Bio-Rad Gene Pulser MXcell™ Electroporation System - Manual (Page 56)

Electroporation Theory
47
Table 17 lists the fractional voltage decrease (% droop) associated with pulse length at
various sample resistances for the high-voltage and low-voltage ranges on the Gene Pulser
MXcell system. For example, pulsing into a 200-
Ω load, the pulse length will be 33.4 msec
with a 5% fractional voltage decrease in the low-voltage range.
Figure 11 shows an exponential decay pulse from a capacitance discharge system. When a
capacitor, charged to an initial voltage V
o
, is discharged into cells, the voltage applied to the
cells decreases over time in an exponential curve such that the voltage V at any given time t
is given by V = V
o
e -(t/RC) . In the special case where t = CR then V
o
/e the value CR is known
as the time constant of the voltage decay. The shorter the time constant the faster the
decay.
Figure 11. Exponential decay pulse and square wave pulse.
Figure 11 also shows a square wave pulse from a capacitance discharge system. The pulse
length is the time the cells are subjected to the discharge. During the pulse the voltage
again decays in an exponentially so that at the end of the pulse the voltage is lower than at
the beginning. We call this drop in voltage the pulse “droop” and measure it as a percentage
of the initial voltage.
Table 17. Fractional voltage decrease (% droop) associated with pulse length.
Low-Voltage Circuit
Fractional voltage
decrease
(% droop) 10 20 5
Sample
Resistance (
Ω)
Pulse length (ms)
20 3.34 7.14 14.6
200 33.4 71.4 146
1000 167 357 730
3500 585 1249 2556