Instructive cartilage regeneration modalities with advanced therapeutic implantations under abnormal conditions

Nanoclay Reinforced Integrated Scaffold for Dual-Lineage Regeneration of Cartilage and Subchondral Bone

Tissue engineering is theoretically considered a promising approach for repairing osteochondral defects. Nevertheless, the insufficient osseous support and integration of the cartilage layer and the subchondral bone frequently lead to the failure of osteochondral repair. Drawing from this, it was proposed that incorporating glycine-modified attapulgite (GATP) into poly(1,8-octanediol-co-citrate) (POC) scaffolds via the one-step chemical cross-linking is proposed to enhance cartilage and subchondral bone defect repair simultaneously. The effects of the GATP incorporation ratio on the physicochemical properties, chondrocyte and MC3T3-E1 behavior, and osteochondral defect repair of the POC scaffold were also evaluated. In vitro studies indicated that the POC/10% GATP scaffold improved cell proliferation and adhesion, maintained cell phenotype, and upregulated chondrogenesis and osteogenesis gene expression. Animal studies suggested that the POC/10% GATP scaffold has significant repair effects on both cartilage and subchondral bone defects. Therefore, the GATP-incorporated scaffold system with dual-lineage bioactivity showed potential application in osteochondral regeneration.

Enhanced hyaline cartilage formation and continuous osteochondral regeneration via 3D-Printed heterogeneous hydrogel with multi-crosslinking inks

The unique gradient structure and complex composition of osteochondral tissue pose significant challenges in defect regeneration. Restoration of tissue heterogeneity while maintaining hyaline cartilage components has been a difficulty of an osteochondral tissue graft. A novel class of multi-crosslinked polysaccharide-based three-dimensional (3D) printing inks, including decellularized natural cartilage (dNC) and nano-hydroxyapatite, was designed to create a gradient scaffold with a robust interface-binding force. Herein, we report combining a dual-nozzle cross-printing technology and a gradient crosslinking method to create the scaffolds, demonstrating stable mechanical properties and heterogeneous bilayer structures. Biofunctional assessments revealed the remarkable regenerative effects of the scaffold, manifesting three orders of magnitude of mRNA upregulation during chondrogenesis and the formation of pure hyaline cartilage. Transcriptomics of the regeneration site in vivo and scaffold cell interaction tests in vitro showed that printed porous multilayer scaffolds could form the correct tissue structure for cell migration. More importantly, polysaccharides with dNC provided a hydrophilic microenvironment. The microenvironment is crucial in osteochondral regeneration because it could guide the regenerated cartilage to ensure the hyaline phenotype.

Articular cartilage repair biomaterials: strategies and applications

Articular cartilage injury is a frequent worldwide disease, while effective treatment is urgently needed. Due to lack of blood vessels and nerves, the ability of cartilage to self-repair is limited. Despite the availability of various clinical treatments, unfavorable prognoses and complications remain prevalent. However, the advent of tissue engineering and regenerative medicine has generated considerable interests in using biomaterials for articular cartilage repair. Nevertheless, there remains a notable scarcity of comprehensive reviews that provide an in-depth exploration of the various strategies and applications. Herein, we present an overview of the primary biomaterials and bioactive substances from the tissue engineering perspective to repair articular cartilage. The strategies include regeneration, substitution, and immunization. We comprehensively delineate the influence of mechanically supportive scaffolds on cellular behavior, shedding light on emerging scaffold technologies, including stimuli-responsive smart scaffolds, 3D-printed scaffolds, and cartilage bionic scaffolds. Biologically active substances, including bioactive factors, stem cells, extracellular vesicles (EVs), and cartilage organoids, are elucidated for their roles in regulating the activity of chondrocytes. Furthermore, the composite bioactive scaffolds produced industrially to put into clinical use, are also explicitly presented. This review offers innovative solutions for treating articular cartilage ailments and emphasizes the potential of biomaterials for articular cartilage repair in clinical translation.

Exosomes loaded with chondrogenic stimuli agents combined with 3D bioprinting hydrogel in the treatment of osteoarthritis and cartilage degeneration

Osteoarthritis (OA) is a challenging joint inflammatory disease that often leads to disability. Immunoregulatory Exosomes (Exos) have shown promise in OA and cartilage degeneration treatment. Engineering Exos to deliver therapeutic agents like Kartogenin (KGN) has displayed potential for restoring cartilage regeneration. However, challenges include the uneven distribution of Exos at the injury site and the release of Exos cargo out of chondrocytes. Hydrogel-loaded uMSC-Exo has demonstrated significant therapeutic effects in wound healing and tissue regeneration. Recently, a new version of three-dimensional (3D) bioprinting of hydrogel significantly restored cartilage regeneration in OA joints. Combining immune regulatory Exos with 3D bioprinting hydrogel (3D-BPH-Exos) holds the potential for immunomodulating cartilage tissue and treatment of OA. It can reduce intracellular inflammasome formation and the release of inflammatory agents like IL-1β, TNF-α, and INF-γ, while also preventing chondrocyte apoptosis by restoring mitochondrial functions and enhancing chondrogenesis in synovial MSCs, osteoprogenitor cells, and osteoclasts. Loading Exos with chondrogenic stimuli agents in the 3D-BPH-Exos approach may offer a faster and safer strategy for cartilage repair while better inhibiting joint inflammation than high doses of anti-inflammatory drugs and cell-based therapies. This review provides a comprehensive overview of hydrogel bioprinting and exosome-based therapy in OA. It emphasizes the potential of 3D-BPH-Exos loaded with chondrogenic stimuli agents for OA treatment, serving as a basis for further research.

Double-edged role of mechanical stimuli and underlying mechanisms in cartilage tissue engineering

Mechanical stimuli regulate the chondrogenic differentiation of mesenchymal stem cells and the homeostasis of chondrocytes, thus affecting implant success in cartilage tissue engineering. The mechanical microenvironment plays fundamental roles in the maturation and maintenance of natural articular cartilage, and the progression of osteoarthritis Hence, cartilage tissue engineering attempts to mimic this environment in vivo to obtain implants that enable a superior regeneration process. However, the specific type of mechanical loading, its optimal regime, and the underlying molecular mechanisms are still under investigation. First, this review delineates the composition and structure of articular cartilage, indicating that the morphology of chondrocytes and components of the extracellular matrix differ from each other to resist forces in three top-to-bottom overlapping zones. Moreover, results from research experiments and clinical trials focusing on the effect of compression, fluid shear stress, hydrostatic pressure, and osmotic pressure are presented and critically evaluated. As a key direction, the latest advances in mechanisms involved in the transduction of external mechanical signals into biological signals are discussed. These mechanical signals are sensed by receptors in the cell membrane, such as primary cilia, integrins, and ion channels, which next activate downstream pathways. Finally, biomaterials with various modifications to mimic the mechanical properties of natural cartilage and the self-designed bioreactors for experiment in vitro are outlined. An improved understanding of biomechanically driven cartilage tissue engineering and the underlying mechanisms is expected to lead to efficient articular cartilage repair for cartilage degeneration and disease.

Construction of ultrasonically treated collagen/silk fibroin composite scaffolds to induce cartilage regeneration

A novel tissue-specific functional tissue engineering scaffold for cartilage repair should have a three-dimensional structure, good biosafety and biological activity, and should be able to promote cartilage tissue regeneration. This study aimed to determine the effect of ultrasound-treated collagen/silk fibroin (Col/SF) composite scaffolds with good mechanical properties and high biological activity on cartilage repair. The characteristics of the scaffolds with different Col/SF ratios (7:3, 8:2, and 9:1) were determined by scanning electron microscopy, Fourier-transform infrared spectroscopy, and porosity, water absorption, and compression tests. In vitro evaluations revealed the biocompatibility of the Col/SF scaffolds. Results suggested that the optimal ratio of Col/SF composite scaffolds was 7:3. The Col/SF scaffolds induced adipose-derived stem cells to undergo chondrogenic differentiation under chondrogenic culture conditions. The efficiency of Col/SF scaffolds for cartilage regeneration applications was further evaluated using an in vivo model of full-thickness articular cartilage defects in New Zealand rabbits. The Col/SF scaffolds effectively promoted osteochondral regeneration as evidenced by macroscopic, histological, and immunohistochemical evaluation. The study demonstrates that ultrasound-treated Col/SF scaffolds show great potential for repairing cartilage defects.

Polyherbal formulation PL02 alleviates pain, inflammation, and subchondral bone deterioration in an osteoarthritis rodent model

Introduction

Osteoarthritis (OA) is a debilitating disease with significant personal and socioeconomic burdens worldwide.

Methods

To address this, we developed a multitargeted formulation called PL02, which includes standardized extracts of Rosa canina L, Hippophae rhamnoides, and collagen peptide. We tested the pharmacological efficacy of PL02 in a rodent model of OA induced by Monosodium iodoacetate (MIA).

Results

Our results demonstrate that oral administration of PL02 has antioxidant effects by down-regulating NOS, reduces pain-related behavior, and mitigates inflammation by inhibiting IL-1b and TNF-α production, as well as downregulating CGRP1 and COX-II. PL02 also exhibits anti-catabolic and chondroprotective activity by significantly downregulating MMP13 and upregulating BCL2. Additionally, PL02 demonstrates chondrogenic activity by significantly upregulating SOX-9 (a master regulator of chondrogenesis), Coll-I, and aggrecan, which are major components of articular cartilage. Furthermore, PL02 prevents microarchitectural deterioration of subchondral bone.

Conclusion

Overall, PL02 is an orally active, multi-targeted therapy that not only alleviates pain and inflammation but also effectively halts cartilage and subchondral bone deterioration. It represents a safe and promising candidate for the treatment and management of OA.

Current application and modification strategy of marine polysaccharides in tissue regeneration: A review

Marine polysaccharides (MPs) are exceptional bioactive materials that possess unique biochemical mechanisms and pharmacological stability, making them ideal for various tissue engineering applications. Certain MPs, including agarose, alginate, carrageenan, chitosan, and glucan have been successfully employed as biological scaffolds in animal studies. As carriers of signaling molecules, scaffolds can enhance the adhesion, growth, and differentiation of somatic cells, thereby significantly improving the tissue regeneration process. However, the biological benefits of pure MPs composite scaffold are limited. Therefore, physical, chemical, enzyme modification and other methods are employed to expand its efficacy. Chemically, the structural properties of MPs scaffolds can be altered through modifications to functional groups or molecular weight reduction, thereby enhancing their biological activities. Physically, MPs hydrogels and sponges emulate the natural extracellular matrix, creating a more conducive environment for tissue repair. The porosity and high permeability of MPs membranes and nanomaterials expedite wound healing. This review explores the distinctive properties and applications of select MPs in tissue regeneration, highlighting their structural versatility and biological applicability. Additionally, we provide a brief overview of common modification strategies employed for MP scaffolds. In conclusion, MPs have significant potential and are expected to be a novel regenerative material for tissue engineering.

Advancing drug delivery to articular cartilage: From single to multiple strategies

Articular cartilage (AC) injuries often lead to cartilage degeneration and may ultimately result in osteoarthritis (OA) due to the limited self-repair ability. To date, numerous intra-articular delivery systems carrying various therapeutic agents have been developed to improve therapeutic localization and retention, optimize controlled drug release profiles and target different pathological processes. Due to the complex and multifactorial characteristics of cartilage injury pathology and heterogeneity of the cartilage structure deposited within a dense matrix, delivery systems loaded with a single therapeutic agent are hindered from reaching multiple targets in a spatiotemporal matched manner and thus fail to mimic the natural processes of biosynthesis, compromising the goal of full cartilage regeneration. Emerging evidence highlights the importance of sequential delivery strategies targeting multiple pathological processes. In this review, we first summarize the current status and progress achieved in single-drug delivery strategies for the treatment of AC diseases. Subsequently, we focus mainly on advances in multiple drug delivery applications, including sequential release formulations targeting various pathological processes, synergistic targeting of the same pathological process, the spatial distribution in multiple tissues, and heterogeneous regeneration. We hope that this review will inspire the rational design of intra-articular drug delivery systems (DDSs) in the future.

From materials to clinical use: advances in 3D-printed scaffolds for cartilage tissue engineering

Osteoarthritis caused by articular cartilage defects is a particularly common orthopedic disease that can involve the entire joint, causing great pain to its sufferers. A global patient population of approximately 250 million people has an increasing demand for new therapies with excellent results, and tissue engineering scaffolds have been proposed as a potential strategy for the repair and reconstruction of cartilage defects. The precise control and high flexibility of 3D printing provide a platform for subversive innovation. In this perspective, cartilage tissue engineering (CTE) scaffolds manufactured using different biomaterials are summarized from the perspective of 3D printing strategies, the bionic structure strategies and special functional designs are classified and discussed, and the advantages and limitations of these CTE scaffold preparation strategies are analyzed in detail. Finally, the application prospect and challenges of 3D printed CTE scaffolds are discussed, providing enlightening insights for their current research.

How Smart are Smart Materials? A Conceptual and Ethical Analysis of Smart Lifelike Materials for the Design of Regenerative Valve Implants

It may soon become possible not just to replace, but to re-grow healthy tissues after injury or disease, because of innovations in the field of Regenerative Medicine. One particularly promising innovation is a regenerative valve implant to treat people with heart valve disease. These implants are fabricated from so-called 'smart', 'lifelike' materials. Implanted inside a heart, these implants stimulate re-growth of a healthy, living heart valve. While the technological development advances, the ethical implications of this new technology are still unclear and a clear conceptual understanding of the notions 'smart' and 'lifelike' is currently lacking. In this paper, we explore the conceptual and ethical implications of the development of smart lifelike materials for the design of regenerative implants, by analysing heart valve implants as a showcase. In our conceptual analysis, we show that the materials are considered 'smart' because they can communicate with human tissues, and 'lifelike' because they are structurally similar to these tissues. This shows that regenerative valve implants become intimately integrated in the living tissues of the human body. As such, they manifest the ontological entanglement of body and technology. In our ethical analysis, we argue this is ethically significant in at least two ways: It exacerbates the irreversibility of the implantation procedure, and it might affect the embodied experience of the implant recipient. With our conceptual and ethical analysis, we aim to contribute to responsible development of smart lifelike materials and regenerative implants.

Failure of cartilage regeneration: emerging hypotheses and related therapeutic strategies

Osteoarthritis (OA) is a disabling condition that affects billions of people worldwide and places a considerable burden on patients and on society owing to its prevalence and economic cost. As cartilage injuries are generally associated with the progressive onset of OA, robustly effective approaches for cartilage regeneration are necessary. Despite extensive research, technical development and clinical experimentation, no current surgery-based, material-based, cell-based or drug-based treatment can reliably restore the structure and function of hyaline cartilage. This paucity of effective treatment is partly caused by a lack of fundamental understanding of why articular cartilage fails to spontaneously regenerate. Thus, research studies that investigate the mechanisms behind the cartilage regeneration processes and the failure of these processes are critical to instruct decisions about patient treatment or to support the development of next-generation therapies for cartilage repair and OA prevention. This Review provides a synoptic and structured analysis of the current hypotheses about failure in cartilage regeneration, and the accompanying therapeutic strategies to overcome these hurdles, including some current or potential approaches to OA therapy.

Macromolecule-based hydrogels nanoarchitectonics with mesenchymal stem cells for regenerative medicine: A review

The role of regenerative medicine in clinical therapies is becoming increasingly vital. Under specific conditions, mesenchymal stem cells (MSCs) are capable of differentiating into mesoblastema (i.e., adipocytes, chondrocytes, and osteocytes) and other embryonic lineages. Their application in regenerative medicine has attracted a great deal of interest among researchers. To maximize the potential applications of MSCs, materials science could provide natural extracellular matrices and provide an effective means to understand the various mechanisms of differentiation for the growth of MSCs. Pharmaceutical fields are represented among the research on biomaterials by macromolecule-based hydrogel nanoarchitectonics. Various biomaterials have been used to prepare hydrogels with their unique chemical and physical properties to provide a controlled microenvironment for the culture of MSCs, laying the groundwork for future practical applications in regenerative medicine. This article currently describes and summarizes the sources, characteristics, and clinical trials of MSCs. In addition, it describes the differentiation of MSCs in various macromolecule-based hydrogel nanoarchitectonics and highlights the preclinical studies of MSCs-loaded hydrogel materials in regenerative medicine conducted over the past few years. Finally, the challenges and prospects of MSC-loaded hydrogels are discussed, and the future development of macromolecule-based hydrogel nanoarchitectonics is outlined by comparing the current literature.

Effect of autogenous osteochondral mosaicplasty on the balance control of patients with cartilage defects of the knee: a pilot study

BACKGROUND

Autogenous osteochondral mosaicplasty (AOM) is a widely used optimal surgical technique for cartilage repair in young patients with focal articular cartilage defects. However, the alterations in balance control in these patients after AOM have not been sufficiently investigated. This study aimed to compare different balance control performances between the patients with knee cartilage defects and healthy controls before and after AOM, as well as evaluate the influence of AOM on balance control in these patients.

METHODS

Static posturographic tests were performed in twenty-four patients who were scheduled for AOM two weeks pre-, three months, and one year postoperatively, along with thirty matched controls, respectively. All participants underwent posturography under four standing conditions: eyes open and closed, without and with foam support to assess the balance control ability. Subsequently, patient-reported outcome measures (PROMs) were synchronously obtained and analyzed.

RESULTS

Compared to the control subjects, less efficient balance control was observed in study patients at three testing phases (p < 0.05), whereas no alterations in postural control were visible in these patients within a year following AOM (p > 0.05). Significant improvements were found in all PROMs such as the International Knee Documentation Committee, the Lysholm Knee Score, and the visual analogue scale in the study patients postoperatively (p < 0.01).

CONCLUSION

The results indicated that patients with knee cartilage defects have a prominent balance control deficit compared to healthy individuals. Furthermore, AOM does not improve balance control in these patients for at least one year postoperatively, and more effective approaches for postural regulation are required for the management of cartilage defect patients.

Cartilage regeneration in zebrafish depends on Nrg1/ErbB signaling pathway

Objective: Cartilage, as the majority of adult mammalian tissues, has limited regeneration capacity. Cartilage degradation consecutive to joint injury or aging then leads to irreversible joint damage and diseases. In contrast, several vertebrate species such as the zebrafish have the remarkable capacity to spontaneously regenerate skeletal structures after severe injuries. The objective of our study was to test the regenerative capacity of Meckel's cartilage (MC) upon mechanical injury in zebrafish and to identify the mechanisms underlying this process. Methods and Results: Cartilage regenerative capacity in zebrafish larvae was investigated after mechanical injuries of the lower jaw MC in TgBAC(col2a1a:mCherry), to visualize the loss and recovery of cartilage. Confocal analysis revealed the formation of new chondrocytes and complete regeneration of MC at 14 days post-injury (dpi) via chondrocyte cell cycle re-entry and proliferation of pre-existing MC chondrocytes near the wound. Through expression analyses, we showed an increase of nrg1 expression in the regenerating lower jaw, which also expresses Nrg1 receptors, ErbB3 and ErbB2. Pharmacological inhibition of the ErbB pathway and specific knockdown of Nrg1 affected MC regeneration indicating the pivotal role of this pathway for cartilage regeneration. Finally, addition of exogenous NRG1 in an in vitro model of osteoarthritic (OA)-like chondrocytes induced by IL1β suggests that Nrg1/ErbB pathway is functional in mammalian chondrocytes and alleviates the increased expression of catabolic markers characteristic of OA-like chondrocytes. Conclusion: Our results show that the Nrg1/ErbB pathway is required for spontaneous cartilage regeneration in zebrafish and is of interest to design new therapeutic approaches to promote cartilage regeneration in mammals.

Thermosensitive hydrogel for cartilage regeneration via synergistic delivery of SDF-1α like polypeptides and kartogenin

Regeneration of injured articular cartilage is limited by low early-stage recruitment of stem cells and insufficient chondrogenic differentiation. Hydrogels are widely used to repair cartilage because they have excellent mechanical and biological properties. In this study, a dual drug-loaded thermosensitive hydroxypropyl chitin hydrogel (HPCH) system was prepared to release stromal-derived factor-1α-like polypeptides (SDFP) and kartogenin (KGN) for stem-cell recruitment and chondrogenic differentiation. The hydrogel had a network structure that promoted cell growth and nutrient exchange. Moreover, it was temperature sensitive and suitable for filling irregular defects. The system showed good biocompatibility in vitro and promoted stem-cell recruitment and chondrogenic differentiation. Furthermore, it reduced chondrocyte catabolism under inflammatory conditions. Animal experiments demonstrated that the dual-drug hydrogel systems can promote the regeneration of articular cartilage in rats. This study confirmed that an HPCH system loaded with KGN and SDFP could effectively repair articular cartilage defects and represents a viable treatment strategy.

Recent Progress in Hydrogel-Based Synthetic Cartilage: Focus on Lubrication and Load-Bearing Capacities

Articular cartilage (AC), which covers the ends of bones in joints, particularly the knee joints, provides a robust interface to maintain frictionless movement during daily life due to its remarkable lubricating and load-bearing capacities. However, osteoarthritis (OA), characterized by the progressive degradation of AC, compromises the properties of AC and thus leads to frayed and rough interfaces between the bones, which subsequently accelerates the progression of OA. Hydrogels, composed of highly hydrated and interconnected polymer chains, are potential candidates for AC replacement due to their physical and chemical properties being similar to those of AC. In this review, we summarize the recent progress of hydrogel-based synthetic cartilage, or cartilage-like hydrogels, with a particular focus on their lubrication and load-bearing properties. The different formulations, current limitations, and challenges of such hydrogels are also discussed. Moreover, we discuss the future directions of hydrogel-based synthetic cartilage to repair and even regenerate the damaged AC.

Thermosensitive and biodegradable PCL-based hydrogels: potential scaffolds for cartilage tissue engineering

Due to a lack of sufficient blood supply and unique physicochemical properties, the treatment of injured cartilage is laborious and needs an efficient strategy. Unfortunately, most of the current therapeutic approaches are, but not completely, unable to restore the function of injured cartilage. Tissue engineering-based modalities are an alternative option to reconstruct the injured tissue. Considering the unique structure and consistency of cartilage tissue (osteochondral junction), it is mandatory to apply distinct biomaterials with unique properties slightly different from scaffolds used for soft tissues. PCL is extensively used for the fabrication of fine therapeutic scaffolds to accelerate the restorative process. Thermosensitive PCL hydrogels with distinct chemical compositions have paved the way for sophisticated cartilage regeneration. This review aimed to collect recent findings regarding the application of PCL in hydrogels blended with natural, synthetic materials in the context of cartilage healing.

Physico-biological evaluation of 3D printed dECM/TOCN/alginate hydrogel based scaffolds for cartilage tissue regeneration

Cartilage damage is the leading cause of osteoarthritis (OA), especially in an aging society. Mimicking the native cartilage microenvironment for chondrogenic differentiation along with constructing a stable and controlled architectural scaffold is considerably challenging. In this study, three-dimensional (3D) printed scaffolds using tempo-oxidized cellulose nanofiber (TOCN), decellularized extracellular matrix (dECM), and sodium alginate (SA) were fabricated for cartilage tissue regeneration. We prepared three groups (dECM80, dECM50, dECM20) of 3D printable hydrogels with different ratios of TOCN and dECM where SA concentration remained the same. Two-step crosslinking was performed with CaCl₂ solution to achieve the highly stable 3D printed scaffolds. Finally, the fundamental physical characterizations showed that increasing the ratio of TOCN with dECM significantly improved the viscoelastic behaviour, stability, mechanical properties, and printability of the scaffolds. Based on the results, the 3D printed dECM50 scaffolds with controlled and identical pore sizes increased the whole-layer integrity and nutrient supply in each layer of the scaffold. Furthermore, evaluation of in vitro and in vivo biocompatibility of the scaffolds with rBMSCs indicated that dECM50 scaffolds provided a suitable microenvironment for cell proliferation and promoted chondrogenesis by remarkably expressing the cartilage-specific markers. This study demonstrates that 3D printed dECM50 scaffolds provide a favourable and promising microenvironment for cartilage tissue regeneration.

Silk-based hydrogel incorporated with metal-organic framework nanozymes for enhanced osteochondral regeneration

Osteochondral defects (OCD) cannot be efficiently repaired due to the unique physical architecture and the pathological microenvironment including enhanced oxidative stress and inflammation. Conventional strategies, such as the control of implant microstructure or the introduction of growth factors, have limited functions failing to manage these complex environments. Here we developed a multifunctional silk-based hydrogel incorporated with metal-organic framework nanozymes (CuTA@SF) to provide a suitable microenvironment for enhanced OCD regeneration. The incorporation of CuTA nanozymes endowed the SF hydrogel with a uniform microstructure and elevated hydrophilicity. In vitro cultivation of mesenchymal stem cells (MSCs) and chondrocytes showed that CuTA@SF hydrogel accelerated cell proliferation and enhanced cell viability, as well as had antioxidant and antibacterial properties. Under the inflammatory environment with the stimulation of IL-1β, CuTA@SF hydrogel still possessed the potential to promote MSC osteogenesis and deposition of cartilage-specific extracellular matrix (ECM). The proteomics analysis further confirmed that CuTA@SF hydrogel promoted cell proliferation and ECM synthesis. In the full-thickness OCD model of rabbit, CuTA@SF hydrogel displayed successfully in situ OCD regeneration, as evidenced by micro-CT, histology (HE, S/O, and toluidine blue staining) and immunohistochemistry (Col I and aggrecan immunostaining). Therefore, CuTA@SF hydrogel is a promising biomaterial targeted at the regeneration of OCD.

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A multifunctional silk-based hydrogel incorporated with metal-organic framework nanozymes (CuTA@SF) was fabricated.

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CuTA@SF hydrogel has antioxidant, anti-inflammation and antibacterial capacities.

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Proteomics analysis confirmed that CuTA@SF hydrogel promoted cell proliferation and ECM synthesis.

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CuTA@SF hydrogel displayed successful osteochondral regeneration in vivo.

Progress of Microfluidic Hydrogel-Based Scaffolds and Organ-on-Chips for the Cartilage Tissue Engineering

Cartilage degeneration is among the fundamental reasons behind disability and pain across the globe. Numerous approaches have been employed to treat cartilage diseases. Nevertheless, none have shown acceptable outcomes in the long run. In this regard, the convergence of tissue engineering and microfabrication principles can allow developing more advanced microfluidic technologies, thus offering attractive alternatives to current treatments and traditional constructs used in tissue engineering applications. Herein, the current developments involving microfluidic hydrogel-based scaffolds, promising structures for cartilage regeneration, ranging from hydrogels with microfluidic channels to hydrogels prepared by the microfluidic devices, that enable therapeutic delivery of cells, drugs, and growth factors, as well as cartilage-related organ-on-chips are reviewed. Thereafter, cartilage anatomy and types of damages, and present treatment options are briefly overviewed. Various hydrogels are introduced, and the advantages of microfluidic hydrogel-based scaffolds over traditional hydrogels are thoroughly discussed. Furthermore, available technologies for fabricating microfluidic hydrogel-based scaffolds and microfluidic chips are presented. The preclinical and clinical applications of microfluidic hydrogel-based scaffolds in cartilage regeneration and the development of cartilage-related microfluidic chips over time are further explained. The current developments, recent key challenges, and attractive prospects that should be considered so as to develop microfluidic systems in cartilage repair are highlighted.

Natural products can modulate inflammation in intervertebral disc degeneration

Intervertebral discs (IVDs) play a crucial role in maintaining normal vertebral anatomy as well as mobile function. Intervertebral disc degeneration (IDD) is a common clinical symptom and is an important cause of low back pain (LBP). IDD is initially considered to be associated with aging and abnormal mechanical loads. However, over recent years, researchers have discovered that IDD is caused by a variety of mechanisms, including persistent inflammation, functional cell loss, accelerated extracellular matrix decomposition, the imbalance of functional components, and genetic metabolic disorders. Of these, inflammation is thought to interact with other mechanisms and is closely associated with the production of pain. Considering the key role of inflammation in IDD, the modulation of inflammation provides us with new options for mitigating the progression of degeneration and may even cause reversal. Many natural substances possess anti-inflammatory functions. Due to the wide availability of such substances, it is important that we screen and identify natural agents that are capable of regulating IVD inflammation. In fact, many studies have demonstrated the potential clinical application of natural substances for the regulation of inflammation in IDD; some of these have been proven to have excellent biosafety. In this review, we summarize the mechanisms and interactions that are responsible for inflammation in IDD and review the application of natural products for the modulation of degenerative disc inflammation.

Recent Advances in "Functional Engineering of Articular Cartilage Zones by Polymeric Biomaterials Mediated with Physical, Mechanical, and Biological/Chemical Cues"

Articular cartilage (AC) plays an unquestionable role in joint movements but unfortunately the healing capacity is restricted due to its avascular and acellular nature. While cartilage tissue engineering has been lifesaving, it is very challenging to remodel the complex cartilage composition and architecture with gradient physio-mechanical properties vital to proper tissue functions. To address these issues, a better understanding of the intrinsic AC properties and how cells respond to stimuli from the external microenvironment must be better understood. This is essential in order to take one step closer to producing functional cartilaginous constructs for clinical use. Recently, biopolymers have aroused much attention due to their versatility, processability, and flexibility because the properties can be tailored to match the requirements of AC. This review highlights polymeric scaffolds developed in the past decade for reconstruction of zonal AC layers including the superficial zone, middle zone, and deep zone by means of exogenous stimuli such as physical, mechanical, and biological/chemical signals. The mimicked properties are reviewed in terms of the biochemical composition and organization, cell fate (morphology, orientation, and differentiation), as well as mechanical properties and finally, the challenges and potential ways to tackle them are discussed.

Engineering exosomes for bone defect repair

Currently, bone defect repair is still an intractable clinical problem. Numerous treatments have been performed, but their clinical results are unsatisfactory. As a key element of cell-free therapy, exosome is becoming a promising tool of bone regeneration in recent decades, because of its promoting osteogenesis and osteogenic differentiation function in vivo and in vitro. However, low yield, weak activity, inefficient targeting ability, and unpredictable side effects of natural exosomes have limited the clinical application. To overcome the weakness, various approaches have been applied to produce engineering exosomes by regulating their production and function at present. In this review, we will focus on the engineering exosomes for bone defect repair. By summarizing the exosomal cargos affecting osteogenesis, the strategies of engineering exosomes and properties of exosome-integrated biomaterials, this work will provide novel insights into exploring advanced engineering exosome-based cell-free therapy for bone defect repair.

Pulsed Electromagnetic Field Therapy and Direct Current Electric Field Modulation Promote the Migration of Fibroblast-like Synoviocytes to Accelerate Cartilage Repair In Vitro

Articular cartilage injuries are a common source of joint pain and dysfunction. As articular cartilage is avascular, it exhibits a poor intrinsic healing capacity for self-repair. Clinically, osteochondral grafts are used to surgically restore the articular surface following injury. A significant challenge remains with the repair properties at the graft-host tissue interface as proper integration is critical toward restoring normal load distribution across the joint. A key to addressing poor tissue integration may involve optimizing mobilization of fibroblast-like synoviocytes (FLS) that exhibit chondrogenic potential and are derived from the adjacent synovium, the specialized connective tissue membrane that envelops the diarthrodial joint. Synovium-derived cells have been directly implicated in the native repair response of articular cartilage. Electrotherapeutics hold potential as low-cost, low-risk, non-invasive adjunctive therapies for promoting cartilage healing via cell-mediated repair. Pulsed electromagnetic fields (PEMFs) and applied direct current (DC) electric fields (EFs) via galvanotaxis are two potential therapeutic strategies to promote cartilage repair by stimulating the migration of FLS within a wound or defect site. PEMF chambers were calibrated to recapitulate clinical standards (1.5 ± 0.2 mT, 75 Hz, 1.3 ms duration). PEMF stimulation promoted bovine FLS migration using a 2D in vitro scratch assay to assess the rate of wound closure following cruciform injury. Galvanotaxis DC EF stimulation assisted FLS migration within a collagen hydrogel matrix in order to promote cartilage repair. A novel tissue-scale bioreactor capable of applying DC EFs in sterile culture conditions to 3D constructs was designed in order to track the increased recruitment of synovial repair cells via galvanotaxis from intact bovine synovium explants to the site of a cartilage wound injury. PEMF stimulation further modulated FLS migration into the bovine cartilage defect region. Biochemical composition, histological analysis, and gene expression revealed elevated GAG and collagen levels following PEMF treatment, indicative of its pro-anabolic effect. Together, PEMF and galvanotaxis DC EF modulation are electrotherapeutic strategies with complementary repair properties. Both procedures may enable direct migration or selective homing of target cells to defect sites, thus augmenting natural repair processes for improving cartilage repair and healing.

A critical review on polydopamine surface-modified scaffolds in musculoskeletal regeneration

Increasing concern about age-related diseases, particularly musculoskeletal injuries and orthopedic conditions, highlights the need for strategies such as tissue engineering to address them. Surface modification has been developed to create pro-healing interfaces, personalize scaffolds and provide novel medicines. Polydopamine, a mussel-inspired adhesive polymer with highly reactive functional groups that adhere to nearly all substrates, has gained attention in surface modification strategies for biomaterials. Polydopamine was primarily developed to modify surfaces, but its effectiveness has opened up promising approaches for further applications in bioengineering as carriers and nanoparticles. This review focuses on the recent discoveries of the role of polydopamine as a surface coating material, with focus on the properties that make it suitable for tackling musculoskeletal disorders. We report the evolution of using it in research, and discuss papers involving the progress of this field. The current research on the role of polydopamine in bone, cartilage, muscle, nerve, and tendon regeneration is discussed, thus giving comprehensive overview about the function of polydopamine both in-vitro and in-vivo. Finally, the report concludes presenting the critical challenges that must be addressed for the clinical translation of this biomaterial while exploring future perspectives and research opportunities in this area.

Targeting Inflammation and Regeneration: Scaffolds, Extracellular Vesicles, and Nanotechnologies as Cell-Free Dual-Target Therapeutic Strategies

Osteoarthritis (OA) affects over 250 million people worldwide and despite various existing treatment strategies still has no cure. It is a multifactorial disease characterized by cartilage loss and low-grade synovial inflammation. Focusing on these two targets together could be the key to developing currently missing disease-modifying OA drugs (DMOADs). This review aims to discuss the latest cell-free techniques applied in cartilage tissue regeneration, since they can provide a more controllable approach to inflammation management than the cell-based ones. Scaffolds, extracellular vesicles, and nanocarriers can be used to suppress inflammation, but they can also act as immunomodulatory agents. This is consistent with the latest tissue engineering paradigm, postulating a moderate, controllable inflammatory reaction to be beneficial for tissue remodeling and successful regeneration.

Fabrication of biphasic cartilage-bone integrated scaffolds based on tissue-specific photo-crosslinkable acellular matrix hydrogels

The fabrication of biphasic cartilage-bone integrated scaffolds is an attractive alternative for osteochondral repair but has proven to be extremely challenging. Existing three-dimensional (3D) scaffolds are insufficient to accurately biomimic the biphasic cartilage-bone integrated microenvironment. Currently, photo-crosslinkable hydrogels based on tissue-specific decellularized extracellular matrix (dECM) have been considered as an important technique to fabricate biomimetic scaffolds, but so far there has been no breakthrough in the photo-crosslinkable hydrogel scaffolds with biphasic cartilage-bone biomimetic microenvironment. Here, we report a novel strategy for the preparation of biomimetic cartilage-bone integrated scaffolds based on photo-crosslinkable cartilage/bone-derived dECM hydrogels, which are able to reconstruct biphasic cartilage-bone biomimetic microenvironment. The biphasic cartilage-bone integrated scaffolds provided a 3D microenvironment for osteochondral regeneration. The cartilage biomimetic scaffolds, consisting of cartilage-derived dECM hydrogels, efficiently regulated chondrogenesis of bone marrow mesenchymal stem cells (BMSCs). The bone biomimetic scaffolds, composed of cartilage/bone-derived dECM hydrogels, first regulated chondrogenesis of BMSCs, followed by endochondral ossification over time. Taken together, the biphasic cartilage-bone integrated tissue could be successfully reconstructed by subcutaneous culture based on cartilage-bone bilayered structural design. Furthermore, the biphasic cartilage-bone biomimetic scaffolds (cell-free) achieved satisfactory cartilage-bone integrated regeneration in the osteochondral defects of rabbits’ knee joints.

Ultra-High Modulus Hydrogels Mimicking Cartilage of the Human Body

The human body is comprised of numerous types of cartilage with a range of high moduli, despite their high hydration. Owing to the limitations of cartilage tissue healing and biological grafting procedures, synthetic replacements have emerged but are limited by poorly matched moduli. While conventional hydrogels can achieve similar hydration to cartilage tissues, their moduli are substantially inferior. Herein, triple network (TN) hydrogels are prepared to synergistically leverage intra-network electrostatic repulsive and hydrophobic interactions, as well as inter-network electrostatic attractive interactions. They are comprised of an anionic 1^st network, a neutral 2^nd network (capable of hydrophobic associations), and a cationic 3^rd network. Collectively, these interactions act synergistically as effective, yet dynamic crosslinks. By tuning the concentration of the cationic 3^rd network, these TN hydrogels achieve high moduli of ≈1.5 to ≈3.5 MPa without diminishing cartilage-like water contents (≈80%), strengths, or toughness values. This unprecedented combination of properties poises these TN hydrogels as cartilage substitutes in applications spanning articulating joints, intervertebral discs (IVDs), trachea, and temporomandibular joint disc (TMJ).

Chondro-Inductive b-TPUe-Based Functionalized Scaffolds for Application in Cartilage Tissue Engineering

Osteoarthritis is a disease with a great socioeconomic impact and mainly affects articular cartilage, a tissue with reduced self-healing capacity. In this work, 3D printed 1,4 butanediol thermoplastic polyurethane (b-TPUe) scaffolds are functionalized and infrapatellar mesenchymal stem cells are used as the cellular source. Since b-TPUe is a biomaterial with mechanical properties similar to cartilage, but it does not provide the desired environment for cell adhesion, scaffolds are functionalized with two methods, one based on collagen type I and the other in 1-pyrenebutiric acid (PBA) as principal components. Alamar Blue and confocal assays display that PBA functionalized scaffolds support higher cell adhesion and proliferation for the first 21 days. However, collagen type I functionalization induces higher proliferation rates and similar cell viability than the PBA method. Further, both functionalization methods induce extracellular matrix synthesis, and the presence of chondrogenic markers (Sox9, Col2a, and Acan). Finally, SEM images probe that functionalized 3D printed scaffolds present much better cell/biomaterial interactions than controls and confirm early chondrogenesis. These results indicate that the two methods of functionalization in the highly hydrophobic b-TPUe enhance the cell-biomaterial interactions and the improvement in the chondro-inductive properties, which have great potential for application in cartilage tissue engineering.

Subchondral bone-inspired hydrogel scaffold for cartilage regeneration

Promoting the in situ regeneration of cartilage without additional cells or cytokines remains challenging. Here, inspired by the unique microstructures of subchondral bone, a cell and cytokine free hydrogel scaffold for cartilage regeneration was developed via a strategy of directional lyophilization and postcrosslinking. This strategy achieved intersecting microchannels in an orderly arrangement and an aligned ladder-like texture in a semi-interpenetrating hydrogel network. The resulting hydrogel had similar mechanical properties to the native cartilage extracellular matrix. Incorporating chitosan into the rigid network also endowed the hydrogel with excellent hemostatic properties. By delicately tuning the components and postcrosslinking conditions, the hydrogel was further endowed with suitable swelling and degradation properties for cartilage regeneration. In vitro tests showed that the highly biocompatible hydrogel scaffold could facilitate the migration and chondrogenic differentiation of bone marrow mesenchymal stem cells. In vivo results further verified that the hydrogel could promote the in situ regeneration of cartilage in a rat model of osteochondral defects. In summary, the subchondral bone-like hydrogel revealed promising prospects in cartilage regeneration and a variety of bioremediation applications.

The effect of LDHs nanoparticles on the cellular behavior of stem cell-laden 3D-bioprinted scaffold

Three-dimensional (3D)-bioprinting as an emerging approach for tissue engineering possesses the promise to create highly mimicked organs or tissues by using computer-aided design. For biomedical applications in tissue engineering in our previous work, we developed an optimized nanocomposite bioink based on methylacrylated gelatin (GelMA), methylacrylated chitosan (ChitMA), and double-layered hydroxide (LDHs) nanoparticles by using 3D-bioprinting technology. Herein, we used the previous formulation to fabricate human bone marrow mesenchymal stem cells (hBMMSCs)-laden nanocomposite bioinks. The effect of LDHs nanoparticles on the cellular behaviors of the encapsulated-hBMMSCs in the scaffolds was evaluated for the first time. Live/Dead, PrestoBlue, and DAPI/Actin analysis were carried out to assess the cell viability, proliferation rate, and cellular morphology of encapsulated hBMMSCs within the scaffolds. In addition, osteogenic differentiation studies were performed culturing the scaffolds for up to 21 days. Results show that LDHs nanoparticles in the GelMA/ChitMA scaffold formulation increased the viability of hBMMSCs, did not cause any adverse effect on the proliferation rate, cell morphology of the hBMMSCs, and increased the Runx2 protein expression of the encapsulated-hBMMSCs in the scaffolds. This study progresses the LDHs containing nanocomposite bioink for cell printing applications in tissue engineering.

A sonication-induced silk-collagen hydrogel for functional cartilage regeneration

Cartilage tissue has limited self-regeneration capacity and current treatment methods often result in fibrocartilage formation. Although collagen has shown the ability to induce chondrogenesis of mesenchymal stem cells (MSCs) and regenerate hyaline cartilage, the application of a pure collagen hydrogel is inherently limited by its fast degradation, poor mechanical properties and excessive cell-mediated shrinkage. To overcome this challenge, we developed a sonication-induced silk-collagen composite hydrogel (COL + SF(S)) and investigated its physicochemical and biological properties compared with a collagen hydrogel (COL) and a non-sonicated silk-collagen composite hydrogel (COL + SF(NS)). The results showed that the sonication treatment of silk fibroin induced antiparallel β-sheet formation and a stronger negative charge on the silk fibroin molecule, which resulted in improved mechanical properties of the COL + SF(S) hydrogel. The COL + SF(S) hydrogel exhibited superior stability during cell culture and promoted the gene expression of SOX9 at the early stage and sulfated glycosaminoglycan (sGAG) deposition without any exogenous growth factor. Moreover, the cartilage regeneration capacity of the COL + SF(S) group was evaluated in rabbit knee defects. The COL + SF(S) group exhibited well-integrated articular hyaline cartilage closely resembling native articular cartilage after 6 months. Overall, the COL + SF(S) hydrogel holds great potential as a scaffold material to regenerate functional hyaline cartilage.

Copper-Lithium-Doped Nanohydroxyapatite Modulates Mesenchymal Stem Cells Homing to Treat Glucocorticoids-Related Osteonecrosis of the Femoral Head

In situ tissue regeneration has been demonstrated to promote bone repair. To identify a better approach for treating osteonecrosis of the femoral head (ONFH), we prepared scaffolds using copper-lithium-doped nanohydroxyapatite (Cu-Li-nHA), which has the potential to modulate mesenchymal stem cells (MSCs) homing. The scaffold was fabricated using the gas foaming method and the migration, angiogenesis, and osteogenesis activities of MSCs were detected using Transwell assays, tube formation assays, alkaline phosphatase and alizarin red S staining, respectively. We then implanted the Cu-Li-nHA scaffold into the femoral heads of ONFH rabbits, and CFSE labeled exogenous MSCs were injected intravenously to verify cell homing. The repair effect was subsequently examined using micro-CT and histological analysis in vivo. The results showed that Cu-Li-nHA significantly promoted MSCs migration and homing by upregulating the HIF-1α/SDF-1 pathway. The Cu-Li-nHA group showed optimal osteogenesis and angiogenesis and greater improvements in new bone formation in ONFH rabbits. To summarize, Cu-Li-nHA promoted homing and induced the osteogenic differentiation of MSCs, thereby enhancing bone regeneration during ONFH repair. Thus, Cu-Li-nHA implantation may serve as a potential therapeutic strategy for ONFH in the future.

Characteristic and Chondrogenic Differentiation Analysis of Hybrid Hydrogels Comprised of Hyaluronic Acid Methacryloyl (HAMA), Gelatin Methacryloyl (GelMA), and the Acrylate-Functionalized Nano-Silica Crosslinker

Developing a biomaterial suitable for adipose-derived stem cell (ADSCs)-laden scaffolds that can directly bond to cartilage tissue surfaces in tissue engineering has still been a significant challenge. The bioinspired hybrid hydrogel approaches based on hyaluronic acid methacryloyl (HAMA) and gelatin methacryloyl (GelMA) appear to have more promise. Herein, we report the cartilage tissue engineering application of a novel photocured hybrid hydrogel system comprising HAMA, GelMA, and 0~1.0% (w/v) acrylate-functionalized nano-silica (AFnSi) crosslinker, in addition to describing the preparation of related HAMA, GelMA, and AFnSi materials and confirming their related chemical evidence. The study also examines the physicochemical characteristics of these hybrid hydrogels, including swelling behavior, morphological conformation, mechanical properties, and biodegradation. To further investigate cell viability and chondrogenic differentiation, the hADSCs were loaded with a two-to-one ratio of the HAMA-GelMA (HG) hybrid hydrogel with 0~1.0% (w/v) AFnSi crosslinker to examine the process of optimal chondrogenic development. Results showed that the morphological microstructure, mechanical properties, and longer degradation time of the HG+0.5% (w/v) AFnSi hydrogel demonstrated the acellular novel matrix was optimal to support hADSCs differentiation. In other words, the in vitro experimental results showed that hADSCs laden in the photocured hybrid hydrogel of HG+0.5% (w/v) AFnSi not only significantly increased chondrogenic marker gene expressions such as SOX-9, aggrecan, and type II collagen expression compared to the HA and HG groups, but also enhanced the expression of sulfated glycosaminoglycan (sGAG) and type II collagen formation. We have concluded that the photocured hybrid hydrogel of HG+0.5% (w/v) AFnSi will provide a suitable environment for articular cartilage tissue engineering applications.

Acellular cartilage matrix biomimetic scaffold with immediate enrichment of autologous bone marrow mononuclear cells to repair articular cartilage defects

Functional repair of articular cartilage defects is always a great challenge in joint surgery clinically. Tissue engineering strategies that combine autologous cell implantation with three-dimensional scaffolds have proven effective for repairing articular cartilage tissue. However, it faces the problem of cell sources and scaffold materials. Autologous chondrocytes and bone marrow are difficult to popularize clinically due to limited donor sources and low mononuclear cell (MNC) concentrations, respectively. The density gradient centrifugation method can increase the concentration of MNCs in fresh bone marrow by nearly a hundredfold and achieve immediate enrichment. In addition, acellular cartilage matrix (ACM), with good biocompatibility and a cartilage-specific microenvironment, is considered to be an ideal candidate scaffold for cartilage regeneration. In this study, hybrid pigs were used to establish articular cartilage defect models of different sizes to determine the feasibility and maximum scope of application of ACM-based biomimetic scaffolds combined with MNCs for inducing articular cartilage regeneration. Importantly, ACM-based biomimetic scaffolds instantly enriched MNCs could improve the repair effect of articular cartilage defects in situ, which established a new model of articular cartilage regeneration that could be applied immediately and suited for large-scale clinical promotion. The current study significantly improves the repair effect of articular cartilage defects, which provides scientific evidence and detailed insights for future clinical applications of ACM-based biomimetic scaffolds combined with MNCs.

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Explore the maximum scope of repairing articular cartilage defect with ACM scaffold.

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Immediate enrichment of mononuclear cells by density gradient centrifugation.

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ACM scaffold enriched MNCs improve the repair effect of articular cartilage defect.

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Enrichment of MNCs expands the maximum scope of repairing articular cartilage defect.

3D Bioprinting of Living Materials for Structure-Dependent Production of Hyaluronic Acid

3D bioprinting of living materials represents an interesting paradigm toward the efficacy enhancement for the biosynthesis of various functional compounds in microorganisms. Previous studies have shown the success of 3D-printed bioactive systems in the production of small molecular compounds. However, the feasibility of such a strategy in producing macromolecules and how the geometry of the 3D scaffold influences the productivity are still unknown. In this study, we printed a series of 3D gelatin-based hydrogels immobilized with fermentation bacteria that can secrete hyaluronic acid (HA), a very useful natural polysaccharide in the fields of biomedicine and tissue engineering. The 3D-printed bioreactor was capable of producing HA, and an elevated yield was obtained with the system bearing a grid structure compared to that either with a solid structure or in a scaffold-free fermentation condition. As for the grid structure, bioreactors with a 90° strut angel and a median interfilament distance displayed the highest HA yield. Our findings highlighted the significant role of 3D printing in the spatial control of microorganism-laden hydrogel structures for the enhancement of biosynthesis efficiency.

Preparation and characterization of a novel drug-loaded Bi-layer scaffold for cartilage regeneration

The incidence of articular cartilage defects is increasing year by year. In order to repair the cartilage tissue at the defect, scaffolds with nanofiber structure and biocompatibility have become a research hotspot. In this study, we designed and fabricated a bi-layer scaffold prepared from an upper layer of drug-dispersed gelatin methacrylate (GELMA) hydrogel and a lower layer of a drug-encapsulated coaxial fiber scaffold prepared from silk fiber (SF) and polylactic acid (PLA). These bi-layer scaffolds have porosity (91.26 ± 3.94%) sufficient to support material exchange and pore size suitable for cell culture and infiltration, as well as mechanical properties (2.65 ± 0.31 MPa) that meet the requirements of cartilage tissue engineering. The coaxial fiber structure exhibited excellent drug release properties, maintaining drug release for 14 days in PBS. In vitro experiments indicated that the scaffolds were not toxic to cells and were amenable to chondrocyte migration. Notably, the growth of cells in a bi-layer scaffold presented two states. In the hydrogel layer, cells grow through interconnected pores and take on a connective tissue-like shape. In the coaxial fiber layer, cells grow on the surface of the coaxial fiber mats and appeared tablet-like. This is similar to the structure of the functional partitions of natural cartilage tissue. Together, the bi-layer scaffold can play a positive role in cartilage regeneration, which could be a potential therapeutic choice to solve the current problems of clinical cartilage repair.

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