Application and prospect of zinc oxide nanoparticle-enhanced biomimetic hydrogels in articular cartilage repair

Abstract

Articular cartilage injury is a common clinical condition in orthopedics. The avascular and aneural nature of the tissue results in extremely poor self-healing ability, which seriously impairs patients' motor function and quality of life. Current clinical treatment methods such as bone marrow stimulation, chondrocyte transplantation, and artificial joint replacement generally have limitations including the repaired tissue being primarily fibrocartilage, poor integration with host cartilage, and the need for secondary surgery, making them difficult to meet clinical needs. Hydrogels have become promising tissue engineering scaffolds for mimicking the natural cartilage extracellular matrix (ECM) due to their high water content, three-dimensional porous structure, and excellent biocompatibility. However, their defects such as insufficient mechanical strength, lack of biological activity, and antibacterial properties limit their clinical application and translation. Zinc oxide nanoparticles (ZnO NPs) are multifunctional bioactive materials that can enhance hydrogel mechanical stability and provide antibacterial, anti-inflammatory, and chondrogenic benefits through controlled Zn²⁺ release. This review summarizes the physicochemical properties and biological functions of ZnO NPs, discusses strategies for constructing ZnO NP-enhanced biomimetic hydrogels, and reviews current in vitro and in vivo evidence supporting their application in cartilage repair. It also highlights key challenges, including biosafety, concentration control, and translational feasibility, providing guidance for the rational design and clinical development of ZnO-based hydrogel scaffolds. Recent studies have explored hydrogel-based strategies to address this limitation by providing a microenvironment that favors hyaline cartilage regeneration over fibrocartilage formation.

Highlights

• •ZnO NPs enhance hydrogel mechanics and biological activity.
• •Zn2+ release supports antibacterial and anti-inflammatory effects.
• •ZnO-modified hydrogels promote chondrogenic differentiation.
• •Dose, morphology, and release kinetics determine biosafety.
• •Clinical translation requires long-term safety validation.

1. Introduction

1.1. Structure, function and injury of articular cartilage

Articular cartilage is a hyaline connective tissue covering the surface of joints, mainly composed of chondrocytes and extracellular matrix (ECM). Among them, ECM accounts for more than 90% of the dry weight of the tissue [1]. The main fibrous component is type II collagen, which forms a porous network structure through cross-linking with proteoglycans such as chondroitin sulfate and hyaluronic acid. This structure endows cartilage with excellent elasticity and compressive resistance, enabling it to buffer impact forces and reduce friction during joint movement [2]. Cartilage tissue is avascular, aneural, and lacks lymphatic vessels. Its nutrition depends on the diffusion of synovial fluid, resulting in extremely weak self-repair ability [3,4]. Once defects occur due to trauma, degenerative lesions, or inflammation, self-healing is difficult. Even minor defects can easily progress to osteoarthritis (OA), causing joint pain, deformity, and even loss of function, seriously affecting the patient's quality of life [5].

1.2. Limitations of current cartilage repair strategies

Currently, commonly used clinical cartilage repair techniques have obvious limitations: Bone marrow stimulation induces bone marrow mesenchymal stem cells (BMSCs) to migrate to the defect area through drilling, but the repaired tissue is primarily fibrocartilage, whose mechanical properties are substantially lower than those of native hyaline cartilage and which is prone to long-term degradation [6]. Osteochondral Autografting (OATS) can restore hyaline-like cartilage in the defect area; however, it is limited by donor site morbidity, restricted availability of graft tissue, and potential mismatch between graft and defect geometry [7]. Autologous Chondrocyte Implantation (ACI)requires in vitro expansion of chondrocytes, with complex processes and high costs, and abnormal cell differentiation is prone to occur after surgery [8]. Artificial joint replacement surgery is traumatic, and the prosthesis has a limited lifespan, requiring secondary revision [9,10]. These techniques all have the risks of postoperative infection and poor interface integration between the repaired tissue and the host cartilage, so more efficient and safe repair strategies are urgently needed. Nanotechnology has been increasingly explored to enhance the performance and longevity of orthopedic implants [11].

1.3. Rise of tissue engineering strategies

The rise of tissue engineering centered on the three elements of “scaffold-cell-growth factor” has opened up a new way for cartilage defect repair. As a carrier for cell colonization, proliferation, and differentiation, the scaffold needs to simulate the structure and function of natural ECM, which is the key to determining the repair effect. An ideal cartilage scaffold should have mechanical properties matching natural cartilage, good biocompatibility, controllable degradation rate, and biological activity that promotes the synthesis of cartilage-specific matrix [12,13]. In recent years, with the advancement of materials science and characterization technology, the research and development of biomimetic scaffolds have gradually become a research hotspot [14,15]. By precisely regulating the microstructure and chemical composition of the scaffold, the active regulation of the cartilage repair microenvironment can be achieved [16]. Biomimetic-smart materials have also emerged as promising candidates for osteochondral regeneration and repair [17].

1.4. Advantages of hydrogels as ideal cartilage scaffolds

Hydrogels are three-dimensional network materials formed by physical or chemical cross-linking of polymer chains, with a water content of 70%-90%, which is highly consistent with that of natural cartilage, and can provide a moist microenvironment similar to that in vivo for cells. The porous structure of hydrogels can promote the transport of nutrients and the excretion of metabolic wastes. Hydrogels exhibit excellent injectability and can fill irregular defect areas through minimally invasive approaches, thereby reducing surgical trauma [18]. The mechanical properties and degradation rate of hydrogels can be regulated by adjusting the type, cross-linking degree, and concentration of polymer substrates [19,20]. However, most unmodified hydrogel systems, including gelatin-sodium alginate composites, still exhibit mechanical moduli well below the range of native articular cartilage. The incorporation of reinforcing agents such as ZnO NPs represents one strategy to bridge this mechanical gap [21]. These characteristics make hydrogels a promising candidate for cartilage tissue engineering scaffolds. Recent reviews have further highlighted the multifaceted roles of hydrogels in ameliorating osteoarthritis through mechanical modulation, anti-inflammation, and tissue regeneration [22].

1.5. Necessity of introducing zinc oxide nanoparticles (ZnO NPs)

Although traditional hydrogels have advantages in structure and biocompatibility, they still have many problems. Traditional hydrogels have insufficient mechanical strength, and are prone to rupture in the dynamically loaded joint environment, making it difficult to support the cartilage repair process. They lack antibacterial properties, and postoperative infection is likely to cause cartilage repair failure. Although the infection rate after cartilage repair surgery is low, once it occurs, the implant needs to be removed and anti-infection treatment is required, which seriously affects the prognosis. The biological activity of traditional hydrogels is limited, and it is difficult to actively regulate cell behavior, requiring additional loading of growth factors or genes, which increases costs and technical complexity [23,24]. ZnO NPs, as a multifunctional nanomaterial, can effectively solve the above problems. The nanoscale size of ZnO NPs allows them to be uniformly dispersed in the hydrogel matrix, and the mechanical properties can be improved by enhancing the interaction between polymer chains [25]. The released Zn²⁺ can exert antibacterial effects by generating reactive oxygen species and destroying bacterial membrane structures [26]. As an essential trace element in the human body, Zn²⁺ can activate the chondrogenic differentiation signaling pathway of BMSCs and promote the synthesis of type II collagen and proteoglycans [27]. Therefore, the combination of ZnO NPs and hydrogels has become an effective strategy to improve the comprehensive performance of cartilage scaffolds. In addition to ZnO NPs, various other nanomaterials have been incorporated into hydrogel systems to enhance their mechanical integrity for cartilage repair. For example, alginate-gelatin composite hydrogels reinforced with silicate nanoparticles or other inorganic nanoparticles have demonstrated improved mechanical properties and biocompatibility. Zinc oxide nanoparticles have attracted particular attention because they not only provide mechanical reinforcement but also exhibit a wide range of bioactive properties.

Currently, there is growing interest in hydrogel-based scaffolds for cartilage repair. However, previous reviews have been limited in their comprehensiveness; most have focused on the general properties of hydrogels or have examined the biological functions of ZnO NPs in isolation, without providing a comprehensive overview of their synergistic effects in articular cartilage repair. This review focuses on the application of ZnO NPs-enhanced biomimetic hydrogels in cartilage repair, systematically expounds the physicochemical properties and biological functions of ZnO NPs, emphasizes their advantages as hydrogel modifiers, systematically analyzes the construction strategies of ZnO NPs composite hydrogels from four dimensions: substrate selection, composite methods, biomimetic design, and AI-driven strategies; interprets the microstructure and mechanical properties of the materials combined with advanced characterization technologies, summarizes the research progress of such materials in in vitro cell experiments and in vivo animal experiments, discusses the challenges in current research, and looks forward to future development directions, so as to provide references for promoting the industrial production and clinical application of such materials. Fig. 1 summarizes the contents of the review in detail.

Fig. 1.

Systematically elaborates on the application of zinc oxide nanoparticle-enhanced hydrogels in articular cartilage repair, covering their properties and biological functions, construction strategies, and applications in cartilage repair.

2. Properties and biological functions of zinc oxide nanoparticles

2.1. Basic physicochemical properties

ZnO NPs have various micro-morphologies, commonly including spherical, rod-like, zinc oxide tetrapods (t-ZnO), nanoflower-like, and streamline-like ZnO (str-ZnO) (Fig. 2) (Table 1). Different morphologies have significant effects on their properties and composite effects with hydrogels [32,33]. For example, The outstanding arm structure of t-ZnO NPs, this open structure has stronger antibacterial and protein adsorption ability than the closed structure [28]. Nanoflower-like ZnO NPs have a larger specific surface area, which can increase the release sites of Zn²⁺ [34]. Compared with tetrapod-like traditional spherical particles, streamlined ZnO NPs can reduce the steric hindrance with hydrogel polymer chains, optimize the rheological properties of the material, and at the same time increase the contact area with cells, improving the gene transfection efficiency [29].

Fig. 2.

ZnO nanoparticles (ZnO NPs) exhibit morphology- and modification-dependent properties that influence dispersion, Zn²⁺ release, and interactions with hydrogel networks. These features contribute to antibacterial activity, anti-inflammatory regulation, chondrogenic support, MMP inhibition, reduced ECM degradation, and improved cartilage repair. However, excessive dose, particle agglomeration, or specific morphologies may induce ROS overproduction, mitochondrial dysfunction, chronic inflammation, reduced cell viability, and tissue damage.

Table 1.

Representative ZnO nanoparticle forms and their documented applications in hydrogel and polymeric matrix systems.

ZnO form/structure	Hydrogel or matrix system	Primary function evaluated	Key readouts	Ref.
ZnO tetrapods	Alginate bioink, 3D printed dressing	Protein handling, antibacterial use	Printability, protein adsorption, construct morphology, ex vivo skin compatibility	[28]
Streamlined ZnO nanoparticles	Injectable MMP responsive GelMA hydrogel, miRNA loaded	miRNA delivery, cartilage matrix homeostasis, cartilage repair	Cellular uptake, miRNA expression, in vivo signal tracking, histology and immunostaining	[29]
ZnO nanoparticles	Thermoresponsive injectable gelatin and oxidized alginate hydrogel	Mechanical reinforcement, cytocompatibility	Rheology and gelation, microstructure, swelling and degradation, cell viability and attachment	[25]
ZnO nanoparticles	Starch and chitosan composite film	Antibacterial and sustained release, mechanical reinforcement, cytocompatibility	Tensile testing, inhibition assay, release profile, cell viability	[30]
ZnO nanoparticles	PEGDA matrix, vat photopolymerization	Antibacterial activity, mechanical performance	Tensile testing, bacterial viability assays, ion leaching, morphology and elemental mappin	[31]

Fig. 2 presents the structural features, functional mechanisms, and biosafety risks of ZnO NPs in hydrogel-based cartilage repair. Shows representative ZnO NP morphologies, including spheres, rods, tetrapods, polyhedral particles, nanoflowers, and streamlined structures. Illustrates surface-modification strategies, such as ligand grafting and hydrogel coating, which contribute to antibacterial activity, anti-inflammatory regulation, and chondrogenic support. Summarizes the interaction between ZnO NPs and hydrogel networks, including cross-linking, chemical bonding, controlled Zn²⁺ release, and MMP inhibition-mediated reduction of ECM degradation. Highlights the role of ZnO-based hydrogels in cartilage repair by improving compatibility, tissue integration, and mechanical strength. Summarizes potential biosafety risks associated with excessive dose or unfavorable morphology, including agglomeration, ROS overproduction, mitochondrial dysfunction, reduced cell viability, and tissue damage. Together, these panels illustrate the dual role of ZnO NPs as functional additives and potential risk factors in hydrogel-based cartilage repair systems.

ZnO NPs are easy to disperse uniformly in the hydrogel matrix, and can also be effectively taken up by BMSCs to regulate intracellular signaling pathways and promote chondrogenic differentiation. When the concentration of ZnO is too high, it is prone to agglomeration, leading to stress concentration inside the hydrogel, which instead reduces its mechanical properties [35].

The surface chemical properties of ZnO NPs can be further optimized through modification. For example, modification with catechol groups can enhance their interaction with polymer substrates and improve their properties [36]. Hydrogel coating can reduce the initial burst release of drugs and improve the biocompatibility of the material [37]. The semiconductor property of ZnO NPs enables them to generate reactive oxygen species (ROS) under light of specific wavelengths, which can be used for synergistic antibacterial effects [26]. Advanced characterization technologies such as XRD and XPS can accurately analyze the crystal structure and surface element composition of ZnO NPs, providing a theoretical basis for their combination with hydrogels and performance regulation.

2.2. Biological importance of zinc ions

Zinc is an essential trace element in the human body and plays a key role in cartilage metabolism. As a coenzyme of various enzymes, Zn²⁺ can regulate cell proliferation and ECM synthesis. Zn²⁺ can inhibit the activity of matrix metalloproteinases, reduce ECM degradation, and maintain the homeostasis of cartilage matrix [38]. In addition, Zn²⁺ is involved in regulating the function of immune cells. By inhibiting the release of pro-inflammatory factors such as TNF-α and IL-6, it reduces the inflammatory response during cartilage repair and creates a good microenvironment for cell proliferation and differentiation [39].

2.3. Toxicity, biocompatibility and degradability

The biosafety of ZnO NPs is the prerequisite for their clinical application. A large number of studies have shown that ZnO NPs have good biocompatibility within an appropriate concentration range [40,41]. They have no significant effect on the survival rate of BMSCs and chondrocytes, and do not cause obvious cell apoptosis or DNA damage. The degradation behavior of ZnO NPs is controllable. In the physiological environment, ZnO NPs can be gradually degraded into Zn²⁺, which is excreted through normal metabolic pathways such as renal excretion in the body, avoiding cumulative toxicity [42]. In the rat cartilage defect model, after implantation of ZnO NPs composite hydrogel, the serum Zn²⁺ concentration was always maintained within the normal physiological range, and no pathological damage was found in major organs such as the liver, kidney, and spleen [39].

However, the toxicity of ZnO NPs is closely related to concentration and surface modification. With the increase of concentration, excessive release of Zn²⁺ may cause oxidative stress damage [42]. Unmodified ZnO NPs have high surface charge, which is easy to bind to cell membranes and reduce cell survival rate. Surface coating with chitosan, polyethylene glycol, etc., or controlling the release rate can further improve their biosafety and lay the foundation for clinical application [37]. The optimal concentration of zinc oxide nanoparticles in hydrogels has attracted significant attention. Ghanbari et al. prepared alginate-formaldehyde/gelatin/ZnO hydrogels, which were non-toxic at ZnO concentrations ranging from 0.01% to 0.05% by weight; the 0.05% ZnO group exhibited a cell survival rate exceeding 90% after 24 h [43].

It is worth noting that several studies have shown that ZnO NPs can induce oxidative stress, manifested as excessive ROS production, lipid peroxidation, and mitochondrial dysfunction, which is particularly pronounced at higher concentrations or under specific particle morphologies. When the concentration of ZnO NPs in in vitro experiments exceeds 10 μg/mL, it is often accompanied by inflammatory responses, such as increased expression of IL-1β and TNF-α. Therefore, ignoring these potential risks may hinder clinical translation, as regulatory authorities require evidence of long-term safety before approving new implantable materials.

2.4. Antibacterial and antibiofilm activity

Infections caused by drug-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) are important reasons for the failure of cartilage repair, which are difficult to be effectively controlled by traditional antibiotics [44]. ZnO NPs have broad-spectrum antibacterial activity and can significantly inhibit both Gram-positive and Gram-negative bacteria [45]. ZnO NPs can generate reactive oxygen species, which oxidize bacterial membrane lipids, leading to increased cell membrane permeability and the leakage of intracellular contents [26]. In addition, Zn²⁺ from ZnO NPs can enter bacterial cells, where it binds to respiratory chain enzymes such as NADH dehydrogenase, disrupting the electron transport chain, interfering with DNA replication, and promoting protein misfolding [46]. Antibacterial hydrogels have been increasingly recognized as effective strategies for combating chronic infections [47].

2.5. Anti-inflammatory properties

The inflammatory response plays a role in clearing necrotic tissue in the early stage of cartilage injury, but persistent chronic inflammation can inhibit cartilage repair and promote the development of osteoarthritis [48]. ZnO NPs can exert anti-inflammatory effects by regulating the function of immune cells: they can act as immunogens, affecting the transcription of genes related to immunity and inflammation in immune cells, thereby regulating the function of immune cells and exerting anti-inflammatory effects [49]. Zn²⁺ inhibits the NF-κB signaling pathway by suppressing the phosphorylation and degradation of IκBα. IκBα is an inhibitory protein that keeps NF-κB sequestered in the cytoplasm. This inhibitory effect prevents the p65 subunit from entering the nucleus, thereby reducing the transcription of pro-inflammatory cytokines such as TNF-α, IL-1β, and IL-6 [50]. In addition, Zn²⁺ upregulates the expression of A20 (also known as TNFAIP3). A20 is a negative feedback regulator that deubiquitinates key signaling molecules upstream of NF-κB, further limiting excessive inflammatory responses.

Compared with other nanomaterials, ZnO NPs offer unique advantages. Silicon dioxide nanoparticles primarily serve a mechanical reinforcement function but lack antimicrobial or chondrogenic activity. Carbon-based materials, such as graphene oxide, exhibit excellent mechanical properties, but their long-term biocompatibility requires further investigation. Silver nanoparticles possess strong antibacterial effects but exhibit high cytotoxicity and cannot promote chondrocyte-like differentiation. In contrast, zinc oxide nanoparticles achieve multiple functions—including mechanical reinforcement, antibacterial activity, anti-inflammatory effects, and chondrogenic promotion—through the release of Zn²⁺.

3. Construction strategies of biomimetic hydrogels based on ZnO NPs

3.1. Selection of hydrogel substrates

The selection of hydrogel substrates directly determines their biocompatibility, mechanical properties, and degradation rate, which needs to be comprehensively considered according to the needs of cartilage repair [51]. Natural polymer substrates have excellent biocompatibility and cell affinity due to their similarity to natural ECM components, but have low mechanical strength and uncontrollable degradation rate [52]. Synthetic polymer substrates have stable mechanical properties and their performance can be regulated through chemical modification, but have poor biological activity [53]. Composite substrates, which combine the advantages of both, have become the mainstream direction of current research (Fig. 3) (Table 2). The chemical materials used in tissue engineering scaffold techniques span a wide range of polymers, ceramics, and composites [58].

Fig. 3.

Structural and mechanical optimization strategies of ZnO-based composite hydro-gels.(A)Schematics of technical route for preparing PAM/PEG, PAM/PEG/SA, PAM/PEG/SA-Na,PAM/PEG/SA/S-CNTs,andPAM/PEG/SA/S-CNTs-Na hydrogels [54]. Copyright ©International Journal of Minerals. (B)(a) Compression curve of different hydrogel samples.(b)Fracture energy of different hydrogel groups.(c)Tensile curve of different hydrogel samples.(d)Tensile stress and elongation break of different hydrogel groups.(e)Elastic modulus and toughness of different hydrogel groups. F. Residual mass ratio of different hydrogel groups. (*P < 0.05, **P < 0.01, ***P < 0.001, n = 6). Source: Reproduced under terms of the CC-BY li-cense [55]. Copyright © 2025 The Authors. Published by American Chemical Society. (C)(a)Microstructure of the upper hydrogel under the cross-section.(b)Real-life image of the double-layer hydrogel.(c)Microstructure of the lower hydrogel under the cross-section.(d)Microstructure of the upper hydrogel in the vertical section.(e)Microstructure at the dividing line of the doublelayer hydrogel in the vertical section.(f)Microstructure of the lower hydrogel in the vertical section [55]. Copyright © 2025 The Authors. Published by American Chemical Society. (D)Schematic diagram of self-degrading gel [56]. Copyright © ACS Omega.

Table 2.

Construction strategies of zinc based hydrogel systems and representative evidence.

Strategy/design	Representative hydrogel base	ZnO format/incorporation route	Reported function	Ref.
Vat photopolymerization composite	PEGDA matrix	ZnO nanoparticles embedded during printing and curing	Antibacterial activity and mechanical performance	[31]
Biodegradable bilayer scaffold	Photocured chondroitin sulfate hydrogel	Porous zinc scaffold combined with chondroitin sulfate hydrogel(cartilage layer); bilayer assembly	Osteochondral regeneration and mechanical support	[27]
Double network bilayer hydrogel	CMCS OSA PVA hydrogel networks	ZnO reinforced lower layer with drug loading; upper layer with complementary drug loading	Controlled release and osteochondral repair related outcomes	[55]
Thermosensitive in situ forming hydrogel	Thermosensitive copolymer hydrogel	ZnO nanoparticles dispersed in injectable formulation	Antimicrobial activity and wound healing support	[57]
Enzyme responsive hydrogel coating	GelMA and HAMA coating hydrogel	ZnO nanostructured titanium with adhesive hydrogel coating	Antibacterial surface with improved biocompatibility	[37]

Fig. 3 illustrates the four principal strategies for incorporating ZnO NPs into hydrogel networks:

Physical blending (Fig. 3A), chemical cross-linking (Fig. 3B),in situ synthesis (Fig. 3C), and enzyme-responsive design (Fig. 3D).

Each strategy yields distinct Zn2+ release profiles and mechanical properties, as discussed in detail in Sections 3.2.1–3.2.4.

3.1.1. Natural polymer substrates

Gelatin, sodium alginate, and hyaluronic acid are commonly used natural substrates. Gelatin is derived from collagen hydrolysis and has good cell adhesion and biodegradability, but its mechanical strength is low and it is easy to swell, so it needs to be modified by cross-linking or composite to improve stability [59]. Sodium alginate forms hydrogels through cross-linking and has excellent injectability, but lacks cell recognition sites and its degradation rate is difficult to regulate, so it needs to be optimized by combining with other components [60]. Hyaluronic acid, as the main component of cartilage ECM, has excellent biocompatibility and can promote chondrocyte proliferation, but its mechanical properties are weak and it is easy to deform under joint load, so it needs to enhance mechanical strength through chemical cross-linking [61]. Gelatin-based biogels with enhanced mechanical toughness have also been developed for biomedical sensor applications [62].

3.1.2. Synthetic polymer substrates

Polyethylene glycol (PEG) and poly(lactic-co-glycolic acid) (PLGA) are widely used. PEG hydrogels have stable chemical properties, their mechanical strength can be regulated by cross-linking density, and they have no immunogenicity, but their cell adhesion is poor, so cell recognition sites such as RGD peptides need to be introduced for improvement [63]. The degradation rate of PLGA hydrogels can be regulated by adjusting the ratio of lactic acid to glycolic acid, and they have excellent mechanical properties, but lack biological activity and are difficult to actively regulate cell behavior, so they need to be combined with natural polymers or active components [64].

3.1.3. Composite polymer substrates

Blending natural and synthetic polymers can achieve performance complementarity. For example, sodium alginate-PLGA composite hydrogels combine the rapid gelation of sodium alginate and the controllable degradation of PLGA. At the same time, the introduction of ZnO NPs further enhances mechanical properties and biological functions, making them the preferred substrate combination for current cartilage scaffolds [65]. Composite hydrogel-nanomaterial scaffolds combined with stem cells have demonstrated potential in mimicking 3D bone microenvironments for bone differentiation [66].

3.2. Composite methods of ZnO NPs and hydrogels

3.2.1. Physical blending

Physical blending is the simplest composite method. Pre-prepared ZnO NPs are dispersed in a polymer solution, and then hydrogels are formed through temperature, light, or ionic cross-linking. This method is simple to operate and low in cost, and is suitable for various hydrogel substrates [67].

3.2.2. Chemical cross-linking

Active groups are introduced on the surface of ZnO NPs through surface modification, enabling them to form covalent bonds with hydrogel polymer chains [68]. Although chemical cross-linking is relatively complex to operate, it can significantly improve the comprehensive performance of composite materials and is currently the mainstream composite method.

3.2.3. In-situ synthesis

Zinc salts and precipitants are added to the hydrogel precursor solution, and ZnO NPs are generated in situ in the hydrogel network by regulating the reaction temperature and pH value. In-situ synthesis can achieve atomic-level uniform distribution of ZnO NPs [69].

The interactions between ZnO NPs and hydrogel networks involve multiple mechanisms. Zn2+ ions released from ZnO NPs can act as ionic crosslinkers in certain hydrogel systems, particularly alginate-based systems, where they form coordination bonds with carboxyl groups on polymer chains. This ionic crosslinking is distinct from covalent crosslinking and is reversible and pH-sensitive, which may enable responsive Zn2+ release in the acidic microenvironment of cartilage defects. In addition, the surface hydroxyl groups and surface charge of ZnO NPs facilitate hydrogen bonding and electrostatic interactions with polymer chains, influencing particle dispersion, network density, and mechanical properties. Surface modification of ZnO NPs, such as functionalization with catechol groups or polyethylene glycol, can further enhance these interactions and improve the stability of the composite system.

The release kinetics of Zn²⁺ depend on the incorporation method and the hydrogel network structure. In physically blended systems, ZnO nanoparticles are typically embedded within the polymer matrix, resulting in an initial rapid release followed by a sustained release phase. In contrast, in chemically cross-linked systems, ZnO nanoparticles are covalently bound or encapsulated through strong interactions, typically exhibiting slower and more controlled release characteristics. Enzyme-responsive hydrogels degrade upon contact with MMPs at the site of cartilage defects, thereby enabling stimulus-induced Zn²⁺ release. The release profile directly influences the efficacy of cartilage repair. Moderate and sustained Zn²⁺ release can maintain long-term activation of chondrogenic signaling pathways and provide continuous antimicrobial protection; conversely, a sudden release may lead to transient Zn²⁺ toxicity, followed by a loss of biological activity. Therefore, regulating release kinetics through rational hydrogel design is crucial for optimizing cartilage regeneration.

To provide a more systematic and critical comparison of ZnO-related hydrogel systems, representative studies are summarized in Table 3. Because direct evidence on ZnO NP-incorporated hydrogels for cartilage repair remains limited, we included both cartilage/osteochondral repair studies and related hydrogel studies that provide evidence for mechanical reinforcement, Zn²⁺ release control, cytocompatibility, antibacterial activity, or anti-inflammatory effects. This classification helps clarify which systems have direct cartilage-repair evidence and which provide supportive but indirect evidence. As shown in Table 3, only a few ZnO-related hydrogel systems have been directly validated in cartilage or osteochondral repair models. Most studies mainly provide supportive evidence regarding mechanical reinforcement, antibacterial activity, cytocompatibility, or controlled release. Moreover, ZnO concentration, Zn²⁺ release kinetics, cytotoxicity thresholds, wear resistance, and long-term in vivo safety are not consistently reported across studies, which limits direct comparison among different material systems.

Table 3.

Comparative analysis of representative ZnO-related hydrogel systems relevant to cartilage and osteochondral repair.

Hydrogel system/ZnO morphology	ZnO concentration/strategy	Release/cytotoxicity profile	Mechanical evidence	Cartilage-repair relevance	Ref.
GelMA/streamlined ZnO	MMP-responsive ZnO hydrogel; miR-17-5p delivery	Zn2+ release; sustained miRNA delivery	Improved rheology; enhanced mechanics	Direct cartilage/OA repair	[29]
CMCS/OSA/PVA bilayer/ZnO NPs	ZnO-reinforced lower layer; drug-loaded design	Sustained curcumin release	Tensile strength; self-healing; adhesion	Direct osteochondral repair	[55]
Gelatin/oxidized alginate/ZnO NPs	0.01–0.05 wt% ZnO	>90% cell viability at 0.05 wt%	>5-fold storage modulus increase	Cartilage scaffold support	[25]
PEGDA/ZnO NPs	3D-printed PEGDA/ZnO	ZnO leaching; antibacterial activity	Printable scaffold; cartilage-like mechanics	Antibacterial scaffold support	[31]
Collagen/ZPVP-ZnO	ZPVP cross-linking	Slower dexamethasone release	Increased hardness; storage modulus	Release/mechanics support	[70]
Alginate/gum acacia/ZnO NPs	ZnO-loaded nanohydrogel	Sustained ZnO release	Hydrogel carrier system	Release/safety support	[71]
Alginate/tetrapodal ZnO	t-ZnO alginate bioink	Cell compatibility; antibacterial activity	3D printability	Morphology/printing support	[28]
3D-printed alginate/ZnO NPs	ZnO NPs in alginate constructs	Cell compatibility; antibacterial activity	Stiffer gels; porous constructs	Antibacterial scaffold support	[65]

3.3. Functional biomimetic design of hydrogels

In biomimetic hydrogel design, extracellular matrix (ECM)-mimicking components are commonly introduced to reproduce the biochemical and structural features of native cartilage microenvironments. ECM generally refers to the native extracellular matrix, whereas decellularized extracellular matrix (dECM) refers to tissue-derived matrix components obtained after removing cellular constituents while preserving tissue-specific biochemical cues [72]. dECM-based injectable hydrogels have attracted attention because of their biocompatibility, injectability, and ability to support cell adhesion, cell infiltration, and tissue reconstruction [72]. In cartilage repair, cartilage-derived dECM hydrogels have been explored for cartilage defect regeneration [73], while double-layer dECM-based injectable systems may support simultaneous repair of cartilage and subchondral bone [74]. Biomimetic design principles inspired by natural tissue healing processes have been increasingly applied in regenerative medicine [75]. Recent reviews have comprehensively summarized the applications and innovations of dECM-based bioactive scaffolds in tissue engineering [76].

3.3.1. Mechanical property biomimetics

The compressive modulus of articular cartilage ranges from approximately 0.5 to 2 MPa, while the storage modulus ranges from 0.1 to 1 MPa; specific values are influenced by age, joint location, and loading conditions. Hydrogels approach the mechanical range of natural articular cartilage by regulating the cross-linking density and ZnO NPs content. For example, incorporating streamline ZnO nanoparticles (str-ZnO) into GelMA significantly enhances the mechanical properties of the hydrogel. 0.5% str-ZnO increases the storage modulus (G′) by 1.6 times, the compressive strength reaches 0.41 MPa, and it can withstand a shear strain of 243.3%, whose mechanical properties are close to the mechanical range of natural articular cartilage [29]. However, most current studies have not reported their wear resistance, leaving a significant gap in this area. The fundamental concepts of biomechanics in bone regeneration and mechanobiology provide important theoretical foundations for scaffold design. The mechanical environment also plays a critical role in in vitro cartilage tissue engineering [77,78].

Additionally, the introduction of ZnO NPs creates physical or chemical cross-linking sites that restrict the mobility of polymer chains, typically resulting in a decrease in the hydrogel's swelling ratio. Additionally, while ZnO NPs can slow down degradation by enhancing network integrity, excessive agglomeration may create local defects, thereby accelerating degradation. Furthermore, the kinetics of Zn²⁺ release are highly dependent on the distribution of ZnO NPs within the hydrogel matrix; that is, uniform dispersion promotes sustained release, whereas aggregation leads to burst release. The introduction of a double cross-linking network can further improve the fatigue resistance of the hydrogel, enabling it to maintain structural stability under repeated loads and meet the needs of long-term joint activities [79].

3.3.2. Structural biomimetics

Natural cartilage ECM presents a hierarchical porous structure. Macropores are conducive to cell migration and nutrient transport, while micropores provide space for matrix synthesis [80]. Similar structures can be constructed in ZnO NPs composite hydrogels through methods such as freeze-drying and porogen removal. For example, the two-step freeze-polymerization method can form a more uniform and hierarchical porous structure in the hydrogel, significantly improving the mechanical properties of the hydrogel and achieving a faster stimulus response rate [81].

3.3.3. Biochemical biomimetics

The biological functionality of hydrogels is improved by introducing cell adhesion peptides, cartilage-specific targeting peptides, or synergistic growth factors. RGD peptides can enhance the adhesion ability between chondrocytes and hydrogels and promote cell spreading [82]. TGF-β3 can further activate the chondrogenic differentiation signaling pathway and improve the efficiency of matrix synthesis [83]. Functional hydrogels with tailored biochemical properties have shown therapeutic potential in various tissue repair contexts [84].

3.3.4. Enzyme-responsive design

High concentrations of MMPs exist in cartilage defect areas, which can degrade ECM and lead to repair failure. The construction of enzyme-responsive hydrogels can achieve specific drug release at the lesion site [85]. After the hydrogel is implanted into the cartilage defect area, the MMPs highly expressed in the injured area can trigger the responsive links in the hydrogel, promoting the precise release of ZnO NPs and RNA drugs, inhibiting MMPs activity, and simultaneously promoting cartilage repair [29]. Stimuli-responsive biomaterials have shown broad potential in various biomedical applications [86].

3.3.5. Gene hydrogel design

Hydrogels are used as carriers for miRNAs, and combined with the gene transfection enhancement effect of ZnO NPs, a gene hydrogel system is constructed. ZnO NPs can form complexes with negatively charged miRNAs through their positive surface charge, protecting miRNAs from nuclease degradation, and can also promote the entry of miRNAs into cells, improving transfection efficiency [87]. For example, hydrogels containing streamline ZnO NPs and miR-17-5p can target and inhibit the expression of cartilage matrix degrading enzyme genes, while promoting ECM synthesis, achieving a dual repair effect of “gene regulation + biological activity [29].

3.4. Potential AI-assisted strategies for ZnO NP-enhanced hydrogel design

Artificial intelligence (AI) has recently attracted increasing attention in biomaterial design, especially in hydrogel formulation optimization, property prediction, high-throughput screening, and experimental design. In hydrogel systems, AI-based methods may help establish relationships between material composition, cross-linking density, rheological behavior, mechanical properties, degradation rate, and biological performance [88]. In additive manufacturing, machine learning has also been used to connect hydrogel formulation, rheological properties, printability, and processing parameters, thereby improving reproducibility and reducing trial-and-error optimization [89]. Recent advances in AI-assisted design, synthesis, and analysis of smart biomaterials have demonstrated the potential of this approach [90].

However, the direct application of AI to ZnO NP-enhanced hydrogels for cartilage repair remains at an early stage. Therefore, AI should currently be regarded as an auxiliary tool rather than an established design strategy in this specific field. Potentially relevant applications include quantitative image analysis of cell viability, cell distribution, histological staining, and cartilage matrix deposition; data-driven screening of hydrogel formulations based on polymer type, cross-linking density, ZnO morphology, ZnO concentration, mechanical modulus, Zn²⁺ release behavior, cytocompatibility, antibacterial activity, and chondrogenic outcomes; and predictive modeling of hydrogel degradation, Zn²⁺ release kinetics, and mechanical attenuation.

From a tissue engineering perspective, machine learning has shown potential in biomaterial design, scaffold fabrication, tissue regeneration, and outcome prediction [91]. In cartilage repair, machine learning has been used to predict mesenchymal stem cell efficacy and patient-specific repair outcomes, suggesting its possible value in evaluating cartilage-regeneration strategies [92]. Nevertheless, such evidence remains indirect for ZnO NP-based hydrogel systems. Future studies should therefore focus on building standardized and experimentally validated datasets that link ZnO-related material parameters with mechanical, biological, and cartilage-repair outcomes. This would allow AI-assisted approaches to move from general prediction toward more reliable design and optimization of ZnO NP-enhanced hydrogels for cartilage repair.

4. Current experimental evidence and performance of ZnO NP composite hydrogels in cartilage repair

4.1. In vitro studies

4.1.1. Cytocompatibility

Cytocompatibility is a basic indicator for evaluating ZnO NPs composite hydrogels. Detection using CCK-8 method and Live/Dead staining shows that ZnO NPs composite hydrogels have no significant effect on cell survival rate, and the cell morphology is normal without obvious apoptotic characteristics [30]. Evaluation of cell adhesion through Phalloidin-TRITC staining proves that str-ZnO hydrogels are beneficial to cell adhesion [29] (Fig. 4).

Fig. 4.

ZnO-based composite hydrogels show cytocompatibility and multifunctional activity in promoting chondrogenesis, gene delivery, and immune modulation. (A)Cytocompatibility assessment by CCK-8 and Live/Dead staining showing good cell viability and normal morphology of L929 cells [93]. Copyright © The Korean Society for Microbiology and Biotechnology. (B)Chondrogenic differentiation of ATDC5 cells cultured with different hydrogels. (a) Alcian Blue staining images after 7 and 14 days. (b) Relative grayscale of GAG staining. (c) Quantitative GAG activity after 7 and 14 days [55]. Copyright © 2025 The Authors. Published by American Chemical Society. (C)Antibacterial activity of PEGDA–ZnO hydrogels determined by bacterial viability and turbidity assays [31]. Copyright© 2023 by the authors. Licensee MDPI, Basel, Switzerland. (D)Gene expression analysis of BMSCs cultured with ZnO-containing hydrogels [29]. Copyright © 2025 The Author(s). Licensed by Springer Nature. (E)Schematic illustration of the anti-inflammatory mechanism of ZnO nanoparticles in LPS-stimulated RAW264.7 macrophages [94]. Copyright © 2025 The Authors. Published by American Chemical Society.

4.1.2. Chondrogenic differentiation induction

The chondrogenic differentiation induction effect of ZnO NPs composite hydrogels is mainly evaluated by detecting the expression of cartilage-related genes and proteins. Zn²⁺ upregulates Ihh expression, thereby modulating PTHrP to maintain the balance between chondrocyte proliferation and differentiation. Concurrently, Zn²⁺ activates the TGF-β/Smad pathway, leading to increased nuclear translocation of Smad2/3, which in turn upregulates SOX9 expression. SOX9 is a key transcription factor in chondrogenesis and further drives the production of downstream chondrocyte-specific matrix proteins. Western blot analysis shows that the intensity of SOX9, COL2A1, and Aggrecan protein bands is significantly higher in the str-ZP Gel group and str-ZPM Gel group, indicating that str-ZnO has the ability to promote the differentiation of BMSCs into chondrogenic cells [29]. ATDC5 cells were used to assess chondrogenic differentiation. Alcian Blue staining showed stronger GAG deposition in the ZnO-containing bilayer hydrogel group, indicating enhanced cartilage differentiation [55].

4.1.3. Verification of antibacterial and anti-inflammatory properties

In terms of antibacterial properties, agar diffusion method and viable count method are mostly used to evaluate the inhibitory effect of composite hydrogels. Co-culturing PEGDA composite hydrogels containing different concentrations of ZnO NPs with Escherichia coli and Staphylococcus aureus respectively, it is found that PEGDA composite hydrogels containing 1.5 wt% ZnO NPs show strong antibacterial activity against both Escherichia coli and Staphylococcus aureus [31]. In the verification of anti-inflammatory properties, using LPS-induced RAW264.7 macrophages as a model, it is found that hydrogels containing ZnO NPs can reduce the secretion of inflammatory factors such as TNF-α and IL-1β by cells, indicating that they can effectively alleviate the inflammatory response and create an anti-inflammatory microenvironment for cartilage repair [95].

4.1.4. Evaluation of gene transfection efficiency

For the gene hydrogel system, luciferase reporter gene and fluorescence microscope are used to evaluate the transfection efficiency of ZnO NPs on miRNAs. By comparing the fluorescence intensity under CLSM at 24 h, the transfection efficiency of spherical ZnO and rod-like ZnO on chondrocytes is evaluated. It is found that the expression level of miR-17 in cells treated with rod-like ZnO is about 3.5 times higher than that in cells treated with spherical ZnO, which is further confirmed by qRT-PCR [29].

4.1.5. Signal pathway research

Decellularized tendon hydrogels containing ZnO NPs treat tendon-derived stem cells (TDSCs), up-regulate the expression of genes related to the TGF-β/Smad pathway, and promote cell proliferation and differentiation [39]. The expression of the Ihh signaling pathway target gene PTHrP is down-regulated after cyclopamine treatment. Although the Zn concentration increases, the expression of cartilage differentiation-related proteins is still inhibited, indicating that str-ZPM Gel may regulate cartilage differentiation by mediating the Ihh/PTHrP pathway [29]. ZnO nanoparticles inhibit inflammation by suppressing NF-κB and JAK1/STAT1/3 signaling, reducing ROS generation, and promoting M2-type macrophage polarization [96].

4.2. In vivo animal experiments

4.2.1. Defect repair effect

Using rat and rabbit knee joint cartilage defect models as research objects, the repair effect of ZnO NPs composite hydrogels is evaluated through gross observation, histological staining, and immunohistochemistry. Gross observation shows that 8 weeks after implantation of ZnO composite hydrogels, the defect area is basically filled with new tissue, the surface is smooth and flat, and the boundary with the surrounding normal cartilage is blurred. In contrast, the defect area in the control group still has obvious depression, which is only partially covered by fibrous tissue [29]. H&E staining, safranin O staining, etc. are used to observe the morphological structure of cartilage tissue, cell distribution, and changes in extracellular matrix components. Immunofluorescence is used to detect CD44 and CD90, FISH is used to detect miR-17-5p, Western blot is used to detect COL2A1, Aggrecan, MMP13, etc., and immunohistochemistry is used to detect Aggrecan, COL2A1, Ihh, PTHrP. The expression of related proteins and factors can be analyzed to evaluate the effect of hydrogels on cartilage repair.

Host-graft integration is a key outcome in cartilage repair research. In the studies reviewed, evidence of improved integration was primarily derived from histological observations. Hydrogels containing ZnO NPs exhibited blurred boundaries between the newly formed tissue and the native cartilage, indicating improved interfacial bonding. Furthermore, the mechanisms by which zinc oxide nanoparticles promote integration—such as enhancing cell migration to the interface, improving extracellular matrix remodeling, or reducing marginal inflammation—have not yet been thoroughly investigated. Therefore, although existing evidence suggests a positive effect, future studies require more rigorous and systematic evaluations of host-graft integration.

4.2.2. In vivo biosafety

The in vivo biosafety of composite hydrogels is evaluated by detecting animal weight changes, pathological sections of major organs, and blood biochemical indicators. During the experiment, the weight of animals in each group increases normally without obvious abnormal behavior (Fig. 5).

Fig. 5.

In vivo performance, biosafety, and functional properties of ZnO NPs–based composite hydrogels.(A)ZnO NPs–based composite hydrogels promote stem cell recruitment and cartilage repair in vivo.(a) Immunofluorescence staining of CD44 and CD90 at 7 and 14 days showing enhanced BMSC recruitment.(b) Macroscopic observation at 4 and 8 weeks demonstrat-ing nearly complete defect filling and smooth cartilage integration.(c) Micro-CT reconstruction confirming effective regeneration of subchondral bone and cartilage tissue [29]. Copyright © 2025 The Author(s). Licensed by Springer Nature. (B)Evaluation of in vivo biosafety of ZnONPs–Gel.(a) Representative H&E staining of major organs from normal and ZnONPs–Gel–implanted mice after 28 days.(b) Serum biochemical and hematological analyses indicate normal liver and kidney function [97]. Copyright© 2022 Lin, Zhao, Xu, Zhou, Wang, Chen and Mei. (C) Quantitative comparison of elastic modulus among OCD, str-ZP Gel, and str-ZPM Gel groups [29]. Copyright © 2025 The Author(s). Licensed by Springer Nature. (D)Controlled release behavior of ZnO nanoparticles from the PEP–ZnO hydrogel [57]. Copyright © 2024 The Authors. Published by American Chemical Society.

Two months after material implantation, the heart, liver, spleen, lung, and kidney of rats are collected and subjected to H&E staining. It is found that there is no obvious organ damage or inflammation in the experimental group with ZnO NPs composite hydrogels, which proves its efficacy and long-term biosafety, and it is suitable for clinical application [29]. Histopathological, biochemical, and hematological assessments at 28 days confirmed that ZnONPs-Gel caused no detectable systemic toxicity, supporting its favorable in vivo biosafety [98].

4.2.3. Functional recovery evaluation

Combining imaging, biomechanical testing, and gait analysis, the recovery of joint function is evaluated.

Atomic force microscopy is used for biomechanical analysis of the repaired cartilage area 8 weeks after surgery. It is found that the elastic modulus value of cartilage repaired with str-ZPM Gel is significantly increased. The CatWalk gait analysis system is used to detect subtle changes in the motor performance of rats. Intuitively, the gait of the str-ZPM Gel group is more coordinated. Micro-computed tomography (micro-CT) is used to evaluate the osteophyte volume in the knee joint. It is found that the formation of osteophytes in the str-ZP Gel group and str-ZPM Gel group is significantly reduced [29].

4.2.4. Hydrogel degradation and drug release

Fluorescence imaging and inductively coupled plasma mass spectrometry (ICP-MS) are used to track the in vivo degradation process of hydrogels and the release behavior of zinc ions [99]. After 4 weeks, fluorescence in situ hybridization (FISH) is used to analyze the collected joints. It is found that the expression level of miR-17 in the str-ZPM Gel group is higher than that in the sph-ZPM Gel group, indicating that miR-17 released by str-ZPM Gel may have a more long-lasting effect on alleviating OA [29]. The PEP hydrogel features a porous structure that gradually degrades, enabling the sustained release of embedded ZnO nanoparticles. As the outer layer erodes, ZnO diffuses outward in a controlled manner, forming a prolonged Zn²⁺ release profile [57].

Although the results from animal models are encouraging, several limitations must be considered when extrapolating these findings to clinical applications. First, the cartilage in rats and rabbits is thinner and remodels more rapidly than human cartilage, which may lead to an overestimation of repair efficacy. Second, these animal models typically involve acute, small-area defects in healthy joints, whereas human patients often present with chronic, large-area defects or a history of osteoarthritis. Third, most current studies lack long-term follow-up, which is crucial for assessing late-stage degeneration, host-graft integration, and potential chronic inflammation. Therefore, future research should shift toward using large animal models, extend follow-up periods, and employ defect sizes that more closely reflect clinical reality.

5. Challenges and prospects

5.1. Main challenges currently faced

Biosafety: The potential cytotoxicity of high-concentration ZnO NPs and the side effects of long-term in vivo retention need to be further studied.

Controllable Release: Achieving long-term and controllable release of Zn²⁺ to continuously exert biological effects instead of burst release.

Accurate Matching of Mechanical Properties: The complex joint environment has higher requirements for the mechanical properties of hydrogels.

Standardization and Large-Scale Production: Problems such as the repeatability and standardization of the controllable synthesis of ZnO morphology, batch consistency of nanomaterials, and sterilization and storage of composite hydrogels.

In addition, several translational challenges remain in manufacturing and clinical applications. First, large-scale production of the material may make it difficult to ensure batch-to-batch consistency. Second, sterilization methods such as autoclaving or ethylene oxide treatment may alter the hydrogel structure, while filtration sterilization is not suitable for pre-gelled scaffolds. Third, there is a lack of research on long-term storage stability, and issues such as loss of injectability or bioactivity remain to be addressed.

For clinical translation, the long-term safety window of Zn²⁺ release should be more clearly defined. Moderate and sustained Zn²⁺ release may support antibacterial activity, inflammatory modulation, and chondrogenic differentiation, whereas excessive or burst release may induce oxidative stress, mitochondrial dysfunction, cytotoxicity, and inflammatory responses. In addition, the potential accumulation, biodistribution, and clearance of ZnO NPs after long-term implantation remain insufficiently understood. Future studies should therefore evaluate local and systemic Zn²⁺ levels, nanoparticle retention, organ distribution, and chronic inflammatory responses over extended periods.

From a regulatory and clinical perspective, ZnO NP-based hydrogels should be considered complex combination biomaterial systems because their biological performance depends on both the hydrogel matrix and Zn²⁺ release behavior. Standardized quality-control parameters should include ZnO particle size and morphology, ZnO concentration, Zn²⁺ release kinetics, residual cross-linking agents, sterility, endotoxin levels, injectability, mechanical durability, and storage stability. In terms of clinical indication selection, these systems may be more suitable for focal cartilage defects or early-stage osteochondral lesions than for advanced osteoarthritis requiring joint replacement. Therefore, future translational studies should define indication-specific requirements and evaluate long-term safety, host-tissue integration, and mechanical durability under clinically relevant loading conditions.

5.2. Future development directions

Morphology Optimization: Current evidence demonstrates that different ZnO morphologies, including tetrapod, streamline, and nanoflower forms, have distinct effects on antibacterial activity, Zn2+ release sites, and gene transfection efficiency. Future studies should systematically optimize ZnO morphology to maximize chondrogenic differentiation and antibacterial performance while minimizing cytotoxicity.

Multifunctional Synergistic Systems: ZnO NPs have been combined with enzyme-responsive hydrogel design and gene therapy approaches, including miR-17-5p delivery, for cartilage repair. Future research should explore additional synergistic combinations, such as pairing ZnO NPs with growth factors like TGF-β3 or with exosomes, to build multi-stimulus-responsive release systems that simultaneously address infection, inflammation, and cartilage regeneration.

3D/4D Bioprinting: The rheological properties of ZnO-containing hydrogels suggest potential suitability for additive manufacturing. Future studies should evaluate whether ZnO NP incorporation affects printability, shape fidelity, and post-printing mechanical stability of cartilage scaffolds with anatomical geometries.

AI-Assisted Design Optimization: AI-assisted design for ZnO NP-enhanced hydrogels remains at an early stage. Future efforts should focus on building standardized, experimentally validated datasets that link ZnO-related material parameters, including morphology, concentration, and crosslinking density, with mechanical, biological, and cartilage-repair outcomes, to support data-driven formulation optimization.

Clinical Translation Research: Given the current translational challenges, future studies should employ large-animal models with ZnO-specific endpoints, including long-term Zn2+ release monitoring, nanoparticle biodistribution assessment, and extended follow-up of cartilage repair quality under clinically relevant loading conditions.

6. Conclusion

In summary, ZnO NP-enhanced biomimetic hydrogels have shown promising potential in articular cartilage repair by integrating mechanical reinforcement, antibacterial activity, anti-inflammatory effects, and chondrogenic differentiation promotion. The current evidence, primarily from in vitro and small-animal studies, demonstrates that ZnO morphology, incorporation strategy, and Zn2+ release kinetics are critical determinants of biological performance. However, significant gaps remain between current evidence and clinical translation, including insufficient quantitative mechanical comparisons with native cartilage, lack of standardized concentration-toxicity data, limited long-term in vivo safety assessment, and the absence of large-animal studies.

Future research should focus on ZnO-specific mechanistic investigations, standardized quality-control parameters, and translational studies that address these gaps. Through interdisciplinary collaboration among materials science, biology, and medicine, ZnO NP-enhanced hydrogels may become a viable strategy for cartilage regenerative medicine.

Informed consent statement

Not applicable.

Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work, the authors used ChatGPT (OpenAI) only for preliminary English translation and language polishing. The AI-assisted tool was not used to generate scientific content, data analysis, figure content, references, interpretations, or conclusions.

Funding

This work was supported by the General Program of the National Natural Science Foundation of China (B22278003), the Innovative Variety Major Cultivation Project of the Beijing Municipal Commission of Science and Technology (Z241100009024027), the Shandong Natural Science Foundation (ZR2024MH037), and the Capital Medical Science and Technology Innovation Achievements Transformation Excellent Promotion Plan Empowerment Project (YC202401QX0320).

CRediT authorship contribution statement

Xiaoqi Dong: Writing – original draft. Jiaming Zhang: Writing – original draft. Shuzheng Liu: Writing – original draft. Jiayu Zhu: Writing – review & editing. Peixun Zhang: Writing – review & editing.

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.

Abbreviations

ECM

extracellular matrix

OA

osteoarthritis

BMSCs

bone marrow mesenchymal stem cells

OATs

Osteochondral Autografting

MMPs

matrix metalloproteinases

ACI

Autologous Chondrocyte Implantation

ZnO NPs

Zinc Oxide Nanoparticles

t-ZnO

zinc oxide tetrapods

ROS

reactive oxygen species

MRSA

methicillin-resistant Staphylococcus aureus

PEG

Polyethylene glycol

PLGA

poly(lactic-co-glycolic acid)

str-ZnO

streamline ZnO

G'

storage modulus

CNN

convolutional neural networks

RNN

recurrent neural networks

TDSCs

tendon-derived stem cells

micro-CT

Micro-computed tomography

ICP-MS

inductively coupled plasma mass spectrometry

FISH

fluorescence in situ hybridization

Appendix ASupplementary data

The following are the Supplementary data to this article:

Data availability

Data sharing is not applicable to this article, as no new datasets were generated or analyzed during the preparation of this review.