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We present a surface-tension driven electrochemical liquid metal (LM) actuator without the gas-producing side-reaction and capable of fabrication/operation in ambient air for practical applications. A hybrid supercapacitor is introduced to inhibit the common counter electrode side reactions, and the use of quasi-solid-state ionic hydrogel instead of liquid electrolyte further enables non-volatile operations. A 2×4 LM droplet array is demonstrated to actuate by a low driving voltage of 3.5 V for a maximum force of ~8.5 mN and a displacement of 0.56 mm in only 1.75 s. With the favorable scaling law of surface tension, further miniaturization could provide new opportunities in applications such as micro-robots, microfluidics, soft robots, and so on.

Compared to traditional actuators (such as piezoelectric ceramics), metal actuators possess the advantages of a low energy consumption, large strain amplitude, and high strain energy density. However, most of the existing metal actuators with an excellent comprehensive performance are composed of precious metals, which are limited by high costs and have almost no possibility for large-scale production in the future. This study focuses on non-precious metal materials and exploits a one-step chemical dealloying method to prepare bulk nanoporous (NP) CoCuAl actuators (NP-CCA) from Al70Co20Cu10 alloy. The microstructure and actuation properties of the NP-CCA were analyzed in detail. The dense continuous nanoscale pores provide an excellent network connectivity for a large strain response, enabling the NP-CCA to achieve a strain amplitude of up to 1.19% (more than eight and two times that of NP-Pt and NP-Ag, respectively), comparable to precious metal actuators. In addition, the NP-CCA possesses a high strain energy density, which is prominent in many precious metal actuation materials (such as NP-Au, NP-Ag, and NP-Pt). [ABSTRACT FROM AUTHOR]

Microfluidic devices are capable of precise drug delivery to the human body. For this purpose, they must be equipped with a compact pump that provides a high flow rate and precise dosing. In this paper, we present a micropump based on a fast electrochemical actuator that meets these requirements. It contains three actuators operating in the peristaltic mode. The device is fabricated from glass and silicon wafers using the standard microfabrication processes. The working part of the pump has a size of about 3 mm3, which is an order of magnitude smaller than other types of diaphragm pumps. The small size of the actuators ensures ultrahigh liquid dosing accuracy of 0.14 nL. At the same time, the high frequency of operation of the actuators makes it possible to develop a specific pumping velocity comparable to other types of pumps. [ABSTRACT FROM AUTHOR]

Compared to traditional actuators (such as piezoelectric ceramics), metal actuators possess the advantages of a low energy consumption, large strain amplitude, and high strain energy density. However, most of the existing metal actuators with an excellent comprehensive performance are composed of precious metals, which are limited by high costs and have almost no possibility for large-scale production in the future. This study focuses on non-precious metal materials and exploits a one-step chemical dealloying method to prepare bulk nanoporous (NP) CoCuAl actuators (NP-CCA) from Al70Co20Cu10 alloy. The microstructure and actuation properties of the NP-CCA were analyzed in detail. The dense continuous nanoscale pores provide an excellent network connectivity for a large strain response, enabling the NP-CCA to achieve a strain amplitude of up to 1.19% (more than eight and two times that of NP-Pt and NP-Ag, respectively), comparable to precious metal actuators. In addition, the NP-CCA possesses a high strain energy density, which is prominent in many precious metal actuation materials (such as NP-Au, NP-Ag, and NP-Pt).

The surface morphology of films of semiconducting polymers: polyaniline, poly-o-toluidine, poly-naphthylamine was studied by scanning electron microscopy. The films were obtained by electrochemical synthesis from solutions of their monomers acidified with mineral acids by the method of cyclic voltammetry. The films are formed on platinum substrates made according to the principle of bimetallic plates. The processes of reversible oxidation and reduction of the obtained films were carried out using an electric current in aqueous solutions of hydrochloric acid, which led to the doping of polymers with chloride anions. It was shown that the surface of films of polyaniline and poly-o-toluidine has a similar structure, consisting of numerous spherical grains ranging in size from 1 to 5 μm. The surface of poly-α-naphthylamine films is significantly different and consists of dendrons and large pores. In this case, in all cases, the observed supramolecular formations for films in the oxidized state are somewhat larger than for films in the reduced state, which is due to a change in the conformation of macromolecules.

Highly functional polymer-gel materials that include highly conductive gels, moving gels, and stimulation-responsive gels are important in a number of applications, including actuators, microrobots, and artificial muscle; hence, the development of new gels with superior properties is an important objective. Herein, we report the fabrication of a flexible electrochemical actuator by the direct transfer of a carbon electrode, formed by screen-printing on a silicone sheet, onto the surface of a poly(acrylic acid) gel containing cupric ions, which was prepared by immersing the poly(acrylic acid) gel in a 0.1 mM copper sulfate solution. Due to the oxidation of copper and the reduction of cupric ions, when potentials of 0.6 V and −0.7 V are alternately applied to the poly(acrylic acid)-cupric-ion gel actuator, it repeatedly expands and contracts along with concomitant copper-ion redox transformations when immersed in 0.1 M aqueous sodium perchlorate. This actuator demonstrated a 0.29% change in expansion ratio, which is 2.3-times larger than that of a previously reported electrochemical actuator (0.13%) formed with a sputtered gold electrode on a conventional polymer substrate.

Submillimeter or micrometer scale electrically controlled soft actuators have immense potential in microrobotics, haptics, and biomedical applications. However, the fabrication of miniaturized and micropatterned open-air soft actuators has remained challenging. In this study, we demonstrate the microfabrication of trilayer electrochemical actuators (ECAs) through aerosol jet printing (AJP), a rapid prototyping method with a 10 μm lateral resolution. We make fully printed 1000 × 5000 × 12 μm 3 ultrathin ECAs, each of which comprises a Nafion electrolyte layer sandwiched between two poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) electrode layers. The ECAs actuate due to the electric-field-driven migration of hydrated protons. Due to the thinness that gives rise to a low proton transport length and a low flexural rigidity, the printed ECAs can operate under low voltages (∼0.5 V) and have a relatively fast response (∼seconds). We print all the components of an actuator that consists of two individually controlled submillimeter segments and demonstrate its multimodal actuation. The convenience, versatility, rapidity, and low cost of our microfabrication strategy promise future developments in integrating arrays of intricately patterned individually controlled soft microactuators on compact stretchable electronic circuits.

Although versatile deformation, high flexibility, and environmental friendliness of electrochemical actuators (EAs) have made them promising in bioinspired soft robots and biomedical devices, the relatively high driving voltages unfortunately impose great restrictions on their applications in low-energy and human-friendly electronics. Here, we find that the uses of a mixed electronic-ionic conductive metal-organic framework (c-MOF), i.e., Ni 3 (hexaiminotriphenylene) 2 (Ni 3 (HITP) 2 ), largely lower the driving voltage of EAs. The as-fabricated EA can work under a driving voltage as low as 0.1 V, representing the lowest value among those for the c-MOF-based EAs reported so far. The Ni 3 (HITP) 2 -based EA shows an excellent actuation performance such as a high bending strain difference of 0.48% (±0.5 V, 0.1 Hz) and long-term durability of >99% after 15,000 cycles due to the improved conductivity up to 1000 S·cm -1 and double-layer capacitance as high as 176.3 F·g -1 stemming from the mixed electronic-ionic conduction of Ni 3 (HITP) 2 .

Microfluidic devices providing an accurate delivery of fluids at required rates are of considerable interest, especially for the biomedical field. The progress is limited by the lack of micropumps, which are compact, have high performance, and are compatible with standard microfabrication. This paper describes a micropump based on a new driving principle. The pump contains three membrane actuators operating peristaltically. The actuators are driven by nanobubbles of hydrogen and oxygen, which are generated in the chamber by a series of short voltage pulses of alternating polarity applied to the electrodes. This process guaranties the response time of the actuators to be much shorter than that of any other electrochemical device. The main part of the pump has a size of about 3 mm, which is an order of magnitude smaller in comparison with conventional micropumps. The pump is fabricated in glass and silicon wafers using standard cleanroom processes. The channels are formed in SU-8 photoresist and the membrane is made of SiNx. The channels are sealed by two processes of bonding between SU-8 and SiNx. Functionality of the channels and membranes is demonstrated. A defect of electrodes related to the lift-off fabrication procedure did not allow a demonstration of the pumping process although a flow rate of 1.5 µL/min and dosage accuracy of 0.25 nL are expected. The working characteristics of the pump make it attractive for the use in portable drug delivery systems, but the fabrication technology must be improved.

An actuator consisting of two poly(acrylic acid) gel films which sandwich a polyimide film coated with Au layers on both sides was developed. The actuator was driven by changes in the volume of the gel induced by redox reaction of p-hydroquinone (QH2)/p-benzoquinone (BQ). The oxidation of QH2 results in decrease in pH, which gives rise to collapse of the gel, whereas the reduction of BQ does the opposite phenomena. The Au film electrodes on both sides of the actuator were used as two electrodes setup. Thus, when the oxidation takes place at one electrode, the reduction does on the other electrode at the opposite side at the same time. In a solution of 5 mM QH2 and 5 mM BQ containing 1 x 10-4 M NaClO4 the actuator bent to left (anode) side at 1.3 V, and bent to right side after the reversal of the polarity. The actuator showed repeated stretching/bending at least 200 cycles. Further, it was able to actuate after 6 times of repeated drying/wetting of the gel. [ABSTRACT FROM AUTHOR]

Nanoporous metal-based actuators show advantages over conventional piezoelectric ceramics and conducting polymers, but are mainly limited to noble metals. Herein, we design a series of ternary Al-Ni-Cu alloys with variable Ni/Cu ratios to investigate their dealloying behaviors as well as electrochemical actuation performances. The change of the Ni/Cu ratio influences the microstructure and phase constituent of the as-cast Al-Ni-Cu alloys. Specially, an Al 3 (Ni, Cu) 2 phase is identified in all the Al-Ni-Cu alloys, and its amount and composition alter with the Ni/Cu ratio. Furthermore, the corrosion-resistant Al 3 (Ni, Cu) 2 phase significantly affects the dealloying behaviors of the ternary Al-Ni-Cu precursors. Additionally, the as-dealloyed Al-Ni-Cu alloys show good electrochemical actuation performance, which is associated with the existence of efficient nanoporous (np) components (np-Cu and np-Ni). The largest strain amplitude and strain rate could reach up to 0.9% and 6.6 × 10−4 s−1, respectively. Compared with noble metal-based actuators, the large strain amplitude and strain rate, as well as the lower material cost, make nanoporous Al-Ni-Cu alloy a potential actuation material for practical applications. [Display omitted] • The change of the Ni/Cu ratio influences the microstructure and phase constituent of the as-cast Al-Ni-Cu alloys. • The corrosion-resistant Al 3 (Ni,Cu) 2 phase significantly affects the dealloying behaviors of ternary Al-Ni-Cu precursors. • The good actuation performance of as-dealloyed Al-Ni-Cu alloys is associated with the existence of np-Cu and np-Ni. [ABSTRACT FROM AUTHOR]

This work presents the cyclical actuation of electric field sensitive microscale hydrogels employing dielectric coated coplanar electrodes. Microscale hydrogels are photopolymerized in-situ, and AC frequency-based actuation combined with pulse width modulation enabled controlled manipulation of hydrogel deformation. Stable actuation cycles are achieved with applied electric potentials from 20 Vpk-pk to 40 Vpk-pk, with a maximum true strain of 29% and a minimum rise time of 4.7 s. The peak and trough osmotic pressure for each system' s cycle is also analytically determined, with a peak pressure at 40 Vpk-pk of 201.1±38.3 kPa. A plateau in the peak-to-trough true strain is observed above 30 Vpk-pk. For comparative purposes a system without dielectric coated electrodes and employing external syringe pumps is also examined, and stable cyclical actuation was achieved for applied electric potentials of 5 Vpk-pk and 10 Vpk-pk. For this system the maximum stable rise time, true strain, and osmotic pressure are 8.1 s, 57%, and 429.2±81.9 kPa, respectively. The difference between the two systems highlights how optimization of the dielectric layer's thickness and uniformity can further enhance actuation performance. The electronically responsive hydrogel-based cyclical actuator developed within this work could be further employed for microfluidic regulation in portable low-power systems. [ABSTRACT FROM AUTHOR]

Abstract: Novel electrochemical fiber actuators based on graphene fiber/polypyrrole (GF/PPy) asymmetric structures have been prepared by a facile controlled electropolymerization of pyrrole (Py) on the GF. The fiber actuator is of high flexibility and durability, and can be further devised to fabricate multi-armed tweezers and net actuators. [Copyright &y& Elsevier]

Highlights: [•] Frequency-based electrical stimulation of hydrogel microactuators. [•] Actuated true strain from 4 to 30%, with response times on the order of seconds. [•] Parylene-N coated electrodes to prevent electrolysis and electrochemical reactions. [•] An analytical RC circuit model to predict a system's characteristic frequency. [•] Experimental capacitance and impedance measurements of multiple systems. [Copyright &y& Elsevier]

Abstract: In this paper we demonstrate a novel electrochemical actuator based on an array of micro-pillars of intercalation compound LiCoO2 with induced crystallographic texture (Lotgering factor f =0.96) to enhance actuation strain. The highly textured LiCoO2 posts were fabricated by hot-press sintering and subsequent dicing, and the contrived texture facilitated both electrochemical lithiation and resulting actuation strain in the longitudinal direction. Compared with traditional actuator materials, such as piezoceramics, the micro-pillar array of LiCoO2 showed an almost one order higher actuation strain (1.2%) at a low applied voltage (<5V). The conceptual demonstration outlined in this paper provides a foundation for the design and application of intercalation compounds as novel smart materials. [Copyright &y& Elsevier]

Abstract: An actuator consisting of poly(acrylic acid) gel in contact with p-hydroquinone (QH2)/p-benzoquinone (BQ), which was immobilized on a Au-coated conductive film electrode, was developed. The driving force of the actuator was changes in volume of the gel induced by pH change resulting from redox reaction of quinone. The pH decreased upon oxidation of QH2, which gives rise to collapse of the gel, whereas it increased upon reduction of Q, which results in expansion. The actuator stretched upon oxidation of QH2 at +1.0V vs. Ag/AgCl in a 10mM quinone solution containing 1×10−4 M NaClO4. It bent at open circuit likely due to diffusion of protons to a bulk solution caused by concentration gradient, but the bending rate increased upon reduction of Q at −0.3V which consumes protons. The actuator showed repeated stretching/bending at least 9 cycles with increasing its working distance upon successive potential switch between +1.0 and −0.3V. Energy conversion efficiency was evaluated to be 10−3%. [Copyright &y& Elsevier]

Abstract: A flexible conducting polymer actuator was fabricated using an electrospun poly(vinyl alcohol) (PVA) nanofibrous mat followed by in situ chemical polymerization with aniline. The PVA/polyaniline (PVA/PANi) hybrid mat consisted of nanostructured PANi grown on the surface of individual PVA nanofibers. The resulting structure with large surface area and porosity enabled the easy diffusion of ions to cause efficient electrochemical reaction. The PVA/PANi hybrid mat showed an electrical conductivity of 2.35S/cm and a single strip produced a maximum linear actuation strain of 1.8% while cycling the potential between specified ranges in 1M methane sulfonic acid solution. The hybrid mat could be stably actuated during extended electrochemical cycling up to 250 cycles. Actuation performance of the hybrid fibrous mat (for a single strip) was explained from the viewpoint of mechanical properties, and it was demonstrated that the flexible structure can generate higher strain than the brittle structure such as a carbon nanotube/conducting polymer composite actuator. Based on the flexible nature and the mechanical stability provided by highly entangled hydrogel nanofibers, the PVA/PANi hybrid mat was conveniently rolled-up into multilayered assembly of cylindrical structures. The interlayer spacing of the rolled-up structure and the huge surface area per unit volume accessible to electrolytes contributed to high current and enabled enhanced actuation performance and fast response. [Copyright &y& Elsevier]

Abstract: In this paper, we present a low-cost, low-voltage and low-power polypyrrole-membrane microfluidic pump with two one-way push button check valves. This polymeric microfluidic pump is fabricated with a conductive polymer (polypyrrole) modified poly dimethylsiloxane (PDMS) membrane encased into a polymethylmethacrylate (PMMA) structure. The pumping action is induced by an electrochemical actuated PPy-PDMS membrane while check valves at the inlet and outlet control the flow direction in the respective pump phases. This pump is self-priming. Accurate control of the output flow rate can be obtained by changing the actuation frequency. With this pump, a maximum pumping rate of 52μL/min was achieved using an input power of 55mW operated at ±1.5V. [Copyright &y& Elsevier]

The objective of this study (Part II) paper is to develop a dry actuator for wider application than the wet actuator. To form a dry actuator, a carbon nanofiber (CNF) actuator is based on a solid polymer electrolyte (SPE). The SPE film is prepared from polymethyl methacrylate (PMMA), an ion-exchange material, a plasticizer, and a solvent by the solution casting method. Ion conductivity studies were carried out to characterize the electrochemical properties of the SPE. Electrochemical impedance spectroscopy was performed to understand the electrochemical cell of the dry actuator. The actuator was tested in a dry environment at various voltages and frequencies and the tip displacement was measured using a laser displacement sensor. Compared to previous single wall carbon nanotube buckypaper actuators, the dry-based CNF actuator requires a little higher voltage to actuate, but it is two orders of magnitude lower in cost. Compared to the liquid-based actuator, the solid electrolyte-based actuator is slower and the displacements are smaller. These results have verified the principle of the CNF dry actuator. Further development of this new smart material could lead to practical smart structures applications in which the CNF hybrid material could be used as a muscle layer on structures, or as the structural material itself. [ABSTRACT FROM AUTHOR]

Abstract: A new redox actuator, consisting of poly(acrylic acid) gel incorporating Cu2+ ion, which was immobilized on Au/polyimide film electrode, was developed. The actuator showed stretching and bending motion upon reduction and re-oxidation, respectively, of Cu2+ in a 1×10−4 M NaClO4 solution. The mechanism of the actuation was attributed to swelling and shrinking of the gel arising from dissociation and formation, respectively, of coordinate and/or electrostatic bonds between Cu2+ ion and carboxylate groups of the gel upon the Cu2+/Cu0 redox reaction. A contraction ratio was calculated to be 1.0% and nearly reversible at least three cycles. Effects of pH and concentration of Cu2+ ions on the actuation were also studied. Changes in film thickness of the gel upon the Cu2+/Cu0 redox reaction was found to be ca. 200%. Energy conversion efficiencies were evaluated to be ca. 10−3%. [Copyright &y& Elsevier]

Biochemical sensors for continuous monitoring require dependable periodic self diagnosis with acceptable simplicity to check its functionality during operation. An in-situ self-diagnostic technique for a dissolved oxygen microsensor is proposed in an effort to devise an intelligent microsensor system with an integrated electrochemical actuation electrode. With a built-in platinum microelectrode that surrounds the microsensor, two kinds of microenvironments, called the oxygen-saturated or oxygen-depleted phases, can be created by water electrolysis, depending on the polarity. The functionality of the microsensor can be checked during these microenvironment phases. The polarographic oxygen microsensor is fabricated on a flexible polyimide substrate (Kapton) and the feasibility of the proposed concept is demonstrated in a physiological solution. The sensor responds properly during the oxygen-generating and oxygen-depleting phases. The use of these microenvironments for in-situ self-calibration is discussed to achieve functional integration, as well as structural integration, of the microsensor system. [ABSTRACT FROM PUBLISHER]

Large deformation of the elastomer membrane, for separating the liquid and electrolysis chamber in an electrochemical actuator, was simulated using Mooney Rivlin model and finite element analyses. Precision gravimetry experiments were conducted in parallel. The pressure in the electrolysis chamber was monitored during the gravimetry experiments. Experimental results are compared with the simulation and good agreement was achieved. It was also found that in the initial stage of the liquid delivery, before the membrane touched the liquid chamber bottom, the delivery rate is constant at a constant current. Hereafter, the delivery rate decreased with time. Simulation and experimental observations are also in good agreement. [ABSTRACT FROM AUTHOR]

Microfluidic devices providing an accurate delivery of fluids at required rates are of considerable interest, especially for the biomedical field. The progress is limited by the lack of micropumps, which are compact, have high performance, and are compatible with standard microfabrication. This paper describes a micropump based on a new driving principle. The pump contains three membrane actuators operating peristaltically. The actuators are driven by nanobubbles of hydrogen and oxygen, which are generated in the chamber by a series of short voltage pulses of alternating polarity applied to the electrodes. This process guaranties the response time of the actuators to be much shorter than that of any other electrochemical device. The main part of the pump has a size of about 3 mm, which is an order of magnitude smaller in comparison with conventional micropumps. The pump is fabricated in glass and silicon wafers using standard cleanroom processes. The channels are formed in SU-8 photoresist and the membrane is made of SiNx. The channels are sealed by two processes of bonding between SU-8 and SiNx. Functionality of the channels and membranes is demonstrated. A defect of electrodes related to the lift-off fabrication procedure did not allow a demonstration of the pumping process although a flow rate of 1.5 µL/min and dosage accuracy of 0.25 nL are expected. The working characteristics of the pump make it attractive for the use in portable drug delivery systems, but the fabrication technology must be improved. [ABSTRACT FROM AUTHOR]

Stimuli-responsive actuating materials offer a promising way to power insect-scale robots, but a vast majority of these material systems are too soft for load bearing in different applications. While strategies for active stiffness control have been developed for humanoid-scale robots, for insect-scale counterparts for which compactness and functional complexity are essential requirements, these strategies are too bulky to be applicable. Here, we introduce a method whereby the same actuating material serves not only as the artificial muscles to power an insect-scale robot for load bearing, but also to increase the robot stiffness on-demand, by bending it to increase the second moment of area. This concept is biomimetically inspired by how insect wings stiffen themselves, and is realized here with manganese dioxide as a high-performing electrochemical actuating material printed on metallized polycarbonate films as the robot bodies. Using an open-electrodeposition printing method, the robots can be rapidly fabricated in one single step in ∼15 minutes, and they can be electrochemically actuated by a potential of ∼1 V to produce large bending of ∼500° in less than 5 s. With the stiffness enhancement method, fast (∼5 s) and reversible stiffness tuning with a theoretical increment by ∼4000 times is achieved in a micro-robotic arm at ultra-low potential input of ∼1 V, resulting in an improvement in load-bearing capability by about 4 times from ∼10 μ N to ∼41 μ N., (© 2021 IOP Publishing Ltd.)

In this study, an electrochemical actuator was prepared by porous nitrogen-doped carbon (NDC) electrode derived from a zeolitic imidazolate framework (ZIF). The microstructure, morphology, and composition of NDC were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The NDC exhibited ZIF structures and the specific capacitance of NDC was 224.1 F∙g−1 in KOH aqueous solution (25 mV∙s−1). Furthermore, the electrochemical actuator based on NDC electrode displayed reliable and large bending deformations under low input voltages. The influence of NDC content on electromechanical performance of actuator was investigated. With the increase of NDC content, the actuating displacement of actuator first increased and then decreased, and the 20%NDC electrode based actuator showed the best actuating performance. The tip peak-to-peak displacement and bending strain of the 20%NDC based actuator were 17.5 mm and 0.20% under the applied voltage of ±3 V at a frequency of 0.1 Hz. Therefore, zeolitic imidazolate frameworks can be applied to provide significant improvements in electrochemical actuators, which can play key roles as technological advances toward bioinspired actuating devices for next-generation soft and wearable electronics. Unlabelled Image • Novel electrochemical actuator based on ZIF-structure carbon electrode was reported. • The synergistic effect of ZIF-structure carbon and PEDOT:PSS led to high actuating displacements. • The influence of amount of ZIF-structure carbon on actuating performance was investigated. [ABSTRACT FROM AUTHOR]

Twisted-yarn-based artificial muscles can potentially be used in diverse applications, such as valves in microfluidic devices, smart textiles, air vehicles, and exoskeletons, because of their high torsional and tensile strokes, high work capacities, and long cycle life. Here, we demonstrate electrochemically powered, hierarchically twisted carbon nanotube yarn artificial muscles that have a contractile work capacity of 3.78 kJ/kg, which is 95 times the work capacity of mammalian skeletal muscles. This record work capacity and a tensile stroke of 15.1% were obtained by maximizing yarn capacitance by optimizing the degree of inserted twist in component yarns that are plied until fully coiled. These electrochemically driven artificial muscles can be operated in reverse as mechanical energy harvesters that need no externally applied bias. In aqueous sodium chloride electrolyte, a peak electrical output power of 0.65 W/kg of energy harvester was generated by 1 Hz sinusoidal elongation.