Hyaluronic acid (HA) is a non-sulfated linear polysaccharide with outstanding biocompatibility, biodegradability, and high viscoelasticity, which has been a promising ECM material for the construction of bio-inks using the bioprinting technology (Pescosolido et al., 2011). However, its high hydrophilicity leads to the weak stability of bioprinting hydrogel constructs. To address this dilemma, many strategies have been developed by physical blending and chemical cross-linking methods with other functional moieties. Nedunchezian et al. developed a kind of HA-based bioprinting HA–biotin–streptavidin (HBS) hydrogels via chemical cross-linking modification and noncovalent bonding interactions for the cartilage repair (Nedunchezian et al., 2021). These hydrogels were further treated with sodium alginate and immersed into CaCl₂ solution to obtain a stable 3D HBSAC (HBSA + Ca²⁺) hydrogel scaffold via ion cross-linking. The physicochemical characteristics of the hybrid bio-ink had been testified with good printability and structural integrity, which was benefited for the chondrogenic differentiation both in vitro and in vivo. Hauptstein et al. prepared a HA-based bio-ink composition for cartilage regeneration (Figure 2). By means of the UV light, thiolated HA and allyl-modified poly (glycidol) were cross-linked to form the hydrogels (Hauptstein et al., 2020). Then unmodified high molecular weight of HA was further blended to form composite hydrogel constructs, which promoted the chondrogenic differentiation of human mesenchymal stromal cells with good biological and mechanical properties.

Gelatin is derived from the collage via hydrolytic degradation and is able to possess outstanding biocompatibility, cytocompatibility, degradability, low melting point, and favorable thermal stability, which has made great progress for the tissue engineering scaffolds (Li et al., 2018). As a bioprinting hydrogel construct, gelatin has the amino acid composition and sequence within its backbone, and therefore displays the unique amphoteric capacity for bio-ink application. Huang et al. prepared the gelatin/hydroxyapatite hybrid materials through the microextrusion 3D bioprinting technique and enzymatic cross-linking method to obtain the hydrogel scaffolds, which could play an important role in improving cell adhesion, proliferation, and growth to promote the chondrogenic differentiation (Huang et al., 2021). In vivo results showed this implanted cell-laden bioprinting scaffold exhibited effective articular cartilage regeneration using a pig model. On account of the precise architecture and suitable porosity by 3D printing technique, these biocompatible scaffolds demonstrated the outstanding chondrogenic differentiation behaviors and potential treatment for the cartilage injury.

Yang et al. developed a 3D-bioprinted difunctional scaffold for articular cartilage regeneration. By means of the photo-crosslinking method, the aptamer and TGF-β3 were encapsulated to form the functional GelMA bio-ink, which could recruit MSCs and promote cell chondrogenic differentiation (Yang et al., 2021a). This bio-ink was further co-printed with PCL for improving the mechanical property. Therefore, this 3D-bioprinted difunctional constructs could not only recruit the MSCs and offer the bionic microenvironment for cell growth and chondrogenesis but also provide the powerful mechanical support for neotissue maturation and cartilage repair using a rabbit model.

Methacrylate gelatin (GelMA) is prepared by the modification of gelatin with the methacrylate groups, which possess similar composition to the natural ECM to facilitate cell adhesion and growth. By means of the photo-crosslinked strategy, these biocompatible GelMA hydrogels were widely used for 3D bioprinting hydrogels with good biological and adjustable physicochemical properties (Lam et al., 2019). In recent years, these GelMA hydrogels were generally combined with other biomaterials to acquire the multifunction for satisfying tissue engineering. For example, Luo et al. demonstrated a cell-laden GelMA-based bio-ink with excellent cell activity and chondrogenesis by the photo-crosslinking method (Luo et al., 2020). By means of precise extrusion bioprinting technology, the printed GelMA hydrogel was obtained with the continuous microfibers and stabilized by the photo-crosslinking method. This bio-ink could express the chondrogenic gene by the immunofluorescence after culturing the constructs into the chondrogenic medium in vitro, and in vivo results showed that these cell-laden bio-ink possessed good printability, structural integration, and high cell viability, which could successfully promote the effects of cartilage regeneration. Lam et al. used a stereolithographic bioprinting approach to achieve a tissue-engineered cartilage bio-ink with the main components of biocompatible GelMA and methacrylated hyaluronic acid (Lam et al., 2019). These bioprinting constructs possessed good stability, chondrogenic viability, and differentiation, which could obviously support cartilage ECM formation and enhance cartilage-typical zonal segmentation for effective treatment of cartilage defects. Lee et al. prepared a hybrid bio-ink comprising GelMA and glycidyl-methacrylated HA (GMHA) for the cartilage repair (Figure 3). After optimizing the ratios, 7% GelMA and 5% GMHA bio-ink was selected with good mechanics, rheology, and printability. In vitro and in vivo results verified the important roles in offering the suitable microenvironment for stem cell chondrogenesis and cartilage tissue regeneration (Lee et al., 2020).

Cellulose comprises glucose units by the β-(1–4)-glycosidic linkages, which is one of the most available natural polymers (Ul-Islam et al., 2014; Ul-Islam et al., 2013; Yin et al., 2017). On account of its advantages on hydrophilicity, water-locking ability, biocompatibility, and biodegradability, cellulose has been greatly applied in the biomedical application. Cellulose is classified with micro-fibrillated, nano-crystalline cellulose and bacterial nanocellulose (BNC). The BNC possesses similar size of collagen fibrils that is widely used for constructing cartilage tissue engineering scaffolds. It is reported that nanocellulose-based bio-inks can be used as hydrogels for 3D bio-printing of living cells. For example, Fan et al. presented a hybrid printing platform of two hybrid hydrogel bio-inks for achieving the simple fabrication and seamless structural integration (Figure 4) (Fan et al., 2020b), wherein cellulose nanocrystals and GelMA/HA were photo-crosslinked to improve mechanical properties and cell growth environment. These hybrid bioprinting scaffolds could encapsulate the chondrogenic cell using two optimized inks with the layer-by-layer method, and the cell viability was basically unaffected during the printing procedure, which suggested its valuable applications for cartilage regeneration.

Apelgren et al. developed a cartilage-specific bio-ink via the aqueous counter collision strategy, which could prolong the fibrils and induce the negative charges (Apelgren et al., 2019). After encapsulating the human nasal chondrocytes into the scaffolds, these cell-laden constructs presented good printability, structural stability, and integrity, thus demonstrating the outstanding chondrocyte proliferation using the nude mice at 2 months. In vivo results showed that a full-thickness skin graft was integrated on these cell-laden bio-inks with potential clinical translation for cartilage repair.

Alginate or sodium alginate (raw form), as an anionic polysaccharide from seaweed source, consists of mannuronic acid and guluronic acid units (Ahn et al., 2012). With a similar structure to the native ECM, alginate has good biocompatibility, high viscosity, and facile gelation for the development of promising bioprinting constructs. However, due to its insufficient ability for cell adhesion and proliferation, alginate is acquired by blending and mixing some other compatible components or cell-adhesion moieties. Based on this, cell-laden bioprinting alginate hydrogels are designed and prepared in recent years. For example, Yu et al. developed a scaffold-free strategy on fabrication of an engineered scaffold-free alginate stretchable tissue strand as a novel robot-assisted bio-ink, which achieved the construction of an 8-cm long tissue strand with the rapid fusion property, thereby avoiding the requirement for solid supports or molds (Yu et al., 2016). Moreover, Kim et al. developed a multilayered composite scaffold through coating an alginate-based bio-ink on the PCL surface, which possessed the powerful mechanics and outstanding cell viability for tissue repair (Kim et al., 2016). Wang et al. prepared a functional alginate–gelatin bio-ink with the interpenetrating network (IPN) for cartilage repair. Embedding of alginate sulfate had a limited effect on the viscosity and shear-thinning properties, which could enable the high-fidelity bioprinting and support the MSC viability (Figure 5) (Wang et al., 2021). The rigidity of the bioprinting constructs was greatly improved compared to that of individual alginate or GelMA bio-ink. In addition, due to high affinity on the heparin-binding growth factors, it possessed a sustained release behavior of TGF-β3 and promoted the chondrogenesis both in vitro and in vivo with high level of cartilage-specific extracellular matrix deposition.

Chitosan is a linear amino-polysaccharide with the backbone of β-(1–4)-linked D-glucosamine, which possesses favorable biocompatibility, biodegradability, high viscosity, and unique antibacterial property for bio-applications (Yang et al., 2018; Li et al., 2020b; Tang et al., 2020; Yang et al., 2021b; Yang et al., 2021c). However, its poor solubility in neutral pH requires the manufacturing process in acidic condition for bio-ink preparation, which greatly limits cell viability and proliferation, especially for the direct bioprinting. To overcome this problem, various alternative strategies have been developed. For example, He et al. prepared a carboxymethyl chitosan-based 3D bioprinting bio-ink for the cartilage repair. Based on the physical crosslinking between the carboxyl groups and calcium, the mechanical properties of hydrogel scaffold were improved. Moreover, in vitro and in vivo experiments showed the favorable cell viability, attachment and proliferation, and obvious chondrogenic expression to verify the cartilage regeneration (He et al., 2020). Chen et al. synthesized a structure-supporting, self-healing, and high permeating hydrogel bio-ink, which was composed of aldehyde HA (AHA)/N-carboxymethyl chitosan (CMC) and gelatin (Gel)/4-arm poly (ethylene glycol) succinimidyl glutarate (PEG-SG) (Chen et al., 2021). The obtained hydrogel bio-inks possessed good printability, self-healing capacity, and high permeability, thus achieving long-term cell viability, attachment, and growth for biological application (Figure 6).

Silk is a natural protein polymer found in the glands of arthropods, and silk fibers were extracted from silk with the main components of silk fibroin and sericin proteins, which has vastly progressed in tissue engineering and drug delivery applications (Lamboni et al., 2015; Gregory et al., 2016). However, this progress has been challenged by the difficulty of developing bioprinting materials, because most bio-inks require toxic chemical cross-linking. To overcome this problem, Singh et al. developed silk fibroin blends with gelatin through the polymeric chain entanglements and physical cross-linking interactions, which were printed as bio-inks with suitable swelling behavior, optimal rheology, and supportive structure (Figure 7) (Singh et al., 2019). By increasing the content of sulfated glycosaminoglycan and collagen, this bioprinting construct allowed the chondrocytes growth and proliferation to generate the cartilage extracellular matrix and upregulate the chondrogenic gene expression, which indicated this cross-linker–free silk-gelatin may be explored as potential candidates in cartilage regeneration.

Ni et al. used the silk fibroin (SF) and hydroxypropyl methyl cellulose (HPMC) to construct a novel cell-laden double network hydrogel through 3D bioprinting technology for cartilage repair (Ni et al., 2020). SF possessed a high β-sheep structure as the first rigid and brittle network and HPMC was acted as the second soft network; in this case, this hydrogel exhibited high strength and remarkable compressive reproducibility, which could support the cell growth and proliferation. After encapsulation of bone marrow mesenchymal stem cell (BMSC), this SF-based bio-inks also expressed the outstanding biochemical property for cartilage tissue regeneration.

Fibrin is a fibrous protein and component of native ECM formed by the interaction between protease thrombin and fibrinogen, which can combine with the platelets from the hemostatic clots for blood coagulation and has been widely used for cartilage engineering tissues due to its biocompatibility, biodegradability, and nanofibrous structural properties. The presence of cell adhesion motif and cell compatibility allows fibrin to benefit in biological printing applications. However, fibrin soft and fragile characteristics lead to the poor structural integrity and cannot maintain shape fidelity for 3D biofabrication and bioprinting constructs. Therefore, it is committed to overcoming this problem by combining fibrin with various polymers. For example, Ning et al. prepared a Schwann cell-laden scaffold using the bioprinting technique with the main components of RGD modified alginate, hyaluronic acid, and fibrin, which exhibited outstanding machinability and biological properties for the tissue regeneration (Ning et al., 2019). Honarvar et al. prepared a kind of polycaprolactone (PCL) scaffolds by coating with fibrin and cartilage acellular ECM using 3D-printing methods. After encapsulating the adipose-derived stem cells (ADSCs) into fibrin/ECM hydrogel and seeding onto the PCL scaffolds, this 3D-printed constructs possessed higher compressive properties and water uptake for benefiting the cell growth, which indicated the important roles in cartilage tissue engineering (Honarvar et al., 2020).

In addition, soy protein is an important natural polymer from shelled and degreased soybeans, which accounts for 6% of the total polymer distribution used in bioprinting applications. The structure and physiological characteristics of soybean protein can be adjusted by various processing. Its improved thermoplastic properties and high biocompatibility allow it to be used for the development of porous scaffolds and tissue engineering applications (Manan et al., 2017).

PEG was a promising material for bioprinting hydrogel due to its good biocompatibility, high solubility, and easy modification. In general, poly (ethylene glycol) diacrylate (PEGDA) was applied for construction of hydrogel bio-ink through UV-induced polymerization. Daniele et al. prepared an interpenetrating network with the main component of GelMA and PEG, which could satisfy the requirement of cell attachment and growth within this composite bioprinting constructs (Daniele et al., 2014). Mouser et al. designed a bioprinting construct with suitable biological and mechanical properties, which consisted of methacrylated hyaluronic acid (HAMA) and methacrylated poly [N-(2-hydroxypropyl) methacrylamide mono/dilactate] (pHPMAlac)/PEG triblock copolymers (Mouser et al., 2017). After co-printing with PCL and encapsulation of chondrocytes, the cell-laden bio-ink displayed the tunable internal architectures, improved the mechanical properties, and promoted the glycosaminoglycan (GAG) and collagen type II contents, which were found to be optimal for cartilage-like tissue formation. Zhou et al. developed 3D bioprinting scaffolds with the components of graphene oxide (GO)-GelMA and PEGDA, which could promote chondrogenic differentiation with the improving glycosaminoglycan and collagen levels (Figure 8) (Zhou et al., 2017).

PLGA which consisted of a copolymer of lactide and glycolide had received significant attentions for bio-ink application due to its adjustable structures, good biocompatibility, and biodegradability (Wang et al., 2022). Gottardi et al. prepared a biphasic bioprinting PLGA scaffold with embedding adult human mesenchymal stem cells for chondrogenic regeneration (Gottardi et al., 2021). The in vitro assay showed that constructs cultured in a two-fluid bioreactor differentiated locally into tendinocytes or chondrocytes according to exposure to the appropriate differentiation medium after culturing for 7, 14, and 21 days. The cartilage matrix marker, collage II, and tendon-specific marker were significantly unregulated in the cartilaginous and tendinous portions at 21 days, indicating the favorable dual fluidic system for fabrication of the tendon enthesis-like biphasic constructs.

PCL was a kind of biodegradable synthetic polyester for wide applications in tissue regeneration because of its outstanding thermoplastic behavior, high mechanics, suitable biodegradability, and easy processability (Zhou et al., 2020). Koo et al. developed a 3D bioprinting cell-laden collagen and hybrid constructs using PCL plates for articular cartilage tissue regeneration, which possessed optimal porous mesh structures and high mechanical properties to support the chondrocyte viability within the bio-inks in vivo (Koo et al., 2018). Visser et al. printed a PCL-based hydrogel scaffold with a highly porous microfiber through an electro-spun technology. This construct with good stiffness and elasticity could support the chondrocyte viability and growth (Visser et al., 2013). Kim et al. developed a multilayered PCL/alginate bio-ink constructs using the cell-printing strategy (Kim et al., 2016). The laden cells could display a substantially more developed cytoskeleton with a homogenous distribution within the constructs, and these cell-laden bio-inks exhibited the outstanding osteogenic activity.

PLA was an aliphatic polymer with high mechanics, suitable processability, and rheological properties for 3D bioprinting of high-resolution scaffolds. Antich et al. developed a versatile bio-ink comprising hyaluronic acid (HA) and PCL components via the 3D bioprinting technology for the cartilage repair (Figure 9) (Antich et al., 2020). This bio-ink possessed good printability, biocompatibility, and biodegradability to allow the cell viability and improve the expression of chondrogenic gene markers and specific matrix deposition, thus demonstrating excellent cartilage tissue formation. Narayanan et al. developed a fibrous hydrogel bio-ink consisting of alginate, PLA, and human adipose-derived stem cells (hASCs) (Narayanan et al., 2016). In vitro results showed the outstanding hASC viability and proliferation over 8 weeks, and in vivo experiments displayed the higher levels of cell proliferation, collagen, and proteoglycans of bioprinted meniscus for promoting chondrogenic differentiation and cartilage repair.