Abstract
It has been observed that when certain electrical potentials are applied across a cell they can induce the formation of pores in the cell membrane and consequently increase the permeability of the cell to macromolecules. This phenomenon is known as electroporation. Since the first report on gene transfer by electroporation1, it has become a standard method for introduction of macromolecules into cells2 3 4. Currently, electroporation is normally done in batches of cells between electrodes and there is little control over the permeabilization of individual cells. Therefore, it is very difficult to study the fundamental biophysics of cell membrane electro-permeabilization and to design optimal electroporation protocols for individual cells2 3 . Although the biophysics of electroporation are still not fully understood, indirect evidence shows that micro aqueous pores with diameters of tens to hundreds of angstroms are created in the cell membrane due to the electrical field induced structural rearrangement of the lipid bilayer5. It occurred to us that if electroporation induces pores in the cell membrane, then in a state of electroporation, a measurable current should flow through the individual cell. From this idea, we have developed a new micro-electroporation technology that employs a “bionic” chip to study and control the electroporation process in individual cells. The micro-electroporation chip, shown schematically in Figure 1, is designed and fabricated using standard silicon microfabrication technology. Each chip is a three-layer device that consists of two translucent poly silicon electrodes and a silicon nitride membrane, which all together form two fluid chambers. The two chambers are interconnected only through a micro hole through the dielectric silicon nitride membrane. In a typical process, the two chambers are filled with conductive solutions and one chamber contains biological cells. Individual cells can be captured in the micro hole and thus incorporated into the electrical circuit between the two electrodes of the chip. When the cell is in its normal state no current flows through the insulating lipid bilayer and consequently between the electrodes. However, when the electrical potential across the electrodes is sufficient to induce electroporation, a measurable current will flow through the pores of the cell membrane and between the electrodes. Measuring the currents through the bionic chip in real time will reveal the information of the state of electropermeabilization in cell membrane. The breakdown potential of irreversible electroporation, the most critical parameter in electroporation process, can be detected by analyzing current signals as well. Figure 2 illustrates a typical electrical signature in an irreversible electroporation process. Once the target cell is electroporated by the application of sufficient electroporation electrical potentials, macromolecules that are normally impermeant to cell membrane can be uploaded into the cell. Figure 3 shows how a cell entrapped in a hole is loaded during electroporation with a fluorescent die. With the ability to manipulate individual cells and detect the electrical potentials that induce electroporation in each cell, the chip can be used to study the fundamental biophysics of membrane electropermeabilization on the single cell level and in biotechnology, for controlled introduction of macromolecules, such as DNA fragments, into individual cells. We anticipate that this new technology will change the way in which electroporation is done and will provide key understanding of the biophysical processes that lead to cell electroporation. This paper will discuss the design, fabrication of the micro-electroporation chip, the experiment system as well as experiments carried out to precisely detect the parameters of electroporation of individual biological cells.
