Tuesday, 26 November 2019

The Future is Now: Electroactive Polymers for Robotic Applications




In recent years, functional polymers got more and more attention, especially from the robotics world. In this blog post, we are going to review the basic concepts of active polymers and make a deep dive into electroactive polymers used in robotic applications.

What are active polymers?

In general, there are non-electrically deformable polymers (NDPs) and electroactive polymers (EAPs). NDPs are actuated by non-electric stimuli such as pH, UV light exposure, or temperature changes. EAPs are actuated by an electric stimulus. EAPs can be divided into two different classes: electronic EAPs (changes by an applied electric field) and ionic EAPs (changes due to diffusion of ions). Major differences between electronic and ionic EAPs. Electronic EAPs need high activation fields (>150 V/µm) and these high values are close to the maximum voltage required to electrically breakdown electronic EAPs. In general, breakdown strength is the maximum dielectric strength of a given material. Furthermore, electronic EAPs can hold a certain deformation upon triggering through a DC voltage. This particular property makes them highly interesting for robotic applications. Electronic EAPs can operate under standard atmospheric conditions and have a fast response time in range of milliseconds upon triggering. Ionic EAPs need a low activation voltage of usually 1 - 5 V to obtain actuation movements. Actuation forces in ionic EAPs are lower compared to electronic EAPs and the response is also considerably slower. Hydrolysis is a concern when operating in aqueous systems. Looking at the deformation mechanism, it is more similar to a muscle deformation. Bending is the main movement of ionic EAPs. The major downside of ionic EPAs is that operation should be either carried in wet environment, or in solid electrolytes. Overall, movements such as bending, stretching, or contracting can be achieved by both, electric and ionic polymers.

Major differences between electronic and ionic EAPs

Electronic EAPs need high activation fields (>150 V/µm) and these high values are close to the maximum voltage required to electrically breakdown electronic EAPs. In general, breakdown strength is the maximum dielectric strength of a given material. Furthermore, electronic EAPs can hold a certain deformation upon triggering through a DC voltage. This particular property makes them highly interesting for robotic applications. Electronic EAPs can operate under standard atmospheric conditions and have a fast response time in range of milliseconds upon triggering. Ionic EAPs need a low activation voltage of usually 1 - 5 V to obtain actuation movements. Actuation forces in ionic EAPs are lower compared to electronic EAPs and the response is also considerably slower. Hydrolysis is a concern when operating in aqueous systems. Looking at the deformation mechanism, it is more similar to a muscle deformation. Bending is the main movement of ionic EAPs. The major downside of ionic EPAs is that operation should be either carried in wet environment, or in solid electrolytes. Overall, movements such as bending, stretching, or contracting can be achieved by both, electric and ionic polymers.

EAPs and robotic applications

In recent years, robotic applications have been increasing and the mechanical as well as electrical functionality of robotic hardware have difficulties to keep up with this fast application development. With the introduction of EAPs into the robotics world, this is about to change. One motivation to use EAPs is for straight-line motions. Such kinds of motions are difficult to make without a complex powertrain behind. Robots which are equipped with several powertrains to make different kind of movements become bulky. This reduces the chance of sending such robots on sensitive missions. Partial removing of powertrains can lead to better energy efficiency too. EAPs are inherently flexible and allow applications in the field of biomimetic machines. In nature, animals have soft and smooth actuation members (e.g. hands and fingers). It is aimed to imitate such soft actuators in robotics as well. Among EAPs, piezoelectric polymers are more and more in use for actuators. Piezoelectric (mechanical stress generates an electric field) polymers use Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)). The high electronegativity of fluorine atoms creates local dipoles (separation of the positive and negative charges) on the polymer backbone. As a consequence, polarized domains are formed by the local dipoles, together with an alignment in the electric field. In general, (P(VDF-TrFE)) copolymers have a Young’s modulus of up to 10 GPa, allowing high mechanical energy densities. Electrostatic strains of up to 2% can be obtained applying a large electric field (~200 MV/m). Apart of robotic actuators, piezoelectric polymers are used in force/pressure sensors, loudspeakers, piezo switches and printed memory applications. One application example is a button made out of a material called Solvene® (Solvay S.A.), where mechanical stresses lead to an electrical field which can be detected. This is shown below in the Youtube-video.

We will see more and more commercial robotic applications using EAPs. Apart of robotics, sensor functions in automotive can be taken over by such polymers from wheel condition monitoring to steering wheel motion monitoring.


Thanks for reading & till next time!
Greetings,
Herwig Juster


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Literature:
[1] Kim et al.: Electroactive Polymers for Robotic Applications, Springer
[2] https://www.youtube.com/watch?v=jUtsrN7C4Uo

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