ellendort wrote...
I need to create holes in the cell membrane for drug delivery.
It doesn't matter that the cell has been lethally damaged in
the process?
By using constant voltages in term of pulse this will create
electric field in between 2 electrodes. The cell will be places
in between this 2 electrodes as the e-field will create the holes.
Parameters vary from cell to cell as in type of the pulse, time,
voltage and the electrode distance.
Generally this cells are the one divided into 2 categories
(2000-5000)ohms - up to 2.5kV - up to 10msec
(20-200)ohms - up to 500V - up to 100msec
The latter example is easy, with the most difficult being
500V and 20 ohms, which implies peak currents of 25A.
As for the pulse i wants to try 2 types of pulse whichever
successful. Exponential decay & square wave. This is why perhaps
need to design the system separately as for the types of the cells
and the pulse i suppose.
Inexpensive SCRs or IGBTs can handle the switching for the 500V
case, but once you've spent the extra effort to make a 500V 25A
IGBT switch, why build a second SCR switch? But you may want
different power sources. For example, a 100-ohm load with a
100ms time constant would imply discharging a 1000uF capacitor.
If charged to 500V for a 5A peak current that's 125J of energy.
By comparison a 500V 5A = 2.5kW, 100ms rectangular pulse takes
250J of energy. That's only twice as much, but it requires a
much larger storage capacitor bank, if you want to avoid much
pulse-amplitude droop. For example, limiting the droop to -10%
or 50V requires C = i t / dV = 10,000uF or 10x more capacitance.
If you point out your spec required 20 ohms 500V and 100ms, now
you'll need 50,000uF, and if you want 5% droop you'll need 0.1F
at 500V, which, just checking, stores 12.5kJ of energy. That's
not very realistic, especially for a tiny little cell membrane.
So it would seem you need to more carefully evaluate and specify
the extremes of your operating range specs. For example, do the
lower resistances, with higher currents, need the full specified
durations? Perhaps an energy-related spec would be better. For
example, the 12.5kJ mentioned above is 100x the 125J number, and
since capacitor size and cost is roughly proportional to energy,
it'll cost you 100x as much to build. A 1000,000uF 500V rated
capacitor will in fact be a huge bank of many large capacitors,
I know, having built just such a beast.
As for the exponential decay i'm using RC circuit as the fixed
capacitor and the resistor will give the constant time
(25mfd*400ohms=10msec). But i'm still facing problem in how to
truncate this pulse as in to stop this pulse at various point
for different cells. (0.2msec - 10msec)
Have you read up on IGBTs yet, as I suggested?
