Electroactive Polymers for Robotics

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Electroactive polymers (EAP) are materials that can exhibit change in shape or size with respect to electrical stimulation. These polymers are also called “artificial muscles” since they possess similar characteristics to that of biological muscles. These polymers are capable of undergoing large induced displacement, which attracts researchers from all disciplines.

Scientists in the field of biomimetics (a field that develops robotic mechanisms with the help of biologically inspired models) are very particular about EAP as they have the potential to imitate the motion of insects and animals. EAP materials are mainly used for the replacement of damaged human muscles; however, the applications of current EAP are limited owing to its low robustness.

Types of Electroactive Polymer-Condutive Polymers

Two major groups of EAP are discussed in the following sections.

Group 1: Conductivity

The key benefits of using these polymers are:

• Easy manufacturing process
• Low cost
• Light weight

Some of the applications of these polymers include the following:

• A thin layer of conducting polymer can be coated on an insulator to prevent the discharge of static electricity.
• Electrical device malfunction due to microwave and radio frequencies could be addressed by replacing the plastic casing with a conducting plastic, which can be used with resins.
• Conducting polymer can also be used in the manufacture of printed circuit boards by replacing the copper plating technique with the polymerization of the conducting plastic.
• Conducting polymers have the compatibility to function as artificial nerves by transporting small electrical signals through the body.

Group 2: Electroactive

The second group utilizes the electroactivity property in various applications.

• Conducting polymers are used in the field of molecular electronics for developing electronic structures (e.g. formation of modulable barrier or spacer in modified polyacetylene).
• The application of conducting polymers in the field of sensors is mainly due to their chemical properties (e.g. development of gas sensors and biosensors).
• The most promising application of conducting polymers are weightless rechargeable batteries (e.g. polymer battery like polypyrrole-lithium cell).
• As conducting polymers can directly convert electrical energy into mechanical energy, they are used in actuators for micromechanical sorting, micropositioners, microvalves and microtweezers, which function based on the energy conversion principle.
• The most futuristic application of conducting polymers includes development of ‘smart’ structures, which modify themselves to function better, e.g. Smart skis used for vibration control, damage assessment on boats, etc.

Example of Their Use

Using an electroactive polymer-acrylic polymer 3M VHB 4910, robotics researchers proposed a flexible and non-intrusive analytical model, which can predict the power generated by an electroactive membrane with respect to the strain applied to the membrane and the physical parameters.

Principle

The electroactive power generator functions based on the equilibrium between electrical and mechanical stresses and a quasi-linear viscoelastic property of the polymer. The polymer material is stretched with an expansion coefficient λact upon the application of electric or mechanical load. The poling voltage V is applied on the electrodes under maximal stretch, which results in the formation of an electrical charge on the polymer.

The polymer is then relaxed and moved until it reaches equilibrium between electrical and elastic stresses. In the active phase, the charge is transferred to voltage source due to the variation of λ. The poling voltage V is then disconnected so that the material returns to its original state. Regardless of this, the material can be pre-strained with an expansion coefficient λp in order to prevent high poling voltage.

Experimental Setup

The chosen polymer film is introduced between two structural flexible frames. The graphite electrodes are formed using a spray consisting of carbon particles. This structure is linked to an electrical circuit. However, a DC/HDC converter is connected between the electrodes in order to develop high constant poling voltage. Finally, the voltage created in the active area and the generated current is measured without connecting any load at the output end.

Result

Promising results have been obtained from the measures of current and voltage in electroactive power generator. The pre-stretch ratio, λp of the tested polymer was 4. The polymer was a square membrane with thickness of 63µm in phase 1. The polymer was electrically stretched during the phase 1 to 2 by applying high voltage of 4kV. The active phase was made to start by reducing the applied voltage to 2kV. Then the current is measured to be 3µA.

Based on this data, the capacity Cmax and Cmin was calculated to be 80.2pF and 66.2pF, respectively. The scavenged energy was found to be 28µJ.

Sources and Further Reading

• Mistral.C.J, Basrour.S, Chaillout.J.J, Bonvilain.A, CEA-LETI, TIMA Laboratory, France, EDA Publishing,2007,pg: 25-27
• Dahiya. R.S, Metta.G, Valle.M, Lorenzelli.L, Adami.A, Piezo-Polymer-FET Devices Based Tactile Sensors for Humanoid Robots, Robotics, Brain and Cognitive Sciences Department, Italian Institute of Technology,Italy, Springer 2010, Volume 54:pp 369-372

This article was updated on 7^th February, 2019.