Harnessing biopolymeric double network hydrogels for cartilage repair: a review of current strategies

Abstract

Hydrogels with a double network (DN) structure that are based on carbohydrates have shown potential as scaffolds for the regeneration of cartilage. We postulate that these hydrogels can aid in the repair of cartilage and sustain the regeneration of tissue because of their ability to hold large quantities of water and large amounts of bioactive molecules. Some recent preclinical studieswith DN hydrogels that have been modified with growth factors, other biological molecules, and or therapeutic agents have shown enhanced growth of the cartilage tissue, hydration, and structural integrity of the tissue. In this review, we detail these advancements as potential carbohydrate-based double network hydrogels for the treatment of cartilage defects and osteoarthritis. Optimized DN hydrogels are tissue engineering scaffolds for cartilage regeneration, with the potential to translate to clinical practice.

Introduction

A comprehensive search for articles was performed on PubMed, Google Scholar, Web of Science, and Scopus, spanning the period from 2016 to 2026. Our selection of sources was based on their original research focusing on biomedical applications of polysaccharide-based double network hydrogels. The cartilage repair applications were extracted.

Carbohydrates are commonly found in natural biomaterials present in many different places. These molecules play a crucial role in various basic biological processes such as cell signaling, inflammation, initiating infections, and even the progression of diseases, primarily by influencing how cells interact with one another. Glycoconjugates, including glycolipids, glycosaminoglycans (GAGs), glycoproteins, and proteoglycans, facilitate these intricate interactions between carbohydrates and cells [1–3]. Carbohydrate-based materials have received particular attention relative to other biopolymers due to their cost-effectiveness, accessibility, biocompatibility, and biodegradability, rendering them optimal polymers for biomaterial applications [4, 5]. Carbohydrate-based double network (DN) hydrogels are a good choice for growing back cartilage. Cartilage, a type of connective tissue that is found in joints, doesn’t heal very well because it doesn’t have blood vessels [6, 7]. For cartilage regeneration to work, scaffolds that mimic the structure of the native extracellular matrix (ECM) and provide mechanical support are needed. DN hydrogels have demonstrated potential in this domain [8].

The primary network within DN hydrogels resembles a polyelectrolyte, meaning it’s the first to form, and is typically strong and rigid. It contains cross-links that break when under stress, allowing it to store considerable energy. This is what gives DN hydrogels most of their stiffness and strength. Conversely, the second network is typically soft, easily stretchable, and poorly connected. This second network is what fills the first and absorbs externally applied stress, and together, these characteristics make the hydrogels strong and pliable [9–11]. The unique structure of DN hydrogels contributes to their ability to effectively absorb energy, making them particularly strong [12]. Similar techniques have been utilized for different soft substances, such as rubber and pliable glues. The design ensures that the first network is more compact, while the second network assists in minimizing the energy required to further expand the first one. The strengthening of the material is also connected to the energy that is released when specific bonds are broken. As a result, we can apply the architecture of DN hydrogels to enhance the production of various other soft materials [13, 14]. Additionally, creating large DN (Macro-DN) resembles the process of producing DN hydrogels at the molecular scale. For example, King and his team created a hard plastic structure using 3D printing as the first layer, and a stretchy silicone rubber as the second layer that can stretch a lot. The new materials demonstrate a gradual breaking process rather than a single break, resulting in high stiffness from the first layer and significant stretchiness from the second layer. So, Macro-DN composites can make materials stronger by using the design idea from small DN hydrogels, which can be used in making various materials [15].

Gong and their team made the first fully crosslinked poly-2-acrylamide-2-methylpropanesulfonic acid (PAMPS) and polyacrylamide (PAAm) double network (DN) hydrogel using a two-step process called continuous free-radical polymerization. This hydrogel exhibited impressive durability, demonstrating a toughness ranging from 102 to 103 J/m², a strain capability of 1000–2000%, and a tensile strength that can withstand 1–10 MPa before failing [16]. During the straining process, the first PAMPS network breaks apart with a little pressure, and a thin gel area starts to form. At the edge of a crack, the PAAm chain helps spread energy to break the PAMPS network further. This helps distribute the stress at the tip of the crack and stops it from growing quickly [17]. Gong’s research team [18] outlined how to make strong DN hydrogels with these basic ideas: (1) Use two different types of networks—one should be stiff and breakable, while the other should be soft and tough; (2) In the soft network, the amount of the soft polymer should be 20 to 30 times greater than the stiff polymer in the first network; (3) The stiff network needs to be tightly connected but made with a low amount, while the soft network can be loosely connected, using a higher amount. Drawing from these concepts, the methods for enhancing DN hydrogels have been improved. The original two-step process for making polymers is usually used for the first network of DN hydrogels, which are strong materials made from charged particles. A primary gel is prepared and then immersed in a solution containing a second component to form a secondary gel within the initial one. If the initial network is composed of a neutral polymer or a weak polyelectrolyte, it exhibits limited swelling potential, resulting in a poorly structured first network. As a result, the double network (DN) hydrogel lacks significant toughness. The “molecular stent” method was created to boost extra ionic pressure by adding ionic micelles or linear polyelectrolytes to a neutral hydrogel. This helps to enhance the strength of DN hydrogels. The variations in strength and other physical properties of the DN hydrogels produced through different cross-linking techniques are presented in Table 1.

Table 1.

Differences in the mechanical properties of hydrogels prepared by different cross-linking methods

DN Hydrogel	Cross-Linking Method	First Network	Second Network	Tensile Strength	Ref.
Xylan/PVA/Borax	Physical-chemical cross-linking	PVA chain with Xylan first physical network	PVA chain and borax chemical network	81 KPa	[135]
Polyacrylamide/Gelatin	Physical-chemical cross-linking	Gelatin network	Polyacrylamide network	0.57 MPa	[136]
Poly(2-acrylamido-2-methylpropanesulfonic acid)/Poly (N-isopropyl acrylamide-co-polyacrylamide)	Chemical cross-linking	Poly(2-acrylamido-2-methylpropane sulfonic acid) network	poly(N-isopropyl-acrylamide-co-acrylamide) network	25 MPa	[134]
Gelatin/Polyacrylic acid/Tannic acid/Aluminum chloride	Physical cross-linking	Gelatin, polyacrylic acid, tannic acid, hydrogen bonding network	Al3+ ligand bonding network with polyacrylic acid	216 KPa	[137]
Agar/Hydrophobic associated polyacrylamide	Physical cross-linking	Agar hydrogel network	Hydrophobically associated polyacrylamide gel network	0.267 MPa	[138]
Chitosan/Polyacrylic acid/Ag+	Physical cross-linking	Chitosan/Ag+ cross-linking network	Polyacrylic acid network	24 MPa	[139]
N-Acryloyl glycinamide/PVA-borax	Radiation Crosslinking	PVA/Dynamic boronic acid lipid bond	poly(N-acryloyl glycinamide)/PVA network	200 KPa	[140]

Carbohydrates, including chitosan, hyaluronic acid, and alginate, are widely used in the fabrication of DN hydrogels for cartilage regeneration. For instance, chitosan, a derivative of chitin, is biocompatible, biodegradable, and possesses antimicrobial properties [19, 20]. It is adaptable and can be improved to better serve the purpose of constructing things such as scaffolding [21]. Hyaluronic acid, on the other hand, is a major component of the native cartilage ECM and plays a crucial role in maintaining tissue homeostasis. Adding it to DN hydrogels develops cell adhesion, proliferation, and differentiation, thereby facilitating cartilage regeneration [22]. The unique architecture of DN hydrogels provides several advantages for cartilage regeneration. The first network, composed of a stiff polymer like chitosan, provides mechanical support and prevents the collapse of the scaffold [23]. The second network, which is made of a more flexible polymer like hyaluronic acid, lets cells get through and nutrients spread. The scaffold can handle compressive forces while allowing tissue to grow and integrate because it is both stiff and flexible, which is similar to the mechanical properties of natural cartilage [24–26]. Carbohydrate double network hydrogels demonstrate superior biocompatibility and mechanical characteristics [27]. The carbohydrates in the hydrogel matrix help cells stick together and grow, which helps new tissue form. Carbohydrates also have anti-inflammatory properties, which can help lessen the inflammatory response that often happens when tissues are healing [28]. Another good thing about carbohydrate-based DN hydrogels is that they can release bioactive molecules in a controlled way [29]. By incorporating growth factors or other therapeutic agents into the hydrogel matrix, researchers can enhance the regenerative potential of the scaffolds. The release kinetics of these bioactive molecules can be tailored by modifying the composition and crosslinking density of the hydrogel, allowing for precise control over the healing process [30–32]. In conclusion, carbohydrate-based DN hydrogels hold significant promise for cartilage regeneration. Their unique architecture, mechanical properties, biocompatibility, and ability to release bioactive molecules make them ideal scaffolds for promoting tissue growth and integration. Further researches are needed to optimize their design and fabrication techniques to ensure their clinical applicability in treating cartilage defects.

General aspects of carbohydrate-based DN hydrogels

To elaborate on the reasons behind selecting carbohydrates to construct DN hydrogels, it is important to understand the basic properties of these carbohydrates commonly used for DN hydrogels. Carbohydrates like sodium alginate (SA), hyaluronic acid (HA), cellulose, chitosan (CS), and others are utilized. The chemical structure of some commonly used carbohydrates is shown in Fig. 1.

Fig. 1.

Structural formulas of some carbohydrates

The carbohydrates found in plants and algae can vary a lot in how they look and behave. The factors influencing this are the origin, cultivation method, harvest time, and extraction process of the plants [33, 34]. Carbohydrates like xanthan gum (XG), guar gum (GG), and curdlan are made by bacteria in a preclinics using a specific process [35, 36]. Considering its capacity to retain water, its ability to undergo uninterrupted degradation effectively, and its compatibility with living organisms, natural hydrogel made from carbohydrates can be used in a variety of medical applications [37, 38]. Unfortunately, conventional techniques often result in weak natural polymer hydrogels, limiting their practical applications. DN hydrogels consist of a combination of two polymer networks that are entangled with each other, permitting the permeation of certain substances. These hydrogels are very strong and tough compared to hydrogels with only one network [39–41]. DN hydrogels get stronger and stiffer when they are stretched or bent because they can absorb energy. This is due to the initial weak but fully connected networks [42]. The addition of the second network to the hydrogel enhances its durability and flexibility, empowering it to withstand external pressures and deformations [38]. Due to their exceptional structural composition, DN hydrogels exhibit strong physical characteristics that enable them to absorb and release energy efficiently [43]. A second network is created to amplify the strength of the initial network, designed with a particular structure to decrease the material’s flexibility. This enhances the ability of the first network to withstand stretching or pulling forces. Additionally, the reinforcement of sacrificial connections within the substance contributes to its enhanced strength. In an interesting study, by utilizing a two-step method, the researchers synthesized a hydrogel by merging PAMPS and PAAm, two chemical compounds. This substance is extremely durable and has the ability to endure high levels of pressure and tension without breaking (Fig. 2).

Fig. 2.

Utilizing a two-step polymerization strategy to construct chemically crosslinked DN hydrogels

The mechanical toughness of the hydrogel previously mentioned is approximately quantified at 102 J/m², with its strain capability extending up to ~2000% and its stress levels oscillating between 1 and 10 MPa [44]. Easy breakage and the resemblance of thin gel necks are common occurrences when the PAMPS network is tensioned. The energy of the PAMPS network is initially transferred through the PAAm segment during crack propagation, leading to stress reduction at the crack tip and decelerating the crack’s velocity [45]. Consequently, the DN approach is very useful for enhancing the strength of natural carbohydrate-based DN hydrogels and cannot be replaced [46]. As mentioned above, DN hydrogels are made from natural carbohydrates. Peptide-based hydrogels and polymers have been used for a very long time, but are limited in their application due to their low mechanical strength and tendency to dissolve in aqueous solutions. DN hydrogels have several advantages over other hydrogels. It is biocompatible and can absorb up to 2% of its weight in surrounding media. Additionally, it is resistant to different types of chemicals, makes a good biostable scaffold, and is easy to handle and process. Natural hydrogels are useful for many things and are better than synthetic hydrogels in a few ways. Their bodies are more robust, durable, and less expensive to manufacture without sacrificing their form or usability.

Natural carbohydrate-based DN hydrogels for cartilage regeneration

The determination to employ carbohydrates within pharmaceutical delivery mechanisms is contingent upon significant considerations. The capacity of organic resources to undergo regeneration is a beneficial attribute. For instance, animals and sea creatures produce chitin and chondroitin [47] from their unused parts, whereas plants possess starch, cellulose, hyaluronic acid, and alginate [48]. Thanks to recent advancements in carbohydrate processing, which involve refining, altering, and examining them, these methods guarantee that natural resources are of superior quality and thoroughly comprehended [19, 49]. Researchers have looked at how carbohydrates can be utilized to make scaffolds for tissue engineering. Carbohydrates are different types of molecules that are similar to other big molecules found in our body. Polysaccharides are natural polymers that are more friendly to our bodies compared to man-made polymers [50, 51]. If synthetic polymers get mixed with leftover initiators, they can become dangerous for the growth of cells [52]. It has been reported that when hydroxypropyl cellulose, a substance derived from carbohydrates, is added to polyurethane, it can reduce platelet adherence. This makes it a suitable choice for materials that are compatible with living organisms [4, 53]. These materials have impressive features such as being strong, tough, and able to bounce back, which is why they are considered good options for building cartilage [54]. In DN hydrogels, adding both weak and strong links makes them tougher. Strong connections are crucial for the strength and capacity of the DN hydrogel to recover its initial form during the loading process [55, 56]. On the other hand, weaker connections play a role in absorbing and dispersing energy to strengthen the stronger connections. The strength of polysaccharide-based hydrogels can be improved by using photocross-linking to introduce a methacrylate functional group to the ${\mathit{OH}}^{-}$ or $R_{3}N$ (R = H, …) groups of the polysaccharide backbone. In addition, using a substance found in sea mussels can make the DN hydrogel more durable and capable of self-repair and recovery [57]. This section of the review will examine various polysaccharides commonly utilized in the fabrication of DN hydrogels and their applications in cartilage regeneration.

Alginate-based DN hydrogels

Alginate represents a significant polysaccharide that is biocompatible and does not elicit immune responses within the organism. Comprising natural saccharides, it can be effectively utilized in medical and diverse applications. Alginate is extensively employed in the field of tissue engineering due to its biocompatibility, ease of utilization within biological systems, and cost-effectiveness. Under standard physiological pH and temperature conditions, the gel undergoes rapid solidification [58, 59]. These favorable conditions are ideal for the survival of living cells and vulnerable biomolecules like proteins and nucleic acids. Alginate is frequently employed for the production of tissue or organ-like structures, in addition to its use in encapsulating cells, medications, enzymes, and biomolecules [60]. Alginate hydrogel is a substance found in the alga, “Spirulina”. Alginate is an extremely abundant and inexpensive natural material that can be synthesized from sugars. It is well-known for its ability to form a gel when exposed to a certain chemical solution. The gel has unique properties such as the ability to absorb liquids and gases while remaining intact. Alginate has many applications and is commonly used in industries such as the healthcare field and food processing. It can also be used in research and other scientific applications, such as bioreactors and bio-printing. Alginate hydrogel is used as an injectable material that helps heal wounds, control bleeding, and as a treatment for cartilage damage. In one interesting study, three types of hydrogels were developed by Baei and coworkers to gradually release medicine. They employed SA alongside CS for this purpose. They tested whether these hydrogels can help rabbits grow new cartilage in damaged joints. According to the findings, all hydrogels had the same starting resistance to being compressed (30–35 kPa), which is useful for keeping the specific cell type associated with cartilage. The SA-catechol DN hydrogel (DN/SA-Cat) exhibited superior strength, toughness, self-healing capability, and recovery compared to the other hydrogels. The synthesized structures had lots of tiny holes, making up about 80–90% of their structure. The SA-Cat/CS and DN/SA-Cat hydrogels stayed strong and didn’t break down as much when they were placed in a liquid with cells, even when an enzyme called lysozyme was present (Fig. 3) [61].

Fig. 3.

Articular cartilage recovery of SA-Cat/CS DN hydrogel. Figure modified with permission from [61]

Furthermore, 3D printing is one of the various methods that are used in fabricating 3D-printed biological structures. The methacrylate carrageenan/sodium alginate (KCMA/SA) hydrogel is a substance that can be used to enclose cells and create 3D-printed biological structures. The network of SA molecules crosslinked with Ca²⁺ in the KCMA/SA bio-ink made it very stretchy since the ionic crosslink network can distribute force evenly and prevent stress build-up and damage. The 3D extrusion printing system generated KCMA/SA hydrogel filaments, which formed a minute structure that penetrated the material. The analysis showed that KC and SA are connected through hydrogen bonding. A higher amount of SA resulted in the enhancement of the hydrogels’ molecular structure. This suggests that KCMA/SA hydrogels have potential in the field of regrowing tubular tissues [62]. Li et al. [56] carried out an experiment utilizing a distinct hydrogel known as a supramolecular-polymer DN gel. The gel was created to measure its ability to aid in the restoration of cartilage tissue. Scientists added a changed version of a substance called HA to the polymer network by mixing it with acrylamide to make it better at growing cartilage. The network formed by the soft peptides in this design functioned in a manner akin to the collagen network present in natural cartilage. The network of polymers can make something stiff and have the right stuff to make cartilage. These hydrogels had great flexibility, strength, quick recovery, and reduced stress. When comparing SN hydrogel to DN hydrogel, the DN hydrogel showed significant improvement in its strength, and it is suggested that they might display advantageous interactions with biological entities. Because of these benefits, the tests done on the living body for cartilage regeneration showed a noticeable improvement in repairing [56].

Cellulose-based DN hydrogels

Scientists are interested in cellulose because it can be easily replenished, widely accessible, not harmful to living organisms, reasonably priced, breaks down naturally, can withstand high temperatures and chemicals, and can be modified into different forms [63, 64]. Each year, plants produce a substantial quantity of cellulose, which acts as a vital carbon reservoir [65]. The linkage of two glucose forms the structure of cellulose rings through oxygen. Each chain has a different number of repeated units according to its source [66]. For example, there are around 10,000 repetitions in native cotton and about 15,000 repetitions in wood. Cellulose is a type of material that is tough to break down because its chain is very long and tightly packed together. Therefore, an important thing to consider when creating biological applications is finding inexpensive and environmentally friendly liquids to make solutions with [67, 68]. In a fascinating study, Zhu et al. created a simple method using the structure of DNA and bacterial cellulose to make bacterial cellulose (BC) DN gels stronger and prevent them from swelling (Fig. 4A). The hydrogels achieved a similar compression strength to natural cartilage, and their ability to swell was reduced to an appropriate level for use in osteochondral tissue engineering. They successfully created a special gel to help regrow bone and cartilage. This type of artificial material that imitates natural tissues has a positive impact and significant benefits in healing both cartilage and bone in the body [69]. After 4 weeks, the hole had filled with healing tissue for the single-layer group, but the bilayer group had less advanced healing tissue in the hole. By the 8-week mark, the implants remained somewhat visible in the single-layer group but were hardly noticeable in the bilayer group. Both groups’ implants fused with the surrounding tissue after a period of 12 weeks, resulting in a smooth and even surface in the damaged area. It is clear that the new chondral tissue formed in the bilayer group had a similar color to the nearby existing chondral tissue (Fig. 4B). Hydrogels made from natural polysaccharides are great for making cartilage tissues because they have a special structure, they can break down naturally over time, and it is posited that they may exhibit advantageous interactions with biological entities [70]. In one research, a method involving chemicals and physical processes was used to make a special type of gel called double-network BC hydrogels. The hydrogels have a natural and porous shape that is similar to living tissue, and they contain a lot of water. This makes them perfect for cells called chondrocytes to stick to and grow. DN hydrogels meet the performance needs of cartilage tissue engineering scaffolds. They have a high strength when compressed (4.53 MPa) and are better at recovering from compression and fatigue compared to single-network hydrogels. Based on experiments, DN hydrogels can help cells stick together and grow because they are safe and not harmful. To make cartilage tissue and grow cells in a 3D environment, DN hydrogels are very useful [71].

Fig. 4.

Schematic and macroscopic images of hydrogels and knees. A A visual representation displaying the process of creating BC-DN hydrogels. B Images of knees that underwent repairs at varying points in time. (Single-layer group and Bilayer group). Figure modified with permission from [69]

With the passage of time, individuals may experience a deterioration of their cartilage. If cartilage is hurt from an injury or illness, it won’t heal by itself. DN hydrogels have been the subject of extensive study as a supple, moist, and robust replacement for emerging artificial cartilage materials. Yang and coworkers [72] synthesized dextrin-based hydrogels that are biocompatible by immersing polyvinyl alcohol (PVA) and polyacrylamide sulfonate (PAMPS) into a matrix of BC nanofibers. The BC-PVA-PAMPS DN hydrogel was as strong and flexible as cartilage because it had more hydrogen bonds and entanglements between its polymer chains. Also, it showed that it could withstand the same amount of force as cartilage after being used 100,000 times because the PVA made it flexible, could absorb energy, and reduced the stress on each BC fiber. Furthermore, green BC/SF DN hydrogel was made by dipping BC into a water-based SF mixture without any crosslinker. The findings show that SF was able to enter the pores of BC and interact with the microfibrils effectively. The synthesized DN hydrogel had a highly porous and interconnected composition. The BC/SF hydrogel is stronger and less likely to break than pure BC. The results from growing cells in a preclinical study show that the BC/SF scaffold is more compatible with the body when the SF is changed, which proves that the BC-based DNhydrogel is a better material for growing cartilage in the body [73]. In a study, Ho and his colleagues made a simulated medical device using a special kind of gel made from cellulose and alginate. They used this gel to create a layer that mimics the annulus fibrosus (AF) and another layer made from cellulose gel for the nucleus pulposus (NP) (Fig. 5A). Using DN structure improves the strength of implanting intervertebral disc (IVD) scaffold, which helps prevent more damage from nearby tissue because of differences in strength. Furthermore, the IVD scaffold has a structure around it that helps keep the GDF-5 growth factor in the area and release it slowly as the hydrogel breaks down. Adding a chelating agent kickstarts the shape recovery process, which makes it easier to implant the scaffold without needing a difficult and complicated surgery (Fig. 5B). In testing how compatible the IVD scaffold is with cells, it was found that there were no toxic effects on MSCs when the scaffold was mixed with a gel-like substance and left to incubate. GDF-5 released from the IVD scaffold kept its ability to control the differentiation of MSCs, as tested with qPCR. The DN hydrogel with RGD peptide helped the cells survive and multiply. In an investigation involving rats exhibiting degeneration of the tail disc, the incorporation of structural support alongside a compound known as GDF-5 and specific peptides significantly mitigated the degeneration process and enhanced functional capacity. This study indicates the potential for modulating the release of beneficial compounds, thereby facilitating the movement and proliferation of endogenous mesenchymal stem cells (MSCs) to aid in the repair of spinal discs (Fig. 5C) [74].

Fig. 5.

Schematic design of bioactive hydrogel. A Creating a unique DN hydrogel and incorporating a substance similar to that found in intervertebral discs. B The research examined the ability of a unique DN hydrogel to return to its initial form within a mouse’s tail disc. C Finding and directing the body’s own stem cells using special peptides in DN hydrogels. Reprinted with permission from [74]

Moreover, studies have demonstrated that the incorporation of Laponite (XLG) in silated hydroxylpropylmethyl cellulose (Si-HPMC) hydrogels enhances their durability while not compromising the passage of oxygen or the viability of cells post-gelation. Si-HPMC/XLG DN hydrogels, when incorporated with chondrocytes and placed in the skin of hairless mice, showed that they could be used to make cartilage tissue [75]. In an interesting investigation, Zhao and his research team developed a distinctive variant of CNFs capable of adhering to methacrylated HA to produce DN hydrogels of HA/CNF via photopolymerization. The resultant DN hydrogels exhibit remarkable mechanical strength and resilience under compressive forces, rendering them appropriate for applications in biological tissues necessitating structural support, such as the articular cartilage present in joints. Moreover, this innovative DN hydrogel fosters an optimal microenvironment conducive to the proliferation and differentiation of bone marrow cells into chondrocytes. Additionally, it demonstrated significant efficacy in repairing damaged cartilage in rodent models. This investigation indicates that the HA/CNF nanocomposite hydrogel holds promise for the development of novel scaffolds for cartilage tissue engineering [76].

Chitosan-based DN hydrogels

Chitosan is extracted from the shells of specific sea creatures, making it a natural product. It has a special property that attracts and holds onto water [77]. Chitosan incorporates various substances known as glycosaminoglycans, which are typically located in the extracellular space surrounding cells. The unique properties of chitosan render it hydrophilic and enable its adhesion to cellular structures. Upon degradation, it does not yield any toxic byproducts. This characteristic is crucial, as it allows for its safe application in medical interventions [78, 79]. Chitosan is frequently employed in the medical field owing to its capacity for degradation, biocompatibility, and lack of adverse effects on human physiology [80]. In 2020, Gan and his research team developed specialized water-based DN-hydrogels utilizing chitosan (CS) and polyvinyl alcohol (PVA) through a process of freezing, thawing, and subsequent soaking. The resultant hydrogels exhibit remarkable resistance to mechanical failure, withstanding forces of approximately 2.70 ± 0.24 MPa (high fracture tensile stress), while also displaying significant toughness quantified at 14.09 ± 206 MJ/m³ (toughness), and demonstrating compressive resistance of 0.88 ± 0.09 MPa (compressive modulus). Furthermore, these hydrogels possess distinct structural attributes that confer exceptional fatigue resistance and the capability for self-healing [81]. Furthermore, CS hydrogel was combined with a type of plastic called poly(ε-caprolactone) to make a special 3D-printed structure. This structure contained certain cells called synovial mesenchymal stem cells (SMSCs) and a type of genetic material called tetrahedral framework nucleic acid. This material mixture was then injected into a specific part of the body called the articular cavity. Tetrahedral framework nucleic acid (TFNA) is a type of DNA material that has the potential to improve the environment needed for regrowth. It can be absorbed by certain cells (called SMSCs) and helps these cells grow and develop into cartilage. CS, a type of charged sugar, can stick to DNA by using electrical attraction and bring in extra TFNA molecules after being injected into a joint cavity inside the body [82]. By utilizing two distinct interactions, the researchers effectively synthesized chitosan/polyethylene glycol-silicotungstic acid (CS/PEG-SiW) double-network hydrogels. These interactions rendered the hydrogels more robust and cohesively structured in comparison to CS/PEG hydrogels (Fig. 6).

Fig. 6.

Schematic outline for the creation of recombined permeable CS/PEG-SiW hydrogels for cartilage tissue building. A Creating a significant number of CS/PEG-SiW hydrogels with a double network composition. B The creation of CS/PEG-SiW DN hydrogels aims to facilitate the growth of new cartilage. Figure modified with permission from [83]

Subsequently, small particles of CS/PEG-SiW were fabricated using mechanical breaking, and these particles can be injected continuously, even in water. The porous CS/PEG-SiW hydrogels were made using a method called “smashed gels recombination”. These hydrogels are structurally strong and contain small interconnected holes. The mixed porous DN hydrogels were safe for cells and created a good place for them to live, grow, and change into cartilage cells. Studies conducted inside a living organism showed that the combined special spongy CS/PEG-SiW hydrogels can help fix damaged cartilage in rats. These findings showed that CS/PEG-SiW structures can be used as a new structure to help grow cartilage tissue [83].

Recently, scientists have created a type of injectable DN hydrogel that is safe for cells, strong and sticky to cartilage. This method uses Gelatin methacryloyl (GelMA)-glycol chitosan (GC) that can be connected with visible light and certain substances (lithium acylphosphinate, tris (2,2′-bipyridyl) dichlororuthenium (II) hexahydrate, and SPS). The synthesized DN hydrogels are especially good at sticking to cartilage, which is mainly because of the inclusion of GC. The Ru/SPS incorporated GelMA-GC DN hydrogels were tested by pulling and pushing them. The tests showed that the hydrogels were as strong as 100 and 46 kPa, respectively (Fig. 7A). When the hydrogel was injected into damaged cartilage in cows and was cross-linked there, it stayed together and became a part of the cartilage even when the joint was moved around outside the body (Fig. 7B). Cells exhibit a high survival rate (>85%) within the hydrogel matrix and possess the capacity to regenerate tissue after a three-week culture period in a controlled preclinical environment [84].

Fig. 7.

Ex vivo adhesion tests of hydrogels. A The utilization of GelMA or GC hydrogel with a photoinitiator in investigating knee bone defects and observing joint movement in an ex vivo context. B The process involves using a tool to make small holes in bone and cartilage, transforming a liquid into a gel, and then solidifying the gel with light and putting the gel into the holes in the bone and cartilage after moving the joint. The results showed good integration (marked with a green arrow) and poor integration (marked with a yellow arrow). Areas where the hydrogel came off are outlined in yellow. Reprinted with permission from [84]

Li et al. used a chemical interaction between cyclodextrin and cholic acid to hold the hydrogel together. The main substance used was a combination of β-cyclodextrin and a modified form of poly(L-glutamic acid). The other substance was chitosan modified with cholic acid. The special gel can fix itself and is easy to inject. It is very good at healing, and more than 90% of the time, it can fix itself. Also, the stem cells taken from fat were able to turn into cartilage cells well when grown in a gel with certain substances that help them change [85]. Moreover, Li et al. developed a novel variant of chitosan double-network hydrogel that is deemed safe for application and exhibits significant physical and biological properties. This was accomplished through the integration of diverse materials and methodologies. These double-network gels possessed a microstructure characterized by numerous interconnected pores of varying dimensions, which enabled liquid absorption and gradual degradation over time. Such attributes facilitated cellular adhesion, proliferation, and growth within the hydrogel. Additionally, the sustained release of kartogenin (KGN) promotes cartilage growth and the synthesis of cartilage components, thereby aiding in the repair of damaged tissues and the formation of new cartilage within living organisms [86].

Hyaluronic acid-based DN hydrogels

Hyaluronic acid, which is a linear anionic polysaccharide, is a crucial glycosaminoglycan found in the ECM of various human tissues [87]. HA helps wounds heal faster by enhancing the growth, movement, and speeding up the formation of new blood vessels [88]. The urgent issue in cosmetic surgery is the repair of segmental costal cartilage defects through autologous cartilage grafting. Concentrating on this issue, Dai and coworkers created a multifunctional DN biohydrogel simulating squid cartilage type II (SGII)/hyaluronic methacrylic acid (HAMA) based on a “curing, shaping, and chemical photopolymerization” gelation system and evaluated its mechanical properties (Fig. 8A). This DN hydrogel cannot only easily fit into the damaged cartilage and is very similar to natural cartilage strength, but also adjusts the microenvironment. local tissue and induce further regeneration of costal cartilage in vivo. This is a useful plan to use a natural repair method for damaged cartilage in medical treatment [89]. Injectable liquid gel can be better for fixing cartilage due to its ability to deliver cells in a way that is not very intrusive. However, many liquid gels that can be injected break down quickly and are not very strong. Additionally, if hydrogels are too stiff, it can make it harder for cells to survive after they are implanted. To solve these problems, Thomas et al. made a special gel that gets stronger at the right temperature when it’s put inside the body. The BDNH looks like the structure of aggrecan, with HA-conjugated PNIPAM making it stiff, and Schiff base polymers can be rendered flexible through the process of crosslinking (Fig. 8B). BDNHs can heal themselves and become stiffer at normal body temperature. The chondrocytes grown in the BDNH hydrogel had healthy cells, grew for a long time, and made cartilage materials. They found that putting special cells in a rabbit’s damaged cartilage helps it heal. This method could be a good way to repair cartilage in the future [90]. Injectable hyaluronic acid gels have limited use for fixing cartilage because they break down quickly and are not very strong. In an interesting study, scientists made a special type of gel using two different reactions. One reaction made it strong, and the other reaction made it versatile, so it can be used in different ways. The DN hydrogel is better than the single cross-linked hydrogel because it can be injected easily, it responds to changes in pH, and it is strong and long-lasting. Furthermore, in vitro tests showed that the injectable DN hydrogel can help cells grow and multiply, which means it could be a good option for fixing damaged cartilage [91]. Furthermore, the alginate-poloxamer/silk fibron (ALG-POL/SF) hydrogel and its mixtures were found to be very strong, stretchy, and durable. Adding HA/chitosan-poly(dioxanone) (HA/CS-PDO) nanoparticles (NPs) made the gel even stronger, but it did not change how stretchy or durable it was. The mixture of gels made with bone morphogenic protein-7 (BMP-7) and HA/CS-PDO NPs can release BMP-7 at different rates for a few weeks. The special gel with BMP-7 helped the cells become cartilage and make cartilage parts. The study shows that this new hydrogel could be effective for fixing cartilage [92].

Fig. 8.

The schematic preparation of SGII/HA-DN hydrogel and BDNH implanted. A Schematic diagram of fabrication of SGII/HAMA-DN hydrogel for regeneration of costal cartilage defects [89]. B Schematic illustration of BDNH implanted with cartilage cells. Figure modified with permission from [90]

Other natural carbohydrate-based DN hydrogels

Gellan gum (GG) is a natural substance (generated by Pseudomonas elodea) that can join together to make a hydrogel when there are certain types of ions around. This hydrogel is safe to use in living things [92]. GG hydrogels have attractive biocompatibility properties, mild gelation conditions, structural similarity to in vivo glycosaminoglycans, and tunable mechanical properties. Although GG has potential for medical use, its high gelation temperatures make it unsuitable for cell encapsulation [93, 94]. An innovative double network hydrogel made from biopolymers and ionically cross-linked GG, as well as hydrophobic polyacrylamide, was effectively created. By using two physical networks together, the double network hydrogel is really strong. It has great elasticity and can withstand significant stretching without breaking, much like natural cartilage. The elastic modulus is approximately 236.4 kPa, while the fracture energy measures around 2199 J/m². The engineered DN hydrogel, owing to the intrinsic characteristics of natural GG and biocompatible chemical cross-linkers, exhibits a beneficial interaction with cells, thereby promoting the proliferation of chondrocytes in a manner aligned with physiological cartilage development. Given its exceptional biocompatibility and mechanical characteristics, the GG/HAP hydrogel represents a viable alternative as a cartilage substitute, thereby aiding in the regeneration of compromised cartilage tissue [95]. Furthermore, the successful preparation of a DN hydrogel by merging HA and GG resulted in a material with favorable mechanical properties and compatibility with cells. A proposed method for creating HA/GG DN hydrogels involves a straightforward and effective approach using both chemical and physical cross-linking. This DN hydrogel can encourage chondrocyte proliferation and support the synthesis of ECM, as a result of its durable structural integrity, alignment with cellular environments, and its inherent biodegradation capability over time. In vivo investigations have substantiated the gel’s efficacy in facilitating the regeneration of cartilage exhibiting properties akin to hyaline cartilage [96]. Curdlan, a polysaccharide polymer, exhibits a multitude of applications, especially within the domain of cartilage regeneration. The molecular architecture of curdlan is characterized by the presence of glycosidic linkages [97]. Wang and his team developed a DN hydrogel with a carboxymethylated curdlan (CMCD) and a PAAm that is both stretchable and able to heal itself (Fig. 9A). The sacrificial structure of CMCD macromolecules, formed by both hydrogen and ionic bonds, allows for a broad range of tunable, diverse, and recoverable mechanical properties in the CMCD/PAAm DN hydrogel. In vitro investigations and in vivo studies have validated the safety and efficacy of the gel in supporting the recovery of cartilage. Its mechanism of action involves mimicking the native cartilage microenvironment, effectively covering the damaged regions while supplying essential nutrients to facilitate healing (Fig. 9B) [98]. Collagen is a very common protein in mammals, making up a big part of the body’s protein. Collagen helps with mechanical strength in tissues and implants, especially when they are stretched [99].

Fig. 9.

Schematic representation of CMCD macromolecules. A Restructure the CMCD network during production. The visible characteristics of the newly formed cartilage: B the control group and C the cohort receiving CMCD/PAAm DN hydrogel treatment. D Histological analysis (H&E staining) of repaired cartilage: D control group and E group treated with CMCD/PAAm DN hydrogel. Reprinted with permission from [98]

Collagen is frequently employed in cellular studies due to its ability to support the proliferation and spreading of fibroblasts on its structural framework. Collagen performs numerous critical functions within the biological system, including the facilitation of cellular adhesion and growth, as well as providing structural support to the body [100]. Furthermore, collagen double network hydrogels have excellent biocompatibility, meaning they are well tolerated by the body and are unlikely to cause an immune response. This is essential for any material used in tissue engineering, as it ensures that the body will accept the hydrogel and that it will not cause adverse reactions [101]. Moreover, these hydrogels can be meticulously engineered to degrade at a rate conducive to cartilage regeneration, allowing for the gradual release of growth factors and other signaling molecules, which collectively contribute to the repair of the affected tissue [102]. Chen et al. developed resilient poly(ethylene glycol) (PEG)/collagen dual-network hydrogels that exhibit superior mechanical strength compared to their single-network counterparts. Additionally, the PEG/collagen dual-network hydrogel is advantageous for biological applications, as the collagen remains bioactive post-crosslinking, thereby establishing a conducive microenvironment for cellular growth in load-bearing tissue. These findings indicate that dual-network hydrogels may serve as promising biomaterials for the fabrication of artificial soft tissues, such as synthetic cartilage. Over extended periods, dual-network hydrogels exhibit limited adhesion to solid substrates, a property attributed to their high water content [103].

Scaffold-host interaction dynamics

The interaction between the hydrogel scaffold and cartilage tissue in DN hydrogels for cartilage repair is characterized by a multifaceted relationship. These DN hydrogels are designed to closely replicate the strength and characteristics of actual cartilage, which is essential for effective tissue healing. DN Hydrogels consist of unique sugars that solidify when exposed to light, along with a synthetic protein. Adjusting the formulation and the way the materials are produced enables us to alter their properties [104]. The ability to adapt strengthens these DN hydrogels compared to those made purely from glycosaminoglycans, which is advantageous for even more non-invasive techniques for cartilage repairs. Adding light-activated materials also makes it easier to control changes after an injection and provides a method that can speed up healing and improve the movement of the joints [71, 105].

To summarize, the relationship between DN hydrogels and their supporting frameworks is vital for promoting the healing of cartilage. Their ability to imitate the strength and structure of genuine cartilage is vital for achieving successful cartilage restoration. These hydrogels provide an effective means of facilitating cartilage repair by offering adaptable support that responds to environmental changes, enhancing treatment efficacy.

Future trends: from the combination of materials to hybrid DN hydrogels

The findings from the studies suggest that unifying different elements leads to a boost in their durability. To address the previously mentioned needs, we must explore a new method distinct from the materials fabrication techniques we’ve already examined. The authors suggest that there is an underexplored area in materials science focused on the study of hybrid hydrogels that combine natural materials with inorganic components. Hybrids consist of two components that are linked on a very small scale. Typically, one of these components is of organic origin, while the other is not. The new mixed materials might have better features than the two individual parts. This opportunity could lead to the development of new materials with the unique properties required for cartilage repair.

By altering the breakdown speed and strength of materials using various chemical or physical techniques on both natural and synthetic structures, hybrids hold significant potential as an effective solution for cartilage repair. With modifications or enhancements, hybrids can adopt various physical forms, such as amorphous structures, semi-crystalline forms, hydrogen bond-linked configurations, or larger, structured assemblies [106].

Hydrogels created from particular substances can uniformly expand and contract when subjected to varying environmental conditions. This indicates that hybrid hydrogels offer numerous advantages by integrating the strengths and weaknesses of their constituent materials. For instance, we can investigate the rate of degradation and enhance biological and strength characteristics by utilizing stronger materials such as silica to restrict swelling in hydrogels in certain areas, or by combining natural polymers derived from sugars with inorganic substances like hydroxyapatite, SiO₂, or processed bone, effectively managing the quantity and types of water-attracting molecules. An additional approach might involve integrating nanotubes with varying chemical compositions to form hybrid structures. This might give these structures useful biological and mechanical traits. This concerns unique sponge-like substances composed of collagen that feature small perforations, into which single-walled carbon nanotubes have been incorporated [107]. Employing single-walled carbon nanotubes improved the ability of cells to grow and generate GAGs in their natural environment. The cells absorbed the nanotubes, facilitating the precise and targeted release of biological materials. Another research project focuses on developing a unique type of cartilage support, utilizing miniature structures modeled after natural designs. This assistance incorporates rosette nanotubes (RNTs) along with a secure substance known as non-woven poly (l-lactic acid) (PLLA) [108].

It was discovered that RNTs resemble natural collagen fibers significantly when they self-assemble in water. They enhance the synthesis of crucial compounds such as glycosaminoglycan, collagen, and proteins. The small surface characteristics and chemical composition of these features aid in the development of cells into cartilage. A different research project examined the effectiveness of 3D artificial scaffolds featuring hexagonal openings, constructed from both organic and inorganic substances, in supporting living tissue in rabbits [109]. The blend of silica and carbohydrates in various materials has demonstrated beneficial health outcomes in numerous natural silica-collagen products. Despite being developed in the past 15 years, these materials have not been applied in therapies for regenerating cartilage. Hybrid materials hold great significance as they offer the versatility to be crafted into various structures and characteristics. Based on our goal of repairing or enhancing soft tissues, such as cartilage, or hard tissues, like bone, we can develop a variety of materials, from pliable gels to firm solids. Additionally, to enhance the comprehension of these materials and their application in developing new medical devices for cartilage repair, it is crucial to investigate the interaction between the organic and inorganic components. This requires careful examination of how the polymer develops and the way the inorganic elements unite. We can modify the biological, mechanical, and degradation characteristics to guarantee that it interacts effectively with living cells, remains safe for cellular structures, and is ultimately harmless for the body.

Cytotoxicity concern

Studying ways to stimulate the growth of new cartilage is an interesting area in the fields of tissue engineering and regenerative medicine [110]. Carbohydrate DN hydrogels work well for fixing cartilage. It looks like these gels can help make new cartilage and fix old cartilage. However, some people are worried that these hydrogels might harm cells, so we need to find out before using them in medicine [111]. Carbohydrate DN hydrogels are a special kind of gel that has two layers of polymers. The top layer is hard and can easily break, while the bottom layer is bendy and can be stretched. These hydrogels can work like your joints and help create new cartilage. They can also help healthy cartilage grow [112]. However, there are reports that these hydrogels can cause harmful effects in the body, leading people to worry about whether they are safe to use for cartilage stabilization [113, 114]. Carbohydrates in DN hydrogels can hurt cells in many ways. This includes stuff used to make the hydrogel, chemicals from making it, and stuff that comes out as the hydrogel breaks down [115]. Certain constituents employed in the fabrication of these hydrogels possess the potential to adversely affect cellular and tissue integrity. Furthermore, in the event of hydrogel disintegration, there exists a risk of releasing toxic substances, exacerbating the prevailing issues. It is imperative to comprehend the mechanisms by which carbohydrate bilayer hydrogels can inflict cellular damage to enhance their safety for applications in cartilage repair [116]. The problem of carbohydrate DN hydrogels being harmful to cells must be resolved to enable their use in medicine. Reducing the amount of harmful chemicals that are produced can be done by changing how the hydrogel breaks down. Chemicals or proteins that help cells stay alive and turn into cartilage. Additionally, before the initiation of human clinical trials, it is imperative to conduct comprehensive evaluations of these hydrogels in both preclinical and clinical settings to ascertain their safety and efficacy [117, 118]. Currently, researchers are exploring the potential of sugar-based DN hydrogels for cartilage regeneration. This is due to their impressive traits that support the growth of chondrocytes. Nonetheless, it is essential to thoroughly evaluate these hydrogels to ensure their safety and efficacy for medical applications. Investigating the mechanisms of cell death and exploring strategies to mitigate this phenomenon may allow bi-carbohydrate network hydrogels to transform the techniques for repairing injured cartilage. They could potentially serve as an effective method for repairing cartilage. Incorporating various polymers along with small particles or active components into a three-dimensional configuration can improve the beneficial properties of DN hydrogels. This may reduce detrimental cellular responses and promote the desired health advantages. Numerous medical applications require adjustments in the dosage or a gradual release of the substance over time. Typically, this is accomplished using water-resistant systems, making it beneficial to manage the way it is dispensed. One potential method for regulating the timing of drug delivery could involve using DN hydrogels that decompose at varying speeds.

Advantages and disadvantages of carbohydrate-based DN hydrogels in cartilage regeneration

Polysaccharides (with the general formula Cx(H₂O)_n) are extensive natural molecules composed of numerous sugar units. Polysaccharides serve as the primary substances required for the survival and proper functioning of most organisms [119]. Polysaccharide-based DN hydrogels are often used in medicine and nanotechnology because they have important benefits compared to synthetic polymers [120, 121]. These perks consist of being beneficial to living beings, naturally breaking down, being safe to use, and allowing for simple modifications [25, 19]. Additionally, variations in the lengths of polysaccharide chains and their branching patterns lead to distinct physical and chemical characteristics. Because of these special features, hydrogels made from polysaccharides are commonly used in treating cartilage damage [122].

They support cell growth, possess resilience and strength, can transform into gels, and are safe for living organisms. Many special materials have been used in medicine to help deliver drugs and study tissue engineering. Investigations into polysaccharide polymers have revealed their effectiveness in drug delivery, enabling targeted delivery and functioning as guides for the medications [79, 123]. Polysaccharide materials are used in tissue engineering as support structures to help grow tissues and imitate the natural environment found in the body. Researchers in tissue engineering have explored methods to modify polysaccharides so that they can serve as specialized scaffolds for cells [124].

These approaches may provide some benefits at times, but they also have polysaccharide-based DN hydrogels and cannot promise long-term healing and enhanced functionality in the impacted region. As a result, it is essential to continue exploring new therapeutic alternatives. Both natural and artificial materials come with their own set of advantages and disadvantages. Natural substances can engage with cell surfaces, and a variety of them are considered safe for use in diverse scenarios [125]. Potential drawbacks might involve difficult processing, concerns about transmitting diseases, and varying rates of degradation across different hosts. Synthetic materials can be produced in bulk with relative ease and can be tailored to achieve particular strength and degradation rates. The majority of them lack the signals necessary for cells to identify one another, which is crucial for establishing connections with neighboring tissues. Common issues often involve difficulty in management and inadequate physical traits. However, there are some drawbacks to utilizing this material. The production method extracts a certain sugar from contaminated seaweed, potentially introducing harmful impurities that may lead to cell growth, damage, and death in the end product. While it’s possible to eliminate these molecules by implementing additional cleaning procedures, doing so requires considerable time and financial resources [126].

For example, one key benefit of alginate hydrogels lies in their excellent compatibility with living cells, especially those that contain a significant quantity of the G segment. This feature helps cells to be packed inside and grow within the hydrogel. Chondrocytes, the special cells in cartilage, can be surrounded by alginate gel and grown to help repair cartilage [127, 128]. The DN hydrogel helps cells grow by providing a good space for them, allowing nutrients to move, and helping produce important materials around the cells [129]. Additionally, alginate hydrogels can be injected, allowing for their application in narrow spaces to address issues in cartilage or joints. This ability to inject allows for accurate positioning and complete coverage of the injured area, helping the healing process. Alginate DN hydrogels can serve as a 3D printing material for addressing issues related to cartilage. Nevertheless, the drawbacks of alginate hydrogels limit their clinical applications. On one hand, the natural sticking ability needed for staying connected to nearby tissue is not strong. Conversely, the structure of sodium alginate lacks sufficient strength to provide robust support under pressure [130].

Challenges and prospects

Through the examples presented, we highlight the distinct cell types that contribute to the construction of cartilage tissue. It’s important to mention that among these, stem cells from bone marrow or umbilical cord are the best options for improving cartilage tissue engineering. Natural cartilage has very few blood vessels and nerves, and its cells (chondrocytes) do not develop properly. As a result, cartilage lacks the ability to heal itself properly. Bioprinting techniques have changed tissue engineering by allowing the creation of personalized artificial tissues that closely match the natural tissues in the body. It has the capability to accurately position minute structures precisely where they are required within the scaffolds, generate products swiftly and effectively, and develop highly intricate and precise designs over both time and space. Selecting the appropriate bioink material is crucial for the precise fabrication of 3D printed constructs used in orthopedic therapies. Therefore, DN hydrogels derived from natural sugars possess customizable chemical characteristics, are simple to work with, are cell-friendly, can naturally decompose, are not toxic to cells, and have a structure akin to the cartilage found in our bodies. As a result, they have gained a lot of interest in supporting the building of cartilage tissue. However, the physical qualities of these hydrogels do not match the necessary features of natural cartilage, which limits their use in medical applications. Combining natural and synthetic hydrogels allows us to leverage the robust qualities of the artificial ones while benefiting from the favorable compatibility of the natural variants. This results in the optimal hydrogel suitable for a range of medical applications. So, combining natural and synthetic materials using the latest technology is a key way to create cartilage models in the lab. Nonetheless, it’s important to acknowledge that fully returning cartilage to its original structure and function poses a significant challenge. Even though there have been great improvements in using 3D bioprinting with natural sugars for creating cartilage, actually using this technology in hospitals is still very difficult. Improving the flow and strength of natural sugar-based inks and structures is very important to solve this problem. More work is needed to make this technology better suited for use in medical settings.

Despite the progress being made, the use of DN hydrogels for cartilage repair is beginning to create problems. Polysaccharide DN hydrogels are mainly used for primary research in the medical area and need to be evaluated for potential use in the medical field. First, there was the use of the hydrogels production method, which used potentially hazardous materials such as initiators, crosslinking agents, surfactants, and other unapproved compounds. It is important for the field of cartilage repair to develop novel and more environmentally friendly methods for polysaccharide DN hydrogels production. Also, there are a lot of studies which focus only on the microstructure, biological compatibility and the biocompatibility of the DN hydrogels. There are still many important research questions unanswered. Most of the research on the DN hydrogels degradation and cartilage repair mechanisms is from in vivo studies (animal studies), and in vitro studies (lab studies), and of course, these studies may not be the direct answers for human application due to metabolic differences. Also, there is a need for research to understand the polysaccharide DN hydrogels and the influence they have on cells and tissues. To perform cell transplantation and tissue regeneration, DN hydrogels must be tested for their ability to handle fatigue and stress and what internal pressures are best for this. The development of polysaccharide DN hydrogel scaffolds is dependent on the investigation of cell migration, angiogenesis, and the body’s response to the bolts and the materials. For use in implants, DN hydrogels must be able to stick and resist repositioning in environments like cartilage. Developing so-called smart DN hydrogels from polysaccharides already in use in the field of medicine will provide it with a higher potential for clinical applicability.

Conclusion and outlook

In recent years, scientists have found that carbohydrate DN hydrogels can be a good material for helping cartilage grow back. The new hydrogels are very good for making body tissues [52, 71]. Carbohydrate DN hydrogels appear to have a bright future in helping cartilage grow back. There is a chance for big advancements in this area. It’s hard for cartilage to grow back because it doesn’t have a strong ability to heal itself [131]. This phenomenon can be attributed to the absence of vascularization and innervation. This characteristic facilitates the regeneration of cartilage when subjected to injury or excessive mechanical stress, potentially leading to conditions such as osteoarthritis. Conventional techniques aimed at stabilizing cartilage, such as microfracture procedures and the application of chondrocytes, may exhibit limited efficacy in the long term [132]. These structures are composed of two cross-linked networks, the first of which gives strength and the second of hardness. This unique structure gives DN hydrogels excellent mechanical properties, making them ideal candidates for cartilage replacement [61]. Carbohydrate DN hydrogels are good for knee cartilage stabilization because it is posited that they may exhibit advantageous interactions with biological entities. These DN hydrogels help chondrocytes, the cells that maintain cartilage, to grow and multiply. Also, the substances in the hydrogels can be changed to make them more similar to the natural cartilage matrix. This makes them even better at helping to regrow tissues [133]. The biocompatibility of carbohydrate double network hydrogels is another key factor that makes them suitable for cartilage regeneration. These gels help cartilage cells grow and multiply. Also, the ingredients of the hydrogels can be changed to resemble the natural material that surrounds cartilage. This makes them even better for helping tissues grow back [134]. These DN hydrogels are easily injected and can easily integrate with the body. This means that DN hydrogels can be placed inside the body with very little penetration, providing a gentler and better way to fix cartilage. This may speed up patients’ recovery and reduce the potential for complications for those undergoing cartilage repair surgery. There is much anticipation for the potential of DN carbohydrate hydrogels in cartilage regeneration. Moreover, finding new ways to change the chemical makeup of hydrogels could help them work better with natural cartilage and improve results for patients over time.

It is conceivable that advancements in three-dimensional (3D) printing technology will play a significant role in the fabrication of bi-carbohydrate network hydrogels for the regeneration of new cartilage. The application of 3D printing enables the production of implants that are tailored to the individual needs of each patient, closely resembling the architecture of natural cartilage. In summary, double network (DN) carbohydrate hydrogels exhibit considerable promise for facilitating the restoration of cartilage in future applications. These innovative materials demonstrate substantial mechanical strength, and it is posited that they may exhibit beneficial interactions with biological systems. Their injectability renders them a viable alternative for the stabilization of cartilage. Maintaining a concentrated effort and achieving advancements in this domain will contribute to significant enhancements in the field of tissue engineering. Such progress is instrumental in providing patients with cartilage-related ailments with improved and prolonged therapeutic options.

Abbreviation

ADA

Alginate-di-aldehyde

AF

Annulus fibrosus

ALG-POL

Alginate-poloxamer

BC

Bacterial cellulose

BDNH

Bioinspired double network hydrogel

BMP-7

Bone morphogenic protein-7

Cat

Catechol

CMCD

Carboxymethylated curdlan

CNF

Cellulose Nanofibril

CS

Chitosan

CTE

Cartilage tissue engineering

DN

Double network

ECM

Extracellular matrix

GAGs

Glycosaminoglycans

GC

Glycol chitosan

GDF-5

Growth/differentiation factor-5

GelMA

Gelatin methacryloyl

GG

Guar gum

HA

Hyaluronic Acid

HAMA

Hyaluronic methacrylic acid

HAP

Hydrophobically associated polyacrylamide

IVD

Intervertebral disc

KC

Carrageenan

KCMA

Methacrylated carrageenan

KGN

Kartogenin

MSCs

Mesenchymal stem cells

mTG

microbial transglutaminase

NP

Nucleus pulposus

PAAm

Polyacrylamide

PAMPS

Poly(2-acrylamido-2-methyl-1-propanesulfonic acid)

PEG

Polyethylene glycol

PDO

Poly(dioxanone)

PVA

Polyvinyl alcohol

qPCR

Quantitative real-time polymerase chain reaction

RGD

Arginyl-glycyl-aspartic acid

SA

Sodium Alginate

SF

Silk Fibroin

SGII

Squid cartilage type II

Si-HPMC

Silated hydroxylpropylmethyl cellulose

SiW

Silicotungstic acid

SKP

Periplasmic protein

SMSCs

Synovial mesenchymal stem cells

SN

Single network

SPS

Sodium persulfate

TFNA

Tetrahedral framework nucleic acid

XG

Xanthan gum

XLG

Laponite

Data availability

No data were used to support this study.

Compliance with ethical standards

Conflict of interest

The authors declare no competing interests.