- Email: [email protected]
Biological fluid interaction with controlled surface properties of organic micro-fluidic devices
ARTICLE IN PRESS
Vacuum 80 (2006) 876–879 www.elsevier.com/locate/vacuum
Biological fluid interaction with controlled surface properties of organic micro-fluidic devices Marshal Dhayala,b,, Jeong Sik Choia, Cheal Ho Sob a
Biological Research Center of Industrial Accelerators, 252 Daeho-dong, Naju, South Korea Center of Optical Micro Device Systems, Dongshin University, 252 Daeho-dong, Naju, South Korea
b
Received 9 September 2005; accepted 27 November 2005
Abstract An organic micro-fluidic device (O-MFD) on silicon/glass substrate was fabricated using plasma polymerisation of acrylic acid (ppAc) with optical lithography including wet and dry etching techniques. The surface chemistry of O-MFD was controlled by depositing ppAc films on MFD. The results showed that these surfaces have more than 50% retention of original monomer functionalities (–COOX) with significant concentration of CQO and C–OX at the surface. The water fluid velocity in 100 mm wide and 100 mm deep micro-channels of ppAc deposited O-MFD was increased by a factor of 10 relatively uncoated surfaces. The mixing of 10% of blood in the water has showed a decrease of about 100 mm/s in the fluid velocity relative to water and 10% red colour dye mixed water. The decrease in the fluid velocity of blood mixed water showed the possibility of blood cells interaction with the highly functional ppAc surfaces of micro-channel in O-MFD and that could have advantages in biological fluids (such as protein) separation. r 2005 Elsevier Ltd. All rights reserved. Keywords: Plasma polymerisation; Bio-MEMS; Micro-fluidics; Acrylic acid; Bonding
1. Introduction The attention of micro/nano technology for applications of micro/nano-electro-mechanical systems (MEMS/ NEMS), bio-MEMS and micro-fluidic devices in medical, chemical, and food science have increased [1–5]. This is because these devices have advantages in biomedical applications due to high sensitivity, need small amount of precious biological samples, and portability of the entire system. The design, fabrication process and applications of bio-MEMS and micro-fluidic devices for each application depends on the type of materials used in the process. For example, materials such as silicon, glass, and related materials [6] are used for semiconductor industry whereas plastic and polymeric materials-based devices are very attractive and have showed usefulness in biological systems [7–11]. Corresponding author. Biological Research Center of Industrial Accelerators, 252 Daeho-dong, Naju, South Korea. Tel.: +82 10 4620 1307; fax: +82 61 330 2836. E-mail address: [email protected] (M. Dhayal).
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.11.068
Recently, the plasma polymerisation process has been used for thin organic film coatings on micro-fluidic device (MFD) to change the surface chemical and physical properties of many types of substrates. This process gives better selectivity at surface such as hydrophobic, hydrophilic, cell adhesive or non-adhesive for different applications [12–14]. We have previously [14] observed a higher fluid velocity after plasma polymerised acrylic acid (ppAc) coatings on the surface relative to uncoated surface and its applications for bonding of these devices at low temperature [15]. Hence, plasma-polymerised coatings are useful for changing the surface properties and can also be used to control the fluid velocity by changing the surface energy or hydrophilic/hydrophobic nature of the surface. In MFD, to move the fluids in the channels, a pumping system (micro-pump) is necessary. A number of different types of pumping system can be applied for pumping of the fluid such as mechanical pump, thermal gradient, piezoelectricity, electrochemical reactions, and electric field [16–18]. In this study we have used asymmetric microelectrode array to generate high electric field to create the fluid flow in the micro-channel of fluidic devices.
ARTICLE IN PRESS M. Dhayal et al. / Vacuum 80 (2006) 876–879
The mechanism of the fluid flow using asymmetric electrode is a well-known phenomena in DC field and the fluid velocity can be written as v ¼ ðEs=kZ) [16], where E is tangential electric field (depends on applied voltage and spacing between electrodes), Z is viscosity of medium, s is surface charge density in the diffuse double layer and k is the reciprocal Debye length [17]. In this study, the ppAc films were only deposited on micro-channel not on the micro-electrode whereas in our previous study [14,15] both micro-channel and microelectrode were coated with ppAc films. The fluid velocity was measured for two different surface conditions. A constant applied voltage and frequency on asymmetric micro-electrode array was used for all the experiments. The aim of this work was to investigate the effect of biocompatible and highly functional ppAc coatings on biological fluid flow in MFD.
Driver Electrode Matching Unit
877
Monomer flow controller Acrylic Acid Monomer
Primary Plasma RF Power Supply
Grid
Turbo Pump
Processing Plasma
Valve
Rotary Pump Sample Holder
Fig. 1. Schematic diagram of plasma polymerisation reactor.
2. Experimental µ-channel
To fabricate the asymmetric micro-electrode array, about 40 nm thick Cr layer was first deposited and followed by a 40 nm thick gold layer on a silicon substrate coated with SiO2 film. The gold coating was coated with a PR coating and exposed to UV radiation covered with a mask. The pattern was transferred first by wet etching of gold in a KI+I2 solution and then by reactive ion etching of Cr in a Cl2+O2 plasma. Finally Cr–gold electrodes were patterned on the substrate. The electrode array fabricated in MFD was consisting of small (4 mm wide) and large (20 mm wide) electrode. The small electrode was separated from the large electrode by 20 and 6 mm gaps in both sides, respectively. The micro-channels of 100 mm wide and about 100 mm deep on silicon and glass substrates were fabricated using wet etching and laser techniques with a total length of about 50 mm. These micro-channels were coated with ppAc thin films. A schematic diagram of the plasma polymerisation process is shown in Fig. 1. The process was similar to the one used in previous experiments [14,19–21] and has advantages to deposit the highly functional films of original monomer functionalities. Flow rate of acrylic acid monomer was controlled using a micro-leveler manually controlled gas flow regulator. Acrylic acid monomer was degassed using freeze-thaw cycle 4 times before admitting to the chamber for the discharge. Oxygen gas attached to the chamber through a gas flow controller system to the reactor was used to clean the reactor after each set of deposition experiment. X-ray photoelectron spectroscopy (using VGScientific ESCALAB 250 spectrometer with monocromatised Al ka X-ray source) was used for a characterisation of the surface chemistry of thin ppAc films deposited on micro-channel. A microscopic picture of micro-device after assembling together micro-electrode and micro-channel is shown in Fig. 2.
100 µm
Asymmetric µ-electrodes Fig. 2. A microscopic picture of fabricated micro-fluidic devices shows a 100 mm width channel and asymmetric micro-electrode array.
3. Results and discussion 3.1. Surface characterisation of ppAc films The surface chemical analysis of the ppAc film was carried out using XPS. The wide scan XPS spectrum showed about 68% carbon and 32% oxygen at the surface. Different carbon environments (hydrocarbon, alcohol/ ether, carbonyl and carboxylic/ester chemical groups) at the surface were analysed by different peak fitting in C1 s narrow scan XPS spectrum as is shown in Fig. 3. The C1 s spectrum was fitted with six peaks and the chemical shift relative to the C–Si peak is quoted in parenthesis; C–Si at 283.7 eV (0 eV), hydrocarbon (C–H/C–C) at 286 eV (2.3 eV), C–C(QO)OX at 286.5 (2.8), alcohol/ether (C–OX) at 287.5 eV (3.8 eV), carbonyl (CQO) at 288.8 eV (5.1 eV), and carboxylic/ester (C(QO)OX) at 290.2 eV (6.5 eV). In the current operating conditions a value of about 17% carbon as carboxylic/ester (C(QO)OX), 11% carbon as
ARTICLE IN PRESS M. Dhayal et al. / Vacuum 80 (2006) 876–879
0.8
Environment C-Si C-H/C-C C-OX C=O C (=O)OX C (=O)OX
Position / eV 283.7 286 287.5 288.8 290.2 286.5
FWHM / eV 2.4 1.9 1.5 1.4 1.6 1.6
Carbon / at% 17.3 31.9 11.2 6.1 16.7 16.7
CPS (103)
0.6 C-OX
C-H/C-C
C(=O)OX C-C(=O)OX
0.4 C=O
C-Si
0.2
0 294
292
290
288 286 284 Binding Energy (eV)
282
280
Fig. 3. Peak fitted XPS narrow scan C1 s spectrum of plasma polymerised acrylic acid film at 40 mTorr pressure and 5 W RF power.
carbonyl (CQO) and 6% carbon as alcohol/ether (C–OX) were observed at the surface of the films deposited using 5 W RF power and 40 mTorr pressure in a similar discharge used in previous experiments [14]. These XPS result showed that the films were highly functionalised and had more than 50% retention of original monomer functionalities (COOX) with a significant concentration of CQO and C–OX at the surface. 3.2. Bonding of micro-electrode and micro-channel The ppAc thin film deposited on micro-channel was aligned with uncoated micro-electrode, assembled together and placed on a clean and hot plate (temperature 100 1C). A load of 1.5 kg (on 1 5 cm2 surface area) was placed on the substrates during the heating for 5 min. After 5 min heating it was removed and placed at a dry place (at room temperature 25 1C) for 12 h. After 12 h thin ppAc films on both micro-electrode and micro-channel were bonded (Tg of ppAc films is about 100 1C). The bond strength was about 1.5 MPa measured by applying a force on both sides of bonded substrates. The increase of the applied load can increase the bond strength, however, it may also damage the micro-electrode structure in MFD. Therefore low load (1.5 kg) was used for the bonding to minimise the defects in the micro-electrode.
scope for different times. First, tap water used as a fluid and in 100 mm wide ppAc coated micro-channel the 480 mm/s fluid velocity was measured by applying a 2 V (peak to peak) bias voltage at 1 kHz frequency on asymmetric micro-electrode array whereas without coating the velocity was about 40 mm/s (see Fig. 4a). An other interesting thing was noticed that even when there was no applied field on micro-electrode about same fluid velocity was measured in O-MFD may be due to hydrophilic nature of ppAc films. This indicates that at the start capillary forces might be acting as a main driving force for the flow of fluid in O-MFD. The asymmetric micro-electrodes (or some other forces such as mechanical micro-pump etc.) were necessary to continue the flow as for only capillary forces the flow could stop after filling the channel and chamber. To study the effects of surface property on biological fluid flow different types of fluids such as blood+water, red colour dye+water, and water were used. Fig. 4b shows fluid velocity in a 100 mm wide micro-channel for three different fluids (only water, 90% water+10% blood, and 90% water+10% red colour dye). In case of 10% blood+90% water result shows a reduction in the fluid velocity up to about 360 mm/s. The decrease in the fluid velocity after mixing the blood into water may be due to change of fluid density (blood+water) or viscosity because the fluid velocity is inversely proportional to the viscosity (v / 1=Z) when other operating conditions are constant. To verify the effect of variation of fluid density/viscosity on fluid flow velocity in micro-channel, the same amount of red colour dye (10%) was mixed in the water and used as a fluid. No significant change was seen in the fluid velocity of water+red colour dye. This indicates that the decrease in the fluid velocity for blood mixed water was not only by the change in the fluid density/viscosity but may be due to interaction of blood cells with the highly reactive functional groups (such as –CQO, –C–OX, –C(O)OX) at the surface. Therefore, when the blood mixed water passed
500
(a)
3.3. Velocity measurement The fluid velocity was measured by tracing the fluid flow in micro-channel of organic MFD (O-MFD) using micro-
500 100 µm
400
400
300
300
200
200
100
100
0
MFD
O-MFD
(b) A
B
C
V (µm/sec)
1
V (µm/sec)
878
0
Fig. 4. Fluid velocity measurements. (a) Two different surface conditions (MFD: uncoated surface and O-MFD: ppAc coated surface); and (b) three different fluids in 100 mm wide micro-channel of O-MFD (A: only water, B: 10% red colour dye mixed water, and C: 10% blood mixed water).
ARTICLE IN PRESS M. Dhayal et al. / Vacuum 80 (2006) 876–879
through the micro-channel in MFD some of blood cells may not move and stick to the surface and only water flows through the channel. This phenomenon may be useful for separation or purification of different types of cells, proteins or other biological samples by controlling the surface chemistry of the devices. This cell and surface interaction possibly depends on the concentration of specific functional groups for the cell attachment at the surface, the interaction time of cell and surface and the pumping pressure (driving force applied on micro-electrode). The impurities at the surface or in the fluid may also influence the flow of biological samples or the performance of the device. Sometimes the impurities in the fluid could make cells unhealthy. This primarily study showed a potential usefulness of controlling the surface properties of these devices for biological use. However, in the future a detailed study is needed and conditions need to be optimised.
4. Conclusion An O-MFD on silicon/glass substrate was fabricated using plasma polymerisation process with optical lithography, wet and dry etching techniques. The XPS surface characterisation of ppAc films shows that these surfaces were having more than 50% retention of original monomer functionalities with significant concentration of CQO and C–OX at the surface. The ppAc coatings on MFD microchannels have shown the usefulness to increase the fluid velocity, bonding micro-electrodes and micro-channel at low temperature. A decrease in the fluid velocity (about 100 mm/s) was also measured for blood mixed water fluid relative to water may be due to blood cells interaction with the highly functional ppAc surfaces of micro-channel in MFD.
879
Acknowledgment We thank Dr. Won in Korean Basic science institute for her help in obtaining XPS data and Dr. Hyung Gon Jeong from BRCIA for valuable discussions. References [1] Varadan VK, Jiang X, Varadan V. Micro stereo lithography and other fabrication techniques for 3D MEMS. New West Susses, England: Wiley; 2001. [2] Bashir R. Adv Drug Deliv Rev 2004;56:1565. [3] Ziaie B, Baldi A, Lei M, Gu Y, Siegel RA. Adv Drug Deliv Rev 2004;56:145. [4] Shawgo RS, Richards AC, Grayson, Li Y, Cima MJ. Curr Opin Solid State Mater Sci 2002;6:329. [5] Whitesides GM, Ostuni E, Takayama S, Jiang X, Ingber DE. Annu Rev Biomed Eng 2001;3:335. [6] Madou MJ. Fundamentals of micro-fabrication: the science of miniaturization. Boca Raton, FL: CRC Press; 2002. [7] Xia Y, Whitesides GM. Annu Rev Mater Sci 1998;28:153. [8] Quake SR, Scherer A. Science 2000;290:1536. [9] Voldman J. Nat Mater 2003;2:433. [10] Bhatia SN, Chen CS. Biomed Microdev 1999;2(2):131. [11] Anderson JR, Chiu DT, Jackman RJ, Cherniavskaya O, McDonald JC, Wu H, et al. Anal Chem 2000;72(14):3158. [12] Hiratsuka A, Muguruma H, Lee KH, Karube I. Biosensors Bioelectron 2004;19:1667. [13] Bouaidat S, Jensen BW, Christensen SF, Jonsmann J. Sensors Actuators A 2004;110:390. [14] Dhayal M, Jeong HG, Choi JS. Appl Surf Sci 2005;252:1710. [15] Dhayal M. Application of low temperature substrate bonding in fabrication of reusable micro fluidic devices, Vacuum, 2005, in press. [16] Ajdari A. Phys Rev E 2000;61(1):R45. [17] Brown ABD, Smith CG, Rennie AR. Phys Rev E 2000;63:16305. [18] Ramos A, Morgan H, Green NG, Castellanos A. J Colloid Interface Sci 1999;217:420. [19] Dhayal M, Alexander MR, Bradley JW. In: 46th technical conference proceeding of society of vacuum coaters, 2003, p. 93. [20] Dhayal M, Forder D, Short RD, Barton D, Bradley JW. Surf Coating Technol 2003;162(2–3):294. [21] Dhayal M, Bradley JW. Surf Coating Technol 2004;184(1):116.
Vacuum 80 (2006) 876–879 www.elsevier.com/locate/vacuum
Biological fluid interaction with controlled surface properties of organic micro-fluidic devices Marshal Dhayala,b,, Jeong Sik Choia, Cheal Ho Sob a
Biological Research Center of Industrial Accelerators, 252 Daeho-dong, Naju, South Korea Center of Optical Micro Device Systems, Dongshin University, 252 Daeho-dong, Naju, South Korea
b
Received 9 September 2005; accepted 27 November 2005
Abstract An organic micro-fluidic device (O-MFD) on silicon/glass substrate was fabricated using plasma polymerisation of acrylic acid (ppAc) with optical lithography including wet and dry etching techniques. The surface chemistry of O-MFD was controlled by depositing ppAc films on MFD. The results showed that these surfaces have more than 50% retention of original monomer functionalities (–COOX) with significant concentration of CQO and C–OX at the surface. The water fluid velocity in 100 mm wide and 100 mm deep micro-channels of ppAc deposited O-MFD was increased by a factor of 10 relatively uncoated surfaces. The mixing of 10% of blood in the water has showed a decrease of about 100 mm/s in the fluid velocity relative to water and 10% red colour dye mixed water. The decrease in the fluid velocity of blood mixed water showed the possibility of blood cells interaction with the highly functional ppAc surfaces of micro-channel in O-MFD and that could have advantages in biological fluids (such as protein) separation. r 2005 Elsevier Ltd. All rights reserved. Keywords: Plasma polymerisation; Bio-MEMS; Micro-fluidics; Acrylic acid; Bonding
1. Introduction The attention of micro/nano technology for applications of micro/nano-electro-mechanical systems (MEMS/ NEMS), bio-MEMS and micro-fluidic devices in medical, chemical, and food science have increased [1–5]. This is because these devices have advantages in biomedical applications due to high sensitivity, need small amount of precious biological samples, and portability of the entire system. The design, fabrication process and applications of bio-MEMS and micro-fluidic devices for each application depends on the type of materials used in the process. For example, materials such as silicon, glass, and related materials [6] are used for semiconductor industry whereas plastic and polymeric materials-based devices are very attractive and have showed usefulness in biological systems [7–11]. Corresponding author. Biological Research Center of Industrial Accelerators, 252 Daeho-dong, Naju, South Korea. Tel.: +82 10 4620 1307; fax: +82 61 330 2836. E-mail address: [email protected] (M. Dhayal).
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.11.068
Recently, the plasma polymerisation process has been used for thin organic film coatings on micro-fluidic device (MFD) to change the surface chemical and physical properties of many types of substrates. This process gives better selectivity at surface such as hydrophobic, hydrophilic, cell adhesive or non-adhesive for different applications [12–14]. We have previously [14] observed a higher fluid velocity after plasma polymerised acrylic acid (ppAc) coatings on the surface relative to uncoated surface and its applications for bonding of these devices at low temperature [15]. Hence, plasma-polymerised coatings are useful for changing the surface properties and can also be used to control the fluid velocity by changing the surface energy or hydrophilic/hydrophobic nature of the surface. In MFD, to move the fluids in the channels, a pumping system (micro-pump) is necessary. A number of different types of pumping system can be applied for pumping of the fluid such as mechanical pump, thermal gradient, piezoelectricity, electrochemical reactions, and electric field [16–18]. In this study we have used asymmetric microelectrode array to generate high electric field to create the fluid flow in the micro-channel of fluidic devices.
ARTICLE IN PRESS M. Dhayal et al. / Vacuum 80 (2006) 876–879
The mechanism of the fluid flow using asymmetric electrode is a well-known phenomena in DC field and the fluid velocity can be written as v ¼ ðEs=kZ) [16], where E is tangential electric field (depends on applied voltage and spacing between electrodes), Z is viscosity of medium, s is surface charge density in the diffuse double layer and k is the reciprocal Debye length [17]. In this study, the ppAc films were only deposited on micro-channel not on the micro-electrode whereas in our previous study [14,15] both micro-channel and microelectrode were coated with ppAc films. The fluid velocity was measured for two different surface conditions. A constant applied voltage and frequency on asymmetric micro-electrode array was used for all the experiments. The aim of this work was to investigate the effect of biocompatible and highly functional ppAc coatings on biological fluid flow in MFD.
Driver Electrode Matching Unit
877
Monomer flow controller Acrylic Acid Monomer
Primary Plasma RF Power Supply
Grid
Turbo Pump
Processing Plasma
Valve
Rotary Pump Sample Holder
Fig. 1. Schematic diagram of plasma polymerisation reactor.
2. Experimental µ-channel
To fabricate the asymmetric micro-electrode array, about 40 nm thick Cr layer was first deposited and followed by a 40 nm thick gold layer on a silicon substrate coated with SiO2 film. The gold coating was coated with a PR coating and exposed to UV radiation covered with a mask. The pattern was transferred first by wet etching of gold in a KI+I2 solution and then by reactive ion etching of Cr in a Cl2+O2 plasma. Finally Cr–gold electrodes were patterned on the substrate. The electrode array fabricated in MFD was consisting of small (4 mm wide) and large (20 mm wide) electrode. The small electrode was separated from the large electrode by 20 and 6 mm gaps in both sides, respectively. The micro-channels of 100 mm wide and about 100 mm deep on silicon and glass substrates were fabricated using wet etching and laser techniques with a total length of about 50 mm. These micro-channels were coated with ppAc thin films. A schematic diagram of the plasma polymerisation process is shown in Fig. 1. The process was similar to the one used in previous experiments [14,19–21] and has advantages to deposit the highly functional films of original monomer functionalities. Flow rate of acrylic acid monomer was controlled using a micro-leveler manually controlled gas flow regulator. Acrylic acid monomer was degassed using freeze-thaw cycle 4 times before admitting to the chamber for the discharge. Oxygen gas attached to the chamber through a gas flow controller system to the reactor was used to clean the reactor after each set of deposition experiment. X-ray photoelectron spectroscopy (using VGScientific ESCALAB 250 spectrometer with monocromatised Al ka X-ray source) was used for a characterisation of the surface chemistry of thin ppAc films deposited on micro-channel. A microscopic picture of micro-device after assembling together micro-electrode and micro-channel is shown in Fig. 2.
100 µm
Asymmetric µ-electrodes Fig. 2. A microscopic picture of fabricated micro-fluidic devices shows a 100 mm width channel and asymmetric micro-electrode array.
3. Results and discussion 3.1. Surface characterisation of ppAc films The surface chemical analysis of the ppAc film was carried out using XPS. The wide scan XPS spectrum showed about 68% carbon and 32% oxygen at the surface. Different carbon environments (hydrocarbon, alcohol/ ether, carbonyl and carboxylic/ester chemical groups) at the surface were analysed by different peak fitting in C1 s narrow scan XPS spectrum as is shown in Fig. 3. The C1 s spectrum was fitted with six peaks and the chemical shift relative to the C–Si peak is quoted in parenthesis; C–Si at 283.7 eV (0 eV), hydrocarbon (C–H/C–C) at 286 eV (2.3 eV), C–C(QO)OX at 286.5 (2.8), alcohol/ether (C–OX) at 287.5 eV (3.8 eV), carbonyl (CQO) at 288.8 eV (5.1 eV), and carboxylic/ester (C(QO)OX) at 290.2 eV (6.5 eV). In the current operating conditions a value of about 17% carbon as carboxylic/ester (C(QO)OX), 11% carbon as
ARTICLE IN PRESS M. Dhayal et al. / Vacuum 80 (2006) 876–879
0.8
Environment C-Si C-H/C-C C-OX C=O C (=O)OX C (=O)OX
Position / eV 283.7 286 287.5 288.8 290.2 286.5
FWHM / eV 2.4 1.9 1.5 1.4 1.6 1.6
Carbon / at% 17.3 31.9 11.2 6.1 16.7 16.7
CPS (103)
0.6 C-OX
C-H/C-C
C(=O)OX C-C(=O)OX
0.4 C=O
C-Si
0.2
0 294
292
290
288 286 284 Binding Energy (eV)
282
280
Fig. 3. Peak fitted XPS narrow scan C1 s spectrum of plasma polymerised acrylic acid film at 40 mTorr pressure and 5 W RF power.
carbonyl (CQO) and 6% carbon as alcohol/ether (C–OX) were observed at the surface of the films deposited using 5 W RF power and 40 mTorr pressure in a similar discharge used in previous experiments [14]. These XPS result showed that the films were highly functionalised and had more than 50% retention of original monomer functionalities (COOX) with a significant concentration of CQO and C–OX at the surface. 3.2. Bonding of micro-electrode and micro-channel The ppAc thin film deposited on micro-channel was aligned with uncoated micro-electrode, assembled together and placed on a clean and hot plate (temperature 100 1C). A load of 1.5 kg (on 1 5 cm2 surface area) was placed on the substrates during the heating for 5 min. After 5 min heating it was removed and placed at a dry place (at room temperature 25 1C) for 12 h. After 12 h thin ppAc films on both micro-electrode and micro-channel were bonded (Tg of ppAc films is about 100 1C). The bond strength was about 1.5 MPa measured by applying a force on both sides of bonded substrates. The increase of the applied load can increase the bond strength, however, it may also damage the micro-electrode structure in MFD. Therefore low load (1.5 kg) was used for the bonding to minimise the defects in the micro-electrode.
scope for different times. First, tap water used as a fluid and in 100 mm wide ppAc coated micro-channel the 480 mm/s fluid velocity was measured by applying a 2 V (peak to peak) bias voltage at 1 kHz frequency on asymmetric micro-electrode array whereas without coating the velocity was about 40 mm/s (see Fig. 4a). An other interesting thing was noticed that even when there was no applied field on micro-electrode about same fluid velocity was measured in O-MFD may be due to hydrophilic nature of ppAc films. This indicates that at the start capillary forces might be acting as a main driving force for the flow of fluid in O-MFD. The asymmetric micro-electrodes (or some other forces such as mechanical micro-pump etc.) were necessary to continue the flow as for only capillary forces the flow could stop after filling the channel and chamber. To study the effects of surface property on biological fluid flow different types of fluids such as blood+water, red colour dye+water, and water were used. Fig. 4b shows fluid velocity in a 100 mm wide micro-channel for three different fluids (only water, 90% water+10% blood, and 90% water+10% red colour dye). In case of 10% blood+90% water result shows a reduction in the fluid velocity up to about 360 mm/s. The decrease in the fluid velocity after mixing the blood into water may be due to change of fluid density (blood+water) or viscosity because the fluid velocity is inversely proportional to the viscosity (v / 1=Z) when other operating conditions are constant. To verify the effect of variation of fluid density/viscosity on fluid flow velocity in micro-channel, the same amount of red colour dye (10%) was mixed in the water and used as a fluid. No significant change was seen in the fluid velocity of water+red colour dye. This indicates that the decrease in the fluid velocity for blood mixed water was not only by the change in the fluid density/viscosity but may be due to interaction of blood cells with the highly reactive functional groups (such as –CQO, –C–OX, –C(O)OX) at the surface. Therefore, when the blood mixed water passed
500
(a)
3.3. Velocity measurement The fluid velocity was measured by tracing the fluid flow in micro-channel of organic MFD (O-MFD) using micro-
500 100 µm
400
400
300
300
200
200
100
100
0
MFD
O-MFD
(b) A
B
C
V (µm/sec)
1
V (µm/sec)
878
0
Fig. 4. Fluid velocity measurements. (a) Two different surface conditions (MFD: uncoated surface and O-MFD: ppAc coated surface); and (b) three different fluids in 100 mm wide micro-channel of O-MFD (A: only water, B: 10% red colour dye mixed water, and C: 10% blood mixed water).
ARTICLE IN PRESS M. Dhayal et al. / Vacuum 80 (2006) 876–879
through the micro-channel in MFD some of blood cells may not move and stick to the surface and only water flows through the channel. This phenomenon may be useful for separation or purification of different types of cells, proteins or other biological samples by controlling the surface chemistry of the devices. This cell and surface interaction possibly depends on the concentration of specific functional groups for the cell attachment at the surface, the interaction time of cell and surface and the pumping pressure (driving force applied on micro-electrode). The impurities at the surface or in the fluid may also influence the flow of biological samples or the performance of the device. Sometimes the impurities in the fluid could make cells unhealthy. This primarily study showed a potential usefulness of controlling the surface properties of these devices for biological use. However, in the future a detailed study is needed and conditions need to be optimised.
4. Conclusion An O-MFD on silicon/glass substrate was fabricated using plasma polymerisation process with optical lithography, wet and dry etching techniques. The XPS surface characterisation of ppAc films shows that these surfaces were having more than 50% retention of original monomer functionalities with significant concentration of CQO and C–OX at the surface. The ppAc coatings on MFD microchannels have shown the usefulness to increase the fluid velocity, bonding micro-electrodes and micro-channel at low temperature. A decrease in the fluid velocity (about 100 mm/s) was also measured for blood mixed water fluid relative to water may be due to blood cells interaction with the highly functional ppAc surfaces of micro-channel in MFD.
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