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.
Carbon nanotubes (CNTs) have attracted extensive attention in the past few years because of their appealing mechanical
and electronic properties. Yarns made through spinning multi-walled carbon nanotubes (MWNTs) have been reported.
Here we report the application of these yarns as electrochemical actuators, force sensors and microwires. When extra
charge is stored in the yarns, change in length. This actuation is thought to be because of electrostatic as well as quantum
chemical effects in the nanotube backbones. We report strains up to 0.7 %. At the same time, the charged yarns can
respond to a change in the applied tension by generating a current or a potential difference that is related to the applied
tension force. As current carriers, the yarns offer a conductivity of ~300 S/cm, which increases linearly with
temperature. We report a current capacity of more than 10<sup>8</sup> A/m<sup>2</sup>, which is comparable to those of macroscopic metal
wires. However, these nanotube yarns have a density (0.8 g/cm<sup>3</sup>) that is an order of magnitude lower than metallic wires.
The MWNT yarns are mechanically strong with tensile strengths reaching 700 MPa. These properties together make
them a candidate material for use in many applications including sensors, actuators and light-weight current carriers.
Twist-spun carbon nanotube yarns actuate when extra charge is added to the yarn. This charge can be stored in a doublelayer
capacitor formed when the yarn is submersed in an electrolyte. The dependence of the actuation stress and strain on
the stored charge must be studied if double layer charging models are to be fully verified over large potential ranges.
However, background currents are generated in the system when an electrical potential is applied, making it hard to
discern the charge stored in the actuator and the charge that passes through the cell due to faradaic processes. A model is
developed to separate the capacitive and faradaic portions of the actuator current. The model is then applied to the
analysis of the actuation data. The consistency of the results paves the way to understanding the real strain-charge
behavior of the actuator.