Abstract:
Systems, methods, apparatus, and circuits for controlling an electrical signal transmitted to a sample load are provided. The electrical signal produced by a capacitor is controlled via a control signal sent to a variable resistance device that is connected in parallel with the sample load. The variable resistance device includes a resistance and a switch in series. The control signal opens and closes the switch, thus providing a variable resistance based on the amount of time the switch is closed.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Patent Application 60/826,422 entitled “RESISTOR PULSE MODULATION,” by Charles W. Ragsdale, filed Sep. 21, 2006, the entire contents of which are incorporated herein by reference. 

   BACKGROUND 
   The present invention relates generally to controlling electrical signals and more particularly to systems and methods for controlling the shape of an electrical pulse in electroporation systems. 
   It is known that exposure of cells or other biological molecules to intense electric fields for brief periods of time temporarily destabilizes membranes. This effect has been described as a dielectric breakdown due to an induced transmembrane potential, and has been termed “electroporation”. Among the procedures that use electroporation are the production of monoclonal antibodies, cell-cell fusion, cell-tissue fusion, insertion of membrane proteins, and genetic transformation. 
   The cells or tissue are exposed to electric fields by administering one or more direct current pulses. These pulses are administered in an electrical treatment that results in a temporary membrane destabilization with minimal cytotoxicity. The intensity of the electrical treatment is typically expressed in terms of the field strength of the applied electric field. This electric field strength is defined as the voltage applied to the electrodes divided by the distance between the electrodes. Electric field strengths used in electroporation typically range from 1000 to 5000 V/cm. 
   For efficient electroporation, it is necessary to control the shape, e.g. time constant of the electrical pulse. For example, electroporation itself occurs within a narrow range of parameters, such as pulse voltage and pulse duration, which is exhibited by a narrow window between electrocution and little or no electroporation. If a pulse with too long a duration or too high a field strength is used, the cells may be lysed (destroyed). If the duration or field strength of a pulse is too low, electroporation efficiency is lost. As an added difficulty, the optimal voltage and time constant varies with the type of cell. The current emphasis on using electroporation to study cells that are sensitive and difficult to transvect makes the control of electroporation conditions particularly important. 
   One problem in selecting the electroporation parameters is that the sample itself (cells plus buffer) is a significant factor in the load imposed on an electroporator and can have a wide range of resistance values. To provide the desired shape, a selection of capacitors (used to store charge for delivery to the sample) may be used to determine a time constant. Parallel resistors can also be switched-in to supplement the adjustment precision. Switchable high-voltage power resistors are large and costly, however. Additionally, the precision of adjustment achievable is still rather coarse to be able to reduce the number of such resistors and switching elements. 
   It is, therefore, desirable to provide systems and methods for controlling the shape of the electrical signal in a more efficient and continuous manner. 
   BRIEF SUMMARY 
   Accordingly, the present invention provides systems, methods and circuits for controlling an electrical signal transmitted to a sample load. The electrical signal produced by a capacitor is controlled via a control signal sent to a variable resistance device that is connected in parallel with the sample load. In one aspect, the variable resistance device includes a resistance and a switch in series. The control signal opens and closes the switch, thus providing a variable resistance based on the amount of time the switch is closed. 
   The variable resistance device may have a constant resistance during an electrical signal, i.e. the amount of time the switch is closed over a certain time period remains constant, or the resistance may vary over a time period for the electrical signal. For example, in an embodiment where the control signal is a pulse width modulated signal, the duty cycle may stay constant or change during the electrical signal. 
   The resistance of the variable resistance device may be a combination of resistors. Also, additional resistances and switches may be in parallel with the sample load as well as the other resistance and switch combinations. All of the resistance and switch combinations may use the same control signal or use different control signals. In one embodiment, the switches are transistors, such as an insulated gate bipolar transistor. 
   As user herein, an electrical signal may be a periodic waveform or be non-periodic, such as a pulse, and each may have different shapes to the waveform, such a square, sine or triangular wave, or an exponential decaying pulse. In one embodiment, an exponentially decaying electrical pulse with a time constant τ tunable with the control signal is provided to a sample load. 
   Reference to the remaining portions of the specification, including the drawings claims and Appendices, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a system having a variable resistance device according to an embodiment of the present invention. 
       FIG. 2  illustrates a variable resistance device according to an embodiment of the present invention. 
       FIG. 3  illustrates a circuit that provides a controllable electrical signal according to an embodiment of the present invention. 
       FIG. 4  illustrates a method for controlling an electrical signal transmitted to a sample load according to an embodiment of the present invention. 
       FIG. 5  illustrates a system having a variable resistance device according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   The present invention provides systems, methods and circuits for controlling an electrical signal transmitted to a sample load. Embodiments utilize a capacitor to produce an electrical signal, which is controlled by a variable resistor device connected with the sample load. Although embodiments are directed to producing an electrical signal in an electroporation, one skilled in the art will appreciate that embodiments may be used in any system with an innate capacitor, such as power supplies and function generators. 
     FIG. 1  illustrates a system  100  according to an embodiment of the present invention. System  100  produces an electrical signal for sample load  120 . A charging circuit  105  is connected with a capacitor  110  so as to charge capacitor  110  when switch  115 , e.g. a high voltage (HV) switch, is open. In one aspect, charging circuit  105  is a power supply or some other constant power source. HV switch  115  connects capacitor  110  to sample load R S    120  and a variable resistor R V    125 , which is connected in parallel with R S    120 . 
   Once charging circuit  105  charges the capacitor  110  to a desired voltage HV switch  115  is closed. HV switch  115  may also be a driver with programmable parameters, such as number of pulses, pulse duration, and bursts of pulses. At this point, an electrical signal is transmitted to R S    120  and to R V    125 . In one aspect the electrical signal is an exponentially decaying pulse, a truncated exponentially decaying pulse, or a square wave signal with droop. 
   The value of the resistance of R V    125  may be tuned to control the electrical signal. For example, for an exponentially decaying pulse, the resistance of R V    125  may be automatically determined such that a desired time constant τ=CR is substantially equal to the desired pulse duration, wherein R=(R S ×R V )/(R S +R V ) and C is the capacitance of capacitor  110 . 
   In one embodiment, system  100  is an electroporator system. In this embodiment, the time constant may be chosen specifically based on the cells or other type of biomolecules in the sample load. As some cells may be sensitive or hard to transvect, a precisely controlled time constant can promote successful electroporation. 
     FIG. 2  illustrates a variable resistance device  200  according to an embodiment of the present invention. Resistance  210  has one terminal connected with a switch  220 , which is controlled by a switching driver  240 . In one aspect, a resistance  230  may be connected between switching driver  240  and switch  220  to provide greater stability. Resistance  220  may be composed of one resistor or multiple resistors in any combination as known to one skilled in the art. 
   One end  201  of device  200  is at a higher potential than another end  209 . Thus, current flows from top to bottom when switch  220  is closed. However, when switch  220  is open, no current flows. Switching driver  240  transmits one or more control signals to switch  220  to control whether switch  220  is open or closed. By controlling when current flows through resistance  210 , the effective resistance of resistance  210  can be controlled. 
   For example, if resistor  210  is 50 ohms, which is generally ideal for a high-throughput electroporator, a 100% duty cycle would result in 50 ohms of resistance. A 100% duty cycle corresponds to switch  220  always being closed and resistor  210  being switched-in continuously. However, a 10% duty cycle that switches-in resistor  210  ten percent of the time would result in a resistance of 10×50=500 ohms. The general formula for the effective resistance is 100/(duty cycle percentage)×actual resistance. Since the switching device controlling the connection of the resistor is either on or off, its losses are low, and it never needs to act as an analog control element. 
   In one embodiment, the control signals from switching driver  240  are pulse width modulated. In one aspect, switch  220  is a transistor, such as an IGBT, MOSFET, or other suitable transistor. Resistance  230  may be connected to the gate of the transistor. In another aspect, switch  220  is a silicon controlled rectifier. 
     FIG. 3  illustrates a circuit  300  according to an embodiment of the present invention. Circuit  300  uses a variable resistance device  325  to control an electrical signal from capacitance  300 . In one aspect, device  200  may be used for device  325 . Capacitance  310  may be composed of multiple capacitors in any suitable arrangement, such in series and/or in parallel with each other. 
   In one embodiment, where the electrical signal is an exponentially decaying pulse, the duty cycle of switch  335  may be adjusted to achieve a desired time constant. For example, if a 7 msec time constant is desired then the required resistance for device  325  may be calculated. If C=50 μF, then the combined resistance R=7 msec/50 μF=140Ω. If R S  was 200Ω, then R V  would be 467Ω. To achieve 467Ω from a resistance of 50Ω for resistance  330 , then the duty cycle=100×50/467, which give a duty cycle of 10.7%. 
   In another embodiment, the duty cycle of the control signal from switching driver  545  varies over time. For example, the duty cycle could be smaller at the beginning of an electrical signal so that the voltage of an electrical pulse droops or decreases at a relatively slow rate. After a certain amount of time, the duty cycle could increase so as to increase the current through resistor  330 , thus increasing the rate of voltage drop for the electrical pulse. Many different combinations of changing the duty cycle could be made as to achieve many different shapes of waveforms, signals, and pulses. 
     FIG. 4  illustrates a method of controlling an electrical signal transmitted to a sample load according to an embodiment of the present invention. In step  410 , the capacitor is charged by a charging circuit, such as a power supply. In step  420 , the capacitor is connected with a sample load and a variable resistance device, such device  325 . The connection may be made by an HV switch. 
   In step  430 , an electrical signal is transmitted from the capacitor to the sample and to the variable resistance device. In one aspect, the electrical signal is of any waveform shape, which may at least be partially determined by the HV switch. In step  440 , the electrical signal is controlled with the control signals that determine the resistance of the variable resistance device. In one aspect, the higher the control signals make the resistance of the variable resistance device, the slower the voltage of the electrical signal drops. 
     FIG. 5  illustrates a system  500  according to an embodiment of the present invention. Charging circuit  505  receives instructions from computer system  550 , which may monitor the voltage at capacitor  510 . The instructions may include whether to continue to charge capacitor  510  or at what level to charge it. In one embodiment, when the voltage at capacitor  510  reaches a sufficient value, computer system  550  signals a HV driver  515  to connect capacitor  510  to the load sample R S    520  and variable resistor device  525 . Computer system  550  may include one or more processors, memory such as RAM, a user interface, and docking ports for receiving external memory units, such as a CD or DVD. 
   Variable resistance device  525  has two switches  535   a  and  535   b , e.g. two IGBT transistors. Each switch  535  drives a pair of tandem connected power resistors  530   a  and  530   b , which may be at the output of an electroporator. In one embodiment, the resistors  530   a  and  530   b  are 100Ω, and resistors  540   a  and  540   b  are 10Ω. In one aspect, switches  535  are driven by switching driver  545  at a 10 KHz rate with a duty cycle dependent on the desired effective resistance for variable resistance device  525 . Accordingly, effective resistances of 50-1500Ω may be achieved. In one aspect, this range is achieved with only about four square inches of board space and perhaps less than $15 worth of parts. 
   Computer system  550  can program switching driver  545  to drive switches  535  at the proper rate and with the proper duty cycle. In one aspect, switching driver  545  contains a processor which can calculate the proper duty cycle or succession of duty cycles required. In another aspect, computer system  550  could include a driver for driving switches  535 . 
   In one embodiment, a desired time constant of an exponentially decaying electrical pulse can be achieved with an almost continuous adjustability, thus allowing a choice of a time constant with very high precision. In one aspect, capacitor  510  includes electronically selectable capacitors, which facilitates the almost continuous range of time constants. For a square wave, the droop level in the positive voltage can also be controlled in a similar manner. 
   In one aspect, since an electronically pulsed load is only on during the brief pulse, any interference is minimized. Also, in another aspect, since the capacitors can be large and the frequency high, the ripple on the output waveform is low. 
   While the invention has been described by way of example and in terms of the specific embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.