Pressure-Sensor array Using as an Experiment Platform for Microfluidics

Pressure-Sensor array Using as an Experiment Platform for Microfluidics Hsin-Hsiung Wang, Po-Chiang Yang and Lung-Jieh Yang Department of Mechanical and Electro-Mechanical Engineering, Tamkang University #151, Ying-Chuan Rd., Tamsui, Taipei, 25137, Taiwan Phone: +886-2-26215656 ext. 2768, Fax: +886-2-26209750, E-mail: [email protected]

Abstract This paper describes a fabrication concept of combining the mature silicon bulk-micromachining and the low-temperature PDMS process to make the microfluidic system chip. The sensing elements for on-site measurement of pressure along the microfluidic system are assigned from the 3×3 pressure sensors chip with the dimension of 1.2cm×1.2cm. The flowing channels and the connected pressure cavities are patterned on PDMS (polymethylsiloxane) by the soft lithography, and bonded with the pressure sensor chip. An example of nonlinear pressure distribution along a strait microchannel with length of 5mm and width of 200µm was successfully demonstrated under the constant air flow rate of 6 ml/min. This concept of micro-sensor array chip has no entry-barrier of conventional silicon surface micromachining, and provides a common experiment platform for implementing various applicable microfluidic systems. Keywords: pressure sensor array, on-site pressure sensor, PDMS, microfluidic system 1 INTRODUCTION Although the first on-site measurement of the nonlinear pressure distribution along a micro-channel with the characteristic dimension of micrometers was first reported in Tranducers’93 [1], there is still plenty of room for improving related microfluidic issues. The most cumbersome difficulty is that we need many runs of the-state-of-art surface micromachining processes and spent many cost-ineffective efforts to purchase the few successful MEMS chips to operate the time-consuming micro-fluidic experiments. In other words, many expertise and talents were expensed on developing the MEMS chips, but not too much concorded experiment results were gotten in rewards [2-8]. The authors in this work, therefore, would like to develop a more concise processing or packaging methodology to encounter the problem mentioned above. The conceptual figure of the packaging idea for the smart microfluidic chips is shown in figure 1. We explore a reliable sensor-array chip as the common platform at first. Such a sensor-array chip should be designed and fabricated by the very mature, current technologies. Herein we choose the silicon bulk micromachinings and piezoresistive pressure sensor technology to meet the requirements of this research.

Besides the technology maturity and reliability, the delicate design of this kind of sensor-array chip should include the compatible considerations of post processing or packaging with the PDMS microfluidic caps before the practical measurements. A key concept for this methodology to work well is that the microfluidic layouts of microchannel branches occupied very small amount of chip area. Then as soon as we reserve and keep invariant of the chip area for pressure caivities and manifolds in advance, the microchannels with different confirmations can be developed as will on the rest portion of the common chip.

Figure 1. The conceptual configuration of packaging idea for the smart microfluidic chips.

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2 DEVICE FABRICATION The new processing sequence is described as follows. First we designed the “common” piezoresistive sensor-array chip and sent it to a MEMS company in Taiwan to provide foundry service of conventional silicon bulk micromachining. The fabricated piezoresistive 3×3 micro pressure sensor-array chip, anodically bonded with a Pyrex #7740 glass underneath, has the dimension of 1.2cm×1.2cm. The fabricated chip after wafer dicing is shown in figure 2. The bonding pads are arranged on the periphery area of the chip surrounding all the 3×3 micro-pressure-sensors on purpose for the PDMS microfluidic cap to be located on the main center portion of the sensor chip.

Figure 2. The fabricated 3×3 piezoresistive pressure sensor array chip. Each of the pressure sensors has the layout of 4-piezoresistors connecting a Wheatstone bridge. Figure 3 shows the calibration curves for 9 pressure sensors on the array chip. We supply a DC bias of 5V as the input voltage and set the range of the pressure loading from 0 psi to 300 psi. The calibration curves of figure 2 demonstrated the linear signal output with

the sensitivity of 200µV/5V/psi only, and the output variation of 9 sensors on the same array chip is about 15%. Such a poor performance of microsensors herein results from the non-uniform control of process in bulk micromachining, and it still needs great modification in the future. According to the calibration curves of figure 3, anyway, the linearity of the piezoresistive pressure sensors available on the “common” chip sustains within 1% FSO in a very repeatable way. 70.00

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The PDMS microfluidic caps on the upper portion of figure 1 are fabricated by the soft lithography with SU-8 resist as the production mould [9]. The spatial resolution of the soft lithography can be adjusted to micrometer fairly repeatably and the accuracy of the dimension of the microfluidic system can be guaranteed promisingly. The layouts of the microfluidic caps depend on the issues users got interested about. For example, we can fabricate a single microchannel with very high length-to-width ratio, or assign some orifices or blockages inside the channel. Using these designs with the critical dimension down to micrometer, a lot of interesting but fundamental phenomena in micro-scaled fluid mechanics can be studied in details.

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Figure 3. The calibration curves of the pressure sensor-array chip. We designed a patterning mask of the SU-8 original mold shown in figure 4(a). By using the sensors with the numbers of #1, #2, #3, #4, #5 and #6 in figure 2, which are located at upper and lower rows of the array chip, we supposed to measure the pressure distribution along the microchannel. The microchannel is assigned to be placed on the center row of the common chip with several connection manifolds. The width and length of the microchannel is 200 and 5000µm, and the inlet/ outlet areas are 800µm in square. The static pressures of different stations along the microchannel with working fluid passing through are by way of the 10µm wide tiny manifolds which are connected to the pressure sensing chambers recessed above the silicon piezoresistive pressure sensors in advance. The width (10µm) of the pressure manifold right aside the connection port should be so appropriate as to be larger than the mean free path (60nm for air at ambient) of working fluid in microchannel on the one hand, and to affect the main flow in the microchannel as slight as possible on the other hand. The interval between two measurement points is 1000µm as shown in figure 4(b). The process flow will be described as followed.

First, we use the photo mask like figure 4(a) to pattern the original SU-8 mold on the silicon substrate (figure 5(a)). Utilizing the syringe needle to define the inlet/ outlet of the microchannel, we spread PDMS gel on the original mold casting as the structure of the microchannel system (figure 5(b)). After baking at 150℃ with 10 minutes in oven, we draw or de-mould the PDMS cap with the microchannel pattern from the original SU-8 mold (figure 5(c)). We use O2 plasma to adjust the PDMS surface to be hydrophilic and good for bonding with the sensor-array chip (figure 5(d)). After aligning the PDMS cap with the sensor-array chip, we apply somewhat pressure on the device to enhance the bonding speed and strength (figure 5(e)).

Figure 4. The pattern of the SU-8 mold: (a)The top view of the microchannel and the pressure chamber; (b)The dimension of the microchannel.

Figure 5. The fabrication of the common chip: (a)Patterning the SU-8 mold on a substrate; (b)Coating the PDMS on the origin mold and defining the inlet/ outlet of the channel with a syringe needle; (c)Drawing the pattern from the SU-8 mold; (d)Using O2 plasma to make hydrophilic surface treatment on the PDMS channel pattern.; (e)Bonding the PDMS channel structure with the pressure sensor. The dimension of the common chip array is 1.2µm square. As shown in figures 1 and 2, the layout of this sensor-array chip is compact and the wire bonding pads are very close to the periphery rim on this chip. We must ensure that the bonding pads as well as the bonding wires will not be overspread by the PDMS cap. Therefore we novelly design a steel frame shown in figure 6 to define the configuration of the PDMS parts. This steel frame, with the area of 5 inches in square and the thickness of 10mm, is done by ordinary electrical discharging machining (EDM). We put the steel frame on the SU-8 mould chip and align the pattern on this chip. Then we can control the volume of PDMS gel very well by pipettes and pour the gel to the cubic cavities confined by the steel frame and got PDMS caps with precise contours. After appropriate baking, we draw the PDMS parts out from the steel frame, and wait for the final packaging step.

. 3 EXPERIMENT SETUP After integrating the “smart” array chip and the double-side printed circuit board (PCB), we proceed with the wire bonding for signal transmission. Each pressure sensor has four signal pads. Two of them are for supplying voltage bias and ground, and others are for collecting output signals.

Figure 6. The steel frame with length of 5 inch in square. 16 casting square cavities with the width of 1cm, depth of 1cm can be processed at the same time. Finally, the common sensor-array chip is bonded with the PDMS channel cap with hydrophilic surface treatment (O2 plasma sweeping) beforehand. Because the processing temperature is very near to the ambient temperature, the microsensors fabricated on the common chip can be preserved very well after the assembly with the PDMS cap. Figure 7 is a completed “smart” chip for microfluidic on-site measurement after bonding the common sensor-array chip with the PDMS cap. The microchannels are made at the middle row of the common chip and connect the inlet and outlet holes. We use the sensors #1 to #3 and #4 to #6, located at upper and lower row in the array chip, to measure the pressure distribution along the microchannel. Restated, the height (h), width (w) and length of the microchannel are 34, 200 and 5000µm respectively, and the hydraulic diameter (w⋅h/(w+h)) of the rectangular microchannel is calculated as 29µm. Moreover, the inlet/outlet areas are 800µm square, and the interval between the two measuring points along the microchannel is 1000µm.

We assign air as the working fluid in the on-site measurement and use a syringe pump as the gas pumping source to control the volume flow rate inside the microchannel. The “smart” sensor-array chip and the syringe pump are connected by a medical component so called “scalp vein sets”. We install another pressure gauge at the inlet of the microchannel to identify the inlet pressure for redundancy. The pressure sensor #3 measures the pressure of the inlet of the microchannel, and the sensor #6, #2, #5, #1 and #4 measure the downstream-wise pressure distribution in the microchannel. Pressure sensors #7, #8 and #9 are spared temporarily in this measurement. We apply a 5V DC bias and record the output voltage signals of all piezoresistive pressure sensors. The sketch of this experimental set is shown as figure 8.

Figure 8. The experimental setup with the common experiment chip performing the on-site measurement of pressure along a microchannel. We control the syringe pump to provide a constant volumetric flow rate of air, e.g. 6 ml/min herein, and keep the gas flow steady for 10 minutes. The output voltage is logged by computer and the interval between two measurement data is 500 ms.

Figure 7. The “smart” sensor-array chip after bonding with the PDMS cap. The two holes are the inlet and outlet ready for working fluids passing through.

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Figure 9. The location of the pressure sensors on the “smart” chip. The arrows indicate the flow direction. 4 RESULTS AND DISCUSSION In the on-site pressure experiment, we initially found that the outputs of the pressure sensors with the numbers of #5 and #6 are failed. Then the functional sensors now available are numbered as #3, #2, #1 and #4 downstream-wise in figure 9. The entrance pressure measured by the pressure gauge is increasing with time when we just start the syringe pump to supply the constant volume flow rate. After 3 minutes, the pressure is no longer increasing with time and the steady pressure is about 4 psi. Similarly at the very beginning of the measurement, the pressure sensors have the voltage outputs rising rapidly, too. But as we having waited for a few minutes, we then captured the steady voltage outputs. Experimentally, the changes of the voltage outputs subject to 5V DC bias are 0.74, 0.46, 0.18 and 0.02 mV for sensor #3, #2, #1, #4. Dividing the output voltages by the individual sensitivity of 0.19, 0.19, 0.22 and 0.21mV/5V/psi for each sensor respectively, we got the local pressure distribution as 3.9, 2.4, 0.8 and 0.1 psi finally. The experimental data with the theoretical prediction curve are plotted in figure 10. We could see that the flowing characteristics of the air flow under the hydraulic diameter of 58 µm herein seems no apparent difference compared to the macro-scale air flow in conventional hydrodynamics (Hagen-Poiseuille law) [10].

Figure 10. The experimental data and its theoretical prediction (Hagen-Poiseuille law) of the pressure distribution along a microchannel with the hydraulic diameter of 58µm and the volumetric flow rate of 6ml/min. 5 CONCLUSION Herein, we proposed the more simplified concept of on-site “smart chip” for microfluidics. The smart chip with 3×3 sensor-array was done by the conventional bulk- micromachinings, and integrated with PDMS fluidic caps afterwards. An example of a single rectangular microchannel with the hydraulic diameter of 29µm was measured its pressure distribution under the volumetric driving of 6 ml/min successfully. We believe that such a concept could be applied to many other microfluidic systems promisingly. ACKNOWLEDGEMENTS This work was financially supported by the National Science Council of Taiwan, Republic of China under the contract number of NSC 92-2212-E-032-004. The authors would like to thank Mr. Min-Shan Lee of Yu-Joh Corp. for his help on wire bonding. The service of EDM foundry provided by the Instrument & Experiment Center (IEC) of Tamkang University is also highly acknowledged. REFERENCES [1] J.Q. Liu, Y.C. Tai, K.C. Pong and C.M. Ho, “Micro-Machined channel / Pressure Sensor Systems for Micro Flow Studies”, Proc. of Transducer’93, pp.995-997. [2] S. Wu, J. Mai, Y. Zohar, Y. C. Tai and C. M. Ho, “A Suspended Microchannel with Integrated Temperature Sensors For High Pressure Flow Studies”, Proceeding of the 11TH IEEE MEMS, Jan 25-29, 1999.

[3] L. Jiang, et al., “Fabrication and characterization of a microsystem for a micro-scale heat transfer study”, Journal of Micromechanics & Microengineering, Vol. 9(4), pp.422-428, 1999. [4] Linan Jiang, Man Wong, Yitshak Zohar, “Phase change in microchannel heat sinks with integrated temperature sensors”, Journal of Microelectromechanical Systems, vol. 8, no. 4, pp. 358-365, 1999. [5] Linan Jiang, Man Wong, and Y. Zohar, “Unsteady characteristics of a thermal microsystem”, Sensors and Actuators A: Physical, vol. 82, pp.108-113, 2000. [6] Xinxin Li, Wing Yin Lee, Man Wong, Yitshak Zohar, “Gas flow in constriction microdevices”, Sensors and Actuators A: Physical, vol. 83, pp. 277-283, 2000. [7] Linan Jiang, Man Wong, Member, and Yitshak Zohar “Forced convection boiling in microchannel heat sink”, Journal of Microelectromechanical Systems, vol. 10, no. 1, pp.80-87, 2001. [8] Man Lee, et al., “Size and Shape Effects on Two-Phase Flow Instabilities in Microchannels”, Proceeding of the 15TH IEEE MEMS, Jan 20-24, pp.28-31, 2002. [9] Mrksich, Milan; Dike, Laura E.; Tien, Joe; Ingber, Donald E.; Whitesides, George M., “Using microcontact printing to pattern the attachment of mammalian cells to self-assembled monolayers of alkanethiolates on transparent films of gold and silver”, Experimental Cell Research vol. 235, Issue 2, September 15, pp. 305-313, 1997. [10] M. Madou, Fundamentals of Microfabrication, CRC press, 1997, p.427. Biographies Hsin-Hsiung Wang was born in 1978 in Taipei, Taiwan. He received his MS degree in the Department of Mechanical and Electro-Mechanical Engineering from Tamkang University, Taiwan, in 2003. He is working toward his PhD degree in the Institute of Mechanical and Electro-Mechanical Engineering, Tamkang University, Taiwan. Po-Chiang Yang was born in 1980 in Taipei, Taiwan. He received his BS degree in the Department of Mechanical Engineering from Tamkang University, Taiwan, in 2001. He is working toward his MS degree in the Institute of Mechanical and Electro-Mechanical Engineering, Tamkang University, Taiwan. Lung-Jieh Yang received his MS degree from Tamkang University, Taiwan in 1991 and PhD degree from the Institute of Applied Mechanics, National Taiwan University, Taiwan, in 1997. He had a 1-year leave at California Institute of

Technology, USA in 2000. He is currently an associate professor of the Department of Mechanical and Electro-Mechanical Engineering and the director of Instrument and Experiment Center, Tamkang University, Taiwan. His current research interests include micro-scaled fluidic dynamics and micromachining technologies.