Tina Shoa

A Dynamic Electromechanical Model for Electrochemically Driven Conducting Polymer Actuators

In this paper, an analytical model is presented to predict the actuation response of electrochemically driven structures. A 2-D impedance model is first presented that uses a conducting polymer transmission line equivalent circuit to predict the charge transfer during actuation. The predicted electrochemical charging is then coupled to a mechanical model to find the actuation response of a bending structure. The advantage of this model compared to existing models is that it represents the 2-D charging of the polymer, namely through the thickness of the polymer structure and along its length. The model considers both ion “diffusion” through the thickness and electronic resistance along the length. The output of the impedance model is charge density in the polymer as a function of position and time, which is then used to estimate free strain via the strain to charge ratio. Given the modulus of the polymer and of passively deformed structures, time-dependent deformation is then determined. The complete electromechanical model is a function of ionic and electronic conductivities, dimensions, volumetric capacitance, elastic modulus, and strain to charge ratio, all of which are measured independently. The full electromechanical model is shown to provide a good description of the response of bending polymer structures when comparing with experimental results. The model can be effectively used as a design tool for electrochemically driven devices.

Conducting polymer based active catheter for minimally invasive interventions inside arteries

An active catheter intended for controllable intravascular maneuvers is presented and initial experimental results are shown. A commercial catheter is coated with polypyrrole and laser micromachined into electrodes, which are electrochemically activated, leading to bending of the catheter. The catheters electro-chemo-mechanical properties are theoretically modeled to design the first prototype device, and used to predict an optimal polypyrrole thickness for the desired degree of bending within ∼30 seconds. We compared the experimental result of catheter bending to the theoretical model with estimated electrochemical strain, showing reasonable agreement. Finally, we used the model to design an encapsulated catheter with polypyrrole actuation for improved intravascular compatibility and performance.

Rate Limits in Conducting Polymers

Conducting polymer actuators are of interest in applications where low voltage and high work density are beneficial. These actuators are not particularly fast however, with time constants normally being greater than 1 second. Strain in these actuators is proportional to charge, with the rate of charging being found to limit the speed of actuation. This rate of charging is in turn limited by a number of factors, the dominant factor depending on the actuator and cell geometry, the potential range, the composition and the timescale of interest. Mechanisms that slow response can be as simple as the RC charging time arising from the actuator capacitance and the series resistances of the electrolyte and the contacts, or may involve polymer electronic or ionic conductivities, which can in turn be functions of potential. Diffusion can also be a factor. An approach is presented to help estimate the relative magnitudes of these rate limiting factors, thereby enabling actuator designs to evaluated and optimized for a given application. The general approach discussed is also useful for analyzing rate limits in carbon nanotube actuators and other related technologies.

Polypyrrole operating voltage limits in aqueous sodium hexafluorophosphate

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.

Microstructuring of Polypyrrole by Maskless Direct Femtosecond Laser Ablation

Ultrafast laser micromachining was optimized for microstructuring polypyrrole as a facile new approach towards tailoring electrochemical and mechanical responses desirable for microactuator, sensors, neural probing, and nerve conduit applications. Laser perforation of high-density and high aspect ratio through-holes generated greater than 5-fold increase in surface area. The flexible machining technique offers micron-size resolution and fast prototyping capability for optimizing properties and opening new directions for polypyrrole-based devices.

Conducting polymer actuator driven catheter: overview and applications

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.

A Model for Mechanical Force Sensing in Conducting Polymers

Conducting polymers as electrochemically activated actuators have been developed for various applications. More recently, conducting polymers have been shown to work as force sensors in an electrolyte by generating a voltage or current when the force applied to them is changed (1-3). The actuation behavior of these materials has been extensively studied; however the mechanism of sensing has not been fully investigated. One possible sensing mechanism is the change in the polymer capacitance as a result of an induced tension. This change in capacitance can be related to perturbation of Donnan equilibrium potential by external load (1,2). In this paper we present a model using this proposition to predict conducting polymer sensing behavior. The model also takes the dependence of the sensing voltage on the polymer oxidation state into account.

Electrically-activated catheter using polypyrrole actuators: cycling effects

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.