Sensor Technology
The sensor
The sensor consists of a lithium-selective electrode in a flow-through cell.
It is disposable, sterilized by gamma irradiation and
foil packed. The sensor is connected to a three-way
tap on the arterial line and a small peristaltic pump
restricts the flow through it to 4.5 ml/min. The flow-through
cell is made of polycarbonate and designed with an eccentric
inlet so that the blood swirls past the tip of the electrode.
The lithium-selective electrode
is made of polyurethane with a central lumen. A wick,
which is soaked in heparinised saline when the cell is
first primed, makes the electrical connection between
the blood in the cell in the vicinity of the tip of the
electrode and the remote reference. This arrangement ensures
adequate constancy of the voltage of the reference which
is far enough from the blood to avoid a significant temperature
effect. The electrode is filled with a reference material
which provides a constant ionic environment and supports
the membrane which is dip cast. The membrane is made of
polyvinyl chloride and contains a lithium ionophore to
make it selectively permeable to lithium ions.
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