Bio-Rad Gene Pulser MXcell™ Electroporation System - Manual (Page 54)

Electroporation Theory
45
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 (Chang et al., 1992).
There are two instrument parameters that describe the changes that cells experience upon
electroporation. The first of these, the electric field strength, E, measured in V/cm, describes
the electrical environment in the electroporation chamber (plate 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 membrane 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.
The Gene Pulser MXcell system is the only electroporation instrument capable of delivering
both exponential decay (page 45) and square wave pulses (page 46) with different protocols
to 24 well sets in a single plate. The system consists of a pulse generator system (the power
module), a plate chamber and electroporation plate with incorporated electrodes (page 5).
Activating the PULSE button on the Gene Pulser MXcell system charges the capacitors in
the unit to a high voltage. Then the system causes current flow from the capacitor into the
sample in the electroporation plate. Discharging the charged capacitor into the sample
generates either the exponential decay or the square wave pulse.
Exponential Decay Pulses
The exponential decay circuit of the Gene Pulser MXcell electroporation system generates
an electrical pulse by discharging a capacitor. When a capacitor is discharged into the
sample, the voltage across the electrodes rises rapidly to the peak voltage then declines
over time t, with an exponential decay waveform (Figure 11 on page 47) according to the
following equation:
V
t
= V
o
[e -(t/RC)]
where V
o
is the initial voltage in the capacitor, V
t
is the voltage at time = t (expressed in ms)
after the pulse, e is the base of the natural logarithm, R is the resistance of the circuit
(expressed in
Ω), and C is the capacitance (expressed in μF). The time required for the initial
voltage to drop to V
o
/e is referred to as the time constant,
Τ, a convenient expression of the
pulse length (expressed in msec). When t =
Τ = R x C, the voltage has declined to 1/e
(~37%) of the initial value, V
o
(V Τ = V
o
/ e).