Preliminary study of lever-based optically driven micro-actuator

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1 Preliminary study of lever-based optically driven micro-actuator Chih-Lang Lin 1*, Yi-Hsiung Li 2, Chin-Te Lin 3, Chia-Chin Chiang 4, Yi-Jui Liu 5, Tien-Tung Chung 3, Patrice L. Baldeck 1,6 1* Inst. of Biomed. Eng. and Mater., Central Taiwan Univ. of Sci. and Tech., Taichung 406, Taiwan 2 P.h.D program in Electrical and Communication Eng., Feng Chia Univ., Taichung 407, Taiwan 3 Dept. of Mechanical Eng., National Taiwan Univ., Taipei 10617, Taiwan 4 Dept. of Mechanical Eng., National Kaohsiung Univ. of Appl. Sci., Kaohsiung 415, Taiwan 5 Dept. of Automatic Control Eng., Feng Chia Univ., Taichung 40724, Taiwan 6 Univ. Grenoble 1 / CNRS, LIPhy UMR 5588, Grenoble, F-38041, France * cllin101943@ctust.edu.tw Abstract This study presents a novel type of optically driven lever-based micro-actuator fabricated using two-photon polymerization 3D-microfabrication technique. The lever is composed of a beam, an arch, and a sphere. First, optical tweezers is applied on the spheres to demonstrate the actuation of the lever. A spring is jointed at the lever for verifying the induced forces. Under the dragging by laser focusing, the lever simultaneously turns and results a torque like a mechanical arm. Then, the demonstration of a photo-driven micro-transducer with a mechanical arm and a gear is preformed. The experimental result indicates that our design enables precise manipulation of the mirco-actuator by optical tweezers at micron scale. This study provides a possibility for driving micron-sized structured mechanisms, such as connecting rods, valves. It is expected to contribute on the investigation of Lab-on-a-chip. Keywords: Micro-lever, Optically driven, Optical tweezers, Micro-actuator, Mechanical arm, Lab-on-a-chip 1. Introduction Lab-on-a-chip is a device which integrates one or several laboratory functions on a cover glass of a few square centimeters in size. Special designed micro-channels, micro-machines, and micro-sensors present critical components for a lab-on-a-chip. A high precision actuator is important to control such kind of device. Optically driven micro/nano structures 1-3 contribute its special characteristics, such as remotely precise manipulation 4, on this field 5-7. Micro-sized particles and objects can be trapped and manipulated in the strongly focused spot of a laser beam. Such a remote photo-driven tool can uniquely elaborate mechanical and sensing functions at micro- and nano-scales without mechanical contact. Using this technique, a number of optically driven micro-objects have been proposed in recent years. For example, photo-driven rotations can be induced either by the net optical torque resulting from the complex shape 8, 9, or by driving a movable part with an optical tweezers trap Third International Conference on Smart Materials and Nanotechnology in Engineering, edited by Jinsong Leng, Yoseph Bar-Cohen, In Lee, Jian Lu, Proc. of SPIE Vol. 8409, SPIE CCC code: X/12/$18 doi: / Proc. of SPIE Vol

2 Two-photon polymerization (TPP) is a convenient technique for fabricating three-dimensional microstructures with arbitrary tri-dimensional shapes 14. Structures with complex shapes are directly obtained by scanning the laser focusing point along predetermined trajectories. Recent studies have focused on the realization and demonstration of more complex devices that integrate a variety of micro-machines, such as spring, micromanipulator, and multilink systems that can be integrated in a microfluidic environment for a lab-on-a-chip. However, there are few possibilities to precisely manipulate the structured micro-machines. In this paper, we report on a novel actuation method which utilizes optical tweezers to drive a lever made by TPP. A lever is one of the six classic simple machines; it is one of the important devices in the structured mechanisms. A lever can either multiply the mechanical force and the distance that the opposite end of the rigid object travels, or change the applied force direction. This study demonstrates the possibility of precisely applying the optical dragging forces using the classic lever function of mechanics in the microscopic world. 2. Fabrication of micro-objects In this study, the proposed micro-actuator is fabricated by using a commercial TPP 3D-microfabrication machine (Teem Photonics Inc.) using a passively Q-switched Nd:YAG microchip laser with 532 nm wavelength, 550 ps pulse-width, and 6.5 khz repetition rate. The laser beam was expanded by a X3 telescope, coupled to an inverted microscope (Olympus IX51), and focused with a microscope objective lens (100x, NA=1.3). Commercial resin (Photomer 3015, Henkel Inc.) is used with the photo-initiator specifically designed for two-photon absorption. Polymerization occurs at the focal point in the resin under less than 0.1 mw of laser power and a 1 ms exposure time. Resin Polymerization point [for TPP] Laser beam PZT motor 100X oil, N.A.=1.3 [for Laser tweezers] Fig. 1. Details of experiment set-up: (1) the laser beam (Q-switched Nd:YAG laser with 532nm wavelength; CW Nd-YAG laser with 1064 nm) entering a objective with 100x oil; (2) left side: resin on a glass for TPP 3D-microfabrication; (3) right side: a chamber for photo-manipulation operation Figure 1 shows the instruments, and their details reveal that, at the focal point, local polymerization of a voxel-based element occurs in the resin, and a three-axis piezoelectric stage (Nanocube, Physik Instrumente) forms the trace to produce the required structures. In a sealed chamber (1 cm in diameter and 0.8 mm in depth) comprising a glass cover and slide spaced by double-sided tape, a few drops of alcohol are added to dissolve the unexposed resin. CCD Proc. of SPIE Vol

3 The micron-sized lever and the spring drawn by AUTOCAD are shown in Fig. 2. The length of long arm and short arm separately are 45 μm and 15μm. A spring is jointed at the end of short arm to form the required arm ratios (1:3). The spring s dimensions are as follows: 0.36 μm in spiral diameter, 12.5 μm in coil diameter, 10 coils, and 2.5 μm in pitch. 45μ m 15μ m 20μ m Fig. 2. AUTOCAD figures of a lever with a spring 3. Optically driven micro-actuator The micro-objects are optically manipulated using optical tweezers. The microscope and objective lens that were previously shown in Fig. 1 were also used for the optical tweezers. A CW Nd-YAG laser at λ=1064 nm, provided the trapping beam (Fig. 1). Before performing the photo-driven micro-lever experiment, the optical exerted forces were calibrated by dragging various sphere sizes. The optical forces (F optic ) applied at the spheres were measured and calculated using the Stoke s law F optic = 6πRηv, where η is the dynamic viscosity (Nt.s/μm 2 ), R is the radius of the spherical object (μm), and v is the escaped velocity (μm/s). This experiment has been performed in our previously study 19. Figure 3 shows the relationship between the optical force and laser power of 7 μm sphere sizes. The sphere was used in the proposed micro-lever design, and its proportional factor 1.13 was used to calculate the exerted optical forces at the micro-lever sphere um slope 1.13 Optical force (pn) Laser power (mw) Fig. 3. The relationship between the optically exerted force and laser power of a 7 μm sphere 19. Proc. of SPIE Vol

4 This study assumes the following: (1) The lever beam and the fulcrum is rigid; (2) the friction force at the fulcrum is much lower than the optical dragging force; and (3) the viscosity between the lever and the solution can be ignored. A demonstration of photo-driven micro-lever with an arm ratio of 1:3 has been performed as shown in Fig. 4. Optical forces (100pN~300pN) are applied at the sphere to pull (Fig. 4-(a)) or compress (Fig. 4-(b)) the spring. In our previous study 19, the quantitatively experimental result indicates that the two cases of pulling and compressing have similar linear proportions. This means that the spring has an appropriate elastic resilience. (a) (b) (c) Fig. 4. Demonstration of photo-driven micro-lever: (a) before turning on the optical tweezers; (b) & (c) optical force is applied at the sphere to pull or compress the spring. The AUTOCAD figures of the micro-actuator structured with a gear, an axis, and an optically driven mechanical arm is shown in Fig. 5. The diameter of gear with 12 teeth is 27 μm. The mechanical arm (length=33 μm) is manipulated using optical tweezers as previous experimental set-up. Figure 6 shows the SEM photo of the micro-actuator. The demonstration of the actuation of a micro-gear rotationally dragged using optically driven mechanical arm is shown in Fig. 7. The feature work is to use this mechanism as a transducer to transmit other gear. 27μ m Laser beam 33μ m Fig. 5. AUTOCAD figures of a gear (12 teeth, diameter: 27μm) with an optically driven mechanical arm (length=33μm). Proc. of SPIE Vol

5 Fig. 6. SEM photo of a micro-actuator composed of a micro-gear and a mechanical arm Fig. 7. Demonstration of a micro-gear rotationally actuated using an optically driven mechanical arm. 4. Conclusion This study successfully demonstrated the possibility of using a photo-driven mechanism to manipulate a first class lever. This technique is proposed to called lever-shaped micro-actuator for biomechanics applications. The optical applied force is approximately 100 to 300 pn with a laser power of 100 to 300 mw. The demonstration of a micro-gear rotationally actuated using optically driven mechanical arm was performed. The result initiates a new feasibility of driving arbitrary micromachines and microsensors. We believe that this study is promising and opens a method for precisely driving micron-sized machines, such as connecting rods, valves, and other structured mechanisms. It is expected to contribute on the investigation of Lab-on-a-chip. Acknowledgements This work is supported by the National Science Council under contracts NSC E MY3. 5. Reference [1] Whyte G., Gibson G., Leach J., and Padgett M., Rotation of micron-sized objects, Opt. Express, 14(25), (2006). [2] Bonin K. D., Kourmanov B., Light torque nanocontrol, nanomotors and nanorockers, Opt. Express, 10(19), Proc. of SPIE Vol

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