The skin has always been the body's primary defense against injury and microbial infections, playing a vital role in maintaining overall health. However, the skin is also the most susceptible and exposed tissue in the human body [1,2], making it prone to various injuries such as trauma, burns, and scalds. If left untreated or not treated appropriately, these wounds can lead to severe complications including infection, tissue decay, and in extreme cases, death [[3], [4], [5]]. Among other things, bacterial-induced wound infections pose a great threat to healing process, as they can lead to complications in the wound healing process [6]. However, the misuse of antibiotics has resulted in the emergence of drug-resistant pathogens, making it crucial to develop alternative treatments [7]. To address this issue, there have been efforts to develop novel antimicrobial materials and advanced therapeutic methods in biomedical research.

Among the various materials used in the biomedical field, hydrogels have proven to be ideal for wound dressings [[8], [9], [10]]. Polyvinyl alcohol (PVA) is one such widely used material due to its biodegradable, non-toxic, biocompatible, and hydrophilic properties [[10], [11], [12]]. However, most PVA-based hydrogels lack antibacterial activity and have poor mechanical properties, limiting their wide application [13]. To enhance the mechanical properties of PVA hydrogels, composite hydrogels are often formed by creating directional structures. It was reported that researchers employed the bi-directional freezing method to create aerogels with bi-directional structures ordered in both vertical and radial directions [14,15]. These aerogels exhibit superior mechanical properties compared to aerogels with disordered pore structure and unidirectional structure [14]. Additionally, it was reported that macroporous hydrogels not only had excellent swelling properties for better absorption of wound secretions and reduction of infections, but also promote cell proliferation, angiogenesis, and collagen deposition, significantly aiding in wound healing [16,17].

Furthermore, to address the lack of antibacterial activity of PVA-based hydrogels, other antibacterial components could be incorporated to enhance the antibacterial activity of the hydrogel itself [18,19]. Chitosan quaternary ammonium salt (CS) is a chitosan derivative with good antibacterial activity. It also promotes tissue adhesion, as well as the adhesion and activation of blood cells and platelets, thereby accelerating hemostasis and wound healing [20,21]. Incorporation of CS into PVA can imbue them with these properties, enhancing their inherent antibacterial activity. In addition, antibacterial agents could be loaded into the hydrogel to further enhance the antibacterial properties of the hydrogel [22,23].

In recent years, graphene and its chemical derivatives have emerged as a new class of nanomaterials and have been widely used in biomedical applications [[24], [25], [26]]. This material not only possesses remarkable qualities such as excellent electrical conductivity, biocompatibility, high surface area, and mechanical strength [27], but also has excellent photothermal conversion capabilities [28]. Some studies have reported that biomaterials with electrical conductivity could play a facilitating role in the wound healing process [[29], [30], [31]]. The high specific surface area also showed specific interaction modes with biomolecules, cells, and even human tissues to enhance the bioactivity of the hydrogel [27]. Compared to graphene, graphene oxide (GO) showed better water dispersion due to its hydrophilic groups, such as hydroxyl, epoxy, and carboxyl groups, and the surface of GO could also inhibit the growth of bacteria [32,33]. One approach to forming reduced GO (rGO) involves the self-polymerization of dopamine (DA) through an oxidative reaction in an alkaline solution (pH = 8.5), resulting in a layer of polydopamine (PDA) encapsulating its surface [26,34]. PDA not only enhances the hydrophilicity of rGO [35], but also possesses excellent photo-thermal conversion ability [36] and biological activity. The abundant catechol groups on its surface allow for strong binding to various organic and inorganic surfaces (e.g., metals, metal oxides, and polymers), which is favorable for the growth of chemical inhibitors [37], such as MOFs. Metal-organic skeleton materials (MOFs), specifically Zeolite Imidazolate Framework-8 (ZIF-8), are a type of bacteriostatic agent [38]. These materials could slowly store and release metal ions such as zinc, copper and cobalt ions and thus used as biocides [39,40]. Zeolite Imidazolate Framework-8 (ZIF-8), as a prominent member of MOFs [41,42], not only exhibited bactericidal and anti-inflammatory properties [40], but the slowly released zinc ions also accelerated wound healing by promoting cell migration, angiogenesis and collagen deposition [39].

In this paper, to enhance the sustained antimicrobial ability of rGO-PDA, ZIF-8 crystals were grown on the surface of rGO-PDA to prepare rGO-PDA@ZIF-8 nanocomposites, taking advantage of the metal chelating ability of catechol groups present on the PDA surface. In addition, composite hydrogels of PVA/CS with directional macroporous structures were prepared by using PVA and CS as the matrix materials of the hydrogels, and polyethylene glycol (PEG) as the pore-forming agent. A bidirectional freeze-casting method was employed to introduce a bidirectional structure. To further enhance the antimicrobial properties, rGO-PDA@ZIF-8/PVA/CS composite hydrogels with directional macroporous structures were prepared by loading rGO-PDA@ZIF-8 as an antibacterial nanofiller on PVA/CS composite hydrogels. The combination of these antimicrobial agents resulted in excellent antibacterial properties, with even 99 % antibacterial efficacy. Moreover, the directional macroporous structure, prepared by pore-forming agent and bidirectional freeze-casting method, not only enhanced the mechanical properties of the hydrogel but also exhibited desirable swelling and water retention rates, which facilitated the absorption of tissue exudate, creating a favorable environment for antibacterial properties and wound healing promotion.