Actuation Mechanisms of Soft Actuator Materials Driven by Electric Field

초록

Soft actuator materials change shape or size in response to stimuli like electricity, which are compliant and well-suited for electromechanical devices. Understanding the actuation mechanisms of these materials is important for developing electromechanical devices. This paper reviews the actuation mechanisms of soft actuator materials, including intrinsic and extrinsic processes. Piezoelectricity, electrostriction, and flexoelectricity are intrinsic processes associated with the crystal symmetry and molecular structure of the material. Piezoelectric polymers are categorized into semi-crystalline, amorphous, composite, and void-charged ones, and they have a linear coupling between strain and electric field. Electrostriction does not require alignment of permanent dipoles like piezoelectric, and it exhibits a quadratic coupling between strain and electric field. Flexoelectricity that has an induced polarization to a strain gradient can appear in all dielectric materials when the dimension is nanoscale. Electrochemical effect (ion migration), electrostatic effect, and friction are the extrinsic processes originating from external factors. The ion and water movement is dominant for the electrochemical effect, and it produces a large deformation, but its response is slow and requires a hydrated condition. Electrostatic effect is very promising for large and fast electrical actuation of SAMs when their mechanical stiffness is below 20 MPa. The friction of two different polarity materials generates electron flow, called the triboelectric effect, which is very useful for triboelectric nanogenerators. Careful interpretation of the SAMs' actuation mechanism is essential. The piezoelectric and electrostrictive responses can be mixed in SAMs, and piezoelectric polarization can be affected by flexoelectric polarization when applied mechanical stress generates nonuniform strain inside materials. Electrostatic effect can be mixed with other actuation mechanisms, and a thorough interpretation of SAMs' actuation mechanisms will allow their proper electromechanical applications, including soft robots, artificial muscles, actuators, sensors, and energy harvesters.

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