Superior antibacterial hydrogel has the benefits of MOP

Biomaterials have an imperative role in biomedical applications. Hydrogels are one of the most promising classes of biomaterials for biomedical use. In the latest article published in the journal Carbohydrate Polymers, Chinese researchers have developed a new hybrid metal-organic polyhedra (MOP)/chitosan (CS) enzyme hydrogel and discussed its application as a superior antimicrobial agent in the treatment of wound healing.

Study: A novel chitosan-based polyhedral/enzyme metal-organic hybrid hydrogel with antibacterial activity to promote wound healing. Image Credit: BBbirdZ/Shutterstock.com

Wound healing with bio-engineered materials

Prompt treatment of a wound can prevent bacterial infections and other serious complications. Therefore, the development of new multifunctional dressing materials can contribute to effective wound care management.

The CS alkaline polysaccharide has received considerable attention in the biomedical field. The amino group present on CS-based hydrogels destroys the bacterial membrane and disrupts bacterial wall mass transport. Although CS-hydrogel has antibacterial properties, its antibacterial effect is limited to the cell wall and further development to expand its antibacterial functions is a difficult task.

The ordered structure of MOP, its good stability, adjustable pore size and modification sites facilitate the formulation of MOPs containing hybrid materials. However, despite their ease of processing, the application of hybrid MOP/biopolymer hydrogels as healing materials remains unexplored.

The oxidation of glucose catalyzed by glucose oxidase (GOx) forms hydrogen peroxide (H2O2) as one of the products. Therefore, GOx is a bacteriostatic agent. However, hydroxyl (OH) free radicals are more potent for antibacterial activity than H2O2. Therefore, combining MOPs with GOx may be an effective strategy to generate OH free radicals to treat wound infections.

New hybrid hydrogel

In the present work, the authors developed a novel CS-based hybrid MOPs/enzyme hydrogel with antimicrobial properties ideal for wound healing treatments. Cross-linked GOx with vanadium (VMOP-2) and CS MOPs using a glutaraldehyde cross-linking agent to form a novel hybrid GOx/VMOP-2/CS (GVCS) hydrogel. The authors characterized the synthesized GVCS hydrogel using Fourier transform infrared (FTIR), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and l energy dispersive X-ray (EDX) analysis. The results revealed a uniform distribution of VMOP-2 and GOx in the hydrogel structure.

In the new GVCS hydrogel, GOx generates H2O2 glucose while VMOP-2 converts the H formed2O2 into OH free radicals, thus exhibiting bactericidal activity against gram-negative bacteria and gram-positive bacteria. In addition, the determination of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and live studies have revealed the cytocompatibility and healing ability of the GVCS hydrogel.

Characterization and biological evaluation of the GVCS hydrogel

SEM images of the GVCS hydrogel (lyophilized) showed a honeycomb structure. Moreover, comparison of SEM images of CS and GVCS hydrogels showed that the addition of GOx and VMOP-2 slightly changed the morphology of the hydrogel. The XPS spectrum of GVCS showed five peaks corresponding to S 2s, C1s, N 1s, V 2p and O 1s.

The FTIR spectrum showed characteristic peaks at 3300 inverse centimeters that correspond to the stretching of the OH and amine (NH) groups. The CH2 asymmetrical and symmetrical stretching vibrations were observed at 2931 and 2869 reverse centimeters, respectively. The peaks at 1646 and 1558 inverse centimeters correspond to the bending of amides and amines. Also, compared to the CS gel, the peaks at 1646 (amide) and 1033 inverse centimeters (C-OH) had increased intensity in the GVCS hydrogel due to the imine bonds formed by cross-linking between the aldehyde groups of glutaraldehyde and the amino groups of chitosan.

TGA studies of the GVCS hydrogel have revealed that weight loss below 110 degrees Celsius is due to loss of free water. Weight loss between 110 and 210 degrees Celsius corroborates bound water loss and short chain decomposition. Moreover, between 210 and 450 degrees Celsius, the weight loss corresponds to a degradation of chitosan followed by a decomposition of VMOP-2 above 390 degrees Celsius. The results show that crosslinking by VMOP-2 improved the thermal stability in the GVCS hydrogel. EDX mapping revealed successful GOx and VMOP-2 doping with uniform distribution in the hydrogel structure.

The antibacterial activity of the GVCS hydrogel against gram-positive bacteria (Escherichia coli) and gram-negative bacteria (Streptococcus aureus) was better than the other hydrogels used (CS, V-CS, GOx-CS). GVCS showed sustained antibacterial activity using glucose at the wound, which infers that GOx and VMOP-2 constantly generate OH free radicals leading to high antibacterial activity.

The cytotoxicity test of the GVCS hydrogel, evaluated by MTT assay on L929 cells, revealed good biocompatibility of the hydrogel. With a decrease in glucose concentration at the wound, the generation of OH free radicals by GVCS hydrogel is reduced, minimizing irritation, thus confirming the biocompatibility of GVCS hydrogel and its suitability as a dressing material.

Separate treatment of approximately 25 square millimeter wounds on the backs of mice with phosphate-buffered saline (PBS), CS, and GVCS hydrogels showed that wound healing was faster in GVCS, suggesting that the GVCS hydrogel promotes wound healing in vivo by generating OH radicals with glucose at the wound site.

Hematoxylin and eosin (H&E) and Masson’s trichrome staining results on skin tissue after GVCS hydrogel treatment for 25 days showed mature granulation tissue at the wound site in GVCS-treated mice, attributing faster angiogenesis and thin fibroblast distribution. Additionally, the dense deposition of collagen at the wound site suggests that the GVCS hydrogel facilitates wound closure.

Conclusion

In this study, the authors synthesized a novel CS-based MOP/enzyme hybrid hydrogel that has multiple advantages over current hydrogel-based dressings. This new hybrid hydrogel also exhibited superior antibacterial therapy.

Each component of the hybrid hydrogel had a significant contribution to the antibacterial effect. GOx helps H2O2 generation which is then converted into OH free radical by VMOP-2 and enhances the antibacterial effect. Additionally, the hybrid hydrogel blocks the supply of nutrients to bacteria by consuming glucose at the wound site.

Reference

Song, J., Zhang, C., Kong, S., Liu, F., Hu, W., Su, F. and Li, S. Novel Chitosan-Based Metal-Organic Polyedrons/Enzyme Hybrid Hydrogel with Antibacterial Activity to Promote wound healing. Carbohydrate polymers (2022). https://www.sciencedirect.com/science/article/pii/S0144861722004271

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