.
3.
Mixing the elastomer and curing aggent of PDMS in the ratio of 5:1 and 20:1 for channnel and valves, respectively. Vacuum degassing of the mixtures for 30 minutes.
4.
Pouring of the PDMS (5:1) on thhe channel mold and baking in oven for 15 minutes. Spin coating of the PDMS (20:1) on the valves moldd.
978-1-4799-8591-3/15/$31.00 ©2015 IEEE
5.
Punching of the inleet and outlet holes in the channel and peeling it from its i mold.
6.
Bonding of the PDM MS channel on the PDMS valves and backing of the whole device in oven at 120 ºC for 45 minutes.
7.
Peeling of the micrrofluidic device and bonding on the opposite face of o the glass substrate with the heaters by using oxxygen plasma (20V, 50W for 30 seconds) and put thhe device overnight by applying, small pressure.
The fabrication stepps, schematically depicted in Fig. 6(a). Figure 6(b) is thee microscopic image of round shape channel obtain after peeling from mold. Figure 6(c) is shown compleete device having microfluidic structure on one side of glass g integrated with three metal heaters on other side of the glass substrate aligned with microfluidic structure. VI.
R RESULTS
The functionalities of o the fabricated device has been tested. The PDMS microflluidic channel was filled with bromine blue liquid in ordder to easily monitor the fluid position under the microscoope during the experiment. The temperature of the heater unnder the valves has been raised slowly by apply voltage and the behavior of single valve has been observed under the miccroscope. Figure 7 (a) shows the microfluidic system at room m temperature, when the channel is open completely, brominee blue liquid is passing through the channel. When, we staart applying voltage on valves heaters, temperature of valvves membrane increasing. It has been noticed the activation of o the valves start around 60°C and the channel closed com mpletely at 92°C, without any leakage of liquid. Figure 7(bb) shows the valve deformation block the liquid passage and close the channel.
2015 XVIII AISEM Annual Conference [3]
Fig. 7. Experimental results (a) Channel is open andd the bromine blue liquid is passing through the channel (b) Heater under u valve is on at 92 °C , it deform the membrane up to 50µm and cllosed the Channel.
The experimental results shows a very good control of liquid flow inside microchannel with therrmally actuated valves and a very good agreement with simulations. The experiment was repeated several times alwaays successfully, demonstrating the reliability of the system. VII. CONCLUSIONS The presented system shows the integratioon of membrane type valves with excellent control of liquid flow inside the microchannel. The design of the valves, channnel and heaters has been carried out with COMSOL L Multiphysics simulations. In order to avoid any leakage off liquid when the channel is closed, the channel was designned with round shape cross-section. The system was fabricateed by using both thick and thin film technologies proocess and its functionalities tested. It has been observed the complete closure of the channel when the valves are acctivated at 92°C, which is in good agreement with the resultts obtained with the simulations. We believe that the techniique reported in this work is very useful for various microfluiidic applications as fluid control inside microchannel. ACKNOWLEDGMENT Authors are highly thankful to Center for Life Nano Science@Sapienza University of Rome, Isttituto Italiano di Tecnologia (Rome, Italy) for the financial suupport. REFERENCES [1]
[2]
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G. de Cesare, A. Nascetti, R. Scipinotti , A. Zahra, D. Caputo,“Onchip detection performed by amorphous a silicon balanced photosensor for lab-on chip application””. Sensing and Bio-Sensing Research Volume 3, March (2015), pp 53–58. 5 [4] D. Caputo, G. de Cesare, A. Nascetti, M. Tucci, “Detailed study of amorphous silicon ultraviolet sensor with chromium silicide window D 55 (1) (2008) 452–456. layer”, IEEE Trans. Electron Devices [5] Zhi Qiang Niu,Wen Yuan Chhen, Shi Yi Shao, Xiao Yu Jia and Wei Ping Zhang “DNA amplification on a PDMS–glass hybrid microchip”, J. Micromech. Miicroeng. Vol.16, (2006) pp 425–433. [6] Petralia S, Verardo R, Klaric E, Cavallaro S, Alessi E, Schneider C “In-Check system: a highly inntegrated silicon lab-on-chip for sample preparation, PCR amplificatioon and microarray detection of nucleic acids directly from biologicaal samples”,.Sensors and Actuators B: Chemical, (2013) 187:99-105. [7] B. Bae, J. Han, R.I. Massel, M.A. Shannon, “A bidirectional electrostatic microvalve with microsecond switching performance”, Journal of Microelectromechannical. Systems, (2007), 16(6), 1461. [8] Kwang W. Oh and Chong Ahhn, "A Review of Microvalves", Journal of Micromechanics and Micrroengineering (JMM), Vol. 16, (2006) pp, R13-39. [9] M.B. Chan-Park, J. Zhang, Y. Yan, C.Y. Yue, “Fabrication of Large SU-8 mold with High Aspectt Ratio Microchannels by UV Exposure Dose Reduction,” Sensors and a Actuators B: Chemical, vol 101, (2004) pp 175-182, A Nascetti, and R. Scipinotti, “a-Si:H [10] D. Caputo, G. de Cesare, A. temperature sensor integratedd in a thin film heater,” Phys. Status Solidi, vol.A 207 (2010) no. 3, pp. 708–711,. [11] A.Zahra, G. de Cesare, D. Capputo, A. Nascetti, “Formation of Round Channel for Microfluidic Applications”, A International Journal of Electrical, Robotics, Electronnics and Communications Engineering Vol:8 ,(2014) No:7 pp. 945-9449
