Elastic micro-cantilever of 30-micrometer size is repeatedly deflected/released by optical tweezers trap, recorded by a high-speed camera (500 frames/sec) and subsequently processed off-line. This paper evaluates the position detection methods of the cantilever head, which is distorted by a diffraction pattern. We developed and tested four methods in our VideoAnalyser software - radial extremes, Hough transform, local corner tracking, and voting normal lines. The time dependence of the head position contains the information about the properties of both: cantilever material and the surrounding environment. Averaging of aligned graphs corresponding to individual cycles significantly improves the signal-to-noise ratio.
Two-photon absorption polymerization (2PP) is a versatile lithographic method for building three-dimensional microdevices with sub-diffraction resolution and tunable elasticity. Using 2PP of organo-ceramic hybrid OrmoComp (trimethylolpropane triacrylate), we prepared microsized polymeric cantilevers consisting of the surface-anchored micropillar, long elastic neck and spherical head. A typical microcantilever has the length of 30 micrometers and thickness of 1-2 micrometers. To investigate the mechanical properties of our elastic microcantilever, we applied a laser optical trapping force on the head of the cantilever and bent the head from its equilibrium position. After switching off the laser trap, the head returns to the equilibrium position. The time-dependent restoring trajectory of the head was recorded by a fast video-tracking technique (500 fps) and subjected to the custom-made drift-correcting image analysis. We find that the microcantilever relaxation process is well-described by a single-exponential relaxation curve with a time constant of 16.5 ± 1.2 ms. Assuming a highly overdamped regime, theoretical calculations yielded an apparent Young´s modulus for our OrmoComp microstructure of 1 MPa, which is 3-orders of magnitude smaller than the reported value for the bulk material (~1 GPa). The possible reasons for such discrepancy are discussed.
Indirect optical micro-manipulation refers to mechanical manipulation of microscopic objects by means of optically trapped micro-tools. Two-photon polymerization is used to prepare the micro-tools, which are then trapped by focused laser beams through their spherical “handle” parts. Simultaneous control of several laser traps by Holographic Optical Tweezers (HOT) allows for positioning (both transfer and rotation) of the micro-tools in 3D. We report on the development of micro-tools and their testing in an automated HOT system. In order to facilitate the manipulation of objects in large systems (exceeding the field-of-view of the trapping microscope objective) the HOT apparatus is equipped with an additional low-resolution microscope. The two live images are processed with the system of several computers communicating with each other via local network and displayed side-by-side on remote client computer to allow interaction with the user. Initially, the user clicks the positions of laser traps matching micro-tool handles. Subsequently, traps are merged into a set with one representative control point serving for manual mouse operations (drag and drop, rotation and by mouse wheel). In the autopilot mode, the micro-tool moves in the given direction and velocity until it reaches the image border where it turns back. In the simulator mode, the manipulation is performed with animated micro-tools instead of real ones captured by camera.
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