Method for multi-beam manipulation of microparticles

Abstract: Irradiating a plurality of laser beams onto different microparticles or different groups of microparticles permits trapping and/or manipulating of these microparticles or groups of microparticles. By introducing an exciting laser beam it is possible to induce chemical reactions for processing or assembling.

Description: This invention relates to a method for multi-beam manipulation of microparticles. More particularly, the invention relates to such a method using a plurality of discrete, non-interfering laser beams. The method is useful in such fields as bioengineering and chemistry, and permits free non-contact manipulation of various kinds of microparticles having sizes of the micrometer order. It is known that such microparticles may be trapped with a laser beam, and attention has been given to applications of this technology for cell manipulation in the field of bioengineering and for quality improvement and reactions of microparticles in the field of chemistry. The present inventors have already proposed methods of micromanipulation useful in the formation of a dynamic pattern with a group of microparticles, microprocessing of microparticles, and manipulation of metal microparticles (Japanese Patent Application No. 1-318,258, Japanese Patent Application No. 2-78,421, Japanese Patent Application No. 2-402,063, and Japanese Patent Application No. 3-104,517). These methods permit manipulation, e.g. trapping, transfer and processing, of a single microparticle or a single group of microparticles in non-contact manner and at will, but do not permit independent individual manipulation of a plurality of microparticles. Burns et al. in Science 249 (17 August 1990), pp. 749-754 describe the use of optical standing wave patterns created by interfering laser beams to promote organisation of arrays of microspheres at the antinode maxima of the interference patterns. It will be appreciated that this method again does not permit independent individual manipulation of a plurality of microspheres. The present invention is based on the finding that a plurality of discrete, non-interfering laser beams may be used to effect trapping, processing and assembly of a plurality of microparticles or a plurality of groups of microparticles, thereby significantly expanding the scope of application of laser scanning. Thus the present invention provides a method for multi-beam manipulation of microparticles which comprises the steps of irradiating a plurality of discrete, non-interfering laser beams onto different microparticles or different groups of microparticles, and trapping and/or manipulating said microparticles or said groups of microparticles. The plurality of discrete, non-interfering laser beams may, for example, be generated by splitting a single laser beam and coaxializing the resulting beams prior to irradiation. Alternatively one may polarize a laser beam and split the same with a polarized beam splitter, coaxializing the resulting plurality of beams prior to irradiation. If desired, an additional exciting laser beam may also be coaxially introduced to induce chemical reaction where appropriate. In the accompanying drawings: Fig. 1 is a block diagram illustrating the configuration of a typical system applicable in the present invention; Fig. 2 is a plan view illustrating a typical manipulation of microparticles according to the present invention; Fig. 3 is a plan view illustrating another typical manipulation of microparticles according to the present invention; and Fig. 4 is a plan view illustrating a further typical manipulation of microparticles according to the present invention. By way of illustration of the invention, the configuration of a system applicable in the method of the present invention is shown in Fig. 1. In this embodiment, a CWND: YAG (Spectron SL902T; wavelength: 1.064 nm; linear polarization) laser is employed to effect trapping. The laser output is converted into a circular polarized beam with a λ/4 plate, and the resultant polarized beam is split into two beams by means of a polarized beam splitter. The two split laser beams are each individually deflected in two axial directions by two galvanomirrors (GSZ Q325DT), then coaxialized using a polarized beam splitter. Since the polarization directions of the two beams are at right angles to each other they are characterised by the absence of mutual interference, i.e. the intensity distribution does not vary with the relative positions of the beams. The laser beams are directed to a microscope (Nikon Uptiphot XF) via a lens system, and are condensed onto a sample through an oil-impregnated objective lens (x100, NA = 1.30). The condensing spot has a size of 1 µm. The galvanomirrors are located at the opening and at the image forming positions of the microscope respectively. Under the effect of deflection caused by the galvanomirrors, the focal position of each beam scans the sample two-dimensionally. The galvanomirrors are controlled by a computer (NEC PC9801 RA): it is possible to move the two beams at will by operation of the keyboard. Laser scanning makes it possible to align a plurality of microparticles with each beam, and even to trap metal microparticles or low-refraction microparticles. Any type of laser scanning can freely be set through appropriate keyboard input. To provide an exciting laser beam, a Q-switch YAG laser (wavelength: 355 nm; pulse width: approx. 30 ps) is provided, the output being condensed on the sample in coaxialization with the trapping laser beams. The progress of microparticle manipulation is observed through a CCD camera and a video recorder. The position of the laser beam and the current status of manipulation are displayed in a superimposed manner on a monitor screen. By way of example, micromanipulation was carried out using the above-mentioned system and a sample prepared by dispersing monodisperse polystyrene microparticles with a diameter of 3 µm in ethylene glycol having acrylic acid (monomer), N,N'-methylene-bisacrylamide (linking agent) and DALOCURE 1116 (photo-polymerization initiator) dissolved therein. Example First, as shown in Fig. 2, two polystyrene latex microparticles of the above-mentioned sample are trapped with two individual beams, and are caused to come into contact with each other by moving the beams. Then the exciting laser beam is irradiated onto the contact point to cause photo-polymerization to start. After a few seconds of exciting laser irradiation, acrylic acid gel is generated on the surfaces of the polystyrene microparticles, thus causing welding of the two microparticles. After confirming the existence of this welding by moving the beams, laser scanning with one of the beams is started to trap the connected microparticles. Then, as shown in Fig. 3, the other beam is moved to trap another microparticle, which is caused to move to an arbitrary position in contact with the two connected microparticles. Exciting laser irradiation of the contact point in the same manner as above again produces welding through photo-polymerization. Repetition of this cycle permits the building of a microparticle-based structure. For the purpose of causing rotary motion of a microparticle structure, as shown in Fig. 4, (a) laser scanning is effected to trap two arbitrary points on the structure; (b) one of the beams is fixed so as to serve as the rotation axis; and (c) the other beam is caused to start circular scanning around the fixed rotation axis as the centre of rotation, whereby the microstructure undergoes rotary motion. It will be appreciated that other laser beam optical systems may be adopted in manipulations as described above, as may organic, inorganic and metal microparticles as alternatives to the above-described organic polymer microparticles. Biological samples such as living cells may also be used. In summary the method of the invention permits independent individual manipulation of a plurality of microparticles or a plurality of groups of microparticles. Thus two non-interfering trapping laser beams may be employed analogously to two human hands. Manipulation is fully controllable by computer. Coaxial introduction of an exciting laser beam permits induction of chemical reactions for processing or assembling. This ability to process, assemble or impart mechanical motion to a plurality of microparticles or a plurality of groups of microparticles renders the method directly applicable to assembling or driving the apparatus of a micromachine, as well as to construction and control of microstructures, useful in physics, chemistry, mechanical engineering and electrical engineering.

Claims: A method for multi-beam manipulation of microparticles which comprises the steps of irradiating a plurality of discrete, non-interfering laser beams onto different microparticles or different groups of microparticles, and trapping and/or manipulating said microparticles or said groups of microparticles. A method as claimed in claim 1 wherein a single laser beam is divided and coaxialized for irradiation. A method as claimed in claim 2 wherein said laser beam is polarized and split by means of a polarized beam splitter, and a plurality of coaxialized beams are irradiated onto the microparticles or groups of microparticles. A method as claimed in claim 2 or claim 3 wherein an additional exciting laser beam is coaxially introduced to induce chemical reaction.

Inventors: KITAMURA NOBORU, MISAWA HIROAKI, SASAKI KEIJI, KITAMURA, NOBORU, MISAWA, HIROAKI, SASAKI, KEIJI
Applicants: JAPAN RES DEV CORP, RESEARCH DEVELOPMENT CORPORATION OF JAPAN
Application year: 1992
Main classification: H05H3
IPC codes: B01J19, G21K1, B22F1, H05H3, B23K26