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Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editor: Gregory W. Faris
  • Vol. 4, Iss. 12 — Nov. 10, 2009
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Optically driven micropump with a twin spiral microrotor

Shoji Maruo, Akira Takaura, and Yohei Saito  »View Author Affiliations


Optics Express, Vol. 17, Issue 21, pp. 18525-18532 (2009)
http://dx.doi.org/10.1364/OE.17.018525


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Abstract

An optically driven micropump that employs viscous drag exerted on a spinning microrotor with left- and right-handed spiral blades on its rotational axis has been developed using two-photon microfabrication. It was demonstrated that the twin spiral microrotor provides a higher rotation speed than a single spiral microrotor. The rotation speed reached 560 rpm at a laser power of 500 mW. The twin spiral microrotor was also applied to a viscous micropump with a U-shaped microchannel. To pump fluid, the twin spiral microrotor located at the corner of the U-shaped microchannel was rotated by focusing a laser beam. The flow field inside the U-shaped microchannel was analyzed using the finite element method (FEM) based on the Navier-Stokes equation to optimize the shape of the microchannel. It was confirmed that the rotation of the twin spiral microrotor generated a unidirectional laminar flow. Finally, a tandem micropump using two twin spiral microrotors was driven by a dual optical trapping system using a spatial light modulation technique.

© 2009 OSA

1. Introduction

Optical tweezers have been widely used for manipulating various kinds of small objects such as micro/nano particles, micromachined parts, viruses and cells [1

1. A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm, and S. Chu, “Observation of a single-beam gradient force optical trap for dielectric particles,” Opt. Lett. 11(5), 288–290 (1986). [CrossRef] [PubMed]

,2

2. D. G. Grier, “A revolution in optical manipulation,” Nature 424(6950), 810–816 (2003). [CrossRef] [PubMed]

]. The remote manipulation of microobjects is a powerful technique for micro-assembly [3

3. P. J. Rodrigo, L. Kelemen, D. Palima, C. A. Alonzo, P. Ormos, and J. Glückstad, “Optical microassembly platform for constructing reconfigurable microenvironments for biomedical studies,” Opt. Express 17(8), 6578–6583 (2009). [CrossRef] [PubMed]

], nano-patterning [4

4. S. Ito, H. Yoshikawa, and H. Masuhara, “Laser manipulation and fixation of single gold nanoparticles in solution at room temperature,” Appl. Phys. Lett. 80(3), 482–484 (2002). [CrossRef]

], and driving microparts [5

5. E. Higurashi, H. Ukita, H. Tanaka, and O. Ohguchi, “Optically induced rotation of anisotropic micro-objects fabricated by surface micromachining,” Appl. Phys. Lett. 64(17), 2209–2210 (1994). [CrossRef]

11

11. S. Maruo and Y. Hiratsuka, “Optically driven micromanipulators with rotating arms,” J. Rob. Mechatronics 19, 565–568 (2007).

]. Optically driven microfluidic devices using colloids, in which microparticles are driven and controlled by optical tweezers to generate and control the flow, is a promising application of optical manipulation technologies, and has received a lot of attention in recent years [12

12. A. Terray, J. Oakey, and D. W. M. Marr, “Microfluidic control using colloidal devices,” Science 296(5574), 1841–1844 (2002). [CrossRef] [PubMed]

15

15. K. Ladavac and D. Grier, “Microoptomechanical pumps assembled and driven by holographic optical vortex arrays,” Opt. Express 12(6), 1144–1149 (2004). [CrossRef] [PubMed]

]. This is because these microfluidic devices can provide unique features such as remote control inside a microchannel and the precise regulation of ultra-low flow rates. Although these devices are simple and versatile, their performance is limited by the shape of colloids. In contrast, our group has used two-photon microfabrication to develop optically driven micropumps using micromachined elements, including a lobed micropump [10

10. S. Maruo and H. Inoue, “Optically driven micropump produced by three-dimensional two-photon microfabrication,” Appl. Phys. Lett. 89(14), 144101 (2006). [CrossRef]

] and a viscous micropump [16

16. S. Maruo and H. Inoue, “Optically driven viscous micropump using a rotating microdisk,” Appl. Phys. Lett. 91(8), 084101 (2007). [CrossRef]

]. Two-photon microfabrication can produce complicated three-dimensional microstructures with sub-100 nm resolution [17

17. S. Maruo, O. Nakamura, and S. Kawata, “Three-dimensional microfabrication with two-photon-absorbed photopolymerization,” Opt. Lett. 22(2), 132–134 (1997). [CrossRef] [PubMed]

21

21. S. Maruo and J. T. Fourkas, “Recent progress in multiphoton microfabrication,” Lasers Photon Reviews 2(1-2), 100–111 (2008). [CrossRef]

] Therefore, precise microfluidic devices can be fabricated by scanning a femtosecond pulsed laser beam inside a photopolymer [10

10. S. Maruo and H. Inoue, “Optically driven micropump produced by three-dimensional two-photon microfabrication,” Appl. Phys. Lett. 89(14), 144101 (2006). [CrossRef]

,16

16. S. Maruo and H. Inoue, “Optically driven viscous micropump using a rotating microdisk,” Appl. Phys. Lett. 91(8), 084101 (2007). [CrossRef]

].

2. Viscous micropump using twin spiral microrotor

2.1 Basic design of viscous micropump using twin spiral microrotor

The optically induced rotation of a microrotor with a single spiral blade was first reported by P. Galajda and P. Ormos [23

23. P. Galajda and P. Ormos, “Rotors produced and driven in laser tweezers with reversed direction of rotation,” Appl. Phys. Lett. 80(24), 4653–4655 (2002). [CrossRef]

]. In their experiments, they demonstrated that the rotation direction of the spiral microrotor is reversed when the trapped position of the microrotor is changed from below to beyond the focus along the optical axis. We employ the phenomenon of the reversed rotation of the single spiral microrotor to create a novel microrotor that has higher rotational speed than a single spiral microrotor. As shown in Fig. 1, in our twin spiral microrotor, two spiral blades with opposite directions are connected along the optical axis inside an outer cylinder. When a laser beam is focused on the center of the twin spiral microrotor, optical torque induced by the optical radiation pressure exerted on the spiral blades is generated in the same direction, because the spiral microrotors are mounted in opposite directions. Therefore, the net optical torque generated by the twin spiral microrotor can exceed that of the single spiral microrotor. In addition, the twin spiral rotor can be trapped at the focus, because the net radiation pressure exerted on the central column is directed toward the focus like a normal cylindrical micro-object.

2.2 Fabrication of twin spiral microrotors using two-photon microfabrication

Two-photon microfabrication was used to construct the twin spiral microrotor. Our fabrication system is based on a mirror scanning method for fabricating a freely movable microrotor separated from a substrate inside a photopolymer. In the fabrication system, a mode-locked Ti:sapphire laser (Mira 900-F, Coherent Inc., wavelength 752 nm; repetition rate 76 MHz; pulse width 200 fs) is used to generate the two-photon absorbed photopolymerization. A laser is equipped with a galvano-scanner system to deflect the beam direction in two dimensions, and it is then focused with an objective lens with a numerical aperture of 1.25. The beam scans the photopolymer laterally while the stage that supports the photopolymer is scanned vertically, thereby moving the point of focus in three dimensions. By controlling both the galvano-scanner system and the stage with 3D computer-aided design data, we can fabricate the desired 3D microstructure, which even includes a freely movable micropart. After the 3D fabrication process, the unsolidified photopolymer is washed away with a solvent, leaving only the created microstructure. In our experiments, we used a commercial epoxy-type photopolymer (SCR-701, D-MEC Ltd.) as the microrotor and microchannel material.

2.3 Optical driving of single and twin spiral microrotors

To demonstrate the validity of our twin spiral microrotor, we examined the driving performance of single and twin spiral microrotors experimentally. We fabricated three types of microrotors: single and twin spiral microrotors, and a twin spiral microrotor with an outer cylinder. Figure 2
Fig. 2 Scanning electron microscope image of a twin spiral microrotor with an outer cylinder. (a) Top view (Media 1) (b) Side view (Media 2).
shows a scanning electron microscope image of the twin spiral microrotor with an outer cylinder. The diameter and height of the microrotor were 4 and 9 µm, respectively. The opposite facing spiral blades were fabricated by the direct writing of a femtosecond pulsed laser beam inside the photopolymer. When extracting the microrotor from unsolidified photopolymer, we used a supercritical CO2 drying process to reduce harmful deformation caused by the surface tension of the rinse [24

24. S. Maruo, T. Hasegawa and N. Yoshimura “Replication of three-dimensional rotary micromechanism by membrane-assisted transfer molding,” Jpn. J. Appl. Phys. 48, 06FH05 (2009).

,25

25. T. Hasegawa and S. Maruo, ““Two-photon microfabrication with a supercritical CO2 drying process toward replication of three-dimensional microstructures,”Proc. of Int,” Symp. on Micro-nanomechatronics and Human Science 2007, 12–15 (2007) (MHS). [CrossRef]

].

To drive the three types of microrotors, we used our optical system for two-photon microfabrication, while operating the Ti:sapphire laser in the CW mode. The surrounding liquid was glycol ether ester, which we used for washing out the liquid photopolymer. Figure 3
Fig. 3 Microrotor rotation speeds dependence on laser power.
shows the microrotor rotation speed dependence on laser power. The rotation speed of each microrotor was proportional to the intensity of the induced laser beam. We found that the rotation speed of the twin spiral microrotor was 3.5 times that of the single spiral microrotor. The maximum speed reached 560 rpm at a laser power of 500 mW (Media 1).

Since the twin spiral microrotor is trapped at the focus, both blades offers unidirectional torque effectively. By contrast, when the single spiral microrotor is trapped at the focus, the upper part of the spiral blade is beyond the focus owing to net radiation pressure exerted on the centeral column. Consequently, the upper part of the spiral blade causes opposite torque, and reduces rotation speed. As a result, the rotation speed of the twin spiral microrotor was more than the double of the single spiral microrotor. Although the rotation speed of the twin spiral microrotor with an outer cylinder was slower than that of the twin spiral microrotor alone, it exceeded 300 rpm (Media 2). These results indicate that twin spiral blades are useful for increasing the rotation speed of a microrotor.

2.4 Finite element analysis of flow distribution of viscous micropump

2.5 Channel width optimization

We examined the pressure field, flow velocity and streamlines of the micropump by changing the width of the microchannel. As a result, we found that when the channel width was small, a large backflow was generated around the microrotor owing to the pressure gradient against the flow direction. On the other hand, when the channel width was wide, the pressure gradient was shallow. In this simulation, we found that the backflow disappeared when the channel width was larger than 6 µm. Figure 4
Fig. 4 Dependence of the maximum flow velocity of the laminar flow on channel width.
shows the dependence of the maximum flow velocity of the laminar flow on the channel width. The results indicated that the flow rate was saturated at a microchannel width of 10 µm with a 4 µm-diameter microrotor. Figure 5
Fig. 5 Simulation results for the flow velocity and streamlines of a micropump with a 10 µm-width microchannel. (a) Flow velocity (b) Streamlines.
shows typical simulation results for the flow velocity and streamlines of the micropump, which has a 10 µm-wide microchannel. Here, the viscous force applied by the spinning microrotor can be efficiently transferred from the inlet to the outlet through the U-shaped microchannel. Fluid can therefore be transported in the forward direction because there is sufficient viscous force against the backpressure caused by the rotation of the microrotor. A steady laminar flow is generated in the linear regions of the U-shaped microchannels.

2.6 Fabrication and driving of viscous micropump using twin spiral microrotor

We fabricated a viscous micropump containing a twin spiral microrotor (diameter: 4 μm) inside a U-shaped microchannel using two-photon microfabrication. In this process, a femtosecond laser beam is focused and scanned inside a photopolymer to produce microchannel, including U-shape groove and upper cover, on a cover glass. Even movable microparts without any anchoring structures can be fabricated by scanning the laser beam along the proper scanning trajectory at high speeds [9

9. S. Maruo, K. Ikuta, and H. Korogi, “Force-controllable, optically driven micromachines fabricated by single-step two-photon microstereolithography,” J. Microelectromech. Syst. 12(5), 533–539 (2003). [CrossRef]

]. We demonstrated that the viscous micropump with a twin spiral microrotor can be used for pumping fluid and the transportation of microobjects. Figure 6
Fig. 6 Particle transportation in a viscous micropump using a twin spiral microrotor (Media 3).
contains a sequence of images taken at 2-s intervals showing a tracer particle being pumped through the channel when the microrotor was rotating at a speed of 300 rpm. In this case, the flow rate of the micropump was estimated at 18 pL/min. The trajectory of the tracer particle movement was similar to the streamlines obtained by the FEM simulation shown in Fig. 5. The flow velocity around the microrotor was higher than that of the linear microchannel, since the viscous drag around the microrotor was dominant.

3. Tandem micropump using two twin spiral microrotors

By connecting multiple viscous micropumps with a crooked microchannel, we can construct a tandem micropump using multiple twin spiral microrotors. As an example, we fabricated a tandem micropump using two microrotors. Since the twin spiral microrotors are rotated at a high speed of over several hundred rpm, multiple focal spots are needed to rotate the two microrotors simultaneously. Therefore, we constructed an optical micromanipulation system using a spatial modulation technique. The optical system consists mainly of a green laser (Verdi-5, Coherent Inc., wavelength: 532 nm, maximum laser power: 5W), and a liquid crystal spatial light modulator (PPM-X8267, Hamamatsu Photonics). The laser beam is diffracted at the spatial light modulator, and forms the desired multiple spots at the Fourier plane of a lens (focal length: 300 mm). The multiple laser spots are then reduced with a lens (focal length: 100 mm) and an objective lens with a numerical aperture of 1.45. Finally, the multiple laser spots are focused on the twin spiral rotors. In our experiments, we inputted the phase distribution of the multiple Fresnel lens into the spatial light modulator to generate multiple focuses at the focal plane. The simultaneous focusing at multiple points makes it possible to drive multiple microrotors at high speed. Figure 7
Fig. 7 Optical microscope image of a tandem micropump driven by dual optical trapping (Media 4).
shows an optical microscope image of the tandem micropump driven by dual optical trapping using the spatial light modulation technique. Both microrotors were stably rotated at each corner of the microchannel. The tandem micropump is useful for the long-distance transportation of microobjects such as living cells and microparticles.

4. Conclusions

An optically driven viscous micropump with a twin spiral microrotor was developed by employing two-photon microfabrication. The twin spiral microrotor could be rotated at a high speed of over 500 rpm by focusing a laser beam, because the optical torque exerted on the left- and right-handed spiral blades was imposed in the same direction. In addition, a tandem micropump including two microrotors in a crooked microchannel was developed. The high-speed microrotors were simultaneously trapped and rotated by optical tweezers using a spatial light modulation technique. These viscous micropumps provide several advantages, including simplicity, ease of miniaturization, a readily controlled flow velocity, the production of a continuous flow, and the safe transportation of biological samples such as cells. In the near future, optically driven micropumps will be utilized for lab-on-a-chip devices where specific features such as an ultralow flow rate, steady continuous flows and the damage-free transportation of biological samples are required.

Acknowledgments

This research was partially supported by research grants from PRESTO, the Japan Science and Technology Agency, and from the Japan Society for the Promotion of Science (Grant-in-Aid Exploratory Research and Scientific Research in Priority Areas: System Cell Engineering by Multi-scale Manipulation, and Grant-in-Aid for Challenging Exploratory Research). The authors also acknowledge Fluid Power Technology Promotion Foundation for its support.

References and links

1.

A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm, and S. Chu, “Observation of a single-beam gradient force optical trap for dielectric particles,” Opt. Lett. 11(5), 288–290 (1986). [CrossRef] [PubMed]

2.

D. G. Grier, “A revolution in optical manipulation,” Nature 424(6950), 810–816 (2003). [CrossRef] [PubMed]

3.

P. J. Rodrigo, L. Kelemen, D. Palima, C. A. Alonzo, P. Ormos, and J. Glückstad, “Optical microassembly platform for constructing reconfigurable microenvironments for biomedical studies,” Opt. Express 17(8), 6578–6583 (2009). [CrossRef] [PubMed]

4.

S. Ito, H. Yoshikawa, and H. Masuhara, “Laser manipulation and fixation of single gold nanoparticles in solution at room temperature,” Appl. Phys. Lett. 80(3), 482–484 (2002). [CrossRef]

5.

E. Higurashi, H. Ukita, H. Tanaka, and O. Ohguchi, “Optically induced rotation of anisotropic micro-objects fabricated by surface micromachining,” Appl. Phys. Lett. 64(17), 2209–2210 (1994). [CrossRef]

6.

P. Galajda and P. Ormos, “Complex micromachines produced and driven by light,” Appl. Phys. Lett. 78(2), 249–251 (2001). [CrossRef]

7.

S. L. Neale, M. P. MacDonald, K. Dholakia, and T. F. Krauss, “All-optical control of microfluidic components using form birefringence,” Nat. Mater. 4(7), 530–533 (2005). [CrossRef] [PubMed]

8.

S. Maruo, K. Ikuta, and H. Korogi, “Submicron manipulation tools driven by light in a liquid,” Appl. Phys. Lett. 82(1), 133–135 (2003). [CrossRef]

9.

S. Maruo, K. Ikuta, and H. Korogi, “Force-controllable, optically driven micromachines fabricated by single-step two-photon microstereolithography,” J. Microelectromech. Syst. 12(5), 533–539 (2003). [CrossRef]

10.

S. Maruo and H. Inoue, “Optically driven micropump produced by three-dimensional two-photon microfabrication,” Appl. Phys. Lett. 89(14), 144101 (2006). [CrossRef]

11.

S. Maruo and Y. Hiratsuka, “Optically driven micromanipulators with rotating arms,” J. Rob. Mechatronics 19, 565–568 (2007).

12.

A. Terray, J. Oakey, and D. W. M. Marr, “Microfluidic control using colloidal devices,” Science 296(5574), 1841–1844 (2002). [CrossRef] [PubMed]

13.

A. Terray, J. Oakey, and D. W. M. Marr, “Fabrication of linear colloidal structures for microfluidic applications,” Appl. Phys. Lett. 81(9), 1555–1557 (2002). [CrossRef]

14.

J. Leach, H. Mushfique, R. di Leonardo, M. Padgett, and J. Cooper, “An optically driven pump for microfluidics,” Lab Chip 6(6), 735–739 (2006). [CrossRef] [PubMed]

15.

K. Ladavac and D. Grier, “Microoptomechanical pumps assembled and driven by holographic optical vortex arrays,” Opt. Express 12(6), 1144–1149 (2004). [CrossRef] [PubMed]

16.

S. Maruo and H. Inoue, “Optically driven viscous micropump using a rotating microdisk,” Appl. Phys. Lett. 91(8), 084101 (2007). [CrossRef]

17.

S. Maruo, O. Nakamura, and S. Kawata, “Three-dimensional microfabrication with two-photon-absorbed photopolymerization,” Opt. Lett. 22(2), 132–134 (1997). [CrossRef] [PubMed]

18.

S. Kawata, H.-B. Sun, T. Tanaka, and K. Takada, “Finer features for functional microdevices,” Nature 412(6848), 697–698 (2001). [CrossRef] [PubMed]

19.

W. Haske, V. W. Chen, J. M. Hales, W. T. Dong, S. Barlow, S. R. Marder, and J. W. Perry, “65 nm feature sizes using visible wavelength 3-D multiphoton lithography,” Opt. Express 15(6), 3426–3436 (2007). [CrossRef] [PubMed]

20.

L. Li, R. R. Gattass, E. Gershgoren, H. Hwang, and J. T. Fourkas, “Achieving λ/20 resolution by one-color initiation and deactivation of polymerization,” Science 324(5929), 910–913 (2009). [CrossRef] [PubMed]

21.

S. Maruo and J. T. Fourkas, “Recent progress in multiphoton microfabrication,” Lasers Photon Reviews 2(1-2), 100–111 (2008). [CrossRef]

22.

A. Takaura, H. Inoue, and S. Maruo, ““Laser-driven viscous micropump using a single microrotor,”Proc. of Int,” Symp. on Micro-nanomechatronics and Human Science 2007, 16–20 (2007) (MHS). [CrossRef]

23.

P. Galajda and P. Ormos, “Rotors produced and driven in laser tweezers with reversed direction of rotation,” Appl. Phys. Lett. 80(24), 4653–4655 (2002). [CrossRef]

24.

S. Maruo, T. Hasegawa and N. Yoshimura “Replication of three-dimensional rotary micromechanism by membrane-assisted transfer molding,” Jpn. J. Appl. Phys. 48, 06FH05 (2009).

25.

T. Hasegawa and S. Maruo, ““Two-photon microfabrication with a supercritical CO2 drying process toward replication of three-dimensional microstructures,”Proc. of Int,” Symp. on Micro-nanomechatronics and Human Science 2007, 12–15 (2007) (MHS). [CrossRef]

OCIS Codes
(090.1760) Holography : Computer holography
(120.4610) Instrumentation, measurement, and metrology : Optical fabrication
(140.7010) Lasers and laser optics : Laser trapping
(170.4520) Medical optics and biotechnology : Optical confinement and manipulation

ToC Category:
Optical Trapping and Manipulation

History
Original Manuscript: August 20, 2009
Revised Manuscript: September 23, 2009
Manuscript Accepted: September 24, 2009
Published: September 29, 2009

Virtual Issues
Vol. 4, Iss. 12 Virtual Journal for Biomedical Optics

Citation
Shoji Maruo, Akira Takaura, and Yohei Saito, "Optically driven micropump with a twin spiral microrotor," Opt. Express 17, 18525-18532 (2009)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-17-21-18525


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References

  1. A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm, and S. Chu, “Observation of a single-beam gradient force optical trap for dielectric particles,” Opt. Lett. 11(5), 288–290 (1986). [CrossRef] [PubMed]
  2. D. G. Grier, “A revolution in optical manipulation,” Nature 424(6950), 810–816 (2003). [CrossRef] [PubMed]
  3. P. J. Rodrigo, L. Kelemen, D. Palima, C. A. Alonzo, P. Ormos, and J. Glückstad, “Optical microassembly platform for constructing reconfigurable microenvironments for biomedical studies,” Opt. Express 17(8), 6578–6583 (2009). [CrossRef] [PubMed]
  4. S. Ito, H. Yoshikawa, and H. Masuhara, “Laser manipulation and fixation of single gold nanoparticles in solution at room temperature,” Appl. Phys. Lett. 80(3), 482–484 (2002). [CrossRef]
  5. E. Higurashi, H. Ukita, H. Tanaka, and O. Ohguchi, “Optically induced rotation of anisotropic micro-objects fabricated by surface micromachining,” Appl. Phys. Lett. 64(17), 2209–2210 (1994). [CrossRef]
  6. P. Galajda and P. Ormos, “Complex micromachines produced and driven by light,” Appl. Phys. Lett. 78(2), 249–251 (2001). [CrossRef]
  7. S. L. Neale, M. P. MacDonald, K. Dholakia, and T. F. Krauss, “All-optical control of microfluidic components using form birefringence,” Nat. Mater. 4(7), 530–533 (2005). [CrossRef] [PubMed]
  8. S. Maruo, K. Ikuta, and H. Korogi, “Submicron manipulation tools driven by light in a liquid,” Appl. Phys. Lett. 82(1), 133–135 (2003). [CrossRef]
  9. S. Maruo, K. Ikuta, and H. Korogi, “Force-controllable, optically driven micromachines fabricated by single-step two-photon microstereolithography,” J. Microelectromech. Syst. 12(5), 533–539 (2003). [CrossRef]
  10. S. Maruo and H. Inoue, “Optically driven micropump produced by three-dimensional two-photon microfabrication,” Appl. Phys. Lett. 89(14), 144101 (2006). [CrossRef]
  11. S. Maruo and Y. Hiratsuka, “Optically driven micromanipulators with rotating arms,” J. Rob. Mechatronics 19, 565–568 (2007).
  12. A. Terray, J. Oakey, and D. W. M. Marr, “Microfluidic control using colloidal devices,” Science 296(5574), 1841–1844 (2002). [CrossRef] [PubMed]
  13. A. Terray, J. Oakey, and D. W. M. Marr, “Fabrication of linear colloidal structures for microfluidic applications,” Appl. Phys. Lett. 81(9), 1555–1557 (2002). [CrossRef]
  14. J. Leach, H. Mushfique, R. di Leonardo, M. Padgett, and J. Cooper, “An optically driven pump for microfluidics,” Lab Chip 6(6), 735–739 (2006). [CrossRef] [PubMed]
  15. K. Ladavac and D. Grier, “Microoptomechanical pumps assembled and driven by holographic optical vortex arrays,” Opt. Express 12(6), 1144–1149 (2004). [CrossRef] [PubMed]
  16. S. Maruo and H. Inoue, “Optically driven viscous micropump using a rotating microdisk,” Appl. Phys. Lett. 91(8), 084101 (2007). [CrossRef]
  17. S. Maruo, O. Nakamura, and S. Kawata, “Three-dimensional microfabrication with two-photon-absorbed photopolymerization,” Opt. Lett. 22(2), 132–134 (1997). [CrossRef] [PubMed]
  18. S. Kawata, H.-B. Sun, T. Tanaka, and K. Takada, “Finer features for functional microdevices,” Nature 412(6848), 697–698 (2001). [CrossRef] [PubMed]
  19. W. Haske, V. W. Chen, J. M. Hales, W. T. Dong, S. Barlow, S. R. Marder, and J. W. Perry, “65 nm feature sizes using visible wavelength 3-D multiphoton lithography,” Opt. Express 15(6), 3426–3436 (2007). [CrossRef] [PubMed]
  20. L. Li, R. R. Gattass, E. Gershgoren, H. Hwang, and J. T. Fourkas, “Achieving λ/20 resolution by one-color initiation and deactivation of polymerization,” Science 324(5929), 910–913 (2009). [CrossRef] [PubMed]
  21. S. Maruo and J. T. Fourkas, “Recent progress in multiphoton microfabrication,” Lasers Photon Reviews 2(1-2), 100–111 (2008). [CrossRef]
  22. A. Takaura, H. Inoue, and S. Maruo, ““Laser-driven viscous micropump using a single microrotor,” Proc. of Int,” Symp. on Micro-nanomechatronics and Human Science 2007, 16–20 (2007) (MHS). [CrossRef]
  23. P. Galajda and P. Ormos, “Rotors produced and driven in laser tweezers with reversed direction of rotation,” Appl. Phys. Lett. 80(24), 4653–4655 (2002). [CrossRef]
  24. S. Maruo, T. Hasegawa and N. Yoshimura “Replication of three-dimensional rotary micromechanism by membrane-assisted transfer molding,” Jpn. J. Appl. Phys. 48, 06FH05 (2009).
  25. T. Hasegawa and S. Maruo, ““Two-photon microfabrication with a supercritical CO2 drying process toward replication of three-dimensional microstructures,” Proc. of Int,” Symp. on Micro-nanomechatronics and Human Science 2007, 12–15 (2007) (MHS). [CrossRef]

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