Research On Motion Control System Of Magnetic Microsphere Based On Permanent Magnetic Tweezer

As well as the optical tweezer and atom force microscope,magnetic tweezer is a kind of single molecule manipulation technology.They play an important role in the research of cell,protein and DNA.Magnet tweezer outputs force to control motion state of the magnetic microsphere with biology materials attached.By analyzing the motion state,the power spectrum and other information of magnetic microsphere,the physical characteristics of cells and macromolecules could be obtained.In order to research different materials,the magnetic force output by the magnetic tweezer should contain different values and directions,thus to realize multidirectional motion of the magnetic microsphere.Based on the research of the development,theory and method of magnetic tweezer techniques,the contrapositive magnetic tweezer and the quadrupole magnetic tweezer are built in this study.And they both realize the spatial position and trajectory control of microspheres.The main research contents of this study are shown as follows:(1)Based on the research of the theory of permanent magnet based far-field magnetic tweezer,the theoretical model of motion control of microsphere in the magnetic tweezer is established.The model of spatial magnetic field distribution of permanent magnetic pole is derived using Maxwell's equations.The relative position between the magnetic pole and the microsphere is changed for the control of magnetic induction intensity and gradient at the position of magnetic microsphere,thus to change the value and direction of the output force and realize the position and trajectory control of the magnetic microsphere.(2)The mechanical structures and the control units of contrapositive and quadrupole magnetic tweezers are designed based on the established spatial magnetic field distribution model.Magnetic microsphere fluorescence imaging unit is also designed to observe the motion of microsphere.In the contrapositive magnetic tweezer,the stepper motors are controlled by the ADC of MSP430F149 to realize the three-dimensional movement of the magnetic pole.In the quadrupole magnet tweezer,the stepping motors are controlled by multiple slave machines through RS485 for the accurately variation of the distance between magnetic pole and sample.The magnetic microsphere fluorescence imaging unit is established to observe and record the position and trajectory of magnetic microspheres using a 30 FPS CCD camera,an objective lens with NA=0.1 and magnification of 4.The experimental results show that the positioning accuracy is about 100 nm by using gray-weighted centroid method and Gaussian surface fitting method for the location of magnetic microspheres.(3)The experimental contrapositive and quadrupole magnetic tweezers are tested respectively.The relationship between the magnetic pole coordinates and the magnetic induction intensity of the sample area is calibrated.The magnetic microspheres are controlled to move in any specified direction,the direction error of motion is within 2°,and a maximum magnetic force of 72.96 p N can be applied to magnetic microspheres with a diameter of 4.5 μm.The calibration experiment of motion direction and the test of specific trajectory have proved that both the two sets of experimental magnetic tweezers can realize the precise control of the position and motion direction of the magnetic microspheres.