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Advances of MXene-based hydrogels for chronic wound healing

Ruijun Song ,  Huixu Xie ,  Guiting Liu

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Citation:  Ruijun Song, Huixu Xie, Guiting Liu. Advances of MXene-based hydrogels for chronic wound healing[J]. Chinese Chemical Letters, 2025, 36(7): 110442. doi: 10.1016/j.cclet.2024.110442 shu

Advances of MXene-based hydrogels for chronic wound healing

English

  • 1. Introduction

    Chronic wounds, such as diabetic foot ulcers, pressure sores and venous leg ulcers, are defined as wounds lasting 4 weeks to 3 months or more and do not have a normal healing phase like acute wounds [1]. There are varying causes of chronic wounds, all of which impose a burden on healthcare systems. In the case of diabetic foot ulcers, which generally exhibit high amputation rates, high recurrence rates and high mortality, the worldwide prevalence was up to 6.3% [2,3]. The wound healing process involves four sequential steps: hemostasis, inflammation, proliferation and remodeling [4], which are regulated by multiple and cascaded physiological processes. However, causes such as persistent bacterial infections, peripheral vascular diseases, immune deficiency and repeated trauma can impede the normal wound healing process, eventually leading to chronic wounds [5-7].

    In recent years, nanomaterials have shown a strong applied potential in the field of chronic wound healing. Among them, thanks to the uniqueness of its physical structure and surface chemical properties, MXene possesses exceptional antibacterial property, fast photothermal response ability and electrical conductivity, making it a research hotspot in various fields [8,9]. More importantly, the excellent physiological stability and biocompatibility allow it to be used for biomedical applications [10]. In clinical practice, MXene plays its role in chronic wounds often with the aid of appropriate dressings. This combination can significantly enhance its effectiveness in promoting wound healing. Hydrogel, an advanced type of dressing known for its good biocompatibility and the ability to absorb wound exudates and provide a moist environment, has been developed based on its similarities to natural extracellular matrix (ECM) and its high water content [11-14]. The abundant hydrophilic groups on the surface of MXene make it a feasible strategy to incorporate it into the hydrogel matrix to prepare bioactive functional hydrogel dressings [15-18]. There have been several studies attempted to combine MXene with hydrogels for chronic wounds and they have exhibited amazing potential. Combined with the antibacterial properties of MXene [19], MXene-based hydrogels can significantly reduce the inflammatory response generated by bacterial infections in chronic wounds [19], and reduce the abuse of antibiotics [20,21]. Then with the near-infrared (NIR)-induced MXene thermal response, precise spatiotemporal control over drug can be easily achieved [22,23]. The last point is that appropriate electrical conductivity enables MXene to have the ability to promote cell proliferation. So, it can be seen that bioactive-functional MXene-based hydrogel dressings have a broad and promising application prospect in the field of wound healing.

    In this review, we summarized the recent advances of MXene-based hydrogels for chronic wound healing (Fig. 1). The preparation and properties of MXene were introduced first, followed by the construction of MXene-based hydrogels. Then, the applications of MXene-based hydrogels in chronic wound healing were discussed in detail, mainly focusing on the performance of controlled drug release, antibacterial infection and cell proliferation promotion. Finally, the challenges of MXene-based hydrogels have been clarified, with the aim of guiding and promoting further clinical practice.

    Figure 1

    Figure 1.  Schematics of MXene-based hydrogels for the treatment of chronic wounds.
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    2. Preparation and properties of MXene

    2.1 Preparation and surface modifications of MXene

    MXene is a type of two-dimensional (2D) nanomaterial consisting of transition metal carbides, nitrides or carbonitrides. It has a specific thickness of several atomic layers and an accordion-like structure and was first synthesized by Gogotsi et al. in 2011 [24]. The molecular formula of MXene can be expressed as Mn + 1XnTx (n = 1 – 3), where M is an early transition metal (such as Ti, V, Nb, Mo), X is carbon and/or nitrogen, and Tx represents surface chemical groups such as -OH, -O-, -Cl and -F. So far, more than 30 MXenes have been prepared by selective etching of A layer in the MAX phase (which is a class of IIIA or IVA elements such as Al, Ga) (Fig. 2A) [24-26]. There are two synthesis methods for MXene, one is the top-down fabrication and the other is the bottom-up approach.

    Figure 2

    Figure 2.  (A) Transformation of MAX phase Mn + 1AXn to MXene Mn + 1XnTx. Copied with permission [27]. Copyright 2018, Accounts of Chemical Research. (B) SEM micrographs of Ti3AlC2 particle and (C) Ti3C2Tz formed after etching in HF. Copied with permission [28]. Copyright 2021, Elsevier.
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    The top-down fabrication method is the most commonly used one which can be divided into two steps. Firstly, multilayer MXene is obtained by selectively etching the A-layer of the 3D MAX phase, in which van der Waals bonds and/or hydrogen bonds maintain the link between the layers. The next step is to exfoliate it into few-layers through some energetic treatments such as ultrasound [29]. MXenes modified by different groups, which exhibit different properties, can be obtained by altering reaction conditions. While hydrofluoric acid (HF) or a fluorine salt are often used for the selective etching of the MAX phase (Figs. 2B and C), the surface of freshly etched MXene is mainly covered with -F, and it can be oxidized into -OH gradually. In this process, the characteristics of MXene change based on the ratio of -F and -OH, indicating that the specific needs of various applications can be met by customizing the surface functionalities of MXenes. For instance, replacing -F with -OH can lead to the improvement of the electrochemical performance of Ti3C2Tx supercapacitors [30]. MXene can also be modified by attaching metal ions. Additional Al(OH)4- can be formed by adding Al3+ during the etching process, which surface functionalization induces the localized surface plasmon resonance (LSPR) effect that enhances the photothermal performance and light-harvesting capability of MXene [31]. There are also some studies that have reported bottom-up synthesis methods, including chemical vapor deposition (CVD) [32,33], template method [34,35] and plasma enhanced pulsed laser deposition (PEPLD) [36,37]. Compared to the top-down method, the bottom-up approach is more cumbersome and difficult to apply for mass production. Therefore, the top-down method is more commonly used for the synthesis of MXene [38]. However, the bottom-up approach possesses the advantages of sufficient atom utilization and more controllable structural design [39], indicating it is promising for optimizing the synthesis of MXene and warranting further research.

    2.2 Properties of MXene

    The 2D structure with fewer monolayers and particular groups on the modified surface can impart MXene with some unique properties. For the application of chronic wound healing, we focused on three key properties of Mxene: excellent photothermal conversion efficiency, antibacterial activity and electrical conductivity. In the following sections, we will elaborate on the mechanisms and effects of each of these properties.

    2.2.1   NIR light response

    NIR light response is one of the main properties of MXene that can contribute to some further applications (Table 1 [40-48]). The primary need for the photothermal conversion of MXene is its excellent light absorption capacity across a broad spectral range. According to the solution of Maxwell's equation, the more conductive a material is, the higher its extinction coefficient, and the better it absorbs electromagnetic waves. Studies have proved that MXene has a higher conductivity and extinction coefficient than graphene oxide (GO), suggesting that it possesses a strong capacity for absorbing electromagnetic waves [49,50]. Some researchers have observed that MXene exhibits a LSPR effect similar to that of noble metal particles [51]. LSPR effect refers to when light is incident on precious metals such as gold, silver, or platinum nanoparticles (NPs), if the incident photon frequency and vibration frequency match the vibration frequency of the NPs' electrons, metal NPs can create very strong absorption of photon energy. This results in a strong resonance absorption peak in the spectrum, and the peak absorption wavelength depends on the microstructure characteristics of the material. A large number of experiments have proven that MXene has a strong absorption spectrum in the NIR region (Figs. 3A and B) [52-54].

    Table 1

    Table 1.  MXene materials that can respond in the near infrared region.
    DownLoad: CSV
    Materials Concentration NIR irradiation conditions Ref.
    Doxorubicin (DOX)-loaded MXene@Hydrogel 20 ppm 808 nm, 1.5 W/cm2 [40]
    Epithelial cell adhesion molecule (EpCAM)-grafted-Ti3C2Tx@gelatin membrane 1 mg/mL 808 nm, 0.54 W/cm2 [41]
    Vitamin E-loaded MXene nanobelt fiber / 808 nm, 0.33 W/cm2 [42]
    DOX-loaded MXene polymethyl methacrylate (PMMA) microspheres 500 µg/mL 808 nm, 1.5 W/cm2 [43]
    Nb2C nanosheets modified bioactive glass scaffolds (NBGS) / 1064 nm [44]
    Emamectin benzoate (EB)@p-phenylenediamine (PDA)@ Ti3C2Tx 0.2 mg/mL 808 nm, 2 W/cm2 [45]
    Ti3C2−OH/Bi2WO6: Yb3+, Tm3+ composites 2.8 wt% Vis-NIR, 30 min [46]
    Ti3C2Tx/ionic liquid ink / 808 nm [47]
    MXene-decorated textile 17.3 wt% 780 nm [48]

    Figure 3

    Figure 3.  (A) Ultraviolet (UV)–visible spectroscopy-NIR absorbance spectra of MXene dispersed in water at different concentrations (0.01, 0.04, 0.06, 0.08, and 0.10 mg/mL). (B) Beer law absorbance plot for absorption at 808 nm. (C) Temperature changes of MXene nanosheet dispersion (0.10 mg/mL) upon NIR irradiation for six laser ON/OFF cycles. Copied with permission [55]. Copyright 2021, Wiley. (D) Real-time infrared thermal images of MXene@agarose containing different concentrations of Ti3C2 under continuous 808 nm irradiation at a power intensity of 1 W/cm2 for 5 min. (E) Real-time infrared thermal images of MXene@agarose containing 30 µg/mL Ti3C2 under continuous 808 nm irradiation with different laser power densities for 5 min. Copied with permission [56]. Copyright 2021, Elsevier.
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    By using a rigorous calculation program, Li et al. demonstrated that the internal light-to-heat conversion efficiency of MXene (Ti3C2) at 808 nm NIR light may reach 100% [57]. The photothermal responsiveness of MXene is highly reproducible, as demonstrated in Fig. 3C, where the target temperature can still be reached under multiple irradiation cycles [55]. And the temperature can also be adjusted by both concentration and irradiation power (Figs. 3D and E) [56]. The excellent photothermal effect of MXene can achieve controlled drug release, promote cell proliferation and angiogenesis, and alsoenhance antibacterial activity [40,58,59], which lays a solid foundation for the application of MXene in promoting chronic wound healing.

    2.2.2   Antibacterial capacity

    Bacterial infection is considered one of the most significant barriers to wound healing [60], and many bacteria have acquired drug resistance due to the overuse of antibiotics [61,62]. In recent years, numerous novel nanomaterials have been discovered to have good antibacterial properties. They have been extensively studied and utilized in a range of fields, particularly in biological applications. MXene, as a novel 2D nanomaterial, has already been proven to have excellent antibacterial properties (Figs. 4D–F) and good biocompatibility [19,63], so it has a broad application prospect in promoting wound healing by enhancing antibacterial effects.

    Figure 4

    Figure 4.  (A) Schematic diagram of bacteria being wrapped by MXene. (B) TEM images of E. coli and B. subtilis treated with 200 µg/mL of Ti3C2Tx for 4 h at low and high magnifications. (C) SEM images of the E. coli (top panel) and B. subtilis (bottom panel) treated with different concentration of Ti3C2Tx, at low and high magnification, respectively. Reproduced with permission [19]. Copyright 2016, American Chemical Society. (D) Bacterial viability against E. coli, S. aureus, and MRSA treated with different samples (*P < 0.05, **P < 0.01, n = 3). Copied with permission [64]. Copyright 2021, American Chemical Society. Cell viability measurements of (E) E. coli and (F) B. subtilis treated with Ti3C2Tx and GO in aqueous suspension. Copied with permission [19]. Copyright 2016, American Chemical Society.
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    Rasool et al. studied the antibacterial mechanism of MXene in detail [19]. The inhibition rate of 200 µg/mL of Ti3C2Tx MXene against Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) was up to 98% and 97% respectively within 4 h. Through scanning electron microscope (SEM) and transmission electron microscope (TEM), it can be observed that bacteria are wrapped by nanoflakes, forming agglomerates (Fig. 4A). This is accompanied by the rupture of the cell membrane and wall, leading to the extravasation of cytoplasm and a decrease in intracellular density [64]. MXene appears to be similar to other two-dimensional nanomaterials, such as GO based on these findings. Its sharp few-layered edge structure can cause the rupture and defects in the bacterial envelope (Figs. 4B and C), which is termed as the "nano knife effect". The stacking and agglomeration of materials will decrease the effectiveness of the "nano knife" [65]. Therefore, the smaller the size of the nanosheet, the better the antibacterial effect [66]. In addition, MXene allows functionalized surface modifications and has a large number of hydrophilic functional groups, makes it easier to adsorb onto the surface of bacteria [67,68].

    The hydrogen bonds formed between the oxygen-containing groups on the surface and the lipopolysaccharides on the cell membrane may prevent the nutrient intake of bacteria. Moreover, MXene can also induce oxidative damage to bacteria through oxidative stress, including reactive oxygen species (ROS) dependent and independent oxidative stress. Finally, as mentioned earlier, MXene has better electrical conductivity than GO. When MXene adheres to the surface of bacteria, it facilitates the transfer of electrons from bacterial intracellular components to the external environment and results in cell death [19].

    2.2.3   Conductivity

    MXene has exceptional electrical conductivity and is tunable due to its unique structure [69-73]. It originates from free electrons in the transition metal carbide or nitride backbone, and it can be tuned by transition metals, X elements, and controlling their surface chemistries [74]. Studies have approved that the conductive core of MXenes is derived from its metallic properties [70], which exhibit a high density of states at the Fermi level and high carrier concentrations, enabling fast electron transport [75,76]. Therefore, the transition metal M is considered to have a significant effect on conductivity. For instance, MXenes with high atomic number (Cr, Mo, and W) are predicted to be topological insulators [77], while bare Ti3C2 exhibits metallic properties [78]. In the single-element transition metal MXenes (Mn + 1Xn (n = 1, 2, 3)), the states near the Fermi level are mainly controlled by the d-electrons of the surface metal atoms. Even more noteworthy is in multi-element transition metal MXenes, the outer transition metal layer has more important electronic properties than the inner metal layer. This is because the electronic structure of the outer metal is affected by the surface groups [74]. Thus, it can be inferred that surface functionalization can also enhance an impact on the electronic properties of MXenes [79]. Bare MXenes are metals with a high density of states (DOS) at the Fermi level. In the process of surface functionalization, due to the transfer of electrons from transition metal to the electronegative surface terminal, the DOS is changed and the Fermi level is dramatically lowered, resulting in electronic tunability [74,80-83]. For example, the common terminations -F and -OH of MXenes have similar effects on the electronic properties of MXenes because they both accept an equilibrium state of electrons. While -O− shows different effects due to its oxidation state allowing the acceptance of two electrons [79]. The intercalant used in the synthesis of few-layered MXene has also been confirmed to affect conductivity by influencing the interlayer spacing. A larger interlayer spacing impedes interflake electron hopping, resulting in a decrease in conductivity. For example, the intercalation of large organic cations such as tetrabutylammonium (TBA+) between MXene flakes will form a large interval between layers and reduce the conductivity [84,85]. In contrast, the intercalation of alkaline ions maintains a high conductivity due to the formation of a narrow gap between layers [86,87].

    3. Construction of MXene-based hydrogel

    While MXene has abundant -F, -OH, -O- and other groups on its surface, it offers numerous sites for chemical bond or hydrogen bond formation [88]. And MXene is negatively charged, which makes it easier to bond with positively charged substances. These characteristics allow MXene to form hydrogel through both chemical and physical bindings (Table 2 [54,89-96]).

    Table 2

    Table 2.  The combination of MXene with different hydrogel matrixs.
    DownLoad: CSV
    Hydrogel matrix Synthesis mechanism Bonding Ref.
    PNIPAM Chemical crosslinking -C-C- covalent bond [54]
    rGO/EDA Chemical crosslinking -C-N- covalent bond [89]
    PAM Chemical crosslinking & physical mixing Ti-O, Ti-C covalent bonds & hydrogen bonds between -OH and -NH2 [90]
    PAA/ACC Physical mixing Chelation between -COO– and Ca2+ & hydrogen bond & electrostatic interaction [91]
    PVA Physical mixing Hydrogen bond between -OH and -OH [92]
    PVA/porphyrin Physical mixing Hydrogen bond between -OH and -COOH [93]
    PNIPAM Physical mixing Hydrogen bond between oxygen-containing groups and -CO-NH2 [94]
    TBC Physical mixing Hydrogen bond & physical absorption [95]
    PAA/PVA Physical mixing / [96]

    Chemical crosslinking is a more common construction method that depends on various bonding reactions between MXene and the cross-linking agent, such as covalent bonds and ion chelation. Xue et al. prepared a NIR light-driven programmable hydrogel network by in situ free radical copolymerization of MXene nanomonomers with thermosensitive poly(N-isopropyl acrylamide) (PNIPAM) hydrogel under light (UV, 365 nm, 20 mW/cm2, 10 min). The characteristic absorption peaks of -NH- and –C=O in the Fourier transform infrared absorption spectrum confirmed that covalent bonds were formed between the PNIPAM matrix and MXene nanomonomers, contributing to the construction of the hydrogel network [54]. Zhu et al. cross-linked MXene with poly(acrylic acid) (PAA) and amorphous calcium carbonate (ACC) to form another polymer-based hydrogel (Fig. 5A). PAA and ACC are not only inserted into the MXene sheet to form covalent bonds but also interact with each other to form hydrogen bonds and ionic chelation (-COO− and Ca2+). These jointly form a stable integrated composite hydrogel [91]. MXene can also form self-assembled hydrogels with the assistance of initiators. Shang et al. reported a method for MXene nanosheets to form an interconnected 3D porous network hydrogel with the assistance of GO and ethylenediamine (EDA) (Fig. 5B). Firstly, the ring-opening reaction of GO and the formation of a hybrid structure of MXene were induced by EDA. Then the hybrid MXene nanosheets formed a 3D network structure through the superposition of π-π bonds and van der Waals forces. After cross-linking, the X-ray diffraction pattern showed that the characteristic TiO2 absorption peak disappeared, which indicated that with the mediation of GO, the cross-linking of the MXene hydrogel was achieved [97]. And MXene can also form gels mediated by metal ions. The addition of Fe2+ into the dispersed MXene nanosheet solution was able to eliminate the electrostatic repulsion between NSs. Subsequently, it acts as a crosslinker, linking the nanosheets together by forming coordination bonds dominated by Fe-O bonds (Fig. 5C). Researchers also found that the interlayer spacing increased slightly after adding Fe2+ into MXene. The difference in spacing was just close to the diameter of Fe2+ ions without a hydration shell (0.122 nm). Therefore it can be inferred that Fe2+ can also be inserted into the MXene layer to assist in the gelation process [98].

    Figure 5

    Figure 5.  (A) Schematic of Go-assisted MXene nanocomposite hydrogels. Copied with permission [91]. Copyright 2021, American Chemical Society. (B) Schematic of MXene-Polymer nanocomposite hydrogels. Copied with permission [97]. Copyright 2019, John Wiley & Sons, Inc. (C) Schematic of MXene-metal hybrid nanocomposite hydrogels. Copied with permission [98]. Copyright 2019, John Wiley & Sons, Inc.
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    Physical binding is mostly accomplished by hydrogen bonding. Zhang et al., for example, loaded few-layer MXene into a hydrophobically associated polyacrylamide (HAPAM) hydrogel by forming hydrogen bonds between the amide groups in HAPAM and -F, -OH groups on the surface of MXene [94]. Likewise, another research team cross-linked MXene and polyvinyl alcohol (PVA) by forming of hydrogen bonds between MXene and PVA. And statistics have shown that physical cross-linking can significantly improve the tensile properties and self-healing ability of PVA hydrogel [92].

    4. Applications of promoting chronic wound healing

    The hydrogel, combined with MXene, can exhibit a variety of biochemical properties. Correspondingly, MXene-based hydrogels can promote chronic wound healing by influencing multiple stages of the healing process. The three key properties mentioned above, high photothermal conversion rate, strong antibacterial ability and electrical conductivity are the basis of function. The composite hydrogel can convert NIR light energy into heat energy, leading to a rise in temperature that accelerates drug release and significantly enhances bactericidal ability. Also, conductivity plays a role in promoting cell proliferation, making MXene-based hydrogel an effective option for promoting chronic wound healing.

    4.1 Application of controlled drug release

    NIR light has the advantages of deep penetration, minimal phototoxicity and spatiotemporal remote control ability [99,100]. When used in conjunction with light-responsive materials, precise spatiotemporal controlled medication release can be achieved. MXene, a newly developed photothermal agent, exhibits a strong light absorption capacity and good biocompatibility in the NIR range, which can be loaded with rich anchor sites [101]. Therefore, utilizing hydrogel-based MXene for drug loading and employing NIR for precise controlled release shows promising potential for applications in the medical field. Given the challenges associated with chronic wound healing, drugs can be selectively loaded, such as anti-inflammatories, pro-angiogenesis. Researchers have developed MXene composite hydrogels with various substrates loaded with different drugs. These hydrogels have shown promising results in enhancing drug release both in vivo and in vitro. The quantity and rate of release can be regulated by adjusting MXene concentration, illumination power and duration (Table 3 [40,56,102-109]).

    Table 3

    Table 3.  The controlled drug release of hydrogel-based MXene.
    DownLoad: CSV
    Materials Loaded drug NIR irradiation conditions Ref.
    MXene@agarose hydrogel Rho-BSA 1.0 W/cm2 [56]
    MXene-poly(N-isopropylacrylamide)-co-N-(hydroxymethyl)acrylamide hydrogel Dex 0.03 W/cm2 [103]
    MXene/sodium alginate hydrogel CIP 0.5 W/cm2 [104]
    MXene/agar-PVA hydrogel CE 0.75 W/cm2 [105]
    MXene/dopamine-hyaluronic acid hydrogel VEGF 0.33 W/cm2 [106]
    MXene/gellan gum hydrogel DOX 1.0 W/cm2 [107]
    MXene/agarose hydrogel DOX 1.5 W/cm2 [40]
    MXene/DNA-pAM hydrogel DOX 1.44 W/cm2 [108]
    KH570-MXene/PNIPAm hydrogel TC 0.5 W [102]
    MXene/hyaluronic hydrogel DFOM, AC 0.5 W [109]

    The principle of controlled release of MXene-based hydrogel through NIR can be divided into two types: one involves the enlargement of pores in the three-dimensional structure of hydrogel due to the temperature rise, leading to the rapid release of drugs [102,110]. Hao et al. loaded MXene onto temperature-sensitive PNIPAm polymer, and then further functionalized the MXene surface with γ-methylacryloxypropyl trimethoxy silane (KH570) to improve the interfacial compatibility of MXene with PNIPAm, so that K-Mx/PNIPAm hydrogel was synthesized (where x represents the mass ratios of MXene of NIPAm monomer). K-M2/PNIPAm hydrogel irradiated with a 1 W laser power could reach 63.6 ℃ in 60 s. The controlled release ability was measured using tetracycline (TC) as a model, shown in Fig. 6A. The results showed that as the concentration of MXene in the hydrogel increased, the drug release rate also increased under the same illumination conditions. The K-M2/PNIPAm hydrogel was irradiated with NIR for 10 min in a liquid-free environment, the release of TC reached 17.36%. After 70 min of irradiation, it increased to 99.66%. Moreover, for the same concentration of K-M/PNIPAm hydrogel, the drug release rate increased with the intensity of NIR light [102].

    Figure 6

    Figure 6.  (A) Schematic diagram of the drug release principle of TC under NIR irradiation. Copied with permission [102]. Copyright 2021, American Chemical Society. (B) The melting image of MXene@Hydrogels with different agarose concentrations under irradiation using an 808 nm laser (1.5 W/cm2). (C) Temperature change and DOX release profile of DOX-loaded MXene@Hydrogel within four on/off cycles of laser irradiation. Copied with permission [40]. Copyright 2021, Elsevier.
    DownLoad: Full-Size Img PowerPoint

    Another approach is to use the photothermal effect of MXene to raise the temperature to melt the hydrogel matrix, thus leading to a drug release. Dong et al. Combined MXene with low-melting-point agarose (MXene@Hydrogel) and loaded it with DOX. With a low concentration of addition, the MXene@Hydrogel could be rapidly heated to 50 ℃ under 808 nm NIR irradiation and melt to release DOX loaded (Figs. 6B and C). While the composite hydrogel without NIR irradiation almost had no DOX release [40].

    Wang et al. loaded liver growth factor (HGF) into MXene@hydrogel (MXene@agarose/HGF) and controlled its release using NIR light [56]. The specific binding of HGF to cellular-mesenchymal epithelial transition factor (c-Met) induces dimerization of c-Met and phosphorylation of multiple tyrosine residues in the cytoplasmic region. These phosphorylation reactions initiate intracellular signal transduction and regulate various cellular functions, including promoting mitosis and migration, inhibiting cell death and inducing epithelial morphogenesis [111]. By measuring the cell diffusion, migration and proliferation abilities under MXene@agarose/HGF treatment, and further in vivo angiogenesis and wound healing experiments, it was confirmed that MXene hydrogel loaded with HGF can promote chronic wound healing by controlling the spatial and temporal drug release through NIR light. A similar effect was observed in MXene-Integrated Microneedle Patches with Adenosine Encapsulation designed by Sun et al. The NIR light response ability of MXene can accelerate the release of adenosine. Animal experiments have demonstrated that microneedle patches can effectively stimulate angiogenesis and can be used for the treatment of chronic wounds [23].

    4.2 Application of antibacterial capacity

    In chronic wounds, such as diabetic ulcers, peripheral neuropathy, difficulty in angiogenesis, and compromised immune function would give rise to bacterial infections, including Gram-negative and Gram-positive bacteria, as well as anaerobic bacteria and some fungi. Biofilms formed by bacteria can largely resist the host immune system and develop drug resistance [112]. Various studies have shown that bacterial infections will cause prolonged inflammatory responses, but the molecular and cellular mechanisms underlying these processes are still not fully understood [113]. What is certain is that in an extended inflammatory response, excessive neutrophil infiltration, increased activity of matrix metalloproteinase, collagenase and elastase, and elevated levels of inflammatory cytokines like interleukin-1 beta (IL-1β), IL-8 and tumor necrosis factor alpha (TNF-α) can inhibit the normal proliferation and collagen synthesis of fibroblasts, degrade ECM and inhibit the level of growth factors [114,115]. It affects wound healing in the subsequent stages, and even exacerbates tissue damage. As mentioned above, MXene has good antibacterial ability due to its special structure and properties, enabling it to act as an antibacterial and anti-inflammatory biomaterial wound healing (Table 4 [20,21,95,116-122]).

    Table 4

    Table 4.  Antibacterial ability of MXene -based hydrogel.
    DownLoad: CSV
    Materials Concentration NIR irradiation conditions Bacterial species Inhibition rate (%) Ref.
    Bi2S3/Ti3C2Tx 200 ppm 0.7 W/cm2, 10 min E. coli 99.92 [116]
    S. aureus 99.86
    Ti3C2 100 µg/mL 0.4 W/cm2, 20 min E. coli > 95 [117]
    S. aureus > 99
    Ag2S/Ti3C2 500 µg/mL 0.67 W/cm2, 20 min S. aureus 99.99 [118]
    MXene/CoNWs/SPEEK 2.12 mg/cm2 1.5 W/cm2, 20 min E. coli 80.10 [119]
    S. aureus 92.74
    TBC-TA@ZIF-8-MXene 2 mg/mL 1 W/cm2, 20 min E. coli 99.8 [95]
    S. aureus 99.9
    CNWs@MXene/ZIF-8 textiles 1.46 mg/cm2 300 mW/cm2, 5 min E. coli 99.99 [120]
    S. aureus 99.99
    MXene-GACS nanofiber membrane 50 µg/mL 1 W/cm2, 5 min E. coli > 99 [20]
    S. aureus > 99
    MRSA > 99
    Cu2O/Ti3C2Tx nanosheets 50 µg/mL 0.54 W/cm2, 10 min E. coli > 99 [121]
    Mo2Ti3C2 300 µg/mL 2 W/cm2, 5 min MRSA 98.6 [21]
    Ti3C2Tx@CuS 0.5 mg/mL 1.5 W/cm, stopped for 10 min every 30 min and lasted for 12 h E. coli 99.60 [122]
    S. aureus 99.10

    The "nano-knife effect" is the primary mechanism, followed by oxidative damage, electron transfer and inhibition of nutrient intake. However, Wu et al. studied the interaction mechanism between MXene and bacterial cell membranes through molecular dynamics (MD) and pointed out that such "nano-knife effect" is difficult to achieve [59]. Since MXene owns a large number of hydrophilic terminal groups (such as -O− and -OH) on its surface, the insertion process requires overcoming a large energy barrier. Consequently, most MXene tends to adsorb onto the surface of the cell membrane. While the thermal energy generated by the NIR light response of MXene adsorbed on the cell membrane is the key factor leading to the damage [59]. This claim is supported by a study conducted by Rosenkranz et al. After treating E. coli and S. aureus with 100 µg/mL of few-layer MXene for 4 h, the bacterial survival rates were over 50% and 30%, respectively. In contrast, the bactericidal rate rapidly increased to 87% and 95% with only 3 W/cm2 NIR irradiation for 5 min [123]. S-CF@PCNx, a smart cotton fabric decorated with a porous coordination network and sprayed with MXene (where x represents the amount of MXene), developed by Nie et al., showed a 99.9999% inactivation rate of E. coli and S. aureus after 30 min of light exposure under a Xe lamp (λ ≥ 420 nm, 500 W, vertical distance: 15 cm). However, after 45 min of co-incubation with S-CF@PCN in the dark, no bacterial inactivation was observed [124].

    MXene-based hydrogels can also be loaded with special antibacterial materials or drugs to achieve a synergistic effect that enhances antibacterial capacity. Silver NPs are a common antibacterial material. Zhu et al. loaded AgNP onto MXene-based hydrogel dressings, showing that Ti3C2Tx MXene has a synergistic antibacterial mode of photothermal sterilization and inhibition of bacterial activity. This combination serves as an outstanding NIR photo-mediated nanoplatform, exhibiting remarkable antibacterial and wound healing properties when exposed to NIR light irradiation. There was a qualitative difference in the antibacterial efficiency of Ti3C2Tx at a concentration of 200 µg/mL in the non-illumination group (exposure to the 1.5 W/cm2, 808 nm NIR for 15 min), with S. aureus survival rates of 53% and 2% respectively. In the absence of NIR irradiation, the bacterial viability of both the Ag and Ti3C2Tx groups was higher than 50%, while that of the Ag/Ti3C2Tx group at the same concentration was reduced to 18%. Under illumination (1.5 W/cm2, 808 nm NIR for 15 min), PTT could also promote the release of silver NPs, and all the bacteria in the Ag/Ti3C2Tx group could be killed. The wound infection model in mice showed that the Ti3C2Tx hydrogel group had fewer inflammatory cells compared to the control group, indicating that it had an effective photothermal ablation effect on bacteria [125]. For antibiotic-resistant bacteria, antimicrobial drugs can be loaded into the MXene-based hydrogels to enhance antimicrobial efficacy. A study reported an antibiotic ciprofloxacin hybrid loaded thermo-sensitive hydrogel (TSG). The nanoblade and PTT-induced physical membrane disruption of Ti3C2 nanosheets may contribute to the intracellular accumulation of ciprofloxacin. By combining chemotherapy and photothermal treatment, the in vitro bactericidal efficiency of TSG against MRSA was up to 99.99999%. And the morphological changes observed by SEM showed that the cellular structure of MRSA was clearly affected by the treatment, including cell membrane rupture and cytoplasmic extravasation [126].

    4.3 Application of promoting cell proliferation

    The latest studies show that MXene also has the ability to promote cell proliferation and differentiation. This has been confirmed in the culture of human mesenchymal stem cells, mouse neural stem cells and fibroblasts [127-130], suggesting its potential application in promoting epithelial regeneration and granulation formation during wound healing.

    MXene has good electrical conductivity, which allows for the construction of a cellular communication network. Furthermore, the role of exogenous electrical stimulation (ES) in wound repair and skin tissue regeneration has been shown to accelerate the wound healing process by promoting angiogenesis, down-regulating the inflammatory response, and promoting granulation tissue formation and collagen synthesis by mimicking endogenous electrical fields [131-134]. Thus, loading ES on MXene-based hydrogel will greatly improve the dressing's ability to promote cell proliferation (Fig. 7). Zheng et al. developed a multifunctional hydrogel scaffold (FOM) with MXene as the main functional component for wound healing of multidrug-resistant (MDR) bacterial infections. Co-cultured L929 cells and human dermal keratinocytes with FOM, and cell proliferation was determined by alive and dead staining and fluorescence intensity on Day 1, 3, and 5. It can be observed that the FOM scaffold can promote cell proliferation to a certain extent. In vitro scratch test results showed that the scratch closure rate of FOM with ES was as high as 73.6% within 24 h (compared with 40.4% in the control group) [17]. Lin et al. reported similar results. They exposed MXene and NIH 3T3 cells to ES during the co-culture. The results showed that the cell fluorescence density of rBC-MXene-2% was much higher than that of the untreated control group when exposed to 100 mV ES. In a full-thickness skin defect model on Sprague-Dawley rats, the rBC/MXene-2% combined with 100 mV ES group had the smallest wound area at any time point, and the wound healing rate reached 93.8% ± 3.2% after 14 days of treatment [135]. According to Zhou et al., the results of the co-incubation culture experiment between the MXene-based hydrogel and cells revealed that, when compared with the control group without MXene, the fluorescence intensity of cell proliferation in the experimental group was significantly increased. This indicates that the MXene-based hydrogel composite material had a positive effect on cell proliferation [64]. The research results of Li et al. are also consistent with this [136]. The proliferation of NIH 3T3 cells was analyzed by AO/PI live/dead staining after co-culture with different concentrations of MXene hydrogel. The results showed that MXene exhibited obvious proliferative activity when the concentration was < 20 µg/mL.

    Figure 7

    Figure 7.  Electrostimulation of NIH 3T3 cells on different hydrogels with varying ES potentials. (A) LSCM images of live/dead staining. Scale bar: 100 µm. (B) The number and (C) viability of NIH 3T3 cells on various hydrogels after 3 days of culture and electrostimulation. Copied with permission [135]. Copyright 2020, Wiley. (D) Fluorescence intensity of human dermal keratinocytes treatment with FOM scaffold and controls for 1, 3 and 5 days (*P < 0.05, **P < 0.01, n = 5). (E) The cell viability of human dermal keratinocytes treatment with FOM scaffold and controls for 1, 3 and 5 days based on CCK-8 assay. Copied with permission [17]. Copyright 2021, Elsevier.
    DownLoad: Full-Size Img PowerPoint

    According to previous studies on the mechanism of conductive polymers. They can allow cells or tissues cultured on them to be stimulated by electrical signals [137], so we could put forward a reasonable conjecture. The possible reason is that MXene-based hydrogel composite material could upregulate related gene expression to promote cell proliferation by constructing the cellular communication network. This is achieved through enhancing the electrical transmission owing to its excellent conductivity. Further QRT-PCR detected that HPEM upregulated the expression of α-actin, COL Ⅲ, and VEGF genes, which play important roles in wound healing. However, the specific mechanism by which MXene promotes cell proliferation remains unclear and requires further research.

    5. Conclusion

    In summary, we reviewed the current research progress of MXene-based hydrogels in the field of chronic wound healing. From the preparation of composite hydrogels to the principle and application in promoting chronic wound healing were comprehensively documented. It can develop a variety of drug-loaded bioactive hydrogels by combining the NIR response ability, antibacterial ability and electrical conductivity of MXene with the hydrogel base. It can promote wound healing from the aspects of antibacterial and cell proliferation, and effective drug loading and controlled release, offering a design idea for the development of innovative wound dressings.

    However, in clinical practice, the darker color of MXene may affect the assessment of wound healing progress, which may limit its application. How to firmly bind MXene to the hydrogel dressing may be the key to solving this problem. And some chemical groups on its surface can be oxidized, which imposes stricter requirements on its storage conditions or customized surface modification. More importantly, it is still uncertain whether the "nano-knife effect" and oxidative stress of MXene can harm normal tissue cells. Of course, these can be balanced by optimizing the concentration of MXene in the hydrogel system. To effectively utilize its pro-healing capability and avoid hazardous side effects, further research is required. We must point out that although there have been some published data that can support the effectiveness of MXene in promoting cell proliferation, the mechanism still remains unclear. The speculations we propose in the paper may just provide some ideas for future studies. While MXene is still a new 2D material with great potential. With the continuous exploration in the field of biomedicine, it is anticipated to contribute to finding a solution to the issue of chronic wound healing.

    Declaration of competing interest

    The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

    CRediT authorship contribution statement

    Ruijun Song: Writing – original draft. Huixu Xie: Supervision. Guiting Liu: Supervision.

    Acknowledgment

    This work was supported by National Natural Science Foundation of China (Nos. 52103039 and 82370977).


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  • Figure 1  Schematics of MXene-based hydrogels for the treatment of chronic wounds.

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    Figure 2  (A) Transformation of MAX phase Mn + 1AXn to MXene Mn + 1XnTx. Copied with permission [27]. Copyright 2018, Accounts of Chemical Research. (B) SEM micrographs of Ti3AlC2 particle and (C) Ti3C2Tz formed after etching in HF. Copied with permission [28]. Copyright 2021, Elsevier.

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    Figure 3  (A) Ultraviolet (UV)–visible spectroscopy-NIR absorbance spectra of MXene dispersed in water at different concentrations (0.01, 0.04, 0.06, 0.08, and 0.10 mg/mL). (B) Beer law absorbance plot for absorption at 808 nm. (C) Temperature changes of MXene nanosheet dispersion (0.10 mg/mL) upon NIR irradiation for six laser ON/OFF cycles. Copied with permission [55]. Copyright 2021, Wiley. (D) Real-time infrared thermal images of MXene@agarose containing different concentrations of Ti3C2 under continuous 808 nm irradiation at a power intensity of 1 W/cm2 for 5 min. (E) Real-time infrared thermal images of MXene@agarose containing 30 µg/mL Ti3C2 under continuous 808 nm irradiation with different laser power densities for 5 min. Copied with permission [56]. Copyright 2021, Elsevier.

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    Figure 4  (A) Schematic diagram of bacteria being wrapped by MXene. (B) TEM images of E. coli and B. subtilis treated with 200 µg/mL of Ti3C2Tx for 4 h at low and high magnifications. (C) SEM images of the E. coli (top panel) and B. subtilis (bottom panel) treated with different concentration of Ti3C2Tx, at low and high magnification, respectively. Reproduced with permission [19]. Copyright 2016, American Chemical Society. (D) Bacterial viability against E. coli, S. aureus, and MRSA treated with different samples (*P < 0.05, **P < 0.01, n = 3). Copied with permission [64]. Copyright 2021, American Chemical Society. Cell viability measurements of (E) E. coli and (F) B. subtilis treated with Ti3C2Tx and GO in aqueous suspension. Copied with permission [19]. Copyright 2016, American Chemical Society.

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    Figure 5  (A) Schematic of Go-assisted MXene nanocomposite hydrogels. Copied with permission [91]. Copyright 2021, American Chemical Society. (B) Schematic of MXene-Polymer nanocomposite hydrogels. Copied with permission [97]. Copyright 2019, John Wiley & Sons, Inc. (C) Schematic of MXene-metal hybrid nanocomposite hydrogels. Copied with permission [98]. Copyright 2019, John Wiley & Sons, Inc.

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    Figure 6  (A) Schematic diagram of the drug release principle of TC under NIR irradiation. Copied with permission [102]. Copyright 2021, American Chemical Society. (B) The melting image of MXene@Hydrogels with different agarose concentrations under irradiation using an 808 nm laser (1.5 W/cm2). (C) Temperature change and DOX release profile of DOX-loaded MXene@Hydrogel within four on/off cycles of laser irradiation. Copied with permission [40]. Copyright 2021, Elsevier.

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    Figure 7  Electrostimulation of NIH 3T3 cells on different hydrogels with varying ES potentials. (A) LSCM images of live/dead staining. Scale bar: 100 µm. (B) The number and (C) viability of NIH 3T3 cells on various hydrogels after 3 days of culture and electrostimulation. Copied with permission [135]. Copyright 2020, Wiley. (D) Fluorescence intensity of human dermal keratinocytes treatment with FOM scaffold and controls for 1, 3 and 5 days (*P < 0.05, **P < 0.01, n = 5). (E) The cell viability of human dermal keratinocytes treatment with FOM scaffold and controls for 1, 3 and 5 days based on CCK-8 assay. Copied with permission [17]. Copyright 2021, Elsevier.

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    Table 1.  MXene materials that can respond in the near infrared region.

    Materials Concentration NIR irradiation conditions Ref.
    Doxorubicin (DOX)-loaded MXene@Hydrogel 20 ppm 808 nm, 1.5 W/cm2 [40]
    Epithelial cell adhesion molecule (EpCAM)-grafted-Ti3C2Tx@gelatin membrane 1 mg/mL 808 nm, 0.54 W/cm2 [41]
    Vitamin E-loaded MXene nanobelt fiber / 808 nm, 0.33 W/cm2 [42]
    DOX-loaded MXene polymethyl methacrylate (PMMA) microspheres 500 µg/mL 808 nm, 1.5 W/cm2 [43]
    Nb2C nanosheets modified bioactive glass scaffolds (NBGS) / 1064 nm [44]
    Emamectin benzoate (EB)@p-phenylenediamine (PDA)@ Ti3C2Tx 0.2 mg/mL 808 nm, 2 W/cm2 [45]
    Ti3C2−OH/Bi2WO6: Yb3+, Tm3+ composites 2.8 wt% Vis-NIR, 30 min [46]
    Ti3C2Tx/ionic liquid ink / 808 nm [47]
    MXene-decorated textile 17.3 wt% 780 nm [48]
    下载: 导出CSV

    Table 2.  The combination of MXene with different hydrogel matrixs.

    Hydrogel matrix Synthesis mechanism Bonding Ref.
    PNIPAM Chemical crosslinking -C-C- covalent bond [54]
    rGO/EDA Chemical crosslinking -C-N- covalent bond [89]
    PAM Chemical crosslinking & physical mixing Ti-O, Ti-C covalent bonds & hydrogen bonds between -OH and -NH2 [90]
    PAA/ACC Physical mixing Chelation between -COO– and Ca2+ & hydrogen bond & electrostatic interaction [91]
    PVA Physical mixing Hydrogen bond between -OH and -OH [92]
    PVA/porphyrin Physical mixing Hydrogen bond between -OH and -COOH [93]
    PNIPAM Physical mixing Hydrogen bond between oxygen-containing groups and -CO-NH2 [94]
    TBC Physical mixing Hydrogen bond & physical absorption [95]
    PAA/PVA Physical mixing / [96]
    下载: 导出CSV

    Table 3.  The controlled drug release of hydrogel-based MXene.

    Materials Loaded drug NIR irradiation conditions Ref.
    MXene@agarose hydrogel Rho-BSA 1.0 W/cm2 [56]
    MXene-poly(N-isopropylacrylamide)-co-N-(hydroxymethyl)acrylamide hydrogel Dex 0.03 W/cm2 [103]
    MXene/sodium alginate hydrogel CIP 0.5 W/cm2 [104]
    MXene/agar-PVA hydrogel CE 0.75 W/cm2 [105]
    MXene/dopamine-hyaluronic acid hydrogel VEGF 0.33 W/cm2 [106]
    MXene/gellan gum hydrogel DOX 1.0 W/cm2 [107]
    MXene/agarose hydrogel DOX 1.5 W/cm2 [40]
    MXene/DNA-pAM hydrogel DOX 1.44 W/cm2 [108]
    KH570-MXene/PNIPAm hydrogel TC 0.5 W [102]
    MXene/hyaluronic hydrogel DFOM, AC 0.5 W [109]
    下载: 导出CSV

    Table 4.  Antibacterial ability of MXene -based hydrogel.

    Materials Concentration NIR irradiation conditions Bacterial species Inhibition rate (%) Ref.
    Bi2S3/Ti3C2Tx 200 ppm 0.7 W/cm2, 10 min E. coli 99.92 [116]
    S. aureus 99.86
    Ti3C2 100 µg/mL 0.4 W/cm2, 20 min E. coli > 95 [117]
    S. aureus > 99
    Ag2S/Ti3C2 500 µg/mL 0.67 W/cm2, 20 min S. aureus 99.99 [118]
    MXene/CoNWs/SPEEK 2.12 mg/cm2 1.5 W/cm2, 20 min E. coli 80.10 [119]
    S. aureus 92.74
    TBC-TA@ZIF-8-MXene 2 mg/mL 1 W/cm2, 20 min E. coli 99.8 [95]
    S. aureus 99.9
    CNWs@MXene/ZIF-8 textiles 1.46 mg/cm2 300 mW/cm2, 5 min E. coli 99.99 [120]
    S. aureus 99.99
    MXene-GACS nanofiber membrane 50 µg/mL 1 W/cm2, 5 min E. coli > 99 [20]
    S. aureus > 99
    MRSA > 99
    Cu2O/Ti3C2Tx nanosheets 50 µg/mL 0.54 W/cm2, 10 min E. coli > 99 [121]
    Mo2Ti3C2 300 µg/mL 2 W/cm2, 5 min MRSA 98.6 [21]
    Ti3C2Tx@CuS 0.5 mg/mL 1.5 W/cm, stopped for 10 min every 30 min and lasted for 12 h E. coli 99.60 [122]
    S. aureus 99.10
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  • 发布日期:  2025-07-15
  • 收稿日期:  2024-06-21
  • 接受日期:  2024-09-10
  • 修回日期:  2024-09-08
  • 网络出版日期:  2024-09-12
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