Electroactive polymers (EAP), often called “artificial muscles”, are polymeric multilayer systems that are usually produced in the laboratory. They can change their shape or perform a mechanical movement when extremely low electrical voltages are applied.
The Department of Functional Materials at Fraunhofer IPA is conducting research on ionic EAPs which function on the basis of electrochemical processes in the electrode layers. Movements are triggered by freely moving ions that diffuse into the layers and thus increase the volume of the electrodes. As the latest addition to ionic EAPs, CNT-based polymer actuators have several advantages. Among other properties, they transmit force well with relatively large deflections, are able to function in air, have integrated sensor functions and are comparatively easy to manufacture. IPA is researching into environmentally friendly and cost-efficient material compositions, fast and scalable manufacturing methods and application-specific integration options for these CNT actuators.
Due to their intrinsic and absolutely noise-free deflections and their flexibility, they are predestined for use in small wearable devices, as well as in soft robotics and in shape-changing surfaces and other technical applications where small, light and flexible actuators are called for.
In the collaborative project “Fraunhofer Project Center for Electroactive Polymers” (FPC) with the Japanese research institute AIST Kansai, which ran from 2014 till 2017, a close exchange of scientific information took place and joint research was carried out on EAP actuators. The main objective was to develop an EAP-operated micropipette for microdosing applications. In this context, Fraunhofer IPA focused on simulation-based processes for optimizing the actuator geometry as well as on electrical contacting methods for integrating the actuators.
Project partners/ funding:
Fraunhofer Gesellschaft e. V.
Thanks to their flexibility and light weight, EAPs open up a whole range of new application possibilities. In the Collaborative Research Centre 1244, an adaptive textile skin with adjustable breathability was developed, which can be used for ultra-light building envelopes. Small flaps and slits in the skin are opened and closed as required by integrated EAP actuators, thus enabling air exchange inside the building to be controlled. The system does not require any complex, heavy or space-consuming mechanics. This project represented the world's first experimental approach to deploy and test the technology of ionic EAP actuators in a building-related application.
Project partners/ funding:
German Research Foundation DFG
In order to use EAPs for a wide range of technical tasks, laboratory-scale manufacturing techniques that have been carried out manually up till now must be transferred to large-scale industrial production processes. At IPA, extensive tests are being performed on automatable printing technologies, such as multi-layer slot die coating or screen printing. In the process, large electrode layers can now be produced much faster and with a more uniform layer thickness. This not only makes the performance characteristics of the actuators more reproducible but also lowers reject rates.
EAP actuators are still in their natural form after assembly. The surfaces of their electrodes are directly exposed to the ambient air and humidity. Their encapsulation is advisable for the following reasons:
a) it creates a strong and anti-abrasive layer which protects against external mechanical and chemical influences,
b) it shields the complex electrochemical processes inside the actuator layers from humidity in the air
c) electrical insulation.
Fraunhofer IPA employs a specially-developed dip-coating process to encapsulate EAPs. Apart from the electrical contact surfaces, the entire actuator is immersed in a solution containing the thermoplastic fluoroplastic PVDF and is then air-dried.
PVDF was chosen mainly for its hydrophobic and chemical resistant properties, but also because this resistant material is used for the polymer matrix of all three actuator layers. Thus, the strong cohesion forces reduce the probability of the protective layer delaminating during operation.
Simulation techniques allow many important material and performance parameters to be calculated and examined under realistic conditions without having to conduct large-scale test series or build several prototypes. The main reason behind using simulation techniques for EAPs is to predict the actuation behavior of different actuator geometries. As a result, shapes can be optimized for specific applications and the actuation mechanism improved through mechanical leverage effects. However, the actuation mechanism of CNT actuators is based on a highly complex interplay of diverse chemical and quantum physical effects.
One approach to modeling the actuation properties of the CNT actuators used is to match experimental data with the theory of common linear elastic models. By correlating parameters, existing or easily obtainable measurement data on stress/displacement and force ratios of CNT actuators can be transferred to a set of virtual material parameters suitable for use in simulation environments.
Deformation of the electrode material is triggered by a virtual thermal load that is applied to all parts of the three-layer actuator in a simulation assembly. The thermal load causes the electrodes to either expand or shrink. The lower electrode is modeled with a negative thermal expansion coefficient, while the upper electrode is assigned a positive thermal expansion.