Non‐Swelling and Anti‐Fouling MXene Nanocomposite Hydrogels for Underwater Strain Sensing

Abstract

as electronic skins, [1][2][3] robotics, [4,5] energy storage devices, [6,7] wearable sensors, [8][9][10] and so forth. As one of the important flexible electronic devices, wearable sensors can convert external stimuli (e.g., human motions, [11,12] temperature, [13] and biochemical signals [14] ) into electrical signals (e.g., resistance, [15] voltage, [16] and capacitance changes [17] ). The wearable sensors have drawn progressively increasing attention due to their promising applications in motion detection, [18] health monitoring, [19] remote health management, [20] humanmachine interfaces, [21] etc. Recently, a large amount of strain-sensitive and pressure-sensitive skin-like sensors have been fabricated through embedding conductive components (e.g., metal nanowires, conductive nanomaterials, and conductive polymers) into flexible substrates. However, these wearable sensors show restrict stretchability and poor fatigue resistance, which is a challenge to achieve interface matching while coupling with human skins, resulting in limited sensing ranges (generally low than 200%) and poor sensing stability. [22,23] Therefore, it is necessary to develop flexible wearable sensors with high stretchability and anti-fatigue properties.As a kind of water-rich polymer material, hydrogels have high stretchability, flexibility, suitable modulus, and biocompatibility, rendering them attractive substances for the fabrication of wearable sensors. [24][25][26][27] The outstanding mechanical properties of hydrogels ensure mechanical matching to dynamic surfaces (such as human skin). For instance, Qin et al. prepared a recyclable strain sensor using ionic conductive gelatin organohydrogels with high stretchability (542%) and skin-like modulus (155 kPa). This hydrogel-based sensor exhibited stable sensing performance and a wide sensing range of strains, even at subzero temperatures. [28] Yang et al. developed a highly stretchable and anti-fatigue double-network hydrogel with high sensitivity and broad sensing ranges of strain and pressure. [2] However, hydrogels exhibit tremendous swelling behavior in underwater environments largely due to that water molecules, as hydrogen bond donors/acceptors, bind to most of the hydrogen bond sites and/or metal-ligand sites, thus greatly reducing their binding strength. This swelling behavior can lead to the destruction of sensing ability. In addition, most hydrogels have good biocompatibility and can support the adhesion of proteins and bacteria.Flexible wearable sensors are of interest for underwater applications such as aquatic robots and marine exploration. Non-swelling hydrogels would be a preferable candidate for underwater sensing with stable sensing performance. Herein, hydrogels comprised of MXene and polyhydroxyethyl methacrylate (PHEMA) are designed and manufactured. Owing to the synergistic effect of phase separation zones and hydrophilic/hydrophobic interaction, MXene/ PHEMA hydrogels exhibit non-swelling performance in various liquid media (e.g., water, seawater) and effectively a...

Hydrogels for underwater adhesion: adhesion mechanism, design strategies and applications

“…147 In addition, the sensing device is able to clearly distinguish the signals emitted by different voices. 151,160 In the event of a diver encountering an unexpected situation, the hydrogel sensor can send a timely distress voice. Unlike the detection of speech signals, Wei et al established a novel underwater communication mechanism based on nger joint bending sensing by combining the developed ionic gel skin sensor with the coding principle of Morse code.…”

“…Therefore, it has extremely low protein adsorption capacity, which can effectively reduce bacterial adhesion. 160 Thanks to the non-swelling and anti-fouling properties, the nanocomposite hydrogel can maintain high mechanical strength and sensing performance after 30 days of immersion in water. And it can accurately monitor the submerged fine and huge movements and realize the recyclability of sensing devices.…”

“…For example, the developed nanocomposite gel sensor was attached to the oscillating parts of marine animal models such as the tail of sharks by Li et al and was used to monitor their movements in simulated seawater. 160 The gel sensor can clearly distinguish the sensing signals of various motions such as swimming, paddling, and feeding of the marine animal model, enabling accurate monitoring of various continuous and complex motions of underwater animals. Moreover, by identifying the intensity and frequency of the signals, various swimming patterns of marine organisms can be evaluated, including resting, slow swimming, fast swimming, leaping out of the water, and making a right turn and left turn.…”

Bio-Inspired Instant Underwater Adhesive Hydrogel Sensors

“…MXene nanosheets were synthesized with the optimized minimally intensive layer delamination (MILD) method mentioned in previous work. 37 Briefly, LiF (0.8 g) and Ti 3 AlC 2 (0.5 g) were gradually added to HCl (10 mL, 9 M) under stirring at 35 °C. The resulted mixture was collected via centrifugation after 24 h and washed with deionized (DI) water until the pH exceeded 5.…”

“…35 Therefore, conductive gels based on MXene nanosheets have promising applications in hydrogel flexible electronics. 36,37 Inspired by barnacles for instant underwater adhesion, we first prepared an underwater adhesion organogel by copolymerizing hydrophobic monomers 2-phenoxyethyl acrylate (PEA) and 2-methoxyethyl acrylate (MEA) with hydrophilic monomer N-(2-hydroxyethyl) acrylamide (HEAA) in dimethylsulfoxide (DMSO) in the presence of MXene (MXene/ PHMP organogel). Compared to other hydrophilic monomers (e.g., acrylic acid, acrylamide), the HEAA monomer contains two hydrophilic function groups, including hydroxyl and amine groups, which would facilitate the adhesion behavior of the hydrogel.…”