The effect of cycling on charge-storage, actuation and sensing behavior of a polypyrrole is studied, having
its application for an electroactive catheter in mind. It is shown that the electrochemical capacitance of a
polypyrrole film decreases by about 15 % over the course of 100 cycles, while the per cycle rate of this
decrease drops by 75 % between the first and the last ten cycles, implying that a steady-state value may
exist. The decrease in capacitance is shown to have a significant effect on actuation strain. In order to
achieve a more constant capacitance and more robust actuation performance, it is proposed to pre-cycle the
potential of the film to exhaust the effect of processes that contribute to the decrease in capacitance and
allow it to reach a more constant value. The ability of a polypyrrole film to generate currents corresponding
to applied external load during actuation is verified and the cycle life time of such a sensor is studied. It is
shown that after an initial decrease, the sensor current reaches a steady-state value as well, and maintains
that value at least over 5600 cycles.
In this paper conducting polymer based active catheters are presented. Design considerations along with the promise and
challenges associated with conducting polymer driven devices are discussed. A conducting polymer driven intravascular
catheter is described briefly and its design challenges such as structural rigidity and angle of bending are studied. Then a
detailed description of a polypyrrole based active catheter that is ultimately intended for in-vivo imaging applications
will be presented. The active catheter contains an optical fibre and is designed to scan the fibre in two dimensions at a
speed of 30 Hz to provide real time imaging. The preliminary design was realized by fabricating polypyrrole actuators
on a commercially available catheter and patterning the polymer using laser machining technique. The initial device was
tested at lower speeds and an image was taken using optical coherence tomography (OCT). The primary challenge to
achieving an effective polypyrrole driven catheter for real time imaging is to demonstrate high speed actuation with
reasonable liftetime. According to our model, electrochemical characteristics of the conducting polymer such as
electronic conductivity, ionic conductivity and electrochemical strain need to be improved to achieve the desired catheter
scanning speed.
Actuation of polypyrrole in aqueous sodium hexafluorophosphate solution has been shown to produce relatively large
strains. However little has been published on appropriate potential range of actuation in this electrolyte. This information
is clearly crucial for applications. Our particular interest is in disposable applications where a relatively small number of
cycles are needed, and maximum strain is desired. The electrochemical degradation as a function of voltage is
investigated by cycling the film between fixed voltages and measuring the charge transfer. The experiment was done on
a glassy carbon substrate in order to reduce effects of change in resistance with oxidation state, preventing actuation. The
dependence of charging on voltage and the rate of reduction in the extent of charging are measured. The voltage range
for effective operation of the device was found to be -0.4 V to 0.8 V versus a Ag/AgCl reference electrode in order to
achieve stable performance over at least 30 minutes. The mechanisms of degradation at potentials beyond 0.8 V appear
to be the substitution of hydroxyl ions in the polymer backbone, as suggested in reports on degradation of polypyrrole in
other electrolytes. An observed reduction in charge transfer rate at potentials lower than -0.4 V is consistent with a
reduction in ionic conductivity at highly reduced states, as has also been suggested in the literature.
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