Automatic cell manipulation and patterning system using dielectrophoresis

Abstract

The ability to manipulate and pattern cells is an important technique used to facilitate cell-based assay and characterization. Seeding cells on a planar substrate is the first step to construct artificial tissues in vitro. Cells should be organized into a pattern similar to the native tissue and cultured on a favorable substrate to facilitate desirable tissue ingrowth. In the field, Lab on a chip (LOC) devices have been widely employed because they are cost-effective and easy to fabricate compared with traditional laboratory methods. A millimeter-scale LOC device with a microchannel is fabricated using lithography. The operating principle of an LOC device is to apply non-contact force, such as magnetic, electrical, hydrodynamic, or gravitational force, on cells in the microchannel. Within the channel, separation, patterning, trapping, or manipulation can be performed using different LOC designs. For cell patterning, dielectrophoresis (DEP) force is commonly utilized because it requires no contact and is less invasive to the cell. To fabricate a DEP-based LOC device, a polydimethyl-siloxane (PDMS) cover is bonded to a glass substrate to form a microchannel. The glass substrate deposited with the electrodes is referred to as a microchip. Cells in the microchannel experience DEP force induced by the non-uniform electric field generated from the electrodes. Although the LOC device can be easily fabricated, their dimensions are limited to millimeter- to submillimeter-scale. Each LOC device is uniquely designed to serve its intended application. In addition, cells entrapped or immobilized on the substrate of the microchannel after culturing cannot be detached for further process. This thesis presents a micromanipulation system that can further advance research on bioassays, cell biology, and biomedical technology. The system consists of five main components: an inversely equipped microchip with its electrodes connected to a function generator, a microchip holder to adjust the position of the microchip to a microscope with a planar movement stage, a biocompatible substrate with the cell droplet, a motorized stage for displacing the substrate and an inverted microscope. To offer increased flexibility, this system can allow different types of microchips and substates to be secured onto the system to construct a desirable microchannel environment. The gap or microchannel between the microchip and the substrate can be precisely controlled and easily detached, offering superior advantages over conventional LOC devices. Images obtained from the microscope would be utilized to facilitate precise alignment and manipulation among the cells, the microchips, and the substrate. In this thesis, microchips with different electrode designs have been fabricated for cell manipulation and cell patterning via negative-DEP (nDEP). The proposed system was first examined to perform precise position and orientation control on mouse embryos with a large scale. This technique is fundamental to numerous processes, such as intracytoplasmic sperm injection and assisted hatching, because the microchip can be conveniently removed. To apply DEP force for cell translation, a microchip with electrodes in a quadrupole configuration was considered to interact with the embryos. With the quadrupole microchip, electrorotation (ER) can be applied for cell orientation. The embryo surrounded by the electrodes can be trapped or rotated accordingly by supplying different signals to the electrodes. Feedback control was applied to rotate and reposition the embryo to the desired orientation and location, respectively, for subsequent operations. A vision-based estimation was employed to evaluate the necessary information of the embryos for the controller, such as position and angular information. A series of experiments was conducted to explore the suitable system configuration. The proposed system was also used to arrange cells into different patterns and enable them to mimic the pattern similar to native tissues for culture. Based on the principle that any type of patterns can be reproduced using multiple grid dots, a microchip consisting of a 4 x 4 dot electrode array and a common surrounding electrode was utilized for cell patterning, which can construct user-defined cell patterns. Unlike the quadrupole electrode design, with the surrounding electrode, each electrode on the microchip can generate an individual ring-like electric field to trap and hold microparticles underneath its center. Each electrode can be turned on or off selectively, and the cell pattern can be constructed with single cells or cell clusters. To test the accessibility of single-cell patterning, polystyrene (PS) beads laid on the substrate were displaced to different electrodes for trapping according to the layout to construct a desirable pattern. The captured image was processed to evaluate information, such as the positions of the electrodes and the PS beads in the image. A separation algorithm was implemented to separate beads adjacent to each other, ensuring that only a single bead was trapped underneath each electrode. An optimal path that can direct the selected bead toward the electrode was evaluated to avoid interference from nearby electric fields. Different strategies were implemented to construct high-quality single-bead patterns and different optimization algorithms were analyzed through simulations to accelerate the patterning procedure. Experiments confirm that PS beads can be successfully patterned on a glass substrate by using the proposed system. Yeast cells were considered to construct large-scale cell pattern with the proposed system to achieve cell-cluster patterning. When multiple cells are present under the energized dot electrode, they would gather together to form a cluster. By controlling the signal for each electrode, cell clusters can be formed into a specific pattern. Factors, such as cell medium, input voltage, and input frequency were examined because they affect cell cluster formation. Different experimental parameters and substrate materials were tested to optimize the performance of the proposed system. Mechanisms to remove abundant cells surrounding the constructed pattern were also incorporated. The flexibility of the system makes it easy to pattern cells in any substrate. With the coordination between the substrate and microchip, large-scale cell patterns were also achieved. A series of characteristics was successfully printed on different substrates by using the proposed system. The proposed system offers an automatic method used to manipulate cells and construct different cell patterns. In contrast to the LOC devices, the adjustable microenvironment created through the proposed system provides high flexibility. The relative position between the electrodes and the substrates can be arbitrarily adjusted, offering an effective method to construct cell patterns. In addition, this system enables cells to be patterned on any type of biocompatible substrate rather than a glass substrate only. The microchip can be easily changed with a different design of electrode microchip according to applications. The microchip scalability of the system allows the manipulation of small (yeast) and large cells (embryo). The proposed system with different microchips can achieve various biomedical applications.