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The design of micro-manipulators such as micro-motors and micro-robots is essential for various applications in nanotechnology, biology and nano-medicine. There are several methods pertaining to micro-manipulators by using chemical reactions, microorganisms, magnetic force, etc. Micro-objects can be manipulated by means of the self-actuation of bacteria. An amount of bacteria is attached to a micro-object, and the micro-object is manipulated through the self-actuation mechanism (Steager, E. B., et al., 2011). Through chemical reactions, micro-tubes can be manipulated. A micro-tube is driven by a bubble which is generated in the micro-tube (Sanchez, S., et al., 2010). Because the self-actuation of bacteria and the aforementioned chemical reactions operate in random directions, manipulation along a desired path is difficult. However, the manipulation of micro-objects by means of magnetic force can be controlled precisely. Micro-manipulators and micro-motors can be driven by magnetic force (Hagiwara, M. et al., 2011; Xia, H. et al., 2010)
In another technique developed recently, optical tweezers are used to manipulate micro-particles, nanowires and living cells. When a laser beam is focused onto an object, the momentum of the light changes. If the reflective index of the object is higher than that of the medium, the optical tweezers tool is generated on the object toward the focal point of the laser beam (Ashkin, A. et al., 1986). Because the optical tweezers can begenerated at a point force, the optical tweezers offers the advantage of enabling the manipulation of a micro-joint like a micro-robot-arm. A micro-lever fabricated by means of two-photon stereolithography was manipulated by optical tweezers and the stiffness of a micro-spring was experimentally measured (Lin, C.L. et al., 2011). A micro-pinch device fabricated by two-photon stereolithography was manipulated by th e optical tweezers and the properties of a protein were experimentally measured (Shimada, N. et al., 2013).
Micro-joints must be able to achieve a variety of movements for a variety of purposes. Among these movements, a translational joint can be used to achieve linear movement. There are several types of translational joints, but most are easy to design based on a kinematic design, which relies on a gap between the moving and fixed parts. However, micro-joint driven by various manipulation methods should overcome the scale effect on a micro scale. On the micro scale, the area effect related to the surface friction force is dominant over the volume effect related to the force of gravity (Trimmer, W. S. N., 1989; Wautelet, M., 2001; Cugat, O. et al., 2003). For precise motion and negligible amounts of surface friction force, the micro-joint should be designed based on an elastic design with an elastic thin plate.
In this paper, a new concept for a micro-translational joint based on elastic design is proposed. The translational joint has two spiral structures and a moving part. The advantage of this elastic translational joint is that it can be fabricated as an integrated structure. Thus, there is no gap in the joint and surface friction is negligible. There are limits on the elastic deformation of the entire spiral structure; however, this can be overcome by increasing the number of turns of the spiral. A finite element method (FEM) simulation was employed to evaluate the proposed elastic translational joint. The result of this simulation shows that the proposed translational joint is capable of precise motion even when moving over relatively long distance.
The proposed translational joint is fabricated with a two-photon stereo-lithography (TPS) process. The TPS process is commonly used for fabrication of 3D micro structures (Yang, D. Y. et al., 2007; Lim, T. W. et al., 2008; Park, S. H. et al., 2006). In the TPS system, a mode-locked titanium-sapphire laser with a wavelength of 780 nm is utilized as the light source, and the laser is tightly focused into the resin by a high-numerical-aperture objective lens (N.A. 1.3. x 100 with immersion oil). The photo-curable resin, SU-8 (Microchem Corp.), was used to form the three-dimensional (3D), movable structure.