EVOM
APPENDIX B: IMPORTANT APPLICATION NOTES
Resistance value of the “blank” insert
When using an
EVOM
with an
STX2
to measure a blank insert, the resistance value is
typically between 120 to 180
Ω
, depending on the specific brand of the insert. Many
users mistakenly think this background resistance is due to the resistance of the blank
filter. However, if the filter membrane is removed from the insert, the resistance reading
of the insert will remain the same. This is because the background resistance reading
is due mainly to the small gap between the bottom of the cell culture insert and the
bottom of the cell culture plate. See Fig. 1. This gap is about 1 mm, with some insert
brands having a slightly larger gap than others. The variation in this gap is the cause
of the difference between blank readings of different brands: the smaller the gap, the
higher the electric resistance.
The resistance of the filter membrane itself is actually
negligible.
If an
Endohm-24SNAP
or
Endohm-12
chamber is used, the blank resistance
becomes near zero because the external electrode is directly underneath the filter and
the gap does not exist.
Special Note: 24 mm diameter (6-well) inserts and STX
electrodes
Note that the resistance readings from 24 mm diameter tissue culture inserts (used in
6-well plates) obtained by using WPI’s STX series of electrodes will be 1.8 to 2.2 times
higher than that obtained using the
Endohm-24
. This is because the STX electrode
cannot deliver a uniform current density over the relatively large membrane through the
small gap between the membrane and the bottom of the well.
Therefore, the resistance
reading of a 24 mm (6-well) diameter insert obtained by STX electrodes should not be used
to calculate the unit area resistance.
STX electrodes are only intended for providing a
qualitative measurement of cell monolayer health and quantitative measurement of cell
confluence. This is not a problem with smaller inserts, such as the 12 mm and 6 mm
diameter inserts, because of the relatively smaller size of the membrane compared to
the electrode. When
EVOM
is used with
the
Endohm
, it gives a repeatable and accurate
value of membrane resistance consistent with that obtained by using a well-designed
Ussing chamber. It is also consistent with that obtained by the
STX2
electrode for the
12 mm and 6 mm diameter filter inserts.
Note:
The unit area resistance is independent of the area of the membrane and may be
used to compare data obtained from inserts of different sizes.
WORLD PRECISION INSTRUMENTS 21
