An Open-Chip for Three-Dimensional Rotation and Translation of Particle Based on Dielectrophoresis
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This study developed a three-dimensional device which can manipulate a small specimen in translation and rotation in a three-dimensional space. The device is based on an open-chip device without packaging; thus, the tip of a scanning microscope. This translational, rotational, and overturning device enables optical or scanning probe microscopy to realize the three-dimensional observation of the specimen’s surface. Especially for the atomic force microscope, it helps scan the back side of the specimen, because the back side is always chemically bonded onto the substrate. This device is based on the three-dimensional dielectrophoretic theory, which applies exact solutions on the force and torque terms. Subsequently, these solutions are applied to dielectrophoretic simulation by using a finite element method (FEM) and to simulate the trapping and rotation of this particle and facilitate three-dimensional device design. Furthermore, microelectromechanical fabrication and laser processing were applied to manufacture electrodes in three-dimensional space. The trapping, rotation, shifting, and overturning of an Aspergillus niger particle were tested to demonstrate the manipulation of this device. The results reveal the rotation at 15–35 Hz had nearly constant period, and the angular velocity was proportional to the triggering frequency. Finally, according to the dielectrophoretic theory, the rotational velocities at frequency ranges of 15–35 Hz were recorded to modify the Clausius–Mossotti factor of A. niger; the results of that procedure can serve to adjust parameters for the advanced manipulation of other particles.
KeywordsTapping mode Atomic force microscope Pressure Vorticity Semianalytical method
Single-cell traps have been successfully used in many biological applications, and numerous methods exist, such as ultrasonic techniques , optical tools , hydrodynamics , and dielectrophoretic forces [4, 5]. Moreover, single-cell rotation is a fundamental technique in modern bioscience, and it enables observation through stereoscopic imaging. Cell rotation is necessary for microscope operations; and cell translation is necessary to orient the target in a focal plane or relative to a focal tip. This is particularly necessary for cells that are not uniform in the buffer liquid. Therefore, cell translation and rotation techniques play crucial roles in stereoscopic imaging, and they involve essential motions in the focal plane and in the plane perpendicular to the focal plane (vertical plane). However, there are no such device which can rotate specimen on the vertical plane, i.e. overturning. Especially for the atomic force microscope, the overturning helps scan and get the contour of the specimen back side, because the back side is always chemically bonded onto the substrate. Furthermore, an open-chip device can present a cell in an open environment with an appropriate buffer; this enables the cell to be observed not only with an optical microscope but also with a scanning probe microscope, where the open environment helps the scanning probe merge in the buffer or liquid environment.
To trap, rotate, and translate a single cell, the trapping forces can be obtained using many methods: ultrasonic, optical, hydrodynamic, or dielectrophoretic forces. The major difference depends on the size of the particle to be trapped. For example, an optical beam is suitable to trap or move nanoparticles, and the dielectrophoresis method or hydrodynamic whirling flow  is suitable for microscale particles. An ultrasonic transducer can trap particles that are approximately tens of micrometers wide . In this study, the dielectrophoresis method [6, 7] was applied to produce the trapping forces, and the controlling cell size was between 0.1 and 20 µm [8, 9]. Furthermore, there are three limitations when an atomic force microscope (AFM) is applied for scanning. The chamber must be opened at the top so that the tip can approach; the cell must be trapped or fixed strongly after its motion; and the device must be small and have a plane surface to be clamped. Therefore, the dielectrophoresis method is the most suitable candidate for manipulating a particle, particularly for controlling biological particles. In this method, dielectrophoresis can operate a particle in a fluidic environment, and the particle is in an ideal liquid medium, which enables the particle to float without friction. The major feature is that the particle is suspended when the AFM tip scans. The particle doesn’t need the chemical bonding to stuck on the substrate; on the other hand, its bonding side could be observed. Moreover, the dielectrophoresis method can be applied in fields of separate particles , can separate DNA, and can trap particles at a specific location. However, according to these literatures, these applications of dielectrophoresis have been limited to two-dimensional motion in a horizontal plane until now. Thus, we developed a device that can operate a particle moving in three-dimensional space with stereoscopic observation.
The dielectrophoresis method was first invented by Kaler and Pohl ; they calculated a range of frequencies and characterized a single living cell. The effective polarizabilities of the yeast cell and a Netrium digitus cell were considered in terms of frequency. Further calculations were completed by Wang et al. , and they proposed a time-averaged dielectrophoresis force in an alternating current electric field of angular frequency. They reported a quantitative analysis of the dielectrophoretic forces acting on particles in some practical electrode configurations, including the translational force experienced by particles in practical rotating electric fields. Jones and Washizu [7, 12, 13] followed their concepts to complete a dyadic tensor representation for multipolar moments for dielectrophoretic force and electrorotational torque. In 1998, Hughes and Morgan [8, 9, 14] studied a two-dimensional and time-dependent electric field, and they mentioned two pivotal results. First, a device capable of both translation and rotation could be designed through an ideal configuration of the electrodes. Second, ten distributions of the electrodes were suggested to control the particles. Reichle et al.  designed a three-dimensional octupole cage which had two layers and eight electrodes, they was driven by rotating electric fields in the MHz range. The feature of this device is that it could trap a particle as suspension; however, they didn’t discuss the overturning function. Moreover, the total dielectrophoretic force, including conventional dielectrophoretic force and traveling-wave dielectrophoretic force, were introduced . Because both forces and torques were applied on the target subject without the device directly contacting the subject, the method was widely used for biological particles and cells. In the total dielectrophoretic mode, when a dielectric particle experiences a nonuniform electric field, the dielectrophoretic force is greater than the initial inertial force.
In this paper, we developed a device that can trap, rotate, translate, and overturn a single particle to realize stereoscopic observation in optical imaging or scanning probe microscopy. In this study, the finite element method (FEM) simulation demonstrated the possibility of an open-chip device through the application of dielectrophoresis equations, including force and torque terms. Conventional forces can trap a single cell at the center of the device, and traveling-wave dielectrophoretic force can rotate and translate the cell in various directions within the three-dimensional space. The device can trap, rotate, shift, and overturn a single particle, and Aspergillus niger was considered the target in a physical experiment. The results revealed the rotation, translation, and overturning at triggering frequencies ranging from 15 to 35 Hz.
Materials and Methods
Total Dielectrophoretic Forces
Dielectrophoresis is introduced in terms of conventional and traveling-wave dielectrophoresis. Dielectrophoresis is a force induced by the effective dipole of a particle, and an electric dipole can be expressed by the electric potential. Thus, the dielectrophoretic force can be written as a function of these electric potentials. The particle must be suspended in some liquid to enable manipulation; therefore, the fluidic force is an essential consideration for particle suspension.
Finite Element Method Analysis
Parameters of the fluidic medium
Parameters of Aspergillus niger and fluidic medium
Frequency of the electric field
Conductivity of the medium
Relative permittivity of the medium
Density of the medium
Viscosity of the medium
Density of the Aspergillus niger
Diameter of the Aspergillus niger
Conductivity of the Aspergillus niger
Relative permittivity of the Aspergillus niger
Parameters of the silicon
This simulation has four limitations: (1) the viscous force and torque on the particle caused by the liquid are not considered; therefore, the moving velocity is not accreted in the simulation; (2) the Clausius–Mossotti factor of A. niger was approximately obtained from a previous study  and that results in an approximation of the gathering time; (3) the shape of the electrodes in the simulation is rectangular, which results in tip discharges at the corners and affects the dielectrophoretic field; thus, the mesh density of the electrodes must be adjusted to avoid this phenomenon; (4) for the vertical plane, only three electrodes exist without the top electrode, resulting in an asymmetric field along the horizontal axis. The fourth electrode could be set up as an AFM probe in the future.
The four electrodes were controlled using a functional program written by LabVIEW; the power was generated by an NI USB-6343 signal generator (National Instrument Company), with a maximum sampling rate of 900,000 simple-rate/s. The apparatus had four analog output channels, and the sampling frequency at each electrode was limited from 10 to 30 kHz, which was confirmed by comparison with the amplitudes of the signals measured using an oscilloscope. An optical microscope (1100–1500 ×) with a charge-coupled device camera was used to observe and record particle movement. The rotational sequence was a program for single traveling waves, in which the desired frequency and amplitude were inputted through a panel controller in LabVIEW. In the test mode, the voltage amplitude and rotation frequency were set to 5 V and 20 kHz, respectively, for the four electrodes.
The rotation, translation, and overturning of the A. niger spores were performed successfully at a specific frequency by using a dielectrophoresis traveling wave. The dielectrophoresis field was expanded into an exact solutions and quantified with finite element analysis. A simulation supported the design of the device and demonstrated the electric field and particle tracing. Subsequently, a three-dimensional device, including four electrodes on the horizontal plane and one electrode on the bottom, demonstrated the A. niger spores in rotation with single, double, and multiple spores, demonstrated translation on a rectangular path, and demonstrated overturning in a vertical plane. According to the rotation test, the Clausius–Mossotti factor was modifiable. In our experience, the positions of the fifth electrode and power supply for the uniform dielectrophoresis were key points for trapping a particle precisely at the center (or any position) of the device and for controlling movement. Furthermore, the translation and rotation velocities were not constant because of the viscosity of the liquid. This device successfully realizes the vertical overturning technique, and that helps observe the back side of the specimen, while the specimen is chemically bonded on the substrate.
The authors thank the Ministry of Science and Technologies of Taiwan, ROC, for the support under contract NSC-104-2628-E-390-001. This manuscript was edited by Wallace Academic Editing.