MXenes, a member of 2D inorganic compounds that contain few-atom-thick layers of transition metal carbides, nitrides, and polar surface functional groups, are extraordinary materials for many applications including stimuliresponsive actuators. Here, an extensive review on MXene-based actuators in comparison with other 2D materials-based actuators is reported, highlighting the main differences in view of chemical structure, mechanical properties, and electrical functionalities. First, since MXenes are newcomers in the field of actuators, their properties are explained including cation and ionic liquid intercalation, high capacitance, good electrical and thermal conductivity, excellent electromagnetic wave absorption, hydrophilicity, and outstanding dispersion in many polar solvents. Second, electro-ionic, electrochemical, electrothermal, photothermal, and humidity-responsive MXene-based actuators are comprehensively addressed with detailed actuation mechanisms, focusing on electro-ionic soft actuators. Third, several applications of those actuators are summarized with an emphasis on soft robotics and future directions of MXene-based actuators are suggested.

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The application of biodegradable and biocompatible materials to triboelectric nanogenerators (TENGs) for harvesting energy from motions of the human body has been attracting significant research interest. Herein, we report diatom bio-silica as a biomaterial additive to enhance the output performance of cellulose nanofibril (CNF)-based TENGs. Diatom frustules (DFs), which are tribopositive bio-silica having hierarchically porous three-dimensional structures and high surface area, have hydrogen bonds with CNFs, resulting in enhanced electrondonating capability and a more roughened surface of the DF−CNF composite film. Hence, DFs were applied to form a tribopositive composite film with CNFs. The DF−CNF biocomposite film is mechanically strong, electron-rich, low-cost, and frictionally rough. The DF−CNF TENG showed an output voltage of 388 V and time-averaged power of 85.5 mW/m 2 in the contact−separation mode with an efficient contact area of 4.9 cm 2 , and the generated power was sufficient for instantaneous illumination of 102 lightemitting diodes. In addition, a cytotoxicity study and biocompatibility tests on rabbit skin suggested that the DF−CNF composite was biologically safe. Moreover, a practical application of the DF−CNF TENG was examined with a self-powered smart mask for human breathing monitoring. This study not only suggests high output performance of biomaterial-based TENGs but also presents the diverse advantages of the DFs in human body-related applications such as self-powered health monitoring masks, skin-attachable power generators, and tactile feedback systems.

The dried structure of paints and coatings are important to understand. In paper coatings, pigments and binder types are known to influence this structure. The influence of a new additive such as cellulose nanofibrils (CNF) on the interaction between the coating components is not thoroughly examined. In this study, the effect of CNF on the rheology of coating color and structure formation of the coating layer was investigated and compared to that of carboxymethyl cellulose (CMC). The addition of these two rheological modifiers made the dried coating layer porous, but the working mechanisms associated with these two additives were different. CMC, which flocculated coating components, limited the rearrangement of the components, resulting in a loosely packed coating structure in wet state. CNF, which did not significantly influence the interactions between the coating components, increased effective volume fraction by absorbing water. The water absorbing characteristics of CNF expanded the dried coating structure.

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Most mechanical energy harvesters produce only small amounts of electrical power from extremely low‐frequency input motions and immediately stop generating electricity when mechanical kinetic energy is exhausted. Here, a steady, long‐lasting and power‐boosted triboelectric nanogenerator is reported that can efficiently harvest from extremely low‐frequency irregular motions of less than 0.1 Hz by utilizing an escapement mechanism and frequency up‐conversion device. The escapement mechanism‐based triboelectric nanogenerator (EM‐TENG) consists of a mechanical energy storage spring, escapement mechanism and a torsional resonator for regular operation and frequency up‐conversion. In addition, the micro‐patterned alternating dielectric surfaces of Nylon and polytetrafluoroethylene (PTFE) and the comb‐type rotator significantly improve the output performance of the rotational EM‐TENG, increasing the current density level approximately 4.2 times compared to flat surfaces. Under an input frequency of 0.067 Hz, the EM‐TENG produces an open circuit voltage of 320 V and a short‐circuit current density of 0.59 mA m‐2. Most importantly, the EM‐TENG can produce long‐lasting and steady output power for 110 s (22 times) under only 5 s of input motion. Therefore, the EM‐TENG might pave the way to effectively harvest energy from extremely low‐frequency motions in nature, such as human motion, structural vibration and ocean waves.

The poor barrier properties and hygroscopic nature of cellulosic paper impede the wide application of cellulosic paper as a packaging material. Herein, a polyvinyl alcohol (PVA)-based polymer coating was used to improve the barrier performance of paper through its good ability to form a film. Alkyl ketene dimer (AKD) was used to enhance the water resistance. The effect of the absorptive characteristics of the base paper on the barrier properties was explored, and it was shown that surface-sized base paper provides a better barrier performance than unsized base paper. Nanoclay (Cloisite Na+) was used in the coating formulation to further enhance the barrier performance. The results show that the coating of PVA/AKD/nanoclay dispersion noticeably improved the barrier performance of the paper. The water vapor transmission rate of the base paper was 533 g/m2·day, and it decreased sharply to 1.3 g/m2·day after the application of a double coating because of the complete coverage of the base paper by the PVA-based polymer coating. The coated paper had excellent water resistance owing to its high water contact angle of around 100°. The grease resistance and mechanical properties of the base paper also improved after coating. This work may provide inspiration for improving the barrier properties of packaging paper through the selection of a suitable base paper and coating formulation.

Due to the micro-sized pores on cellulosic substrate surface and the hygroscopic nature of cellulosic fibers, paper has poor barrier properties. Dispersion coating can improve the barrier properties of cellulosic paper noticeably by forming a continuous, non-porous polymer film on paper surface. In this work, the excellent film-forming performance of polyvinyl alcohol (PVA) was used to seal the surface pores of paper, thus enhancing the barrier properties. Alkyl ketene dimer (AKD) was also added as a coating component to improve the water resistance of paper. Results showed that after PVA/AKD coating hydrophilic base paper changed to hydrophobic one, as proved by water contact angle (WCA) measurements. The water vapor transmission rate (WVTR) of base paper decreased sharply from 543 g/m2·day to 2 g/m2·day in the case of PVA/AKD triple coating, where the threshold of WVTR was reached. Meanwhile, the pristinely non-grease resistant base paper converted to a product with the highest grease resistance level. Furthermore, both elongation at break and tensile strength of base paper improved markedly after PVA/AKD coating. It was concluded that these improved properties were contributed by the combined use of PVA and AKD in the coating.