The meniscus is a pair of crescent, wedge-shaped fibrocartilaginous pads located between the femoral condyle and tibial plateau that serve a variety of functions, such as distributing loads, absorbing shock, maintaining stability, and contributing to cartilage lubrication and nutrition (Makris et al., 2011; Rongen et al., 2014). From a macroscopic point of view, the medial and lateral menisci possess their own anatomical variations, but the anterior horns of both are connected by the anterior intermeniscal ligament (Rongen et al., 2014; Figure 1A). Since vascularization of the meniscus decreases as the meniscus matures, the limited healing capacity of the inner zone of the meniscus is directly related to the poor blood supply, and nutrients can only be received from passive synovial fluid diffusion (Arnoczky and Warren, 1982; Petersen and Tillmann, 1995; Makris et al., 2011). Microscopically, it is reasonable to distinguish the peripheral red zone from the inner avascular white zone (Murphy et al., 2019). The inner zone is characterized by chondrocyte-like cells embedded in collagen type II and glycosaminoglycans (GAGs). In contrast to the inner zone, the peripheral zone presents abundant collagen type I deposition and many more elongated fibroblast-like cells (Makris et al., 2011; Jacob et al., 2020). In addition, the cells within the superficial zone are postulated to produce and secrete lubricant and anti-adhesive proteins or act as progenitor cells with regenerative potential (Lee et al., 2008; Jacob et al., 2020; Figure 1B).

Generally, the meniscus plays a critical role in maintaining normal knee joint mechanics and functions. A number of studies have been performed to measure the mechanical strength of meniscal tissue in humans (Table 1). The specific mechanical properties of the meniscus are mainly determined by highly spatially oriented collagen fibers (Rongen et al., 2014). Indeed, the most important role of the collagen-proteoglycan meniscal matrix is its capacity to provide mechanical support, such as resistance to tension, compression and shear stress (Fithian et al., 1990). Specifically, the meniscus transfers 50–90% of the joint reaction forces under weight-bearing conditions (Walker and Erkman, 1975; Ahmed and Burke, 1983), with load transfer and absorption occurring via well-characterized mechanisms. In general, circumferential stresses within meniscal tissue are generated after joint surface contact, transferring compressive loads into horizontal tensile stress. Excessive energy being absorbed by collagen can also be released via the expulsion of synovial fluid (Storm et al., 2005; Rongen et al., 2014). Other important functions of the meniscus include providing lubrication, supplying nutrients to the cartilage and supporting proprioception (Jacob et al., 2020).

In addition to trauma, other risk factors affect meniscal tissue, such as genetic susceptibility, obesity and knee malalignment (Englund et al., 2012). Twisting or shearing motions with a varus or valgus force account for the mechanism of most meniscal tears (Pihl et al., 2017). For younger patients, acute traumatic injury is a major cause of meniscal tears, and as in elderly patients, degenerative meniscal tears might act as key factors in the development of knee OA (Englund et al., 2007, 2012). The healing capacity of the meniscus after injury is basically dictated by the tear pattern and location. For instance, horizontal and radial tears involving the inner zone are thought to have the least healing potential owing to incursion into the avascular inner zone (Kwon et al., 2019). Unfortunately, meniscal injury is often followed by knee OA, which is known as the “meniscal pathway.” Briefly, loss of meniscal mechanical support leads to dramatically increased structural stress on articular cartilage, causing loss of cartilage, subchondral bone changes, and bone marrow lesions (Englund et al., 2012). In addition, the subsequent increased proinflammatory state within the knee joint after meniscal tears contributes to the progression of OA (Bigoni et al., 2017).

Conventional therapies for meniscal tears include both non-surgical and surgical approaches (Li et al., 2020a). Arthroscopic meniscectomy is the most commonly used surgical procedure for meniscal injuries (Katz and Martin, 2009; Kim et al., 2011b). However, it inevitably results in progressive cartilage degeneration and OA, and the curative effect on degenerative meniscal tears remains a matter of debate (Fairbank, 1948; Roos et al., 1998; Monk et al., 2017). Meniscal allograft transplantation (MAT) may further restore knee function, but this advantage is countered by the disadvantages of the insufficient number of donors and risk of disease transmission, immune rejection and non-matching (Noyes and Barber-Westin, 2010; Noyes and Barber-Westin, 2016; Parkinson et al., 2016). Chondroprotective evidence also needs to be validated (Vrancken et al., 2013). In search of a clinical solution for meniscal injury and joint homeostasis restoration, tissue engineering and scaffold-based regenerative medicine strategies have become some of the most promising approaches (Bandyopadhyay and Mandal, 2019). In this context, this review focuses on details of the polymeric aspects of meniscal therapy and advanced, novel polymeric scaffold-based strategies for meniscal repair and regeneration.

Tissue engineering techniques often involve the application of scaffolds, cells and biochemical and biomechanical stimuli to create engineered tissues (Kwon et al., 2019). These three main components collectively form many combinations and have obtained some promising advances. Therefore, to better understand the interaction among these three main components, cells and physical and biochemical signals all need to be introduced. Cells are important players in meniscal tissue engineering. Stem/progenitor/multipotent cell sources in meniscal tissue engineering can be obtained from various tissues, including bone marrow, synovium, and adipose tissue (Bilgen et al., 2018). Another large family of cells originates from mature connective tissue, such as the meniscus and cartilage (Peretti et al., 2004; Zellner et al., 2017). Biochemical stimuli also play an important role in engineering meniscal scaffolds. To increase extracellular matrix (ECM) production in engineered meniscal tissue, the administration of biochemical stimuli, such as growth factors, has long been used. A variety of growth factors, including platelet-derived growth factor (PDGF), bone morphogenetic protein-2 (BMP-2), transforming growth factor-β (TGF-β), insulin-like growth factor-1 (IGF-1), and fibroblast growth factor (FGF), have shown efficacy in improving meniscal regeneration (Bhargava et al., 1999; Pangborn and Athanasiou, 2005; Gunja and Athanasiou, 2010; Puetzer et al., 2013). Changes in oxygen tension have also yielded mixed effects on engineered meniscal tissue, which showed improved ACAN and COL2A1 expression under hypoxic conditions (Liang et al., 2017). On the other hand, the development of biomechanical stimuli for meniscal tissue engineering has focused on replicating heterogeneity and matrix-level arrangement of the tissue (Kwon et al., 2019). For example, compression and hydrostatic pressure have been used to improve the functional properties of meniscal neotissue (MacBarb et al., 2013; Zellner et al., 2015; Puetzer and Bonassar, 2016).

Generally, scaffold design is of pivotal importance to accelerate meniscal tissue repair and regeneration. Polymer selection and biophysical and biochemical properties all need to be taken into consideration when designing an optimal tissue-engineered meniscal scaffold.

It is well known that biocompatibility and bioactivity are major considerations that may lead to scar tissue formation if not achieved (Murphy et al., 2018). In meniscal tissue engineering, a microenvironment conducive to cell adhesion, cell proliferation and matrix synthesis is necessary (Tan and Cooper-White, 2011). Natural ECM is a sophisticated 3D network that can support cells and control cellular responses, such as migration, proliferation and differentiation, via autocrine and paracrine mechanisms (Murphy et al., 2018; Subbiah and Guldberg, 2019). Therefore, compositionally, the scaffold needs to create an ECM-mimicking microenvironment with biocompatibility and minor immune rejection and degrade into harmless products along with meniscal tissue growth. The biophysical properties of natural ECM can also modulate cell behaviors (Elliott et al., 2019; Gaharwar et al., 2020). In regard to the architecture, the anisotropic orientation as well as suitable pore size and porosity are required to provide an optimal structure for cell ingrowth. In addition, scaffolds are also required to have appropriate mechanical properties, which enable the scaffold to preserve the normal contact biomechanics of the knee. In summary, biomaterial design and fabrication should mimic the biomechanics and components of natural ECM in meniscal regeneration (Ma, 2008; Zhou and Lee, 2011). Furthermore, the processing techniques should be convenient and versatile enough for clinically customized application. The specific design criteria of meniscal scaffolds are summarized in Table 2. To apply polymeric scaffold-based regenerative strategies in the context of the meniscus, clarification of the key role of scaffolds in meniscal tissue remodeling and maturation is needed. Collectively, a polymeric scaffold should not only provide a supportive microenvironment but also favor the migration, proliferation and differentiation of meniscogenic cells (Figure 2). In this review, we will summarize recent developments in polymeric scaffolds in terms of compositions, structures, processing technologies and bioactivities.

Natural polymers, such as polysaccharides and proteins, are considered to have great potential for meniscal tissue engineering due to their excellent biocompatibility, processability and bioactivity (Murphy et al., 2018). These polymers are also characterized by some drawbacks, including limited tunability, uncontrollable degradation, undesirable immunogenicity and poor mechanical properties, and are thus susceptible to failure in meniscal repair and regeneration (Chocholata et al., 2019; Li et al., 2020a; Table 3).

Owing to their poor mechanical strength, unstable degradation and limited sources, natural polymers are still insufficient for meniscal repair and regeneration. Therefore, synthetic polymers with favorable mechanical properties, reproducibility and controllable degradation have been widely used to produce meniscal scaffolds (Makris et al., 2011; da Silva Morais et al., 2020; Table 3). However, one of the limitations of synthetic scaffolds is their paucity of bioactive cues, which could be overcome by adding biological coatings (Silva et al., 2020).

As a biodegradable polymer with excellent biocompatibility and mechanical strength, poly(glycolic acid) (PGA) has been widely used in the biomechanical and medical fields since the 1970s and can serve as a scaffolding material to repair articular cartilage in clinical practice (Siclari et al., 2018; Otsuki et al., 2019; Cojocaru et al., 2020). In meniscal tissue engineering, a 3D, meniscal-like, PGA-hyaluronan implant with high porosity was developed, and this scaffold showed high biocompatibility and improved the expression of chondrogenic genes during coculture with human meniscal cells in vitro. In addition, in a sheep model, the scaffold showed greater proteoglycan and collagen type I production than the control group (Cojocaru et al., 2020). For in vivo evaluation, a meniscus-shaped scaffold made of PGA covered with a polylactic acid/caprolactone [P(LA/CL)] sponge was implanted into the right knee in a medial meniscus resection minipig model. The results showed that the scaffold provided appropriate initial strength and could prevent cartilage degeneration with relatively low inflammation. However, the compressive stress and elastic modulus of the scaffold were significantly inferior to those of the native meniscus (Otsuki et al., 2019).

Poly(lactic acid) (PLA) is a thermoplastic aliphatic polyester obtained from the polymerization of lactic acid and/or the ring-opening polymerization of lactide, which has suitable biocompatibility and biodegradability but also some drawbacks, such as a high cost, long degradation time, low utility and limited molecular weight (Castro-Aguirre et al., 2016; Murariu and Dubois, 2016). In an interesting study, core-shell coaxial nanofibrous scaffolds were prepared by electrospinning. The PLA core provided mechanical strength, while the collagen shell facilitated cellular adhesion and matrix synthesis. In vivo experiments showed excellent integration between the scaffold and native tissue (Baek et al., 2019). The filamentous PLA structure was printed by 3D printing and modified by surface modification with active functional groups. Then, IPN hydrogels populated with hMSCs were applied to the surface-modified PLA structure for in vitro and in vivo research. At 28 days after implantation in a rat model, the structure of PLA remained relatively intact, which was consistent with the degradation curve in vitro. The integrity of the PLA scaffold ensured minimal mechanical stress on the hMSCs, allowing optimal function to be achieved (Gupta et al., 2020a).

Compared with that of PLA, the degradation time of poly(lactic-co-glycolic acid) (PLGA) can be controlled according to the glycolic acid content, while the higher the ethyl ester ratio is, the easier the polymer is to degrade (Zhou et al., 2012). PLGA is a linear copolymer composed of different proportions of glycolic acid and lactic acid monomers (Gentile et al., 2014). PLGA is an attractive polymer used in drug delivery and tissue engineering due to its favorable biodegradability, flexible processability, tunable degradation, surface functionalization and targeted drug delivery (Danhier et al., 2012; Mir et al., 2017; Zhao et al., 2021). PLGA is biodegradable because its ester linkages can be hydrolyzed in aqueous solution, and as byproducts, glycolic acid and lactic acid can be cleared from the body via normal metabolic pathways (Lü et al., 2009). Gu et al. (2012) used cartilage-derived morphogenetic protein-2 (CDMP-2) and TGF-β1 to preculture autologous myoblasts and then construct myoblast cell-seeded PLGA scaffolds, which presented accelerated healing after meniscal defect implantation. Regarding functionalized PLGA scaffolds, Kwak et al. (2017) reported a PRP-pretreated PLGA mesh scaffold seeded with human ACs to regenerate meniscal tissue. At 6 weeks after subcutaneous implantation, increased cell attachment and cartilaginous tissue formation were observed in the cell-seeded scaffold between native devitalized meniscal discs (Kwak et al., 2017). Other researchers have developed several PLGA-based approaches as drug delivery platforms. For instance, TGF-β3 and connective tissue growth factor (CTGF) loaded in PLGA microparticles were incorporated into 3D-printed PCL anatomical meniscal scaffolds and demonstrated promising reparative results (Lee et al., 2014; Figure 3).

PCL is a hydrophobic polyester with a low melting point (56–61°C), slow degradation, favorable compatibility and satisfactory mechanical strength (Abedalwafa et al., 2013; Teng et al., 2014). However, its hydrophobicity and inadequate wettability may lead to poor cell attachment and proliferation (Mondal et al., 2016; Silva et al., 2020). For that reason, the surface modification of PCL is crucial for its biological application (Mondal et al., 2016). In one study, galactose was incorporated into electrospun PCL nanofibrous scaffolds for meniscal cell culture, whereas the composite scaffold resulted in increased cell attachment and proliferation (Gopinathan et al., 2015). In addition, PCL has been used to prepare nanofiber scaffolds with a high effective surface area-to-volume ratio to facilitate the release of biomolecules and ensure greater interaction between seeded cells and biomolecules (Qu et al., 2019). For example, Qu et al. (2019) developed an aligned protein-containing scaffold based on electrospun PCL-PLGA fibers using bovine serum albumin (BSA) to stabilize the loaded TGF-β3. Compared with the high-dose TGF-β3-loaded scaffold, the low-dose TGF-β3-loaded nanofiber scaffold effectively activated the fibrochondrogenic differentiation of synovium-derived stem cells (SDSCs). The results indicated that fibrochondrogenesis and chondrogenesis differed by growth factor concentration, with the former requiring a lower dose (Qu et al., 2019).

Polyurethane (PU) possesses elasticity, thermoplasticity and excellent biocompatibility and has already been applied in meniscal tissue engineering (Venkatesan and Kim, 2014; Koch et al., 2018; Vedicherla et al., 2018; Bharadwaz and Jayasuriya, 2020). For example, Actifit^® implants have been studied as a cellular component delivery vehicle. Vedicherla et al. (2018) developed a cell-seeded PU scaffold. Fresh chondrocytes (FCs) and minced cartilage (MC) were cultured on the scaffold, and the tissue integration effect was evaluated in a caprine meniscal explant model. The results exhibited better matrix deposition and tissue integration in both the FC and MC groups than in the acellular scaffold group (Vedicherla et al., 2018). MSCs are also promising for meniscal repair due to their potential for fibrochondrogenesis and their ability to secrete reparative growth factors (Koch et al., 2018). An MSC-loaded PU scaffold was produced as a replacement for large, full-thickness meniscal defects. At 12 weeks after surgery, the vessel density in the scaffold group was superior to that in the cell-free groups. Additionally, significantly greater proteoglycan deposition and integration with the surrounding meniscal tissue were observed in the MSC-loaded group than in the acellular group. However, this advantage of MSC loading disappeared after 12 weeks (Koch et al., 2018). It has also been reported that MSC-seeded PU scaffolds exhibit little additional clinical benefit in the protection of articular cartilage (Olivos-Meza et al., 2019).

Another synthetic polymer that should be addressed is polycarbonate urethane (PCU). PCU is a flexible, biocompatible, biostable and wear-resistant material that can be incorporated in 3D-printed, porous structural scaffolds (Williams et al., 2015; Abar et al., 2020). In addition, as a hydrophilic material, PCU can mimic the lubrication mechanism in native synovial joints (Wan et al., 2020). A medial meniscus PCU prosthesis, called NUsurface^® (Active Implants Corp., Memphis, TN, United States), has been undergoing clinical trials and has become available on the market (Drobnič et al., 2019). Another novel meniscus-shaped, wear-resistant full implant made of PCU was also developed. The study showed that the posterior horn of the implant was under maximum pressure at 3 months, and the deformation at 12 months after implantation was acceptable. However, one implant failed due to a complete tear during posterior angular extension. Therefore, it is essential to strengthen the posterior horn of the implant to prevent fixation failure of one horn under extension. The damage progression in the implant group was similar to that in the allograft group but significantly worse than that in the non-operated group (Vrancken et al., 2017).

PVA is a polymer synthesized from partially or completely hydroxylated polyvinyl acetate (Baker et al., 2012). As a bioinert, non-carcinogenic, moist, biocompatible composite material (Hayes and Kennedy, 2016; Marrella et al., 2018), PVA possesses good formability, mechanical properties and manufacturability (Kobayashi et al., 2005; Moradi et al., 2017a; Marrella et al., 2018) and has been widely used in the field of regenerative tissue engineering. Polyvinyl alcohol hydrogel (PVA-H) has viscoelastic properties similar to those of cartilage and meniscal cartilage and does not wear out even after millions of compression cycles (Moradi et al., 2017a). As early as 2005, Kobayashi et al. (2005) developed an artificial meniscus with high-water-content (90%) PVA-H and conducted a preliminary study in a rabbit model. This study showed that the articular cartilage of the knee was still in good condition 2 years after the operation (Kobayashi et al., 2005). As a physically crosslinked gel, PVA-H does not contain toxic monomers that may be present after chemical crosslinking. However, its poor tensile properties limit its practical use, especially in strong fibrous tissues, such as the meniscus, tendons and ligaments (Holloway et al., 2010, 2014). Therefore, it is particularly important to modify PVA-H with new materials. A 3D biomimetic meniscal scaffold was designed using 3:1 SF/PVA. Autoclaved eggshell membrane (AESM) powder (1–3% w/v) was used as a biomechanical enhancer, and the composite scaffold presented with good load-bearing performance and improved meniscal tissue regeneration (Pillai et al., 2018).

In addition, various copolymers based on poly(ethylene oxide) (PEO) have been developed and applied in the field of drug delivery due to their good biocompatibility and fast, non-toxic degradation (Kim et al., 2014). PEO can be used as a sacrificial, water-soluble scaffold material to increase porosity and promote cell infiltration (Baker et al., 2008). As Qu’s group reported, collagenase-loaded PEO electrospun fibers may trigger matrix degradation. When applied in meniscal tears in vitro, PEO degradation was accompanied by the release of enzymes in the local wound edge and successfully increased both tissue porosity and cell migration (Qu et al., 2013).

The decellularization method has historically been used to isolate the ECM via cell removal and is widely applied in tissue regeneration (Gilbert et al., 2006). Due to their meniscus-specific chemical composition and architecture, implants derived from decellularized materials have been used as scaffolds in meniscal tissue engineering (Murphy et al., 2018; Ruprecht et al., 2019). Recently, decellularized materials have been explored for use in decellularized meniscal scaffolds (DMSs) to support the regeneration of meniscal tissue. For instance, Stabile et al. (2010) developed fresh-frozen meniscus allografts with increased porosity and promising mechanical integrity that presented potential for clinical application. DMECM presents components similar to those of the native meniscus and can be formed into various structures, thereby promoting cell infiltration and remodeling (Chen M. et al., 2019; Ruprecht et al., 2019). Researchers have demonstrated that meniscus-derived matrix scaffolds are capable of promoting the infiltration of endogenous meniscal cells and MSCs (Ruprecht et al., 2019). Although numerous studies have proven that decellularized matrix (DCM) may be able to regulate stem cell differentiation, Liang et al. (2018) demonstrated that synovial fluid-derived mesenchymal stem cell (SF-MSC)-loaded meniscus-derived DCM was incapable of inducing the differentiation of SF-MSCs into MFCs without TGF-β3 and IGF-1 supplementation. However, DCM materials suffer from poor performance in load-bearing applications. To tackle this problem, our team previously developed a hybrid scaffold for regenerating the meniscus in a rabbit model, combining acellular meniscus extracellular matrix (AMECM) and demineralized cancellous bone (DCB). The AMECM/DCB constructs demonstrated favorable mechanical properties and a promising capacity to promote fibrochondrocyte proliferation and GAG secretion (Yuan et al., 2016). Another common solution to improve the stiffness of implants is to hybridize them with other synthetic polymers. For example, Guo and coworkers used DMECM and a 3D-printed PCL scaffold to create a hybrid construct for rabbit meniscal regeneration. The hybrid scaffold displayed biomechanical strength similar to that of the native meniscus and facilitated whole meniscal regeneration in both rabbit and sheep meniscus repair models (Guo et al., 2021). Additionally, as another tissue engineering approach, DMECM-based injectable hydrogels have been developed. One early work by Yuan et al. (2017) developed hMSC-loaded DMECM hydrogels and found that the cell-laden DMECM hydrogel successfully retained the viability of hMSCs in nude rat meniscal injury for 8 weeks, resulting in neomeniscal tissue formation and preventing joint space narrowing, pathological mineralization and OA development.

Recently, researchers have fabricated numerous hybrid scaffolds made from two or more types of polymeric materials. While a single natural or synthetic polymer can only provide limited advantages for tissue-engineered meniscal scaffolds, the final product produced by a mixture of natural and synthetic polymers tends to possess comprehensive advantages that no single polymer can provide (Bakhshandeh et al., 2017). For example, natural polymers, such as chitosan, collagen and gelatin, usually contain biological molecules that can interact with cells, providing superior biological performance for hybrid polymeric scaffolds (Donnaloja et al., 2020), while synthetic polymers provide tunable physical properties such as mechanical support and a controllable degradation rate (Zhou et al., 2012; Figure 4).

Loading hybrid scaffolds with tissue-derived cells has the advantage of the encapuslated cells replenishing ECM loss and filling in defects as scaffold degradation occurs over time (Bilgen et al., 2018). For example, Chen M. et al. (2019) constructed a wedge-like, 3D-printed, MFC-loaded hybrid scaffold with a PCL scaffold as a backbone and then injected an optimized MECM-based Ca-alginate hydrogel (2%). In vivo experiments confirmed that the PCL-hydrogel-MFC group was similar to the sham group in terms of biochemical content, histological structures and biomechanical properties, which demonstrated an ideal capability for regeneration of the whole meniscus (Chen M. et al., 2019). In another study, by Cengiz et al. (2020), to meet the requirements of biomimetic meniscal scaffolds and cell coculture, PCL was blended with SF and entrapped in a 3D-printed cage (EiC) scaffold. Human stem cells or meniscocytes were cultured on the EiC scaffold and implanted subcutaneously in nude mice. The SF-based EiC scaffold showed better cell infiltration as well as a milder inflammatory response (Cengiz et al., 2020).

On the other hand, hybrid scaffolds combining polymers and biofactors can locally deliver signal molecules to create a favorable microenvironment. For instance, a PCL/SF hybrid scaffold based on 3D printing was developed by Li et al. (2020d) and exhibited a balance between the mechanical properties and degradation rate. The SF sponge provided cartilage protection due to its high elasticity and low interfacial shear force, the PCL provided excellent mechanical support, and the conjugation of a peptide with SMSC-specific affinity (LTHPRWP; L7) increased cellular recruitment and retention. In vivo experiments showed that this meticulously tailored scaffold greatly enhanced meniscal regeneration while protecting cartilage (Li et al., 2020d). In summary, hybrid scaffolds have been used for meniscal tissue engineering. The general design strategy consists of utilizing synthetic polymers as a supporting framework, with natural polymers more likely serving as an additive microenvironment to mimic extracellular microenvironments, while cells and bioactive factors may further assist in improving cell recruitment, proliferation, and differentiation and ultimately improve regeneration.

Promising advances have been made in technology for the fabrication of meniscal scaffolds in terms of particulate leaching, freeze drying, solvent casting, electrospinning, and 3D printing (Esposito et al., 2013; Sun et al., 2017; Li et al., 2020e). Table 4 is a summary of the most relevant examples of meniscal scaffold fabrication techniques. Currently, in the fabrication of sponge scaffolds using physical and/or chemical treatments, the porous scaffold can provide a desirable microenvironment for the cells culturing, however, the pore size, porosity and interconnectivity cannot be controlled, and these scaffolds are often hindered by pore blockage effects in vivo (Sun et al., 2017). Another alternative manufacturing technology, electrospinning technology can be used to produce collagen-mimicking nanoscale fibers but is also limited in terms of controlling the structure and porosity of the construct (Megelski et al., 2002; Sun et al., 2017).

As an emerging additive manufacturing technology, 3D printing is able to produce components with ideal shapes and structures, allowing the accurate construction of specified 3D hierarchical structures (Bahcecioglu et al., 2019; Fu et al., 2020). Hence, among all of these scaffolding technologies, 3D printing is more promising in meniscal tissue engineering. Indeed, 3D bioprinting is a branch of the development of 3D printing technology and represents the technology’s entry into the field of tissue engineering and regenerative medicine (Matai et al., 2020). 3D bioprinting is an automated, tissue-friendly manufacturing method that very accurately simulates the actual arrangement of the cells and ECM of the targeted tissue, with the ability to construct the 3D tissue equivalent of the desired shape and structure, reproducing the complexity of human tissue (Murphy et al., 2018; Chae et al., 2021). Generally, bioprinting technologies mainly applied in meniscus/cartilage tissue engineering can be classified as follows: 3D plotting/direct ink writing, stereolithography (SLA), selective laser sintering (SLS), fused deposition modeling (FDM), and extrusion-based bioprinting (Moroni et al., 2018). Table 5 presents several commonly used bioprinting techniques and provides some comparative information. At present, there are many studies using FDM to produce a PCL framework and then combining hydrogels to fabricate composite meniscal scaffolds. However, FDM techniques suffer from poor surface quality and difficulties in combining biopolymers owing to their high extrusion temperatures (Agarwal et al., 2021). The SLA technique is characterized by high resolution; however, a limited selection of photopolymers restricts its application in tissue engineering, and it has not yet been applied in meniscal tissue engineering (Qu et al., 2021). Tissue-engineered meniscal scaffolds produced by extrusion bioprinting techniques have high yields and excellent structural integrity, and this technique is the most common 3D bioprinting technology in the field of meniscal regeneration.

From a polymeric materials science point of view, numerous polymers have been extensively investigated to serve as bioinks in 3D printing for tissue engineering. Bioinks, mainly hydrogels that contain cells and various biological components, play an important role in creating a compatible microenvironment for cellular activity (Roseti et al., 2018; Samadian et al., 2020). Some single-component polymer bioinks, such as SF (Bandyopadhyay and Mandal, 2019), alginate (Narayanan et al., 2018), and GelMA (Grogan et al., 2013) bioinks, have been used for 3D meniscal bioprinting. However, a single polymer cannot reproduce the complexity of ECM in natural tissue and thereby provide a good microenvironment for cells in vivo. Researchers have made immense efforts to develop an optimal bioink that simultaneously meets the requirements of biocompatibility and printability. Recently, tissue-specific meniscal dECM (me-dECM) bioinks based on 3D cell printing were designed by Chae et al. (2021) and helped preserve the complexity of the natural ECM, thus demonstrating the potential for tissue regeneration and special biological functions. This printable bioink supported the proliferation and differentiation of encapsulated stem cells, and induced fibrochondrocytes were also observed in vitro (Chae et al., 2021). In addition to combining hydrogels and solid polymers to mimic the biphasic structure of the meniscus and produce an optimal cellular microenvironment, 3D bioprinting has also been used to incorporate growth factors and cells in hydrogel-based scaffolds (Caterson et al., 2001; Sun et al., 2017). In a pilot in vivo study performed by Sun et al. (2020), a ready-to-implant anisotropic meniscal scaffold fabricated by 3D bioprinting was implanted in a goat meniscus transplantation model. This 3D-bioprinted meniscus substitute contained microspheres encapsulating CTGF and TGF-β3 and was mixed with BMSCs. The in vitro and in vivo results demonstrated cell phenotypes and matrix formation resembling those of the native meniscus, as well as chondroprotection of the goat articular cartilage (Sun et al., 2020).

In addition to the implications of the aforementioned traditional bioprinting techniques for the engineering of meniscus substitutes, some advanced techniques have also generated enthusiasm about their potential applications in meniscal tissue engineering. For example, the inkjet technique, which is suitable for printing cellularized scaffolds, can be used to print customized cellularized menisci. The process of inkjet printing usually consists of two parts: droplet formation on the target substrate and droplet interaction with related materials (Li et al., 2020c). Recently, in a proof-of-concept study, a cellularized human meniscus was produced by a microvalve-based inkjet technique. Primary human bone marrow mesenchymal stem cells were isolated, embedded with collagen bioink and customized inkjet printing, resulting in a patient-specific meniscal scaffold with superior anatomical structure and favorable cell compatibility (Filardo et al., 2019). In addition, some new bioprinting technologies based on non-contact (acoustoelectric, optical and magnetic) guidance of cell chemotaxis are gradually coming into play. Traditional scaffold technology guides the migration, adhesion and orientation of cells through the conformation of its own scaffold fibers and controls the expression of extracellular matrix according to the orientation of the fibers. A new acoustic electrophoretic three-dimensional (3D) biomanufacturing method, which uses radiation forces generated by superimposing ultrasonic bulk acoustic waves (BAWs) to preferentially organize arrays of cells in monolayer and multilayer hydrogel structures, was developed. The researchers investigated the effects of the parameters on the cell array configuration by controlling the frequency and amplitude of the ultrasonic wave, the signal voltage, the viscosity of the bioink, and the time of action. Finally, a physiologically related 3D construction of the meniscus was presented (Chansoria et al., 2019).

In conclusion, the excellent ability of 3D bioprinting technology to precisely control the fiber diameter, orientation, and microarchitecture endows the resulting constructs with promising mechanical properties and favorable biological functions. Therefore, it is one of the most attractive and promising technologies for tissue engineering applications, particularly meniscal regeneration.

Restoration the heterogeneities in anatomical, structural, mechanical and biochemical characteristics of meniscus is still a challenging goal albeit significant advances achieved in biomaterials science. Meniscal regeneration involves the two distinct regions that with different composition, structures and function. Hence, it requires an in-depth understanding of the zonal difference within meniscus and thus design such an optimal scaffold possessed with the anisotropic variation of (i) biomaterial composition, (ii) microarchitectures, and (iii) regional-specific pro-regenerative effects.

In this context, recent 3D fabrication techniques which can construct zone-dependent features, combined with polymeric biomaterials, have shed light on this topic. Firstly, biomimicry of meniscal spatial variation can be achieved by varying scaffolding materials. For example, in a study by Bahcecioglu et al. (2019), anatomically shaped PCL meniscal scaffolds were impregnated with cell-laden GelMA-agarose in the inner region and GelMA in the outer region. The in vitro results showed that inner GelMA-agarose hydrogel enhance chondrogenic differentiation, while the increasing COL I and decreasing COL II expression in the outer region was observed (Bahcecioglu et al., 2019). This structurally and biochemically correct scaffold presented with an advantageous solution on meniscus zonal reconstruction. Anatomical selected meniscus ECM were also exhibiting potential to develop anisotropic scaffolds able to induce the zonal fibrocartilage differentiation (Rothrauff et al., 2017). In addition to the biopolymers selections, the cell-loaded meniscal constructs in bioreactors is another approach to fulfill the zonal construction requirements. The application of bioreactors is commonly referred to applying controlled biophysical stimulus, especially valuable for hard tissue regeneration (Aguilar et al., 2019). Ideally, a zone-dependent mechanical stimulation in terms of tensile stimulus dominating on outer zone and compressive stimulus dominating on inner region can be a promising strategy for zonal differentiation (Puetzer and Bonassar, 2016). As an example, in Bahcecioglu et al. (2019a), fibrochondrocytes loaded in PCL scaffold were arranged to form scaffolds. The authors tested the effect of different cell-laden PCL/hydrogel using a custom-made bioreactor and compared the outcomes against static methods. The scaffolds treated with the dynamic stimulation resulted in increased ratio of COL II expression in the inner region (agarose impregnated along with compressive method), and enhanced ratio of COL I expression in the outer region (GelMA impregnated along with tensile method). From this instance, it is evident that bioreactor combined with spatial biopolymer incorporation enhance the formation of zone-specific tissues compared to the static seeding.

Furthermore, the region-specific infusion of various grow factors has also been explored for meniscal zonal reconstruction. For example, Lee et al. (2014) designed a 3D printed anatomic-mimicking PCL scaffold infused with different growth factors (Figure 3). The spatial-temporal releasing growth factors allowed dual induction of chondrogenic and fibrochondrogenic differentiation in the same construct (Lee et al., 2014). Another example is shown in Figure 5 where a dual growth factors functionalized scaffold was placed within a perfusion bioreactor (Zhang et al., 2019). The bioreactor was used to provide a consistent dynamic biomechanical stimulus to accelerate the spatial regulation of fibrochondrocyte differentiation. The authors used this biomechanical and biochemical combined strategy in an attempt to construct a physiological anisotropic meniscus. Their results showed that the cells expressed zonal-specific differences in the expression of the COL I and COL II, and also presented with a long-term chondroprotection of the knee joint when implanted into a rabbit model.

In short, relevant studies have brought hope on zonal meniscal reconstruction which indicate that more attempts should paid on biomaterials innovations, manufacturing technologies, biomechanical stimulation, and other novel techniques.

Polymeric materials science of the past focused on biocompatibility, including foreign body reactions (FBRs), degradability, and toxicity. However, current studies are aimed at solving one hurdle in terms of tissue engineering, that is, immune compatibility. In general, more advanced polymeric scaffolds are needed not only to fulfill the criteria of the present studies but also to regulate both the innate and adaptive immune responses.

Synthetic polymers, such as polyesters, are frequently used as cellular frameworks to support neotissue formation (Lee et al., 2019; Patel et al., 2019). For a long time, the immune compatibility of synthetic polymers focused on reducing the unwanted effects of FBR, fibrotic encapsulation, and toxicity (Chung et al., 2017). Numerous studies have confirmed that FBR may be a consistent process involved in neutrophil infiltration, monocyte/macrophage influx, fibroblast-mediated collagen and capillary bed formation, and the systemic response may also contribute to this process (Chung et al., 2017). On the other hand, natural polymers are manipulated not to induce an FBR response but to be a primary promoter for the regenerative process. For example, Ji’s and Lei’s groups found that glycidyl methacrylate-modified thermosensitive hydroxypropyl chitin hydrogels polarize macrophages both in vitro and in vivo at the cartilage defect site to accelerate a pro-regenerative process (Ji et al., 2020). It is believed that the bioactive motifs within ECM or ECM-like biological polymers can directly modulate the immune response (Rowley et al., 2019). For instance, the fibronectin sites FNI_10–12 and FNII_1–5 have been demonstrated to be able to modulate cellular responses in the regenerative cascade (Sawicka et al., 2015).

Apart from the composition of polymers, the factors involved in the immune response to biomaterials require further investigation. Interestingly, Veiseh et al. (2015) proved that spherical polymers (≥1.5 mm) in diameter triggered more severe fibrotic reactions than smaller particles or other shaped constructs, which indicated that the geometry and dimension of biomaterials are also vital impact factors of the FBR response. Currently, polymeric scaffolds are emerging as orchestrators to modulate the immune response that can influence tissue regeneration and repair processes (Chung et al., 2017). Recent research on biomaterial-mediated immunomodulation has two main directions. First, we focused on the modification of engineering scaffolds. A variety of surface chemical features have been studied, including the functional groups, surface charge, hydrophilicity, and molecular weight of the compound (Lee et al., 2019). Delivering anti-inflammatory drugs or cytokines is a second direction for engineered scaffolds to stimulate immunomodulation responses. For example, decellularized cartilage extracellular matrix has been decorated with the anti-inflammatory cytokine IL-4, and this modified scaffold promoted M2 macrophage polarization and osteochondral repair in a rat model (Tian et al., 2021). In addition, acellular cartilage matrix scaffolds functionalized with Wharton’s jelly mesenchymal stem cell-derived exosomes successfully reduced the inflammatory response in the joint cavity of a rabbit osteochondral model and improved cell proliferation, migration and polarization in vitro.

In summary, in recent decades, various biomaterial-mediated meniscal tissue engineering strategies have mainly focused on modulating the activity of different cells involved in meniscal regeneration to induce meniscal tissue formation. However, more attention should be given to the topic of biomaterial design to modulate the immune response and avoid undesirable inflammatory responses (Lee et al., 2019). To date, there are no systematic studies on the impact of immunomodulatory biomaterials on meniscal regeneration. We believe that this kind of engineered scaffold could have a profound impact on meniscal regeneration and associated patient recovery.