Electrostatic actuators are simple and lightweight devices that emulate human muscles.
However, their usage has primarily been restricted to moving small devices since they need high
voltages to generate significant forces.
Now, however, it may be possible to use electrostatic actuators in artificial muscles thanks to
research from Tokyo Institute of Technology(Tokyo Tech) that made use of ferroelectric materials
to create an electrostatic actuator that can generate a strong force at a low driving voltage.
Electrostatic actuators are devices that use electric fields to move objects.
These devices consist of two oppositely charged electrodes that generate a force whenever
an electric field develops between them.
By altering the shape of their electrodes and filling the gap between them with flexible,
soft materials, various configurations for electrostatic actuators have been developed
in which the force can emulate that of operating muscles.
The force generated by electrostatic actuators depends on the voltage applied to their electrodes
and the charges accumulated at the interface between the electrodes and the dielectric
material.
Therefore, to generate sufficient forces to support human movement and activities, these
devices have to be supplied with a large voltage, which can be hazardous to the body.
Aiming to increase the force generated by actuators while keeping the voltage low, researchers
from Tokyo Tech have increased the accumulated charge by utilizing ferroelectric materials
that spontaneously polarize.
The study 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.
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.
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.
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.