Inflammation is the immune system's multifaceted biological reaction to infection and tissue injury [1]. Additionally, inflammatory responses are necessary for the healing of tissues and wounds [2], but the inflammatory response is a double-edged sword. The inflammation status is modulated by a dynamic equilibrium between pro- and anti-inflammatory cytokines. A disturbance in the balance may cause damage to the body [3,4]. Macrophages play pivotal roles in inflammation, they can regulate inflammation processes by alteration of cellular polarization to various phenotypes [5]. The strict regulation of macrophage M1/M2 polarization involves various signaling pathways and regulatory networks. Understanding macrophage polarization provides insights into their functional changes during inflammation and underlying molecular mechanisms, offering theoretical foundations for treating inflammatory diseases.

Nowadays, biomaterials have aroused wide public concern for their remarkable physicochemical and structural properties, making them ideal for facilitating tissue repair and regeneration [6]. Macrophages are the initial immune cells to reach wound sites and engage with biomaterials, exhibiting heterogeneity and plasticity [7]. They possess the capacity to either inhibit or facilitate inflammation, contingent upon the secretion and profile of cytokines [8]. Responding to the local microenvironment or stimulus, macrophages can polarize into two main phenotypes: M1 type (classically activated) and M2 type (selectively activated) [9]. Additionally, Toll-like receptor (TLR) agonists like lipopolysaccharide (LPS) or Th1-type cytokines like IFN-γ may trigger M1 production [10]. The involvement of M1 cells is of utmost importance during the initial stage of the inflammatory response. They have the ability to absorb foreign pathogens early in the inflammatory response, release pro-inflammatory chemicals, stimulate T-cells, regulate Th1 immune response, and exacerbate initial inflammatory. In the final stages of inflammation, M2 cells, primarily stimulated by IL-4 or IL-13, can release anti-inflammatory molecules, control the Th2 immune response, and contribute in tissue repair process. The balance of the macrophage M1-M2 polarization is crucial when inflammation or injury occurs [11]. Once the infection or inflammation is sufficiently enough, more unpolarized macrophages will differentiate into M1 state and release pro-inflammatory factors to counteract the stimulus. Conversely, M2 macrophages release a variety of anti-inflammatory substances to reduce inflammation and promote tissue remodeling, revascularization, and other beneficial processes [12]. Controlling macrophage polarization towards M2 type has demonstrated significant efficacy in suppressing inflammation and facilitating tissue regeneration [13]. Nevertheless, In terms of immuno-engineering for biomaterials, regulating macrophage polarization phenotype towards M2 still remains a challenge [14]. Therefore, the design of immunomodulatory biomaterials is vital for effectively regulating macrophage polarization, thereby providing notable benefits in the progression of chronic wound healing towards its typical state.

Hydrogels, especially those incorporating hyaluronic acid (HA), a natural polysaccharide and a key component of the extracellular matrix, have attracted significant interest in wound healing. They maintain a humid environment, absorb wound exudates, and protect against pathogen colonization [14,15]. Despite these advantages, HA hydrogels often exhibit poor mechanical properties, rapid degradation [16] and limited activity [17]. Combining HA with other materials, such as polyphenols [[18], [19], [20]], can enhance these properties. Dopamine, a polyphenolic organic compound, with roles in several inflammatory and immune-related disorders, also promotes wound healing by scavenging reactive oxygen species, reducing inflammation, and promoting neovascularization [[21], [22], [23], [24], [25]]. However, the impact of HA-based hydrogels on inflammation and wound healing mechanisms, particularly the immunomodulatory effect of dopamine-modified HA hydrogels on macrophage cytokine expression was poorly investigated. Elaborating macrophages polarization modulated by dopamine concentration especially the mechanism in signaling pathways could provide guidance for the application of dopamine in biomaterials.

In this study, we propose a facile method to remedy mechanical strength defects of HA-based gels. Our aim is to reveal how HA-DA regulates RAW264.7 polarization and immune response. The hydrogel network initially comprises methacrylate-modified HA (HAMA), further modified by dopamine. A secondary crosslinking network forms with a small amount of oxidizing agent under alkaline conditions. The study explores macrophage polarization behaviors influenced by dopamine concentrations in vitro and in vivo and delves into the mechanisms of inflammatory regulation, particularly signaling pathways (Scheme 1). This study synthesizes a double network hydrogel combining hyaluronic acid and oxidized dopamine and elaborates on the regulation of macrophage polarization through the NF-κB signaling pathway depending on dopamine concentrations, paving the way to understand inflammation mechanisms based on dopamine-modified biomaterials.