Hydrogel actuators, that convert external energy, such as pH, light, heat, magnetic field, and ion strength, into mechanical motion, have been utilized in sensors, artificial muscles, and soft ...robotics. For a practicality of the hydrogel actuators in a wide range of fields, an establishment of robust mechanical properties and rapid response are required. Several solutions have been proposed, for example, setting porous and anisotropy structures to hydrogels with nanocomposite materials to improve the response speed and deformation efficiency. In this review paper, we focused on hydrogel actuators including various nanocomposite by categorizing the dimensional aspects of additive materials. Moreover, we described the role of diverse additive materials in terms of the improvement of mechanical property and deformation efficiency of the hydrogel actuators. We assumed that this review will provide a beneficial guidance for strategies of developing nanocomposite hydrogel actuators and outlooks for the future research directions.
Most triblock copolymer-based physical hydrogels form three-dimensional networks through micellar packing, and formation of polymer loops represents a topological defect that diminishes hydrogel ...elasticity. This effect can be mitigated by maximizing the fraction of elastically effective bridges in the hydrogel network. Herein, we report hydrogels constructed by complexing oppositely charged multiblock copolymers designed with a sequence pattern that maximizes the entropic and enthalpic penalty of micellization. These copolymers self-assemble into branched and bridge-rich network units (netmers), instead of forming sparsely interlinked micelles. We find that the storage modulus of the netmer-based hydrogel is 11.5 times higher than that of the micelle-based hydrogel. Complementary coarse grained molecular dynamics simulations reveal that in the netmer-based hydrogels, the numbers of charge-complexed nodes and mechanically reinforcing bridges increase substantially relative to micelle-based hydrogels.In the formation of physical hydrogels from triblock copolymers, formation of polymer loops can result in topological defects. Here, the authors report the development of hydrogels from oppositely charged copolymers which assembled to form branched and bridge-rich networks.
Thermoresponsive poly(
-isopropylacrylamide) (PNIPAAm) hydrogels have been attracting attention in a variety of functional materials, such as biomaterials, because they exhibit a volume phase ...transition phenomenon near physiological temperatures. However, the slow kinetics and small volume shrinkage of bulk PNIPAAm hydrogels upon heating greatly limit their practical application. Here, we report PNIPAAm hydrogels with phase-separated structures that exhibited ultrafast shrinking upon heating. The phase separation into a PNIPAAm-rich phase and a water-rich phase was formed through aqueous polymerization in the presence of NaClO
salt. Through structural analysis of the hydrogels, a topologically heterogeneous and porous structure was observed, which was highly dependent on the NaClO
concentration in the polymerization step. Compared to conventional PNIPAAm hydrogels, the phase-separated hydrogels exhibited much faster and larger shrinkage upon heating. Simultaneously, the hydrogels quickly released a large amount of water owing to the effective water channels inside them. The present method can be widely applied to general hydrogels, and it can address the numerous limitations of hydrogels in terms of operating programmability and deformation efficiency.
Aqueous zinc metal batteries (AZMBs) are emerging energy storage systems that are poised to replace conventional lithium‐ion batteries owing to their intrinsic safety, facile manufacturing process, ...economic benefits, and superior ionic conductivity. However, the issues of inferior anode reversibility and dendritic plating during operation remain challenging for the practical use of AZMBs. Herein, a gel electrolyte based on zwitterionic poly(sulfobetaine methacrylate) (poly(SBMA)) dissolved with different concentrations of ZnSO4 is proposed. Two‐dimensional correlation spectroscopy based on Raman analysis reveals an enhanced interaction priority between the polar groups in SBMA and the dissolved ions as electrolyte concentration increases, which establishes a robust interaction and renders homogeneous ion distribution. Attributable to the modified coordination, zwitterionic gel polymer electrolyte with 5 mol kg−1 of ZnSO4 (ZGPE‐5) facilitates stable zinc deposition and improves anode reversibility. By taking advantage of preferential coordination, a symmetrical cell evaluation employing ZGPE‐5 demonstrates a cycle life over 3600 h, where ZGPE‐5 also exerts a beneficial effect on the full cell cycling when assembled with Zn0.25V2O5 cathode. This study elucidates changes in the internal ion behavior that are dependent on electrolyte concentrations and pave the way for durable AZMBs.
A zwitterionic polymer‐based gel electrolyte is proposed for an advanced aqueous zinc metal battery. By controlling the concentration of dissolved salt, electrochemical behaviors can be modified, attributable to the change in the coordination priority order. Accordingly, the interaction between charge carriers and the gel matrix is intensified, which contributes to the improved reversibility of the zinc anode.
As a new class of materials, implantable flexible electrical conductors have recently been developed and applied to bioelectronics. An ideal electrical conductor requires high conductivity, ...tissue‐like mechanical properties, low toxicity, reliable adhesion to biological tissues, and the ability to maintain its shape in wet physiological environments. Despite significant advances, electrical conductors that satisfy all these requirements are insufficient. Herein, a facile method for manufacturing a new conductive hydrogels through the simultaneous exfoliation of graphite and polymerization of zwitterionic monomers triggered by microwave irradiation is introduced. The mechanical properties of the obtained conductive hydrogel are similar to those of living tissue, which is ideal as a bionic adhesive for minimizing contact damage due to mechanical mismatches between hard electronics and soft tissues. Furthermore, it exhibits excellent adhesion performance, electrical conductivity, non‐swelling, and high conformability in water. Excellent biocompatibility of the hydrogel is confirmed through a cytotoxicity test using C2C12 cells, a biocompatibility test on rat tissues, and their histological analysis. The hydrogel is then implanted into the sciatic nerve of a rat and neuromodulation is demonstrated through low‐current electrical stimulation. This hydrogel demonstrates a tissue‐like extraneuronal electrode, which possesses high conformability to improve the tissue–electronics interfaces, promising next‐generation bioelectronics applications.
A new conductive hydrogel is developed through the simultaneous exfoliation of graphite and polymerization of zwitterionic monomers triggered by microwave irradiation. This hydrogel demonstrates tissue‐like extraneuronal electrodes satisfying requirements of bioelectronics such as low storage modulus, high viscoelasticity, low toxicity, reliable adhesion, and conformability to biological tissues, and maintenance of conductivity and hydrogel volume in wet environments.
Extraneural Electrodes
In article number 2203431, Youn Soo Kim and co‐workers introduce an ideal conductive hydrogel for tissue‐like extraneural electrodes with high conformability to improve the ...tissue–electronic interface. This hydrogel exhibits excellent adhesion, biocompatibility, non‐swelling, and electrical conductivity in water. The hydrogel is implanted into the sciatic nerve of rats, and neuromodulation is successfully demonstrated through low‐current electrical stimulation.
The effect of Hofmeister anions on the lower critical solution temperature (LCST) of two structural isomers, namely poly(
N
-isopropylacrylamide) (PNIPAAm) and poly(2-isopropyl-2-oxazoline) (PiPOx), ...is studied. Following the preparation of these two polymers with the same molecular weights and chain ends, their thermal hysteresis and LCST properties are observed in the aqueous and Hofmeister salt solutions. We investigate the effects of the salt type and salt concentration on the LCST for both polymers. More specifically, the LCST of PiPOx sharply increases as the concentration of SCN
−
increases, whereas that of PNIPAAm slightly increases prior to decreasing again. It is also confirmed that the boundary between the salting-out and salting-in effects is different for both polymers according to the type of salt present. Furthermore, Raman spectroscopy and two-dimensional correlation spectroscopy are used to examine the conformational changes in both polymers with increasing SCN
−
(chaotropes) and Cl
−
(kosmotropes) concentrations. Since the interaction mechanism between the polymer and anion is remarkably different even between isomeric polymers, it is necessary to rationally design the polymer structure to enhance the anion effect of the thermoresponsive polymer.
In chaotropic solution, the different lower critical solution temperature (LCST) increments of two structural isomers, namely, poly(
N
-isopropylacrylamide) (PNIPAAm) and poly(2-isopropyl-2-oxazoline) (PiPOx), is studied.
The Tucker–Lewis index (TLI; Tucker & Lewis,
1973
), also known as the non-normed fit index (NNFI; Bentler & Bonett,
1980
), is one of the numerous incremental fit indices widely used in linear mean ...and covariance structure modeling, particularly in exploratory factor analysis, tools popular in prevention research. It augments information provided by other indices such as the root-mean-square error of approximation (RMSEA). In this paper, we develop and examine an analogous index for categorical item level data modeled with item response theory (IRT). The proposed Tucker–Lewis index for IRT (TLIRT) is based on Maydeu-Olivares and Joe's (
2005
)
M
2
family of limited-information overall model fit statistics. The limited-information fit statistics have significantly better Chi-square approximation and power than traditional full-information Pearson or likelihood ratio statistics under realistic situations. Building on the incremental fit assessment principle, the TLIRT compares the fit of model under consideration along a spectrum of worst to best possible model fit scenarios. We examine the performance of the new index using simulated and empirical data. Results from a simulation study suggest that the new index behaves as theoretically expected, and it can offer additional insights about model fit not available from other sources. In addition, a more stringent cutoff value is perhaps needed than Hu and Bentler's (
1999
) traditional cutoff criterion with continuous variables. In the empirical data analysis, we use a data set from a measurement development project in support of cigarette smoking cessation research to illustrate the usefulness of the TLIRT. We noticed that had we only utilized the RMSEA index, we could have arrived at qualitatively different conclusions about model fit, depending on the choice of test statistics, an issue to which the TLIRT is relatively more immune.
The recent energy crisis has resulted in numerous energy-harvesting methods receiving significant attention in the past decades. To overcome this crisis, we successfully develop a first-ever rotating ...water triboelectric nanogenerator (TENG) based on water-electrification and rotating fluid inertia. The proposed TENG is a fully packaged design composed of partially filled cylinder and gear systems. To the best of our knowledge, the correlation of inner fluid motion and electrical voltage output performance using fluid dynamics analysis is demonstrated for the first time. In addition, we propose guidelines for optimum design and operation of a TENG using a non-dimensional factor G, which is based on the angular velocity of the cylinder and the volume ratio of the water and cylinder. In addition, a multiphase fluid flow simulation is introduced to demonstrate fluid dynamic motion and the electrical potential based on instantaneous water motion. Furthermore, a portable hand-driven device combined with a gear train that can light 30 LEDs instantaneously is introduced to demonstrate the wide applicability of the proposed TENG. Thus, our study supports a simple model where a rotating cylinder is filled with water and can be used effectively to expand new types of energy-harvesting methods.
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•New triboelectric nanogenerator generation mechanism using water electrification and inertia.•Optimization of the dimension and operating conditions for the nanogenerator using nondimensional factor G.•A multiphase dynamic numerical simulation for water energy harvesting.
Single-atom catalysts have already been widely investigated for the nitrogen reduction reaction (NRR). However, the simplicity of a single atom as an active center encounters the challenge of ...modulating the multiple reaction intermediates during the NRR process. Moving toward the single-atom-dimer (SAD) structures can not only buffer the multiple reaction intermediates but also provide a strategy to modify the electronic structure and environment of the catalysts. Here, a structure of a vanadium SAD (V-O-V) catalyst on N-doped carbon (O-V2-NC) is proposed for the electrochemical nitrogen reduction reaction, in which the vanadium dimer is coordinated with nitrogen and simultaneously bridged by one oxygen. The oxygen-bridged metal atom dimer that has more electron deficiency is perceived to be the active center for nitrogen reduction. A loop evolution of the intermediate structure was found during the theoretical process simulated by density functional theory (DFT) calculation. The active center V-O-V breaks down to V-O and V during the protonation process and regenerates to the original V-O-V structure after releasing all the nitrogen species. Thus, the O-V2-NC structure presents excellent activity toward the electrochemical NRR, achieving an outstanding faradaic efficiency (77%) along with the yield of 9.97 μg h–1 mg–1 at 0 V (vs RHE) and comparably high ammonia yield (26 μg h–1 mg–1) with the FE of 4.6% at −0.4 V (vs RHE). This report synthesizes and proves the peculiar V-O-V dimer structure experimentally, which also contributes to the library of SAD catalysts with superior performance.
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