MXene-composited highly stretchable, sensitive and durable hydrogel for flexible strain sensors

Recently a new, large family of two-dimensional (2D) early transition metal carbides and carbonitrides, called MXenes, was discovered. MXenes are produced by selective etching of the A element from the MAX phases, which are metallically conductive, layered solids connected by strong metallic, ionic, and covalent bonds, such as Ti2 AlC, Ti3 AlC2 , and Ta4 AlC3 . MXenes -combine the metallic conductivity of transition metal carbides with the hydrophilic nature of their hydroxyl or oxygen terminated surfaces. In essence, they behave as "conductive clays". This article reviews progress-both -experimental and theoretical-on their synthesis, structure, properties, intercalation, delamination, and potential applications. MXenes are expected to be good candidates for a host of applications. They have already shown promising performance in electrochemical energy storage systems. A detailed outlook for future research on MXenes is also presented.

Density functional theory (DFT) computations were performed to investigate the electronic properties and Li storage capability of Ti(3)C(2), one representative MXene (M represents transition metals, and X is either C or/and N) material, and its fluorinated and hydroxylated derivatives. The Ti(3)C(2) monolayer acts as a magnetic metal, while its derived Ti(3)C(2)F(2) and Ti(3)C(2)(OH)(2) in their stable conformations are semiconductors with small band gaps. Li adsorption forms a strong Coulomb interaction with Ti(3)C(2)-based hosts but well preserves its structural integrity. The bare Ti(3)C(2) monolayer exhibits a low barrier for Li diffusion and high Li storage capacity (up to Ti(3)C(2)Li(2) stoichiometry). The surface functionalization of F and OH blocks Li transport and decreases Li storage capacity, which should be avoided in experiments. The exceptional properties, including good electronic conductivity, fast Li diffusion, low operating voltage, and high theoretical Li storage capacity, make Ti(3)C(2) MXene a promising anode material for Li ion batteries.

Injectable self-healing hydrogel dressing with multifunctional properties including anti-infection, anti-oxidative and conductivity promoting wound healing process will be highly desired in wound healing application and its design is still a challenge. We developed a series of injectable conductive self-healed hydrogels based on quaternized chitosan-g-polyaniline (QCSP) and benzaldehyde group functionalized poly(ethylene glycol)-co-poly(glycerol sebacate) (PEGS-FA) as antibacterial, anti-oxidant and electroactive dressing for cutaneous wound healing. These hydrogels presented good self-healing, electroactivity, free radical scavenging capacity, antibacterial activity, adhesiveness, conductivity, swelling ratio, and biocompatibility. Interestingly, the hydrogel with an optimal crosslinker concentration of 1.5 wt% PEGS-FA showed excellent in vivo blood clotting capacity, and it significantly enhanced in vivo wound healing process in a full-thickness skin defect model than quaternized chitosan/PEGS-FA hydrogel and commercial dressing (Tegaderm™ film) by upregulating the gene expression of growth factors including VEGF, EGF and TGF-β and then promoting granulation tissue thickness and collagen deposition. Taken together, the antibacterial electroactive injectable hydrogel dressing prolonged the lifespan of dressing relying on self-healing ability and significantly promoted the in vivo wound healing process attributed to its multifunctional properties, meaning that they are excellent candidates for full-thickness skin wound healing.