CHAPTER 1 INTRODUCTION
1.1 BACKGROUND
In order to control particles moving, there are many methods forcing on manipulating particles. In addition, when it comes to a biological cell, the non-contact forces, such as the best choose, are required to avoid the physical contacting to ruin the cell. Accordingly, there are the ultrasonic, optical, hydrodynamic, and electromagnetic forces. The optical method is performed by optical beam; the hydrodynamic force is operated through the whirling flow; and the electromagnetic forces can be separated into electric method and magnetic method. However, there are disadvantages among these different kinds of forces from methods, such as that the force of the optical beam is too small to trigger or to move a big specimen while whose diameter is greater than 100 μm (Takeshi Hayakawa, et al., 2015).
In addition, the dielectrophoresis is chosen as the applying force in our study. The dielectrophoresis has become an important method of manipulating a particle especially in the field of controlling the biological particles (Benhal, 2013; Jones, 2003). In this method, the dielectrophoresis can operate a particle in a fluidic environment, and the particle is in the ideal medium, liquid, which helps particle float and frictionless. Moreover, the dielectrophoresis method can be applied in fields of isolating particles, separating DNA, traping particles at a specific location, etc. However,
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according to references, these applications of the dielectrophoresis are limited in the two-dimensional motion in horizontal plane until now. In this paper, we try to develop a device which can operate a particle moving in three-dimensional space.
The dielectrophoresis were firstly found by Phol (Phol, 1978). Wang et al. calculated the amplitude and the phase in the operation system, and they also analyzed torques and forces on particles in an electric field with special concept (Wang et al., 1994). They consider the time averaged design in which the time averaged concept is defined by
1 tt t
time-depended electric field, and they pointed out two important results (Hughes and Morgan, 1998): Firstly, a device which accomplishes both translation and rotation could be designed through an ideal configuration of the electrodes. Secondly, there are ten different distributions of the electrodes suggested to control the particles. In Figure 1.1, (a) ~ (j) represent5
polynomial, bone, square, pointed pyramidal, truncated pyramidal, pin, oblate ellipse, circular, prolate ellipse, and narrow prolate ellipse electrodes.
In addition to the dielectrophoresis, the total dielectrophoretic force including conventional dielectrophoretic force and traveling-waved dielectrophoretic force were introduced, and they applied both forces and torques on the target subject without the device contacting the subject directly. And so that it could be widely used in biological particles and cells (Benhal, 2013; Jones, 2003). In the total dielectrophoretic mode, when a dielectric particle experiences a non-uniform electric field, the dielectrophoretic force is greater than the initial inertial force. Afterward, the particles starts to move under the electric pathes. Here this phenomenon is called dielectrophoresis (Jones, 2003). That applying an AC electric to the electrodes makes a particle stably translate and rotate in the electric field is so called the traveling-waved dielectrophoresisp (Chen, 2008; Jones and Washizu, 1996). electroorientation. The above two methods, the electrorotation and the
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electrooientation, are based on the same theory; and both of these dielectrophoretic forces are called dielectrophoresis.
In recent years, researchers have scanned many surfaces of the materials, and these results have reached to the sub-nano scale. Among these materials, the biological cells have not been mentioned for many times. There are some difficulties when performing a three-dimensional scan on the biological particles, including the medium and environment. In this case the particles need a normal saline to avoid the particle being destroyed. The most common way to scan a particle’s surface is using the Scanning Tunneling Microscope (STM) which is operating in vacuum. Once the particle is a biological cell, the particle should be coated with other material to avoid the cell exploding in the vacuum environment. Coating a thing is as soft as the biological cell may make the cell deform. Or the coating material is bonded with each other.
Those make the depth of the surface become shallow, and hence the result is unreal.
There are three different kinds of the microscope, such as Atomic Force Microscope (AFM), Transmission Electron Microscope (TEM), and STM, can image micro-scalar particles or even nano-scalar particles. A STM requires a conductive specimen for scanning. A TEM requires a vacuum environment to execute the scanning. Because of the ability of performing a scan in the normal condition and in the water, the AFM is become a popular
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equipment. An AFM is a microscope which can execute the nano-scale scanning with piezoelectric components in the open air or in the open liquid environment, and it was widely applied in measuring the contour of biological specimen (Shih and Shih, 2015). The AFM contains a laser, a detector, a vibration base, and a microcantilever with a tip. The major mechanism can assemble as Figure 1.2. An AFM base is a piezoelectric vibrating base. A microcantilever is fixed on the AFM base, and a tip locates at the free end of the microcantilever. When a laser hits the back of the microcantilever, the laser deflects to the detector. While the detector receives the signal, it transmits the signal to the computer which runs the further nanometers to touch or contact the specimen, the tip is dragged down by the van der Waals' force. The advantage of the contact mode is that it directly touched the sample. The distortion of the particle’s image would be less than non-contact mode. This advantage is more obvious when the specimen is rigid. The responsive signal of the contact mode is proportional to the
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displacement of the tip and noise. The disadvantage is that the tip can directly touch the specimen if the cantilever is with high stiffness. This will make the specimen be destroyed. On the other hand, the dynamic mode can also be separated into many details. Generally, the dynamic mode is a way that vibrating the cantilever and the tip moves toward the specimen. It is generally called the tapping mode (TM). And it uses a piezoelectric material to make the microcantilever do a simple harmonic motion. While the tip approaches the sample, the amplitude of vibration becomes small and the natural frequency shifts high. The method measuring the amplitude is also called the amplitude mode (AM). The method detecting the change of the vibrating frequency is also called the frequency mode (FM). To avoid the directly physical contact specimen during the process, the dynamic mode will be chosen when it scans biological specimen.