Two-Dimensional MoO₂ Nanosheet Composite Hydrogels with High Transmittance and Excellent Photothermal Property for Near-Infrared Responsive Actuators and Microvalves

Abstract

Stimulus-responsive intelligent hydrogel actuators have highly promising applications in the fields of soft robotics, smart manipulators, and flexible devices. Near-infrared (NIR) light is considered an ideal method to trigger the response behavior remotely and precisely. In order to realize the excellent optical transmittance and photothermal property of NIR-responsive hydrogels at the same time, two-dimensional nonlayered MoO2 nanosheets (2D-MoO2) with excellent photothermal efficiency (62% under an NIR light irradiation of 808 nm), splendid chemistry stability, and low preparation cost are used as photothermal agents and incorporated into the poly­(N-isopropylacrylamide) (PNIPAM) hydrogel network, forming the 2D-MoO2/Laponite/PNIPAM ternary nanocomposite hydrogel (TN hydrogel). It is remarkable that compared with the GO and MXene hydrogels with the same agent content (1.0 mg mL–1) and thickness (1 mm) whose transmittance values are only ∼5% at 600 nm, the TN hydrogel shows a similar NIR-responsive temperature, but much higher optical transmittance (∼53%). Besides, of the three hydrogels with similar transmittance, the TN hydrogel shows a much higher NIR-responsive temperature. The TN hydrogel with a low loading of 2D-MoO2 (1.5 mg mL–1) can produce a significant temperature increase of ∼30 °C after the application of 0.8 W cm–2 NIR light irradiation for 15 s. Impressively, the TN hydrogel exhibits excellent anti-fatigue property, keeping a fast response and temperature rise behavior even after 50 times of heating–cooling cycles. The flexibly controllable and reversible deformation is realized by a well-designed bilayer structure even in harsh environments. The transparent and asymmetric bilayer hydrogel is further used as a soft manipulator to capture objects visually and accurately. The NIR light-controlled microvalve based on this composite hydrogel is also demonstrated. This work provides a novel kind of transparent hybrid NIR response hydrogel for the further development of smart, programmable, reversible hydrogel-based actuators and soft robotics.

An ultrasound-induced MXene doped PAM–SA super-tough hydrogel

“…In recent years, hydrogels, a class of polymeric materials with cavernous network structures, have been widely used in tissue engineering, 1,2 agriculture, 3,4 flexible electronics 5,6 and flexible actuators 7,8 due to their outstanding bio-compatibility and mechanical properties similar to those of biological tissues. 9 The rapid development of hydrogel materials in applications has driven researchers to seek more advanced methods for preparing hydrogels and optimizing their properties.…”

Advances and Challenges of Hydrogel Materials for Robotic and Sensing Applications

“…The synthesized hydrogel/ MoO 2 composites have shown potential for applications in photothermal actuators and microfluidic devices. 70 Different from the above photochemical or photothermal agents that absorb light within a narrow range of wavelength, it would be ideal if novel photoresponsive materials exhibited broadband light absorption from the UV−visible to infrared region. Such advanced materials could be constructed by rationally designing the porous structure, hierarchical networks, and nanowire alignments of the known material libraries.…”

“…Recently, nonlayered MoO 2 nanosheets have been studied in terms of their LSPR mechanism, NIR absorption efficiency, chemical stability, and toxicity. The synthesized hydrogel/MoO 2 composites have shown potential for applications in photothermal actuators and microfluidic devices . Different from the above photochemical or photothermal agents that absorb light within a narrow range of wavelength, it would be ideal if novel photoresponsive materials exhibited broadband light absorption from the UV–visible to infrared region.…”

Multifunctional gradient hydrogel with ultrafast thermo-responsive actuation and ultrahigh conductivity

“…7,8 However, it is really challenging to integrate ultrafast actuation and ultrahigh conductivity into a hydrogel, severely limiting its application. Although hydrogel actuators have attracted much attention due to their exible and controllable shape transitions under various external stimuli, such as temperature, [9][10][11] near infrared (NIR) light, [12][13][14] pH, 15,16 humidity, 17 ionic strength (IS), 18 and electrical 19 or magnetic elds, 20 conventional hydrogel actuators have almost no or low electrical conductivity. Also, conductive hydrogels, by incorporating conductive polymers, 21,22 conductive nanoparticles 23,24 or conductive ionic compounds 25,26 into hydrogel systems, have been widely developed due to their outstanding electrical conductivity and mechanical properties; however, conventional conductive hydrogels can hardly exhibit fast actuation performance.…”