Is 3D Printing Promising for Osteochondral Tissue Regeneration?

Advancement of 3D biofabrication in repairing and regeneration of cartilage defects

Three-dimensional (3D) bioprinting, an additive manufacturing technology, fabricates biomimetic tissues that possess natural structure and function. It involves precise deposition of bioinks, including cells, and bioactive factors, on basis of computer-aided 3D models. Articular cartilage injuries, a common orthopedic issue. Current repair methods, for instance microfracture procedure (MF), autologous chondrocyte implantation (ACI), and osteochondral autologous transfer surgery have been applied in clinical practice. However, each procedure has inherent limitation. For instance, MF surgery associates with increased subchondral cyst formation and brittle subchondral bone. ACI procedure involves two surgeries, and associate with potential risks infection and delamination of the regenerated cartilage. In addition, chondrocyte implantation's efficacy depends on the patient's weight, joint pathology, gender-related histological changes of cartilage, and hormonal influences that affect treatment and prognosis. So far, it is a still a grand challenge for achieving a clinical satisfactory in repairing and regeneration of cartilage defects using conditional strategies. 3D biofabrication provide a potential to fabricate biomimetic articular cartilage construct that has shown promise in specific cartilage repair and regeneration of patients. This review reported the techniques of 3D bioprinting applied for cartilage repair, and analyzed their respective merits and demerits, and limitations in clinical application. A summary of commonly used bioinks has been provided, along with an outlook on the challenges and prospects faced by 3D bioprinting in the application of cartilage tissue repair. It provided an overall review of current development and promising application of 3D biofabrication technology in articular cartilage repair.

In Vitro Engineered ECM-incorporated Hydrogels for Osteochondral Tissue Repair: A Cell-Free Approach

Prevalence of osteoarthritis has been increasing in aging populations, which has necessitated the use of advanced biomedical treatments. These involve grafts or delivering drug molecules entrapped in scaffolds. However, such treatments often show suboptimal therapeutic effects due to poor half-life and off-target effects of drug molecules. As a countermeasure, a 3D printable robust hydrogel-based tissue-repair platform is developed containing decellularized extracellular matrix (dECM) from differentiated mammalian cells as the therapeutic cargo. Here, pre-osteoblastic and pre-chondrogenic murine cells are differentiated in vitro, decellularized, and incorporated into methacrylated gelatin (GelMA) solutions to form osteogenic (GelO) and chondrogenic (GelC) hydrogels, respectively. Integrating the bioactive dECM from differentiated cell sources allows GelO and GelC to induce differentiation in human adipose-derived stem cells (hASCs) toward osteogenic and chondrogenic lineages. Further, GelO and GelC can be covalently adhered using a carbodiimide coupling reaction, forming a multi-layered hydrogel with potential application as a bioactive osteochondral plug. The designed multi-layered hydrogel can also induce differentiation of hASCs in vitro. In conclusion, the bioactive dECM carrying 3D printed robust hydrogel offers a promising new drug and cell-free therapeutic strategy for bone and cartilage repair and future osteoarthritis management.

Wood-Derived Hydrogels for Osteochondral Defect Repair

Repairing cartilage tissue is a serious global challenge. Herein, we focus on wood skeletal structures that are highly porous for cell penetration yet have load-bearing strength, and aim to synthesize wood-derived hydrogels with the ability to regenerate cartilage tissues. The hydrogels were synthesized by wood delignification and the subsequent intercalation of citric acid (CA), which is involved in tricarboxylic acid cycles and essential for energy production, and N-acetylglucosamine (NAG), which is a cartilage glycosaminoglycan, among cellulose microfibrils. CA and NAG intercalation increased the amorphous region of the cellulose microfibrils and endowed them with flexibility while maintaining the skeletal structure of the wood. Consequently, the CA-NAG-treated wood hydrogels became twistable and bendable, and the acquired stiffness, compressive strength, water content, and cushioning characteristics were similar to those of the cartilage. In rabbit femur cartilage defects, CA-NAG-treated wood hydrogels induced the differentiation of surrounding cells into chondrocytes. Consequently, the CA-NAG-treated wood hydrogels repaired cartilage defects, whereas the collagen scaffolds, delignified wood materials, and CA-treated wood hydrogels did not. The CA-NAG-treated wood hydrogels exhibit superior structural and mechanical characteristics over conventional cellulose-fiber-containing scaffolds. Furthermore, the CA-NAG-treated wood hydrogels can effectively repair cartilage on their own, whereas conventional natural and synthetic polymeric materials need to be combined with cells and growth factors to achieve a sufficient therapeutic effect. Therefore, the CA-NAG-treated wood hydrogels successfully address the limitations of current therapies that either fail to repair articular cartilage or sacrifice healthy cartilage. To our knowledge, this is the pioneer study on the utilization of thinned wood for tissue engineering, which will contribute to solving both global health and environmental problems and to creating a sustainable society.

Osteochondral Regeneration With Anatomical Scaffold 3D-Printing-Design Considerations for Interface Integration

There is a clinical need for osteochondral scaffolds with complex geometries for restoring articulating joint surfaces. To address that need, 3D-printing has enabled scaffolds to be created with anatomically shaped geometries and interconnected internal architectures, going beyond simple plug-shaped scaffolds that are limited to small, cylindrical, focal defects. A key challenge for restoring articulating joint surfaces with 3D-printed constructs is the mechanical loading environment, particularly to withstand delamination or mechanical failure. Although the mechanical performance of interfacial scaffolds is essential, interface strength testing has rarely been emphasized in prior studies with stratified scaffolds. In the pioneering studies where interface strength was assessed, varying methods were employed, which has made direct comparisons difficult. Therefore, the current review focused on 3D-printed scaffolds for osteochondral applications with an emphasis on interface integration and biomechanical evaluation. This 3D-printing focus included both multiphasic cylindrical scaffolds and anatomically shaped scaffolds. Combinations of different 3D-printing methods (e.g., fused deposition modeling, stereolithography, bioprinting with pneumatic extrusion of cell-laden hydrogels) have been employed in a handful of studies to integrate osteoinductive and chondroinductive regions into a single scaffold. Most 3D-printed multiphasic structures utilized either an interdigitating or a mechanical interlocking design to strengthen the construct interface and to prevent delamination during function. The most effective approach to combine phases may be to infill a robust 3D-printed osteal polymer with an interlocking chondral phase hydrogel. Mechanical interlocking is therefore recommended for scaling up multiphasic scaffold applications to larger anatomically shaped joint surface regeneration. For the evaluation of layer integration, the interface shear test is recommended to avoid artifacts or variability that may be associated with alternative approaches that require adhesives or mechanical grips. The 3D-printing literature with interfacial scaffolds provides a compelling foundation for continued work toward successful regeneration of injured or diseased osteochondral tissues in load-bearing joints such as the knee, hip, or temporomandibular joint.

Highly Flexible Methyl Cellulose/Gelatin Hydrogels for Potential Cartilage Tissue Engineering Applications

Cartilage damage resulting from trauma demonstrates a poor capacity for repair due to its avascular nature. Cartilage tissue engineering offers a unique therapeutic option for cartilage recovery. In this study, methylcellulose (MC)/gelatin (GEL) hydrogels (MC10G20, MC12.5G20, MC15G20, and MC17.5G20) were developed to assess and compare their chemical, mechanical, and biological characteristics for cartilage repair. First, the interaction between MC and GEL after blending and subsequent crosslinking with EDC/NHS was confirmed by using FTIR. Mechanical tests under compression test revealed that hydrogels' resistance to both elastic and plastic deformation increased with higher wt.% of MC. The % strain of the hydrogels doubled with the addition of MC, likely due to abundant hydrogen bonding between polymeric chains. Furthermore, the compressive modulus of MC/GEL hydrogels was approximately 0.2 MPa, closely matching modulus of human cartilage tissue. Similarly, the % water retention capacity of the hydrogels increased over the 7 days as the MC content increased. Additionally, SEM images showed that the incorporation of MC to GEL introduced porosity with the diameters ranging from 10 to 50 μm, similar to the size of pores in native cartilage. In vitro cell culture studies confirmed the biocompatibility of MC/GEL hydrogels. Fluorescence staining showed a 2.5-fold increase in F-actin staining following the incorporation of MC into the hydrogels. Overall, this study highlights the potential of MC/GEL hydrogels for cartilage tissue engineering, however, further research is required to assess its full potential.

Smart Biopolymer Scaffolds Based on Hyaluronic Acid and Carbonyl Iron Microparticles: 3D Printing, Magneto-Responsive, and Cytotoxicity Study

This study deals with utilization of the hyaluronic acid (HA) and carbonyl iron (CI) microparticles to fabricate the magneto-responsive hydrogel scaffolds that can provide triggered functionality upon application of an external magnetic field. The various combinations of the HA and CI were investigated from the rheological and viscoelastic point of view to clearly show promising behavior in connection to 3D printing. Furthermore, the swelling capabilities with water diffusion kinetics were also elucidated. Magneto-responsive performance of bulk hydrogels and their noncytotoxic nature were investigated,, and all hydrogels showed cell viability in the range 75-85%. The 3D printing of such developed systems was successful, and fundamental characterization of the scaffolds morphology (SEM and CT) has been presented. The magnetic activity of the final scaffolds was confirmed at a very low magnetic field strength of 140 kA/m, and such a scaffold also provides very good biocompatibility with NIH/3T3 fibroblasts.

A modular approach to 3D-printed bilayer composite scaffolds for osteochondral tissue engineering

Prolonged osteochondral tissue engineering damage can result in osteoarthritis and decreased quality of life. Multiphasic scaffolds, where different layers model different microenvironments, are a promising treatment approach, yet stable joining between layers during fabrication remains challenging. To overcome this problem, in this study, a bilayer scaffold for osteochondral tissue regeneration was fabricated using 3D printing technology which containing a layer of PCL/hydroxyapatite (HA) nanoparticles and another layer of PCL/gelatin with various concentrations of fibrin (10, 20 and 30 wt.%). These printed scaffolds were evaluated with SEM (Scanning Electron Microscopy), FTIR (Fourier Transform Infrared Spectroscopy) and mechanical properties. The results showed that the porous scaffolds fabricated with pore size of 210-255 µm. Following, the ductility increased with the further addition of fibrin in bilayer composites which showed these composites scaffolds are suitable for the cartilage part of osteochondral. Also, the contact angle results demonstrated the incorporation of fibrin in bilayer scaffolds based on PCL matrix, can lead to a decrease in contact angle and result in the improvement of hydrophilicity that confirmed by increasing the degradation rate of scaffolds containing further fibrin percentage. The bioactivity study of bilayer scaffolds indicated that both fibrin and hydroxyapatite can significantly improve the cell attachment on fabricated scaffolds. The MTT assay, DAPI and Alizarin red tests of bilayer composite scaffolds showed that samples containing 30% fibrin have the more biocompatibility than that of samples with 10 and 20% fibrin which indicated the potential of this bilayer scaffold for osteochondral tissue regeneration.

Controlled Mechanical Property Gradients Within a Digital Light Processing Printed Hydrogel-Composite Osteochondral Scaffold

Tissue engineered scaffolds are needed to support physiological loads and emulate the micrometer-scale strain gradients within tissues that guide cell mechanobiological responses. We designed and fabricated micro-truss structures to possess spatially varying geometry and controlled stiffness gradients. Using a custom projection microstereolithography (μSLA) system, using digital light projection (DLP), and photopolymerizable poly(ethylene glycol) diacrylate (PEGDA) hydrogel monomers, three designs with feature sizes < 200 μm were formed: (1) uniform structure with 1 MPa structural modulus ( E ) designed to match equilibrium modulus of healthy articular cartilage, (2) E  = 1 MPa gradient structure designed to vary strain with depth, and (3) osteochondral bilayer with distinct cartilage ( E  = 1 MPa) and bone ( E  = 7 MPa) layers. Finite element models (FEM) guided design and predicted the local mechanical environment. Empty trusses and poly(ethylene glycol) norbornene hydrogel-infilled composite trusses were compressed during X-ray microscopy (XRM) imaging to evaluate regional stiffnesses. Our designs achieved target moduli for cartilage and bone while maintaining 68-81% porosity. Combined XRM imaging and compression of empty and hydrogel-infilled micro-truss structures revealed regional stiffnesses that were accurately predicted by FEM. In the infilling hydrogel, FEM demonstrated the stress-shielding effect of reinforcing structures while predicting strain distributions. Composite scaffolds made from stiff μSLA-printed polymers support physiological load levels and enable controlled mechanical property gradients which may improve in vivo outcomes for osteochondral defect tissue regeneration. Advanced 3D imaging and FE analysis provide insights into the local mechanical environment surrounding cells in composite scaffolds.

Materials Suitable for Osteochondral Regeneration

Osteochondral defects affect articular cartilage, calcified cartilage, and subchondral bone. The main problem that they cause is a different behavior of cell tissue in the osteochondral and bone part. Articular cartilage is composed mainly of collagen II, glycosaminoglycan (GAG), and water, and has a low healing ability due to a lack of vascularization. However, bone tissue is composed of collagen I, proteoglycans, and inorganic composites such as hydroxyapatite. Due to the discrepancy between the characters of these two parts, it is difficult to find materials that will meet all the structural and other requirements for effective regeneration. When designing a scaffold for an osteochondral defect, a variety of materials are available, e.g., polymers (synthetic and natural), inorganic particles, and extracellular matrix (ECM) components. All of them require the accurate characterization of the prepared materials and a number of in vitro and in vivo tests before they are applied to patients. Taken in concert, the final material needs to mimic the structural, morphological, chemical, and cellular demands of the native tissue. In this review, we present an overview of the structure and composition of the osteochondral part, especially synthetic materials with additives appropriate for healing osteochondral defects. Finally, we summarize in vitro and in vivo methods suitable for evaluating materials for restoring osteochondral defects.

Multilayered Shape-Morphing Scaffolds with a Hierarchical Structure for Uterine Tissue Regeneration

Owing to dysfunction of the uterus, millions of couples around the world suffer from infertility. Different from conventional treatments, tissue engineering provides a new and promising approach to deal with difficult problems such as human tissue or organ failure. Adopting scaffold-based tissue engineering, three-dimensional (3D) porous scaffolds in combination with stem cells and appropriate biomolecules may be constructed for uterine tissue regeneration. In this study, a hierarchical tissue engineering scaffold, which mimicked the uterine tissue structure and functions, was designed, and the biomimicking scaffolds were then successfully fabricated using solvent casting, layer-by-layer assembly, and 3D bioprinting techniques. For the multilayered, hierarchical structured scaffolds, poly(l-lactide-co-trimethylene carbonate) (PLLA-co-TMC, "PLATMC" in short) and poly(lactic acid-co-glycolic acid) (PLGA) blends were first used to fabricate the shape-morphing layer of the scaffolds, which was to mimic the function of myometrium in uterine tissue. The PLATMC/PLGA polymer blend scaffolds were highly stretchable. Subsequently, after etching of the PLATMC/PLGA surface and employing estradiol (E2), polydopamine (PDA), and hyaluronic acid (HA), PDA@E2/HA multilayer films were formed on PLATMC/PLGA scaffolds to build an intelligent delivery platform to enable controlled and sustained release of E2. The PDA@E2/HA multilayer films also improved the biological performance of the scaffold. Finally, a layer of bone marrow-derived mesenchymal stem cell (BMSC)-laden hydrogel [which was a blend of gelatin methacryloyl (GelMA) and gelatin (Gel)] was 3D printed on the PDA@E2/HA multilayer films of the scaffold, thereby completing the construction of the hierarchical scaffold. BMSCs in the GelMA/Gel hydrogel layer exhibited excellent cell viability and could spread and be released eventually upon biodegradation of the GelMA/Gel hydrogel. It was shown that the hierarchically structured scaffolds could evolve from the initial flat shape into the tubular structure completely in an aqueous environment at 37 °C, fulfilling the requirement for curved scaffolds for uterine tissue engineering. The biomimicking scaffolds with a hierarchical structure and curved shape, high stretchability, and controlled and sustained E2 release appear to be very promising for uterine tissue regeneration.

Exploring the Biomedical Potential of PLA/Dysprosium Phosphate Composites via Extrusion-Based 3D Printing: Design, Morphological, Mechanical, and Multimodal Imaging and Finite Element Modeling

The present investigation demonstrates the feasibility of dysprosium phosphate (DyPO4) as an efficient additive in polylactide (PLA) to develop 3D printed scaffolds through the material extrusion (MEX) principle for application in bone tissue engineering. Initially, uniform sized particles of DyPO4 with tetragonal crystal setting are obtained and subsequently blended with different concentrations of PLA to extrude in the form of filaments. A maximum of 20 wt % DyPO4 in PLA matrix has been successfully drawn to yield a defect free filament. The resultant filaments were 3D printed through material extrusion methodology. The structural and morphological analysis confirmed the successful reinforcement of DyPO4 throughout the PLA matrix in all of the 3D printed components. All of the PLA/DyPO4 composites exhibited magnetic resonance imaging and computed tomography contrasting properties, which were dependent on the dysprosium content in the PLA matrix. The detailed mechanical evaluation of the 3D printed PLA/DyPO4 composites ensured good strength accomplished by the reinforcement of 5 wt % DyPO4 in PLA matrix, beyond which a gradual decline in the strength is noticed. Representative volume elements models were developed to realize the intrinsic property of the PLA/DyPO4 composite, and finite element analysis under both static and dynamic loading conditions has been performed to account for the reliability of experimental results.

Vat Polymerization by Three-Dimensional Printing and Curing of Antibacterial Zinc Oxide Nanoparticles Embedded in Poly(ethylene glycol) Diacrylate for Biomedical Applications

Digital light processing (DLP) is a vat photopolymerization 3D printing technique with increasingly broad application prospects, particularly in personalized medicine, such as the creation of medical devices. Different resins and printing parameters affect the functionality of these devices. One of the many problems that biomedical implants encounter is inflammation and bacteria growth. For this reason, many studies turn to the addition of antibacterial agents to either the bulk material or as a coating. Zinc oxide nanoparticles (ZnO NPs) have shown desirable properties, including antibacterial activity with negligible toxicity to the human body, allowing their use in a wide range of applications. In this project, we developed a resin of poly(ethylene glycol) diacrylate (PEGDA), a cross-linker known for its excellent mechanical properties and high biocompatibility in a 4:1 weight ratio of monomers to water. The material's mechanical properties (Young's modulus, maximum elongation, and ultimate tensile strength) were found similar to those of human cartilage. Furthermore, the ZnO NPs embedding matrix showed strong antibacterial activity against Escherichia coli (E. coli) and Staphylococcus aureus (S.A.). As the ZnO NPs ratio was changed, only a minor effect on the mechanical properties of the material was observed, whereas strong antibacterial properties against both bacteria were achieved in the case of 1.5 wt.% NPs.