Inga Põldsalu

Ionic electroactive polymer artificial muscles in space applications

A large-scale effort was carried out to test the performance of seven types of ionic electroactive polymer (IEAP) actuators in space-hazardous environmental factors in laboratory conditions. The results substantiate that the IEAP materials are tolerant to long-term freezing and vacuum environments as well as ionizing Gamma-, X-ray, and UV radiation at the levels corresponding to low Earth orbit (LEO) conditions. The main aim of this material behaviour investigation is to understand and predict device service time for prolonged exposure to space environment.

Inversion-based control of ionic polymer-metal composite actuators with nanoporous carbon-based electrodes

Ionic polymer‐metal composite (IPMC) actuators are multilayer composites that change their shape and size in response to low-voltage driving signals. Because these devices possess both actuating and sensing properties, these composites have already been proposed for numerous applications. Recently, high-strain nanoporous carbon electrodes were developed with the aim of improving the performance and stability of IPMC actuators. In order to prevent damage to the device or injury, precise control of the actuator is essential. Thus far, there are no reports on the dynamic response of IPMC actuators with electrodes composed of a porous carbon material. This study fills this gap in the research by presenting the results of testing both openand closed-loop controllers of this novel actuator for position control. Inversion-based open-loop controller is first tested on the actuator to evaluate the performance of the actuator in sensorless control applications. Then, the displacement of the tip point of the actuator is used as the feedback signal for closed-loop control to compensate for the errors experienced in open-loop control.

Lifetime measurements of ionic electroactive polymer actuators

This article is focused on proposing a unified methodology for automating the measurement procedures of ionic electroactive polymer actuators. The proposed methodology and large-scale automation would make testing ionic electroactive polymer actuators less labor-intensive and allow analyzing many ionic electroactive polymer actuators simultaneously. Defining a clear framework for testing ionic electroactive polymer actuators performance and reliability would make the testing process reproducible and provide better comparison between ionic electroactive polymer actuators of either different or similar classes. Our methodology separates two types of degradation: degradation during operation and spontaneous self-degradation.

Thermal migration of molecular lipid films as a contactless fabrication strategy for lipid nanotube networks

We demonstrate the contactless generation of lipid nanotube networks by means of thermally induced migration of flat giant unilamellar vesicles (FGUVs), covering micro-scale areas on oxidized aluminum surfaces. A temperature gradient with a reach of 20 μm was generated using a focused IR laser, leading to a surface adhesion gradient, along which FGUVs could be relocated. We report on suitable lipid-substrate combinations, highlighting the critical importance of the electrostatic interactions between the engineered substrate and the membrane for reversible migration of intact vesicles.

Fabrication of ion-conducting carbon-polymer composite electrodes by spin-coating

We report a fabricating method for ion-conducting carbon electrodes on top of industrially produced PVDF membrane by spin-coating. Spin-coating is desirable due to its potential application in large-scale actuator manufacturing and its possibility to produce very thin electrodes. The industrial grade membrane was chosen in order to investigate more accurately the results of spin-coating without considering the deviations present in a hand-made membrane. Spin-coating and surface resistivity measurements via four-point probe were described in further detail. The production process of electrode suspension and suspension dispensing were developed and fine-tuned. The spin coater was programmed to obtain electrodes with uniform electrical properties. The arrangement of the spin coater was slightly altered to remove swelling and bubble formation effects concurrent with usage of the porous membrane. Electrodes produced with the developed method were measured and analyzed. Thickness of the film was measured with micrometer screw gauge and four-point probe was used to measure sheet resistivity, in addition film was studied under scanning electron microscope. In best cases the coefficient of variation for sheet conductivity was 6.2%. For all electrode sheet conductivities the median coefficient of variation was 7%. The thickness of the electrodes varied from 6 to 23 μm. As a proof of concept for the developed method a working actuator with spin-coated electrodes was produced.

Long-term degradation of the ionic electroactive polymer actuators

The research is focused on lifetime and degradation of ionic electroactive polymer actuators (IEAP). The lifetime measurements were carried out using identical methodology upon the different IEAP types. The experiment conducted with large number of samples shows that two types of degradation have serious effect to the IEAPs: degradation during operation and spontaneous self-degradation. Additionally, two ways of occasional damage decrease their overall reliability. In the scope of the current paper we describe degradation of two different types of IEAP actuators: with carbonaceous electrodes and with conducting polymer electrodes. Nevertheless, the common evolutionary trends, rather than the comparative data analysis or formal statistics of all particular samples, are given. Analyzing the electromechanical and electrical impedances of the samples during their whole lifetime, we have found that observing the electric current gives adequate information about the degradation level of any IEAP actuator. Moreover, tracking this electrically measurable parameter enables detecting the occasional damage of an actuator.

Pulse-width-modulated charging of ionic and capacitive actuators

We report on using a pulse-width-modulated (PWM) signal for driving the ionic electroactive polymer (IEAP) actuators. The traditional approach for driving the IEAP actuators involves generation of complex analog signals. The proposed control method is substantially different: a digital PWM driving waveform is applied using an H-bridge driver. The two outputs of the H-bridge driver are switched between three states - they are either connected to the positive or negative power supply terminal, or disconnected, with a high-impedance output. An H-bridge can also be used for short-circuiting of the actuator, in turn improving the power-efficiency of the IEAP actuator. This control method is particularly beneficial in applying IEAP actuators in soft robotics.

Micro-mechanics of ionic electroactive polymer actuators

Commonly, modeling of the bending behavior of the ionic electroactive polymer (IEAP) actuators is based on the classical mechanics of cantilever beam. It is acknowledged, that the actuation of the ionic electroactive polymer (IEAP) actuators is symmetric about the centroid - the convex side of the actuator is expanding and the concave side is contracting for exactly the same amount, while the thickness of the actuator remains invariant. Actuating the IEAP actuators and sensors under scanning electron microscope (SEM), in situ, reveals that for some types of them this approach is incorrect. Comparison of the SEM micrographs using the Digital Image Correction (DIC) method results with the precise strain distribution of the IEAP actuators in two directions: in the axial direction, and in the direction of thickness. This information, in turn, points to the physical processes taking place within the electrodes as well as membrane of the trilayer laminate of sub-millimeter thickness. Comparison of the EAP materials, engaged as an actuator as well as a sensor, reveals considerable differences between the micro-mechanics of the two modes.

An ionic liquid-based actuator as a humidity sensor

The ionic electromechanically active polymer actuators with ionic liquid electrolytes working in air also respond to ambient relative humidity. The physicochemical parameters, especially viscosity and conductivity, of ionic liquids are strongly dependent on temperature as well as on their water content. By measuring the impedance between the actuator electrodes, the relative humidity value can be extracted from the signal. The sensing actuator has the ability of converting electric energy into mechanical work and additionally to sense the parameters of the surrounding environment.

Carbon-polymer-ionic liquid composite as a motion sensor

High surface area carbon, ionic liquid and polymer are incorporated in an electromechanically active composite. This laminate bends when voltage (typically less than 3 V) is applied between the electrodes, and generates voltage and current when bent with an external force. By suitable optimization, the material can be used either as an actuator, energy storage element (supercapacitor) or sensor. Strain caused by bending promotes dislocation of ions in the micropores of carbon. As a result, the charge separation occurs because ions of ionic liquid are likely trapped in the micropores of diameters close to the ion sizes.