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Small, highly-mobile "swimming" robots are desired for underwater monitoring operations, including pollution
detection, video mapping and other tasks. Actuator materials of all types are of interest for any application where space
is limited. This constraint certainly applies to the small-scale swimming robot, where multiple small actuators are
needed for forward/backward propulsion, steering and diving/surfacing. A number of previous studies have
demonstrated propulsion of floating objects using IPMC type polymer actuators [1-3] or piezoceramic actuators [4, 5].
Here, we show how propulsion is also possible using a multi-layer polypyrrole bimorph actuator. The actuator is based
on our previously published work showing very fast resonance actuation in polypyrrole bending-type actuators [6].
The bending actuator is a tri-layer structure, in which the gold-PVDF (porous poly(vinylidene fluoride) membrane)
substrate was coated on both sides with polypyrrole layers to form an electrochemical cell. Polypyrrole films on gold
coated PVDF were grown galvanostatically at a current density of 0.10 mA/cm2 for 12 hours from propylene carbonate
(PC) solution containing 0.1 M Li+TFSI-, 0.1 M pyrrole and 1% (w/w) water. The polypyrrole deposited PVDF was
thoroughly rinsed with acetone and stored in 0.1 M Li+TFSI- / PC solution. The edges of the bulk film were trimmed
off and the bending actuators were prepared as rectangular strips typically 2mm wide and 25 mm long.
These actuators gave fast operation in air (to 90 Hz), and were utilised as active flexural joints on the tail fin of a fishshaped
floating "boat". The actuators were attached to a simple truncated shaped fin and the deflection angle was
analysed in both air and liquid for excitation with +/- 1V square wave at a range of frequencies. The mechanical
resonance of the fin was seen to be 4.5 Hz in air and 0.45 Hz in PC, which gave deflection angles of approximately 60°
and 55° respectively.
The boat contained a battery, receiver unit and electronic circuit attached to the actuator fin assembly. Thus, the boat
could be operated by remote control, and by varying the frequency and duty cycle applied to the actuator, the speed and
direction of the boat could be controlled. The boat had a turning circle as small as 15 cm in radius and a maximum
speed of 2m/min when operating with a tail frequency of approximately 0.7 Hz. The efficiency of the flapping tail fin
was analysed and it was seen that operation at this frequency corresponded with a Strouhal number in the optimal range.
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