Ionic actuators based on novel sulfonated ethylene vinyl alcohol copolymer membranes

Electrochemically-driven actuators: from materials to mechanisms and from performance to applications

Soft actuators, pivotal for converting external energy into mechanical motion, have become increasingly vital in a wide range of applications, from the subtle engineering of soft robotics to the demanding environments of aerospace exploration. Among these, electrochemically-driven actuators (EC actuators), are particularly distinguished by their operation through ion diffusion or intercalation-induced volume changes. These actuators feature notable advantages, including precise deformation control under electrical stimuli, freedom from Carnot efficiency limitations, and the ability to maintain their actuated state with minimal energy use, akin to the latching state in skeletal muscles. This review extensively examines EC actuators, emphasizing their classification based on diverse material types, driving mechanisms, actuator configurations, and potential applications. It aims to illuminate the complicated driving mechanisms of different categories, uncover their underlying connections, and reveal the interdependencies among materials, mechanisms, and performances. We conduct an in-depth analysis of both conventional and emerging EC actuator materials, casting a forward-looking lens on their trajectories and pinpointing areas ready for innovation and performance enhancement strategies. We also navigate through the challenges and opportunities within the field, including optimizing current materials, exploring new materials, and scaling up production processes. Overall, this review aims to provide a scientifically robust narrative that captures the current state of EC actuators and sets a trajectory for future innovation in this rapidly advancing field.

Characterization and Actuation of Ionic Polymer Metal Composites with Various Thicknesses and Lengths

Ionic polymer metal composites (IPMCs) with various thicknesses of 1, 2, 4, and 6 nafion films (denoted as 1-film, 2-film, 4-film and 6-film, respectively) are fabricated, and their characterization and actuation performances are then investigated. The effects of the thickness of the IPMCs on their morphology, surface resistance, and water uptake capability are studied. Their actuation performances are further evaluated by examining the tip force and displacement in terms of the length and the thickness of the IPMCs, under a direct current (DC) power of 3.0 or 4.5 V. In comparison with the 1-film, the 2-film shows a six-fold increase in the maximum tip force, but the response time increases from 2 to 9 s. The 4-film doubles the maximum tip force of the 2-film at 21 s. On the other hand, a reduction of the length of the IPMC from 30 to 15 mm also results in a double-maximum tip force, but this never increases the response time. Repeated actuations of the IPMCs with various thicknesses are performed by three actuation methods of no treatment, treatment in deionized water, and treatment in a NaCl solution. The relationships between the repeated actuation methods and actuations of the IPMCs with various thicknesses are also investigated.

Sulfonated Copper Phthalocyanine/Sulfonated Polysulfone Composite Membrane for Ionic Polymer Actuators with High Power Density and Fast Response Time

Ionic polymer composite membranes based on sulfonated poly(arylene ether sulfone) (SPAES) and copper(II) phthalocyanine tetrasulfonic acid (CuPCSA) are assembled into bending ionic polymer actuators. CuPCSA is an organic filler with very high sulfonation degree (IEC = 4.5 mmol H⁺/g) that can be homogeneously dispersed on the molecular scale into the SPAES membrane, probably due to its good dispersibility in SPAES-containing solutions. SPAES/CuPCSA actuators exhibit larger ion conductivity (102 mS cm^-1), tensile modulus (208 MPa), strength (101 MPa), and strain (1.21%), exceptionally faster response to electrical stimuli, and larger mechanical power density (3028 W m^-3) than ever reported for ion-conducting polymer actuators. This outstanding actuation performance of SPAES/CuPCSA composite membrane actuators makes them attractive for next-generation transducers with high power density, which are currently developed, e.g., for underwater propulsion and endoscopic surgery.

Fabrication of a silver nano powder embedded kraton polymer actuator and its characterization

A novel silver nano powder (Ag Pw) embedded kraton (KR) ionic polymer actuator was fabricated.

Ionic polymer-metal composite actuators based on triple-layered polyelectrolytes composed of individually functionalized layers

Ionic polymer-metal composite (IPMC) actuators based on two types of triple-layered Nafion composite membranes were prepared via consecutive solution recasting and electroless plating methods. The triple-layered membranes are composed of a Nafion layer containing an amphiphilic organic molecule (10-camphorsulfonic acid; CSA) in the middle section (for fast and large ion conduction) and two Nafion/modified inorganic composite layers in the outer sections (for large accumulation/retention of mobile ions). For construction of the two types of IPMCs, sulfonated montmorillonite (MMT) and polypyrrole (PPy)-coated alumina fillers were incorporated into the outer layers. Both the triple-layered IPMCs exhibited 42% higher tip displacements at the maximum deflections with a negligible back-relaxation, 50-74% higher blocking forces, and more rapid responses under 3 V dc, compared with conventional single-layered Nafion-IPMCs. Improvements in cyclic displacement under a rectangular voltage input of 3 V at 1 Hz were also made in the triple-layered configurations. Compared with single-layered IPMCs consisting of the identical compositions with the respective outer composite layers, the bending rates and energy efficiencies of both the triple-layered IPMCs were significantly higher, although the blocking forces were a bit lower. These remarkable improvements were attributed to higher capacitances and Young's moduli as well as a more efficient transport of mobile ions and water through the middle layer (Nafion/CSA) and a larger accumulation/retention of the mobile species in the outer functionalized inorganic composite layers. Especially, the triple-layered IPMC with the PPy-modified alumina registered the best actuation performance among all the samples, including a viable actuation even at a low voltage of 1.5 V due to involving efficient redox reactions of PPy with the aid of hygroscopic alumina.

The enhanced actuation response of an ionic polymer–metal composite actuator based on sulfonated polyphenylsulfone

Novel SPPSU-based IPMC membranes with different ion exchange capacities were synthesized. SPPSU4-based IPMCs showed the best actuation performance.

Preparation and Actuation of Electro-Active Artificial Muscle Based on Sulfonated SEBS

In this study, electro-active artificial muscle was developed based on sulfonated poly (styrene-b-(ethylene-co-butylene)-b-styrene) (S-SEBS). The S-SEBS membrane was prepared by a solution casting method using THF as solvent. The physical properties were tested in terms of ion-exchange capacity, water uptake and linear expansion. Fourier transform infrared (FT-IR) was used to study the composition of S-SEBS. The results showed that the S-SEBS exhibited electro-active property with blocking force about 2.3gf/g.

A novel electrolyte-responsive membrane with tunable permeation selectivity for protein purification

A novel electrolyte-responsive membrane, RC-g-PSBMA, was successfully prepared from regenerated cellulose (RC) membrane through surface-initiated atom transfer radical polymerization (ATRP) of a zwitterionic monomer, sulfobetaine methacrylate (SBMA). Different degrees of polymerization for the grafted SBMA polymers (i.e., PSBMA) on the RC membrane were easily obtained by adjusting the ATRP reaction conditions. The electrolyte-responsive behavior of RC-g-PSBMA was first evaluated through the permeation experiments with sodium chloride (NaCl) solutions of different concentrations. It was found that the permeability of RC-g-PSBMA showed a clear dependence on NaCl concentration in the solutions. To further examine the potential of RC-g-PSBMA for protein purification, bovine serum album (BSA) was chosen as a model protein and polystyrene nanoparticles (NPs) of different sizes were used as representative impurities in the solutions. The rejection rates of BSA and NPs by RC-g-PSBMA were examined with the solutions containing BSA and NPs at different NaCl concentrations. The results showed that the rejection rates of BSA were at a very low level regardless of the concentration of NaCl in the solutions, indicating that the membrane allowed BSA to permeate. However, the rejection rates of NPs of different sizes by RC-g-PSBMA changed remarkably with the concentration of NaCl in the solutions. The study has demonstrated the possibility to separate BSA from NPs of different sizes by using the same membrane but simply altering the concentration of NaCl in the solutions. Membranes with such properties will have a great potential for protein purification as well as for many other separation applications.