BC-535 Preliminary, Rev. 060126
14
grounding the Faraday cage through the headstage (see
Headstage connections
, page 11), we do not
recommend this procedure, but instead suggest that the cage be grounded through the amplifier
circuit ground.
Several Faraday cage designs are available. The most common commercial design is that of a
copper or aluminum wire mesh supported on an aluminum frame. This frame attaches to the
conducting top of a floor-standing vibration isolation table which completes the cage enclosure. Entry
is through large front panel doors. This design is most often used in conjunction with patch clamp
setups since the large enclosure can house a microscope as well as several other devices.
An option exclusively presented by Warner Instruments consists of a Faraday cage with an
enclosed vibration isolation table. This unique combination is specifically designed with the bilayer user
in mind. The assembly requires little lab space, rests comfortably on a sturdy work surface, and
actively isolates the tabletop from the cage enclosure. The cage is easily assembled and has several
design features simplifying bilayer work.
Regardless of the Faraday cage employed, the headstage and membrane support system (e.g.
cups and chambers) are contained within the cage which acts as the electromagnetic shield. Other
devices such as a perfusion system or stirring apparatus may also be housed within the cage, but
some investigators place these components on the outside (with proper grounding) to reduce their
noise contribution.
Vibration isolation
The isolation and damping of mechanical noise is critical to increasing the signal to noise ratio of
a BLM workstation. The significance of this becomes apparent when one considers that the acoustic
coupling of normal speech to the buffers on each side of the bilayer is large enough to present a
significant capacitance current artifact in the data.
Several approaches have been employed to eliminate large amplitude mechanical vibrations in an
experimental setup. These include specially designed vibration isolation tables or optical benches.
These floor standing benches employ a heavy table top resting on pneumatic supports. Alternatively,
investigators have placed heavy concrete slabs (commonly referred to as balance tables) or large steel
sheets on partially inflated inner tubes or tennis balls. We recommend the use of a high quality
commercial table since these devices provide more long term stability and more effectively damp
vibrational noise inputs.
Another, more subtle, source of noise in electrophysiological recording systems is associated with
vibration of the headstage. This movement can produce a rapidly fluctuating stray capacitance which
appears as increased noise in the amplifier output. This effect can be minimized by shock mounting
the headstage to its support. Since it is advantageous to keep the associated moment arm as small as
possible, the headstage should be directly mounted to its support rather than through a long
connecting rod. Warner Instruments has developed the
HST-1
headstage holder system expressly for
this purpose.
Membrane support
The general approach to the formation of a planar lipid bilayer membrane involves spanning lipids
across a small hole or aperture in a membrane support. A cocktail of lipids, usually suspended in a
Warner
Instruments
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