A sensor-actuator coupled device was developed using solid polymer electrolyte membrane (SPM) as an active tracheal
tube for ventilator. Active tracheal tube is a novel type of tube for ventilator that removes patient's phlegm
automatically upon sensing the narrowing of trachea by phlegm. This type of active tube is extremely useful in clinical
settings as currently the sole measure to remove phlegm from patient's tube is to do it manually by a nurse every few
hours.
As SPM works both as a sensor and an actuator, an effective compact device was developed. SPM based
sensor-actuator coupled device was fabricated with modified gold plating method. Prepared SPM was fixed as an array
on a plastic pipe of diameter 22 mm and was connected to a ventilator circuit and driven by a ventilator with a volume
control ventilation (VCV) mode. SPM was connected both to a sensing unit and an actuation unit.
Generated voltage developed by the membrane with the setting of the maximum pressure from 5 cmH2O to 20 cmH2O
was in order of several hundred &mgr;V. SPM sensor demonstrated a biphasic response to the ventilator flow. The sensor
data showed nearly linearly proportional voltage development to the intra-tracheal pressure.
The sensed signal was filtered and digitized with an A/D converting unit on a PC board. A real time operating program
was used to detect the sensed signal that indicates the narrowing of trachea. The program then activated a driving
signal to control the actuation of the membrane. The signal was sent to a D/A converting unit. The output of the D/A
unit was sent to an amplifier and the galvanostat unit which drives the membrane with constant current regardless of the
change in the load.
It was demonstrated that the sensor-actuator unit detects the narrowing of trachea within several hundreds milli-seconds
and responds by actuating the same membrane with the driving voltage of 3-4 V and driving current of several hundred
milli-ampere for each membrane. SPM array actuated the obstructing material of 2 g to expel from the trachea tube.
Also, a theoretical model of the propagating wave generated by SPM was examined.
Solid polymer electrolyte membrane (SPM) acts not only as an actuator but as a small, voltage generating, and fast response sensor. Sensing characteristics of SPM as applied to a flow sensor for a ventilator was studied. SPM was prepared by chemically plating with gold on the surface of Nafion membrane. A new technique using Nafion R-1100 resin was applied to fabricate SPM with an arbitrary thickness between 200-1000 μm. Flow sensing unit and signal amplifier was constructed to measure the induced voltage by bending SPM with air-flow from the ventilator. Induced voltage by SPM ranged 1-100 μV over a ventilator air-flow range of 20-100 L/min. SPM sensor showed linear increase of induced voltage by the increase of flow. This relationship was tested over a range of SPM thickness, length and width. The result was compared with an electro-mechanical coupling model of SPM transducer: data showed consistent result on the relation between the induced voltage and membrane length and thickness while a discrepancy was observed in the relation of membrane width and induced voltage. The result, however, was consistent with the assumption of capacitive component model.
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