Bio-Rad Gene Pulser Xcell™ Electroporation Systems - Manual (Page 50)

3.11.2 Pulse Trac Diagnostic Algorithm
This test algorithm tests and selects the optimal capacitor circuit of the Gene Pulser Xcell unit and the
CE Module module in the range of 25–3,275 µF. This is the key bank of capacitors used in low
voltage/high capacitance precision pulse delivery. The test algorithm tightens the already rigorous
capacitor tolerance from ±20% to ±10% (other unit designs can have a capacitor variance as high as
±40%) in the 200–1075 µF range. The high voltage capacitors in the Gene Pulser Xcell unit are not part
of this system since they are pre-selected to the same 10% tolerance that this test algorithm provides.
The Pulse Trac test algorithm is activated upon startup of the Gene Pulser Xcell when the Capacitor
Extender module is installed.
Section 4
Overview of Electroporation Theory
Electroporation is a physical process in which cells are exposed to a high-voltage electric field resulting
in a temporary rearrangement of the cell membrane. As a result, the cells become permeable and may
take up solutes from their surrounding environment, including nucleic acids, proteins, carbohydrates,
and small molecules. While much work has been done to determine how cells become permeabilized
during the process of electroporation, the membrane changes that occur are still largely hypothetical
(see Chang, et al., 1992).
The Gene Pulser Xcell is the only electroporation instrument capable of delivering both exponential decay
and square wave pulses (Figure 4.1). The system consists of a pulse generator system (the main unit with
either or both the CE Module and PC Module), a shocking chamber, and a cuvette with incorporated
electrodes (see Section 2). Activating the pulse button on the Gene Pulser Xcell charges the capacitors in
the unit and, if attached, in the CE Module, to a high voltage; the instrument then discharges the current
in the capacitor into the sample in the cuvette. Discharging the charged capacitor generates either the
exponential decay or the square wave pulse.
There are two instrument parameters that describe the changes that the cells experience upon
electroporation. The first of these, the electric field strength, E, measured in volts/cm, describes the
electrical environment in the electroporation chamber. Standard electrodes used in electroporation
consist of two parallel plates separated by a distance, d (cm); therefore,
E = V / d,
where V is the applied voltage and d is the distance between the electrodes. In practical terms, the field
strength is manipulated by altering the voltage of the instrument or by changing the distance between
the electrodes. Because the electric conductance of the cell cytoplasm is much higher than that of the
cell membrane, placing the cell in an electric field creates a voltage potential across the cell membrane.
As the field strength increases, the transmembrane voltage experienced by the cell increases as does
the likelihood that a pore will form in the membrane due to breakdown of the lipid bilayer, allowing
molecules to enter the cell from the outside (Hui, 1995; Neumann, et al, 2000).
The second parameter that affects the cell is the length of time that it is exposed to the electric field.
For exponential decay pulses, this is controlled by the capacitance of the instrument and the resistance
within the circuit. For square wave pulses the pulse length is controlled directly by setting the time that
the cells are exposed to the electric field. These are discussed for each pulse type below.
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