Three-Dimensional Bioprinting of Articular Cartilage: A Systematic Review

Engineering gene-activated bioprinted scaffolds for enhancing articular cartilage repair

Untreated articular cartilage injuries often result in severe chronic pain and dyskinesia. Current repair strategies have limitations in effectively promoting articular cartilage repair, underscoring the need for innovative therapeutic approaches. A gene-activated matrix (GAM) is a promising and comprehensive therapeutic strategy that integrates tissue-engineered scaffold-guided gene therapy to promote long-term articular cartilage repair by enhancing gene retention, reducing gene loss, and regulating gene release. However, for effective articular cartilage repair, the GAM scaffold must mimic the complex gradient structure of natural articular cartilage. Three-dimensional (3D) bioprinting technology has emerged as a compelling solution, offering the ability to precisely create complex microstructures that mimic the natural articular cartilage. In this review, we summarize the recent research progress on GAM and 3D bioprinted scaffolds in articular cartilage tissue engineering (CTE), while also exploring future challenges and development directions. This review aims to provide new ideas and concepts for the development of gene-activated bioprinted scaffolds with specific properties tailored to meet the stringent requirements of articular cartilage repair.

High-throughput bioprinting of spheroids for scalable tissue fabrication

Tissue biofabrication mimicking organ-specific architecture and function requires physiologically-relevant cell densities. Bioprinting using spheroids can achieve this, but is limited due to the lack of practical, scalable techniques. This study presents HITS-Bio (High-throughput Integrated Tissue Fabrication System for Bioprinting), a multiarray bioprinting technique for rapidly positioning multiple spheroids simultaneously using a digitally-controlled nozzle array (DCNA). HITS-Bio achieves an unprecedented speed, ten times faster compared to existing techniques while maintaining high cell viability ( > 90%). The utility of HITS-Bio was exemplified in multiple applications, including intraoperative bioprinting with microRNA transfected human adipose-derived stem cell spheroids for calvarial bone regeneration ( ~ 30 mm3) in a rat model achieving a near-complete defect closure (bone coverage area of ~ 91% in 3 weeks and ~96% in 6 weeks). Additionally, the successful fabrication of scalable cartilage constructs (1 cm3) containing ~600 chondrogenic spheroids highlights its high-throughput efficiency (under 40 min per construct) and potential for repairing volumetric defects.

Improving biological and mechanical properties of bioprinted PCL-alginate-chondrocyte scaffolds for patellofemoral cartilage tissue regeneration

In this study, polycaprolactone (PCL) scaffolds have been employed as structural framework scaffolds for patellofemoral cartilage tissue regeneration. The biomechanical and biological properties of different scaffolds were investigated by varying alginate concentrations and the number of scaffold layers. Patellofemoral cartilage defects result in knee pain and reduced mobility, and they are usually treated with conventional methods, often with limited success. Generally, tissue-engineered PCL-alginate scaffolds fabricated by bioprinting technology show promise for enhanced cartilage regeneration due to the biocompatibility and mechanical stability of PCL. In addition, alginate is known for its cell encapsulation capabilities and for promoting cell viability. Biological and morphological assessments, utilizing water contact angle, cell adhesion tests, MTT assays, and scanning electron microscopy (SEM), informed the selection of the optimized scaffold. Comparative analyses between the initial optimal scaffolds with the same chemical composition also included flexural and compression tests and fracture surface observations using SEM. The controlled integration of PCL and alginate offers a hybrid approach, that assembles the mechanical strength of PCL and the bioactive properties of alginate for tissue reconstruction potential. This study aims to identify the most effective scaffold composition for patellofemoral articular cartilage tissue engineering, emphasizing cell viability, structural morphology, and mechanical integrity. The results showed that the optimum biomechanical and biological properties of scaffolds were obtained with a 10% alginate concentration in the monolayer of PCL structure. The findings contribute to regenerative medicine by advancing the understanding of functional tissue constructs, bringing us closer to addressing articular cartilage defects and related clinical challenges.

Advancing Cartilage Tissue Engineering: A Review of 3D Bioprinting Approaches and Bioink Properties

Articular cartilage is crucial in human physiology, and its degeneration poses a significant public health challenge. While recent advancements in 3D bioprinting and tissue engineering show promise for cartilage regeneration, there remains a gap between research findings and clinical application. This review critically examines the mechanical and biological properties of hyaline cartilage, along with current 3D manufacturing methods and analysis techniques. Moreover, we provide a quantitative synthesis of bioink properties used in cartilage tissue engineering. After screening 181 initial works, 33 studies using extrusion bioprinting were analyzed and synthesized, presenting results that indicate the main materials, cells, and methods utilized for mechanical and biological evaluation. Altogether, this review motivates the standardization of mechanical analyses and biomaterial assessments of 3D bioprinted constructs to clarify their chondrogenic potential.

Arthroscopic device with bendable tip for the controlled extrusion of hydrogels on cartilage defects

Advanced tools for the in situ treatment of articular cartilage lesions are attracting a growing interest in both surgery and bioengineering communities. The interest is particularly high concerning the delivery of cell-laden hydrogels. The tools currently available in the state-of-the-art hardly find an effective compromise between treatment accuracy and invasiveness. This paper presents a novel arthroscopic device provided with a bendable tip for the controlled extrusion of cell-laden hydrogels. The device consists of a handheld extruder and a supply unit that allows the extrusion of hydrogels. The extruder is equipped with a disposable, bendable nitinol tip (diameter: 4 mm, length: 92 mm, maximum bending angle: 90°) that guarantees access to hard-to-reach areas of the joint, which are difficult to get to, with conventional arthroscopic instruments. The tip accommodates a biocompatible polymer tube that is directly connected to the cartridge containing the hydrogel, whose plunger is actuated by a volumetric or pneumatic supply unit (both tested, in this study). Three different chondrocyte-laden hydrogels (RGD-modified Vitrogel®, methacrylated gellan gum, and an alginate-gelatine blend) were considered. First, the performance of the device in terms of resolution in hydrogel delivery was assessed, finding values in the range between 4 and 102 µL, with better performance found for the pneumatic supply unit and no significant differences between straight tip and bent tip conditions. Finite element simulations suggested that the shear stresses and pressure levels generated during the extrusion process were compatible with a safe deposition of the hydrogels. Biological analyses confirmed a high chondrocyte viability over a 7-day period after the extrusion of the three cell-laden hydrogel types, with no differences between the two supply units. The arthroscopic device was finally tested ex vivo by nine orthopedic surgeons on human cadaver knees. The device allowed surgeons to easily deliver hydrogels even in hard-to-reach cartilage areas. The outcomes of a questionnaire completed by the surgeons demonstrated a high usability of the device, with an overall preference for the pneumatic supply unit. Our findings provide evidence supporting the future arthroscopic device translation in pre-clinical and clinical scenarios, dealing with osteoarticular treatments.

Organ bioprinting: progress, challenges and outlook

Bioprinting, as a groundbreaking technology, enables the fabrication of biomimetic tissues and organs with highly complex structures, multiple cell types, mechanical heterogeneity, and diverse functional gradients. With the growing demand for organ transplantation and the limited number of organ donors, bioprinting holds great promise for addressing the organ shortage by manufacturing completely functional organs. While the bioprinting of complete organs remains a distant goal, there has been considerable progress in the development of bioprinted transplantable tissues and organs for regenerative medicine. This review article recapitulates the current achievements of organ 3D bioprinting, primarily encompassing five important organs in the human body (i.e., the heart, kidneys, liver, pancreas, and lungs). Challenges from cellular techniques, biomanufacturing technologies, and organ maturation techniques are also deliberated for the broad application of organ bioprinting. In addition, the integration of bioprinting with other cutting-edge technologies including machine learning, organoids, and microfluidics is envisioned, which strives to offer the reader the prospect of bioprinting in constructing functional organs.

3D puzzle-inspired construction of large and complex organ structures for tissue engineering

3D printing as a powerful technology enables the fabrication of organ structures with a programmed geometry, but it is usually difficult to produce large-size tissues due to the limited working space of the 3D printer and the instability of bath or ink materials during long printing sessions. Moreover, most printing only allows preparation with a single ink, while a real organ generally consists of multiple materials. Inspired by the 3D puzzle toy, we developed a "building block-based printing" strategy, through which the preparation of 3D tissues can be realized by assembling 3D-printed "small and simple" bio-blocks into "large and complex" bioproducts. The structures that are difficult to print by conventional 3D printing such as a picture puzzle consisting of different materials and colors, a collagen "soccer" with a hollow yet closed structure, and even a full-size human heart model are successfully prepared. The 3D puzzle-inspired preparation strategy also allows for a reasonable combination of various cells in a specified order, facilitating investigation into the interaction between different kinds of cells. This strategy opens an alternative path for preparing organ structures with multiple materials, large size and complex geometry for tissue engineering applications.

Bioinks adapted for in situ bioprinting scenarios of defect sites: a review

In situ bioprinting provides a reliable solution to the problem of in vitro tissue culture and vascularization by printing tissue directly at the site of injury or defect and maturing the printed tissue using the natural cell microenvironment in vivo. As an emerging field, in situ bioprinting is based on computer-assisted scanning results of the defect site and is able to print cells directly at this site with biomaterials, bioactive factors, and other materials without the need to transfer prefabricated grafts as with traditional in vitro 3D bioprinting methods, and the resulting grafts can accurately adapt to the target defect site. However, one of the important reasons hindering the development of in situ bioprinting is the absence of suitable bioinks. In this review, we will summarize bioinks developed in recent years that can adapt to in situ printing scenarios at the defect site, considering three aspects: the in situ design strategy of bioink, the selection of commonly used biomaterials, and the application of bioprinting to different treatment scenarios.

3D printed bionic ear and microtia-anotia: Medical and forensic implications

Advances in science and technology result in continuous upgrading of the medical and clinical fields. These upgrades have been beneficial to many patients suffering from congenital and developmental diseases causing compromised functionality as well as the structural integrity of the affected organs. One such congenital anomaly is the microtia-anotia spectrum, which results in deformities of the external ear, affecting the hearing capability of an individual. The collaboration and integration of electronics in human biology are exemplified by the development of a 3D printed cyborg bionic ear from the patient's own cells, which is implanted to ameliorate microtia and improve the patient's hearing capacity. Since the late nineteenth century, forensic scientists have explored the external ear, establishing the significance of ear biometrics in the identification process. Similarly, ear prints can also play an important role in identifying the felon of a crime. In this regard, we examine the structure and functionality of implanted ears. This communication is an attempt to enlighten investigators on the forensic importance and limitations of the use of bionic ears for identification.

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.

Ex Vivo Maturation of 3D-Printed, Chondrocyte-Laden, Polycaprolactone-Based Scaffolds Prior to Transplantation Improves Engineered Cartilage Substitute Properties and Integration

OBJECTIVE

The surgical management of nasal septal defects due to perforations, malformations, congenital cartilage absence, traumatic defects, or tumors would benefit from availability of optimally matured septal cartilage substitutes. Here, we aimed to improve in vitro maturation of 3-dimensional (3D)-printed, cell-laden polycaprolactone (PCL)-based scaffolds and test their in vivo performance in a rabbit auricular cartilage model.

DESIGN

Rabbit auricular chondrocytes were isolated, cultured, and seeded on 3D-printed PCL scaffolds. The scaffolds were cultured for 21 days in vitro under standard culture media and normoxia or in prochondrogenic and hypoxia conditions, respectively. Cell-laden scaffolds (as well as acellular controls) were implanted into perichondrium pockets of New Zealand white rabbit ears (N = 5 per group) and followed up for 12 weeks. At study end point, the tissue-engineered scaffolds were extracted and tested by histological, immunohistochemical, mechanical, and biochemical assays.

RESULTS

Scaffolds previously matured in vitro under prochondrogenic hypoxic conditions showed superior mechanical properties as well as improved patterns of cartilage matrix deposition, chondrogenic gene expression (COL1A1, COL2A1, ACAN, SOX9, COL10A1), and proteoglycan production in vivo, compared with scaffolds cultured in standard conditions.

CONCLUSIONS

In vitro maturation of engineered cartilage scaffolds under prochondrogenic conditions that better mimic the in vivo environment may be beneficial to improve functional properties of the engineered grafts. The proposed maturation strategy may also be of use for other tissue-engineered constructs and may ultimately impact survival and integration of the grafts in the damaged tissue microenvironment.

Three-Dimensional Bioprinting for Cartilage Tissue Engineering: Insights into Naturally-Derived Bioinks from Land and Marine Sources

In regenerative medicine and tissue engineering, the possibility to: (I) customize the shape and size of scaffolds, (II) develop highly mimicked tissues with a precise digital control, (III) manufacture complex structures and (IV) reduce the wastes related to the production process, are the main advantages of additive manufacturing technologies such as three-dimensional (3D) bioprinting. Specifically, this technique, which uses suitable hydrogel-based bioinks, enriched with cells and/or growth factors, has received significant consideration, especially in cartilage tissue engineering (CTE). In this field of interest, it may allow mimicking the complex native zonal hyaline cartilage organization by further enhancing its biological cues. However, there are still some limitations that need to be overcome before 3D bioprinting may be globally used for scaffolds' development and their clinical translation. One of them is represented by the poor availability of appropriate, biocompatible and eco-friendly biomaterials, which should present a series of specific requirements to be used and transformed into a proper bioink for CTE. In this scenario, considering that, nowadays, the environmental decline is of the highest concerns worldwide, exploring naturally-derived hydrogels has attracted outstanding attention throughout the scientific community. For this reason, a comprehensive review of the naturally-derived hydrogels, commonly employed as bioinks in CTE, was carried out. In particular, the current state of art regarding eco-friendly and natural bioinks' development for CTE was explored. Overall, this paper gives an overview of 3D bioprinting for CTE to guide future research towards the development of more reliable, customized, eco-friendly and innovative strategies for CTE.

Progress in 3D Bioprinting Technology for Osteochondral Regeneration

Osteochondral injuries can lead to osteoarthritis (OA). OA is characterized by the progressive degradation of the cartilage tissue together with bone tissue turnover. Consequently, joint pain, inflammation, and stiffness are common, with joint immobility and dysfunction being the most severe symptoms. The increase in the age of the population, along with the increase in risk factors such as obesity, has led OA to the forefront of disabling diseases. In addition, it not only has an increasing prevalence, but is also an economic burden for health systems. Current treatments are focused on relieving pain and inflammation, but they become ineffective as the disease progresses. Therefore, new therapeutic approaches, such as tissue engineering and 3D bioprinting, have emerged. In this review, the advantages of using 3D bioprinting techniques for osteochondral regeneration are described. Furthermore, the biomaterials, cell types, and active molecules that are commonly used for these purposes are indicated. Finally, the most recent promising results for the regeneration of cartilage, bone, and/or the osteochondral unit through 3D bioprinting technologies are considered, as this could be a feasible therapeutic approach to the treatment of OA.

Advances in Tissue Engineering of the Temporomandibular Joint Disc: An Overview of Current Status and Future Directions

Advances in tissue engineering have progressed to potentially offer a solution to temporomandibular joint disc (TMJ) disorders not amenable to conservative therapies. Conclusive treatment options for patients with end-stage disc disorders requires discectomy and reconstruction of the articular disc with various materials. Tissue engineering TMJ disc is a promising alternative to the limited and sometimes inadequate clinical options in the management of such disorders. However, tissue engineering is far from completion for the TMJ disc regeneration. This review briefly discusses the properties of native disc, the mechanism by which TMJ disorders manifest, and how a tissue engineered disc could assuage the problems inherent in the management of such disorders. Furthermore, the review addresses and provides updates to relevant themes of tissue engineering in regards to the TMJ disc, namely, the scaffolds, cells and biomarkers, hurdles in tissue engineering of the disc, and its application in translation to the clinical practice and future directions.

Articular cartilage reconstruction with TGF-β1-simulating self-assembling peptide hydrogel-based composite scaffold

Transforming growth factor-β (TGF-β) is an important inducing factor for the differentiation of mesenchymal stem cells and the secretion of collagen II, but the inaccessibility and instability limit its application in clinical practice. In this study, the TGF-β1-simulating peptide LIANAK (CM) was connected with the self-assembling peptide Ac-(RADA)₄-CONH₂ (RAD) to obtain the functionalized self-assembling peptide Ac-(RADA)₄-GG-LIANAK-CONH₂ (RAD-CM). The results indicated that the CM-functionalized RAD hydrogel contributed to the enhanced expressions of chondrogenic genes and extracellular matrix deposition. The self-assembling peptides were then combined with decellularized cartilage extracellular matrix (DCM) to construct a composite scaffold for articular cartilage repair. The CM-functionalized composite scaffold RAD/RAD-CM/DCM (R/C/D) exhibited good bioactivity and structural stability and exhibited satisfactory performance in promoting neocartilage restoration and the reconstruction of the osteochondral unit. This study provides a promising strategy for in situ cartilage regeneration via the stable presentation of TGF-β1-simulating peptide. STATEMENT OF SIGNIFICANCE: Deficiency of effective chondrogenic inducers (especially, the TGF-β family) significantly limits the self-regeneration of cartilage in osteochondral defect cases. Oligopeptide LIANAK, named CM, could simulate TGF-β1's bioactivity with particular structure, but traditional chemical crosslinking to polymer scaffolds resulted in risks of safety and complication, which is unfavorable for clinical applications. Here, self-assembling peptide RAD was used to load CM, to obtain a TGF-β1 mimetic peptide hydrogel. Depending on the homology (amino acids) of RAD and CM, the synthesis of the whole peptide only needs simply extended sequences of CM following that of RAD by automated solid-phase peptide synthesis. The modified peptide effectively demonstrated osteochondrogenic bioactivity, ensured the convenience, safety, and mass production, which displayed great potential in tissue engineering research and translational medicine.

In vitro Cartilage Regeneration Regulated by a Hydrostatic Pressure Bioreactor Based on Hybrid Photocrosslinkable Hydrogels

Because of the superior characteristics of photocrosslinkable hydrogels suitable for 3D cell-laden bioprinting, tissue regeneration based on photocrosslinkable hydrogels has become an important research topic. However, due to nutrient permeation obstacles caused by the dense networks and static culture conditions, there have been no successful reports on in vitro cartilage regeneration with certain thicknesses based on photocrosslinkable hydrogels. To solve this problem, hydrostatic pressure (HP) provided by the bioreactor was used to regulate the in vitro cartilage regeneration based on hybrid photocrosslinkable (HPC) hydrogel. Chondrocyte laden HPC hydrogels (CHPC) were cultured under 5 MPa HP for 8 weeks and evaluated by various staining and quantitative methods. Results demonstrated that CHPC can maintain the characteristics of HPC hydrogels and is suitable for 3D cell-laden bioprinting. However, HPC hydrogels with concentrations over 3% wt% significantly influenced cell viability and in vitro cartilage regeneration due to nutrient permeation obstacles. Fortunately, HP completely reversed the negative influences of HPC hydrogels at 3% wt%, significantly enhanced cell viability, proliferation, and extracellular matrix (ECM) deposition by improving nutrient transportation and up-regulating the expression of cartilage-specific genes, and successfully regenerated homogeneous cartilage with a thickness over 3 mm. The transcriptome sequencing results demonstrated that HP regulated in vitro cartilage regeneration primarily by inhibiting cell senescence and apoptosis, promoting ECM synthesis, suppressing ECM catabolism, and ECM structure remodeling. Evaluation of in vivo fate indicated that in vitro regenerated cartilage in the HP group further developed after implantation and formed homogeneous and mature cartilage close to the native one, suggesting significant clinical potential. The current study outlines an efficient strategy for in vitro cartilage regeneration based on photocrosslinkable hydrogel scaffolds and its in vivo application.

Additive Manufacturing Strategies for Personalized Drug Delivery Systems and Medical Devices: Fused Filament Fabrication and Semi Solid Extrusion

Novel additive manufacturing (AM) techniques and particularly 3D printing (3DP) have achieved a decade of success in pharmaceutical and biomedical fields. Highly innovative personalized therapeutical solutions may be designed and manufactured through a layer-by-layer approach starting from a digital model realized according to the needs of a specific patient or a patient group. The combination of patient-tailored drug dose, dosage, or diagnostic form (shape and size) and drug release adjustment has the potential to ensure the optimal patient therapy. Among the different 3D printing techniques, extrusion-based technologies, such as fused filament fabrication (FFF) and semi solid extrusion (SSE), are the most investigated for their high versatility, precision, feasibility, and cheapness. This review provides an overview on different 3DP techniques to produce personalized drug delivery systems and medical devices, highlighting, for each method, the critical printing process parameters, the main starting materials, as well as advantages and limitations. Furthermore, the recent developments of fused filament fabrication and semi solid extrusion 3DP are discussed. In this regard, the current state of the art, based on a detailed literature survey of the different 3D products printed via extrusion-based techniques, envisioning future directions in the clinical applications and diffusion of such systems, is summarized.

Biomaterial Integration in the Joint: Pathological Considerations, Immunomodulation, and the Extracellular Matrix

Defects of articular joints are becoming an increasing societal burden due to a persistent increase in obesity and aging. For some patients suffering from cartilage erosion, joint replacement is the final option to regain proper motion and limit pain. Extensive research has been undertaken to identify novel strategies enabling earlier intervention to promote regeneration and cartilage healing. With the introduction of decellularized extracellular matrix (dECM), researchers have tapped into the potential for increased tissue regeneration by designing biomaterials with inherent biochemical and immunomodulatory signals. Compared to conventional and synthetic materials, dECM-based materials invoke a reduced foreign body response. It is therefore highly beneficial to understand the interplay of how these native tissue-based materials initiate a favorable remodeling process by the immune system. Yet, such an understanding also demands increasing considerations of the pathological environment and remodeling processes, especially for materials designed for early disease intervention. This knowledge would avoid rejection and help predict complications in conditions with inflammatory components such as arthritides. This review outlines general issues facing biomaterial integration and emphasizes the importance of tissue-derived macromolecular components in regulating essential homeostatic, immunological, and pathological processes to increase biomaterial integration for patients suffering from joint degenerative diseases. This article is protected by copyright. All rights reserved.

Development and Prospect of Esophageal Tissue Engineering

Currently, patients with esophageal cancer, especially advanced patients, usually use autologous tissue for esophageal alternative therapy. However, an alternative therapy is often accompanied by serious complications such as ischemia and leakage, which seriously affect the prognosis of patients. Tissue engineering has been widely studied as one of the ideal methods for the treatment of esophageal cancer. In view of the complex multi-layer structure of the natural esophagus, how to use the tissue engineering method to design the scaffold with structure and function matching with the natural tissue is the principle that the tissue engineering method must follow. This article will analyze and summarize the construction methods, with or without cells, and repair effects of single-layer scaffold and multi-layer scaffold. Especially in the repair of full-thickness and circumferential esophageal defects, the flexible design method and the binding force between the layers of the scaffold are very important. In short, esophageal tissue engineering technology has broad prospects and plays a more and more important role in the treatment of esophageal diseases.

Insights into the present and future of cartilage regeneration and joint repair

Knee osteoarthritis is the most common joint disease. It causes pain and suffering for affected patients and is the source of major economic costs for healthcare systems. Despite ongoing research, there is a lack of knowledge regarding disease mechanisms, biomarkers, and possible cures. Current treatments do not fulfill patients' long-term needs, and it often requires invasive surgical procedures with subsequent long periods of rehabilitation. Researchers and companies worldwide are working to find a suitable cell source to engineer or regenerate a functional and healthy articular cartilage tissue to implant in the damaged area. Potential cell sources to accomplish this goal include embryonic stem cells, mesenchymal stem cells, or induced pluripotent stem cells. The differentiation of stem cells into different tissue types is complex, and a suitable concentration range of specific growth factors is vital. The cellular microenvironment during early embryonic development provides crucial information regarding concentrations of signaling molecules and morphogen gradients as these are essential inducers for tissue development. Thus, morphogen gradients implemented in developmental protocols aimed to engineer functional cartilage tissue can potentially generate cells comparable to those within native cartilage. In this review, we have summarized the problems with current treatments, potential cell sources for cell therapy, reviewed the progress of new treatments within the regenerative cartilage field, and highlighted the importance of cell quality, characterization assays, and chemically defined protocols.

The Role of Mesenchymal Stromal Cells in the Management of Knee Chondral Defects

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Mesenchymal stromal cells (MSCs) are a subset of progenitor cells that help to promote tissue healing and regeneration through the secretion of various cytokines and growth factors. Although technically pluripotent, MSCs in vivo rarely repair damaged tissue through direct differentiation and engraftment.

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Augmentation of traditional marrow stimulation techniques with MSCs has been theorized to improve repair tissue quality and defect fill. Clinical trials evaluating this technique are limited but have shown modest improvements compared with isolated marrow stimulation.

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Various scaffolds also have been utilized in combination with MSCs to treat focal chondral defects. Although the techniques described are heterogeneous, many have shown promising early clinical outcomes.

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Newer techniques involving 3-dimensional bioprinted scaffolds seeded with MSCs allow for the recreation of complex architecture, more closely resembling articular cartilage. These techniques are evolving and have not yet been studied in human clinical trials.

Advances in Engineered Three-Dimensional (3D) Body Articulation Unit Models

Indeed, the body articulation units, commonly referred to as body joints, play significant roles in the musculoskeletal system, enabling body flexibility. Nevertheless, these articulation units suffer from several pathological conditions, such as osteoarthritis (OA), rheumatoid arthritis (RA), ankylosing spondylitis, gout, and psoriatic arthritis. There exist several treatment modalities based on the utilization of anti-inflammatory and analgesic drugs, which can reduce or control the pathophysiological symptoms. Despite the success, these treatment modalities suffer from major shortcomings of enormous cost and poor recovery, limiting their applicability and requiring promising strategies. To address these limitations, several engineering strategies have been emerged as promising solutions in fabricating the body articulation as unit models towards local articulation repair for tissue regeneration and high-throughput screening for drug development. In this article, we present challenges related to the selection of biomaterials (natural and synthetic sources), construction of 3D articulation models (scaffold-free, scaffold-based, and organ-on-a-chip), architectural designs (microfluidics, bioprinting, electrospinning, and biomineralization), and the type of culture conditions (growth factors and active peptides). Then, we emphasize the applicability of these articulation units for emerging biomedical applications of drug screening and tissue repair/regeneration. In conclusion, we put forward the challenges and difficulties for the further clinical application of the in vitro 3D articulation unit models in terms of the long-term high activity of the models.

Tethered TGF-β1 in a Hyaluronic Acid-Based Bioink for Bioprinting Cartilaginous Tissues

In 3D bioprinting for cartilage regeneration, bioinks that support chondrogenic development are of key importance. Growth factors covalently bound in non-printable hydrogels have been shown to effectively promote chondrogenesis. However, studies that investigate the functionality of tethered growth factors within 3D printable bioinks are still lacking. Therefore, in this study, we established a dual-stage crosslinked hyaluronic acid-based bioink that enabled covalent tethering of transforming growth factor-beta 1 (TGF-β1). Bone marrow-derived mesenchymal stromal cells (MSCs) were cultured over three weeks in vitro, and chondrogenic differentiation of MSCs within bioink constructs with tethered TGF-β1 was markedly enhanced, as compared to constructs with non-covalently incorporated TGF-β1. This was substantiated with regard to early TGF-β1 signaling, chondrogenic gene expression, qualitative and quantitative ECM deposition and distribution, and resulting construct stiffness. Furthermore, it was successfully demonstrated, in a comparative analysis of cast and printed bioinks, that covalently tethered TGF-β1 maintained its functionality after 3D printing. Taken together, the presented ink composition enabled the generation of high-quality cartilaginous tissues without the need for continuous exogenous growth factor supply and, thus, bears great potential for future investigation towards cartilage regeneration. Furthermore, growth factor tethering within bioinks, potentially leading to superior tissue development, may also be explored for other biofabrication applications.

The treatment of knee cartilage lesions: state of the art

The management and repair of knee cartilage lesions currently represents a challenge for the orthopaedic surgeon. Identifiable causes are the characteristics of the involved tissues themselves and the presence of poor vascularization, which is responsible for overall reduced repair capacity. The literature reports three types of cartilage lesions' treatment modalities: chondroprotection, chondroreparation and chondrogeneration. The preference for one or the other therapeutic option depends on the pattern of the lesion and the clinical conditions of the patient. Each treatment technique is distinguished by the quality of the restorative tissue that is generated. In particular, the chondrorigeneration represents the last frontier of regenerative medicine, as it aims at the complete restoration of natural cartilage. However, the most recent literature documents good results only in the short and medium terms. In recent years the optimization of chondroregeneration outcomes is based on the modification of the scaffolds and the search for new chondrocyte sources, in order to guarantee satisfactory long-term results.

3D bioprinting of photo-crosslinkable silk methacrylate (SilMA)-polyethylene glycol diacrylate (PEGDA) bioink for cartilage tissue engineering

Articular cartilage damage poses huge burden on healthcare sector globally due to its extremely weak inherent regenerative ability. Three-dimensional (3D) bioprinting for development of cartilage mimic constructs using composite bioinks serves as an emerging perspective. However, difficulty in development of suitable bioink and chemical crosslinking associated inherent toxicity hamper widespread adoption of this technique. To circumvent this, a photo-polymerizable hydrogel-based bioink which helps in recapitulation of the complex cartilage microenvironment is pertinent. Herein, a photo-crosslinkable bioink containing different concentrations of silk methacrylate (SilMA) and polyethylene glycol diacrylate (PEGDA) was mixed with chondrocytes for biofabrication of 3D bioprinted cartilage constructs. The rheological properties, printability of bioink and physico-chemical characterization of printed hydrogel constructs were examined along with cartilaginous tissue formation. The printed SilMA-PEGDA hydrogel constructs possessed proper internal porous structure and demonstrated most reliable rheological properties, printability along with good mechanical, and degradation properties suitable for cartilage regeneration. Live/dead staining showed cytocompatibility of the 3D-bioprinted SilMA-PEGDA constructs. Moreover, a marked increase in cell number and DNA content was observed within the cartilaginous tissue as indicated by cell viability and DNA content quantitation. Biochemical evaluation confirmed the neocartilage formation within SilMA-PEGDA bioprinted constructs as revealed by enhanced deposition of cartilage specific extracellular matrix-sulphated GAG (sGAG) and collagen type II (>2-fold increase, p < 0.001) with time. Finally, immunohistochemical analysis indicated expression of collagen type II and aggrecan which corroborated with cartilaginous tissue formation. Taken together, we conclude that SilMA-PEGDA bioink could be suitable candidate for bioprinting chondrocytes to support cartilage tissue repair and regeneration.

The Developing Field of Scaffold-Free Tissue Engineering for Articular Cartilage Repair

Articular cartilage is critical for proper joint mobility as it provides a smooth and lubricated surface between articulating bones and allows for transmission of load to underlying bones. Extended wear or injury of this tissue can result in osteoarthritis, a degenerative disease affecting millions across the globe. Because of its low regenerative capacity, articular cartilage cannot heal on its own and effective treatments for injured joint restoration remain a challenge. Strategies in tissue engineering have been demonstrated as potential therapeutic approaches to regenerate and repair damaged articular cartilage. Although many of these strategies rely on the use of an exogenous three-dimensional scaffolds to regenerate cartilage, scaffold-free tissue engineering provides numerous advantages over scaffold-based methods. This review highlights the latest advancements in scaffold-free tissue engineering for cartilage and the potential for clinical translation.

Microvalve Bioprinting of MSC-Chondrocyte Co-Cultures

Recent improvements within the fields of high-throughput screening and 3D tissue culture have provided the possibility of developing in vitro micro-tissue models that can be used to study diseases and screen potential new therapies. This paper reports a proof-of-concept study on the use of microvalve-based bioprinting to create laminar MSC-chondrocyte co-cultures to investigate whether the use of MSCs in ACI procedures would stimulate enhanced ECM production by chondrocytes. Microvalve-based bioprinting uses small-scale solenoid valves (microvalves) to deposit cells suspended in media in a consistent and repeatable manner. In this case, MSCs and chondrocytes have been sequentially printed into an insert-based transwell system in order to create a laminar co-culture, with variations in the ratios of the cell types used to investigate the potential for MSCs to stimulate ECM production. Histological and indirect immunofluorescence staining revealed the formation of dense tissue structures within the chondrocyte and MSC-chondrocyte cell co-cultures, alongside the establishment of a proliferative region at the base of the tissue. No stimulatory or inhibitory effect in terms of ECM production was observed through the introduction of MSCs, although the potential for an immunomodulatory benefit remains. This study, therefore, provides a novel method to enable the scalable production of therapeutically relevant micro-tissue models that can be used for in vitro research to optimise ACI procedures.

Bioprinting Technology in Skin, Heart, Pancreas and Cartilage Tissues: Progress and Challenges in Clinical Practice

Bioprinting is an emerging additive manufacturing technique which shows an outstanding potential for shaping customized functional substitutes for tissue engineering. Its introduction into the clinical space in order to replace injured organs could ideally overcome the limitations faced with allografts. Presently, even though there have been years of prolific research in the field, there is a wide gap to bridge in order to bring bioprinting from "bench to bedside". This is due to the fact that bioprinted designs have not yet reached the complexity required for clinical use, nor have clear GMP (good manufacturing practices) rules or precise regulatory guidelines been established. This review provides an overview of some of the most recent and remarkable achievements for skin, heart, pancreas and cartilage bioprinting breakthroughs while highlighting the critical shortcomings for each tissue type which is keeping this technique from becoming widespread reality.

Chondro-inductive hyaluronic acid/chitosan coacervate-based scaffolds for cartilage tissue engineering

Injuries related to articular cartilage are among the most challenging musculoskeletal problems because of poor repair capacity of this tissue. The lack of efficient treatments for chondral defects has stimulated research on cartilage tissue engineering applications combining porous biocompatible scaffolds with stem cells in the presence of external stimuli. This work presents the role of rat bone marrow mesenchymal stem cell (BMSC) encapsulated-novel three-dimensional (3D) coacervate scaffolds prepared through complex coacervation between different chitosan salts (CHI) and sodium hyaluronate (HA). The 3D architecture of BMSC encapsulated scaffolds (HA/CHI) was shown by scanning electron microscopy (SEM) to have an interconnected structure to allow cell-cell and cell-matrix interactions. Chondrogenic induction of encapsulated BMSCs within HA/CHI coacervates demonstrated remarkable cellular viability in addition to the elevated expression levels of chondrogenic markers such as sex determining region Y-box 9 protein (SOX9), aggrecan (ACAN), cartilage oligomeric matrix protein (COMP) and collagen type II (COL2A1) by immunofluorescence staining, qPCR and ELISA test. Collectively, HA/CHI coacervates are promising candidates for future use of these scaffolds in cartilage tissue engineering applications.

3D bio-printed biphasic scaffolds with dual modification of silk fibroin for the integrated repair of osteochondral defects

Repair of osteochondral defects is still a challenge, especially the regeneration of hyaline cartilage. Parathyroid hormone (PTH) can inhibit the hypertrophy of chondrocytes to maintain the phenotype of hyaline cartilage. Here, we aimed to construct a bio-printed biphasic scaffold with a mechanical gradient based on dual modification of silk fibroin (SF) for the integrated repair of osteochondral defects. Briefly, SF was grafted with PTH (SF-PTH) and covalently immobilized with methacrylic anhydride (SF-MA), respectively. Next, gelatin methacryloyl (GM) mixed with SF-PTH or SF-MA was used as a bio-ink for articular cartilage and subchondral bone regeneration. Finally, the GM + SF-PTH/GM + SF-MA osteochondral biphasic scaffold was constructed using 3D bioprinting technology, and implanted in a rabbit osteochondral defect model. In this study, the SF-PTH bio-ink was synthesized for the first time. In vitro results indicated that the GM + SF-MA bio-ink had good mechanical properties, while the GM + SF-PTH bio-ink inhibited the hypertrophy of chondrocytes and was beneficial for the production of hyaline cartilage extracellular matrix. Importantly, an integrated GM + SF-PTH/GM + SF-MA biphasic scaffold with a mechanical gradient was successfully constructed. The results in vivo demonstrated that the GM + SF-PTH/GM + SF-MA scaffold could promote the regeneration of osteochondral defects and maintain the phenotype of hyaline cartilage to a large extent. Collectively, our results indicate that the integrated GM + SF-PTH/GM + SF-MA biphasic scaffold constructed by 3D bioprinting is expected to become a new strategy for the treatment of osteochondral defects.

Generation and Evaluation of Novel Biomaterials Based on Decellularized Sturgeon Cartilage for Use in Tissue Engineering

Because cartilage has limited regenerative capability, a fully efficient advanced therapy medicinal product is needed to treat severe cartilage damage. We evaluated a novel biomaterial obtained by decellularizing sturgeon chondral endoskeleton tissue for use in cartilage tissue engineering. In silico analysis suggested high homology between human and sturgeon collagen proteins, and ultra-performance liquid chromatography confirmed that both types of cartilage consisted mainly of the same amino acids. Decellularized sturgeon cartilage was recellularized with human chondrocytes and four types of human mesenchymal stem cells (MSC) and their suitability for generating a cartilage substitute was assessed ex vivo and in vivo. The results supported the biocompatibility of the novel scaffold, as well as its ability to sustain cell adhesion, proliferation and differentiation. In vivo assays showed that the MSC cells in grafted cartilage disks were biosynthetically active and able to remodel the extracellular matrix of cartilage substitutes, with the production of type II collagen and other relevant components, especially when adipose tissue MSC were used. In addition, these cartilage substitutes triggered a pro-regenerative reaction mediated by CD206-positive M2 macrophages. These preliminary results warrant further research to characterize in greater detail the potential clinical translation of these novel cartilage substitutes.

Three-dimensional Printing in Orthopaedic Surgery: Current Applications and Future Developments

Three-dimensional (3D) printing is an exciting form of manufacturing technology that has transformed the way we can treat various medical pathologies. Also known as additive manufacturing, 3D printing fuses materials together in a layer-by-layer fashion to construct a final 3D product. This technology allows flexibility in the design process and enables efficient production of both off-the-shelf and personalized medical products that accommodate patient needs better than traditional manufacturing processes. In the field of orthopaedic surgery, 3D printing implants and instrumentation can be used to address a variety of pathologies that would otherwise be challenging to manage with products made from traditional subtractive manufacturing. Furthermore, 3D bioprinting has significantly impacted bone and cartilage restoration procedures and has the potential to completely transform how we treat patients with debilitating musculoskeletal injuries. Although costs can be high, as technology advances, the economics of 3D printing will improve, especially as the benefits of this technology have clearly been demonstrated in both orthopaedic surgery and medicine as a whole. This review outlines the basics of 3D printing technology and its current applications in orthopaedic surgery and ends with a brief summary of 3D bioprinting and its potential future impact.

Recent progress in extrusion 3D bioprinting of hydrogel biomaterials for tissue regeneration: a comprehensive review with focus on advanced fabrication techniques

Over the last decade, 3D bioprinting has received immense attention from research communities to bridge the divergence between artificially engineered tissue constructs and native tissues.

Hybrid Printing Using Cellulose Nanocrystals Reinforced GelMA/HAMA Hydrogels for Improved Structural Integration

3D printing of soft-tissue like cytocompatible single material constructs with appropriate mechanical properties remains a challenge. Hybrid printing technology provides an attractive alternative as it combines a cell-free ink for providing mechanical support with a bioink for housing embedded cells. Several hybrid printed structures have been developed, utilizing thermoplastic polymers such as polycaprolactone as structural support. These thermoplastics demonstrated limited structural integration with the cell-laden components, and this may compromise the overall performance. In this work, a hybrid printing platform is presented using two distinct hydrogel inks that share the same photo-crosslinking chemistry to enable simple fabrication and seamless structural integration. A mechanically reinforced hydrogel ink is developed comprising cellulose nanocrystals and gelatin methacryloyl/hyaluronic acid methacrylate (GelMA/HAMA) as the structural component, and GelMA/HAMA as the cytogel containing a mouse chondrogenic cell line, ATDC5. Hybrid printed constructs with encapsulated cells are fabricated using the two optimized inks, and the structural integration of the constructs is evaluated by cyclic mechanical compression. Finally, the cell viability of encapsulated ATDC5 cells in the hybrid printed structures is evaluated.

Hybrid Bioprinting of Zonally Stratified Human Articular Cartilage Using Scaffold-Free Tissue Strands as Building Blocks

The heterogeneous and anisotropic articular cartilage is generally studied as a layered structure of "zones" with unique composition and architecture, which is difficult to recapitulate using current approaches. A novel hybrid bioprinting strategy is presented here to generate zonally stratified cartilage. Scaffold-free tissue strands (TSs) are made of human adipose-derived stem cells (ADSCs) or predifferentiated ADSCs. Cartilage TSs with predifferentiated ADSCs exhibit improved mechanical properties and upregulated expression of cartilage-specific markers at both transcription and protein levels as compared to TSs with ADSCs being differentiated in the form of strands and TSs of nontransfected ADSCs. Using the novel hybrid approach integrating new aspiration-assisted and extrusion-based bioprinting techniques, the bioprinting of zonally stratified cartilage with vertically aligned TSs at the bottom zone and horizontally aligned TSs at the superficial zone is demonstrated, in which collagen fibers are aligned with designated orientation in each zone imitating the anatomical regions and matrix orientation of native articular cartilage. In addition, mechanical testing study reveals a compression modulus of ≈1.1 MPa, which is similar to that of human articular cartilage. The prominent findings highlight the potential of this novel bioprinting approach for building biologically, mechanically, and histologically relevant cartilage for tissue engineering purposes.

Aspiration-assisted bioprinting of the osteochondral interface

Osteochondral defects contain damage to both the articular cartilage and underlying subchon- dral bone, which remains a significant challenge in orthopedic surgery. Layered structure of bone, cartilage and the bone-cartilage interface must be taken into account in the case of biofabrication of the osteochondral (OC) interface. In this study, a dual layered OC interface was bioprinted using a newly developed aspiration-assisted bioprinting (AAB) technique, which has been the first time that scaffold-free bioprinting was applied to OC interface engineering. Tissue spheroids, made of human adipose-derived stem cells (ADSCs), were differentiated in three dimensions (3D) into chondrogenic and osteogenic spheroids, which were confirmed by immunostaining and histology qualitatively, and biochemistry assays and gene expression, quantitatively. Remarkably, the OC interface was bioprinted by accurate positioning of a layer of osteogenic spheroids onto a sacrificial alginate support followed by another layer of chondrogenic spheroids overlaid by the same support. Spheroids in individual zones fused and the maintenance of phenotypes in both zones confirmed the successful biofabrication of the histomorphologically-relevant OC interface. The biofabrication of OC tissue model without the use of polymeric scaffolds unveils great potential not only in regenerative medicine but also in drug testing and disease modeling for osteoarthritis.

Biological perspectives and current biofabrication strategies in osteochondral tissue engineering

Articular cartilage and the underlying subchondral bone are crucial in human movement and when damaged through disease or trauma impacts severely on quality of life. Cartilage has a limited regenerative capacity due to its avascular composition and current therapeutic interventions have limited efficacy. With a rapidly ageing population globally, the numbers of patients requiring therapy for osteochondral disorders is rising, leading to increasing pressures on healthcare systems. Research into novel therapies using tissue engineering has become a priority. However, rational design of biomimetic and clinically effective tissue constructs requires basic understanding of osteochondral biological composition, structure, and mechanical properties. Furthermore, consideration of material design, scaffold architecture, and biofabrication strategies, is needed to assist in the development of tissue engineering therapies enabling successful translation into the clinical arena. This review provides a starting point for any researcher investigating tissue engineering for osteochondral applications. An overview of biological properties of osteochondral tissue, current clinical practices, the role of tissue engineering and biofabrication, and key challenges associated with new treatments is provided. Developing precisely engineered tissue constructs with mechanical and phenotypic stability is the goal. Future work should focus on multi-stimulatory environments, long-term studies to determine phenotypic alterations and tissue formation, and the development of novel bioreactor systems that can more accurately resemble the in vivo environment.

Intraoperative Bioprinting: Repairing Tissues and Organs in a Surgical Setting

3D bioprinting directly into injured sites in a surgical setting, intraoperative bioprinting (IOB), is an effective process, in which the defect information can be rapidly acquired and then repaired via bioprinting on a live subject. In patients needing tissue resection, debridement, traumatic reconstruction, or fracture repair, the ability to scan and bioprint immediately following surgical preparation of the defect site has great potential to improve the precision and efficiency of these procedures. In this opinion article, we provide the reader with current major limitations of IOB from engineering and clinical points of view, as well as possibilities of future translation of bioprinting technologies from bench to bedside, and expound our perspectives in the context of IOB of composite and vascularized tissues.

Bio-inspired hydrogel composed of hyaluronic acid and alginate as a potential bioink for 3D bioprinting of articular cartilage engineering constructs

Bioprinting is a promising tool to fabricate well-organized cell-laden constructs for repair and regeneration of articular cartilage. The selection of a suitable bioink, in terms of composition and mechanical properties, is crucial for the development of viable cartilage substitutes. In this study, we focused on the use of one of the main cartilage components, hyaluronic acid (HA), to design and formulate a new bioink for cartilage tissue 3D bioprinting. Major characteristics required for this application such as printability, biocompatibility, and biodegradability were analyzed. To produce cartilage constructs with optimal mechanical properties, HA-based bioink was co-printed with polylactic acid (PLA). HA-based bioink was found to improve cell functionality by an increase in the expression of chondrogenic gene markers and specific matrix deposition and, therefore, tissue formation. These results indicate that it is a promising bioink candidate for cartilage tissue engineering based in 3D bioprinting. STATEMENT OF SIGNIFICANCE: The recent appearance of 3D printing technology has enabled great advances in the treatment of osteochondral disorders by fabrication of cartilage tissue constructs that restore and/or regenerate damaged tissue. In this attempt, the selection of a suitable biomaterial, in terms of composition and mechanical properties, is crucial. In this study, we describe for first time the development of a bioink based on the main component of cartilage, HA, with suitable biological and mechanical properties, without involving toxic procedure, and its application  in cartilage tissue bioprinting. Hybrid constructs prepared by co-printing  this  bioink and thermoplastic polymer PLA provided an optimal niche for chondrocyte growth and maintenance as well as mechanical properties necessary to support load forces exerted in native tissue. We highlight the translation potential of this HA-based bioink in the clinical arena.

Human articular cartilage repair: Sources and detection of cytotoxicity and genotoxicity in photo-crosslinkable hydrogel bioscaffolds

Three-dimensional biofabrication using photo-crosslinkable hydrogel bioscaffolds has the potential to revolutionize the need for transplants and implants in joints, with articular cartilage being an early target tissue. However, to successfully translate these approaches to clinical practice, several barriers must be overcome. In particular, the photo-crosslinking process may impact on cell viability and DNA integrity, and consequently on chondrogenic differentiation. In this review, we primarily explore the specific sources of cellular cytotoxicity and genotoxicity inherent to the photo-crosslinking reaction, the methods to analyze cell death, cell metabolism, and DNA damage within the bioscaffolds, and the possible strategies to overcome these detrimental effects.

Photopolymerizable gelatin and hyaluronic acid for stereolithographic 3D bioprinting of tissue-engineered cartilage

To create artificial cartilage in vitro, mimicking the function of native extracellular matrix (ECM) and morphological cartilage-like shape is essential. The interplay of cell patterning and matrix concentration has high impact on the phenotype and viability of the printed cells. To advance the capabilities of cartilage bioprinting, we investigated different ECMs to create an in vitro chondrocyte niche. Therefore, we used methacrylated gelatin (GelMA) and methacrylated hyaluronic acid (HAMA) in a stereolithographic bioprinting approach. Both materials have been shown to support cartilage ECM formation and recovery of chondrocyte phenotype. We used these materials as bioinks to create cartilage models with varying chondrocyte densities. The models maintained shape, viability, and homogenous cell distribution over 14 days in culture. Chondrogenic differentiation was demonstrated by cartilage-typical proteoglycan and type II collagen deposition and gene expression (COL2A1, ACAN) after 14 days of culture. The differentiation pattern was influenced by cell density. A high cell density print (25 × 106  cells/mL) led to enhanced cartilage-typical zonal segmentation compared to cultures with lower cell density (5 × 106  cells/mL). Compared to HAMA, GelMA resulted in a higher expression of COL1A1, typical for a more premature chondrocyte phenotype. Both bioinks are feasible for printing in vitro cartilage with varying differentiation patterns and ECM organization depending on starting cell density and chosen bioink. The presented technique could find application in the creation of cartilage models and in the treatment of articular cartilage defects using autologous material and adjusting the bioprinted constructs size and shape to the patient. © 2019 The Authors. Journal of Biomedical Materials Research Part B: Applied Biomaterials published by Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B:2649-2657, 2019.

Bioprinting functional tissues

Despite the numerous lives that have been saved since the first successful procedure in 1954, organ transplant has several shortcomings which prevent it from becoming a more comprehensive solution for medical care than it is today. There is a considerable shortage of organ donors, leading to patient death in many cases. In addition, patients require lifelong immunosuppression to prevent graft rejection postoperatively. With such issues in mind, recent research has focused on possible solutions for the lack of access to donor organs and rejections, with the possibility of using the patient's own cells and tissues for treatment showing enormous potential. Three-dimensional (3D) bioprinting is a rapidly emerging technology, which holds great promise for fabrication of functional tissues and organs. Bioprinting offers the means of utilizing a patient's cells to design and fabricate constructs for replacement of diseased tissues and organs. It enables the precise positioning of cells and biologics in an automated and high throughput manner. Several studies have shown the promise of 3D bioprinting. However, many problems must be overcome before the generation of functional tissues with biologically-relevant scale is possible. Specific focus on the functionality of bioprinted tissues is required prior to clinical translation. In this perspective, this paper discusses the challenges of functionalization of bioprinted tissue under eight dimensions: biomimicry, cell density, vascularization, innervation, heterogeneity, engraftment, mechanics, and tissue-specific function, and strives to inform the reader with directions in bioprinting complex and volumetric tissues. STATEMENT OF SIGNIFICANCE: With thousands of patients dying each year waiting for an organ transplant, bioprinted tissues and organs show the potential to eliminate this ever-increasing organ shortage crisis. However, this potential can only be realized by better understanding the functionality of the organ and developing the ability to translate this to the bioprinting methodologies. Considering the rate at which the field is currently expanding, it is reasonable to expect bioprinting to become an integral component of regenerative medicine. For this purpose, this paper discusses several factors that are critical for printing functional tissues including cell density, vascularization, innervation, heterogeneity, engraftment, mechanics, and tissue-specific function, and inform the reader with future directions in bioprinting complex and volumetric tissues.

Trends in Scientific Reports on Cartilage Bioprinting: Scoping Review

BACKGROUND

Satisfactory therapeutic strategies for cartilaginous lesion repair do not yet exist. This creates a challenge for surgeons and biomedical engineers and leads them to investigate the role of bioprinting and tissue engineering as viable treatments through orthopedic surgery, plastic surgery, and otorhinolaryngology. Recent increases in related scientific literature suggest that bioprinted cartilage may develop into a viable solution.

OBJECTIVE

The objectives of this review were to (1) synthesize the scientific advances published to date, (2) identify unresolved technical problems regarding human application, and (3) identify more effective ways for the scientific community to transfer their findings to clinicians.

METHODS

This scoping review considered articles published between 2009 and 2019 that were identified through searching PubMed, Scopus, Web of Science, and Google Scholar. Arksey and O'Malley's five-step framework was used to delimit and direct the initial search results, from which we established the following research questions: (1) What do authors of current research say about human application? (2) What necessary technical improvements are identified in the research? (3) On which issues do the authors agree? and (4) What future research priorities emerge in the studies? We used the Cohen kappa statistic to validate the interrater reliability.

RESULTS

The 13 articles included in the review demonstrated the feasibility of cartilage bioprinting in live animal studies. Some investigators are already considering short-term human experimentation, although technical limitations still need to be resolved. Both the use and manufacturing process of stem cells need to be standardized, and a consensus is needed regarding the composition of hydrogels. Using on-site printing strategies and predesigned implants may allow techniques to adapt to multiple situations. In addition, the predictive capacity of implant behavior may lead to optimal results.

CONCLUSIONS

Cartilage bioprinting for surgical applications is nearing its initial use in humans. Current research suggests that surgeons will soon be able to replace damaged tissue with bioprinted material.