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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 9 — Apr. 26, 2010
  • pp: 9733–9738
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The PDMS-based microfluidic channel fabricated by synchrotron radiation stimulated etching

Tingchao He, Changshun Wang, Tsuneo Urisu, Takeshi Nagahiro, Ryugo Tero, and Rong Xia  »View Author Affiliations


Optics Express, Vol. 18, Issue 9, pp. 9733-9738 (2010)
http://dx.doi.org/10.1364/OE.18.009733


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Abstract

Micro pattern on PDMS surface has been achieved by using synchrotron radiation (SR) stimulated etching. The experimental results indicated that SR stimulated etching has many advantages, such as extremely high etching rate (as large as 40-50 μ m per 10 min), area-selectivity and anisotropy at room temperature, high spatial resolution. Combining the SR stimulated etching with photolithography, a PDMS-based microfluidic channel was obtained. The aim of this work is to develop a three-dimensional microfluidic channel with a special through hole, which is beneficial for cell differentiation, functionality and longevity and cannot be fabricated by conventional direct tooling techniques.

© 2010 OSA

1. Introduction

In the present work, a PDMS-based microfluidic channel was presented by the fabrication of synchrotron radiation (SR) stimulated etching utilizing a SR etching beam line consisting of differential pumping and an etching chamber. Experimental results indicate that the SR stimulated etching method is very useful for the fabrication of PDMS-based microfluidic channel owing to its unique features, such as surprisingly high etching rate, low damage to substrates, anisotropy etching and high spatial resolution and aspect ratio because of the short wavelengths.

2. Experimental

The SR etching of PDMS film was carried out at the beam line 4A1 of the SR facility (UVSOR) at the Institute for Molecular Science in Japan. The photon energy region is 50-95 eV and the pulse duration is between 20 ps and 1 ns with a harmonic cavity system. The end station consists of a parabolic focusing mirror chamber and an etching chamber as shown in Fig. 1
Fig. 1 End station of the beam line containing the etching chamber and the focussing mirror chamber.
. The SR beam from the light source is focused by a Pt coated vertical pre-mirror, and then tightly focussed by another Pt coated vertical parabolic mirror (FM in Fig. 1) to the aperture with 1 mm diameter and 10 mm length. The light finally irradiates onto the surface of the PDMS sample in the etching chamber. The SR beam spot size on the sample surface is about 0.5 mm in diameter. This aperture is very important to keep sufficiently high vacuum (<10−5 torr) in the focusing mirror chamber when the XeF2 gas pressure increases up to about 1 torr in the etching chamber. The reaction gas (100% XeF2) pressure in the etching chamber is controlled by repeating open-close sequence of the valves controlled by the computer with two timers. The PDMS films in the experiments were fabricated from a 10:1 (weight ratio) mixture of Sylgard Silicone Elastomer 184W/C and Sylgard Curing Agent 184 (Dow Corning Corp.).

3. Results and discussion

When the PDMS was etched by directly irradiating the SR beam onto the sample surface without mask, a high speed, area selective and anisotropic etching was easily realized. Under the condition of 0.5 torr XeF2 gas pressure, 150 mA ring current (the photoemission current measured by the Pt detector set at the sample position was about 312 µA) and 10 min irradiation time, the maximum etching depth can reach 500 μm and the hole size was about 0.5 mm. However, some rugged structures were observed at the surrounding of the etched hole due to the heating effect of the SR beam. It is also found that PDMS was not etched by XeF2 gas without SR irradiation or not etched by SR irradiation without XeF2 gas. In the following etching experiments, in order to avoid the damage due to the SR beam heating effect, the beam intensity was attenuated using a slit set close to the center of the beam line. The photoemission current was measured to be 30 μA for ring current 110 mA. The following etching was carried out by irradiating the SR beam through three kinds of masks and etching time was 10 min. The pressure of reaction gas XeF2 was controlled in the range of 0.18 to 0.22 torr. Two copper masks, with width of the square window of 20 or 30 μm, were purchased from Okenshoji Inc. The other mask as shown in Fig. 2 (a)
Fig. 2 (a), microscope image of mask with 1 μm size formed by FIB technique; (b), top view of the pattern formed on the PDMS film surface covered with 1 μm size mask; (c) the depth profile of pattern measured with noncontact 3-dimension measuring meter
, with width of the square window of 1 μm, was obtained by etching a piece of gold foil with a thickness of 10 μm using focussed ion beam (FIB) technique, which has been used for milling or deposition in the surface of materials in well defined patterns in the micrometer and nanometer scale (micro- and nanoprinting) [12

12. W. Brostow, B. P. Gorman, and O. Olea-Mejia, “Focused ion beam milling and scanning electron microscopy characterization of polymer metal hybrids,” Mater. Lett. 61(6), 1333–1336 (2007). [CrossRef]

,13

13. C. Aubry, T. Trigaud, J. P. Moliton, and D. Chiron, “Polymer gratings achieved by focused ion beam,” Synth. Met. 127(1-3), 307–311 (2002). [CrossRef]

].

Figure 2(b) shows the top view of the etched pattern on the PDMS surface covered with the mask of 1μm size at 134 mA storage ring current. Figure 2(c) shows depth profile measured with noncontact 3-dimension profile meter (Mitaka). From the cross-section profile of the etched pattern, it is found that the lateral dimensions of the structure viewed from above after removal of the etching mask were 3.7 μm at the bottom and 10 μm at the top of the hole, respectively. The difference of the mask pattern and the etched pattern should be induced by the diffraction effect due to the uncontrollable gap between the mask and PDMS sample surface. When covering PDMS films using the other two masks, it was found that the lateral dimensions of the pattern were 21- 24 μm for the mask of 20 μm size, while the lateral dimensions of the pattern were 32- 35 μm square for the mask of 30 μm size. Compared with the case of the mask of 1 μm size, the diffraction in the case of the mask of 20 or 30 μm size became less obvious.

Figure 3
Fig. 3 The dependence of etched depth on the ring current using the masks with different size: (a), 1 μm; (b), 20 μm; (c) 30 μm.
shows the dependence of the etched depth on the storage ring current. From the repeated measurements made on the sample a maximum error bar of ± 7% was estimated. As expected, etched depth of pattern on PDMS film was found to increase with storage ring current. However, a nonlinear relationship was provided between etched depth and ring current. This was probably because it was difficult to control the gas flow at low XeF2 pressure. It was obvious that the etched depth decreased as the mask size became smaller due to the greater attenuation of SR irradiation. Furthermore, the maximum etched depth for the mask of 30 μm was up to 42 μm. Although we did not discuss the etching reaction mechanism in this work, the high spatial resolution, anisotropic and high speed etching at room temperature indicated that the etching should be induced by the electric excitation of PDMS [14

14. T. Urisu and H. Kyuragi, “Synchrotron radiation-excited chemical-vapor deposition and etching,” J. Vac. Sci. Technol. B 5(5), 1436–1440 (1987). [CrossRef]

16

16. C. Wang, X. Pan, C. Sun, and T. Urisu, “Area-selective deposition of self-assembled monolayers on SiO2/Si(100) patterns,” Appl. Phys. Lett. 89(23), 233105–233107 (2006). [CrossRef]

], since PDMS is a silicon-based polymer. The small pattern size and large etched depth obtained in the etching experiments indicated that the SR etching method will be very useful for the fabrication of microfluidic devices containing special features with micrometer size, such as through holes or irregular shapes.

In order to confirm the feasibility of SR etching for its application in the fabrication of special features, a PDMS-based microfluidic channel containing a through hole was presented by combining SR etching with photolithography. A three-dimensional structure on a piece of round PDMS thin film with 1 mm thickness and 10 mm diameter was fabricated by a molding method. There is a conical shape depression on the upper side surface of PDMS, and a cross road shape microfluidic circuit, which is observed like a wind mill, on the lower side surface as shown in Fig. 4(a)
Fig. 4 (a), The fabricated PDMS-based microfluidic channel; (b) the etched depth profile of single through hole; (c) the optical microscope image of the PC12 cell incubated on the smooth PDMS film surface.
. The circular pattern observed in Fig. 4(a) is the bottom flat area of the conical shape depression with 100 μm in diameter. The height of the cross road shaped microfluidic circuit was about 5 μm. The mold of the conical shape depression was made by machining of acrylic resin, and that of the cross road shape micro fluidic circuit was fabricated by photolithography [17

17. P. Camelliti, J. O. Gallagher, P. Kohl, and A. D. McCulloch, “Micropatterned cell cultures on elastic membranes as an in vitro model of myocardium,” Nat. Protoc. 1(3), 1379–1391 (2006). [CrossRef]

]. The silicon wafer was spin-coated with negative photoresist SU-8 and was exposed to UV light through a photo-mask. The thickness of PDMS film forming the circular bottom of the conical shape depression was about 20 μm. The micro through hole with about 20 μm diameter, which connects the upper and the lower side patterns was fabricated by the XeF2 assisted SR etching in ultra high vacuum as shown in Fig. 4(a). In this SR experiment, we utilized a gold foil mask with a through hole of 20 μm diameter, which was also fabricated by using FIB technique. The mask was carefully set on the PDMS film under a microscope in order to make the through hole as close to the center of the conical shape depression as possible. The SR irradiation time was 20 min at the ring current of 200 −192 mA. Figure 4(b) shows the etched depth profile of the single through hole, and the measured depth was up to 24μm, which indicated that a through hole was formed. In order to confirm the compatibility of cells and fabricated PDMS-based microfluidic channel, a preliminary experiment is carried out to culture the PC12 cells on the smooth PDMS film surface. Figure 4(c) shows the optical microscope image of the PC12 cells cultured on the smooth PDMS substrate, on which collagen IV is coated as an extra cellar matrix. It can be seen that the cells grow well on the surface of PDMS.

4. Conclusion

In conclusion, high spatial resolution, area selective and anisotropic etching of elastic material PDMS film has been demonstrated. Extremely high etching rate (as large as 40-50 μm per 10 min) and small patterning size (in order of several micrometers) can be easily realized by using SR etching, which will have potential values in a wide variety of materials and surface chemistries. Utilizing the SR etching technique and photolithography, a microfluidic channel with three-dimensions was fabricated.

Acknowledgements

References and links

1.

G. M. Whitesides, “The origins and the future of microfluidics,” Nature 442(7101), 368–373 (2006). [CrossRef] [PubMed]

2.

A. E. Kamholz, B. H. Weigl, B. A. Finlayson, and P. Yager, “Quantitative analysis of molecular interaction in a microfluidic channel: the T-sensor,” Anal. Chem. 71(23), 5340–5347 (1999). [CrossRef] [PubMed]

3.

A. E. Kamholz, E. A. Schilling, and P. Yager, “Optical measurement of transverse molecular diffusion in a microchannel,” Biophys. J. 80(4), 1967–1972 (2001). [CrossRef] [PubMed]

4.

A. Folch, B. H. Jo, O. Hurtado, D. J. Beebe, and M. Toner, “Microfabricated elastomeric stencils for micropatterning cell cultures,” J. Biomed. Mater. Res. 52(2), 346–353 (2000). [CrossRef] [PubMed]

5.

G. B. Lee, S. H. Chen, G. R. Huang, W. C. Sung, and Y. H. Lin, “Microfabricated plastic chips by hot embossing methods and their application for DNA separation and detection,” Sens. Actuators B 75, 142–148 (2001).

6.

S. G. Li, Z. G. Xu, A. Mazzeo, D. J. Burns, G. Fu, M. Dirckx, V. Shilpiekandula, X. Chen, N. C. Nayak, E. Wong, S. F. Yoon, Z. P. Fang, K. Youcef-Toumi, D. Hardt, S. B. Tor, C. Y. Yue, and J. H. Chun, “Review of production of microfluidic devices: material, manufacturing and metrology,” Proc. SPIE6993, 69930F1–12 (2008).

7.

C. Gaertner, H. Becker, B. Anton, A. P. O'Neill, and O. Roetting, “Polymer based microfluidic devices: examples for fluidic interfaces and standardization concepts,” Proc. SPIE 4982, 99–104 (2003). [CrossRef]

8.

J. Narasimhan and I. Papautsky, “Rapid fabrication of hot embossing tools using PDMS,” Proc. SPIE 4982, 110–119 (2003). [CrossRef]

9.

Z.-C. Peng, Z.-G. Ling, J. Goettert, J. Hormes, and K. Lian, “Interconnected multilevel microfluidic channels fabricated using low-temperature bonding of SU-8 and multilayer lithography,” Proc. SPIE 5718, 209–215 (2005). [CrossRef]

10.

A. Bubendorfer, X. M. Liu, and A. V. Ellis, “Microfabrication of PDMS microchannels using SU-8/PMMA moldings and their sealing to polystyrene substrates,” Smart Mater. Struct. 16(2), 367–371 (2007). [CrossRef]

11.

M. Liu, J. Sun, Y. Sun, C. Bock, and Q. Chen, “Thickness-dependent mechanical properties of polydimethylsiloxane membranes,” J. Micromech. Microeng. 19(3), 035028 (2009). [CrossRef]

12.

W. Brostow, B. P. Gorman, and O. Olea-Mejia, “Focused ion beam milling and scanning electron microscopy characterization of polymer metal hybrids,” Mater. Lett. 61(6), 1333–1336 (2007). [CrossRef]

13.

C. Aubry, T. Trigaud, J. P. Moliton, and D. Chiron, “Polymer gratings achieved by focused ion beam,” Synth. Met. 127(1-3), 307–311 (2002). [CrossRef]

14.

T. Urisu and H. Kyuragi, “Synchrotron radiation-excited chemical-vapor deposition and etching,” J. Vac. Sci. Technol. B 5(5), 1436–1440 (1987). [CrossRef]

15.

C. S. Wang and T. Urisu, “Synchrotron radiation stimulated etching SiO2 thin films with a contact cobalt mask,” Appl. Surf. Sci. 242(3-4), 276–280 (2005). [CrossRef]

16.

C. Wang, X. Pan, C. Sun, and T. Urisu, “Area-selective deposition of self-assembled monolayers on SiO2/Si(100) patterns,” Appl. Phys. Lett. 89(23), 233105–233107 (2006). [CrossRef]

17.

P. Camelliti, J. O. Gallagher, P. Kohl, and A. D. McCulloch, “Micropatterned cell cultures on elastic membranes as an in vitro model of myocardium,” Nat. Protoc. 1(3), 1379–1391 (2006). [CrossRef]

OCIS Codes
(340.0340) X-ray optics : X-ray optics
(340.6720) X-ray optics : Synchrotron radiation

ToC Category:
X-ray Optics

History
Original Manuscript: January 21, 2010
Revised Manuscript: April 11, 2010
Manuscript Accepted: April 11, 2010
Published: April 23, 2010

Virtual Issues
Vol. 5, Iss. 9 Virtual Journal for Biomedical Optics

Citation
Tingchao He, Changshun Wang, Tsuneo Urisu, Takeshi Nagahiro, Ryugo Tero, and Rong Xia, "The PDMS-based microfluidic channel fabricated by synchrotron radiation stimulated etching," Opt. Express 18, 9733-9738 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-9-9733


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References

  1. G. M. Whitesides, “The origins and the future of microfluidics,” Nature 442(7101), 368–373 (2006). [CrossRef] [PubMed]
  2. A. E. Kamholz, B. H. Weigl, B. A. Finlayson, and P. Yager, “Quantitative analysis of molecular interaction in a microfluidic channel: the T-sensor,” Anal. Chem. 71(23), 5340–5347 (1999). [CrossRef] [PubMed]
  3. A. E. Kamholz, E. A. Schilling, and P. Yager, “Optical measurement of transverse molecular diffusion in a microchannel,” Biophys. J. 80(4), 1967–1972 (2001). [CrossRef] [PubMed]
  4. A. Folch, B. H. Jo, O. Hurtado, D. J. Beebe, and M. Toner, “Microfabricated elastomeric stencils for micropatterning cell cultures,” J. Biomed. Mater. Res. 52(2), 346–353 (2000). [CrossRef] [PubMed]
  5. G. B. Lee, S. H. Chen, G. R. Huang, W. C. Sung, and Y. H. Lin, “Microfabricated plastic chips by hot embossing methods and their application for DNA separation and detection,” Sens. Actuators B 75, 142–148 (2001).
  6. S. G. Li, Z. G. Xu, A. Mazzeo, D. J. Burns, G. Fu, M. Dirckx, V. Shilpiekandula, X. Chen, N. C. Nayak, E. Wong, S. F. Yoon, Z. P. Fang, K. Youcef-Toumi, D. Hardt, S. B. Tor, C. Y. Yue, and J. H. Chun, “Review of production of microfluidic devices: material, manufacturing and metrology,” Proc. SPIE 6993, 69930F1–12 (2008).
  7. C. Gaertner, H. Becker, B. Anton, A. P. O'Neill, and O. Roetting, “Polymer based microfluidic devices: examples for fluidic interfaces and standardization concepts,” Proc. SPIE 4982, 99–104 (2003). [CrossRef]
  8. J. Narasimhan and I. Papautsky, “Rapid fabrication of hot embossing tools using PDMS,” Proc. SPIE 4982, 110–119 (2003). [CrossRef]
  9. Z.-C. Peng, Z.-G. Ling, J. Goettert, J. Hormes, and K. Lian, “Interconnected multilevel microfluidic channels fabricated using low-temperature bonding of SU-8 and multilayer lithography,” Proc. SPIE 5718, 209–215 (2005). [CrossRef]
  10. A. Bubendorfer, X. M. Liu, and A. V. Ellis, “Microfabrication of PDMS microchannels using SU-8/PMMA moldings and their sealing to polystyrene substrates,” Smart Mater. Struct. 16(2), 367–371 (2007). [CrossRef]
  11. M. Liu, J. Sun, Y. Sun, C. Bock, and Q. Chen, “Thickness-dependent mechanical properties of polydimethylsiloxane membranes,” J. Micromech. Microeng. 19(3), 035028 (2009). [CrossRef]
  12. W. Brostow, B. P. Gorman, and O. Olea-Mejia, “Focused ion beam milling and scanning electron microscopy characterization of polymer metal hybrids,” Mater. Lett. 61(6), 1333–1336 (2007). [CrossRef]
  13. C. Aubry, T. Trigaud, J. P. Moliton, and D. Chiron, “Polymer gratings achieved by focused ion beam,” Synth. Met. 127(1-3), 307–311 (2002). [CrossRef]
  14. T. Urisu and H. Kyuragi, “Synchrotron radiation-excited chemical-vapor deposition and etching,” J. Vac. Sci. Technol. B 5(5), 1436–1440 (1987). [CrossRef]
  15. C. S. Wang and T. Urisu, “Synchrotron radiation stimulated etching SiO2 thin films with a contact cobalt mask,” Appl. Surf. Sci. 242(3-4), 276–280 (2005). [CrossRef]
  16. C. Wang, X. Pan, C. Sun, and T. Urisu, “Area-selective deposition of self-assembled monolayers on SiO2/Si(100) patterns,” Appl. Phys. Lett. 89(23), 233105–233107 (2006). [CrossRef]
  17. P. Camelliti, J. O. Gallagher, P. Kohl, and A. D. McCulloch, “Micropatterned cell cultures on elastic membranes as an in vitro model of myocardium,” Nat. Protoc. 1(3), 1379–1391 (2006). [CrossRef]

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