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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 14 — Jul. 9, 2007
  • pp: 9009–9014
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2D optical manipulation and assembly of shape-complementary planar microstructures

Peter John Rodrigo, Lóránd Kelemen, Carlo Amadeo Alonzo, Ivan R. Perch-Nielsen, Jeppe Seidelin Dam, Pál Ormos, and Jesper Glückstad  »View Author Affiliations


Optics Express, Vol. 15, Issue 14, pp. 9009-9014 (2007)
http://dx.doi.org/10.1364/OE.15.009009


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Abstract

Optical trapping and manipulation offer great flexibility as a non-contact microassembly tool. Its application to the assembly of microscale building blocks may open new doors for micromachine technology. In this work, we demonstrate all-optical assembly of microscopic puzzle pieces in a fluidic environment using programmable arrays of trapping beams. Identical shape-complimentary pieces are optically fabricated with submicron resolution using two-photon polymerization (2PP) technique. These are efficiently assembled into space-filling tessellations by a multiple-beam optical micromanipulation system. The flexibility of the system allows us to demonstrate both user-interactive and computer-automated modes of serial and parallel assembly of microscale objects with high spatial and angular positioning precision.

© 2007 Optical Society of America

1. Introduction

2. Design and fabrication of micropuzzle pieces

2.1 Geometrical considerations for the micropuzzle pieces

Not unlike its macroscopic analogue, the tiling of micropuzzles we perform here known as tessellation follows a simple rule of bringing two-dimensional shapes together to cover a plane or substrate without overlapping and without leaving gaps. There exist a wide variety of tessellations to choose from but the particular one selected here is shown in Fig. 1(a). With this design preference, identically shaped puzzle pieces whose geometry is depicted in Fig. 1(b) can be fabricated, thereby simplifying both optical procedures of fabrication and manipulation. The target puzzle pattern exhibits a characteristic checkerboard arrangement of horizontally and vertically symmetric tiles. It falls under the “p4g“ category [9] of the seventeen possible plane symmetry groups (a mathematical classification of 2D repetitive patterns).

Fig. 1. Illustration of (a) the desired tessellation to be optically assembled and (b) a single micropuzzle element, which is a symmetric cutout of a circle of radius r.

Other simpler tessellations are those that can be formed by tiling one type of congruent regular polygons such as equilateral triangles, squares or hexagons. For the optical manipulation part, the puzzle piece design in Fig. 1 seems a reasonable choice since laterally elongated microobjects have been oriented by similarly elongated optical traps with good rotational selectivity [10

10. A. T. O'Neil and M. J. Padgett, “Rotational control within optical tweezers by use of a rotating aperture,” Opt. Lett. 27, 743–745 (2002). [CrossRef]

]. Here we straightforwardly realize the sufficient trap geometry for controlling both the position and the angular orientation of a puzzle piece by using adjacently paired optical traps with identical tophat intensity profiles. The size of the composite trap is adjusted to match that of the microobject. Our optical trapping graphical user interface (GUI) developed in LabVIEW readily offers these trap-grouping and trap-scaling features together with the necessary control over clusters of traps, i.e. xy-position and angular rotation about the centroid of each paired traps.

2.2 Microfabrication via 2PP technique

Fig. 2. Selected trajectories for the focused femtosecond laser beam in the 2PP fabrication of the puzzle pieces with characteristic radius r = 2.5 μm. Inset shows the corresponding SEM micrographs of the 2PP-fabricated structures.

After the whole 2PP-writing process, a 2-min post-exposure bake is performed at 95 °C. The sample is cooled down and the portion of substrate with polymerized puzzle pieces is placed into a reservoir with the SU8 layer facing up. Then 100 μL of a developer solution (Micro Resist Technology GmbH, Berlin, Germany) was gently added into the reservoir to remove the unsolidified SU8. In this process, puzzle pieces slowly drift from their original locations and eventually settle onto the glass substrate. The developer is replaced with another fresh 100-μL solution, which does not cause further positional drift. Then the developer is removed and the reservoir is rinsed gently with ethanol and dried. To obtain fluid-borne microstructures from the substrate, we first add few microliters of 5% surfactant solution (Tween 20, Sigma-Aldrich) to the reservoir. Then, while viewing the sample under a low-magnification microscope, we detach and extract the puzzle pieces using a customized mechanical micromanipulator made from glass capillary tube connected to a microsyringe by silicon rubber tubing. The free and sharper end of the capillary tube has approximately 30-μm diameter opening and is controllably positioned by a motorized 3D manipulator arm. An SEM image of the micropuzzle pieces is shown in the inset of Fig. 2. The estimated 400-500 nm lateral resolution of the 2PP method based on the implemented parameters enabled us to produce the pieces with acceptable morphology and uniformity. We estimate the thickness of the structures to be 1.0 ± 0.2 μm.

3. Optical micromanipulation: results and discussion

3.1 Optical trapping performance for position and orientation control of microstructures

In testing the viability of our multiple-beam trapping system as a microassembly tool, we first measure the degrees of spatial localization and angular orientation that can be achieved for a single puzzle piece. To do so, we initially position a puzzle element at the center position, (x, y) = (0, 0), of the imaging field of view using horizontally aligned pair of optical traps (Fig. 3). With our LabVIEW GUI, we capture a video (10 frames/sec), whose frames are subsequently sent to an image analysis subroutine that measures the angle θ of the longer symmetry-axis and centroid coordinates (x, y) of the microobject. Plots of θ(t) and the radial distance ρ(t), ρ 2 = x 2 + y 2, in Fig. 3 show that, in the presence of a composite trap with a total power of ~6 mW (i.e. a pair of non-overlapping tophat-profiled beams imaged onto the sample plane), translational control of the microstructure can be achieved with submicron resolution. Angular control is precise to within a few degrees. While the puzzle element is trapped for ~95 sec, the measured standard deviations for ρ and θ are 39.6 nm and 3.41 deg, respectively. Immediately after the laser trap is switched off, the puzzle piece observably wanders away with unstable angular orientation due to Brownian motion.

Fig. 3. Plot of the angular orientation and radial position of a puzzle piece as a function of time in the presence and absence of paired trapping beams. Relative positions of the paired traps are indicated by the two circular markers overlaid with the image of the trapped and horizontally oriented microobject (inset).

3.2 All-optical assembly of micropuzzles

Fig. 4. (AVI, 2.4 MB, 3x speed) User-interactive pick-and-place optical assembly of sixteen micropuzzle pieces into a 4×4 tiling. The linear and angular speeds at which the pieces are translated and rotated are ~2 μm/sec and ~20 deg/sec, respectively.

At higher particle densities, parallel trapping and assembly becomes more feasible as more micropuzzle pieces can be found within a single microscope scene. Using this approach, tessellations of the microscale pieces have been formed in a 4×4-tiling configuration (see Fig. 5) using a single GUI execution that moves all the trapping beams, and thereby the constituent pieces, simultaneously along pre-defined paths as specified by the user. This mode results in shorter assembly time of a few seconds.

It is also noted that the larger tiled structure consisting of several constituents can be translated and rotated by the array of traps (~6 mW trapping power for each puzzle piece) as one entity at linear and angular speeds of approximately 1.5 μm/sec and 10 deg/sec, respectively, without compromising structural integrity. This means that the sample stage may be moved at similar speeds instead of the grouped traps and can serve as a means of bringing the tessellated pieces to a farther transverse location in the sample where other free pieces could be gathered and thereby increasing the size of the tessellation. Such procedure has been applied in the computer-automated mode of microassembly.

Fig. 5. (AVI, 0.9 MB) Larger tessellation of sixteen micropuzzle pieces optically assembled in parallel into a 4×4 arrangement. Once assembled (~7 sec from 1st to 3rd frame), adjacent elements remain intact while the superstructure is displaced and rotated.

Fig. 6. (AVI, 2.2 MB, 2x speed) Computer-automated “hunt-and-collect” procedure for tiling micropuzzle pieces. The dashed rectangle highlights the selected detection area where pieces coming from the left due to constant-speed sample stage movement are automatically detected. Once detected, trapping beams with appropriate target trajectories are immediately assigned.

Several methods are also available if microassemblies of permanently bonded components are desired. One example is the use of polymerizable liquid medium that enable a focused laser to locally fuse microelements either by one-photon or two-photon absorption at elements’ adjacent edges while they are held in place by optical traps [11

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

]. Microelements may also be attached together by surface functionalization or critical point drying methods.

4. Conclusion

To address the need for microassembly methods particularly for objects with dimensions much below 100 μm, we demonstrated the use of real-time adjustable multiple optical traps to assemble micropuzzles with user-interactive or computer-automated control. The demonstration of a fully autonomous search-and-collect routine highlighted the potential usefulness of real-time reconfigurable optical traps for working with low-density distributions of microparticles. We characterized the accuracy with which the optical traps are able to position and orient microscale components. A few miliwatts of trapping power can achieve precise placement of microstructures with only tens of nanometers and few degrees of variations in position and angle, respectively. This can be enhanced further by increase in trapping power. The reliance on complementarity of shapes of the puzzle components, accurately fabricated by the two-photon polymerization technique, may also be applied to future extension of optical microassembly using microscale building blocks with more intricate geometries.

Acknowledgments

We acknowledge the support from the EU-FP6-NEST program (ATOM3D), the ESF-Eurocores-SONS program (SPANAS), the Danish Technical Scientific Research Council (FTP), the Hungarian Scientific Research Fund (grant T 046747 for P. O.) and the National Office for Research and Technology, Hungary (grant NKFP1/0007/2005 for L. K.). We thank Erzsébet Mihalik, head of the Department of Botany and Botanic Garden, University of Szeged, for the scanning electron micrographs.

References and links

1.

M. Gauthier, D. Heriban, D. Gendreau, S. Regnier, P. Lutz, and N. Chaillet, “Micro-factory for submerged assembly: interests and architectures,” Proc. 5th Int. Workshop on Microfactories (2006). [PubMed]

2.

J. J. Talghader, J. K. Tu, and J. S. Smith, “Integration of fluidically self-assembled optoelectronic devices using silicon-based process,” IEEE Photon. Technol. Lett. 7, 1321–1323 (1995). [CrossRef]

3.

K. Hosokawa, I. Shimoyama, and H. Miura, “Two-dimensional micro-self-assembly using the surface tension of water,” Sens. Actuators A 57, 117–125 (1996). [CrossRef]

4.

R. L. Eriksen, V. R. Daria, and J. Glückstad, “Fully dynamic multiple-beam optical tweezers,” Opt. Express 10, 597–602 (2002). [PubMed]

5.

P. J. Rodrigo, R. L. Eriksen, V. R. Daria, and J. Glückstad, “Interactive light-driven and parallel manipulation of inhomogeneous particles,” Opt. Express 10, 1550–1556 (2002). [PubMed]

6.

I. R. Perch-Nielsen, P. J. Rodrigo, C. A. Alonzo, and J. Glückstad, “Autonomous and 3D real-time multi-beam manipulation in a microfluidic environment,” Opt. Express 14, 12199–12205 (2006). [CrossRef] [PubMed]

7.

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

8.

L. Kelemen, S. Valkai, and P. Ormos, “Integrated optical motor,” Appl. Opt. 45, 2777–2780 (2006). [CrossRef] [PubMed]

9.

http://mathworld.wolfram.com/WallpaperGroups.html

10.

A. T. O'Neil and M. J. Padgett, “Rotational control within optical tweezers by use of a rotating aperture,” Opt. Lett. 27, 743–745 (2002). [CrossRef]

11.

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

OCIS Codes
(140.7010) Lasers and laser optics : Laser trapping
(170.4520) Medical optics and biotechnology : Optical confinement and manipulation
(220.4610) Optical design and fabrication : Optical fabrication
(230.4000) Optical devices : Microstructure fabrication

ToC Category:
Trapping

History
Original Manuscript: April 25, 2007
Revised Manuscript: June 19, 2007
Manuscript Accepted: June 22, 2007
Published: July 5, 2007

Virtual Issues
Vol. 2, Iss. 8 Virtual Journal for Biomedical Optics

Citation
Peter John Rodrigo, Lóránd Kelemen, Carlo Amadeo Alonzo, Ivan R. Perch-Nielsen, Jeppe Seidelin Dam, Pál Ormos, and Jesper Glückstad, "2D optical manipulation and assembly of shape-complementary planar microstructures," Opt. Express 15, 9009-9014 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-14-9009


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References

  1. M. Gauthier, D. Heriban, D. Gendreau, S. Regnier, P. Lutz and N. Chaillet, "Micro-factory for submerged assembly: interests and architectures," Proc. 5th Int. Workshop on Microfactories (2006). [PubMed]
  2. J. J. Talghader, J. K. Tu, and J. S. Smith, "Integration of fluidically self-assembled optoelectronic devices using silicon-based process," IEEE Photon. Technol. Lett. 7, 1321-1323 (1995). [CrossRef]
  3. K. Hosokawa, I. Shimoyama and H. Miura, "Two-dimensional micro-self-assembly using the surface tension of water," Sens. Actuators, A: Physical 57, 117-125 (1996). [CrossRef]
  4. R. L. Eriksen, V. R. Daria, and J. Glückstad, "Fully dynamic multiple-beam optical tweezers," Opt. Express 10, 597-602 (2002). [PubMed]
  5. P. J. Rodrigo, R. L. Eriksen, V. R. Daria, and J. Glückstad, "Interactive light-driven and parallel manipulation of inhomogeneous particles," Opt. Express 10, 1550-1556 (2002). [PubMed]
  6. I. R. Perch-Nielsen, P. J. Rodrigo, C. A. Alonzo, and J. Glückstad, "Autonomous and 3D real-time multi-beam manipulation in a microfluidic environment," Opt. Express 14, 12199-12205 (2006). [CrossRef] [PubMed]
  7. S. Maruo, O. Nakamura, and S. Kawata, "Three-dimensional microfabrication with two-photon-absorbed photopolymerization," Opt. Lett. 22, 132-134 (1997). [CrossRef] [PubMed]
  8. L. Kelemen, S. Valkai, and P. Ormos, "Integrated optical motor," Appl. Opt. 45, 2777-2780 (2006). [CrossRef] [PubMed]
  9. http://mathworld.wolfram.com/WallpaperGroups.html>
  10. A. T. O'Neil and M. J. Padgett, "Rotational control within optical tweezers by use of a rotating aperture," Opt. Lett. 27, 743-745 (2002). [CrossRef]
  11. A. Terray, J. Oakey and D. W. M. Marr, "Fabrication of linear colloidal structures for microfluidic applications," Appl. Phys. Lett. 81, 1555-1557 (2002). [CrossRef]

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