Antibacterial and Biocompatible Hydrogel Dressing Based on Gelatin, CastorOil-Derived Biocidal Agent

Favored antibacterial activity associated with excellent biocompatibility, mechanical durability, and exudate handling needs to be addressed by modern dressing to achieve the desired wound healing. This paper deals with developing a new green and facile approach to manufacturing non-leachable antibacterial gelatin-based films for wound dressing. Therefore, a reactive methoxy-silane-functionalized quaternary ammonium compound bearing a fatty amide residue originating from castor oil (Si-CAQ) was initially synthesized. The antibacterial dressings were then fabricated via solgel and condensation reactions of the mixture containing gelatin, Si-CAQ, (3-glycidyloxypropyl) trimethoxysilane, and poly(vinyl alcohol).

By utilizing bioactive polymers as starting materials and eliminating organic solvents during the dressing preparation, desirable clinical safety could be ensured. The gelatin-based films presented appropriate mechanical properties, such as flexibility and strength, in both dried and hydrated states (tensile strength >6 MPa and elongation >100). It is due to the in situ generations of the inorganic silicon domain in the organic framework via the sol–gel cross-linking process. The prepared dressings exhibited desirable features, including excellent biocompatibility (cell viability >95%), proper wound-exudate-managing characteristics (equilibrium water contact (EWA) 280–350% and water vapor transmission rate (WVTR) 2040–2200 g/m2/day), fluid handling capacity (FHC) (3–3.35 g), as well as commendable hemocompatibility. The promising bactericidal activity of the dressing against Bacillus subtilis, methicillin-resistant Staphylococcus aureus, and Escherichia coli strains with a contact-killing efficacy of 100% could prevent infection development at the wounded area. As evaluated by the wound scratch assay, the desired fibroblast cell growth, migration, and proliferation indicated the capability of the dressing to facilitate the healing process by encouraging fibroblast cell migration to the damaged area. In vivo wound-healing results showed that the prepared biocidal dressing stimulates wound healing and enhances epithelialization, collagen maturation, and vascularization of wounds due to their antibacterial effects and accelerated cellular functions.

Designed Route for the Synthesis of the Si-CAQ Biocidal Agent

Rapid and effective healing of damaged areas to re-establish the anatomy and function of skin and prevent subsequent severe healthcare problem is still a great challenge in modern wound-care medicine.To this aim, covering the wound area at different stages with a proper wound dressing material is a commonly accepted practice in clinical therapy and research studies.In this way, developing dressing materials that possess an appropriate combination of physical protection and encouraging the self-healing process of wounds is of paramount importance. The favored dressing materials must meet several prerequisite conditions and boost the self-healing process by providing an optimal healing environment over the wounded area. The bacterial infection during the healing process postpones the routine healing of wounds by a remarkable increase in exudate formation and tissue necrosis and interfering with angiogenesis and epithelialization during the wound repair process.Previous studies have demonstrated that bacteria colonization and subsequent biofilm formation could occur within 24 h.Therefore, covering wounds with dressings that prevent bacterial adhesion, survival, or multiplication during the first few hours of injury plays a crucial role in avoiding wound infection.

Inducing antimicrobial activity into dressing materials through impregnating the dressing with bactericidal agents like antibiotics or silver ions has been considered in many reports. However, heavy metal poisoning, developing bacterial resistance, short lifetimes, and allergic reactions have limited their practical application. Thus, it is of particular significance to create durable, long-lasting, and contact-active antibacterial wound dressings by chemical anchoring of the bactericidal agent to the dressing materials. Among different biocidal agents, quaternary ammonium salts (QAS) as contact-active biocidal agents are regarded as robust candidates owing to their unique biological and structural flexibility.Tan and co-workers developed a cross-linked polyurethane containing the Gemini quaternary ammonium salt with excellent antibacterial activity without significant cytotoxicity against mammalian cells.Inducing antibacterial property in wound-dressing materials through chemical anchoring of various types and contents of QAS derivatives has also been reported by Yeganeh et al.Nevertheless, QAS-decorated antibacterial materials can hardly meet the high biocidal activity along with physicomechanical and biocompatibility requirements since the QAS could not kill bacteria without affecting the mammalian cell viability.

FTIR spectra of (a) castor oil, (b) TACO, and (c) Si-CAQ.

Previous research has established that the alkyl chain length connected to the quaternary ammonium moieties, the position of QAS, and the hydrophobic/hydrophilic balance of the bactericidal components as the nature and flexibility of the polymeric matrix all contribute to the antimicrobial efficacy and cytocompatibility.Tuning the positive charge content and the amphiphilic balance has been introduced as a possible way of enhancing the antibacterial activity of QAS components.An in-depth inspection of previous studies revealed that the QAS compound bearing a long hydrophobic chain exhibits a superior antibacterial efficiency. However, improving the antibacterial property through an increase in hydrophobicity might lead to an increase in cytotoxicity.Hence, designing and engineering the structure of QAS components could provide potent antibacterial activity while minimizing toxicity.

Herein, they developed a new reactive methoxy-silane-functionalized QAS compound with a long hydrophobic chain residue from castor oil (CO) (Si-QRC). Besides the hydrophobic effect on antibacterial activity, the proven analgesic and anti-inflammatory properties of castor oil and its primary derivative, ricinoleic acid, on wound-healing applications also inspired us to use CO as a source of a long hydrophobic chain for the synthesis of Si-QRC.Theoretically, combining the electrostatic interaction of positive ammonium moieties with the membrane-destabilizing property of a long hydrophobic chain originating from castor oil could enhance the biocidal activity of the newly designed QAS compound. Embedding the designed QAS component in the bioactive and hydrophilic gelatin-based network is also believed to provide potent antimicrobial activity while minimizing toxicity. Gelatin is an inexpensive biopolymer that has been widely used in wound treatment because of its hemostatic properties, excellent biocompatibility, low immunogenicity, and high wound-exudate-absorbing capacity and facilitating cell adhesion migration and proliferation at the wound site.

1H NMR spectra of (a) castor oil, (b) TACO, and (c) Si-CAQ compounds.

Nevertheless, providing the necessary tensile strength and occlusive characteristics by membranes made of gelatin is not an easy task because of its structural rigidity, low flexibility, rapid in vivo degradation by proteases, and considerable weakening of the overall mechanical strength within exudate absorption. On the other hand, hydrogels made from biopolymers like gelatin are susceptible to bacteria and cannot protect the damaged area from pathogenic bacteria. The integration of gelatin with a synthetic hydrogel and an organic–inorganic polymeric network through sol–gel and condensation reactions is considered in the present work to address shortcomings of gelatin-based films. Poly(vinyl alcohol) (PVA) was chosen as a typical synthetic hydrogel because its unique structural properties offer excellent exudate absorption capacity, sufficient elasticity, and the ability to participate in the sol–gel reaction. Its several advantages include effective in situ generations and distribution of silica domains within the polymeric matrix, aqueous media reaction at low temperature, and the availability of various hydrolyzable alkoxysilane components, which inspired us to choose sol–gel chemistry. On the other hand, introducing inorganic siloxane domains into the polymer could enhance the mechanical properties of polymeric matrices.

Schematic Illustration of Dressing Preparation

A facile, green, and environmentally friendly procedure was developed in this work to prepare multifunctional gelatin-based hydrogel films. First, the Si-CAQ as a bactericidal agent was synthesized by a two-step reaction from castor oil. A novel nonleaching and long-lasting antibacterial dressing with outstanding wound-exudate-handling performance, superior mechanical properties, and excellent cytocompatibility was prepared by the simultaneous sol–gel hydrolysis condensation reaction of the mixture containing gelatin, Si-CAQ, (3-glycidyloxypropyl) trimethoxysilane, and PVA. It was deduced that the hybrid organic–inorganic gelatin–PVA–siloxane network made through the sol–gel technique could support the essential cellular functions by gelatin, viscoelastic properties by the PVA–siloxane domain, and the potent nonreleasing antibacterial activity by embedded Si-CAQ.

Cross-section image of the XGP-SiQ30 sample showing the distribution of silicon and chloride.

Tensile testing results of dressings under (a) dry and (b) hydrated conditions.
Contact-active biocidal activity of the prepared dressings against (a) B. subtilis and (b) E. coli.
Growth of bacteria in the presence of leachates extracted from (a) XGP-SiQ0, (b) XGP-SiQ15, and (c) XGP-SiQ30 against E. coli.

(a) Proliferation of fibroblast cells after 48 h of direct contact with the dressing samples. (b) Hemolysis ratios of XGP-SiQ0, XGP-SiQ15, and XGP-SiQ30 samples. The difference between quantities with similar superscripts is not significant (P < 0.05).

Morphology of stained proliferated fibroblast cells on the standard TCP, XGP-SiQ0, and XGP-SiQ30 dressings. Cell nuclei were stained with DAPI; F-actin was stained with Alexa Fluor 488-phalloidin.

In vitro scratch healing: fibroblast cell migration by optical microscopy on TCP, XGP-SiQ0, and XGP-SiQ30 in three different time points.

(a) Photographs and (b) wound closure rate of wounds treated with gauze (control), 3M dressing, XGP-SiQ0, and XGP-SiQ30 for 3 weeks.

Masson’s trichrome staining of the treated wounds at 14 and 21 days post wounding. Scale bars indicate 200 μm.

  1. Antibacterial and Biocompatible Hydrogel Dressing Based on Gelatin- and Castor-Oil-Derived Biocidal Agent Reza Gharibi, Ali Shaker, Alireza Rezapour-Lactoee, and Seema Agarwal ACS Biomaterials Science & Engineering Article ASAP DOI: 10.1021/acsbiomaterials.1c00706