Electroactive Polymer-Based Composites for Artificial Muscle-like Actuators: A Review

Abstract

Unlike traditional actuators, such as piezoelectric ceramic or metallic actuators, polymer actuators are currently attracting more interest in biomedicine due to their unique properties, such as light weight, easy processing, biodegradability, fast response, large active strains, and good mechanical properties. They can be actuated under external stimuli, such as chemical (pH changes), electric, humidity, light, temperature, and magnetic field. Electroactive polymers (EAPs), called 'artificial muscles', can be activated by an electric stimulus, and fixed into a temporary shape. Restoring their permanent shape after the release of an electrical field, electroactive polymer is considered the most attractive actuator type because of its high suitability for prosthetics and soft robotics applications. However, robust control, modeling non-linear behavior, and scalable fabrication are considered the most critical challenges for applying the soft robotic systems in real conditions. Researchers from around the world investigate the scientific and engineering foundations of polymer actuators, especially the principles of their work, for the purpose of a better control of their capability and durability. The activation method of actuators and the realization of required mechanical properties are the main restrictions on using actuators in real applications. The latest highlights, operating principles, perspectives, and challenges of electroactive materials (EAPs) such as dielectric EAPs, ferroelectric polymers, electrostrictive graft elastomers, liquid crystal elastomers, ionic gels, and ionic polymer-metal composites are reviewed in this article.

Keywords: artificial muscles; conductive polymers; dielectric EAPs; electroactive polymers (EAPs); electrostrictive graft elastomers; ionic gels; ionic polymer-metal composites; liquid crystal elastomers.

Figure 1

External stimuli for activation of polymer actuators.

Figure 2

Electroactive polymers groups.

Figure 3

(a) Work principle of the DE actuator and (b) actuation of 3D printed DE actuator (U = 4.67 kV). Reprinted/adapted with permission from Ref. [16]. 2019, Elsevier.

Figure 4

Polarization process of molecular dipoles of elastomer films under an external electric field.

Figure 5

Some topologies of DEG. (a) Equibiaxial DEG (ideal uniformly stretched generator, and practical multipoint stretching embodiment); (b) pure shear (namely, strip-biaxial) DEG; (c) diamond DEG; (d) cone DEG; and (e) circular diaphragm DEG [21].

Figure 6

Schematic of P(VC-E)-4 deformation mechanism driven by the high electric field. Reprinted/adapted with permission from Ref. [27]. 2022, Elsevier.

Figure 7

(a) Illustration of the formation of PCB particles. Catalyzed by DBTDL, hydroxyl group of CB surface initially crosslinks with IPDI to produce a N=C=O group functionalized CB, then couples with OH-PDMS, thus coating a layer of PDMS film to CB surface. (b) Illustration of the fabrication of DEA. The PCB particles are filled in VRS precursor and cured to produce a PCB/VRS hybrid film. The conductive silicone paste is brushed on both PCB/VRS film surfaces to make a trilayer DEA. (c) Prestrained by an elastic spring, the hybrid DEA takes an out-of-plane actuation under a pulse electrical field. Reprinted/adapted with permission from Ref. [28]. 2022, Elsevier.

Figure 8

Schematic representation of piezoelectric effects depending on stress and voltage: (a) applying a stress, (b) applying a voltage.

Figure 9

Piezoelectric polymers classification.

Figure 10

Schematic structure of electrostrictive graft elastomers.

Figure 11

The electroclinic effect in ferroelectric liquid crystalline elastomers. The polarized mesogenic side groups will turn by “θ” or “−θ” depending on the direction of the electric current flow E (electric field out of or into the plane of material). Δh—a decrease in the thickness of the polarized mesogenic side groups during the application of the electric field.

Figure 12

Schematic illustration of the optical response of the DTAB-decorated LC film to the adsorption of the ssDNA probe. Reprinted/adapted with permission from Ref. [48]. 2022, Elsevier.

Figure 13

Work principle for the ionic EAPs.

Figure 14

Conduction mechanisms of electroactive polymers.

Figure 15

Schematics of three types of hydrogels based on alginate: (a) ionically crosslinked alginate chains, (b) covalently crosslinked polyacrylamide chains, (c) covalently crosslinked amine groups on polyacrylamide chains and carboxyl groups on alginate chains.

Figure 16

Formation of the PVA-CNC hydrogel [58].

Figure 17

Non-aqueous liquid electrolytes based on 1-ethyl 3-methylimidazolium bis(nonafluorobutane-1-sulfonyl imidate) ionic liquid. Reprinted/adapted with permission from Ref. [59]. 2020, Elsevier.

Figure 18

Ionic actuation mechanisms of conductive polymer actuators depending on the anion size and mobility: (a) small anion mobile, (b) large and immobile anion.

Figure 19

(A) Schematic illustration of gelation processes for six types of conductive polymer hydrogels: PPy, PAni, PIn-4-NH2, PIn-5-NH2, PIn-6-NH2, and PIn-7-NH2. (B) Photographs of the conductive polymer hydrogels (from left to right: PPy/PEDOT:PSS hydrogel, PAni/PEDOT:PSS hydrogel, PIn-4-NH2/PEDOT:PSS hydrogel, PIn-5-NH2/PEDOT:PSS hydrogel, PIn-6-NH2/PEDOT:PSS hydrogel, PIn-7-NH2/PEDOT:PSS hydrogel). (C) Crosslinking mechanism of the conductive polymer/PEDOT:PSS hydrogels. The addition of the PEDOT:PSS as the dopant and gelator possess a large amount of negatively charged sulfonic acid functional group, which can form electrostatic interaction with the positively charged conductive polymer chains. Reprinted/adapted with permission from Ref. [70]. 2020, Elsevier.

Figure 20

Electrostatically nano-assembled carbon-nanotubes-decorated poly(methyl methacrylate) (PMMA) particles for fabrication of transparent conductive polymer composites. Reprinted/adapted with permission from Ref. [71]. 2020, Elsevier.

Figure 21

Work principle of ionic polymer–metal composites.