While electroactive polymer applications have shown good efficiencies and lower power consumption when compared to conventional electromagnetic solutions, they operate at high operating voltages that necessitate a ‘step-up’ power supply. As previously stated, one of the challenges for electroactive polymer commercialization was obtaining a low-cost, small-form-factor power supply.
Many new robotic and teleoperated applications necessitate a high degree of mobility or dexterity, which is difficult to achieve with current actuator technology. Natural muscle is an actuator with many desirable properties for such applications, including high energy density, fast response time, and large stroke. Electrostriction of polymer dielectrics with compliant electrodes can be used in electrically controllable, muscle-like actuators.
Electrostatic actuators are lightweight, simple devices that mimic human muscles. However, because they require high voltages to generate significant forces, their application has been limited to moving small devices. However, thanks to research from Tokyo Institute of Technology (Tokyo Tech), it may be possible to use electrostatic actuators in artificial muscles. Tokyo Tech used ferroelectric materials to create an electrostatic actuator that can generate a strong force at a low driving voltage.
When we applied an electric field of 0.25 MV m-1, the device contracted by 6.3 mm, which is about 19% of its original length. Visual observation showed that the device moves when a voltage of 20 V is applied. This means that even a dry cell battery can power the present actuator.
Prof. Nishimura
Electrostatic actuators are devices that move objects by using electric fields. These devices are made up of two oppositely charged electrodes that produce a force when an electric field forms between them. Various configurations for electrostatic actuators have been developed by changing the shape of their electrodes and filling the gap between them with flexible, soft materials, allowing the force to mimic that of operating muscles.
The force generated by electrostatic actuators is determined by the voltage applied to their electrodes and the charges accumulated at the electrode-dielectric material interface. As a result, in order to generate sufficient forces to support human movement and activities, these devices must be powered by a high voltage, which can be dangerous to the body.
Aiming to increase the force generated by actuators while keeping the voltage low, Professor Suzushi Nishimura and his team from Tokyo Tech have increased the accumulated charge by utilizing ferroelectric materials that spontaneously polarize. The study, which was a collaboration between researchers from Tokyo Tech and ENEOS Corporation, Japan, has been published in Advanced Physics Research.
When ferroelectric materials are subjected to an electric field, charge separation (polarization) occurs. However, unlike conventional paraelectric materials, ferroelectrics retain their polarization even after the removal of the electric field, enabling them to maintain a high number of accumulated charges at a low voltage.
Furthermore, since the polarization of ferroelectric materials is independent of the voltage, the generated force is linearly proportional to the applied voltage (Figure 1). “Ferroelectric media are superior to ordinary paraelectric media for use in electrostatic actuators in two respects. One is that they can generate a higher force by maintaining a large polarization even at low voltage, and the other is that their voltage response is almost linear, resulting in good device controllability,” explains Prof. Nishimura.
The researchers used liquid crystals in the special nematic phase (i.e., a phase where the long axes of molecules are arranged in parallel lines but not layers) as a ferroelectric material. The material was found to be able to flow like a liquid at room temperature while possessing a rod-shaped molecular structure like that of solid crystals–necessary characteristics that give these materials a large dipole moment (i.e., magnitude of polarization) and the fluidity required for their use in artificial muscles.
In tests, the ferroelectric liquid crystal was found to generate forces across electrodes that were 1,200 times higher than that of conventional paraelectric materials such as insulating oils. With the ferroelectric liquid crystals and a 3D-printed double-helical coil electrode, the researchers developed an electrostatic actuator capable of producing contraction and expansion—like muscles would—at low voltages (Figure 2).
“When we applied an electric field of 0.25 MV m-1, the device contracted by 6.3 mm, which is about 19% of its original length,” says Prof. Nishimura. “Visual observation showed that the device moves when a voltage of 20 V is applied. This means that even a dry cell battery can power the present actuator.”
These findings demonstrate that ferroelectric materials with spontaneous polarization are promising for developing electrostatic actuators suitable for artificial muscles. The researchers are now planning to optimize the viscoelasticity of the liquid crystal material to further improve the operation of the electrostatic actuator.
