Devices called electrostatic actuators move objects by using electric fields. These devices consist of two electrodes with opposite charges that produce a force when an electric field forms between them. Various configurations of electrostatic actuators have been devised where the force can resemble that of contracting muscles by changing the shape of their electrodes and filling the space between them with flexible, pliable materials.

However, it is possible to use electrostatic actuators in artificial muscles thanks to research from the Tokyo Institute of Technology (Tokyo Tech) that used ferroelectric materials to create an electrostatic actuator that can generate strong force at a low driving voltage.

Scientists increased the accumulated charge to increase the force generated by actuators. They did this by using ferroelectric materials that spontaneously polarize.

Charge separation (polarization) occurs when an electric field is applied to ferroelectric materials. Unlike normal paraelectric materials, ferroelectric materials retain their polarization long after the electric field is removed, allowing them to support many accumulated charges at a low voltage. The resultant force is also linearly proportional to the applied voltage, since the polarization of ferroelectric materials is independent of voltage.

Professor Suzushi Nishimura said: “Ferroelectric media are superior to ordinary paraelectric media for electrostatic actuators in two respects. One is that they can generate higher power by maintaining large polarization even at low voltage. The other is that their voltage response is nearly linear, resulting in good controllability of the device.”

Scientists then used liquid crystals in the special nematic phase as a ferroelectric material. The material was found to flow like a liquid at room temperature while possessing a rod-shaped molecular structure like that of solid crystals—necessary properties that give these materials significant dipole moment and the fluidity necessary for their use in artificial muscle.

In experiments, it was found that ferroelectric liquid crystals produced forces across electrodes 1200 times greater than those produced by traditional paraelectric materials such as insulating oils. The scientists have created an electrostatic actuator that can contract and expand like muscles at low voltages using ferroelectric liquid crystals and a 3D-printed double spiral electrode.

prof. Nishimura said: “When we applied an electric field of 0.25 MV m-1, the device shrank 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 to it. Even a dry cell battery can power the current actuator.”

These findings show that ferroelectric materials with spontaneous polarization hold promise for developing electrostatic actuators suitable for artificial muscles.

Magazine reference:

  1. Suzushi Nishimura et al. Decreasing the driving voltage of the electrostatic actuator and increasing generated force using spontaneous polarization of ferroelectric nematic liquid crystals. Advanced physics research. DOI: 10.1002/apxr.202200017