Innovative hydrogel solutions for articular cartilage regeneration: a comprehensive review

Abstract

Articular cartilage damage is predominantly caused by trauma, osteoarthritis (OA), and other pathological conditions. The limited intrinsic capacity of cartilage tissue to self-repair necessitates timely intervention following acute injuries to prevent accelerated degeneration, leading to the development of planar arthritis or even osteoarthritis. Unfortunately, current therapies for articular cartilage damage are inadequate in effectively replacing or regenerating compromised cartilage due to the absence of suitable tissue-engineered artificial matrices. However, there is promise in utilizing hydrogels, a category of biomaterials characterized by their elasticity, smooth surfaces, and high water content, for cartilage regeneration. Recent advancements in hydrogel engineering have focused on improving their bioactive and physicochemical properties, encompassing innovative composition designs, dynamic modulation, and intricate architectures. This review provides a comprehensive analysis of hydrogels for articular cartilage repair, focusing on their innovative design, clinical applications, and future research directions. By integrating insights from the latest research studies and clinical trials, the review offers a unique perspective on the translation of hydrogels for articular cartilage repair, underscoring their potential as promising therapeutic agents.

Introduction

Highlights

• Joint cartilage injury is one of the main causes of osteoarthritis.

• Natural hydrogel, synthetic hydrogel, and mixed hydrogel all play an important role in articular cartilage regeneration.

• Hydrogel can be used as a good scaffold material for cartilage and bone tissue engineering.

• Hybrid hydrogels commonly used for repairing articular cartilage injuries and promoting cartilage regeneration.

Articular cartilage plays a vital role in lubricating joint surfaces, absorbing shock, and cushioning stress to protect against wear and tear. Damage to the cartilage can result from various factors such as sports injuries, inflammation, aging, obesity, and tumors, eventually leading to joint function deterioration and osteoarthritis¹. Treatment options for cartilage injuries include conservative methods like medication, rehabilitation, physiotherapy, and surgical interventions such as arthroscopic debridement and graft repair². However, both approaches only offer temporary relief and do not regenerate the cartilage. Therefore, the repair and reconstruction of damaged articular cartilage are crucial for restoring joint function³. Surgical methods like bone stimulation drilling, autograft, and allograft have limitations and shortcomings, prompting the exploration of alternative treatments^4–7. Mesenchymal stem cells (MSCs) have emerged as a promising option for cartilage repair due to their ability to differentiate into cartilage cells, abundance, and minimal adverse reactions⁸. Researchers have been studying efficient chondrogenic differentiation processes of MSCs for the treatment of cartilage injuries, with promising results in clinical practice⁹. Recent studies have evaluated the safety and efficacy of intravenous infusion of umbilical cord-derived MSCs in patients with chronic heart failure, psoriasis, and severe COVID-19¹⁰. Results have shown improvements in left ventricular function, reduced lung lesion volume, and enhanced clinical outcomes without significant adverse events. Additionally, MSCs have been shown to promote cartilage regeneration through various mechanisms¹¹. Overall, MSC-based therapies show potential in treating cartilage injuries and other conditions, providing hope for more effective and durable solutions for patients in need of joint repair and functional restoration (Fig. 1).

Figure 1.

Schematic representation of the role of MSCs in cartilage regeneration.

MSCs promote cartilage regeneration by diverse mechanisms. MSCs are able to proliferate and differentiate into chondrocytes to replace damaged cells. In addition, MSCs can also secrete cytokines to maintain chondrocyte phenotypes, promote their proliferation and extracellular matrix (ECM) composition. More importantly, MSCs can exert immunomodulatory effects on diverse immune cells upon exposure to injured tissue or inflammatory factors.

In the field of tissue engineering, the repair of articular cartilage damage has advanced through the use of three key elements: seed cells, scaffold materials, and cell growth factors^12–14. Scaffold materials aim to mimic the extracellular matrix and create an optimal environment for cell growth, proliferation, and differentiation at the site of the cartilage defect^15–17. The performance of tissue-engineered materials relies on how closely the scaffold materials resemble the structure and composition of the extracellular matrix^18,19. Some commercial collagen scaffolds, such as NeoCart (Histogenics)²⁰ and NOVOCART 3D—AesculapOrthopaedics (BBraun)²¹, are similar to cartilage’s ECM components. Their presence appropriately stimulates chondrogenesis and the maintenance of the cellular phenotypes of chondrocytes. These scaffolds affect the adhesion and proliferation of the cell and cell proliferation. Unfortunately, these scaffolds often do not meet the necessary requirements, as they quickly lose their structure (sensitivity to an aquatic environment) and transform into a gel-like form. They are also not mechanically strong enough to support the cells and regenerated tissue. Thus, this kind of membrane does not have suitable properties to create hyaline cartilage. Hence, the development of cartilage repair materials that mimic the mechanical properties of cartilage, load with stem cell or bioactive components, and exhibit similar characteristics as cartilage is of paramount clinical significance. Hydrogels have gained significant attention as scaffold materials for cartilage and bone tissue engineering due to their similarity to the extracellular matrix. Degradable hydrogels have proven effective in articular cartilage repair and regeneration through different methods^22,23. One method involves in situ repair using injectable hydrogels, where a biodegradable hydrogel containing drugs or cells is injected into the damaged part of the cartilage to mold itself at the defect site. This method reduces scarring and minimizes patient pain^24,25. Tissue-engineered scaffolds can also be prepared for implantation using conventional techniques. Three-dimensional porous scaffolds can be fabricated using methods such as mold casting, particle leaching, freeze-drying, and gas foaming^26,27. These scaffolds simulate the porous microenvironment of natural cartilage and provide mechanical support and stimulation for cell binding and growth factor integration^17,28. Additionally, implantable repairs can be achieved using three-dimensional (3D) printing techniques to create cartilage scaffolds. 3D printing allows the layer-by-layer deposition of biomaterials, cells, and active biomolecules to create functional scaffolds for cartilage tissue engineering, facilitating the repair of defective cartilage (Fig. 2).

Figure 2.

Cartilage repair process.

Hydrogels offer numerous advantages for articular cartilage regeneration compared to other frequently used materials. These advantages include biocompatibility, as hydrogels possess a high water content and a structure similar to the extracellular matrix, making them suitable for supporting cell growth and tissue regeneration; mechanical properties that can be tailored to mimic natural cartilage, enabling them to withstand compressive loads and resist deformation within the joint’s mechanical environment; cell affinity, as hydrogels promote cell adhesion, proliferation, and differentiation without triggering immune responses; bioactive functionality, with the ability to load and release factors that aid in cartilage repair and regeneration; scaffold degradability, allowing for controlled breakdown to match the growth of new cartilage and facilitate tissue regeneration; and innovative design options, with the ability to engineer specific structures and compositions to enhance performance in cartilage repair. When compared to other materials, hydrogels excel in terms of superior biocompatibility, mechanical mimicry, bioactive functionality, scaffold degradability, and customizable design, offering a versatile platform for tissue engineering applications. In summary, hydrogels present themselves as a promising option for articular cartilage regeneration due to their unique features and benefits. The focus of this paper is to provide a comprehensive review of commonly used hydrogels for articular cartilage repair, discussing their current clinical applications and future research directions. The goal is to analyze the effectiveness of these hydrogels and identify areas of improvement and further study.

The overview of each section is briefly presented as following for readers to quickly understand the article and locate the parts of interest.

Section 1: Introduction

1. Introduces the current status of treatment of cartilage damage and the application of hydrogel in cartilage regeneration.

   Section 2: Literature search process

2. Provide key aspects of the literature search process as reflected in the review.

   Section 3: Physical and chemical properties of hydrogel

3. Discusses fundamental characteristics and attributes of hydrogels stemming from their physical and chemical properties.

4. Provides a foundational understanding of these properties governing behavior and functionality of hydrogels.

5. Lays groundwork for further discussions on hydrogel applications in tissue engineering, drug delivery, and biomedical fields.

   Section 4: Natural or synthetic hydrogels in cartilage injury repair

6. Focuses on comparing and contrasting the use of natural and synthetic hydrogels in cartilage injury repair.

7. Provides a comprehensive overview of their role in cartilage repair, highlighting unique properties, applications, and implications for tissue engineering and regenerative medicine.

   Section 5: Cartilage regeneration with hydrogels

8. Discusses the role of hydrogels in regenerating cartilage tissue, particularly in articular cartilage injuries and osteoarthritis.

9. Provides a comprehensive overview of the current state of cartilage regeneration with hydrogels.

10. Highlights the potential of hydrogel-based therapies in addressing cartilage injuries and advancing regenerative medicine.

    Section 6: Applications of novel hydrogels in cartilage regeneration surgery

11. Focuses on discussing innovative uses of advanced hydrogels in surgical procedures for cartilage regeneration.

12. Provides insights into novel hydrogel applications, potentially revolutionizing current treatment approaches.

13. Highlights potential to improve clinical outcomes for patients with cartilage injuries and degenerative conditions.

    Section 7: Prospections and conclusion

14. Summarizes key findings and implications from preceding sections about hydrogel use in cartilage regeneration.

15. Synthesizes key insights, implications, and future prospects regarding hydrogels in cartilage regeneration.

16. Emphasizes the significance of hydrogels in advancing tissue engineering and regenerative medicine.

Literature search process

We used the methodical and integrative approach to compile and synthesize existing research on hydrogels for articular cartilage regeneration. The following are the key aspects that characterize this process as reflected in the review:

1. Targeted research focus: We centered on the specific application of hydrogels in cartilage repair, indicating that the literature search was directed towards studies that explore this niche area within biomedical engineering and tissue engineering.

2. Comprehensive keyword strategy: We employed a well-defined set of keywords and phrases related to hydrogels, cartilage regeneration, and tissue engineering. This would include terms such as ‘hydrogel’, ‘cartilage injury’, ‘biocompatibility’, and ‘3D bioprinting’, ensuring a broad yet relevant search scope.

3. Diverse database utilization: Multiple academic databases were utilized to gather a wide range of literature, ensuring that both foundational studies and the latest advancements in hydrogel technology were included.

4. Systematic screening and selection: We used a systematic approach to screen articles based on predefined inclusion and exclusion criteria. We evaluated the relevance of studies based on their abstracts and methodologies to ensure that only pertinent research was included.

5. Synthesis of findings: The literature search culminated in a synthesis of findings, where we organized the information thematically. This synthesis highlights the advantages of hydrogels, their applications in cartilage repair, and the challenges faced in clinical translation, providing a comprehensive overview of the current state of research.

6. Identification of gaps and future directions: Throughout the literature search, we identified gaps in the existing research, which informed the conclusions and recommendations for future studies. This aspect is crucial for guiding subsequent research efforts in the field.

In summary, the literature search process described is characterized by its systematic, comprehensive, and integrative nature, aimed at providing a thorough understanding of the role of hydrogels in cartilage regeneration while identifying key insights and future research directions.

Chondrogenic factors play a crucial role in the repair process. They can recruit endogenous mesenchymal stem cells for cartilage repair or be used to induce the differentiation of implanted mesenchymal stem cells into chondrocytes. This leads to the production of cartilage-related extracellular matrices, promoting the regeneration, and repair of damaged cartilage.

Physical and chemical properties of hydrogel

Currently, commercial hydrogels used for clinical cartilage repair are mostly simple in structure, such as fibrin, hyaluronic acid (HA), collagen, and polyvinyl alcohol (PVA). Natural biomacromolecules and their derivatives have good biocompatibility and biodegradability but lack mechanical strength, while synthetic polymer hydrogels offer precise control over structure and properties but often lack cell affinity and biodegradability. Combining natural and synthetic hydrogels can help mitigate their drawbacks. Despite various effective hydrogels for cartilage repair, the complexity of structures poses challenges for clinical application due to the need for specialized formulas, equipment, and higher costs.

Classification of hydrogels

Hydrogels can be physical, forming through entanglement and bonds, or chemical, forming complex networks through reactions. Physical hydrogels, or ‘pseudogels’, offer elasticity, biocompatibility, and degradability with applications in medicine, biotechnology, and materials science^29–32. Chemical hydrogels have three-dimensional structures and high water absorption, used in tissue engineering, drug delivery, and more. An example includes HACC/PAAc.Fe3+hydrogel with high mechanical strength and self-healing abilities³³. Wang et al.³⁴developed a chitosan and polyisopropylacrylamide hybrid hydrogel for drug delivery with near-infrared light initiation. Unlike physical hydrogels, chemical hydrogels are irreversible ‘true gels’ with enhanced stability. The classification of hydrogels is shown in Table 1.

Table 1.

The classification of hydrogels.

Classification criteria	Category	Characteristics
Force when gel occurs	Physical hydrogel	It has low crosslinking degree and strength, but has good flexibility and
		reversibility, and is suitable for preparing soft gel materials
	Chemical hydrogel	It has high mechanical strength and stability, and is suitable for
		preparing high strength gel materials
Responding to environmental stimuli	Traditional hydrogel	The mechanical properties, recoverability, and self-healing ability are poor, which
		cannot meet the requirements of their applications in the fields of medicine and pharmacy
	Stimulus responsive hydrogel	Able to respond quickly to environmental changes
Source of preparation materials	Natural polymer hydrogel	Good biocompatibility, good environmental sensitivity, low price, but poor stability
	Synthetic polymer hydrogel	Has good chemical stability, customizability, biocompatibility,
		mechanical properties, permeability, large-scale production, and scalability

Temperature-sensitive hydrogels

A temperature-responsive hydrogel is a type of hydrogel that exhibits volume changes in response to temperature fluctuations. These temperature-sensitive hydrogels can be categorized into two groups, based on their critical phase transition temperatures: high and low^35,36. The three-dimensional network structure of this hydrogel can be modulated by temperature alterations, making it suitable for intelligent drug delivery systems, as well as for promoting cell loading and growth. Notably, temperature-responsive hydrogels offer distinct advantages in cartilage repair. When the ambient temperature is below the volumetric phase transition temperature, the hydrogel contracts. Conversely, when the temperature surpasses the volumetric phase transition temperature, the hydrogel swells. In the former case, the hydrogel is referred to as a thermal expansion type temperature-sensitive hydrogel, with the volumetric phase transition temperature denoted as the high critical phase transition temperature^37,38. Conversely, when the hydrogel shrinks upon heating, it is classified as a heat-shrinkable temperature-sensitive hydrogel, with the corresponding volume phase transition temperature known as the low critical phase transition temperature^39,40. The heat-shrinkable hydrogel is the most commonly used in practice, and the low critical phase transition temperature can be adjusted by altering the hydrogel’s composition and structure, enabling it to closely approximate the human body temperature of 37°C. The temperature-sensitive properties of this hydrogel render it in a solution state at room temperature, making it injectable for clinical drug administration. Once inside the body, it subsequently gels within a predetermined timeframe, often employed as a drug carrier for controlled release functions⁴¹. Rothrauff et al. conducted experiments utilizing pepsin-soluble acellular tendon, methacrylate (GelMA) functionalized cartilage extracellular matrix, and methacrylic acid/gelatin temperature-responsive hydrogel as controls. When bone marrow mesenchymal stem cells were implanted and cultured in a chondrocyte culture medium, the methacrylic acid ECM hydrogel exhibited less contraction compared to the nonmethacrylic acid ECM hydrogel. Gene expression analyses, biochemical composition evaluations, and histological examinations revealed that GelMA hydrogel facilitated the deposition of proteoglycan and type II collagen during the formation process. As a result, GelMA hydrogel demonstrated a similar performance to pepsin in dissolving cartilage and tendon hydrogel and, therefore, holds promise in cartilage tissue engineering.

pH-sensitive hydrogels

A pH-responsive hydrogel is a biomaterial that exhibits sensitivity to changes in pH and typically contains acidic or basic groups^42–45. Alterations in pH levels prompt protonation or ionization within these hydrogels, resulting in changes to their charge distribution, internal interactions, and overall structure. Due to the significant variations in pH across different regions of the human body, pH-responsive hydrogels have garnered attention for their potential in targeted drug delivery. Solid tumors, characterized by high rates of glycolysis, often have lower extracellular pH levels compared to normal tissues. Consequently, pH-responsive hydrogels have been employed as intelligent drug delivery systems for treating cartilage damage, with the ability to achieve minimally invasive treatment⁴⁶. In this approach, a polymer aqueous solution is injected into the affected tissue, leading to the in-situ formation of a gel network structure through pH stimulation. This mechanism facilitates controlled drug release, subsequently reducing pain and associated clinical complications, and even supporting cartilage regeneration by serving as a tissue engineering scaffold⁴⁷. Wang et al.⁴⁸ synthesized a pH-responsive hydrogel composed of hydroxyethyl methacrylate, vinyl pyrrolidone, and methacrylic phosphate choline. This hydrogel can transition from a flowable polymer state to a gelled state within the pH range of 6.0–7.4. Their investigation revealed that this hydrogel system can function as an intelligent carrier for glucocorticoids and exhibit therapeutic effects in the inflammatory environment akin to osteoarthritis. Additionally, pH-responsive hydrogels can be employed in tissue engineering applications to repair damaged cartilage. Yu et al.⁴⁹ developed a novel hydrogel utilizing carboxymethyl chitosan (CMCS) and a self-synthesized dynamic agent (Dy). This hydrogel exhibits favorable pH-responsive swelling properties and self-healing capabilities. Swelling behavior varies depending on specific pH values, and the self-healing ability remains unaffected by the environment. Consequently, this hydrogel can be effectively harnessed as a tissue scaffold to promote damaged cartilage repair.

Light-sensitive hydrogels

Photosensitive hydrogels, which exhibit a bulk phase transition upon exposure to light radiation, are scientifically referred to as light-sensitive hydrogels⁵⁰. These hydrogels typically incorporate photosensitive groups within their structure, enabling isomerization or dissociation upon light irradiation. Consequently, local temperature elevation or conformational changes occur within the hydrogel, leading to alterations in the hydrogel’s solubility. The mechanism of in situ gelation in photopolymerized hydrogels holds significant importance in applications such as drug release systems and tissue engineering. Light-sensitive hydrogels are favored for fabricating tissue engineering scaffolds due to their robust in situ formation capabilities and compatibility with collagen and other materials possessing favorable mechanical properties and biocompatibility, thereby closely mimicking the mechanical characteristics of natural cartilage⁵¹. Lim et al.⁵² demonstrated the synthesis of a visible light-responsive bifunctionalized tyramine methacryl gelatin (GelMA Tyr) utilizing tris (2,2-bipyridyl) dichlororuthenium (II) and sodium persulfate as initiators. The precursor hydrogel solution was locally administered into irregular cartilage defects under ultraviolet light, validating the hydrogel’s extrusion printing functionality and in situ molding efficacy. The amalgamation of the hydrogel with tissue is achieved through interactions with proteins inherent to natural cartilage, thus fostering a conducive microenvironment for cartilage regeneration.

Hydrogels can be classified as synthetic polymer hydrogels or natural polymer hydrogels, with natural polymers being more biocompatible and biodegradable, making them safer for biomedical applications^53,54 (Fig. 3). Various studies have shown modifications to natural polymers like HA-β3, alginate, and chitosans, leading to improved mechanical strength and promoting cell differentiation for applications such as tissue support and cartilage repair⁵⁵. Synthetic hydrogels are easily manipulated but require strict control of materials to avoid biocompatibility issues⁵⁶. Research has demonstrated the effectiveness of synthetic polymers such as polycaprolactone polyethylene glycol polycaprolactone (PCL-PEG-PCL) membrane and fumaric acid modified PEG macrocycle hydrogels in enhancing cell adhesion, proliferation, and cartilage defect repair⁵⁷. By combining the advantages of natural and synthetic hydrogels, researchers aim to create hydrogels with enhanced properties for various applications.

Figure 3.

Physical and chemical properties of natural hydrogel and synthetic polymeric scaffold.

The natural hydrogel is characterized by the presence of laminin, entactin, and collagen IV, highlighting its compatibility with living tissues and ability to naturally degrade. It is presented as a favorable substrate with minimal risk to the patient, although concerns are raised about its consistency under varying environmental conditions. The synthetic polymeric scaffold exhibits customizable physical and chemical properties, facilitating improved functionality and a wide array of applications. The fabrication process of this scaffold involves intricate procedures such as polymerization, purification, and cross-linking, which collectively establish a specifically engineered microenvironment to facilitate stem cell growth and differentiation.

Biocompatibility

Biocompatibility refers to the capacity of a biomaterial to elicit an appropriate host response in a specific application context. Tissue engineering materials benefit from their similarity in structure and composition to the native extracellular matrix (ECM). In this regard, hydrogel scaffolds not only replicate the properties of natural ECM, but also allow for cell attachment and migration to facilitate the transfer of biofactors and the diffusion of nutrients. However, the biocompatibility of synthetic biomaterials is generally suboptimal, necessitating the modification of their properties or the incorporation of natural biomaterials. Various techniques, such as cross-linking and altering the properties of the material through conjugation methods or modifications to polymer ion charges, can augment the biocompatibility of the scaffold⁵⁸. Research has indicated that the presence of cations can have an adverse impact on the biocompatibility of hydrogels, while negative charges can enhance the biocompatibility of D-oligopeptide hydrogel⁵⁹. Furthermore, the integration of collagen chitosan porous scaffolds with γ-Radiation and carbodiimide (CAR) crosslinking has been shown to improve the biocompatibility of chitosan gelatin composite scaffolds manufactured via electrospinning technology⁶⁰. Presently, natural or synthetic hydrogels are commonly employed to fabricate cellular scaffolds possessing a high specific surface area in close proximity to the target tissue^61,62. For hydrogels with inadequate biocompatibility, the synthesis of conjugated polymers and the addition of ions are frequently utilized to enhance their biocompatibility^63,64.

Biodegradability

The biodegradable properties of physicochemical, mechanical, and biological materials undergo changes over time, as do the degradation products that are compatible with the original tissue. Ideally, biodegradable biomaterials should be composed of degradable materials that are nontoxic and rapidly metabolized and cleared from the body. Degradation of hydrogel tissue-engineered scaffolds can occur through physical or chemical mechanisms, as well as biological processes involving biologically enzyme-catalyzed degradation and hydrolysis of the hydrogel^65,66. Proper degradation behavior is crucial for maintaining mechanical support. If the degradation rate is too fast, the mechanical integrity of the scaffold may prematurely disintegrate before adequate tissue regeneration occurs. Conversely, if the degradation rate is too slow, tissue healing can be delayed. To enhance the degradability of hydrogels, various techniques can be employed. Surface modification involves introducing hydrophilic groups on the hydrogel surface, facilitating nonhomogeneous hydrolysis and thereby accelerating the degradation rate^67,68. Hydrogels derived from naturally biodegradable polymers such as collagen, chitosan, and gelatin possess favorable bioactivity, nontoxicity, and exceptional hydrophilicity^61,69,70. Blending these polymers creates additional water transport channels, accelerating hydrolysis. Additionally, incorporating degradable alkaline particles increases the pH level in the surrounding environment, augmenting the degradation rate⁷¹. Controlling the biodegradability of hydrogels to match the rate of cell growth and tissue repair is vital for their rational design. Studies have shown that excessively fast degradation can reduce the retention of extracellular matrix (ECM) proteins, while excessively slow degradation hinders cell remodeling, hindering tissue formation. Hydrogels with a balanced biodegradation rate promote the formation of new cartilage in cells and achieve higher mechanical properties after long-term cultivation⁷². Thus, it is crucial for the biodegradability of hydrogels to correspond with the rate of cell growth and tissue repair. If the degradation rate of the hydrogel is too slow, new tissue growth will be impeded, while excessively fast degradation will result in complete loss of mechanical integrity.

Self-repairability

Self-repairing hydrogels have emerged as a promising solution for augmenting the longevity of materials, thus extending their service life. Particularly, self-healing flexible hydrogels have gained considerable attention in recent years, owing to their remarkable stretchability, resilience, and exceptional performance in the development of highly-sensitive and rapidly-responsive sensors. Furthermore, the potential of self-repairing hydrogels in biomedical applications has also attracted significant interest. These hydrogels demonstrate timely and repetitive autonomous response to damage, exhibiting micro-to-macro response⁷³. Additionally, they possess mechanical, rheological, and biocompatibility properties that are consistent with their intended application. Notably, these hydrogels retain their original mechanical and rheological attributes following the self-healing process⁷⁴. The self-healing capability of hydrogels is typically achieved through dynamic covalent bonding or by means of supramolecular interactions.

Mechanical properties

The mechanical properties of hydrogels are influenced by various factors, such as crosslinking density, porosity, swelling ratio, temperature, and pH value. Among these factors, crosslinking density and porosity are considered to be the primary determinants of the mechanical properties of hydrogels. Crosslinking density refers to the average distance between crosslinking points within the hydrogel network, directly impacting its mechanical strength and elastic modulus. Porosity, on the other hand, represents the ratio of pore volume to total volume within the hydrogel network, influencing the swelling characteristics and permeability of the hydrogel. To evaluate the mechanical properties of hydrogels, several indicators are commonly used, including elastic modulus, tensile strength, and elongation at break. Elastic modulus measures the ratio of stress to strain within the elastic deformation range, providing insights into the rigidity and resistance to deformation of the hydrogel. Tensile strength represents the maximum stress that the hydrogel can endure during stretching, reflecting its strength and durability. Elongation at break assesses the extent of elongation the hydrogel undergoes before reaching the maximum stress during the tensile process, indicating its plasticity and toughness.

In addition to the aforementioned factors, the mechanical properties of hydrogels are also influenced by various aspects, such as the preparation process, ambient temperature and humidity, and solvent type. For instance, different preparation methods can affect crosslinking density and porosity, thereby impacting the mechanical properties. Moreover, ambient temperature and humidity can influence the swelling and permeability characteristics of hydrogels, ultimately affecting their mechanical properties. Similarly, the choice of solvent can influence the swelling behavior and elastic modulus of hydrogels, thereby affecting their mechanical properties. In summary, the mechanical properties of hydrogels are governed by a multitude of interacting and interdependent factors. To obtain hydrogels with superior mechanical properties, it is necessary to carefully select and optimize the appropriate preparation process, crosslinking agent type, solvent type, and other relevant parameters based on desired applications. Furthermore, rigorous control of the hydrogel preparation process is essential to ensure the quality and stability of the resulting hydrogel. Currently, notable methods for enhancing the mechanical properties of hydrogels include double network hydrogels, slip ring hydrogels, and nanocomposite hydrogels. Double network hydrogels involve the synthesis of a tightly crosslinked rigid network followed by swelling in a monomer solution to generate a loosely crosslinked flexible network. The mechanical strengthening of these hydrogels primarily depends on the resilient nature of the rigid network (Fig. 4). Depending on the type of crosslinking, double network hydrogels can be classified as fully chemically crosslinked hydrogels, physical-chemical mixed crosslinked hydrogels, or fully physically crosslinked hydrogels. For the purpose of rendering the fractured rigid network recoverable, physical crosslinking is often employed to form the rigid network.

Figure 4.

Schematic description of engineering of tough double network hydrogels.

The engineering of tough double network hydrogels. The combination of a rigid and brittle first network with a second, flexible and stretchable network, resulting in a composite double network gel that is both strong and tough, representing an advancement in hydrogel material science.

Natural or synthetic hydrogels hydrogels in cartilage injury repair

Injectable micro gel and bioadhesive hydrogel can be used to improve cell survival rate and retention rate (Fig. 5). At present, there are many kinds of materials, and they are used to make hydrogel scaffolds and repair cartilage damage. Hydrogels can be classified into two groups: Natural hydrogels include agarose, chitosan, collagen protein, gelatin hydrogel, fibrin, hyaluronic acid, and alginate; synthetic hydrogels include polyethylene glycol and polyvinyl alcohol. The advantages and disadvantages of common types of hydrogels were displayed in Table 2.

Figure 5.

Schematic description of utilizing injectable microgels and bioadhesive hydrogels to increase cell survival and retention.

Table 2.

The advantages and disadvantages of common types of hydrogels.

Types	Advantages	Disadvantages
Physical crosslinked hydrogel	It has reversibility, meaning that it can return to the solution state when external conditions change	The mechanical properties may be relatively poor, as its cross-linking network
	This means that it can be reused and environmentally friendly. In addition,	is formed by noncovalent bonds such as electrostatic and hydrogen bonding
	its preparation method is relatively simple and cost-effective	This may lead to its instability in some applications
Chemically crosslinked hydrogel	The mechanical properties are more stable because their cross-linking network is constructed	The production method is relatively complex and costly.
	by chemical bonds. This means that they can be used under a wider range of conditions and can withstand	In addition, due to their permanent cross-linking network, they cannot be reused
	greater mechanical pressure. Furthermore, due to their stable structure, they are permanent
Biocompatible hydrogel	It has excellent compatibility with biological tissues and is therefore very useful in	The prices are usually high, and their performance may be affected by the biomaterials
	biomedical applications, such as tissue engineering and drug delivery. In addition,	used in the production process. In addition, their mechanical properties and
	they usually have good biodegradability, so they can naturally degrade in the body	stability may not be as good as chemical crosslinked hydrogels
Nanocomposite hydrogel	It has excellent mechanical properties and chemical stability, and can also contain multiple functions.	The price of such hydrogels is usually high, and their preparation process may be complex.
	For example, they can contain drugs, genetic substances, or photosensitizers to achieve various therapeutic	In addition, due to their nanostructure, they may cause certain pollution to the environment
	functions. In addition, due to their nanostructure, they can also be used to prepare nanomaterials

The delivery of an injectable hydrogel mixed with mesenchymal stem cells (MSCs) into a joint space, potentially for regenerative purposes. The detailed composition of the biomaterial include an injectable hydrogel with embedded MSCs alongside microparticles or microgels also containing MSCs. Chemical structures of acrylate, catechol, aldehyde, and a reaction involving o-nitrobenzyl alcohol are depicted to highlight the material’s polymerization and cross-linking properties.

Agarose

Agarose, a natural polysaccharide material, is recognized for its cellular biocompatibility, gradual biodegradability, and cost-effectiveness. At temperatures approaching 37°C, agarose undergoes gelation to form a stable hydrogel structure when the solution temperature is reduced. Notably, the elastic modulus of the resultant hydrogel exhibits a significant increase with escalating concentrations of agarose. Utilization of hydrogels incorporating growth factors and infrapatellar fat pads has shown to enhance the proliferative capacity of CD90-unadherent and rapidly adherent cells, leading to enhanced growth of sGAG-rich matrices, thereby presenting a promising therapeutic approach for cartilage defect repair⁷⁵. Furthermore, a research investigation evaluating the biological characteristics of hydrogels derived from diverse polysaccharides such as gellan gum (GG), alginate, and agarose in conjunction with hyaluronic acid (HA) for cartilage regeneration revealed that each polysaccharide substantially fostered tissue regeneration. In particular, GG and agarose contributed to cartilage regeneration through the inhibition of inflammatory mediators, stimulation of genes linked to cartilage formation and autophagy, thereby supporting cartilage regeneration⁷⁶. Balestri et al.⁷⁷ introduced a three-dimensional in vitro model composed of collagen and agarose to explore bone tendon muscle interface regeneration. The two-phase and three-phase hydrogels constituting the 3D model demonstrated suitability for cell proliferation, promoting cell viability, expression of tissue-specific markers, and deposition of new matrix components within a 21-day period.

Chitosan

Chitosan (CS) is a naturally occurring alkaline polysaccharide known for its excellent biocompatibility, affordability, and availability. In wound healing, CS has been shown to enhance tissue regeneration, reduce inflammation, promote neovascularization, and regulate blood circulation⁷⁸. Hydrogel materials made from CS create a moist healing environment that prevents infections, minimizes immune system damage, and expedites healing by absorbing fluids⁷⁹. CS hydrogels can act as carriers for controlled release of bioactive substances, benefiting applications such as tissue repair and regeneration⁸⁰. Studies have explored the effectiveness of CS-based hydrogels in cartilage repair, such as the use of CS/PVA gel with BMSCs and the incorporation of proteins like TGF-β1 for cartilage ECM deposition⁸¹. Other researchers have investigated composite hydrogels involving CS and other materials like alginate and hyaluronic acid for cartilage regeneration⁸². Novel approaches include the development of injectable hydrogels for cartilage repair, such as those triggered by visible blue light or containing cross-linking agents like 1,6-diisocyanate and polyethylene glycol⁸³. These advanced materials have shown promise in enhancing cell proliferation, ECM secretion, and tissue repair processes in vitro and in vivo, providing new avenues for cartilage tissue engineering and regeneration⁸⁴.

Collagen protein

Collagen, a protein found in animal connective tissues, is highly valued for its biomaterial properties in medical applications. It is known for its biocompatibility and low immunogenicity. Hydrogels, known for their soft, absorbent, and stable nature, provide a three-dimensional mesh structure that promotes cell growth. Collagen hydrogels blend the benefits of collagen and hydrogels, offering controlled mechanical strength, maintaining collagen’s stability, and reducing immunogenic responses. These properties make collagen hydrogels well-suited for tissue engineering and therapeutic applications in the medical field. Snyder et al. conducted modifications on Fibrin/HA using methacrylic anhydride (MA) to enhance its mechanical properties. They discovered that incubating bone marrow-derived mesenchymal stem cells (BMSCs) with fibronectin/HA-MA hydrogels suppressed the expression of collagen type 1 alpha 1 while promoting Sox9 expression, indicating the potential to facilitate early chondrogenesis. These results suggest that fibronectin/HA-MA hydrogels have the ability to induce BMSC differentiation into chondrocytes, making them a promising option for articular cartilage repair⁸⁵. Choi et al.⁸⁰ successfully attached TGF-β1 protein covalently to an injectable MeGC hydrogel system, effectively controlling the delivery of TGF-β1 while preserving type II collagen. The TGF-β1-functionalized hydrogel system showed significant capabilities in promoting cartilage regeneration. Portalati et al. created a collagen hybrid hydrogel that combines the bioactivity of type II collagen and the superior mechanical properties of type I collagen, while incorporating chondroitin sulfate and hyaluronic acid to enhance its properties. This superior hydrogel demonstrated excellent mechanical properties, potentially crucial for the development of articular cartilage engineering scaffolds⁸⁶. Wong et al. investigated Col II hydrogel-associated auricular chondrocytes and observed their expression of chondrogenic molecules. The histologic and biomechanical characteristics of auricular chondrocytes cultured with Col II hydrogel scaffolds resembled those of articular chondrocytes. Notably, the synthesis of abundant type II collagen and glycosaminoglycans (GAGs) played a pivotal role in effective cartilage repair⁸⁷. Tsai et al. employed microbial transglutaminase to cross-link gelatin with type II collagen and GAGs, creating articular cartilage extracellular matrix (cECM) encapsulated in a transglutaminase-catalyzed hydrogel. When human adipose-derived stem cells (hADSCs) were cultured in the ECM-enriched hydrogel, the low cytotoxicity of the enzymatic cross-linking reaction was observed. Furthermore, the hydrogels showed great potential in promoting the proliferation and chondrogenic differentiation of stem cells, significantly enhancing hyaline cartilage regeneration⁸⁸. Kilmer et al. investigated the effects of embedding 3:1 mixed type I and type II collagen (Col I/II) hydrogels or all type I collagen (Col I) hydrogels in mesenchymal stem cells (MSCs) on chondrogenic differentiation. Their results indicated that Col I/II hydrogels stimulated the synthesis of GAGs and facilitated cartilage repair in MSCs compared to Col I hydrogels or pellets⁸⁹.

Gelatin Hydrogel

Gelatin is a natural polymer material containing various functional groups such as -COOH, -NH2, -OH, and -SH, which can form coordination compounds with metal ions and has the advantage of being degradable. However, there are drawbacks such as inadequate mechanical properties and limited thermal stability. Currently, the research on gelatin film modification primarily focuses on incorporating cross-linking agents to modify gelatin through cross-linking. Physical, chemical, and enzymatic methods are commonly employed for cross-linking. Beck et al. decellularized and lysed porcine cartilage and utilized methacrylation and ultraviolet cross-linking to obtain methacrylic acid solubilized decellularized cartilage (MeSDCC) gels, in which rat bone marrow mesenchymal stem cells were encapsulated. The MeSDCC gels were discovered to significantly enhance the mechanics of cartilage tissue, enhance matrix synthesis, and induce cartilage gene expression compared with gelatin methacrylate (GelMA)⁹⁰. Li et al. found that visible light-cured microgels composed of gelatin-norbornene (GelNB) and polyethylene glycol (PEG) cross-linkers were fabricated using microfluidic technology, and hBMSCs were placed in them for culture and detection. The results demonstrated that in the GelNB microgels, hBMSCs exhibited an abnormally high degree of chondrogenesis, in which the expression of type II collagen was greatly elevated⁹¹. Levato et al. demonstrated that hydrogels utilizing gelatin were used to culture pluripotent articular cartilage-resident chondrogenic progenitor cells (ACPCs) and BMSCs. The findings revealed a significant reduction in the gene expression level of type X collagen, a hypertrophic marker, in ACPCs following culture. Conversely, there was a marked increase in the expression of PRG4. Furthermore, ACPCs exhibited superior capacity for new cartilage production compared to chondrocytes⁹². Tsai et al. showed that utilization of microbial transglutaminase was used to crosslink gelatin to construct hydrogels and incorporate articular cECM into them. Human ADSCs were encapsulated in the ECM-enriched hydrogel, and it was discovered by assay that the hASCs showed great potential for proliferation and cartilage differentiation in the hydrogel and did not elicit significant inflammatory responses⁸⁸. Zheng et al. proved that a composite hydrogel with an interpenetrating network (IPN) structure was constructed by combining GelMA and silk fibroin (SF) and utilizing UV irradiation and under ethanol treatment. The composite IPN hydrogel demonstrated low swelling and excellent biocompatibility by enhancing the expression of the genes of Col-2, Acan, and Sox-9, as well as promoting chondrogenesis⁹³. Shen et al.⁹⁴ introduced an interpenetrating hydrogel composed of GelMA and glycidyl methacrylate silk fibroin (SG). Compared with GelMA hydrogel, GelMA/SG has the required mechanical properties and can significantly promote the viability of chondrocytes. In addition, GelMA/SG as a bioink has good printability and high intensity for digital light processing (DLP) bioprinting, while chondrocytes significantly proliferate in vitro culture.

Fibrin

Fibronectin is a biopolymer composed of monomeric fibrinogen, consisting of three pairs of disulfide-linked polypeptide chains, Aα-, Bβ-, and γ-. Fibrin gels are characterized by nontoxicity of degradation products, excellent biocompatibility, and the ability to promote cellular secretion of extracellular matrix. However, the gel state formed is unstable and the preparation conditions and processes are demanding. Snyder et al. designed a hydrogel-based on cartilage fibronectin/hyaluronic acid (HA) and modified with MA (HA-MA) and utilized BMSCs as a regenerative agent for the treatment of OA. The results showed that BMSCs cultured in fibronectin/HA-MA hydrogel showed a decrease in the expression of collagen type 1 alpha 1 and an increase in the expression of Sox9. Collectively, the findings demonstrated that the hydrogel could induce BMSC differentiation into chondrocytes thereby facilitating articular cartilage repair⁸⁵. Yu et al. showed that a hydrogel containing recombinant human bone marrow cell-derived factor 1a (rhSDF-1a) composed of fibronectin and hyaluronic acid was designed and evaluated for its utility in cartilage defects. Analysis of the assay revealed that stimulation of local CPC recruitment prior to treatment using chondrogenic factor markedly elevated the biochemical and mechanical properties of cartilage tissue formed at cartilage defects⁹⁵.

Hyaluronic acid

HA derivatives can be prepared in the form of gels, nano and drug carriers, and hybrid materials that have found extensive applications in organ regeneration, tissue engineering, targeted drug release, disease diagnosis, prevention of postoperative adhesions, promotion of wound healing, and immunomodulation. Unterman et al. designed a polyethylene glycol hydrogel with noncovalent HA binding capacity and encapsulated MSCs in it. Treatment with HA-interacting hydrogel was found to facilitate the generation of defective cartilage tissue by assay⁹⁶. Chung et al. found that hUCB-MSCs and four different hydrogels were cultured and transplanted into a rat model of right knee defects. The application of 4% hyaluronic acid hydrogel was determined, through assay at the 16-week post-transplantation stage, to exhibit superior cartilage repair and to resemble adjacent uninjured articular cartilage in terms of cellular arrangement and collagen organization pattern. Moreover, the 4% hyaluronic acid hydrogel group markedly promoted the production of type II collagen without significant rejection⁷⁹. Parmar et al. showed that HA or CS binding peptides were utilized for the modification of streptococcal collagen and cross-link it with matrix metalloproteinase 7 (MMP7)-sensitive peptides to form a degradable hydrogel. This hydrogel exhibited significant enhancement in hMSCs viability and facilitated cartilage differentiation. Further analysis revealed that hydrogels functionalized with CS-binding peptides promoted MMP7 expression, while HA-binding peptides predominantly promoted chondrogenic differentiation of hMSCs⁹⁷. Park et al. analyzed the repair of damaged cartilage after transplantation of hUCB-MSCs and HA hydrogel, demonstrating that HA hydrogel-enhanced the chondrogenic differentiation ability of hUCB-MSCs. Additionally, they found that low cell concentration was more effective than high cell concentration in promoting cartilage repair⁹⁸. Feng et al. demonstrated that conjugated a tunable number of sulfate groups on HA thereby reducing the rate of hyaluronidase degradation. Sulfated HA hydrogels were found to immensely facilitate chondrogenesis and inhibit hypertrophy of encapsulated hMSCs. Furthermore, intra-articular injection of sulfated HA hydrogels effectively decreased the potential for cartilage wear and hypertrophy in animal osteoarticular joints⁹⁹.

Sodium alginate

Sodium alginate (Alginate sodium, SA), a natural polysaccharide extracted from brown seaweeds like kelp, is a linear copolymer formed by linking β-D-mannuronic acid (M unit) and α-L-guluronic acid (G unit) with β-1,4-glycosidic bond. The carboxyl group of the G unit can establish physical cross-links with divalent metal ions such as Ca²⁺, Ba²⁺, and Fe²⁺. Moreover, SA exhibits excellent biocompatibility, degradability, and cost-effectiveness while being environmentally friendly, but its application range is constrained by suboptimal mechanical properties. Fernandez et al. developed an alginate hydrogel capable of supporting nanohydroxyapatite (nHA)-mediated activation of genes involved in nonviral gene transfer (TGF-β3 and BMP2) and thus controlling the phenotype of MSCs. The assays demonstrated that the delivery of TGF-β3 and BMP2 genes significantly enhances sGAG and collagen production, potentially promoting hypertrophy and intrachondral pathway progression of MSCs¹⁰⁰. Chen et al. showed that autologous nasal chondrocytes (NCs) were cultured in 3D alginate hydrogel, and it was observed that NCs could proliferate stably in 3D alginate matrix and exhibited a higher synthesis rate of GAGs. In addition, the expression of collagen type II alpha 1 chain (Col2A1), aggrecan (ACAN), and SRY-box transcription factor 9 (SOX9) was raised while the expression of Col1A1 was significantly down-regulated in cultured NCs. In conclusion, NCs are a good source of seed cells for cartilage regeneration, and diatomate hydrogel can serve as an appropriate delivery system¹⁰¹. Li et al. identified that the repair of injured cartilage was achieved using a nonrechargeable hydrogel incorporating both naringin and BG. The findings demonstrated that the Naringin-BG hydrogel effectively facilitated cartilage regeneration by upregulating the expression of aggrecan, SRY-box 9, and type II collagen α, while concurrently mitigating inflammatory responses through M2 macrophage polarization. In addition, Naringin-BG hydrogel prevents ECM degradation by decreasing the expression of metalloproteinase-13 substrate, nitric oxide synthase, and metalloproteinase-1 substrate and thereby preventing ECM degradation¹⁰². Yu et al. showed that injectable SA/BG hydrogels were mixed with injectable thermo-responsive SA/agarose (AG)/quercetin (Que) hydrogels to obtain injectable hydrogels containing Que and BG (Que-BG hydrogels) for articular cartilage regeneration. The injectable Que-BG hydrogel not only promoted macrophage M2 polarization but also effectively reduced inflammation, and inhibited ECM degradation by down-regulating the expression of iNOS, MMP13, and MMP1 in degenerative chondrocytes. In conclusion, the injectable Que-BG hydrogel elevated the bioavailability of Que, maintained the chondrocyte phenotype, inhibited ECM degradation, and reduced the inflammatory response¹⁰³.

Polyethylene glycol

The molecular polymer PEG is commonly employed to modulate the physicochemical properties of substances and finds extensive applications in various fields such as food, cosmetics, and forging. PEG can effectively enhance the mechanical properties of hydrogels. Nguyen et al. discovered that by incorporating CS and matrix metalloproteinase-sensitive peptide (MMP-pep) into PEG hydrogels, they were able to promote collagen II secretion while inhibiting proteoglycan expression¹⁰⁴. Furthermore, a three-layer hydrogel structure was constructed, that is, polyethylene glycol (PEG)-based hydrogel with CS and MMP-pep bound in the top layer, CS incorporated in the middle layer, and hyaluronic acid incorporated in the bottom layer. Such hydrogels were tested and found to help elevate the formation of native-like articular cartilage with spatially variable mechanical and biochemical properties¹⁰⁵. Sridhar et al. utilized sulfated TGF-β1 functionalized PEG descending ice sheet hydrogels and crosslinked them with an MMP degrading peptide. It was observed that this hydrogel significantly enhanced the deposition of GAG and collagen, as well as the production of cartilage matrix¹⁰⁶. Zhao et al. extracted porcine articular chondrocytes and encapsulated them in polyethylene glycol-dimethacrylate copolymer (PEGDM) hydrogels, demonstrating that the novel cartilage formed in the photochemically crosslinked gel was morphologically similar to natural articular cartilage and could be surrounded by new ECM. Furthermore, the contents of total DNA, glycosaminoglycans, and hydroxyproline were immensely elevated in the newly generated cartilage¹⁰⁷. Zhang et al. prepared a triblock copolymer poly(lactic-co-ethylene glycol)-block-poly(lactic-co-ethylene glycol) (PLGA-PEG-PLGA) thermogel and used it as a scaffold for BMMSCs. The analysis conducted following the implantation of BMMSC-coated thermogels into injured cartilage revealed an upregulation in the expression of glycosaminoglycans and type II collagen. Additionally, the regenerated cartilage exhibited a strong bond with both the surrounding normal cartilage and subchondral bone.

Polyvinyl alcohol

PVA hydrogels have been widely studied on account of their high mechanical strength, good biodegradability, better biocompatibility, low price, and nontoxicity and nonhazardous. Matsumura et al. developed artificial cartilage materials by applying amorphous hydroxyapatite (HA) on the surface of biologically inert poly (vinyl alcohol) hydrogel (PVA-H) using a pulsed laser deposition technique, and the HA coating on PVA-H was found to immensely enhance the affinity between bone and PVA-H by testing¹⁰⁸. Scholten et al. showed that porous PVA hydrogel scaffolds and alginate microspheres were fabricated using a two-step water-in-oil emulsification process, which effectively controlled the mechanical properties of the scaffolds and promoted cell migration. These hydrogels have shown promising results in repairing cartilage defects¹⁰⁹. Qi et al. found that a thermosensitive CS/PVA composite hydrogel was prepared and encapsulated with BMSCs transfected with hTGF-b1. It was detected that this hydrogel induced the upregulation of aggrecan and Collagen II in BMSCs transfected with hTGF-b1, thereby facilitating the surface restoration of damaged cartilage⁷⁸. Kanca et al. found that the in vitro tribological properties of cartilage-PVA and PVP hybrid hydrogels using a custom-designed multidirectional wear device, and the results demonstrated that the PVA/PVP hybrid hydrogel has a lower coefficient of friction compared to cartilage-stainless steel articulation, which is more similar to cartilage-cartilage articulation. Nevertheless, such hydrogels are susceptible to abrasion/deformation and only hydrogels prepared at higher concentrations can decrease the apparent volume loss¹¹⁰. Peng et al. found that a novel porous hydrogel of PVA/CS was designed and the ratio was set at 6:4. It was found that this hydrogel exhibited excellent mechanical properties and stable physical and chemical properties. Moreover, it is noncytotoxic and significantly boosts cell proliferation⁸².

Swelling properties

Hydrogels are water dispersion systems characterized by a three-dimensional network structure formed through the cross-linking of water-soluble polymer chains using cross-linking agents. When exposed to water, hydrogels rapidly absorb water and undergo swelling, which involves intricate physical and chemical mechanisms. The molecular structure of hydrogels typically consists of linear polymer chains that are interconnected by cross-linking agents, resulting in the formation of a highly hydrophilic three-dimensional network. This network structure enables hydrogels to absorb and retain large quantities of water due to the presence of numerous hydrophilic groups such as hydroxyl (-OH) or carboxyl (-COOH) within the polymer chains, further enhancing their water absorption capacity. The water absorption mechanism of hydrogels primarily hinges upon the openness and hydrophilicity of their three-dimensional network structure. When the hydrogel comes into contact with water, water molecules interact with the hydrophilic groups through hydrogen bonds, leading to the expansion of the hydrogel network. This process is reversible, and as ambient humidity decreases, the gel network contracts and releases water. Moreover, this water absorption process is accompanied by a significant increase in volume, commonly referred to as ‘swelling’. This particular property renders hydrogels exceptionally well-suited for myriad applications, including drug delivery, tissue engineering, and biomedical diagnosis. The swelling process of hydrogels is progressive and can be categorized into three stages. The first stage is characterized by rapid water absorption, during which the hydrogel swiftly takes in water and begins to increase in volume. Subsequently, the slow growth stage ensues, where water absorption stabilizes gradually. The final phase is the swelling equilibrium stage, where the hydrogel’s network structure has completed expansion and can no longer absorb additional water. The speed and extent of the swelling process are contingent upon various factors, encompassing the properties of polymer chains, the type and concentration of cross-linking agents, and the environmental humidity. Comprehending how these factors influence the swelling process of hydrogels is indispensable for optimizing their performance and applicability.

3D bioprinting potential

3D bioprinting is an advanced technology that employs 3D printing techniques for the purpose of fabricating biologically functional tissues. This innovative approach holds the potential to create diverse tissue types, including bones, muscles, skin, and blood vessels. Notably, recent years have witnessed remarkable advancements in the field of 3D bioprinting, culminating in several successful application cases^111,112. To implement this technology, specialized software is employed to design intricate 3D models, while specific biomaterials serve as ‘ink’ for the printing process. These biomaterials possess the capability to accommodate cells, growth factors, and other substances, which are pivotal in facilitating tissue growth and repair^113,114. Once the models are successfully printed, they are meticulously nurtured within a controlled environment, ensuring optimal tissue growth and maturation. The potential applications of 3D bioprinting technology are wide-ranging. For instance, it can provide invaluable aid to individuals who have experienced the loss of body parts due to illness, accidents, or the natural effects of aging. Likewise, it holds the potential to produce medical models that enhance physicians’ comprehension of patients’ conditions and surgical procedures. Furthermore, 3D bioprinting technology can be harnessed to create meticulous laboratory models for the examination of human tissue development and pathologies. Nonetheless, there exist certain challenges and limitations regarding 3D bioprinting technology. Foremost, the utilization of specialized biomaterials and cell types entails concerns surrounding their availability and safety. Additionally, the high cost associated with 3D bioprinting technology may render it prohibitive for widespread adoption. Moreover, further research and development are imperative to optimize its clinical applicability. The fusion of 3D bioprinting and hydrogel technology engenders the potential for refined and intricate biofabrication. By way of illustration, 3D bioprinting technology enables the printing of tissues or organs with specific shapes and structures, which can subsequently be enveloped by hydrogels to promote cell growth and differentiation. Furthermore, hydrogels can serve as drug carriers, encapsulating therapeutic agents and facilitating their controlled release within the printed 3D structures¹¹⁵.

Cartilage regeneration with hydrogels

Articular cartilage injury is a prevalent orthopedic condition which can be caused by various factors, such as trauma, osteoarthritis, or injuries related to joint surgery^116,117. Its main clinical symptoms include joint pain and impaired mobility. It is crucial to promptly treat acute injuries to the cartilage as it has limited self-repair capabilities. If left untreated, the degeneration of cartilage tissue can accelerate significantly, leading to the development of planar arthritis or even osteoarthritis. From a clinical perspective, articular cartilage injury remains the primary reason for disability among adults and is an enduring biomedical challenge^118–120.

Since the emergence of the concept of tissue engineering, there has been a persistent effort by researchers to apply tissue engineering techniques in the repair of cartilage defects, with the goal of mimicking natural processes of cartilage repair. This involves the modulation of autologous seed cells, scaffolds, and cellular bioactive substances at the intended site, in order to promote the healing of cartilage tissues and restore their normal mechanical movement and metabolic effects^121–124. Tissue engineering can be classified into two categories: in situ and in vitro technologies, based on the origin of synthesis. In situ, tissue engineering technology is a technique developed from traditional in vitro tissue engineering technology for regeneration and repair. It does not rely on exogenous seed cells, but rather utilizes novel scaffold materials with unique structural and physicochemical properties. These materials recruit stem cells around the damaged area, directing their migration to the site of injury, where they can complete the repair process in situ^125,126. The in situ tissue engineering technology offers a novel concept and direction for the repair of cartilage damage, and holds great potential for cartilage repair applications^127,128. Regardless of the in situ or in vitro techniques, the use of seed cells is essential. Seed cells play a crucial role in their conversion, migration, and colonization within damaged areas, facilitating metabolic function and promoting cellular repair^129,130 (Fig. 6). Hydrogels have been widely adopted as carriers of cells, growth factors, and other regulatory substances in tissue engineering scaffolds, or as permanent implants to replace damaged cartilage. Numerous in vitro and in vivo experiments have demonstrated the promising potential of hydrogels in cartilage repair^53,131. While various hydrogels with complex structures have shown excellent cartilage repair effects, and advanced methods have been developed for hydrogel scaffold preparation, commercialized hydrogels for cartilage repair are still limited in number and relatively simple in structure. These complexities can introduce challenges when trying to meet current Good Manufacturing Practices and Quality System Regulations. Moreover, advanced biomanufacturing technology is required for the preparation of composite hydrogels, demanding special formulations and equipment, resulting in longer preparation times and higher costs compared to simpler structured hydrogel products. Moreover, the effects of different components, morphology, structure, and physicochemical properties of hydrogels on cartilage repair remain unclear, and confounding factors abound, further complicating the clinical translation of complex hydrogel structures.

Figure 6.

Hydrogel encapsulation may bring the treatment of MSCs and their secretome to the next level.

MSCs are derived from various tissues, including bone marrow, adipose, umbilical cord, peripheral blood, etc. MSCs can be encapsulated by hydrogel in various formation (single-cell hydrogel encapsulation, MSC spheroid hydrogel encapsulation, and MSCs hydrogel encapsulation). Also, MSC-EVs can be encapsulated by hydrogel for proregenerative, proangiogenic, immunomodulation, and antifibrotic effects. MSC, mesenchymal stem cell; MSC-EVs, mesenchymal stem cell-derived extracellular vesicles.

Applications of novel hydrogels in cartilage regeneration surgery

Currently, there is no definitive treatment for repairing cartilage injuries or defects. Surgical interventions typically include microfracture surgery (a standard approach), autologous osteochondral transplantation, autologous chondrocyte implantation (ACI), and autologous matrix-induced chondrogenesis (AMIC). These surgical methods have inherent limitations in clinical application. Translation of novel hydrogels in cartilage regeneration surgery is an arduous endeavor both during the regulatory approval process and after receiving marketing approval. Exciting progress in the clinical translation of novel hydrogels, is evident from currently ongoing clinical trials and will be the focus of this section (Table 3). The utilization of hydrogels in a clinical setting has the capacity to enhance current treatment modalities. In a study by Sharma et al.¹³², a PEGDA-based hydrogel was developed and injected into focal cartilage defects in the medial femoral condyle of 15 patients undergoing standard microfracture surgery. Despite the acellular nature of the injected hydrogel, the concurrent microfracture surgery facilitated the infiltration of autologous cells into the hydrogel. Comparative analysis between patients treated solely with microfracture and those who received the hydrogel-enhanced therapy revealed superior tissue filling and enhanced tissue organization, as assessed by MRI. Additionally, patients in the hydrogel-treated group reported diminished pain levels, a pivotal clinical parameter within this patient cohort. In a separate investigation, Niemeyer et al. explored the treatment of sizable cartilage defects in the knee through hydrogel-based autologous chondrocyte implantation in a prospective, 2-year, single-arm, multicenter phase III trial involving 100 patients. They devised a biocompatible and in-situ cross-linkable albumin-hyaluronan-based hydrogel to serve as a carrier substance for matrix-assisted autologous chondrocyte implantation (M-ACI) procedures. This hydrogel formulation allows for arthroscopic or minimally invasive application, even in challenging defect sites. Notably, hydrogel-based ACI, compared to baseline, emerged as a viable therapeutic avenue for patients with substantial knee cartilage defects¹³³. Injectable hydrogels are a quickly progressing area of biomedical research, having already led to numerous approvals in the US and in Europe¹³⁴. PAAG-OA (Contura), an intra-articular injection of poly(acrylamide) hydrogel, is still being evaluated in two new trials (NCT04179552 and NCT04045431) in Denmark for knee osteoarthritis¹³⁵. Overall, a significant portion of current clinical trials seek to determine what additional benefits and disease indications are possible for already-approved formulations. Nevertheless, several trials of novel formulations are paving the way to the clinic for more experimental and multifunctional injectable hydrogels. These new hydrogel clinical trials, such as NCT05186935 and NCT06028763, evaluate their in vivo safety and compare short-term and potential mid-term effects with standard microfracture surgery, with the potential to advance hydrogel materials into clinical therapeutic stages¹³⁶.

Table 3.

Novel hydrogels in clinical trials for cartilage regeneration.

Material	ID	Study title	Phase
A biocompatible and in situ cross-linkable albumin-hyaluronan-based hydrogel	NCT03319797	Treatment of large cartilage defects in the knee by hydrogel-based autologous chondrocyte implantation	3
Polyacrylamide hydrogel	NCT04179552	PAAG-OA treatment for knee osteoarthriti	N/A
Polyacrylamide hydrogel	NCT04045431	Treatment of knee osteoarthritis with PAAG-OA	N/A
Polyacrylamide hydrogel	NCT03067090	Aquamid reconstruction for osteoarthritis of the knee	N/A
Fibrin microsphere	NCT05051332	Articular cartilage defect, articular cartilage degeneration	3
Dextran + hyaluronic acid	NCT05186935	Advanced cartilage treatment with injectable-hydrogel validation of the effect	N/A
A biocomposite hydrogel, containing MSCs and growth factors (TGF-β1 and BMP-4)	NCT06028763	Treatment of ankle cartilage using injectable biocomposite hydrogel	N/A
polynucleotide gel	NCT05322005	Treatments in degenerative meniscopathy with injection of polynucleotides	N/A

The current emphasis on translation might herald a new generation of functional hydrogels to treat a wide array of cartilage regeneration indications. The scaling and manufacturing of hydrogels for commercial products is still a challenging process. These commercially available hydrogels have relatively simple structures compared to those with more complex compositions. The development of hydrogels with complex structures faces challenges in their clinical translation. This is due to the need for specialized formulas and equipment, which increases the preparation time and cost compared to hydrogels with simple structures. Additionally, the impact of different components, morphological structures, and physical and chemical properties of hydrogels on cartilage repair is not yet fully understood. The multitude of factors involved further complicates the clinical translation of hydrogels with complex structures. At present, most hydrogels are produced in small quantities for preclinical research, but before approval and commercialization, large-scale reactions and processes must be designed and optimized through the officially recognized Good Manufacturing Process (GMP)¹³⁴. Nanotechnological and biological components of hydrogels might present unique regulatory challenges, especially as are incorporated into next-generation formulations. So while the advantages of nanoparticles in hydrogel formulations are readily apparent in the preclinical literature, there is the issue that nanomedicines have generally been difficult to translate to the clinic¹³⁷. Similarly, many bioactive hydrogels used in preclinical studies rely on natural biopolymers, but these biopolymers show batch-by-batch changes, which may impair standardized preparation.

Prospections and conclusion

An ideal hydrogel for tissue-engineered cartilage should possess a range of essential characteristics for effective performance. These characteristics encompass the mechanical properties required for mimicking natural cartilage, exemplified by the ability to absorb compressive loads, resist longitudinal deformation, and endure the intricate mechanical environment within the joint cavity. Furthermore, the hydrogel’s design should incorporate a suitable micro-porous structure conducive to cell infiltration, migration, nutrient diffusion, and waste exchange to support tissue regeneration. It is imperative that the raw materials and degradation products utilized in hydrogel formulation demonstrate attributes of safety, nontoxicity, and biological compatibility. Controlled biodegradability becomes paramount to ensure synchronization between the degradation of the hydrogel scaffold and the growth of new cartilage. Moreover, the hydrogel should exhibit favorable cell affinity and specific functionalities, including the capacity to load and release bioactive factors, thereby creating a biomimetic microenvironment that fosters cell adhesion, proliferation, and differentiation without provoking immune or inflammatory reactions. An added advantage lies in streamlining the hydrogel preparation process to enhance efficiency, reduce production costs, and facilitate storage and transportation logistics.

Despite the promising applications of hydrogels, several challenges that hinder their clinical translation were identified. The scaling and manufacturing of hydrogels for commercial use remain complex, particularly for those with intricate compositions. The need for specialized equipment and formulations increases preparation time and costs, making it difficult to produce hydrogels in large quantities for clinical applications. Furthermore, the variability in natural biopolymers can lead to inconsistencies in hydrogel properties, complicating standardization and regulatory approval processes. In this review, we concluded by outlining the essential characteristics of an ideal hydrogel for cartilage tissue engineering, including good mechanical properties, a suitable micro-porous structure for cell infiltration, and safety in terms of raw materials and degradation products. It is important to continue research to optimize hydrogel formulations and manufacturing processes, as well as to better understand the interactions between hydrogels and biological systems. In summary, this review serves as a valuable resource for researchers and clinicians interested in the field of cartilage regeneration, providing insights into the current state of hydrogel technology, its applications, and the challenges that must be addressed to facilitate successful clinical outcomes.

Ethical approval

None.

Consent

None.

Source of funding

None.

Author contribution

S.L.,Y.G., and Y.K.: original draft preparation, allocation, revision, supplement, and edition. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest disclosure

The authors declare no conflict of interest.

Research registration unique identifying number (UIN)

None.

Guarantor

Shenglong Li.

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Provenance and peer review

Not commissioned, externally peer-reviewed.