Perspectives in Multiphasic Osteochondral Tissue Engineering

Advances in Tissue Engineering and Its Future in Regenerative Medicine Compared to Traditional Reconstructive Techniques: A Comparative Analysis

Tissue engineering represents a revolutionary approach in regenerative medicine, offering promising alternatives to traditional reconstructive techniques. This systematic review explores recent advances in tissue engineering, comparing their efficacy, postoperative outcomes, and patient satisfaction to conventional methods. A comprehensive literature search was conducted across PubMed, Cochrane Library, and Google Scholar, covering studies published from 2000 to 2024. Fourteen studies were selected for final analysis based on inclusion criteria focusing on outcomes such as scar quality, postoperative pain, and patient satisfaction. The review demonstrated that tissue engineering techniques consistently provided superior cosmetic outcomes with minimal scarring compared to traditional methods. Patients undergoing tissue-engineered procedures experienced mild-to-moderate postoperative pain with rapid resolution, whereas traditional techniques resulted in moderate to severe pain requiring extended management. Furthermore, patients treated with tissue engineering reported high satisfaction rates due to improved cosmetic and functional outcomes. Despite challenges such as ensuring adequate vascularization, controlling scaffold degradation, and overcoming regulatory and cost barriers, ongoing research and development are essential to fully realize the potential of these innovative therapies. Tissue engineering offers significant advantages over traditional reconstructive techniques and has the potential to profoundly improve patient care in regenerative medicine.

Biofabrication Strategies for Oral Soft Tissue Regeneration

Gingival recession, a prevalent condition affecting the gum tissues, is characterized by the exposure of tooth root surfaces due to the displacement of the gingival margin. This review explores conventional treatments, highlighting their limitations and the quest for innovative alternatives. Importantly, it emphasizes the critical considerations in gingival tissue engineering leveraging on cells, biomaterials, and signaling factors. Successful tissue-engineered gingival constructs hinge on strategic choices such as cell sources, scaffold design, mechanical properties, and growth factor delivery. Unveiling advancements in recent biofabrication technologies like 3D bioprinting, electrospinning, and microfluidic organ-on-chip systems, this review elucidates their precise control over cell arrangement, biomaterials, and signaling cues. These technologies empower the recapitulation of microphysiological features, enabling the development of gingival constructs that closely emulate the anatomical, physiological, and functional characteristics of native gingival tissues. The review explores diverse engineering strategies aiming at the biofabrication of realistic tissue-engineered gingival grafts. Further, the parallels between the skin and gingival tissues are highlighted, exploring the potential transfer of biofabrication approaches from skin tissue regeneration to gingival tissue engineering. To conclude, the exploration of innovative biofabrication technologies for gingival tissues and inspiration drawn from skin tissue engineering look forward to a transformative era in regenerative dentistry with improved clinical outcomes.

Recent Advances in Scaffolds for Guided Bone Regeneration

The rehabilitation of alveolar bone defects of moderate to severe size is often challenging. Currently, the therapeutic approaches used include, among others, the guided bone regeneration technique combined with various bone grafts. Although these techniques are widely applied, several limitations and complications have been reported such as morbidity, suboptimal graft/membrane resorption rate, low structural integrity, and dimensional stability. Thus, the development of biomimetic scaffolds with tailor-made characteristics that can modulate cell and tissue interaction may be a promising tool. This article presents a critical consideration in scaffold's design and development while also providing information on various fabrication methods of these nanosystems. Their utilization as delivery systems will also be mentioned.

Microporous/Macroporous Polycaprolactone Scaffolds for Dental Applications

This study leverages the advantages of two fabrication techniques, namely, melt-extrusion-based 3D printing and porogen leaching, to develop multiphasic scaffolds with controllable properties essential for scaffold-guided dental tissue regeneration. Polycaprolactone-salt composites are 3D-printed and salt microparticles within the scaffold struts are leached out, revealing a network of microporosity. Extensive characterization confirms that multiscale scaffolds are highly tuneable in terms of their mechanical properties, degradation kinetics, and surface morphology. It can be seen that the surface roughness of the polycaprolactone scaffolds (9.41 ± 3.01 µm) increases with porogen leaching and the use of larger porogens lead to higher roughness values, reaching 28.75 ± 7.48 µm. Multiscale scaffolds exhibit improved attachment and proliferation of 3T3 fibroblast cells as well as extracellular matrix production, compared with their single-scale counterparts (an approximate 1.5- to 2-fold increase in cellular viability and metabolic activity), suggesting that these structures could potentially lead to improved tissue regeneration due to their favourable and reproducible surface morphology. Finally, various scaffolds designed as a drug delivery device were explored by loading them with the antibiotic drug cefazolin. These studies show that by using a multiphasic scaffold design, a sustained drug release profile can be achieved. The combined results strongly support the further development of these scaffolds for dental tissue regeneration applications.

Osteochondral Interface: Regenerative Engineering and Challenges

Osteochondral (OC) defects are debilitating for patients and represent a significant clinical problem for orthopedic surgeons as well as regenerative engineers due to their potential complications, which are likely to lead to osteoarthritis and related diseases. If they remain untreated or are treated suboptimally, OC lesions are known to impact the articular cartilage and the transition from cartilage to bone, that is, the cartilage-bone interface. An important component of the OC interface, that is, a selectively permeable membrane, the tidemark, still remains unaddressed in more than 90% of the published research in the past decade. This review focuses on the structure, composition, and function of the OC interface, regenerative engineering attempts with different scaffolding strategies and challenges ahead of us in recapitulating the native OC interface. There are different schools of thought regarding the structure of the native OC interface: stratified and graded. The former assumes the cartilage-to-bone interface to be hierarchically divided into distinct yet continuous zones of uncalcified cartilage-calcified cartilage-subchondral bone. The latter assumes the interface is continuously graded, that is, formed by an infinite number of layers. The cellular composition of the interface, either in respective layers or continuously changing in a graded manner, is chondrocytes, hypertrophic chondrocytes, and osteoblasts as moved from cartilage to bone. Functionally, the interface is assumed to play a role in enabling a smooth transition of loads exerted on the cartilage surface to the bone underneath. Regenerative engineering involves, first, a characterization of the native OC interface in terms of the composition, structure, and function, and, then, proposes the appropriate biomaterials, cells, and biomolecules either alone or in combination to eventually form a structure that mimics and functionally behaves similar to the native interface. The major challenge regarding regeneration of the OC interface appears to lie, in addition to others, in the formation of tidemark, which is a thin membrane separating the OC interface into two distinct zones: the avascular OC interface and the vascular OC interface. There is a significant amount of literature on regenerative approaches to the OC interface; however, only a small portion of them consider the importance of tidemark. Therefore, this review aims at highlighting the significance of the structural organization of the components of the OC interface and increasing the awareness of the orthopedics community regarding the importance of tidemark formation after clinical interventions or regenerative engineering attempts.

Multicompartmental Scaffolds for Coordinated Periodontal Tissue Engineering

Successful periodontal repair and regeneration requires the coordinated responses from soft and hard tissues as well as the soft tissue-to-bone interfaces. Inspired by the hierarchical structure of native periodontal tissues, tissue engineering technology provides unique opportunities to coordinate multiple cell types into scaffolds that mimic the natural periodontal structure in vitro. In this study, we designed and fabricated highly ordered multicompartmental scaffolds by melt electrowriting, an advanced 3-dimensional (3D) printing technique. This strategy attempted to mimic the characteristic periodontal microenvironment through multicompartmental constructs comprising 3 tissue-specific regions: 1) a bone compartment with dense mesh structure, 2) a ligament compartment mimicking the highly aligned periodontal ligaments (PDLs), and 3) a transition region that bridges the bone and ligament, a critical feature that differentiates this system from mono- or bicompartmental alternatives. The multicompartmental constructs successfully achieved coordinated proliferation and differentiation of multiple cell types in vitro within short time, including both ligamentous- and bone-derived cells. Long-term 3D coculture of primary human osteoblasts and PDL fibroblasts led to a mineral gradient from calcified to uncalcified regions with PDL-like insertions within the transition region, an effect that is challenging to achieve with mono- or bicompartmental platforms. This process effectively recapitulates the key feature of interfacial tissues in periodontium. Collectively, this tissue-engineered approach offers a fundament for engineering periodontal tissue constructs with characteristic 3D microenvironments similar to native tissues. This multicompartmental 3D printing approach is also highly compatible with the design of next-generation scaffolds, with both highly adjustable compartmentalization properties and patient-specific shapes, for multitissue engineering in complex periodontal defects.

3D-printed mesoporous bioactive glass/GelMA biomimetic scaffolds for osteogenic/cementogenic differentiation of periodontal ligament cells

Integrated regeneration of periodontal tissues remains a challenge in current clinical applications. Due to the tunable physical characteristics and the precise control of the scaffold microarchitecture, three-dimensionally (3D) printed gelatin methacryloyl (GelMA)-based scaffold has emerged as a promising strategy for periodontal tissue regeneration. However, the optimization of the printing biomaterial links the formulation and the relationship between the composition and structures of the printed scaffolds and their comprehensive properties (e.g. mechanical strength, degradation, and biological behaviors) remains unclear. Here, in this work, a novel mesoporous bioactive glass (BG)/GelMA biomimetic scaffold with a large pore size (∼300 μm) was developed by extrusion-based 3D printing. Our results showed that the incorporation of mesoporous bioactive glass nanoparticles (BG NPs) significantly improved shape fidelity, surface roughness, and bioactivity of 3D-printed macroporous GelMA scaffolds, resulting in the enhanced effects on cell attachment and promoting osteogenic/cementogenic differentiation in human periodontal ligament cells. The excellent maintenance of the macropore structure, the visibly improved cells spreading, the release of bioactive ions (Si4+, Ca2+), the upregulation of gene expressions of osteogenesis and cementogensis, and the increase in alkaline phosphatase (ALP) activity and calcium nodules suggested that BG NPs could endow GelMA-based scaffolds with excellent structural stability and the ability to promote osteogenic/cementogenic differentiation. Our findings demonstrated the great potential of the newly formulated biomaterial inks and biomimetic BG/GelMA scaffolds for being used in periodontal tissue regeneration and provide important insights into the understanding of cell-scaffold interaction in promoting the regeneration of functional periodontal tissues.

Novel approaches for periodontal tissue engineering

The periodontal complex involves the hard and soft tissues which support dentition, comprised of cementum, bone, and the periodontal ligament (PDL). Periodontitis, a prevalent infectious disease of the periodontium, threatens the integrity of these tissues and causes irreversible damage. Periodontal therapy aims to repair and ultimately regenerate these tissues toward preserving native dentition and improving the physiologic integration of dental implants. The PDL contains multipotent stem cells, which have a robust capacity to differentiate into various types of cells to form the PDL, cementum, and alveolar bone. Selection of appropriate growth factors and biomaterial matrices to facilitate periodontal regeneration are critical to recapitulate the physiologic organization and function of the periodontal complex. Herein, we discuss the current state of clinical periodontal regeneration including a review of FDA-approved growth factors. We will highlight advances in preclinical research toward identifying additional growth factors capable of robust repair and biomaterial matrices to augment regeneration similarly and synergistically, ultimately improving periodontal regeneration's predictability and long-term efficacy. This review should improve the readers' understanding of the molecular and cellular processes involving periodontal regeneration essential for designing comprehensive therapeutic approaches.

Silk fibroin-based biomaterials for cartilage/osteochondral repair

Osteoarthritis (OA) is a common joint disease with a high disability rate. In addition, OA not only causes great physiological and psychological harm to patients, but also puts great pressure on the social healthcare system. Pathologically, the disintegration of cartilage and the lesions of subchondral bone are related to OA. Currently, tissue engineering, which is expected to overcome the defects of existing treatment methods, had a lot of research in the field of cartilage/osteochondral repair. Silk fibroin (SF), as a natural macromolecular material with good biocompatibility, unique mechanical properties, excellent processability and degradability, holds great potential in the field of tissue engineering. Nowadays, SF had been prepared into various materials to adapt to the demands of cartilage/osteochondral repair. SF-based biomaterials can also be functionally modified to enhance repair performance further. In this review, the preparation methods, types, structures, mechanical properties, and functional modifications of SF-based biomaterials used for cartilage/osteochondral repair are summarized and discussed. We hope that this review will provide a reference for the design and development of SF-based biomaterials in cartilage/osteochondral repair field.

In vivo evaluation of additively manufactured multi-layered scaffold for the repair of large osteochondral defects

The repair of osteochondral defects is one of the major clinical challenges in orthopaedics. Well-established osteochondral tissue engineering methods have shown promising results for the early treatment of small defects. However, less success has been achieved for the regeneration of large defects, which is mainly due to the mechanical environment of the joint and the heterogeneous nature of the tissue. In this study, we developed a multi-layered osteochondral scaffold to match the heterogeneous nature of osteochondral tissue by harnessing additive manufacturing technologies and combining the established art laser sintering and material extrusion techniques. The developed scaffold is based on a titanium and polylactic acid matrix-reinforced collagen "sandwich" composite system. The microstructure and mechanical properties of the scaffold were examined, and its safety and efficacy in the repair of large osteochondral defects were tested in an ovine condyle model. The 12-week in vivo evaluation period revealed extensive and significantly higher bone in-growth in the multi-layered scaffold compared with the collagen-HAp scaffold, and the achieved stable mechanical fixation provided strong support to the healing of the overlying cartilage, as demonstrated by hyaline-like cartilage formation. The histological examination showed that the regenerated cartilage in the multi-layer scaffold group was superior to that formed in the control group. Chondrogenic genes such as aggrecan and collagen-II were upregulated in the scaffold and were higher than those in the control group. The findings showed the safety and efficacy of the cell-free "translation-ready" osteochondral scaffold, which has the potential to be used in a one-step surgical procedure for the treatment of large osteochondral defects.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s42242-021-00177-w.

Joint Surface Lesions in the Knee Treated with an Acellular Aragonite-Based Scaffold: A 3-Year Follow-Up Case Series

OBJECTIVE

The study aimed to evaluate the clinical outcome and repair capacity of a cell-free aragonite-based scaffold in patients with an isolated symptomatic joint surface lesion (JSL) of the knee.

DESIGN

Thirteen patients (age 33.5 ± 8.9; female 23%; body mass index 25.3 ± 3.4, K/L [Kellgren-Lawrence] 1.8) with a JSL (2.6 ± 1.7 cm² [1.0-7.5 cm²]) of the distal femur were enrolled in a single-center prospective case series. Safety and clinical outcome was assessed by the KOOS (Knee Injury and Osteoarthritis Outcome Score), IKDC (International Knee Documentation Committee), Lysholm, and Tegner activity scale at baseline and 6, 12, 18, 24, and 36 months follow-up. The MOCART 2.0 and scaffold integration were evaluated on magnetic resonance imaging at 12, 24, and 36 months postoperatively.

RESULTS

Primary outcome (KOOS pain) improved with 36.5 ± 14.7 points at 12 months (P = 0.002) and 41.2 ± 14.7 points at 36 months (P = 0.002) follow-up. Similar increasing trends were observed for the other KOOS subscales, IKDC, and Lysholm score, which were significantly better at each follow-up time point relative to baseline (P < 0.05). Activity level increased from 2.75 ± 1.6 to 4.6 ± 2.2 points at final follow-up (P = 0.07). The MOCART was 61.7 ± 12.6 at 12 months and 72.9 ± 13.0 at 36 months postoperatively. Sixty-six to 100% implant integration and remodeling was observed in 73.3% cases at 36 months. No serious adverse events were reported.

CONCLUSION

The study demonstrated that the biphasic aragonite-based scaffold is a safe and clinically effective implant for treating small-medium sized JSLs of the distal femur in a young and active patient cohort. The implant showed satisfying osteointegration and restoration of the osteochondral unit up to 3 years postimplantation.

Scaffolds in Periodontal Regenerative Treatment

Successful periodontal regeneration requires the hierarchical reorganization of multiple tissues including periodontal ligament, cementum, alveolar bone, and gingiva. The limitation of conventional regenerative therapies has been attracting research interest in tissue engineering-based periodontal therapies where progenitor cells, scaffolds, and bioactive molecules are delivered. Scaffolds offer not only structural support but also provide geometrical clue to guide cell fate. Additionally, functionalization improves bioactive properties to the scaffold. Various scaffold designs have been proposed for periodontal regeneration. These include the fabrication of biomimetic periodontal extracellular matrix, multiphasic scaffolds with tissue-specific layers, and personalized 3D printed scaffolds. This review summarizes the basic concept as well as the recent advancement of scaffold designing and fabrication for periodontal regeneration and provides an insight of future clinical translation.

Biomimetic and mechanically supportive 3D printed scaffolds for cartilage and osteochondral tissue engineering using photopolymers and digital light processing

Successful 3D scaffold designs for musculoskeletal tissue engineering necessitate full consideration of the form and function of the tissues of interest. When designing structures for engineering cartilage and osteochondral tissues, one must reconcile the need to develop a mechanically robust system that maintains the health of cells embedded in the scaffold. In this work, we present an approach that decouples the mechanical and biochemical needs and allows for the independent development of the structural and cellular niches in a scaffold. Using the highly tuned capabilities of digital light processing-based stereolithography, structures with complex architectures are achieved over a range of effective porosities and moduli. The 3D printed structure is infilled with mesenchymal stem cells and soft biomimetic hydrogels, which are specifically formulated with extracellular matrix analogs and tethered growth factors to provide selected biochemical cues for the guided differentiation towards chondrogenesis and osteogenesis. We demonstrate the ability to utilize these structures to (a) infill a focal chondral defect and mitigate macroscopic and cellular level changes in the cartilage surrounding the defect, and (b) support the development of a stratified multi-tissue scaffold for osteochondral tissue engineering.

Bilayered scaffold with 3D printed stiff subchondral bony compartment to provide constant mechanical support for long-term cartilage regeneration

BACKGROUND/OBJECTIVE

We seek to figure out the effect of stable and powerful mechanical microenvironment provided by Ti alloy as a part of subchondral bone scaffold on long-term cartilage regeneration.Methods: we developed a bilayered osteochondral scaffold based on the assumption that a stiff subchondral bony compartment would provide stable mechanical support for cartilage regeneration and enhance subchondral bone regeneration. The subchondral bony compartment was prepared from 3D printed Ti alloy, and the cartilage compartment was created from a freeze-dried collagen sponge, which was reinforced by poly-lactic-co-glycolic acid (PLGA).

RESULTS

In vitro evaluations confirmed the biocompatibility of the scaffold materials, while in vivo evaluations demonstrated that the mechanical support provided by 3D printed Ti alloy layer plays an important role in the long-term regeneration of cartilage by accelerating osteochondral formation and its integration with the adjacent host tissue in osteochondral defect model at rabbit femoral trochlea after 24 weeks.

CONCLUSION

Mechanical support provided by 3D printing Ti alloy promotes cartilage regeneration by promoting subchondral bone regeneration and providing mechanical support platform for cartilage synergistically.

TRANSLATIONAL POTENTIAL STATEMENT

The raw materials used in our double-layer osteochondral scaffolds are all FDA approved materials for clinical use. 3D printed titanium alloy scaffolds can promote bone regeneration and provide mechanical support for cartilage regeneration, which is very suitable for clinical scenes of osteochondral defects. In fact, we are conducting clinical trials based on our scaffolds. We believe that in the near future, the scaffold we designed and developed can be formally applied in clinical practice.

Hard Dental Tissues Regeneration-Approaches and Challenges

With the development of the modern concept of tissue engineering approach and the discovery of the potential of stem cells in dentistry, the regeneration of hard dental tissues has become a reality and a priority of modern dentistry. The present review reports the recent advances on stem-cell based regeneration strategies for hard dental tissues and analyze the feasibility of stem cells and of growth factors in scaffolds-based or scaffold-free approaches in inducing the regeneration of either the whole tooth or only of its component structures.

Characterization of Properties, In Vitro and In Vivo Evaluation of Calcium Phosphate/Amino Acid Cements for Treatment of Osteochondral Defects

Novel calcium phosphate cements containing a mixture of four amino acids, glycine, proline, hydroxyproline and either lysine or arginine (CAL, CAK) were characterized and used for treatment of artificial osteochondral defects in knee. It was hypothesized that an enhanced concentration of extracellular collagen amino acids (in complex mixture), in connection with bone cement in defect sites, would support the healing of osteochondral defects with successful formation of hyaline cartilage and subchondral bone. Calcium phosphate cement mixtures were prepared by in situ reaction in a planetary ball mill at aseptic conditions and characterized. It was verified that about 30–60% of amino acids remained adsorbed on hydroxyapatite particles in cements and the addition of amino acids caused around 60% reduction in compressive strength and refinement of hydroxyapatite particles in their microstructure. The significant over-expression of osteogenic genes after the culture of osteoblasts was demonstrated in the cement extracts containing lysine and compared with other cements. The cement pastes were inserted into artificial osteochondral defects in the medial femoral condyle of pigs and, after 3 months post-surgery, tissues were analyzed macroscopically, histologically, immunohistochemically using MRI and X-ray methods. Analysis clearly showed the excellent healing process of artificial osteochondral defects in pigs after treatment with CAL and CAK cements without any inflammation, as well as formation of subchondral bone and hyaline cartilage morphologically and structurally identical to the original tissues. Good integration of the hyaline neocartilage with the surrounding tissue, as well as perfect interconnection between the neocartilage and new subchondral bone tissue, was demonstrated. Tissues were stable after 12 months’ healing.

Current and future trends in periodontal tissue engineering and bone regeneration

Periodontal tissue engineering involves a multi-disciplinary approach towards the regeneration of periodontal ligament, cementum and alveolar bone surrounding teeth, whereas bone regeneration specifically applies to ridge reconstruction in preparation for future implant placement, sinus floor augmentation and regeneration of peri-implant osseous defects. Successful periodontal regeneration is based on verifiable cementogenesis on the root surface, oblique insertion of periodontal ligament fibers and formation of new and vital supporting bone. Ultimately, regenerated periodontal and peri-implant support must be able to interface with surrounding host tissues in an integrated manner, withstand biomechanical forces resulting from mastication, and restore normal function and structure. Current regenerative approaches utilized in everyday clinical practice are mainly guided tissue/bone regeneration-based. Although these approaches have shown positive outcomes for small and medium-sized defects, predictability of clinical outcomes is heavily dependent on the defect morphology and clinical case selection. In many cases, it is still challenging to achieve predictable regenerative outcomes utilizing current approaches. Periodontal tissue engineering and bone regeneration (PTEBR) aims to improve the state of patient care by promoting reconstitution of damaged and lost tissues through the use of growth factors and signaling molecules, scaffolds, cells and gene therapy. The present narrative review discusses key advancements in PTEBR including current and future trends in preclinical and clinical research, as well as the potential for clinical translatability.

Biomimetic Hybrid Systems for Tissue Engineering

Tissue engineering approaches appear nowadays highly promising for the regeneration of injured/diseased tissues. Biomimetic scaffolds are continuously been developed to act as structural support for cell growth and proliferation as well as for the delivery of cells able to be differentiated, and also of bioactive molecules like growth factors and even signaling cues. The current research concerns materials employed to develop biological scaffolds with improved features as well as complex preparation techniques. In this work, hybrid systems based on natural polymers are discussed and the efforts focused to provide new polymers able to mimic proteins and DNA are extensively explained. Progress on the scaffold fabrication technique is mentioned, those processes based on solution and melt electrospinning or even on their combination being mainly discussed. Selection of the appropriate hybrid technology becomes vital to get optimal architecture to reasonably accomplish the final applications. Representative examples of the recent possibilities on tissue regeneration are finally given.

Advances and prospects in biomimetic multilayered scaffolds for articular cartilage regeneration

Due to the sophisticated hierarchical structure and limited reparability of articular cartilage (AC), the ideal regeneration of AC defects has been a major challenge in the field of regenerative medicine. As defects progress, they often extend from the cartilage layer to the subchondral bone and ultimately lead to osteoarthritis. Tissue engineering techniques bring new hope for AC regeneration. To meet the regenerative requirements of the heterogeneous and layered structure of native AC tissue, a substantial number of multilayered biomimetic scaffolds have been studied. Ideal multilayered scaffolds should generate zone-specific functional tissue similar to native AC tissue. This review focuses on the current status of multilayered scaffolds developed for AC defect repair, including design strategies based on the degree of defect severity and the zone-specific characteristics of AC tissue, the selection and composition of biomaterials, and techniques for design and manufacturing. The challenges and future perspectives of biomimetic multilayered scaffold strategies for AC regeneration are also discussed.

Perspectives on Synthetic Materials to Guide Tissue Regeneration for Osteochondral Defect Repair

Regenerative engineering holds the potential to treat clinically pervasive osteochondral defects (OCDs). In a synthetic materials-guided approach, the scaffold's chemical and physical properties alone instruct cellular behavior in order to effect regeneration, referred to herein as "instructive" properties. While this alleviates the costs and off-target risks associated with exogenous growth factors, the scaffold must be potently instructive to achieve tissue growth. Moreover, toward achieving functionality, such a scaffold should also recapitulate the spatial complexity of the osteochondral tissues. Thus, in addition to the regeneration of the articular cartilage and underlying cancellous bone, the complex osteochondral interface, composed of calcified cartilage and subchondral bone, should also be restored. In this Perspective, we highlight recent synthetic-based, instructive osteochondral scaffolds that have leveraged new material chemistries as well as innovative fabrication strategies. In particular, scaffolds with spatially complex chemical and morphological features have been prepared with electrospinning, solvent-casting-particulate-leaching, freeze-drying, and additive manufacturing. While few synthetic scaffolds have advanced to clinical studies to treat OCDs, these recent efforts point to the promising use of the chemical and physical properties of synthetic materials for regeneration of osteochondral tissues.

An oriented-collagen scaffold including Wnt5a promotes osteochondral regeneration and cartilage interface integration in a rabbit model

Articular cartilage regeneration remains a major challenge in orthopedics. Noncanonical Wnt5a is a particularly attractive growth factor in this context; Wnt5a inhibits chondrocyte hypertrophy but maintains chondrogenesis. We designed a novel, vertically oriented-collagen scaffold. The effect of Wnt5a on MSCs and chondrocytes and the therapeutic effects of the Wnt5a/oriented-collagen scaffold in terms of osteochondral repair and cartilage integration were evaluated. In vitro, the proliferation, migration, and differentiation of MSCs and chondrocytes treated with Wnt5a, and the mechanisms thereof, were assessed. mRNA microarray analysis was performed to compare the expression profiles of MSCs before and after Wnt5a treatment. In vivo, full-thickness cylindrical osteochondral defects (4 mm in diameter, 3 mm in depth) were created in the patellar grooves of 24 New Zealand white rabbits and implanted with oriented-collagen scaffolds (n = 8), Wnt5a/oriented-collagen scaffolds (n = 8), or nothing (n = 8). After 6 and 12 weeks, integration and tissue responses were evaluated. The proliferation, migration, chondrogenic differentiation, and extracellular matrix formation of/by MSCs and chondrocytes improved greatly after treatment with Wnt5a. Western blotting showed that the PI3K/AKT/JNK signaling pathway was activated. Microarray analysis revealed that the Wnt5a group exhibited a significant upregulation of the PI3K pathway. Reactome GSEA pathway interaction analysis revealed that such upregulation was associated with collagen and extracellular matrix formation. In vivo, the Wnt5a/oriented-collagen scaffold group exhibited optimal interface integration, cartilage regeneration, and collagenous fiber arrangement, accompanied by significantly increased glycosaminoglycan and collagen accumulations in the zones of regeneration and integration, compared to the other groups. Gene expression analysis showed that the levels of mRNAs encoding genes involved in cartilage formation were significantly increased in the Wnt5a/oriented, collagen scaffold group (all P < .05). Wnt5a promoted the proliferation, migration, and chondrogenic differentiation of MSCs and chondrocytes via the activation of the PI3K/AKT/JNK signaling pathway. The Wnt5a/oriented-collagen constructs enhanced the structure-specific regeneration of hyaline cartilage in a rabbit model and may be a promising treatment for the repair of human cartilage defects.

Advances in Hybrid Fabrication toward Hierarchical Tissue Constructs

The diversity of manufacturing processes used to fabricate 3D implants, scaffolds, and tissue constructs is continuously increasing. This growing number of different applicable fabrication technologies include electrospinning, melt electrowriting, volumetric-, extrusion-, and laser-based bioprinting, the Kenzan method, and magnetic and acoustic levitational bioassembly, to name a few. Each of these fabrication technologies feature specific advantages and limitations, so that a combination of different approaches opens new and otherwise unreachable opportunities for the fabrication of hierarchical cell-material constructs. Ongoing challenges such as vascularization, limited volume, and repeatability of tissue constructs at the resolution required to mimic natural tissue is most likely greater than what one manufacturing technology can overcome. Therefore, the combination of at least two different manufacturing technologies is seen as a clear and necessary emerging trend, especially within biofabrication. This hybrid approach allows more complex mechanics and discrete biomimetic structures to address mechanotransduction and chemotactic/haptotactic cues. Pioneering milestone papers in hybrid fabrication for biomedical purposes are presented and recent trends toward future manufacturing platforms are analyzed.

A review of calcium phosphate cements and acrylic bone cements as injectable materials for bone repair and implant fixation

Treatment of bone defects caused by trauma or disease is a major burden on human healthcare systems. Although autologous bone grafts are considered as the gold standard, they are limited in availability and are associated with post-operative complications. Minimally invasive alternatives using injectable bone cements are currently used in certain clinical procedures, such as vertebroplasty and balloon kyphoplasty. Nevertheless, given the high incidence of fractures and pathologies that result in bone voids, there is an unmet need for injectable materials with desired properties for minimally invasive procedures. This paper provides an overview of the most common injectable bone cement materials for clinical use. The emphasis has been placed on calcium phosphate cements and acrylic bone cements, while enabling the readers to compare the opportunities and challenges for these two classes of bone cements. This paper also briefly reviews antibiotic-loaded bone cements used in bone repair and implant fixation, including their efficacy and cost for healthcare systems. A summary of the current challenges and recommendations for future directions has been brought in the concluding section of this paper.

Role of the Calcified Cartilage Layer of an Integrated Trilayered Silk Fibroin Scaffold Used to Regenerate Osteochondral Defects in Rabbit Knees

The repair of osteochondral defects remains challenging, given the complexity of native osteochondral tissue and the limited self-repair capacity of cartilage. Osteochondral tissue engineering is a promising strategy. Here, we fabricated a biomimetic osteochondral scaffold using silk fibroin and hydroxyapatite, including a calcified cartilage layer (CCL). We studied the role played by the CCL in terms of cell viability in vivo. We established osteochondral defects in rabbit knees to investigate the effects of CCL-containing scaffolds with or without adipose tissue-derived stem cells (ADSCs). We evaluated osteochondral tissue regeneration by calculating gross observational scores, via histological and immunohistochemical assessments, by performing quantitative biochemical and biomechanical analyses of new osteochondral tissue, and via microcomputed tomography of new bone at 4, 8, and 12 weeks after surgery. In terms of surface roughness and integrity, the CCL + ADSCs group was better than the CCL and the non-CCL + ADSCs groups at all time points tested; the glycosaminoglycan and collagen type II levels of the CCL + ADSCs group were highest, reflecting the important role played by the CCL in cartilage tissue repair. Subchondral bone smoothness was better in the CCL + ADSCs group than in the non-CCL + ADSCs and CCL groups. The CCL promoted smooth subchondral bone regeneration but did not obviously affect bone strength or quality. In conclusion, a biomimetic osteochondral scaffold with a CCL, combined with autologous ADSCs, satisfactorily regenerated a rabbit osteochondral defect. The CCL enhances cartilage and subchondral bone regeneration.

Osteochondral tissue regenerated via a strategy by stacking pre-differentiated BMSC sheet on fibrous mesh in a gradient

The reconstruction of osteochondral tissue remains a challenging task in clinical therapy because of its heterogeneous structure. The best way to face the challenge is to develop a biomimetic construct to mimic the multilayered gradient from cartilage, to calcified cartilage and subchondral bone. In this study, bone marrow mesenchymal stromal cells (BMSCs) were cultured on electrospun fibrous meshes and cell sheets were incubated. The fibrous meshes were composed of 50% poly(L-lactide) and 50% gelatin, displaying excellent biocompatibility, cell affinity and degradability. Differentiation of BMSC sheets on fibrous meshes was induced chondrogenically or osteogenically. In particular, the BMSC sheets were able to be efficiently induced differentiating towards calcified cartilage by using a 1:1 (v/v) mixed medium of chondrogenic and osteogenic inductive media. Thus, a gradient 3D construct was built by stacking the differently pre-differentiated cell/mesh complexes layer by layer from top to bottom to mimic the cartilage-to-bone transition. With this gradient construct being implanted in the rabbit knee osteochondral defect, it was confirmed that it could promote the tissue regeneration with intact cartilage layer formation in comparison with the multilayered construct without a gradient. The strategy of using properly pre-differentiated BMSC sheet on fibrous mesh to build the osteochondral interface was thus suggested as being feasible and effective in mimicking its hierarchical complexity, and favored the repairing of injured joint cartilage.

The Impact of Melt Electrowritten Scaffold Design on Porosity Determined by X-Ray Microtomography

IMPACT STATEMENT

Melt electrowriting is an AM technology that bridges the gap between solution electrospinning and melt microextrusion technologies. It can be applied to biomaterials and tissue engineering by making a spectrum of scaffolds with various laydown patterns at dimensions not previously studied. Using submicrometer X-ray tomography, a "fingerprint" of porosity for such scaffolds can be obtained and used as an important measure for quality control, to ensure that the scaffold fabricated is the one designed and allows the selection of specific scaffolds based on desired porosities.

3D bioprinted hydrogel model incorporating β-tricalcium phosphate for calcified cartilage tissue engineering

One promising strategy to reconstruct osteochondral defects relies on 3D bioprinted three-zonal structures comprised of hyaline cartilage, calcified cartilage, and subchondral bone. So far, several studies have pursued the regeneration of either hyaline cartilage or bone in vitro while-despite its key role in the osteochondral region-only few of them have targeted the calcified layer. In this work, we present a 3D biomimetic hydrogel scaffold containing β-tricalcium phosphate (TCP) for engineering calcified cartilage through a co-axial needle system implemented in extrusion-based bioprinting process. After a thorough bioink optimization, we showed that 0.5% w/v TCP is the optimal concentration forming stable scaffolds with high shape fidelity and endowed with biological properties relevant for the development of calcified cartilage. In particular, we investigate the effect induced by ceramic nano-particles over the differentiation capacity of bioprinted bone marrow-derived human mesenchymal stem cells in hydrogel scaffolds cultured up to 21 d in chondrogenic media. To confirm the potential of the presented approach to generate a functional in vitro model of calcified cartilage tissue, we evaluated quantitatively gene expression of relevant chondrogenic (COL1, COL2, COL10A1, ACAN) and osteogenic (ALPL, BGLAP) gene markers by means of RT-qPCR and qualitatively by means of fluorescence immunocytochemistry.

Dimension-Based Design of Melt Electrowritten Scaffolds

The electrohydrodynamic stabilization of direct-written fluid jets is explored to design and manufacture tissue engineering scaffolds based on their desired fiber dimensions. It is demonstrated that melt electrowriting can fabricate a full spectrum of various fibers with discrete diameters (2-50 µm) using a single nozzle. This change in fiber diameter is digitally controlled by combining the mass flow rate to the nozzle with collector speed variations without changing the applied voltage. The greatest spectrum of fiber diameters was achieved by the simultaneous alteration of those parameters during printing. The highest placement accuracy could be achieved when maintaining the collector speed slightly above the critical translation speed. This permits the fabrication of medical-grade poly(ε-caprolactone) into complex multimodal and multiphasic scaffolds, using a single nozzle in a single print. This ability to control fiber diameter during printing opens new design opportunities for accurate scaffold fabrication for biomedical applications.

Micro/Nano Scaffolds for Osteochondral Tissue Engineering

To develop an osteochondral tissue regeneration strategy it is extremely important to take into account the multiscale organization of the natural extracellular matrix. The structure and gradients of organic and inorganic components present in the cartilage and bone tissues must be considered together. Another critical aspect is an efficient interface between both tissues. So far, most of the approaches were focused on the development of multilayer or stratified scaffolds which resemble the structural composition of bone and cartilage, not considering in detail a transitional interface layer. Typically, those scaffolds have been produced by the combined use of two or more processing techniques (microtechnologies and nanotechnologies) and materials (organic and inorganic). A significant number of works was focused on either cartilage or bone, but there is a growing interest in the development of the osteochondral interface and in tissue engineering models of composite constructs that can mimic the cartilage/bone tissues. The few works that give attention to the interface between cartilage and bone, as well as to the biochemical gradients observed at the osteochondral unit, are also herein described.

Biphasic Scaffolds from Marine Collagens for Regeneration of Osteochondral Defects

Background: Collagens of marine origin are applied increasingly as alternatives to mammalian collagens in tissue engineering. The aim of the present study was to develop a biphasic scaffold from exclusively marine collagens supporting both osteogenic and chondrogenic differentiation and to find a suitable setup for in vitro chondrogenic and osteogenic differentiation of human mesenchymal stroma cells (hMSC). Methods: Biphasic scaffolds from biomimetically mineralized salmon collagen and fibrillized jellyfish collagen were fabricated by joint freeze-drying and crosslinking. Different experiments were performed to analyze the influence of cell density and TGF-β on osteogenic differentiation of the cells in the scaffolds. Gene expression analysis and analysis of cartilage extracellular matrix components were performed and activity of alkaline phosphatase was determined. Furthermore, histological sections of differentiated cells in the biphasic scaffolds were analyzed. Results: Stable biphasic scaffolds from two different marine collagens were prepared. An in vitro setup for osteochondral differentiation was developed involving (1) different seeding densities in the phases; (2) additional application of alginate hydrogel in the chondral part; (3) pre-differentiation and sequential seeding of the scaffolds and (4) osteochondral medium. Spatially separated osteogenic and chondrogenic differentiation of hMSC was achieved in this setup, while osteochondral medium in combination with the biphasic scaffolds alone was not sufficient to reach this ambition. Conclusions: Biphasic, but monolithic scaffolds from exclusively marine collagens are suitable for the development of osteochondral constructs.

A review on gradient hydrogel/fiber scaffolds for osteochondral regeneration

Osteochondral tissue regeneration is a complicated field due to the distinct properties and healing potential of osseous and chondral phases. In a natural osteochondral region, the composition, mechanics, and structure vary smoothly from bony to cartilaginous phase. Therefore, a homogeneous scaffold cannot satisfy the complexity of the osteochondral matrix. In essence, a natural extracellular matrix is composed of fibrous proteins elongated into a gelatinous background. A hydrogel/fiber scaffold possessing gradient in both phases would be of the utmost interest to imitate tissue arrangement of a native osteochondral interface. However, there are limited research works that exploit hydrogel/fiber scaffolds for osteochondral restoration. In the present review, currently used fibrous or gelatinous scaffolds for osteochondral damages are discussed. Moreover, superiority of using gradient hydrogel/fiber composites for osteochondral regeneration and practical approaches to develop those scaffolds is debated.

Osteochondral Tissue Engineering: Translational Research and Turning Research into Products

Osteochondral (OC) defect repair is a significant clinical challenge. Osteoarthritis results in articular cartilage/subchondral bone tissue degeneration and tissue loss, which in the long run results in cartilage/ostecochondral defect formation. OC defects are commonly approached with autografts and allografts, and both these options have found limitations. Alternatively, tissue engineered strategies with biodegradable scaffolds with and without cells and growth factors have been developed. In order to approach regeneration of complex tissues such as osteochondral, advanced tissue engineered grafts including biphasic, triphasic, and gradient configurations are considered. The graft design is motivated to promote cartilage and bone layer formation with an interdigitating transitional zone (i.e., bone-cartilage interface). Some of the engineered OC grafts with autologous cells have shown promise for OC defect repair and a few of them have advanced into clinical trials. This chapter presents synthetic osteochondral designs and the progress that has been made in terms of the clinical translation.

Multiscale design and synthesis of biomimetic gradient protein/biosilica composites for interfacial tissue engineering

Continuous gradients present at tissue interfaces such as osteochondral systems, reflect complex tissue functions and involve changes in extracellular matrix compositions, cell types and mechanical properties. New and versatile biomaterial strategies are needed to create suitable biomimetic engineered grafts for interfacial tissue engineering. Silk protein-based composites, coupled with selective peptides with mineralization domains, were utilized to mimic the soft-to-hard transition in osteochondral interfaces. The gradient composites supported tunable mineralization and mechanical properties corresponding to the spatial concentration gradient of the mineralization domains (R5 peptide). The composite system exhibited continuous transitions in terms of composition, structure and mechanical properties, as well as cytocompatibility and biodegradability. The gradient silicified silk/R5 composites promoted and regulated osteogenic differentiation of human mesenchymal stem cells in an osteoinductive environment in vitro. The cells differentiated along the composites in a manner consistent with the R5-gradient profile. This novel biomimetic gradient biomaterial design offers a useful approach to meet a broad range of needs in regenerative medicine.

Fiber-reinforced scaffolds in soft tissue engineering

Soft tissue engineering has been developed as a new strategy for repairing damaged or diseased soft tissues and organs to overcome the limitations of current therapies. Since most of soft tissues in the human body are usually supported by collagen fibers to form a three-dimensional microstructure, fiber-reinforced scaffolds have the advantage to mimic the structure, mechanical and biological environment of natural soft tissues, which benefits for their regeneration and remodeling. This article reviews and discusses the latest research advances on design and manufacture of novel fiber-reinforced scaffolds for soft tissue repair and how fiber addition affects their structural characteristics, mechanical strength and biological activities in vitro and in vivo. In general, the concept of fiber-reinforced scaffolds with adjustable microstructures, mechanical properties and degradation rates can provide an effective platform and promising method for developing satisfactory biomechanically functional implantations for soft tissue engineering or regenerative medicine.

Bioprinting of osteochondral tissues: A perspective on current gaps and future trends

Osteochondral tissue regeneration has remained a critical challenge in orthopaedic surgery, especially due to complications of arthritic degeneration arising out of mechanical dislocations of joints. The common gold standard of autografting has several limitations in presenting tissue engineering strategies to solve the unmet clinical need. However, due to the complexity of joint anatomy, and tissue heterogeneity at the interface, the conventional tissue engineering strategies have certain limitations. The advent of bioprinting has now provided new opportunities for osteochondral tissue engineering. Bioprinting can uniquely mimic the heterogeneous cellular composition and anisotropic extra-cellular matrix (ECM) organization, while allowing for targeted gene delivery to achieve heterotypic differentiation. In this perspective, we discuss the current advances made towards bioprinting of composite osteochondral tissues and present an account of challenges-in terms of tissue integration, long-term survival, and mechanical strength at the time of implantation-required to be addressed for effective clinical translation of bioprinted tissues. Finally, we highlight some of the future trends related to osteochondral bioprinting with the hope of in-clinical translation.

Fabrication and Evaluation of Electrospun, 3D-Bioplotted, and Combination of Electrospun/3D-Bioplotted Scaffolds for Tissue Engineering Applications

Electrospun scaffolds provide a dense framework of nanofibers with pore sizes and fiber diameters that closely resemble the architecture of native extracellular matrix. However, it generates limited three-dimensional structures of relevant physiological thicknesses. 3D printing allows digitally controlled fabrication of three-dimensional single/multimaterial constructs with precisely ordered fiber and pore architecture in a single build. However, this approach generally lacks the ability to achieve submicron resolution features to mimic native tissue. The goal of this study was to fabricate and evaluate 3D printed, electrospun, and combination of 3D printed/electrospun scaffolds to mimic the native architecture of heterogeneous tissue. We assessed their ability to support viability and proliferation of human adipose derived stem cells (hASC). Cells had increased proliferation and high viability over 21 days on all scaffolds. We further tested implantation of stacked-electrospun scaffold versus combined electrospun/3D scaffold on a cadaveric pig knee model and found that stacked-electrospun scaffold easily delaminated during implantation while the combined scaffold was easier to implant. Our approach combining these two commonly used scaffold fabrication technologies allows for the creation of a scaffold with more close resemblance to heterogeneous tissue architecture, holding great potential for tissue engineering and regenerative medicine applications of osteochondral tissue and other heterogeneous tissues.

Poly (lactic acid)-based biomaterials for orthopaedic regenerative engineering

Regenerative engineering converges tissue engineering, advanced materials science, stem cell science, and developmental biology to regenerate complex tissues such as whole limbs. Regenerative engineering scaffolds provide mechanical support and nanoscale control over architecture, topography, and biochemical cues to influence cellular outcome. In this regard, poly (lactic acid) (PLA)-based biomaterials may be considered as a gold standard for many orthopaedic regenerative engineering applications because of their versatility in fabrication, biodegradability, and compatibility with biomolecules and cells. Here we discuss recent developments in PLA-based biomaterials with respect to processability and current applications in the clinical and research settings for bone, ligament, meniscus, and cartilage regeneration.

A novel aragonite-based scaffold for osteochondral regeneration: early experience on human implants and technical developments

INTRODUCTION

Chondral and osteochondral lesions represent a debilitating disease. Untreated lesions remain a risk factor for more extensive joint damage. The objective of this clinical study is to evaluate safety and early results of an aragonite-based scaffold used for osteochondral unit repair, by analysing both clinical outcome and MRI results, as well as the benefits of the procedure optimization through novel tapered shaped implants.

METHODS

A crystalline aragonite bi-phasic scaffold was implanted in patients affected by focal chondral-osteochondral knee lesions of the condyle and trochlea. Twenty-one patients (17 men, 4 women with a mean age of 31.0 ± 8.6 years) without severe OA received tapered shaped implants for the treatment of 2.5 ±1.7 cm² sized defects. The control group consisted of 76 patients selected according to the same criteria from a database of patients who previously underwent implantation of cylindrical-shaped implants. The clinical outcome of all patients was evaluated with the IKDC subjective score, the Lysholm score, and all 5 KOOS subscales administered preoperatively and at 6 and 12 months after surgery, while MRI evaluation was performed at the 12 month follow-up.

RESULTS

A statistically significant improvement in all clinical scores was documented both in the tapered implants and the cylindrical group. No difference could be detected in the comparison between the improvement obtained with the two implant types, neither in the clinical nor in imaging evaluations. A difference could be detected instead in terms of revision rate, which was lower in the tapered implant group with no implant removal - 0% vs 8/76-10.5% failures in the cylindrical implants.

CONCLUSIONS

This study highlighted both safety and potential of a novel aragonite-based scaffold for the treatment of chondral and osteochondral lesions in humans. A tapered shape relative to the cylindrical shaped implant design, improved the scaffold's safety profile. Tapered scaffolds maintain the clinical improvement observed in cylindrical implants while reducing the postoperative risk of revision surgery. This aragonite-based implant was associated with a significant clinical improvement at the 12 month follow-up. Moreover, MRI findings revealed graft integration with good bone and cartilage formation.

Nanoengineered biomaterials for repair and regeneration of orthopedic tissue interfaces

Orthopedic interface tissue engineering aims to mimic the structure and function of soft-to-hard tissue junctions, particularly bone-ligament, bone-tendon, and bone-cartilage interfaces. A range of engineering approaches has been proposed to mimic the gradient architecture, physical properties and chemical characteristics of interface tissues using conventional polymeric biomaterials. Recent developments in nanomaterials and nanofabrication technologies introduce a range of synthesis and fabrication tools to effectively engineer the structure and function of native tissue interfaces. In this review, we will focus on nanoengineered strategies used to replicate the structural and functional aspects of native biological tissues for engineering bone-cartilage, bone-ligament, and bone-tendon interfaces. This review will also highlight some of the emerging applications and future potential of nanomaterials and fabrication technologies in engineering tissue interfaces.

STATEMENT OF SIGNIFICANCE

A major challenge in engineering interfaces is to control the physical and structural characteristics of an artificial environment. The use of nanomaterials and nanoengineered strategies allow for greater control over the changes in structure and function at molecular and nanometer length scale. This review focuses on advanced nanomaterials and nanofabrication approaches developed to emulate bone-cartilage, bone-ligament, and bone-tendon interface regions. Some of the emerging nanoengineered biomaterials proposed to mimic tissue interfaces are also highlighted.

Review of the biomechanics and biotribology of osteochondral grafts used for surgical interventions in the knee

A review of research undertaken to evaluate the biomechanical stability and biotribological behaviour of osteochondral grafts in the knee joint and a brief discussion of areas requiring further improvement in future studies are presented. The review takes into consideration osteochondral autografts, allografts, tissue engineered constructs and synthetic and biological scaffolds.