Bio-Rad MicroPulser™ Electroporator - Manual (Page 7)

The capacitance discharge circuit of the MicroPulser generates an electrical pulse with an
exponential decay waveform (Figure 2). When the capacitor is discharged into the sample, the
voltage across the electrodes rises rapidly to the peak voltage (also known as the initial voltage,
V
0
), and declines over time, t, as follows,
V
t
= V
0
[e -(t/ τ) ] Equation 1
where τ = R x C, the time constant, a convenient expression of the pulse length. The resistance
of the circuit, R, is expressed in ohms, and the capacitance of the apparatus, C, is expressed
in microfarads. According to Equation 1,
τ is the time over which the voltage declines to 1/e
(~37%) of the peak value. The internal circuitry of the MicroPulser is designed to provide
optimum electroporation of E. coli and S. cerevisiae, as well as many other microorganisms,
in which the optimum transformation efficiency occurs at a time constant of approximately
5 msec. These electroporation conditions are achieved by using a 10 microfarad capacitor
and by placing a 600 ohm resistor in parallel with the sample cuvette along with a 30 ohm
resistor in series with the sample cuvette.
Fig. 2. Exponential decay pulse from a capacitance discharge system. When the capacitor,
charged to an initial voltage, V
o
is discharged into cells, the voltage applied to the cells decreases over
time so that at time t = τ, the voltage is (1/e) x V
o
of the initial value.
In addition to the time constant, the electric field strength is the other instrument param-
eter that is important in determining transformation efficiency. The electric field strength, E,
is the voltage applied between the electrodes and is described by
E = V/d Equation 2
where V is the voltage applied and d is the distance (cm) between the electrodes. The strength
of the electric field and the size of the cells determine the voltage drop across each cell, and
it is this voltage drop that may be the important manifestation of the voltage effect in elec-
troporation.
3
V
0
V
0
e
Time (msec)
Voltage (V)