Skin regeneration axis under metal-based nanozymes: from chronic wound healing to structural and functional restoration

Graphical abstract

Highlights

• •Material selection and fabrication strategies of metal-based nanozymes are summarized.
• •The potential of metal-based nanozymes in full-process skin regeneration is interpreted.
• •Future clinical translation of metal-based nanozymes for skin regeneration is discussed.

Abstract

Background

Diabetic infected wounds, representing a prototypical model of chronic non-healing wounds, leading to severe impairment of skin regeneration. However, complete skin restoration requires more than just wound closure; it crucially involves the inhibition of pathological scarring (fibrosis) and the functional regeneration of skin appendages (e.g., hair follicles). Metal-based nanozymes (MNZs), due to their highly stable, tunable enzyme-mimicking catalytic activity and excellent integration potential, have emerged as promising candidates, with initial studies demonstrating their utility in these broader regenerative aspects. But existing reviews primarily focus on MNZs’ role in basic wound healing, overlooking the structure and functional regeneration potential of skin under MNZs intervention.

Aim of review

This review addresses the knowledge gap by analyzing the recent literature to establish a comprehensive “Skin Regeneration Axis” for MNZs application. Specifically, the objective is to integrate and evaluate MNZs’ potential across the three essential dimensions of full-process skin repair: chronic wound healing, structural pathological scar inhibition, and functional appendage regeneration. This integration fully reports MNZs’ potential for holistic skin restoration.

Key scientific concepts of review

This review summarizes the fundamentals of MNZs, including their catalytic mechanisms, classifications, and synthetic strategies. It articulates a multidimensional regenerative paradigm for MNZs, elucidating their roles across the entire spectrum of skin regeneration, which constitutes the core concepts of this review: (1) Chronic Wound Healing via controlling inflammation and oxidative stress, (2) Pathological Scar Inhibition through targeted molecular pathway regulation, and (3) Structural Appendage Regeneration (e.g., hair follicles). The review also highlights critical translational challenges and proposes integrated strategies for accelerating the clinical application of MNZs for holistic skin repair.

Introduction

The skin, as the largest organ of the human body, serves as a protective barrier against external pathogens. Both acute and chronic injuries can disrupt this barrier, resulting in traumatic skin defects. In recent years, the incidence of skin wounds, particularly non-healing chronic wounds, has been steadily rising, posing a serious global public health challenge. Diabetic foot ulcers (DFUs) represent one of the most common forms of chronic wounds. Globally, more than 18 million new cases of DFUs are reported annually, severely compromising patients’ quality of life and imposing a substantial economic burden on healthcare systems [1]. The lifetime risk of developing DFUs in individuals with diabetes is estimated at 19%-34%, with recurrence rates reaching as high as 65% within 3–5 years, and lifetime amputation rates approaching 20% [2]. Chronic wound persistence is typically associated with sustained infection, excessive inflammation, and impaired tissue regeneration [3]. In normal healing, the inflammatory phase rapidly transitions into the proliferative and remodeling phases. By contrast, chronic wounds remain arrested in the inflammatory stage, characterized by massive neutrophil infiltration and persistent overexpression of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6), which in turn suppress macrophage M2 polarization and tissue repair functions [4]. Moreover, wounds are highly susceptible to secondary bacterial infections, particularly by Staphylococcus aureus and Pseudomonas aeruginosa, which can form biofilms on the wound surface. These biofilms resist host immune defenses and antimicrobial therapies, thereby exacerbating tissue damage and chronic inflammation, and constituting a major barrier to effective treatment. In addition, diabetic wounds display a distinctive pathological microenvironment, including hyperglycemia-induced endothelial dysfunction, impaired circulation, heightened oxidative stress, and accumulation of advanced glycation end products (AGEs), which damage the extracellular matrix (ECM) [5]. These pathological changes delay angiogenesis and granulation tissue formation, while inhibiting fibroblast and keratinocyte migration and proliferation, ultimately impeding wound closure. To date, no clinical treatment has been able to simultaneously address infection control, inflammatory regulation, and tissue regeneration in such wounds.

Although certain wounds can achieve closure under therapeutic interventions, in most cases they fail to fully restore structure and function, ultimately resulting in scar tissue replacing normal skin. This incomplete regeneration arises primarily from two factors: first, excessive fibrosis disrupts the dermal architecture and mechanical properties [6]; second, regeneration of skin appendages (such as hair follicles, sebaceous glands, and sweat glands) is impaired, leading to the loss of essential physiological functions [7]. Scar formation is driven by aberrant fibroblast activation, imbalanced collagen deposition, and dysregulated matrix remodeling. During normal healing, the ratio of type I to type III collagen remains dynamically regulated and gradually returns to normal alignment during the remodeling phase. In contrast, under chronic inflammatory conditions, transforming growth factor (TGF)-β1 persistently activates the SMAD signaling pathway, inducing fibroblasts to overproduce type I collagen and fibronectin while suppressing matrix metalloproteinase (MMP) expression, thereby causing disordered collagen deposition and alignment [8]. Concurrently, myofibroblasts exhibit abnormal proliferation and sustained expression of α-smooth muscle actin (SMA), resulting in increased tissue tension and contracture, which further compromises the biomechanical stability and appearance of skin [6]. On the other hand, regeneration of skin appendages depends on specific stem cell populations-including hair follicle stem cells, dermal papilla cells (DPCs), and basal keratinocytes—as well as precise regulation of their microenvironment. However, in chronic inflammatory states, these cells fail to become effectively activated due to microenvironmental disruption, stem cell exhaustion, or dysregulation of key signaling pathways such as Wnt/β-catenin, Shh, and Notch, ultimately leading to impaired regeneration of hair follicles, sebaceous glands, and other structures [9]. Moreover, the absence of a dermal-epidermal junction in scar tissue hinders the integration of nerves, blood vessels, and skin appendages, aggravating functional deficits. Therefore, restoration of skin function relies not only on wound closure but also on the hierarchical reconstruction of its tissue architecture and appendageal systems, a challenge that continues to drive regenerative medicine toward multidimensional pro-regenerative strategies.

In recent years, metal-based nanozymes (MNZs) have emerged as robust candidates for skin regeneration owing to their tunable enzyme-mimicking activities. Composed of metal elements or oxides (e.g., CeO₂, Fe₃O₄, MoS₂), MNZs mimic natural enzymes such as peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT) to modulate critical biochemical microenvironments [10], [11]. Unlike conventional therapies, MNZs facilitate holistic repair by concurrently scavenging reactive oxygen species (ROS), eliminating bacterial biofilms, and regulating key cellular behaviors including angiogenesis and stem cell differentiation [12], [13], [14], [15], [16], [17]. These properties allow MNZs to drive the regenerative process from basic wound closure to complex structural and functional restoration (Scheme 1), with their comparative advantages over other nanozymes detailed in Table 1. Although recent reviews have discussed nanozymes in wound healing [13], [18], [19], a comprehensive summary focusing specifically on MNZs and their impacts across the full process of skin regeneration-particularly addressing both structural and functional restoration-remains lacking.

Scheme 1.

Scheme of therapeutic modalities of metal-based nanozyme in skin regeneration.

Table 1.

Comparison of different categories of nanozymes.

Property	Metal-Based Nanozymes	Carbon-Based Nanozymes	MOF-Based Nanozymes
Material Composition	Metal elements/Alloys/Oxides/Sulfides	Carbon materials	Metal-organic frameworks (Metal nodes + Organic linkers)
Catalytic Activity	High	Moderate (dependent on surface functionalization)	Tunable (dependent on metal nodes and pore structure)
Stability	Oxides/Sulfides > Metal elements (oxidation resistance)	Excellent (acid/alkali and high-temperature resistance)	Variable (some sensitive to moisture/acid)
Biocompatibility	Can be surface-modified	Excellent (e.g., low toxicity of carbon dots)	Degradable (some exhibit good biocompatibility)
Synthesis Cost	Moderate to High (costly noble metals)	Low (inexpensive carbon sources)	Moderate (cost of organic linkers)
Catalytic Mechanism	Metal valence state change / Surface electron transfer	Surface functional groups (e.g., –OH, –COOH)	Metal nodes + Confinement effect within pores
Tuning Strategies	Size/Shape/Crystal facet control	Doping or surface functionalization	Linker design / Pore size engineering
Environmental Robustness	Oxides resistant to high temp./acid/alkali	Highly tolerant	Variable (some susceptible to hydrolysis)

Thus, this review provides a comprehensive overview of recent advances in the application of MNZs for diabetic infected wounds, pathological scarring, and skin appendage regeneration, with an emphasis on their therapeutic potential across the continuum from wound closure to functional and structural skin restoration. We begin by outlining the commonly employed strategies for MNZ synthesis and material design, discussing how different metal cores and fabrication approaches influence their performance, along with their respective strengths and limitations. The review then categorizes MNZs by metal type and summarizes their distinct roles in infection control, scar regulation, and the regeneration of appendages, highlighting the mechanisms through which they contribute to both structural and functional repair. Building on this foundation, we also analyze the current obstacles in translating MNZs into clinical practice and propose directions for the design of next-generation MNZ-based therapies. Using diabetic infected wounds as a representative model of refractory healing, this review underscores the multifaceted regulatory capacity of MNZs across the “wound healing-scar modulation-hair regeneration” trajectory, offering both conceptual insights and forward-looking perspectives to guide their future development in skin regenerative medicine.

Synthesis of metal-based nanozymes

Classification

MNZs exhibit distinct advantages in skin regeneration, yet their therapeutic potential varies depending on their ability to navigate complex pathological microenvironments. According to different classification schemes, MNZs can be categorized into several types, each playing unique roles in chronic wound healing, anti-scarring, and the regeneration of skin appendages (Table 2).

Table 2.

Classification of metal-based nanozymes.

Classification Basis	Subcategory	Representative Materials	Characteristics	Mimicking Enzyme Activity
By Chemical Form	Metal elements	Au, Ag, Pt, Fe	-Catalytic -Noble metals offer high stability	-GOx -OXD
	Metal oxides	Fe3O4, CuO, CeO2, MnO2	-Metal valence state changes -pH/ROS responsiveness	-POD -CAT -SOD
	Metal sulfides/selenides	MoS2, CuS, CoSe2	-Enhanced stability via S/Se coordination -Some exhibit photothermal effects	-POD -GPx
	Metal phosphates/carbonates	Cu2(OH)PO4	-Phosphate/carbonate groups enable slow release of metal ions -Stable structure	-OXD -POD
	Metal halides	AgCl, CuI	-Halogen ions modulate solubility	-OXD
By Metal Element	Transition metals	Fe3O4, CuO	-Flexible valence states -High catalytic activity	-POD -OXD -CAT
	Noble metals	Au, Ag, Pt	-Catalysis dominated by surface effects -High stability	-GOx -OXD -SOD
	Rare-earth metals	CeO2	-Antioxidant/anti-radiation	-SOD -CAT
	Alkaline-earth metals	MgO	-Auxiliary catalysis -pH modulation	/
By Structure	Single-metal	Fe3O4, CeO2	-Single active site -Simple synthesis	-POD -CAT
	Bimetallic/Alloys	Fe-Co, Au-Ag	-Synergistic effects	-Multi-enzyme activity
	Single-atom nanozymes	Fe-N-C, Cu–N-C	-Atomically dispersed	-Highly specific catalysis
By Catalytic Function	POD-mimics	Fe3O4, MoS2	H2O2 → ·OH (Strong oxidizing activity)	-POD
	OXD-mimics	CuO, Au	O2 → ROS (Antibacterial properties)	-OXD
	CAT-mimics	MnO2, CeO2	H2O2 → H2O + O2 (Alleviates hypoxia)	-CAT
	SOD-mimics	CeO2, Co3O4	·O2-→ H2O2 + O2 (Antioxidant)	-SOD
	GPx-mimics	CoSe2	-Reduces ROOH -Consumes GSH	-GPx
Special Composite Types	Metal-Carbon Hybrids	Fe3O4-Graphene, Cu–N-C	-Carbon support enhances conductivity	-Multi-enzyme synergy
	Metal-Organic Coordination Polymers	Fe-Gallic acid, Cu-Amino acid	-Organic ligands modulate active centers -Responsiveness	-Dynamic catalysis
	Biomineralized Materials	Ferritin-Fe3O4	-Biomolecule-guided synthesis -High biocompatibility	-Targeted catalysis

GOx: Glucose oxidase; OXD: Oxidase; POD: Peroxidase; CAT: Catalase; SOD: Superoxide dismutase; GPx: Glutathione peroxidase.

From a chemical composition perspective, elemental metals such as Ag nanoparticles exert antibacterial effects through surface catalytic activity, providing a sterile environment for wound sites [20]; metal oxides such as Fe₃O₄ and CeO₂ modulate oxidative stress via valence state changes, accelerating re-epithelialization in chronic wounds [21], [22]; metal sulfides, such as copper sulfide, leverage photothermal therapy (PTT) to promote angiogenesis [23]. Classified by metal element, transition metal-based nanozymes (e.g., copper oxides) catalyze ROS generation through dynamic valence switching, regulating the inflammatory microenvironment and suppressing fibrosis progression [24]; noble metals such as Au nanoparticles activate growth factor signaling pathways to promote appendage regeneration, including hair follicles [25]; rare-earth metal-based materials (e.g., CeO₂) protect newly formed tissue from oxidative damage through antioxidant properties, while alkaline-earth metals such as calcium carbonate deliver calcium ions in a pH-responsive manner to support epidermal barrier reconstruction [26]. From the perspective of catalytic functionality, POD-mimetic enzymes reduce oxidative damage by decomposing excess H₂O₂ in wounds [27], CAT-mimetic enzymes alleviate tissue hypoxia and facilitate fibroblast migration [28], and SOD-mimetic enzymes suppress scar formation by scavenging superoxide radicals [21]. Specialized composite nanozymes, such as metal–organic coordination polymers (e.g., Cu-amino acid complexes), release copper ions in response to the wound microenvironment, synergistically promoting antibacterial activity and collagen remodeling [29]; biomineralized materials (e.g., ferritin-loaded Fe₃O₄) exploit bio-guided targeting to injured sites, enabling functional regeneration of appendages such as sweat and sebaceous glands [30]. Collectively, these properties position MNZs as multifunctional tools for achieving full-thickness skin regeneration and functional restoration.

Metal selection

Copper

The choice of metal in MNZs critically determines their catalytic activity and biomedical functions. Different metal-based nanozymes promote regeneration through distinct catalytic mechanisms and biological effects (Table 3).

Table 3.

Advantages and disadvantages of metal of nanozymes in skin regeneration applications.

Metal	Advantages	Disadvantages
Zn	-Activates metalloenzyme systems to promote epithelial regeneration -Immunomodulation improves the inflammatory microenvironment -Long-lasting antibacterial efficacy	-Excess Zn2+ can inhibit macrophage function -May induce contact dermatitis
Cu	-Broad-spectrum antibacterial activity -Promotes angiogenesis -Regulates macrophage polarization -Epigenetic regulation facilitates regeneration	-Binding to serum proteins may lead to systemic toxicity -Long-term use can exacerbate oxidative stress
Fe	-Induces ferroptosis to clear senescent cells -Dynamically modulates ROS levels -Inhibits TGF-β signaling to reduce scarring	-Iron overload can trigger lipid peroxidation -Blood contact carries risk of thrombosis
Ag	-Synergistic photothermal-catalytic antibacterial action -Sustained release of Ag+ prolongs therapeutic duration	-Dermal accumulation may cause argyria (skin discoloration) -Inhibits mitochondrial function in keratinocytes, delaying re-epithelialization
Mg	-Promotes barrier reconstruction -Degradation products help modulate pH -Facilitates hair follicle regeneration	-Rapid degradation can cause localized sharp pH increase -Rare risk of emphysema-like complications
Mn	-Alleviates hypoxia -Scavenges superoxide radicals -Inhibits scar contracture	-Potential neurotoxicity risk -Can cause MRI artifacts
Ce	-Exhibits self-regenerating antioxidant activity	-Potential hepatic accumulation -Lacks long-term safety data

Copper (Cu)-based nanozymes exert unique effects in wound repair owing to the redox properties of copper ions. The Cu⁺/Cu²⁺ valence cycling endows them with both POD-and SOD-like activities [31], enabling catalysis of H₂O₂ to generate ·OH for biofilm bacterial eradication, while concurrently scavenging excess ·O₂^-to mitigate oxidative stress. Copper ions also promote angiogenesis by upregulating the hypoxia-inducible factor-1 (HIF-1)/VEGF pathway [32], [33], and may enhance collagen crosslinking through activation of lipoxygenase (LOX) [34], thereby improving the mechanical strength of newly formed tissue. Notably, Cu-based nanozymes can precisely modulate macrophage polarization toward the M2 phenotype, converting the pro-inflammatory state of chronic wounds into a pro-regenerative environment [35]. Additionally, Cu may influence appendage regeneration through epigenetic regulation, such as histone deacetylase (HDAC)-mediated modulation of stem cell function [36]. However, copper ions readily bind to serum proteins, posing risks of systemic toxicity [37], and prolonged exposure may exacerbate oxidative stress via Fenton reactions, potentially impairing tissue repair.

Zinc

Zinc (Zn)-based nanozymes are widely applied in skin regeneration. As an essential trace element in the human body, Zn²⁺ participates in protein synthesis and cell proliferation by activating metalloproteinase systems [38]. Zn-based nanozymes, such as ZnO nanoparticles, not only enhance platelet aggregation to accelerate hemostasis [39], but also modulate zinc finger protein expression to inhibit the nuclear factor κB (NF-κB) pathway [40], markedly reducing pro-inflammatory cytokines including tumor necrosis factor-α (TNF-α) and interleukin (IL)-6 [41], thereby improving the inflammatory microenvironment of chronic wounds. Released Zn²⁺ can disrupt bacterial membrane potential and, in combination with catalytically generated ROS, efficiently eliminate drug-resistant bacteria such as Staphylococcus aureus [42]. More importantly, zinc ions may regulate fibroblast growth factor (FGF) and insulin-like growth factor (IGF)-1 signaling pathways [43], [44], possibly promoting keratinocyte migration and stimulating collagen III synthesis, ultimately reducing scar formation. However, excessive Zn²⁺ can impair macrophage phagocytic function, delaying infection clearance [45]. Additionally, some patients may develop contact dermatitis upon zinc application, which is associated with zinc-induced metal hypersensitivity reactions [46].

Iron

Iron (Fe)-based nanozymes can promote regeneration through a dual mechanism involving ferroptosis and enzymatic catalysis. The Fe³⁺/Fe²⁺ redox cycle confers both CAT-and POD-like activities [47], dynamically regulating ROS levels in hypoxic wounds-simultaneously scavenging inflammation-associated radicals and generating bactericidal ·OH at infection sites. Iron ions also induce ferroptosis in senescent cells via the Fenton reaction, thereby creating space for tissue regeneration [48]. Recent studies indicate that Fe can activate the Nrf2/ARE pathway to enhance cellular antioxidant capacity [49], [50], and suppress myofibroblast activation by modulating the TGF-β/Smad signaling pathway [51], [52], ultimately reducing hypertrophic scar formation. However, iron overload can catalyze lipid peroxidation, and exposure to blood may increase thrombosis risk [53], [54], warranting caution in patients with venous ulcer-type wounds.

Silver

Silver (Ag)-based nanozymes combine the intrinsic antibacterial properties of silver ions with the catalytic functions of nanozymes. Ag⁺ disrupts the bacterial respiratory chain by binding to thiol groups [54]. Assisted by PTT, these nanozymes can penetrate biofilms to eradicate persistent infections, while the controlled release of silver ions may modulate TGF expression, potentially influencing wound remodeling and scar formation [55]. Moreover, low-dose Ag⁺ can activate Notch signaling to promote the proliferation of hair follicle progenitor cells [56], offering possibilities for appendage regeneration. However, silver nanoparticles tend to accumulate in the dermis, potentially causing localized gray-blue skin discoloration [57]. Excessive silver can also impair keratinocyte mitochondrial function, delaying re-epithelialization [58], [59]. Therefore, dosage and safety remain critical limitations for their clinical application.

Magnesium

Magnesium (Mg)-based nanozymes participate in the entire regenerative process through ionic regulation. Mg²⁺, as an essential cofactor for ATPases, directly promotes fibroblast proliferation and migration [60]. The alkaline microenvironment generated during its degradation can neutralize the acidic pH of chronic wounds, while released magnesium ions mitigate tissue damage by inhibiting NOD-like receptor protein 3 (NLRP3) [61]. Moreover, Mg can modulate the Wnt/β-catenin pathway [62], offering significant potential for hair follicle regeneration; it also enhances epidermal barrier function by regulating the transient receptor potential cation channel subfamily M member 7 (TRPM7) ion channel [63], [64]. In endothelial cells exposed to magnesium alloys, Yes-associated protein (YAP) exhibits pronounced transcriptional activity: YAP can bind to promoter and enhancer regions of IL-6 to influence its expression, magnesium alloys affect mRNA methylation levels, and YAP regulates mRNA methylation-related factors such as METTL3 and METTL14, thereby modulating inflammatory gene expression [65]. These findings suggest a potential role for the Hippo-YAP pathway in magnesium-mediated inflammatory regulation, which may in turn influence scar formation. However, rapid degradation can lead to abrupt local pH elevation, and magnesium implant–associated gas formation has been reported in clinical cases [66], [67].

Manganese

Manganese (Mn)-based nanozymes offer unique advantages in addressing wound hypoxia. Their CAT-like activity decomposes H₂O₂ to generate oxygen [68], alleviating tissue hypoxia and promoting re-epithelialization. Simultaneously, the Mn²⁺/Mn³⁺/Mn⁴⁺ multivalence confers SOD-like activity [69], enabling the scavenging of superoxide radicals generated during ischemia–reperfusion injury. Studies have also shown that manganese ions can activate the cGAS-STING pathway, enhancing innate immunity [70]. However, manganese poses neurotoxicity risks at relatively low thresholds: occupational exposure studies report blood manganese concentrations ranging from 9-22.5 μg/L in exposed individuals, compared with an average of 11 μg/L (range 3.9–22.5 μg/L) in controls, indicating a potential risk of neurological symptoms at the upper end of this range [71]; patients with peripheral neuropathies require strict monitoring. Furthermore, local manganese precipitation may introduce artifacts during MRI examinations [72].

Others

Beyond the commonly used metals, rare-earth-based nanozymes, such as CeO₂ nanoparticles, exhibit antioxidant properties through Ce³⁺/Ce⁴⁺ self-cycling [73]. During the inflammatory phase, their SOD-like activity scavenges ·O₂^-to protect tissue, while in the proliferative phase, POD-like activity promotes angiogenesis. Moreover, multimetallic MNZs integrate multiple functionalities through intermetallic synergistic effects. For example, Zn²⁺/Fe³⁺ co-regulation can simultaneously enhance antibacterial activity and cell migration, whereas Cu⁺/Mn²⁺ combinations achieve a dynamic balance between ROS generation and scavenging. Such designs provide comprehensive solutions for refractory wounds, including diabetic foot ulcers, combining anti-infection, inflammation modulation, pro-angiogenesis, and regenerative stimulation. By precisely controlling metal valence, coordination environment, and release kinetics, MNZs are driving wound therapy from passive repair toward functional regeneration.

Fabrication method

Co-precipitation

MNZs can be synthesized via a variety of methods, each with distinct advantages and limitations; commonly used materials and techniques are summarized in Table 4. Co-precipitation is among the earliest methods applied for MNZ synthesis, typically involving the reaction of metal salts with precipitants (e.g., NaOH, NH₄OH) under controlled conditions to generate metal oxide nanoparticles in situ in aqueous or alcohol-water mixed solutions [74]. This method is simple to operate, uses inexpensive reagents, and is suitable for large-scale production; for instance, CeO₂ and Fe₃O₄ nanozymes are frequently prepared this way [75], [76]. Its advantages include low equipment requirements, environmental friendliness, and suitability for producing spherical or particulate materials. However, co-precipitation offers limited precision in particle size control, is prone to aggregation, and yields products with restricted crystallinity and dispersibility, making it challenging to achieve high surface area or complex architectures [74].

Table 4.

Synthesis methods for metal-based nanozymes.

Method	Common Raw Materials	Brief Procedure Description	Advantages	Disadvantages
Co-precipitation	Oxides of Ce, Fe, Zn, Mn, etc.	Metal salts react with a precipitating agent (e.g., NaOH) in a water/alcohol mixed solution, followed by centrifugation, washing, and drying to obtain nanoparticles.	-Simple operation -Low cost -Suitable for large-scale production -Environmentally friendly	-Poor particle size control -Prone to agglomeration -Limited crystallinity and dispersity -Difficulty constructing complex structures
Hydrothermal/Solvothermal	Oxides of Mn, Co, Fe, Zn, etc.	Metal precursors are crystallized in a sealed autoclave under high temperature and pressure (80-200°C) to form nanostructures with specific morphologies.	-High crystallinity -Controllable morphology -Applicable to diverse metal systems	-Long reaction time -High equipment requirements -Some solvents are toxic
Sol-Gel	Polymetallic oxides (Fe, Ce, Zr, etc.)	Metal alkoxides undergo hydrolysis and condensation to form a gel, which is then dried/calcined to obtain porous nanomaterials.	-Homogeneous composition -Tunable porosity -Suitable for polymetallic doping -High specific surface area	-Expensive precursors -Humidity sensitivity -Complex process -Prone to cracking during drying
Single-Atom Dispersion	Fe, Cu, Co, etc.	Metal precursors are coordinated with a support (e.g., N-doped carbon) followed by thermal treatment to form atomically dispersed active sites.	-100% atom utilization -High catalytic selectivity -Designable multifunctional sites	-Stringent synthesis conditions -Low yield -Difficult characterization -Challenging to scale up
Microwave-Assisted	Pt, Ag metal sulfides, etc.	Microwave irradiation rapidly heats the reaction system, completing nucleation and growth within minutes to yield small-sized nanoparticles.	-Rapid reaction -Energy-efficient -Uniform particle size -Suitable for batch production	-Non-uniform heat distribution -Strict dielectric property requirements -Complex scale-up process
Template-Assisted	Hollow structures (Ni, Cu, ZnO, etc.)	Soft/hard templates (e.g., SiO2) guide metal deposition; subsequent template removal yields porous or core–shell structures.	-Precise morphology control -High specific surface area -Enhanced mass transport	-Difficult template removal -Potential residual impurities -High process cost

Hydrothermal/Solvothermal method

The hydrothermal/solvothermal method promotes reactions in a high-temperature, high-pressure sealed environment, enabling the preparation of metal nanostructures with controllable morphology and high crystallinity, particularly suitable for synthesizing oxide nanozymes with complex shapes such as nanorods, nanosheets, or nanoflowers [11], [77]. For example, manganese and cobalt oxide nanozymes are often synthesized using this approach to achieve high catalytic activity. Advantages of this method include excellent crystal integrity, strong reaction controllability, and compatibility with a variety of metal precursors. However, its limitations include long reaction times (typically several hours to over ten hours), high demands on equipment such as autoclaves, and potential environmental or safety concerns associated with certain solvents or byproducts [11], [77].

Sol-gel method

The sol–gel method is based on the hydrolysis and condensation of metal alkoxides or metal–organic complexes to form a three-dimensional network gel, which is subsequently dried and calcined to yield metal oxide nanostructures [78]. This approach allows for uniform doping of material components and is particularly suitable for synthesizing multimetallic co-doped nanozymes, commonly applied to Fe-, Ce-, and Zr-based materials [76]. Advantages of the sol–gel method include high molecular-level uniformity, facile control over nanoporous structures, and suitability for producing porous materials with large surface areas. However, the method has limitations such as relatively high cost, strong sensitivity of precursors to humidity, complex operational procedures, and the tendency for shrinkage during processing to induce cracks [78], [79].

Single-atom synthesis

Single-atom synthesis aims to stably anchor metal ions onto specific supports, such as N-doped carbon or graphene, forming catalytic sites with high atomic utilization [80]. Common strategies include coordination complexation followed by thermal treatment, in situ adsorption, and chemical vapor deposition. For instance, MN₄-type structures have been widely applied to modulate inflammation and ROS levels in infected wounds [81]. Advantages of this approach include maximized metal utilization, high catalytic selectivity, and the ability to design multifunctional active centers. However, it suffers from stringent synthesis conditions, low yield, challenges in structural characterization, and limited stability, necessitating further optimization for scalable production [82].

Microwave-assisted synthesis

Microwave-assisted synthesis employs rapid and uniform microwave heating to significantly shorten reaction times and improve crystal quality, commonly used for preparing metal oxide and metal sulfide nanozymes [83]. Reactions typically proceed within minutes, allowing for rapid construction of nanoparticles with narrow size distributions and good dispersibility. Advantages include fast heating rates, energy efficiency, environmental friendliness, and suitability for batch production. Limitations arise from the penetration depth of microwaves and the dielectric properties of different reaction systems, which can lead to “hot spot” effects, uneven reactions, and challenges in process scale-up [84].

Template-assisted synthesis

Template-assisted synthesis utilizes soft templates (e.g., polymers, micelles) or hard templates (e.g., SiO₂, Al₂O₃) to guide the formation of hollow, core–shell, or porous nanozyme structures. This approach enhances surface area, facilitates mass transport, and exposes more catalytic sites, making it suitable for designing stimuli-responsive or controlled-release nanozymes [85]. For example, Ni-Cu nanospheres and hollow ZnO spheres are often constructed via templating methods. Advantages include precise morphological control and highly designable structures, whereas the choice of template and its removal process can be complex and may introduce residues or damage the material architecture [86].

Structure-activity relationship of MNZs

The catalytic efficacy of MNZs in skin regeneration is governed not only by their chemical composition but fundamentally by their physical architecture. Recent advances have shifted from simple component screening to precise structural design, revealing that the geometric arrangement of atoms, ranging from size and shape to crystal facets and surface functionalization, deterministically dictates catalytic kinetics, substrate selectivity, and biological outcomes [87]. Understanding these structure–activity relationships is crucial for designing next-generation MNZs with enzyme-like precision.

Size effects

The size of MNZs directly influences their specific surface area and electronic structure. As the particle size decreases, the proportion of surface atoms with coordinative unsaturation increases, typically enhancing catalytic activity per unit mass. When the size is reduced to the ultrasmall cluster scale, less than 5 nm, quantum confinement effects emerge, leading to discrete electronic energy levels [88]. For instance, ultrasmall Au nanoclusters exhibit intrinsic SOD-like activity absent in larger gold nanoparticles, attributed to their specific electronic structure that facilitates electron transfer with superoxide anions [88]. Similarly, Co-doped carbon quantum dots leverage their ultrasmall size to rapidly penetrate bacterial membranes, inducing membrane hyperpolarization and efficient bactericidal effects [89]. The ultimate limit of size reduction is the single-atom catalyst, which achieves 100% atomic utilization. Unlike the heterogeneous surface of nanoparticles, single-atom nanozymes feature isolated metal atoms anchored on supports, such as N-doped carbon, forming well-defined coordination environments like Fe-N₄ and Zn-N₄ that mimic the active centers of natural metalloenzymes like heme [90], [91]. This unique configuration often yields catalytic activities orders of magnitude higher than their bulk counterparts. For example, bioinspired single-atom Fe nanozyme has been shown to exhibit superior POD-like activity for wound disinfection by optimizing the adsorption energy of hydrogen peroxide intermediates, while minimizing off-target toxicity [92], [93].

Shape and morphology modulation

Beyond size, the morphology of MNZs determines the exposure of active sites and the mass transport dynamics of substrates. A summary of the impact of different morphological dimensions on catalytic performance is presented in Table 5. One-dimensional structures, such as nanorods and nanowires, provide continuous channels for electron transport, which is beneficial for multi-electron redox reactions. Moreover, anisotropic growth often leads to the selective exposure of highly active crystal facets. For instance, cerium oxide nanorods predominately expose unstable (110) and (100) facets, which possess lower oxygen vacancy formation energies compared to the stable (111) facets found in spherical particles, thereby exhibiting enhanced antioxidant activity [94]. Two-dimensional materials, such as manganese phosphorus trichalcogenide and molybdenum disulfide, offer ultra-high surface-to-volume ratios with nearly all atoms exposed to the reaction medium, eliminating pore diffusion limitations found in bulk materials. The abundant sulfur or phosphorus vacancies on the edges of these nanosheets often serve as key catalytic centers for ROS scavenging [95]. Furthermore, three-dimensional structures like hollow nanocages and nanoframes introduce a cage effect, where the internal cavity can act as a nanoreactor to enrich local substrate concentrations, increasing the collision frequency between substrates and active sites. For example, hollow PdPt₃ nanocages exhibit significantly higher catalytic turnover rates for ROS elimination compared to solid counterparts due to this confinement effect and the lattice strain induced by their ultrathin walls [96]. Additionally, rough surfaces, such as those found on nanoflowers, enhance mechanical adhesion to bacterial membranes, synergizing physical damage with chemical catalysis [97].

Table 5.

Activities of MNZs across different morphological dimensions.

Dimension	Representative Morphology	Physicochemical Mechanism	Catalytic Advantage & Application
0D	Single Atoms	-100% atomic utilization -Uniform coordination environment simulating natural enzyme active centers	-Maximized intrinsic activity -High specificity for targeted therapy
0D	-Quantum Dots -Clusters	-Quantum confinement effect -Discrete energy levels facilitating electron transfer	-Emergent enzyme activities absent in larger particles -Ultrasmall size allows renal clearance
1D	-Nanorods -Nanowires	-Directional electron transport channels -Selective exposure of high-energy facets	-Enhanced ROS scavenging efficiency -Improved tissue penetration for inflammation control
2D	Nanosheets	-All-surface atom exposure -Elimination of pore diffusion limits -Abundant edge defects	-Rapid substrate access boosting turnover rates -Edge vacancies serve as active sites for redox reactions
3D	Hollow Nanocages	-“Cage effect” for substrate enrichment -Lattice strain in ultrathin walls optimizing electronic structure	-Significantly amplified reaction rates -High loading capacity for drug delivery
3D	-Nanoflowers -Hierarchical	-High surface roughness -Multiple light scattering for photothermal enhancement	-Stronger physical adhesion to bacterial membranes -Synergistic photothermal-catalytic antibacterial effects

Crystal facet engineering

Crystal facet engineering allows for the atomic-level tuning of surface reactivity, as different crystal facets possess distinct atomic packing densities and dangling bond configurations, resulting in varying surface energies and adsorption capabilities for reactants. The catalytic specificity of MNZs can be tailored by exposing specific facets. Research on palladium nanocrystals has revealed that the (111) facet is more favorable for the chemical adsorption and activation of oxygen molecules compared to the (111) facet, thus exhibiting stronger oxidase-like activity [98]. Conversely, the (111) facet often shows superior antioxidant performance. By synthesizing MNZs with specific facet terminations, researchers can fine-tune the material to favor either ROS generation for antibacterial therapy or ROS scavenging for anti-inflammatory therapy. Beyond perfect facets, introducing surface defects such as oxygen vacancies and grain boundaries can further modulate activity. Oxygen vacancies in metal oxides like manganese dioxide and cerium oxide act as electron traps that facilitate the activation of hydrogen peroxide or oxygen, significantly lowering the activation energy for catalytic reactions [99].

Surface coating and microenvironment

While the inorganic core dictates intrinsic activity, the surface coating determines bio-interface interactions, stability, and selectivity. Modifying MNZs with biological ligands such as DNA or peptides can confer substrate specificity akin to the lock-and-key mechanism of natural enzymes. For instance, modifying gold nanoparticles with specific peptide sequences can orient adsorbed substrate molecules, enhancing catalytic efficiency by over 10-fold [100]. Encapsulating MNZs within MOFs or hydrogels creates a defined microenvironment. The porous shell of MOFs can act as a molecular sieve, allowing small substrate molecules like hydrogen peroxide to access the catalytic core while excluding larger interfering biomolecules, thereby improving selectivity in complex biological fluids [101]. Furthermore, chiral surface coatings, such as chiral polymer imprinting, can even endow MNZs with enantioselectivity, enabling them to distinguish between chiral substrates like L-versus D-amino acids, a feature critical for precise biological intervention [102].

In conclusion, the therapeutic potential of MNZs is intimately tied to their multiscale structural features. From the atomic-level coordination of single atoms to the macroscopic architecture of hollow cages, each structural dimension offers a unique lever to tune catalytic kinetics and biological interactions. By mastering these structure–activity relationships, specifically through size reduction, morphology control, facet engineering, and surface functionalization, researchers can transition from trial-and-error synthesis to the rational design of high-performance, pathology-specific nanozymes for skin regeneration.

Metal-based nanozymes application in skin regeneration

Diabetic infected wounds

Infected diabetic wounds exhibit distinct pathological features. First, the persistent hyperglycemic microenvironment leads to endothelial dysfunction and microcirculatory impairment, causing local ischemia and hypoxia [29]. This not only delays tissue repair but also creates favorable conditions for pathogen proliferation. Second, diabetic patients often display immune dysregulation, including reduced neutrophil chemotaxis and abnormal macrophage polarization, which diminishes pathogen clearance [29]. Concurrently, sustained inflammation generates excessive ROS, resulting in oxidative stress-induced tissue damage. Third, biofilm formation by common drug-resistant bacteria, such as Staphylococcus aureus and Pseudomonas aeruginosa, further impedes antibiotic penetration and immune cell activity, while hyperglycemia enhances bacterial virulence factor expression, exacerbating infection severity. Additionally, accumulation of senescent cells and impaired growth factor signaling hinder re-epithelialization and disrupt ECM remodeling, manifesting as poor granulation tissue formation and prolonged wound healing.

These pathological changes collectively create a vicious cycle of impaired healing and highlight key targets for therapeutic intervention. MNZs, owing to their unique catalytic properties and bioactivity, offer innovative solutions for addressing the complex pathophysiology of infected diabetic wounds (Table 6). Their multi-enzyme mimetic activity efficiently scavenges excessive ROS, alleviating oxidative stress-induced damage. Tunable surface physicochemical properties allow penetration of biofilm structures, synergistically enhancing antibiotic efficacy. Moreover, certain MNZs possess immunomodulatory functions, promoting macrophage polarization toward an anti-inflammatory phenotype and restoring immune function. By stimulating angiogenic factor secretion and cell migration, MNZs can markedly improve local ischemia and hypoxia, accelerating tissue regeneration. These multifunctional attributes enable MNZs to simultaneously target multiple critical aspects of impaired healing.

Table 6.

Applications of metal-based nanozymes in diabetic infected wounds.

Metal	Author(s)	Publication Year	Key Therapeutic Design
Cu	Feng[103]	2023	-pH-responsive catalysis (antibacterial in acidic environment / pro-angiogenic in alkaline environment) -Photothermal enhancement
	Huang[106]	2023	-Photothermal antibacterial activity -Catalytic oxygen generation to alleviate hypoxia -Promotion of angiogenesis
	Shang[151]	2023	-Ultrasound-enhanced five-enzyme cascade (SOD, CAT, GOx, POD, NOS) -Anti-inflammatory -Hypoglycemic -Antibacterial -Pro-angiogenic
	Tian[109]	2023	-Triple multi-enzyme activity (POD/GSH-Px/SOD) -Bactericidal in acidic environment -Promotes healing in neutral environment
	Zhao[114]	2023	-Mimics antioxidant system (GPx/SOD) -Potent antibacterial activity (against MRSA) -Anti-inflammatory -ROS scavenging
	Chen[110]	2024	-Polyphenol ligand-enhanced antioxidant activity -ROS scavenging -Reduced cytotoxicity -Protects cells from oxidative damage
Zn	Dai[122]	2024	-Mitochondria-targeted ROS clearance -Synergistic antibacterial action (Zn/C-dots) -Pro-angiogenic -Anti-inflammatory
Fe	Li[123]	2023	-Dual nanozyme cascade (GOx/POD) -Acidifies microenvironment via glucose consumption -Fe3+/Fe2+ cycling enhances antibacterial efficacy
	You[22]	2025	-Heterojunction for ROS clearance -Induces bacterial ferroptosis -Photothermal enhancement -Microneedle delivery
Pt	Zhou[127]	2023	-Self-regulating pH -Cascade catalysis (GOD/POD/OXD/CAT/SOD) -Oxygen supply -Dual-stage antibacterial/anti-inflammatory/pro-healing
	Sun[129]	2025	-Biofilm-targeting -Synergistic sonodynamic/sonothermal therapy -Catalytic oxygen generation -Antibacterial/anti-biofilm
Au	Ren[130]	2025	-Photothermal antibacterial activity -POD-like activity -Catalytic stability
Mn	Cai[136]	2024	-Biomimetically mineralized nanozyme -Dual anti-inflammatory/antibacterial action -ROS scavenging -Pro-angiogenic -Microneedle patch delivery
	Wang[137]	2025	-Honeycomb-like MnO2@PDA nanozyme -Oxygenation effect alleviates hypoxia -Antioxidant -Antibacterial/tissue-adhesive hydrogel
Multi-metal (Zn/Cu)	Song[142]	2023	-Combined photothermal/chemodynamic therapy -ROS generation -Immune activation
Multi-metal (Ag/Fe)	Wang[158]	2023	-Core-shell structure -Silver ion release for bactericidal activity -Catalytic oxygen generation alleviates hypoxia -EGF promotes proliferation -Thermoresponsive hydrogel carrier
Multi-metal (Ag/Pt)	Chen[161]	2024	-Broad-spectrum antibacterial -ROS scavenging/oxygen generation -Anti-inflammatory hydrogel
Multi-metal (Mo/W)	Du[238]	2024	-Enhanced NIR-II absorption -Synergistic photothermal/enzymatic catalysis -Deep-tissue antibacterial activity
Multi-metal (Mo/Fe/Cu)	Li[156]	2024	-pH-switchable cascade reaction (GOx/POD/OXD/CAT/SOD) -Antibacterial/antioxidant/pro-oxygenation -Fluorescent hydrogel
Multi-metal (Fe/Cu)	Qin[239]	2024	-Photothermal biofilm disruption -Autonomous bacterial capture -POD-like anti-inflammatory activity -Cu2+ promotes angiogenesis
Multi-metal (Ru/Ni)	Zhang[165]	2024	-Bifunctional nanozyme (POD/CAT) -Cascade catalysis (glucose consumption/ROS generation/oxygen production)
Multi-metal (Au/CuS)	Gu[162]	2025	-Self-powered patch -Real-time glucose/pH monitoring -Nanozyme cascade antibacterial (GOx/POD) -Synergistic electrical stimulation
Multi-metal (Pt/Zn)	Hou[164]	2025	-Nanozyme hybridization (oxidase/POD/CAT) -Sustained ROS-mediated antibacterial action -Colorimetric infection monitoring -Microneedle delivery
Multi-metal (Cu/Fe/Au)	Sun[139]	2025	-Cascade catalysis for hypoglycemia/·OH generation -Zwitterionic hydrogel promotes repair
Multi-metal (Au/Cu)	Tan[140]	2025	-Quadruple enzyme mimic (POD/CAT/GOD/GSH-Px) -Self-supply of H2O2/O2 -Photothermal antibacterial/pro-healing
Multi-metal (Fe/Mo)	Xiao[166]	2025	-Photosynthesis-like nanozyme -·OH generation via Fenton reaction -GSH depletion -Synergistic photothermal antibacterial activity
Multi-metal (Cu/Mn)	Yan[150]	2025	-Metal-phenolic nanozyme -Photothermal controlled release -MnO2/TA antioxidant activity -Cu2+ promotes angiogenesis -Microneedle delivery

Single-metal MNZs

In the treatment of infected diabetic wounds, ROS accumulation, hypoxia, chronic inflammation, and pathogen infection often coexist, rendering single-mechanism interventions insufficient. Cu-based MNZs, as biomimetic materials with multiple enzyme-mimetic activities, have therefore been extensively studied for multi-mechanistic synergistic interventions. Early research primarily focused on regulating ROS levels to alleviate oxidative stress, but more recent studies have developed systems that simultaneously achieve antibacterial activity, pro-angiogenesis, and immunomodulation.

Initial Cu nanozyme systems were largely centered on ROS scavenging or ROS generation as the core strategy; however, a single redox mechanism is inadequate to address the complex pathology of diabetic wounds. The development of Alg/CuP composite hydrogels marked a critical advancement. This system employs a responsive enzyme activity switching mechanism: under the acidic conditions typical of acute infection (pH ∼ 5.5), CuP exhibits dominant POD-like activity, catalyzing endogenous H₂O₂ into hydroxyl radicals ·OH for potent bacterial eradication. Conversely, in the alkaline microenvironment of chronic non-healing wounds, it switches to CAT-like activity, decomposing H₂O₂ into O₂ to alleviate hypoxia and oxidative stress [103]. Mechanistically, this switch is often attributed to the proton-dependent coordination state of copper centers, where high proton concentration facilitates electron transfer to peroxide bonds, while hydroxide ions promote the disproportionation pathway [104]. The composite system further integrates NIR-induced PTT and sustained Cu²⁺ release. The released copper ions not only act as biocides but also function as essential cofactors for HIF-1α stabilization [105]. By inhibiting prolyl hydroxylase (PHD), intracellular copper prevents the ubiquitin-proteasomal degradation of HIF-1α, thereby upregulating downstream angiogenic factors like VEGF and promoting neovascularization even under normoxic conditions [10]. This “microenvironment-responsive, multi-mechanism synergistic” strategy represents a new paradigm for intelligent diabetic wound therapy. Building on this, the introduction of a dual-ligand self-assembled Cu-2,5-dimercaptoterephthalic acid (DCA) nanozyme further improves Cu ion stability and functional diversity (Fig. 1a) [106].

Fig. 1.

Design and applications of Cu-based MNZs. a) Design, preparation, and characterization of Cu-DCA (Reproduced with permission [106] Copyright @2023, Springer Nature); b) Antibacterial and catalytic functions of Cu-GA (Reproduced with permission [109] Copyright @2023, Elsevier); c) Preparation and characterization of TA-Cu (Reproduced with permission [110] Copyright @2024, American Chemical Society).

Unlike previous systems where copper ions might leak uncontrollably, the DCA ligand framework provides a robust coordination environment (Cu-S bonds) that modulates the redox potential of the metal center. This coordination facilitates a glutathione peroxidase (GPx)-like activity cycle, where the Cu center shuttles between Cu²⁺ and Cu⁺ states to consume excess intracellular GSH while scavenging ROS [107]. The Cu-DCA system not only retains ROS-regulating and PTT antibacterial functions but also provides drug delivery and sustained-release capabilities through its porous MOF structure. This enables coordinated regulation of hypoxia, bacterial infection, and regenerative impairment by delivering small-molecule modulators directly to the wound bed. This strategy exemplifies a transition from passive responsiveness to active therapeutic modulation, highlighting the importance of nanozyme structure—specifically the ligand field effect—in orchestrating functional output. Similarly, structurally simpler Cu-gallic acid (GA) nanorods leverage naturally derived polyphenol ligands to achieve bidirectional ROS regulation under varying pH conditions (Fig. 1b). The polyphenol-metal coordination networks allow for electron delocalization, which lowers the activation energy for both ROS generation and scavenging [108]. In acidic environments, the GA ligands facilitate electron transfer from the Cu center to H₂O₂, enhancing POD-like activity for efficient bacterial killing. Under neutral conditions, the redox potential shifts, favoring SOD-like and CAT-like activities where the complex acts as an electron sink to scavenge superoxide and peroxide radicals, thereby promoting healing [103], [109]. This minimalist structural design parallels the conceptual framework of the Alg/CuP hydrogel but emphasizes structural stability and synthetic accessibility through green chemistry approaches. Cu-tannic acid (TA) integrated systems follow a similar principle (Fig. 1c), utilizing simple ligand-mediated control to modulate the valence state and charge transfer of two-dimensional Cu nanosheets [110]. The TA ligands, rich in phenolic hydroxyl groups, form strong coordination bonds with copper, inducing a Ligand-to-Metal Charge Transfer (LMCT) [111]. This effect optimizes the d-orbital electron density of copper, enhancing its ability to undergo reversible redox cycling (Cu²⁺/Cu⁺) essential for multi-enzyme mimicry [112]. This confers high ROS-scavenging capacity (SOD/CAT-mimicry) and biocompatibility without requiring complex structures or external stimuli, achieving excellent cellular protection and antioxidant function at the molecular level [110]. By reducing oxidative damage to DNA and lipids, these systems preserve the viability of fibroblasts and keratinocytes crucial for wound closure. Additionally, cationic polymers such as ε-polylysine (EPL) can be co-assembled with Cu to develop antibacterial-antioxidant composite interface systems. Through electrostatic interactions, the positively charged EPL enhances affinity for the negatively charged bacterial membranes, effectively concentrating the nanozyme at the pathogen surface [113]. This proximity effect amplifies the localized ROS toxicity generated by the Cu centers while minimizing off-target damage to host cells. Simultaneously, the system retains SOD-and GPx-like ROS-scavenging activity to protect host tissue from inflammation-induced oxidative stress. Mechanistically, the Cu-EPL complex disrupts bacterial membrane potential and metabolic pathways, by interfering with the respiratory chain, improving therapeutic outcomes against antibiotic-resistant MRSA infections. Compared with the aforementioned systems, this design emphasizes the physical–chemical integration of the structural interface—combining electrostatic targeting with catalytic prowess—as a key functional determinant [114].

Against the backdrop of the growing maturity of Cu-based nanozyme research, other MNZ systems have been gradually introduced for regulating the microenvironment of diabetic infected wounds. Although less extensively studied than copper-based systems, these materials, each leveraging unique valence state modulation, biological functions, and structural design strategies, provide important complements and are driving wound therapy from single-element modulation toward multi-metal synergistic mechanisms. Zinc, a trace element naturally involved in wound healing, possesses anti-inflammatory, antibacterial, and pro-angiogenic functions. Zn single-atom-loaded carbon dots (Zn/C-dots), in which single-atom catalytic sites coupled with a carbon-based support to establish a dual-catalytic system, thereby synergistically enhance intracellular ROS scavenging, inhibition of inflammatory mediators, and antibacterial activity. Compared with Cu-based systems, this design emphasizes mitochondrial targeting and precise intracellular ROS regulation. Mechanistically, Zn single atoms act as “electron shunts” within the mitochondrial electron transport chain (ETC), effectively scavenging superoxide anions produced during electron leakage [115], [116], [117]. This preservation of mitochondrial membrane potential prevents the opening of permeability transition pores and the subsequent release of cytochrome c, thereby blocking the intrinsic apoptotic pathway in endothelial cells [118]. Furthermore, released Zn²⁺ ions function as intracellular signal modulators by inhibiting glycogen synthase kinase-3β (GSK-3β) phosphorylation [119], [120]. This inhibition prevents the degradation of β-catenin, allowing its nuclear translocation to activate the Wnt/β-catenin signaling pathway [121], which is critical for promoting endothelial cell migration and neovascularization [122]. This work expands the application of MNZs from exogenous scavenging toward cellular functional empowerment via specific signal pathway activation. In the case of Fe-based materials, researchers have primarily explored redox-state transitions and cascade reaction amplification mechanisms. By partially reducing Fe³⁺ to Fe²⁺ and coupling with GOx and POD-like activities, a catalytic cascade system was constructed to achieve self-amplified antibacterial activity under the hyperglycemic conditions of diabetic wounds [123]. Compared with single-mode ROS regulation, this design simultaneously reduces glucose levels, modulates pH, and activates POD reactions in acidic conditions. Specifically, the cascade induces a “starvation-enhanced chemodynamic” effect. Glucose consumption deprives bacteria of energy while generating H₂O₂, which is then converted into ·OH by Fe-based POD activity. This process triggers bacterial ferroptosis by inducing lipid peroxidation of the cell membrane and specifically interfering with the bacterial ETC, thereby blocking ATP synthesis and leading to metabolic collapse (Fig. 2) [123], [124], [125]. Incorporation into microneedle arrays further enhances tissue penetration and therapeutic efficacy [126]. Construction of Fe₃O₄/MXene heterostructures expands functional boundaries by introducing defect-induced ferroptosis mechanisms, enabling simultaneous Fe ion release and integration into gelatin methacrylate (GelMA) microneedles to achieve tissue penetration and localized disruption of iron homeostasis for combined antibacterial and ROS regulation [22]. Pt-based nanozyme research represents another integration pathway, extending its scope from antioxidation to oxygen supply and comprehensive infection regulation. By combining the multi-enzyme-like activities of Pt nanozymes (GOD, POD, SOD, CAT, etc.) with the oxygen-carrying capacity of perfluorooctyl bromide (PFOB), a dynamic therapeutic platform was developed to achieve “early-stage bactericidal activity + late-stage ROS homeostasis”. This system features an automatic reaction-pathway switch governed by the wound pH and oxygen tension: in the hypoxic, acidic early stage, it prioritizes ROS generation for sterilization; as inflammation subsides, it shifts to CAT-like activity, decomposing H₂O₂ to replenish O₂. This reoxygenation reverses the metabolic shift from glycolysis to oxidative phosphorylation in host cells, rescuing them from metabolic stress [127], [128]. Furthermore, integration of Pt nanozymes with the cationic antimicrobial peptide HHC-36 enabled targeted biofilm disruption and sonosensitization antibacterial mechanisms, further expanding the functional potential of Pt in infection control [129]. Gold-based systems share similar mechanistic pathways with Pt but place greater emphasis on surface-interface engineering. For instance, a Prussian blue (PB)-Au composite was synthesized via tannic acid and sodium citrate reduction, with Au nanoclusters deposited onto PB frameworks. When incorporated into a hydrogel, this system achieved POD-like activity modulation, enhanced antibacterial stability, and PTT-augmented antibacterial effects. The deposition of Au nanoclusters modulates the electron transfer kinetics at the PB surface, lowering the activation energy for H₂O₂ decomposition. Additionally, the localized surface plasmon resonance of Au contributes to a photothermal effect that disrupts bacterial membrane integrity, increasing permeability to reactive radical species [130], [131]. Although less comprehensive in functionality compared to Pt systems, it provides new directions in structural controllability and biocompatibility. Finally, Mn-based nanozymes have emerged as popular candidates for anti-inflammatory applications owing to their intrinsic SOD-and CAT-mimicking properties. Using CAT enzyme as a template, a CAT-MnSH microneedle patch was constructed through assembly with Mn sulfide, enabling not only clearance of endogenous and exogenous ROS but also H₂S-mediated regulation of macrophage M2 polarization, angiogenesis, neurogenesis, and tissue repair. Mechanistically, Mn-based nanozymes efficiently scavenge mitochondrial ROS (mtROS) to maintain organelle integrity [132]. The reduction in mtROS directly inhibits the assembly and activation of the NLRP3 inflammasome [133], thereby blocking the caspase-1-mediated maturation and secretion of pro-inflammatory cytokines like IL-1β [134], [135]. Simultaneously, the released H₂S acts as a gasotransmitter to persulfidate key signaling proteins, further driving the immune microenvironment toward a regenerative M2 phenotype [136]. Although Mn rapidly reacts to generate gas, this very reactivity contributes to instability, posing challenges for clinical translation. One solution involves loading MnO₂ into chitosan-based hydrogels, with PDA coating to enhance stability. Additionally, the introduction of quaternary ammonium and catechol moieties confers excellent antibacterial and tissue adhesion properties, enabling Mn-based systems to regulate ROS while simultaneously alleviating hypoxia [137].

Fig. 2.

Design and application of Fe-based MNZs, a) design, synthesis, and therapeutic paradigm; b) Representative images of MNZs; c) wound healing efficacy (Reproduced with permission [123] Copyright @ 2023, Elsevier).

Despite these optimizations, critical limitations persist. Serum proteins in wound exudates often compete for copper binding, disrupting catalytic function. Additionally, the heterogeneous acidity of diabetic ulcers can hinder the precise transition of pH-responsive switches. Future research must therefore pivot towards designing bio-orthogonal ligands and adaptive interfaces that maintain robust stability and specificity within the complex, protein-rich wound microenvironment.

Multi-metal MNZs

In the development of multi-metal MNZs, copper-centered composite systems remain predominant, often combined with gold, zinc, or iron to achieve multi-enzyme cascade reactions and photo/thermal-responsive synergistic therapy. For instance, a multi-enzyme-mimicking hydrogel system integrated the Fenton-like catalytic activity of Cu with the GOx-like activity of Au nanoparticles, thereby modulating glucose metabolism and endogenous H₂O₂ utilization. Specifically, the Au nanoparticles function as GOx mimics, catalyzing the aerobic oxidation of glucose into gluconic acid and H₂O₂. This process achieves a dual therapeutic effect: it induces “starvation therapy” by depleting the bacterial nutrient source, while the generated gluconic acid lowers the local pH. This acidification creates an optimal microenvironment for the Cu-mediated Fenton-like reaction, significantly lowering the activation energy for H₂O₂ decomposition [138], thereby efficiently generating ·OH at the wound site, accelerating re-epithelialization and collagen deposition in diabetic wounds [139]. In another study, an Au-Cu bimetallic aerogel further integrated POD-, CAT-, GOx-, and GPx-like activities, enabling self-supplied oxygen, glucose regulation, ROS clearance, and antibacterial effects. Its CuO₂ shell undergoes acid-triggered disproportionation, acting as an in situ reservoir that continuously replenishes H₂O₂ substrate specifically within the acidic infection microenvironment. This overcomes the limitation of insufficient endogenous H₂O₂ in chronic wounds. Simultaneously, the Au component facilitates electron transfer to the copper centers, enhancing the catalytic turnover rate for ROS generation, coordinating multi-enzyme pathways to eliminate pathogens at early stages while creating a favorable microenvironment for subsequent tissue repair [140], [141]. Cu-Zn composites added further structural complexity and ROS regulation capacity. By releasing Cu²⁺ and H₂O₂ from CuO₂ under acidic conditions and leveraging Zn single-atom catalytic sites anchored on carbon defects to accelerate ·OH production, this system enabled synergistic chemical dynamic therapy (CDT) and PTT antibacterial action [122]. Mechanistically, the atomically dispersed Zn active sites modulate the adsorption energy of peroxide intermediates, acting as electron shuttles to boost the Fenton-like activity of Cu ions. Furthermore, the released Zn²⁺ ions act as intracellular signal modulators, likely by inhibiting the NF-κB pathway to alleviate inflammation and activating the Wnt/β-catenin pathway to accelerate epithelial migration, accompanied by inflammation alleviation and immune activation, leading to significant improvements in refractory infected wounds [142], [143], [144]. Similarly, a Cu-Mn combination was incorporated into microneedle patches, where a thermosensitive hyaluronic acid hydrogel provided responsive release. MnO₂ contributed to antioxidation by acting as a sacrificial electron donor to scavenge superoxide anions [145], [146], thereby protecting mitochondrial integrity and inhibiting the NLRP3 inflammasome activation cascade [147]. Cu²⁺ promoted angiogenesis via the “PHD-HIF-1α-VEGF” axis. Intracellular copper ions inhibit PHDs, preventing the degradation of HIF-1α and ensuring the sustained transcription of VEGF [148], [149]. Together they established a closed-loop antibacterial–antioxidant–pro-regenerative therapeutic strategy [150].

Expansion into ternary Cu-Au-Fe systems further exemplified this trend. A Cu-Au-L-Arg composite, immobilized on phosphorus-doped graphitic carbon nitride and combined with hyaluronic acid spray and ultrasound stimulation, activated five enzyme-mimicking activities-SOD, CAT, GOx, POD, and NOS-achieving synchronized ROS regulation, oxygen supply, glucose modulation, and angiogenesis [151]. This platform established the therapeutic paradigm of “multi-pathway synergy and microenvironment-triggered response”. Specifically, the ultrasound stimulation creates a piezoelectric field that enhances electron transfer between the graphitic carbon nitride support and the metal centers, boosting the catalytic turnover of the nanozymes [152], [153]. The L-Arg component serves as a substrate for the NOS-like activity, generating NO in situ [154]. This gasotransmitter then acts on vascular smooth muscle cells to induce vasodilation and upregulates VEGF expression via the cGMP pathway, directly promoting angiogenesis [155]. A similar approach was reported in a Mo-Fe/Cu-I-Ag nanozyme system, which realized a complete therapeutic cascade from glucose consumption and ROS production to hypoxia alleviation and immune regulation [156]. In this system, the Mo-Fe component likely mimics the electron transport chain, facilitating the conversion of glucose-derived electrons to generate ROS [157], while the Cu/Ag components disrupt bacterial cell wall integrity through local ion release. This multi-step cascade effectively starves bacteria of glucose while subjecting them to oxidative stress, a dual-strike mechanism that prevents resistance development. Furthermore, the system modulates the TNF-α signaling pathway, reducing pro-inflammatory cytokine release and favoring a transition to the proliferative phase. Such systems demonstrated enhanced adaptability to dynamic pathological microenvironments and superior structural integration.

Beyond copper, Au-Fe-Ag systems have also gained attention. An Ag@Fe-GA core–shell structure was employed to load epidermal growth factor (EGF), which was delivered via a hydrogel platform to achieve triple functions of antibacterial action, oxygen supply, and cell proliferation promotion-particularly effective for chronic wounds under hypoxic conditions [158]. The core–shell architecture protects the EGF cargo from proteolytic degradation in the wound exudate. Meanwhile, the Fe-GA shell acts as a “gatekeeper” responsive to MMPs overexpressed in chronic wounds, ensuring controlled release. The Ag core contributes to long-term bacteriostasis by binding to bacterial DNA and inhibiting replication [159], complementing the immediate ROS-mediated killing provided by the Fe shell [160]. Another study utilized Ag-Pt hybrid particles anchored on a GO substrate, markedly enhancing antibacterial efficacy and ROS scavenging, thereby offering excellent management of bacteria-infected wounds [161]. The GO substrate provides a high surface area for anchoring Ag-Pt hybrids and facilitates electron mobility, which lowers the activation energy for superoxide disproportionation. This enhances the SOD-like activity, protecting host cells from oxidative damage while the Ag release targets bacterial membranes. Additionally, efforts have been directed toward more advanced architectures such as intelligent responsive and wearable systems. A monitoring device based on an Au/CuS anode and Pt cathode not only enabled antibacterial activity via enzyme cascades but also allowed real-time glucose and pH monitoring, improving dynamic therapeutic regulation (Fig. 3a and 3b) [162]. This self-powered system utilizes the bio-catalytic oxidation of glucose at the anode to generate an electric current, which can be correlated with glucose concentration. Simultaneously, the pH-dependent potential shift of the electrode allows for the monitoring of wound acidity, a key indicator of infection status. The generated electric field can also stimulate keratinocyte migration via electrotaxis [163], further accelerating wound closure. Likewise, microneedle-based systems integrating infection recognition, MRSA eradication, and cascade enzyme activity offered a pathway toward precision nanozyme therapy (Fig. 3c) [164]. These microneedles can be engineered to change color in response to bacterial proteases or pH changes, providing a visual readout of infection. Upon activation, they release encapsulated nanozymes that penetrate deep into the biofilm, leveraging their size and surface chemistry to disrupt the extracellular polymeric substance matrix and kill dormant bacteria. Finally, systems such as Ru-NiB [165], Fe-Mo[166], and TA-Fe/Cu metal-phenolic hybrids [108] continue to drive the evolution of multi-metal MNZs from material complexity toward functional precision.

Fig. 3.

Precision medical applications of multimetallic MNZs. a) A monitoring system based on an Au/CuS anode and a Pt cathode for real-time glucose and pH detection (Reproduced with permission [162] Copyright @ 2025, Wiley-VCH); b) application of the detection system in promoting wound healing in animal models (Reproduced with permission [162] Copyright @ 2025, Wiley-VCH); c) multifunctional MNZs integrating infection recognition, MRSA eradication, and cascade enzymatic activity (Reproduced with permission [164] Copyright @ 2025, American Chemical Society).

Despite the versatility of multi-metal MNZs, their practical application is hindered by critical challenges. A primary bottleneck is the precise synthetic control over metal arrangement; slight deviations often cause phase segregation, leading to antagonistic rather than synergistic effects. Elucidating catalytic mechanisms is also arduous, requiring advanced characterization to decouple complex metal–metal interactions. Moreover, the potential leakage of multiple metal ions raises biocompatibility concerns in diabetic tissues. Future efforts must focus on high-precision atomic assembly and machine learning to rationally design stable, synergistic active sites for targeted therapy.

Pathological scar

Therapeutic application in in vivo pathological scar models

In the application of MNZs for skin regeneration, recent research has shifted from the sole objective of “accelerating wound closure” toward the higher-order goal of “promoting wound healing while preventing scar formation” (Table 7). Most studies have focused on constructing synergistic pathways that integrate ROS scavenging, inflammation modulation, and fibroblast regulation, among which Mo-, Zr-, and Ce-based MNZs demonstrate remarkable potential.

Table 7.

Applications of metal-based nanozymes in scar therapy.

Metal	Author(s)	Publication Year	Key Therapeutic Design
Zn	Bai[184]	2022	-Piezocatalytic zinc oxide@graphdiyne heterostructure -Ultrasound-activated ROS generation and antibacterial activity -Promotes collagen alignment and reduces scarring
Cu	Xu[179]	2023	-Photothermal antibacterial activity -ROS scavenging -Promotes angiogenesis -Significantly reduces scar marker α-SMA expression
Fe	Chen[181]	2024	-Efficient ·OH generation for antibacterial action -Regulation of anti-inflammatory factors -Significantly reduces scar area
Ce	Yuan[187]	2024	-Zinc-doped hollow mesoporous ceria hydrogel -Dynamic regulation of enzymatic activity -Promotes scar-free healing
Mn	Gao[188]	2025	-Janus nanoparticles -Dual-enzyme activity for ROS clearance -Controlled release of FGF to promote repair
Zr	Feng[172]	2025	-Efficient ROS scavenging -Promotes M2 macrophage polarization -Facilitates ordered collagen deposition
Multi-metal (Cu/Mo)	Shan[180]	2022	-Spatiotemporal catalytic ROS generation -PDA-mediated ROS clearance/oxygen production -Reduces inflammation and scarring
Multi-metal (Fe/Ca)	Yan[240]	2023	-pH-responsive system -Antibacterial in acidic environment/wound healing promotion in neutral environment -Photothermal-enhanced catalysis
Multi-metal (Co/Ce)	Zhang[192]	2025	-NIR-responsive microneedle system -Photothermal antibacterial/ROS scavenging/angiogenesis promotion -Collagen remodeling

Mo-based materials, owing to their strong antioxidant enzyme-mimicking capacity, have shown outstanding performance in fibrosis regulation (Fig. 4a). The Mo_4/3B₂ nanoscaffold, with both SOD-and GPx-like activities, effectively scavenges excess ROS in wounds, thereby alleviating oxidative stress–induced inflammation and fibroblast hyperactivation [167]. Mechanistically, this inhibition acts on the “ROS-TGF-β1 loop”. ROS are essential second messengers that oxidize the latency-associated peptide to release active TGF-β1 from its latent complex [168], [169]. By sustaining a reductive intracellular environment, Mo-based nanozymes prevent latency-associated peptide (LAP) oxidation, thereby locking TGF-β1 in its inactive state and blocking the downstream phosphorylation of Smad2/3 proteins [8]. Beyond this, metabolomics revealed that the system can reprogram amino acid pathways and restore purine metabolism homeostasis [170]. Specifically, glycine and glutamate are the primary amino acid residues constituting the collagen triple helix structure [171]. The downregulation of their biosynthetic pathways by Mo-based nanozymes effectively deprives myofibroblasts of the necessary raw materials, inducing a state of “metabolic starvation” that limits excessive ECM deposition. Similarly, Zr-doped Prussian blue nanozymes (Zr-PB) suppress fibrosis by enhancing ROS clearance. Zr-PB not only outperforms conventional PB in SOD/CAT-like activities but also induces M2 macrophage polarization and restores mitochondrial function, creating an antifibrotic immune environment [172]. The doping of Zr ions modulates the electronic spin state and charge density of the Fe centers within the crystal lattice, significantly lowering the activation energy barrier for superoxide disproportionation [173]. This potent antioxidant capability preserves mitochondrial membrane potential, preventing the opening of the mitochondrial permeability transition pore and the leakage of cytochrome c [174]. This mitochondrial protection is crucial for immune reprogramming, as it supports the metabolic shift from glycolysis, characteristic of pro-inflammatory M1 macrophages, to oxidative phosphorylation, thereby stabilizing the anti-inflammatory M2 phenotype [159]. At the tissue level, Zr-PB markedly reduces the expression of collagen I/III and MMP1, thereby inhibiting abnormal collagen accumulation and excessive ECM remodeling, demonstrating strong anti-scarring performance.

Fig. 4.

Designs and applications of MNZs for anti-scarring therapy, a) anti-scarring treatment paradigm and therapeutic outcomes of molybdenum-based MNZs (Reproduced with permission [167]; Copyright @ 2024, American Chemical Society); b) comprehensive anti-scarring framework and application outcomes of cerium-based MNZs, encompassing “immune modulation-cellular reprogramming–fibrosis restriction” (Reproduced with permission [175]; Copyright @ 2025, Springer Nature).

Ce-based nanozyme systems further exhibit multifaceted regulatory mechanisms, particularly in redox balance, inflammatory transition, and fibroblast fate modulation. A hydrogel spray system based on TA-modified CeO₂ nanozymes, possessing SOD/CAT cascade activity, not only restores oxygenation and ROS homeostasis in diabetic wounds but also reduces scar indices, decreases epidermal thickness, and normalizes collagen organization to resemble healthy skin architecture [21]. The modification with TA induces a LMCT, which accelerates the reversible conversion between C^e3+ and Ce⁴⁺ states, ensuring continuous regeneration of catalytic active sites for sustained ROS scavenging. In another approach, ε-polylysine-modified CeO₂ hydrogels enabled rapid antibacterial activity and ROS scavenging at the early stage, followed by bFGF-mediated fibroblast proliferation and c-Jun siRNA suppression of fibroblast overactivation, thereby blocking fibrotic pathways at the molecular level (Fig. 4b) [175]. Crucially, the CeO₂ core acts upstream to inhibit the assembly and activation of the NLRP3 inflammasome in macrophages by scavenging mitochondrial ROS, thereby cutting off the secretion of pro-fibrotic cytokines like IL-1β. The subsequent release of c-Jun siRNA acts downstream to specifically silence the JNK signaling pathway in fibroblasts. This prevents the formation of the activator protein-1 transcription factor complex, which is a key regulator of collagen and MMP gene expression, effectively halting pathological ECM remodeling [176]. This system provides a temporally ordered “immune regulation–cellular reprogramming–fibrosis inhibition” framework for scar prevention.

Collectively, Mo-, Zr-, and Ce-based MNZs, through antioxidant enzyme-mimicking activities, macrophage immune modulation, fibroblast regulation, and collagen expression control, establish a systematic anti-fibrosis strategy while promoting wound repair. These advances provide both material and mechanistic foundations for the design of scar-free skin regeneration. Yet, a more profound limitation lies in the lack of temporal precision in current MNZ therapies. Scar formation is a dynamic process where ROS play dual roles—acting as vital signaling molecules in early healing yet driving fibrosis in later stages. Indiscriminate, constitutive ROS scavenging by MNZs may inadvertently disrupt essential re-epithelialization or immune clearance. Furthermore, current strategies largely neglect the dense, avascular fibrotic matrix, which acts as a physical barrier limiting the deep tissue penetration and retention of nanozymes. Future research must shift from general antioxidant approaches to temporally regulated systems that specifically modulate mechanotransduction pathways and overcome ECM transport barriers.

Therapeutic clues for anti-fibrotic effect in wound models

Although research specifically targeting scars with MNZs remains limited, many studies on wound healing also reveal the potential of MNZs in scar management. This reliance on general wound models is necessitated by the current paucity of standardized and stable animal models for pathological scarring, particularly for conditions like keloids, for which no consensus model currently exists. Consequently, evaluating anti-fibrotic outcomes within the dynamic process of wound healing represents the most viable approach to explore the holistic regenerative and anti-scarring potential of MNZs. In infected wound treatment, some MNZs, initially designed for antibacterial activity and microenvironmental modulation, indirectly reduce scar formation through their anti-inflammatory, antioxidative, and tissue remodeling functions. This “non-purposeful anti-scar” phenomenon has been observed across multiple metal-based systems.

For instance, copper-based MNZs often leverage multi-enzyme mimicry and ROS regulation to mitigate fibrosis. Copper-rich PB nanozymes, under PTT assistance, significantly downregulate α-SMA levels in scar tissue, indicating their potential to inhibit fibroblast-to-myofibroblast transition and thus intervene in scar formation. Copper ions released from the PB framework may interfere with the TGF-β1/Smad signaling pathway, preventing the nuclear translocation of Smad2/3 complexes required for α-SMA transcription [8]. Furthermore, the mild hyperthermia induces the expression of heat shock protein 70 (HSP70), which blocks the NF-κB signaling pathway, thereby reducing the secretion of pro-fibrotic cytokines [177], [178]. Moreover, their anti-inflammatory and pro-angiogenic capabilities may reduce excessive collagen deposition, diminishing scar thickness and density [179]. Similarly, Cu₂MoS₄-PDA hybrid microneedle patches dynamically regulate ROS levels (Fig. 5a), maintaining antibacterial efficacy while avoiding oxidative stress-induced fibrosis. This dynamic regulation likely involves the PDA coating acting as an electron reservoir, buffering the redox potential to prevent excessive ROS accumulation that triggers the NLRP3 inflammasome. By maintaining ROS within a physiological window, the system prevents the oxidative activation of latent TGF-β1, ensuring sufficient microbial clearance without excessive oxidative damage, preventing fibroblast overactivation and abnormal collagen deposition, thereby lowering the risk of scar formation [180].

Fig. 5.

Anti-scar therapeutic designs of MNZs in wound models. a) fabrication of microneedle patches by hybridizing Cu₂MoS₄ with PDA (Reproduced with permission [180]; Copyright @ 2023, Wiley-VCH); b) development of a Co–Ce–Mn hybrid microneedle system (Reproduced with permission [192]; Copyright @ 2025, Wiley-VCH).

In contrast, iron-based MNZ systems, with abundant Fenton reaction sites and anti-inflammatory capacity, exhibit broader effects in dermal tissue remodeling. Single-atom Fe nanozymes in infection models not only clear bacteria but also suppress TNF-α and IL-1β expression, subsequently normalizing fibroblast function and leading to progressively diminished scars [181]. The single-atom Fe sites likely mimic the active center of SOD, efficiently scavenging superoxide anions that sustain chronic inflammation. This reduction in oxidative stress inhibits the downstream, thereby blocking NF-κB activation and suppressing the transcription of pro-fibrotic cytokines. Their potent POD-mediated bactericidal activity, combined with VEGF and EGF upregulation, may promote orderly re-epithelialization and vascular reconstruction, reducing fibrotic collagen deposition under chronic inflammatory conditions [181]. Another single-atom Fe nanozyme, employing NIR-induced photothermal effects and ·OH generation, disrupts biofilms while upregulating VEGF, highlighting its capacity to modulate late-stage vascularization in scar formation [182]. The strong synergy of PTT and ROS-mediated antibacterial effects not only eliminates infection sources but also promotes VEGF-driven vascular remodeling, potentially mitigating abnormal vascular networks in scars. Additionally, the modulation of intracellular ROS levels by Fe nanozymes may influence the Kelch-like ECH-associated protein 1 (Keap1)-Nuclear factor erythroid 2-related factor 2 (Nrf2) pathway [183], enhancing the expression of endogenous antioxidant enzymes to further counteract fibrotic stress.

Zinc, cobalt, cerium, and manganese-based systems also exhibit anti-scar potential at the tissue level. A ZnO-based piezo-catalytic system, although primarily designed for rapid bacterial clearance, significantly improved collagen alignment, enhancing scar structure [184]. This suggests that the piezoelectric field generated by ZnO under mechanical stress may align collagen fibers through electromechanical coupling [152]. Furthermore, Zn²⁺ ions released from the matrix act as essential cofactors for MMPs, balancing the degradation and synthesis of the ECM to prevent hypertrophic scarring. This regulating fibroblast migration and secretory behavior can promote orderly collagen deposition, improving scar tissue architecture. Co single-atom nanozymes, delivered via immune cell membranes, suppress inflammation and support epidermal restoration, yielding reduced scarring [185]. The IL-4 release specifically targets the STAT6 signaling pathway in macrophages, driving their polarization towards the anti-inflammatory M2 phenotype [186]. These M2 macrophages secrete anti-fibrotic factors that antagonize TGF-β signaling, thereby inhibiting the differentiation of fibroblasts into myofibroblasts [159]. This system releases IL-4 to modulate macrophage phenotype, inhibit excessive inflammation, and promote epithelial reconstruction, indirectly restraining TGF-β and MMP signaling to prevent fibrosis and ECM overaccumulation—a representative immune-modulation-mediated anti-scar mechanism.

Similarly, Ce/Zn alloy nanozyme hydrogels demonstrated near-scarless healing by day 14, outperforming controls [187]. Through multi-enzyme catalytic activity, these materials shorten the inflammatory phase and accelerate wound closure while reducing collagen deposition, likely via ROS-mediated TGF-β inhibition and ECM metabolic balance. The Ce³⁺/Ce⁴⁺ redox couple acts as a regenerative antioxidant cycle to scavenge ROS, preventing the oxidation of the LAP and thus keeping TGF-β in its latent state. MnO₂-based systems also show anti-scar effects; Janus structures combining MnO₂ with acidic fibroblast growth factor (aFGF) on SiO₂ platforms modulate oxidative stress and promote angiogenesis, resulting in scar tissue reduction and hair regeneration [188]. MnO₂ acts as a catalase mimic to generate oxygen from endogenous H₂O₂, alleviating hypoxia which is a known trigger for fibrosis [189]. Simultaneously, aFGF binds to fibroblast growth factor receptors, activating the PI3K/Akt pathway to promote cell survival and ordered proliferation, creating a microenvironment conducive to appendage regeneration rather than scarring [190], [191]. Building on these MNZs, integrated multi-metal systems further enable scar modulation. For example, a Co-Ce-Mn hybrid microneedle system combines antibacterial, antioxidative, and immune-regulatory mechanisms, exhibiting minimal scar width in histological analysis (Fig. 5b) [192]. By synergistically clearing ROS, promoting angiogenesis, and enhancing macrophage M2 polarization, this composite nanoarchitecture establishes a low-inflammation, high-blood-flow, high-remodeling wound environment, supporting epidermal restoration and substantial scar width reduction. Its anti-scar mechanism relies on the holistic regulation of the “immune-fibrosis axis,” where the trimetallic synergy halts the self-perpetuating cycle of inflammation, oxidative stress, and pathological remodeling.

A critical ambiguity in current research lies in distinguishing true anti-fibrotic modulation from the passive benefits of accelerated wound closure. In acute wound models, reduced scarring often results solely from shortened inflammatory exposure rather than specific interference with pathological fibrotic pathways. Consequently, it remains unclear whether MNZs actively reprogram fibroblast phenotypes or simply expedite the transition to the remodeling phase. Moreover, current evaluations excessively focus on reducing total collagen mass, neglecting the pivotal role of collagen architecture—specifically fiber orientation, cross-linking density, and Type I/III ratios—in restoring tissue mechanics. Future studies must decouple healing kinetics from remodeling quality, utilizing second-harmonic generation imaging to validate the structural integrity and functional restoration of the ECM lattice.

Hair regeneration

Therapeutic application in androgenetic alopecia

Hair regeneration holds crucial significance in functional skin regeneration (Table 8). However, current studies have largely focused on androgenetic alopecia (AGA) as the primary disease model. Oxidative stress is regarded as a key driver of AGA progression, where excessive ROS disrupt the follicular microenvironment and suppress stem cell function. As previously noted, MNZs possess the ability to scavenge ROS and improve the follicular niche, and when combined with microneedle systems, they can enhance delivery efficiency and bioavailability within follicular regions. Along this mechanistic axis, MNZs with different metal compositions have been harnessed to construct delivery platforms with pro-regenerative bioactivity.

Table 8.

Applications of metal-based nanozymes in promoting hair regeneration.

Metal	Author(s)	Publication Year	Key Therapeutic Design
Ce	Yuan[17]	2021	-Nanozyme microneedles -ROS scavenging/angiogenesis promotion -Superior to minoxidil -No skin damage
	Li[21]	2025	-Oxygenating hydrogel spray -Remodels hair follicle microenvironment
Pt	Hu[199]	2025	-Nanozyme microneedles -Efficient ROS scavenging/oxygen generation -Promotes hair follicle stem cell differentiation
Mn	Zhang[203]	2022	-Machine learning-optimized nanozyme -Low IC50 SOD activity -Deep delivery for efficient regeneration
Multi-metal (Ni/Cu)	Wang[196]	2025	-Nanozyme microneedles -Dual-enzyme activity for oxygen production -Synergistic effect with minoxidil
Multi-metal (Mn/Au)	Gao[188]	2025	-Janus nanoparticles -Dual-enzyme activity for ROS clearance -Controlled release of aFGF
Multi-metal (Rh/Co)	Liu[211]	2024	-Paired-atom nanozyme -NAD + regeneration -Macrophage M2 polarization

For example, encapsulating CeO₂ nanozymes within microneedle patches effectively overcomes the skin barrier and releases Ce-MNZs with CAT-and SOD-mimicking activity, thereby clearing excessive ROS and alleviating oxidative stress surrounding hair follicles [17]. Importantly, the microneedles also induce mechanical microinjury, stimulating local vasodilation and improving blood perfusion and oxygenation in follicular regions. The Ce³⁺/Ce⁴⁺ redox cycling at the nanozyme surface acts as a continuous antioxidant engine, preserving the mitochondrial membrane potential of hair follicle stem cells (HFSCs) against oxidative insults [193], [194]. Simultaneously, the mechanical cues from microneedle insertion trigger the release of platelet-derived growth factor and activate the Wnt/β-catenin signaling pathway, a master regulator of follicle cycling, thereby synergizing with the chemical antioxidant effect to drive the transition from telogen to anagen [195]. This dual effect-antioxidation plus angiogenesis-proved superior to minoxidil in animal models, yielding more stable restoration of hair growth (Fig. 6a). Similarly targeting redox balance, a microneedle system co-loaded with Ni-Cu nanozymes and minoxidil exhibited enhanced synergistic effects (Fig. 6b) [196]. The Ni-Cu nanozymes decomposed H₂O₂ under SOD-and CAT-like activities, releasing O₂, significantly reducing ROS burden and restoring follicular stem cell proliferation. The bimetallic Ni-Cu interface likely facilitates electron transfer, lowering the activation energy for H₂O₂ disproportionation. The in situ generated oxygen directly alleviates the hypoxic microenvironment typical of androgenetic alopecia, shifting the metabolism of HFSCs from anaerobic glycolysis to more efficient oxidative phosphorylation [197]. This metabolic reprogramming provides the ATP surge required for rapid cell division during the anagen phase [198]. Meanwhile, the microneedle delivery achieved a transdermal efficiency as high as 83.4%, and the accompanying mechanical stimulation promoted angiogenesis, thereby optimizing nutrient supply to follicles. This combination therapy markedly improved hair regrowth coverage, hair shaft diameter, and CD31⁺ vascular density in the dermal papilla, highlighting a mechanism of follicular regeneration via microenvironment remodeling and stem cell activation. Platinum nanozymes have also been incorporated into microneedle platforms for AGA treatment. In high oxidative stress environments, Pt-MNZs reduce ROS to O₂, which in turn enhances mitochondrial oxidative phosphorylation and drives follicular stem cell differentiation [199]. Unlike Ce-or Ni-Cu-based systems, Pt nanozymes emphasize a sequential cascade of “ROS reduction-metabolic activation-differentiation induction”. By acting as an electron sink, Pt nanozymes prevent the ROS-mediated oxidation of key signaling proteins such as β-catenin, ensuring its nuclear translocation and transcriptional activity [200]. Furthermore, the restoration of redox homeostasis inhibits the expression of DKK-1, a Wnt antagonist often upregulated in balding scalps, thereby removing the molecular brake on hair growth [201], [202]. Microneedles provide transient permeation channels and prolong local retention, and notably, a thrice-weekly regimen achieved superior regeneration compared to daily minoxidil, underscoring the central role of oxygen metabolism regulation in hair therapy. In addition, a research team applied machine learning to identify a novel Mn-based nanozyme, subsequently formulated into a microneedle patch for AGA therapy [203]. This MNZ displayed an exceptionally low IC₅₀ and potent SOD-like activity, effectively eliminating excess ROS in hair regeneration zones. Despite the absence of auxiliary drugs or oxygen-supplementing mechanisms, this system achieved superior outcomes compared to minoxidil, even at reduced treatment frequencies. Its efficacy relied primarily on efficient ROS clearance and deep-skin diffusion capacity. The Mn-based nanozyme likely mimics the active center of manganese superoxide dismutase, the primary antioxidant enzyme in the mitochondrial matrix [204], [205]. By specifically scavenging mitochondrial superoxide, it prevents the opening of the mitochondrial permeability transition pore and cytochrome c release, protecting HFSCs from intrinsic apoptosis and preserving the regenerative pool [206]. Its efficacy demonstrated the potential of single-factor nanozyme regulation in follicular microenvironment remodeling.

Fig. 6.

Application of MNZ-based microneedle patches in hair regeneration. a) comparison of CeO₂ nanozyme microneedle patches with minoxidil (Reproduced with permission [17]; Copyright @ 2021, American Chemical Society); b) a microneedle system co-loaded with Ni-Cu nanozymes and minoxidil for hair regeneration (Reproduced with permission [196]; Copyright @ 2025, American Chemical Society).

Current MNZ strategies for androgenetic alopecia suffer from a fundamental “mechanistic disconnect”. They predominantly target downstream oxidative stress while neglecting the upstream hormonal driver, dihydrotestosterone, induced follicle miniaturization. This renders the treatment palliative, alleviating microenvironmental damage without halting the underlying pathological regression. Furthermore, the physical barrier of the sebum-filled follicular infundibulum severely restricts the passive diffusion required to reach the deep-seated bulge stem cell niche. Future research must evolve from simple antioxidants to “dual-mechanism” systems that integrate 5α-reductase inhibition or androgen receptor blockade, coupled with transfollicular delivery technologies to ensure precise accumulation at the regenerative unit.

Therapeutic clues for hair regeneration from non-hair-related models

Current research specifically targeting AGA with MNZs remains relatively limited. This scarcity compels a broader examination of non-hair-related models, particularly wound healing, where hair regeneration outcomes have been unexpectedly reported. This shift in perspective is crucial, not only as a supplementary evidence base but because the de novo formation of hair follicles, wound-induced hair neogenesis, within healed tissue serves as the standard for distinguishing complete skin regeneration from simple fibrotic repair. Consequently, capturing these regenerative signals in general wound models provides unique and stringent evidence for the capability of MNZs to drive structural restitution. However, despite this significance, studies explicitly focusing on appendage regeneration within the context of MNZ-treated wounds remain sparse.

The elimination of ROS within the wound microenvironment and the restoration of hypoxic balance have been repeatedly demonstrated to enhance tissue repair. When hair follicles are maintained in an optimal oxygenation and low-inflammation niche, their regenerative potential may also be triggered. This was indirectly verified for the first time in a study using CeO₂ nanocomposites for the treatment of radiation-induced injury [207]. By anchoring miR-129 and stabilizing nanozyme release, the system markedly attenuated radiation-induced ROS accumulation and HIF-1α activation, thereby improving neovascularization and epithelial reconstruction. Notably, follicle density was restored and even exceeded that of the control group [207]. It is speculated that the synergistic effects of ROS mitigation, angiogenesis, and miRNA modulation established a stable, low-inflammatory and oxygen-rich regenerative window for the follicular microenvironment. Mechanistically, miR-129 targets the mRNA, suppressing its expression and the subsequent inflammatory cascade, while the CeO₂ core scavenges superoxide anions, preventing DNA double-strand breaks in follicular progenitors [208], [209]. In another study, a CeO₂-modified multifunctional hydrogel was applied to diabetic chronic wounds [21]. Although the primary focus was on antioxidation, hypoxia alleviation, and macrophage M2 polarization, RNA sequencing and histological analyses revealed not only complete wound closure but also the formation of abundant hair follicles and sweat glands. The combined ROS scavenging and oxygen-supplying cascade catalytic activity of SOD-CAT likely improved the niche for follicular stem cells; simultaneously, M2-polarized macrophages secreted a repertoire of growth factors that might directly or indirectly induce follicular redevelopment and activation. Specifically, M2 macrophages are known to secrete Wnt ligands that activate the Wnt/β-catenin pathway in adjacent epidermal cells, a prerequisite for de novo folliculogenesis [210]. Similarly, paired-atom nanozymes demonstrated restoration of follicular appendages, although their major emphasis was on excessive NADH oxidation and immune modulation. Remarkably, following skin repair, a significant increase in hair follicles and sweat glands was observed [211]. RhCo-MNZ, via NOX-like activity, promoted NAD⁺ generation and drove macrophage polarization toward the M2 phenotype, thereby improving the regenerative environment. This phenomenon may be attributed to mitochondrial metabolic recovery driven by NAD⁺ regeneration, coupled with immune relief, which collectively facilitated follicular stem cell activation [212]. The restored NAD⁺/NADH ratio activates Sirtuins, which deacetylate and inhibit NF-κB, suppressing inflammation while promoting the expression of genes involved in mitochondrial biogenesis and stem cell maintenance [213], [214]. Beyond MNZs themselves, their drug-loading capability can further enhance hair regenerative capacity. By engineering asymmetric MnO₂–Au Janus nanoparticles, ROS scavenging, oxygen delivery, and aFGF release were integrated, markedly accelerating wound tissue repair [188]. Although the study centered on skin structural restoration, hair growth was explicitly documented in parallel with scar contraction. This suggests that under the triple intervention of ROS clearance, improved oxygen supply, and growth factor availability, aFGF may stimulate dormant hair follicles to re-enter the anagen phase, thereby inducing new hair growth. The binding of aFGF to FGFR2b on follicular epithelial cells activates downstream ERK1/2 and Akt signaling cascades, which are essential for cell proliferation and protection against oxidative stress-induced apoptosis [215], [216].

A significant conceptual gap exists when extrapolating hair regeneration potential from general wound healing models. While MNZs successfully accelerate skin closure via angiogenesis and immunomodulation, these broad regenerative cues do not guarantee the reactivation of the quiescent HFSC niche, which requires precise, high-threshold signaling distinct from homeostatic repair. Consequently, observing hair growth at wound margins often confounds true follicle neogenesis with the passive migration of existing progenitors. Future investigations must distinguish between generalized tissue maintenance and specific anagen induction, focusing on validating the targeted activation of molecular switches within the bulge niche rather than relying on non-specific metabolic support.

Matching MNZs to pathological scenario

The diversity of metal elements and their distinct catalytic behaviors necessitate a critical translational assessment regarding the optimal selection of MNZs for specific skin regeneration scenarios. Given that no universal solution exists, the selection of the metal core must be strictly aligned with the dominant pathological barriers of the target clinical stage. A stage-dependent selection framework is summarized in Table 9.

Table 9.

Comparative selection of MNZs for different pathological scenarios.

Clinical Scenario	Dominant Pathology	Therapeutic Goal	Recommended Metal Core	Limitations / Cautions
Infected Diabetic Wound	-Biofilm -Acute inflammation -Ischemia	-Sterilization & Angiogenesis -Destructive ROS generation	-Cu: POD-like killing + HIF-1α stabilization -Ag: Broad-spectrum membrane disruption -Fe: Fenton reaction triggers bacterial ferroptosis	-Cu: High doses may cause cytotoxicity -Ag: Potential for argyria; inhibits keratinocyte migration -Fe: Risk of thrombosis/lipid peroxidation
Pathological Scarring	-Excessive ROS -TGF-β1 activation -Hypoxia	-Anti-fibrosis & Remodeling -Protective ROS scavenging	-Ce: Regenerative Ce3+/Ce4+ cycle permanently scavenges ROS -Mn: CAT-like activity alleviates hypoxia-induced fibrosis -Mo: Biodegradable and metabolic regulation of collagen	-Cu/Fe: Residual Fenton activity may worsen scarring if not chelated -Ce: Long-term retention in RES (liver/spleen) requires ultrasmall designs
Hair Regeneration	-Oxidative stress -Stem cell quiescence -Vascular insufficiency	-Niche Maintenance & Activation -Metabolic reprogramming	-Zn: Activates Wnt/β-catenin pathway for anagen induction -Ce: Protects stem cell mitochondria from ROS -Pt: Stable CAT/SOD mimicry improves oxygenation	-Ag: Cytotoxicity to follicle stem cells -Pt/Au: High cost; non-biodegradability limits chronic use -Cu: Narrow therapeutic window; excess may cause hair damage

In the early stage of diabetic infected wounds, the primary objective is the rapid eradication of drug-resistant biofilms and the neutralization of acute inflammation, necessitating destructive high-intensity ROS generation to overwhelm bacterial defense systems. Cu and Ag emerge as superior first-line candidates; Cu-based MNZs are particularly advantageous for diabetic wounds due to their dual benefit of generating ·OH via POD-like activity for sterilization while stabilizing HIF-1α to promote angiogenesis, addressing the concurrent challenges of infection and ischemia. Fe serves as a secondary choice effective for ferroptosis-mediated bacterial killing, though it requires caution in patients with vascular complications due to potential risks of thrombosis. Conversely, pure antioxidant nanozymes (e.g., Ce, Mn) are generally contraindicated as monotherapies in this phase, as they may inadvertently protect bacteria from oxidative killing unless conjugated with antibiotics.

As the wound transitions to the proliferation and remodeling phase, the therapeutic goal shifts from destruction to protection and regulation, aiming to inhibit the ROS-driven TGF-β1/Smad signaling loop that drives scarring. Consequently, metals with strong oxidative capabilities (Cu, Fe) must be avoided or strictly controlled to prevent residual Fenton activity from exacerbating oxidative stress and driving collagen hyper-deposition. Instead, Ce and Mn represent the optimal choices; Ce-based MNZs provide sustainable regenerative Ce³⁺/Ce⁴⁺ redox cycles to continuously scavenge ROS and lock TGF-β1 in a latent state, while Mn-based MNZs uniquely generate oxygen to alleviate the hypoxic microenvironment fueling fibrosis. Additionally, Mo is gaining prominence as due to its excellent biocompatibility and rapid renal clearance, making it suitable for preventing long-term foreign body reactions in scar tissue.

For hair follicle regeneration, such as in AGA, the microenvironment requires subtle metabolic reprogramming rather than drastic redox shifts, focusing on preserving the mitochondrial function of stem cells and stimulating specific signaling pathways. Ce and Zn stand out as first-line choices; Ce offers critical mitochondrial protection to prevent stem cell apoptosis, while Zn functions uniquely as a bioactive ion to activate zinc-finger proteins and the Wnt/β-catenin pathway, directly stimulating the anagen transition. Pt and Au serve as effective alternative choices due to their stable antioxidant properties and PTT-mediated blood flow enhancement, but their non-biodegradability poses concerns for chronic, repeated scalp applications.

In summary, the design of MNZs must evolve from “material-centric” to “pathology-centric”. Future clinical formulations are likely to employ sequential therapy (e.g., Cu/Ag in the initial phase for debridement, followed by Ce/Mn in subsequent phases for remodeling) or stimuli-responsive adaptive systems (e.g., pH-triggered switching from Fe-mediated killing to Ce-mediated repair) to match the dynamic biological needs of skin regeneration.

Biosafety and translation

While the therapeutic efficacy of MNZs in skin regeneration, ranging from infection control to appendage restoration, has been extensively validated in preclinical models, their transition from “bench to bedside” remains hampered by complex biological barriers. Unlike organic drugs with well-defined metabolic kinetics, inorganic MNZs face rigorous scrutiny regarding their absorption, distribution, metabolism, and excretion profiles. The clinical translation of MNZs is currently navigating the Valley of Death, where the equilibrium between catalytic potency and long-term biosafety constitutes the decisive factor. It is necessary critically evaluates the biosafety landscape, metabolic fate, and translational potential of MNZs applications (Table 10).

Table 10.

Evaluation of clinical translational prospects for MNZs in skin regeneration.

Metal	Key Clearance Pathway	Major Toxicity / Risk Factors
Fe	Endogenous Metabolism (Ferritin/Hemoglobin)	-Local iron overload -Ferroptosis
Zn	Endogenous Metabolism (Zinc pool)	-Low risk -Essential nutrient
Mo	Renal Clearance (Oxidizes to MoO42-)	-Low -Degradation products are non-toxic
Ru	Renal Clearance (if ultrasmall < 5.5 nm)	-Low -Rapid excretion
Ce	RES Accumulation (Liver/Spleen)	-Long-term biopersistence -Lysosomal damage
Cu	Biliary Excretion	-Cuproptosis -Hepatotoxicity -Narrow safety margin
Ag	Biliary / Skin Deposition	-Argyria -Cytotoxicity to keratinocytes -Neurotoxicity
Mn	Biliary Excretion	-Neurotoxicity (Manganism) at high doses
Au	RES Accumulation	-Biopersistence -Immunogenicity risks
Pt	RES / Renal (size dependent)	-Biopersistence (if large)
Co	Renal / Fecal	-High intrinsic cytotoxicity -Necrosis risk

Barrier-status dependent biosafety assessment

A “one-size-fits-all” safety evaluation is inappropriate for skin regeneration because the integrity of the skin barrier varies drastically across different therapeutic indications. Consequently, biosafety risks must be categorized into three distinct scenarios based on the specific barrier status.

In the context of diabetic ulcers, deep burns, or infected trauma, the stratum corneum and basement membrane are compromised or entirely absent. This zero barrier state means that topically applied MNZs have direct access to the dermal capillary network and immune cells. The primary concern here is not merely local skin irritation but the high risk of systemic translocation. Nanoparticles can easily enter the bloodstream and accumulate in distal organs such as the liver, spleen, and kidneys. For instance, silver nanoparticles applied to open wounds have been detected in systemic circulation, raising risks of argyria and organ toxicity [217]. Therefore, MNZs designed for open wounds must possess rapid clearance capabilities, such as renal filtration, or inherent biodegradability to prevent systemic accumulation after they fulfill their local catalytic function.

In hypertrophic scars or keloids, the skin barrier is technically restored but structurally abnormal, often characterized by a thickened but dysfunctional stratum corneum and a disorganized collagen matrix. The challenge in this scenario lies in balancing transdermal delivery efficiency with the risk of local retention. While the thickened barrier impedes passive diffusion, therapeutic strategies often employ microneedles or physical enhancers to force MNZs into the dermis. The safety risk focuses on chronic local inflammation or granuloma formation caused by non-degradable particles trapped in the dense scar tissue, rather than immediate systemic toxicity. Ideally, MNZs for anti-scarring applications should be locally metabolized within the dermal matrix to avoid long-term tissue persistence that could trigger secondary foreign body reactions.

Treating AGA involves applying MNZs to intact scalp skin, which presents a formidable barrier against particle entry. To overcome the stratum corneum, MNZs are frequently integrated with dissolving microneedles or designed specifically for follicular penetration. The safety concern here is unique, as MNZs must target the hair follicle bulb without diffusing excessively into the surrounding interfollicular dermis or systemic circulation. Since these treatments often require long-term, repeated application, the cumulative toxicity of metal ions in the scalp environment becomes a critical regulatory hurdle. Consequently, the safety focus must be on ensuring high biocompatibility with stem cells and avoiding follicular toxicity even at low, chronic doses.

Metabolic fate and organ-specific clearance

The long-term biopersistence of inorganic nanomaterials is the most significant regulatory hurdle. The metabolic fate of MNZs is dictated by their chemical composition, size, and surface chemistry, dividing them into three primary categories based on clearance pathways.

Fe and Zn-based nanozymes represent the safest class for clinical translation due to the body’s innate homeostatic mechanisms. Iron-based nanozymes, such as Fe₃O₄ and Prussian Blue, benefit significantly from the body's sophisticated iron handling system. Upon degradation, released iron ions are sequestered by ferritin or incorporated into hemoglobin, effectively bypassing heavy metal accumulation toxicity [218]. Zn ions released from ZIF-8 or ZnO frameworks in acidic wound environments function as essential cofactors for MMPs involved in re-epithelialization [219], [220]. The degraded components enter the physiological zinc pool, supporting diverse enzymatic functions without generating xenobiotic waste [38], [221], [222].

For non-essential metals, rapid excretion is preferred over accumulation to minimize systemic risk. Molybdenum (Mo)-based nanozymes exhibit a unique biodegradable advantage. Under physiological conditions, they slowly oxidize into molybdate ions (MoO₄^2-), a non-toxic form that is readily filtered by the glomerulus and excreted via urine [223]. This “use-and-dispose” mechanism resolves the issue of chronic toxicity, positioning Mo-based materials as top-tier candidates for translation. In the case of chemically stable noble metals like Ru and Rh, which pose accumulation risks if the particles are large, engineering them into ultrasmall nanodots (<5.5 nm) allows them to fall below the renal filtration threshold [224]. For example, ultrasmall RuO₂ nanozymes have been shown to escape reticuloendothelial system (RES) capture and be cleared renally, protecting against acute kidney injury while maintaining high multienzyme-like activity [225].

The most challenging category involves chemically inert or non-biodegradable nanomaterials that exceed the renal filtration limit (>10 nm) [226]. Large nanoparticles, particularly those composed of Au and Ag, tend to undergo opsonization and are subsequently phagocytosed by Kupffer cells in the liver and macrophages in the spleen [227]. While Au is biologically inert, its biopersistence raises regulatory concerns regarding potential long-term immunogenicity. Similarly, Au-Pt nanodots, despite their high efficiency in photothermal-antibacterial therapy, require careful long-term safety evaluation [228]. CeO₂, although celebrated for its auto-regenerative antioxidant cycle (Ce³⁺/Ce⁴⁺), faces similar challenges. As a lanthanide, the human body lacks an active excretion mechanism for cerium, causing large particles to persist in the RES and potentially cause lysosomal damage over time [217]. Recent strategies focusing on ultrasmall (<4 nm) PEGylated ceria have shown promise in enabling renal clearance, thus improving the safety profile [229], [230].

Adverse reactions and toxicity thresholds

The catalytic mechanism of MNZs is often a double-edged sword, where the very ROS-generating activity used to eradicate biofilms can indiscriminately damage host fibroblasts and keratinocytes if uncontrolled. While metabolically safe, Fe-based nanozymes pose a local risk of inducing ferroptosis, an iron-dependent form of programmed cell death, if local iron ion concentrations exceed the buffering capacity of ferritin [182]. While beneficial for tumor ablation, this mechanism can be detrimental to wound healing if not precisely controlled via slow-release formulations. Similarly, Ag nanoparticles, while historically established as antimicrobials, carry significant risks when applied to open wounds. Systemic absorption can lead to argyria due to Ag deposition, and studies have shown that silver nanoparticles exhibit higher embryotoxicity compared to Au or Pt counterparts in zebrafish models, necessitating strict limits on the duration of use [231].

Cu-based nanozymes present another specific toxicity profile. Although they are potent angiogenic agents, they possess a narrow therapeutic window. Excess intracellular copper can trigger cuproptosis, a form of cell death linked to the lipoylation of TCA cycle proteins [232]. To mitigate this, novel Vitamin C-functionalized Cu nanozymes have been developed to enhance biocompatibility while maintaining potent antibacterial and immunomodulatory effects [233]. Furthermore, neurotoxicity is a critical concern with Mn nanozymes, which are effective SOD mimics for inflammation control. However, free Mn ions can traverse the blood–brain barrier, and chronic exposure to high doses carries a risk of manganism, a neurodegenerative disorder resembling Parkinson's disease. Manganese-gallic acid coordination polymers aims to provide safer, inflammation-specific regulation [234]. Additionally, nanozymes have been explored for mitigating neuropathic pain, highlighting the need for precise neuro-safety profiling [235].

Clinical translation status and hurdles

Despite the flourishing landscape of preclinical research, the translation of MNZs into approved clinical therapies remains slow. Currently, no specific “metal nanozyme drug” has received FDA approval for dermatological use, although Ag-based dressings, classified as device, are common. To date, only one nanozyme-based therapeutic, NCT06492811, has progressed to clinical trials, currently in Phase II for diabetic wound treatment using a nanozyme-hydrogel dressing [236]. Several bottlenecks impede this transition. First, regulatory ambiguity remains a significant hurdle; the classification of MNZs is often blurred. If an MNZ acts primarily via chemical catalysis to generate ROS, it falls under the definition of a drug; however, if integrated into a dressing acting as a physical barrier with auxiliary antimicrobial properties, it might be classified as a medical device. This ambiguity complicates the approval pathway and increases development risks. Second, there is a critical gap in long-term safety data. Most studies focus on acute efficacy, 2 to 4 weeks, due to the short application time of wound scenario. Data on the chronic effects (years) of biopersistent metals (Au, Ce) in the skin and visceral organs are largely missing [217]. For chronic conditions like psoriasis or anti-aging maintenance, proving long-term safety is paramount. Finally, manufacturing consistency poses a formidable challenge. Laboratory synthesis often relies on delicate control, such as single-atom anchoring or specific crystal facets, at the milligram scale. Scaling this to kilogram-level industrial production while maintaining strict batch-to-batch consistency in size, surface ligand density, and catalytic activity remains a major chemical engineering obstacle.

To bridge the gap between laboratory promise and clinical reality, future development must pivot from maximizing catalytic activity to optimizing biosafety. One promising direction is the prioritization of biodegradable designs, utilizing metals that degrade into non-toxic metabolites, such as Mo converting to molybdate, Fe to physiological iron, or biodegradable nickel disulfide [237]—offering the smoothest regulatory path. Concurrently, for noble metals and rare earths, engineering renal-clearable architectures with ultrasmall sizes (<5 nm) is essential to ensure rapid excretion and avoid biopersistence [225]. Furthermore, the development of smart responsiveness is crucial. Smart nanozymes that only activate their toxic (ROS-generating) modes in specific pathological microenvironments, such as acidic or high H₂O₂ infection sites, while remaining catalytically silent in healthy tissue, will significantly widen the therapeutic window. In conclusion, while the versatility of MNZs offers a revolutionary toolkit for skin regeneration, their success depends on a rigorous “safety-by-design” approach. The field is currently witnessing a paradigm shift towards biodegradable and renal-clearable nanozymes (e.g., Fe, Mo, ultrasmall Ru), which are poised to overcome the “Valley of Death” and enter clinical trials in the near future.

Conclusion and perspectives

In summary, MNZs have emerged as a versatile therapeutic class in skin regeneration, bridging the gap between basic wound closure and complex organ reconstruction. By mimicking natural enzymes such as SOD, CAT, and POD, MNZs function through a “Skin Regeneration Axis”: accelerating chronic wound healing via ROS scavenging and hypoxia alleviation, inhibiting pathological scarring through redox-mediated TGF-β downregulation, and promoting appendage regeneration by activating follicular stem cell niches. Despite these advances, the transition from promising nanomaterials to standard-of-care therapies is obstructed by distinct scientific and translational hurdles.

At the fundamental research level, current MNZ strategies often lack the biological precision required for intricate organogenesis. A primary challenge lies in the “black box” of regenerative signaling; it remains unclear whether MNZs play an instructive role in triggering differentiation pathways or merely a permissive role by creating a low-ROS environment. This mechanistic ambiguity is compounded by the scarcity of research specifically targeting pathological scarring and appendage regeneration compared to simple epithelialization. Furthermore, MNZs often exhibit catalytic promiscuity, lacking the lock-and-key substrate specificity of natural enzymes, which risks off-target oxidation in complex wound exudates. Additionally, static catalytic activities fail to match the dynamic spatiotemporal needs of healing, where the transition from a pro-inflammatory antimicrobial phase to a pro-remodeling antioxidant phase requires precise “on-off” switching.

Translational barriers further impede clinical adoption, particularly regarding biosafety and manufacturing. The long-term retention of non-essential metals poses risks of chronic immunotoxicity, necessitating a shift toward renal-clearable ultrasmall architectures or biodegradable metals (e.g., Fe, Mo) that metabolize into non-toxic ions. Moreover, maintaining batch-to-batch consistency in catalytic activity during kilogram-scale production remains a significant engineering bottleneck for regulatory approval (Fig. 7).

Fig. 7.

Roadmap of MNZ research and translation challenge with their potential solution.

Looking forward, the next generation of MNZs must evolve from simple catalysts into intelligent, multimodal regenerative platforms to break the therapeutic ceiling of standalone catalytic therapy. Future efforts should prioritize converging MNZs with advanced biotechnologies: integrating with stem cell therapies to engineer pro-survival niches, utilizing MNZs as non-viral vectors for gene therapy to overcome signaling deficits in fibrotic tissue, or embedding them into smart bio-electronic dressings for feedback-regulated precision. By addressing the mechanistic voids in appendage regeneration and solving the biosafety bottlenecks of metal retention, MNZs are poised to transition from passive tools for wound management to active drivers of true skin organogenesis.

Funding

This work was supported by grants from Zhejiang Provincial Medical and Healthy Science Foundation of China (No. WKJ-ZJ-2530), Zhejiang Provincial Natural Science Foundation of China (No. LQN26H150014), National Natural Science Foundation of China (No. 82172206), Zhejiang Provincial Medical and Healthy Science Foundation of China (No. 2024KY110 and 2023RC183), Key Research and Development Program of Zhejiang Province (No. 2024C03068).

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.

Biographies

Ze-Ming Zhuang is currently a Ph.D. student in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. His research interest primarily focuses on the application of biomaterials in skin regeneration. He was primarily responsible for the core Conceptualization of the review, performing the systematic Literature Search/Investigation, and Writing the Original Draft Preparation.

KaiGuo is currently a Ph.D. student in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. His research interest primarily focuses on collagen crosslinking and application. He also contributed significantly to Conceptualization, carried out substantial Literature Search, and was crucial for the Data Curation and synthesis of the literature findings.

Zhang-RuiWu is currently an M.D. student in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. Her research interest primarily focuses on scar and fibrosis treatment. She took the lead in organizing and Visualizing the key information, performed detailed Data Curation, and provided the Writing -Review & Editing.

Yi Wang is currently a postgraduate student in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. Her research interest primarily focuses on scar treatment. She contributed through extensive Literature Search and Data Curation.

Dr. Xin-CaoZhong is a Resident Doctor in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. Her research interest primarily focuses on scar and RNA therapy. She focused on the Validation of the literature and the refinement of the Methodology

Chun-Ye Chen is currently an M.D. student in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. Her research interest primarily focuses on small molecule drug for skin therapy. She assisted with in-depth Data Curation and the initial Writing -Review & Editing of the manuscript.

Dr.Zi-Xuan Feng is a Resident Doctor in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. Her research interest primarily focuses on bioinformatics for skin disease. She assisted with in-depth Data Curation and the initial Writing -Review & Editing of the manuscript.

Yan-ZeYu is a postgraduate student in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. His research interest primarily focuses on bioinformatics for skin disease. He performed Literature Investigation and assisted with the classification of Resources.

Dr. Hai-Qi Zhang is an Associate Research Professor in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. Her research interest primarily focuses on biomaterials application for skin regeneration. She performed Literature Investigation and assisted with the classification of Resources.

Dr. Min-Hong Tan is an Associate Research Professor in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. His research interest primarily focuses on biomaterials application for skin regeneration. He provided substantial support for Literature Search and assisted with Formal Analysis of the compiled information.

Dr. Tao Zhang is an Resident Doctor in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. His research interest primarily focuses on polymer dressing skin application. He provided substantial support for Literature Search and assisted with Formal Analysis of the compiled information.

Dr. Xiao-Ying Lin is a Research Assistant Professor in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. Her research interest primarily focuses on scar mechanism. She was primarily responsible for the collection, Data Curation of relevant literature, and securing Funding Acquisition.

Dr. Yong Wang is an Associate Research Professor in the Department of Plastic Surgery at Sir Run Run Shaw Hospital, Zhejiang University School of Medicine. His research interest primarily focuses on collagen crosslinking and collagen-based dressing invention. He was responsible for the overall Conceptualization, providing essential Supervision and Project Administration, and ensuring the intellectual content and clarity of the manuscript through comprehensive Writing -Review & Editing.

Prof. Wei-QiangTan , MD, PhD, is Professor, Chief Physician, and Doctoral Supervisor at Zhejiang University School of Medicine. He serves as Director of the Department of Plastic Surgery at Sir Run Run Shaw Hospital. His research focuses on diabetic wound healing, including biomaterials, tissue regeneration, and mechanisms of microenvironment remodeling. He has published over 100 papers, with 79 SCI-indexed articles as first or corresponding author, including several in journals with impact factors above 20. He was responsible for the overall Conceptualization, securing Funding Acquisition, providing essential Supervision and Project Administration, and ensuring the intellectual content and clarity of the manuscript through comprehensive Writing -Review & Editing.