Guide to Electrophysiological Recording • 45
Chapter 4
any glass may potentially modify channel currents, one must be aware of this fact
and control for it regardless of the glass one uses. We recommend two glasses for
noise-critical work: Corning #7052 and quartz. Both have been successfully
sealed to many different cell types. Quartz, with its significantly lower loss factor,
has produced the lowest noise recordings known to us. However, because of its
extremely high-softening temperature, quartz requires a special puller like the P-
2000 from the Sutter Instrument Company.
Even if one uses electrically superior glasses, low noise will not be obtained unless
the outer surface of the glass is coated with a hydrophobic substance, such as Dow
Corning Sylgard #184. This substance prevents the bathing solution from creeping
up the outer wall of the pipette glass. This is important since a thin film of
solution on the outer surface of the glass produces a distributed resistance that
interacts with the glass capacitance to produce a noise source that rises with
frequency. Since it becomes the dominant noise source, it must be eliminated.
While many other hydrophobic substances have been used, none, to the best of our
knowledge, produces as low noise as does Sylgard #184. Sylgard also decreases
the capacitance of the pipette wall and so reduces the lossiness of the wall as well.
It has been shown experimentally that Sylgard will improve the noise of any glass
but it will not turn a poor electrical glass into a good one. Low-loss glasses coated
with Sylgard give significantly less noise than poor glasses coated with Sylgard.
Obviously, the closer to the tip that the Sylgard can be applied, the lower the noise.
However, under some conditions a thin film of Sylgard may flow right to the tip of
the electrode, interfering with seal formation. This problem can be reduced by
using partially-cured, thickened Sylgard for coating. Alternatively, or in addition,
the tip of the electrode can be gently “polished” using a microforge to burn off the
contaminating Sylgard.
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