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Soft-lithographic methods for the fabrication of dielectrophoretic devices using molds by proton beam writing
Microelectronic Engineering 87 (2010) 835–838
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Soft-lithographic methods for the fabrication of dielectrophoretic devices using molds by proton beam writing Y. Shiine a, H. Nishikawa a,*, Y. Furuta a, K. Kanamitsu a, T. Satoh b, Y. Ishii b, T. Kamiya b, R. Nakao c, S. Uchida c a
Dept. of Electrical Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan Japan Atomic Energy Agency, 1233 Watanuki-machi, Takasaki, Gunma 370-1292, Japan c Dept. of Electrical and Electronic Engineering, Tokyo Metropolitan University, 1-1 Minami Osawa, Hachioji, Tokyo 192-0397, Japan b
a r t i c l e
i n f o
Article history: Received 14 September 2009 Received in revised form 16 December 2009 Accepted 16 December 2009 Available online 24 December 2009 Keywords: PBW SU-8 PDMS Soft lithography Dielectrophoretic devices
a b s t r a c t Proton beam writing (PBW) was applied to the fabrication of dielectrophoretic (DEP) devices equipped with high-aspect-ratio pillar arrays. With coupled use of soft lithography for micro-fluidic channels, we successfully fabricated a device equipped with SU-8 pillar arrays produced by PBW, which is covered with a poly-dimethylsiloxane (PDMS) micro-fluidic channel. For more simplified prototyping of the device, we modified a SU-8 mold for simultaneous replication of both pillar arrays and micro-fluidic channel on PDMS. Replication of pillar arrays is limited to the aspect ratio of less than three. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Proton beam writing (PBW) is a direct write process using focused beam of MeV-protons [1,2]. The focused MeV-proton beam has the following advantages over other techniques using sources such as electrons, X-rays, and UV light. First, the MeV-proton beam is suitable for fabrication of high-aspect-ratio structures, since the MeV-proton shows lower scattering than that of electrons at the same range in the material. Second, the penetration depth into resist materials can be controlled with the beam energy. In addition, arbitrary patterns can be drawn with an electrostatic beam scanning without expensive masks. The dielectrophoretic (DEP) device is typically comprises of a micro-fluidic channel with a pair Au electrode [3]. Conventional 2D DEP device utilizes the electric field gradient at the electrode edge thereby trapping of microbes occurs. In order to increase the trapping sites, we have introduced the dielectric pillar arrays into the gap by means of PBW [3]. When the high-aspect-ratio microstructures such as pillar arrays were introduced to the DEP devices, it was demonstrated that a spatially modulated electric field by the pillar arrays can serve as an efficient trapping sites for microbes such as Escherichia coli (E. coli) [3]. In the course of such applications, the PBW has been demonstrated as a quite use-
* Corresponding author. Fax: +81 3 5859 8217. E-mail address: [email protected] (H. Nishikawa). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.12.071
ful tool with flexibility in producing variety of high-aspect-ratio microstructures due to the maskless features [4]. However, since the PBW is serial and a relatively slow lithographic process, it is time consuming to expose large area of resists for the whole device including micro-fluidic channels with a length of several tens of millimeters. Such a drawback can be overcome by a coupled use of soft-lithography techniques [5] with a mold produced by PBW on poly-dimethylsiloxane (PDMS). In this paper, we introduce soft-lithographic methods for the fabrication of 3D-DEP devices equipped with high-aspect-ratio pillars, which were then combined with a PDMS micro-fluidic channel replicated from a SU-8 mold fabricated by PBW. The method has been further extended to the replication of both pillars and micro-fluidic channel to PDMS using a modified SU-8 mold. 2. Experiments Fig. 1 illustrates the process flow of fabricating DEP devices, including (a) PBW of SU-8 layer on silica for patterning pillar arrays and a mold for a micro-fluidic channel, (b) soft lithography of micro-fluidic channel by the SU-8 mold, and (c) sealing the pillar arrays on silica with the PDMS, where the silica substrate with pillar arrays is bonded to PDMS with a micro-fluidic channel. Proton beam writing was performed at beam energy of 1.0 MeV using a dedicated PB writer at the Center for Flexible Micromachining (CFM), Shibaura Institute of Technology. The beam was focused
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Y. Shiine et al. / Microelectronic Engineering 87 (2010) 835–838
down to 1.3 1.3 lm2, which was estimated from the secondary electron image of a Ni mesh. The PBW process was applied to exposure of a SU-8 layer, either for direct patterning of the pillar arrays for DEP, or molds for micro-fluidic channels for the replication to PDMS, as shown in Fig. 1a. Development process of SU-8 after PBW was described elsewhere [4]. Soft lithography [5,6] was performed by pouring a PDMS prepolymer (Sylgard 184, Dow Corning) onto a SU-8 mold with surface microstructures produced by PBW. As shown in Fig. 1b, the process involves pouring a mixture of PDMS prepolymer and curing agent onto the mold. Then, it is cured at 150 °C for 11 min until it is cross-linked. The patterned PDMS replica was peeled off from the mold and was then bonded to silica substrate with SU-8 pillar, as shown in Fig. 1c. Prior to the bonding, both the silica substrate and the PDMS replica were subjected to a plasma treatment for 3 min for irreversible sealing [6]. The microstructured surfaces of the SU-8 and PDMS were observed by an optical microscope, or scanning electron microscope (SEM, Shimadzu, SSX-550). Shown in Fig. 2 is a fabrication process of a modified SU-8 mold for simultaneous replication of both pillar arrays and a channel. Since the SU-8 is a negative resist where exposed area is insoluble against developer, the SU-8 hole arrays in Fig. 2b are fabricated by scanning the proton beam as shown in Fig. 2a. By pouring the PDMS prepolymer into the arrays of holes in Fig. 2c, the positive relief pattern is expected to be formed as arrays of pillars after curing. Fabrication of an enclosed pillar arrays in a channel is completed by sealing the PDMS with a silica substrate, as shown in Fig. 2d.
Fig. 1. Fabrication processes of a DEP device by (a) proton beam writing of SU-8 for pillar arrays and a mold, coupled with (b) a soft lithography using SU-8 mold on PDMS, followed by (c) sealing the pillar arrays on silica with PDMS.
PB
Scanning
Si
Si Development Exposed area
SU-8
(a) PBW on SU-8
(b) SU-8 mold on Si Peeled off
PDMS
PDMS
Si silica (c) Pour PDMS prepolymer
(d) Sealing silica substrate
and cure. Fig. 2. Fabrication process of a modified SU-8 mold (thickness: 15 lm) by PBW for simultaneous replication of pillar and micro-fluidic channel on PDMS.
Fig. 3. SEM images of (a) the arrays with an area of 1.0 mm 80 lm of SU-8 pillars (rectangular of 2.5 6.8 lm in shape and 13 lm in height) on Si by proton beam writing at 1.0 MeV with a fluence of 100 nC/mm2 and (b) the magnified image.
Y. Shiine et al. / Microelectronic Engineering 87 (2010) 835–838
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3. Results and discussion
Fig. 4. SEM images of (a) SU-8 mold of 200 lm in width and 20 lm in height written by PBW (beam energy: 1.0 MeV, fluence: 40 nC/mm2) and (b) replicated PDMS micro-fluidic channel.
Fig. 3a shows a SEM image of the part of pillar arrays with an area of 1.0 mm 80 lm by PBW (1.0 MeV, 100 nC/mm2) onto a 15-lm thick SU-8 on a silica substrate. High-aspect-ratio pillars (rectangular of 2.5 6.8 lm in shape and 13 lm in height) were observed with smooth and vertical surface, as shown in Fig. 3b. Fig. 4a shows the SEM image of a SU-8 mold with 200 lm in width and 2.0 mm in length and 20 lm in height. Following the process illustrated in Fig. 1b, the pattern of the SU-8 mold was successfully transferred to PDMS, as shown in Fig. 4b. In order to obtain the enclosed micro-fluidic channel, the SU-8 pillars on silica was then sealed with the PDMS micro-fluidic channel. Aligning the PDMS micro-fluidic channel was manually performed under a microscope observation. From an optical microscope image in Fig. 5a, the pillar arrays on silica were properly sealed with a PDMS micro-fluidic channel. Fig. 5b shows a photograph of the DEP device with tubing at inlet and outlet ports. Thanks to the plasma treatment, the tight sealing was achieved and no leakage was observed. The sealing process was manually performed and was successful for the simple device in Fig. 5a with a set of pillar arrays and a channel. However, it is expected for more complex channels with multiple channels that poor alignment in horizontal direction may result in the failure of the device. Therefore, for more simplified alignment of the device, we have tried the simultaneous replication of pillar array and channel by introducing a modified mold. Fig. 6a shows a SEM image of a SU-8 mold with arrays of holes with rectangle of 7.0 5.3 lm2 in shape and 12 lm in depth. As shown in Fig. 6b, the holes were successfully replicated to PDMS as arrays of pillars. Under current experimental conditions, the replication is limited to relatively low-aspect-ratio (<3) structures. For higher-aspect-ratio structures, we found that most pillars were fractured and PDMS remained inside the hole of the mold. In order to produce array of pillars inside the micro-channel, trial has been made to replicate to PDMS using the mold shown in Fig. 7a. As shown in Fig. 7b, while the transfer of the low-as-
Fig. 5. (a) Optical microscope image of a sealed pillar arrays on silica and (b) a photograph of the entire DEP device.
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Fig. 6. SEM images of (a) a SU-8 mold on Si with arrays of holes in rectangle of 7.0 5.3 lm2 with a depth of 12 lm, fabricated by PBW(beam energy: 1.0 MeV, fluence: 40 nC/mm2) and (b) replicated PDMS pillar arrays. Shown in inset is a magnified image of the pillar (rectangle of 6.9 5.3 lm2 at the bottom and 12 lm in height).
Fig. 7. SEM images of (a) 12-lm height SU-8 mold with arrays of holes (8.2 8.8 lm) on Si produced by PBW (beam energy: 1.0 MeV, fluence: 40 nC/mm2) and (b) replicated PDMS.
pect-ratio micro-fluidic channel seems satisfactory, the arrays of the pillar were almost fractured. Since the most critical part is the pillar arrays at the time of peeling from Fig. 2c and d, it is important to lower the adhesion force between the SU-8 mold and PDMS and alignment of vertical direction during separation to avoid fracture of PDMS replica, as previously reported [7]. Finally, we address the lifetime issue of the SU-8 molds produced by PBW. During the experiments of the replications, the molds were subjected to the process of casting of the PDMS, peeling off and then cleaning with a commercially available solvent (Dynasolve 225, Dynaloy). However, any degradation of the SU-8 mold was not observed by inspection using an optical microscope, even after the replications of several times which, we believe, is good enough for prototyping processes. For more exact prediction of the lifetime of the SU-8 mold, we need further experiments, which are underway.
4. Conclusions A soft lithography technique combined with PBW was successfully applied to the fabrication of the 3D-DEP device equipped with high-aspect-ratio pillar arrays in micro-fluidic channel. By the coupled use of the soft lithography technique with PBW, the prototyping capability of the PBW was demonstrated for the fabrication of the 3D-DEP devices by sealing the high-aspect-ratio pillar arrays with PDMS micro-fluidic channel. In quest of more simplified pro-
cess, we tried a modified SU-8 mold by which the pillar arrays and channels are simultaneously replicated. The most critical part of the process is pattern transfer of the high-aspect pillar from the SU-8 mold to PDMS. Replication of relatively low-aspect-ratio (<3) pillar arrays was successful. Trials of successful replication of both pillar arrays and a micro-fluidic channel are underway for DEP devices with more complex structures. Acknowledgments This work was supported by ‘‘Academic Frontier” Project and Grant-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science, and Technology, Japan. References [1] F. Watt, M.B.H. Breese, A.A. Bettiol, J.A. van Kan, Mater. Today 10 (6) (2007) 20– 29. [2] Y. Furuta, N. Uchiya, H. Nishikawa, J. Haga, T. Sato, M. Oikawa, Y. Ishii, T. Kamiya, J. Vac. Sci. Technol. B 25 (6) (2007) 2171–2174. [3] Y. Furuta, H. Nishikawa, T. Sato, Y. Ishii, T. Kamiya, R. Nakao, S. Uchida, Microelectron. Eng. 86 (4–6) (2009) 1396–1400. [4] Y. Furuta, H. Nishikawa, T. Satoh, Y. Ishii, T. Kamiya, R. Nakao, S. Uchida, Nucl. Instrum. Methods Phys. Res. B 267 (2009) 2285–2288. [5] J. Cooper McDonald, George M. Whitesides, Acc. Chem. Res. 35 (7) (2002) 491– 499. [6] J.M.K. Ng, I. Gitlin, A.D. Stroock, G.M. Whitesides, Electrophoresis 23 (2002) 3461–3473. [7] P.G. Shao, J.A. van Kan, K. Ansari, A.A. Bettiol, F. Watt, Nucl. Instrum. Methods Phys. Res. B260 (2007) 479–482.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Soft-lithographic methods for the fabrication of dielectrophoretic devices using molds by proton beam writing Y. Shiine a, H. Nishikawa a,*, Y. Furuta a, K. Kanamitsu a, T. Satoh b, Y. Ishii b, T. Kamiya b, R. Nakao c, S. Uchida c a
Dept. of Electrical Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan Japan Atomic Energy Agency, 1233 Watanuki-machi, Takasaki, Gunma 370-1292, Japan c Dept. of Electrical and Electronic Engineering, Tokyo Metropolitan University, 1-1 Minami Osawa, Hachioji, Tokyo 192-0397, Japan b
a r t i c l e
i n f o
Article history: Received 14 September 2009 Received in revised form 16 December 2009 Accepted 16 December 2009 Available online 24 December 2009 Keywords: PBW SU-8 PDMS Soft lithography Dielectrophoretic devices
a b s t r a c t Proton beam writing (PBW) was applied to the fabrication of dielectrophoretic (DEP) devices equipped with high-aspect-ratio pillar arrays. With coupled use of soft lithography for micro-fluidic channels, we successfully fabricated a device equipped with SU-8 pillar arrays produced by PBW, which is covered with a poly-dimethylsiloxane (PDMS) micro-fluidic channel. For more simplified prototyping of the device, we modified a SU-8 mold for simultaneous replication of both pillar arrays and micro-fluidic channel on PDMS. Replication of pillar arrays is limited to the aspect ratio of less than three. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Proton beam writing (PBW) is a direct write process using focused beam of MeV-protons [1,2]. The focused MeV-proton beam has the following advantages over other techniques using sources such as electrons, X-rays, and UV light. First, the MeV-proton beam is suitable for fabrication of high-aspect-ratio structures, since the MeV-proton shows lower scattering than that of electrons at the same range in the material. Second, the penetration depth into resist materials can be controlled with the beam energy. In addition, arbitrary patterns can be drawn with an electrostatic beam scanning without expensive masks. The dielectrophoretic (DEP) device is typically comprises of a micro-fluidic channel with a pair Au electrode [3]. Conventional 2D DEP device utilizes the electric field gradient at the electrode edge thereby trapping of microbes occurs. In order to increase the trapping sites, we have introduced the dielectric pillar arrays into the gap by means of PBW [3]. When the high-aspect-ratio microstructures such as pillar arrays were introduced to the DEP devices, it was demonstrated that a spatially modulated electric field by the pillar arrays can serve as an efficient trapping sites for microbes such as Escherichia coli (E. coli) [3]. In the course of such applications, the PBW has been demonstrated as a quite use-
* Corresponding author. Fax: +81 3 5859 8217. E-mail address: [email protected] (H. Nishikawa). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.12.071
ful tool with flexibility in producing variety of high-aspect-ratio microstructures due to the maskless features [4]. However, since the PBW is serial and a relatively slow lithographic process, it is time consuming to expose large area of resists for the whole device including micro-fluidic channels with a length of several tens of millimeters. Such a drawback can be overcome by a coupled use of soft-lithography techniques [5] with a mold produced by PBW on poly-dimethylsiloxane (PDMS). In this paper, we introduce soft-lithographic methods for the fabrication of 3D-DEP devices equipped with high-aspect-ratio pillars, which were then combined with a PDMS micro-fluidic channel replicated from a SU-8 mold fabricated by PBW. The method has been further extended to the replication of both pillars and micro-fluidic channel to PDMS using a modified SU-8 mold. 2. Experiments Fig. 1 illustrates the process flow of fabricating DEP devices, including (a) PBW of SU-8 layer on silica for patterning pillar arrays and a mold for a micro-fluidic channel, (b) soft lithography of micro-fluidic channel by the SU-8 mold, and (c) sealing the pillar arrays on silica with the PDMS, where the silica substrate with pillar arrays is bonded to PDMS with a micro-fluidic channel. Proton beam writing was performed at beam energy of 1.0 MeV using a dedicated PB writer at the Center for Flexible Micromachining (CFM), Shibaura Institute of Technology. The beam was focused
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down to 1.3 1.3 lm2, which was estimated from the secondary electron image of a Ni mesh. The PBW process was applied to exposure of a SU-8 layer, either for direct patterning of the pillar arrays for DEP, or molds for micro-fluidic channels for the replication to PDMS, as shown in Fig. 1a. Development process of SU-8 after PBW was described elsewhere [4]. Soft lithography [5,6] was performed by pouring a PDMS prepolymer (Sylgard 184, Dow Corning) onto a SU-8 mold with surface microstructures produced by PBW. As shown in Fig. 1b, the process involves pouring a mixture of PDMS prepolymer and curing agent onto the mold. Then, it is cured at 150 °C for 11 min until it is cross-linked. The patterned PDMS replica was peeled off from the mold and was then bonded to silica substrate with SU-8 pillar, as shown in Fig. 1c. Prior to the bonding, both the silica substrate and the PDMS replica were subjected to a plasma treatment for 3 min for irreversible sealing [6]. The microstructured surfaces of the SU-8 and PDMS were observed by an optical microscope, or scanning electron microscope (SEM, Shimadzu, SSX-550). Shown in Fig. 2 is a fabrication process of a modified SU-8 mold for simultaneous replication of both pillar arrays and a channel. Since the SU-8 is a negative resist where exposed area is insoluble against developer, the SU-8 hole arrays in Fig. 2b are fabricated by scanning the proton beam as shown in Fig. 2a. By pouring the PDMS prepolymer into the arrays of holes in Fig. 2c, the positive relief pattern is expected to be formed as arrays of pillars after curing. Fabrication of an enclosed pillar arrays in a channel is completed by sealing the PDMS with a silica substrate, as shown in Fig. 2d.
Fig. 1. Fabrication processes of a DEP device by (a) proton beam writing of SU-8 for pillar arrays and a mold, coupled with (b) a soft lithography using SU-8 mold on PDMS, followed by (c) sealing the pillar arrays on silica with PDMS.
PB
Scanning
Si
Si Development Exposed area
SU-8
(a) PBW on SU-8
(b) SU-8 mold on Si Peeled off
PDMS
PDMS
Si silica (c) Pour PDMS prepolymer
(d) Sealing silica substrate
and cure. Fig. 2. Fabrication process of a modified SU-8 mold (thickness: 15 lm) by PBW for simultaneous replication of pillar and micro-fluidic channel on PDMS.
Fig. 3. SEM images of (a) the arrays with an area of 1.0 mm 80 lm of SU-8 pillars (rectangular of 2.5 6.8 lm in shape and 13 lm in height) on Si by proton beam writing at 1.0 MeV with a fluence of 100 nC/mm2 and (b) the magnified image.
Y. Shiine et al. / Microelectronic Engineering 87 (2010) 835–838
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3. Results and discussion
Fig. 4. SEM images of (a) SU-8 mold of 200 lm in width and 20 lm in height written by PBW (beam energy: 1.0 MeV, fluence: 40 nC/mm2) and (b) replicated PDMS micro-fluidic channel.
Fig. 3a shows a SEM image of the part of pillar arrays with an area of 1.0 mm 80 lm by PBW (1.0 MeV, 100 nC/mm2) onto a 15-lm thick SU-8 on a silica substrate. High-aspect-ratio pillars (rectangular of 2.5 6.8 lm in shape and 13 lm in height) were observed with smooth and vertical surface, as shown in Fig. 3b. Fig. 4a shows the SEM image of a SU-8 mold with 200 lm in width and 2.0 mm in length and 20 lm in height. Following the process illustrated in Fig. 1b, the pattern of the SU-8 mold was successfully transferred to PDMS, as shown in Fig. 4b. In order to obtain the enclosed micro-fluidic channel, the SU-8 pillars on silica was then sealed with the PDMS micro-fluidic channel. Aligning the PDMS micro-fluidic channel was manually performed under a microscope observation. From an optical microscope image in Fig. 5a, the pillar arrays on silica were properly sealed with a PDMS micro-fluidic channel. Fig. 5b shows a photograph of the DEP device with tubing at inlet and outlet ports. Thanks to the plasma treatment, the tight sealing was achieved and no leakage was observed. The sealing process was manually performed and was successful for the simple device in Fig. 5a with a set of pillar arrays and a channel. However, it is expected for more complex channels with multiple channels that poor alignment in horizontal direction may result in the failure of the device. Therefore, for more simplified alignment of the device, we have tried the simultaneous replication of pillar array and channel by introducing a modified mold. Fig. 6a shows a SEM image of a SU-8 mold with arrays of holes with rectangle of 7.0 5.3 lm2 in shape and 12 lm in depth. As shown in Fig. 6b, the holes were successfully replicated to PDMS as arrays of pillars. Under current experimental conditions, the replication is limited to relatively low-aspect-ratio (<3) structures. For higher-aspect-ratio structures, we found that most pillars were fractured and PDMS remained inside the hole of the mold. In order to produce array of pillars inside the micro-channel, trial has been made to replicate to PDMS using the mold shown in Fig. 7a. As shown in Fig. 7b, while the transfer of the low-as-
Fig. 5. (a) Optical microscope image of a sealed pillar arrays on silica and (b) a photograph of the entire DEP device.
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Y. Shiine et al. / Microelectronic Engineering 87 (2010) 835–838
Fig. 6. SEM images of (a) a SU-8 mold on Si with arrays of holes in rectangle of 7.0 5.3 lm2 with a depth of 12 lm, fabricated by PBW(beam energy: 1.0 MeV, fluence: 40 nC/mm2) and (b) replicated PDMS pillar arrays. Shown in inset is a magnified image of the pillar (rectangle of 6.9 5.3 lm2 at the bottom and 12 lm in height).
Fig. 7. SEM images of (a) 12-lm height SU-8 mold with arrays of holes (8.2 8.8 lm) on Si produced by PBW (beam energy: 1.0 MeV, fluence: 40 nC/mm2) and (b) replicated PDMS.
pect-ratio micro-fluidic channel seems satisfactory, the arrays of the pillar were almost fractured. Since the most critical part is the pillar arrays at the time of peeling from Fig. 2c and d, it is important to lower the adhesion force between the SU-8 mold and PDMS and alignment of vertical direction during separation to avoid fracture of PDMS replica, as previously reported [7]. Finally, we address the lifetime issue of the SU-8 molds produced by PBW. During the experiments of the replications, the molds were subjected to the process of casting of the PDMS, peeling off and then cleaning with a commercially available solvent (Dynasolve 225, Dynaloy). However, any degradation of the SU-8 mold was not observed by inspection using an optical microscope, even after the replications of several times which, we believe, is good enough for prototyping processes. For more exact prediction of the lifetime of the SU-8 mold, we need further experiments, which are underway.
4. Conclusions A soft lithography technique combined with PBW was successfully applied to the fabrication of the 3D-DEP device equipped with high-aspect-ratio pillar arrays in micro-fluidic channel. By the coupled use of the soft lithography technique with PBW, the prototyping capability of the PBW was demonstrated for the fabrication of the 3D-DEP devices by sealing the high-aspect-ratio pillar arrays with PDMS micro-fluidic channel. In quest of more simplified pro-
cess, we tried a modified SU-8 mold by which the pillar arrays and channels are simultaneously replicated. The most critical part of the process is pattern transfer of the high-aspect pillar from the SU-8 mold to PDMS. Replication of relatively low-aspect-ratio (<3) pillar arrays was successful. Trials of successful replication of both pillar arrays and a micro-fluidic channel are underway for DEP devices with more complex structures. Acknowledgments This work was supported by ‘‘Academic Frontier” Project and Grant-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science, and Technology, Japan. References [1] F. Watt, M.B.H. Breese, A.A. Bettiol, J.A. van Kan, Mater. Today 10 (6) (2007) 20– 29. [2] Y. Furuta, N. Uchiya, H. Nishikawa, J. Haga, T. Sato, M. Oikawa, Y. Ishii, T. Kamiya, J. Vac. Sci. Technol. B 25 (6) (2007) 2171–2174. [3] Y. Furuta, H. Nishikawa, T. Sato, Y. Ishii, T. Kamiya, R. Nakao, S. Uchida, Microelectron. Eng. 86 (4–6) (2009) 1396–1400. [4] Y. Furuta, H. Nishikawa, T. Satoh, Y. Ishii, T. Kamiya, R. Nakao, S. Uchida, Nucl. Instrum. Methods Phys. Res. B 267 (2009) 2285–2288. [5] J. Cooper McDonald, George M. Whitesides, Acc. Chem. Res. 35 (7) (2002) 491– 499. [6] J.M.K. Ng, I. Gitlin, A.D. Stroock, G.M. Whitesides, Electrophoresis 23 (2002) 3461–3473. [7] P.G. Shao, J.A. van Kan, K. Ansari, A.A. Bettiol, F. Watt, Nucl. Instrum. Methods Phys. Res. B260 (2007) 479–482.