Flexible MXene hydrogel enables wearable human-computer interaction

In recent years, conductive hydrogels have attracted extensive attention in the field of smart wearable electronics. Although conductive hydrogels have been widely studied, the balance between mechanical properties (tensile properties, strength and toughness) and electrical properties (conductivity, sensitivity and stability) still faces great challenges. In addition, the poor swelling of hydrogels in water will cause the loss of mechanical and electrical properties, resulting in their inability to be further applied in water environments. Therefore, designing conductive hydrogels with excellent mechanical properties, electrical properties and anti-swelling is crucial for the development of underwater flexible devices.

Professor Lai Yuekun's team from Fuzhou University published an article titled "Flexible MXene-Based Hydrogel Enables Wearable Human-Computer Interaction for Intelligent Underwater Communication and Sensing Rescue" in the journal Advanced Functional Materials, reporting the use of a hydrophobic association method to construct a conductive hydrogel with excellent mechanical and electrical properties. The conductive hydrogel is made of gelatin, acrylamide, octadecyl methacrylate, SDS, NaCl and MXene, and exhibits high stretchability (1224%), high sensing linearity (R²=0.999) and a wide strain range (2% ~ 700%). In addition, the researchers further used surface encapsulation technology to successfully achieve underwater monitoring and effective underwater information transmission in various liquid environments, which can provide early warnings of potential dangers. This work provides new ideas for the design and construction of smart wearable sensors and underwater monitoring devices (Figure 1).

Figure 1 Schematic diagram of the synthesis of conductive hydrogel and its use in underwater communication and rescue warning

As shown in Figure 2, the hydrogel has excellent mechanical properties. The hydrophobic monomers and hydrophilic monomers are polymerized through micelles. The hydrophobic part of the polymer chain forms a dynamic hydrophobic association with the surfactant micelle, which serves as a physical cross-linking center to further improve the mechanical properties of the hydrogel.

Figure 2 Mechanical properties characterization

As shown in Figure 3, the conductive hydrogel exhibited a high linear sensitivity of 0.999 and a wide strain sensing range of 2% to 700%. 1200 loading-unloading cycles at 500% strain showed stable resistance responsiveness and repeatability. It also exhibited a wide range of pressure sensing performance.

Figure 3 Sensing performance characterization

By using PDMS/Triton X-100 to construct a surface waterproof layer, the encapsulation layer exhibits excellent underwater stability and underwater adhesion. The hydrophilic hydrogel encapsulated by this encapsulation layer also exhibits excellent underwater anti-swelling properties. This method provides a new method for hydrophilic conductive hydrogels to achieve underwater sensing (Figure 4).

Figure 4 Surface encapsulation layer

Due to the underwater stability of the composite device, it is used in underwater communication and motion monitoring. By defining Morse code, information transmission can be easily realized. Underwater, it can realize contact sensing such as bending and compression, and can also realize non-contact water level detection in different environments (Figure 5).

Figure 5 Underwater communication and sensing

The hydrogel device is integrated into the alarm system. When the sensing current exceeds the set value, the LED light starts flashing and alarms. The water level can be set as the alarm water level limit. When the height difference between the device and the water surface exceeds the set value, the alarm light starts flashing, showing underwater early warning capabilities (Figure 6).

Figure 6 Underwater intelligent early warning system