Microlens-array-enabled on-chip opticaltrapping and sorting

Xing Zhao,1 Yuyang Sun,2 Jing Bu,1 Siwei Zhu,3 and X.-C. Yuan1,*1Institute of Modern Optics, Key Laboratory of Optoelectronic Information Science and Technology,

Ministry of Education of China, Nankai University, Tianjin 300071, China2School of Electrical and Electronic Engineering, Nanyang Technological University,

Nanyang Avenue, 639798, Singapore3Nankai University Affiliated Hospital, Tianjin 300121, China

*Corresponding author: xcyuan@nankai.edu.cn

Received 20 September 2010; revised 9 November 2010; accepted 5 December 2010;posted 6 December 2010 (Doc. ID 135380); published 14 January 2011

An on-chip optical trapping and sorting system composed of a microchamber and a microlens array(MLA) is demonstrated. The MLA focuses the incident light into multiple confocal spots to trap the par-ticles within themicrochamber. The SiO2=ZrO2 solgel material is introduced in the fabrication ofMLA forits unique optical and chemical characters. Moreover, in order to prove the effectiveness of the system,experimental demonstration of multibeam trapping and locked-in transport of micropolymer particles inthe microchamber is implemented. The system may easily be integrated as microfluidic devices, offeringa simple and efficient solution for optical trapping and sorting of biological particles in lab-on-a-chiptechnologies. © 2011 Optical Society of AmericaOCIS codes: 140.7010, 160.6060, 170.4520, 220.4000, 230.3120, 350.4855.

1. Introduction

Optical trapping and sorting technologies are play-ing important roles in life science, diagnostics, anal-ytical chemistry, and bioanalytical chemistry. Tomanipulate micrometer-sized particles, such as mi-croparticles and mammalian cells, one can use con-ventional optical trapping systems with the effectsof gradient force and scattering force assisted by re-fraction, diffraction, holography, and coherence tech-niques [1–4]. To manipulate submicrometer- and/ornanometer-sized particles, such as nanoparticlesand proteins or DNA molecules, one has to use opti-cal trapping systems with newmechanisms to exceedthe diffraction limit, for example, near-field evanes-cent trapping and localized-surface-plasmon-inducedscattering field trapping [5,6]. After trapping theseparticles, one can separate or sort them through la-

beled or label-free methods [7–9]. With the require-ment of portable and convenient analysis anddiagnostics, lab-on-a-chip techniques are becominga new focus of frontier research in optical trappingand sorting, for example optofluidic devices. Com-pared with the above new mechanisms, integrationof conventional optical trapping and sorting systemsis difficult because it will be a challenging task to in-tegrate complex and bulk optics into a tiny chip atlow cost. In the last few years, many on-chip trappingconfigurations with waveguides have been reported,for instance, chips with subwavelength slot wave-guides for optical manipulation of nanoparticles andbiomolecules [5], flexible planar waveguide chips foroptical trapping with the new mechanism of wave-guide loss [10], optofluidic ring resonator switchesfor optical particle transport [11], and optofluidictrapping and transport on solid core waveguideswithin a microfluidic device [12]. However, progressof on-chip integration of conventional optical trap-ping and sorting systems has been absent.

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During the application of biological and chemicalanalysis, the optofluidic chip is subject to a complexchemical and optical environment. However, conven-tional materials, such as SU-8 photoresist commonlyused for fabrication of microstructures in the lab,are not durable enough to deal with these challenges[12,13]. Therefore, a material with good environ-mental durability, that can easily be fabricated aswell, is absolutely required for the optofluidic chipapplications.

In this paper, we demonstrate an on-chip opticaltrapping and sorting platform composed of a micro-chamber and the microlens array (MLA), whichfocuses the incident light into multiple confocal spotsby refraction to trap the polymer microparticles. Thesystem could offer optical trapping and label-freehigh-throughput sorting of the micro-objects withinthe chamber. The MLA-enabled on-chip optical trap-ping and sorting have advantages over waveguide-based on-chip integrations in terms of designflexibility, ease of fabrication and operation, andhigh-throughput trapping and sorting capabilities.Furthermore, precision coating and complex packa-ging protection is no longer required as comparedto the integrated focusing micromirror-array chips[13,14]. The inorganic–organic hybrid SiO2=ZrO2 sol-gel material is introduced in the fabrication of theMLA, which provides highly tolerable optical proper-ties and high chemical durability. Finally, on-chipoptical trapping and experimental deflection of poly-mer particles within the microchamber is presented.

2. Experiments

A. MLA-Enabled On-Chip Optical Trapping and Sorting

It is known that MLAs have the ability to focusoptical spots to nearly uniform intensity. In our pre-vious work, the array of confocal spots generated byan MLA was projected onto an objective lens formultiple trapping and sorting [15]. However, a parti-cularly designed MLA could also be utilized todirectly create an optical lattice near the surface ofan element, without the optical projection system.As shown in Fig. 1, the integrated optical trappingsystem is composed of a microchamber and a spe-cially designed MLA fabricated on a glass substrate.The incident laser beam will be focused by the MLAinto a light spot array within the microchamber. Thesample in the microchamber includes two types ofparticles of different size or refractive index. IfMLA parameters and chamber thickness are appro-priately chosen, with the pump driving force comingfrom the inlet, the particles will flow and be sortedwhile passing through the optical potential welllandscape. After sorting, the separated particles willbe driven to the outlet and collected.

B. Design and Fabrication of MLA

Extensive photolithographic and etching processesare required in the conventional fabrication of MLAs.Furthermore, it is difficult to obtain uniform and

homogeneous MLAs. In our system, an inorganic–organic hybrid SiO2=ZrO2 solgel material is intro-duced for the fabrication of MLA. The solgel materialused in the fabrication of the MLA offers advantagessuch as simple fabrication, low cost, highly tolerantoptical properties, good mechanical strength, highchemical and environmental durability and no pro-duction of toxic gases. The solgel MLA can be usedas an end product because it can be fabricated in asingle step without etching [16,17].

In the fabrication of the MLA, the solgel materialwas synthesized as a negative photoresist. It wassynthesized from two solutions. Solution I was a si-licon oxide network, which was formed by the hydro-lysis of 3-(trimethoxysilyl) propyl methacrylate inisopropanol and acidified water, with a volume ratioof 20∶10∶1. Solution II was a zirconia oxide network,which was formed by the hydrolysis of zirconium n-propoxide [ZrðOC3H7Þ4] in propanol, nitric acid, andhydrochloric acid with a volume ratio of 3∶4∶8∶1.The two solutions were then mixed and aged at roomtemperature for 48h with vigorous stirring. This ne-gative-tone silicon zirconium material was made UVphotosensitive by adding the photoinitiator IRGA-CURE 184 (Ciba; Basel, Switzerland) with 4wt:%to the solgel. After being filtered to remove large im-purity particles, it was then spin-coated at 2500 rpmfor 35 s onto a glass substrate to achieve a thicknessof 0:9 μm and followed by a prebake on a hotplate at80 °C for 3 min. A 325nm He–Cd UV laser directwriting system with a lateral resolution of 3 μm(maximum output power ∼200mW) was employed.The laser beam was guided into a microscope andthen focused with an objective lens onto the sample,and the intensity of the laser beam was controlled byan acoustic optical modulator (AOM). The calibratedrelationship between the SiO2=ZrO2 solgel filmthickness and the AOM value is shown in Fig. 2.After development in acetone for 12 s, the desiredMLA structures were formed in the solgel thin film.

Fig. 1. (Color online) Schematic of on-chip optical trapping usingMLA.

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Figure 3 shows a scanning electron microscope(SEM) photo of the fabricated MLA.

C. On-chip Optical Trapping and Deflecting Experiment

In the optical trapping and sorting experiment, a mi-crofluidic chamber with a depth of 70 μm was formedbetween two glass plates, which were separated by aspacer. An aqueous suspension of 3:1 μm microparti-cles was sandwiched in the microchamber. In the sys-tem, shown in Fig. 4, a continuous wave 1W Yb fiberlaser (IPG Photonics Corporation, USA) at 1070nmwavelength was employed to trap the particles. Thelaser beam was collimated before being focused bythe MLA on the chip. A high-intensity fiber light pipewas employed as the illumination source to let theparticles and optical spots display clearly on themonitor. Two dichroic mirrors (Thorlabs Inc. BB1-E03) were positioned along the optical path. Onewas used to combine the trapping infrared beam(1070nm) and the illumination beam for passingthrough the microchamber andMLA, while the otherwas used to pass the illumination beam and block theinfrared beam entering from the CCD camera. Themagnification factor of the objective lens is selectedaccording to the size of particles and the optical spot

size, and usually 60× or 100× is used in our system.Furthermore, a long working distance (about15–20mm) objective lens is adopted to be suitablefor a large range of microchamber thicknesses.

In order to achieve the highest power efficiency forthe trapping system, it is important to make the focalplane of the MLA lie within the microchamber. Tothis end, the microchamber and fabricated MLAwere held on two respective three-dimensional mi-crostages, which enabled us to precisely control theair gap between the MLA and the microchamber.Subsequently, the study of locked-in transport of par-ticles in the chamber was conducted. As continuoustransport requires an additional driving force tocreate a flow of particles with uniform velocity, themicrochamber (with samples) was mounted on amotorized stage in this case.

3. Results and Discussions

A. Focal Length of the Microlens

The diameter of the individual microlenses in therefractive MLA is designed to be 22 μm. By meansof design calculations, we can obtain the focal lengthof the fabricated MLA. The radius of curvature (R)and focal length (f ) of the microlens are given by

R ¼ D2 þ 4h2

8h; ð1Þ

n2R ¼ ðn1 − n2Þf ; ð2Þwhere h is the “sag” of the lens, n1 is the refractiveindex of the fabrication material for the MLA (1.42for the SiO2=ZrO2 solgel material), and n2 is the re-fractive index of water for microfluidic trapping andsorting, as shown in Fig. 5. According to (1) and (2),the focal length of the fabricated MLA is 161:1 μm inair and 1000 μm in water. Thus, it can be seen thatthe air gap between the MLA and the microchamberin Fig. 4 has to be precisely controlled to ensure that

Fig. 2. Measured calibration curve showing the dependence ofthe SiO2=ZrO2 solgel film thickness on the AOM value.

Fig. 3. SEM photo of fabricated MLA.

Fig. 4. (Color online) MLA-enabled on-chip optical trapping andsorting system.

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the focal plane is placed in the chamber at a depthof 70 μm.

B. On-chip Optical Trapping and Deflecting of thePolymer Particles

In the presented inverted microscopic system with-out an objective lens, the scattering force acting ona particle is a dominant force that is much strongerthan the gradient force exerted by the optical spotarray at different planes. When the laser wasswitched on with an output power of 300mW, it wasobserved that all particles were levitated andtrapped regularly corresponding to each individuallight spot of the optical lattice, as shown in Fig. 6.Furthermore, when the laser was switched off, in

seconds all the particles fell down to the original bot-tom plane and became randomly located.

As shown in Fig. 7, sequential images of motions ofthe mixed particles were taken at 0:5 s intervals.With a laser power of 400mW and with a flow veloc-ity of around 10 μm=s, it was observed that there wasa strong deflection for the 3:1 μm polymer particles(two of them highlighted by a square) when theypassed through the optical landscape. The 3:1 μmpolystyrene particles highlighted by a square wereobviously deflected and locked into the axis of opticallattice from the top of the image to bottom. Figure 8shows the abstract of trajectories of the particles todemonstrate the continuous locked-in transportcaused by the optical lattice. Since the deflection tra-jectories of the particles are completely determinedby intrinsic physical attributes like geometrical sizeand refractive index, it is believed that different par-ticles or cells mixed in a continuous flow will havetotally different deflection trajectories. Therefore,the on-chip system as shown in Fig. 4 consists of a

Fig. 5. (Color online) Schematic of individual microlens.

Fig. 6. (Color online) On-chip multiple optical trapping of 3:1 μmpolymer particles.

Fig. 7. (Color online) Sequential images of motions of mixed par-ticles taken with 0:5 s intervals.

Fig. 8. (Color online) Trajectories of the 3:1 μm polystyreneparticles being locked-in transported.

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passive continuous microfluidic sorting which re-quires no human interaction.

4. Conclusion

In this paper, we demonstrate an MLA enabled on-chip optical trapping and sorting system composedof a microchamber. The MLA is fabricated by laserdirect writing a SiO2=ZrO2 solgel material on a glasssubstrate in a single step without etching. The solgelMLA could be a good solution for future optofluidicchips owing to excellent optical properties, goodmechanical strength, and high chemical and environ-mental durability over the commonly used photo-resist. Furthermore, on-chip optical trapping anddeflection of the micro polymer particles within themicrochamber were experimentally demonstrated.Multibeam trapping and locked-in transport of theparticles within the microchamber prove that suchan on-chip optical trapping and sorting configurationeffectively integrates conventional refractive opticaltrapping and sorting system at low cost. This systemis easy to operate and reliable for potential applica-tions in life science, medical diagnosis, and analyticalchemistry.

This work was supported by the National NaturalScience Foundation of China (NSFC) under grant60778045. The fabrication work was partially sup-ported by the Ministry of Science and Technologyof China under grant 2010CB327702. The authorswould like to thank theMinistry of Science and Tech-nology of China under grant 2009DFA52300 for sup-porting China–Singapore collaborations.

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