Bi-layered micro-fibre reinforced hydrogels for articular cartilage regeneration

Trilayered biomimetic hydrogel scaffolds with dual-differential microenvironment for articular osteochondral defect repair

Commonly, articular osteochondral tissue exists significant differences in physiological architecture, mechanical function, and biological microenvironment. However, the development of biomimetic scaffolds incorporating upper cartilage, middle tidemark-like, and lower subchondral bone layers for precise articular osteochondral repair remains elusive. This study proposed here a novel strategy to construct the trilayered biomimetic hydrogel scaffolds with dual-differential microenvironment of both mechanical and biological factors. The cartilage-specific microenvironment was achieved through the grafting of kartogenin (KGN) into gelatin via p-hydroxyphenylpropionic acid (HPA)-based enzyme crosslinking reaction as the upper cartilage layer. The bone-specific microenvironment was achieved through the grafting of atorvastatin (AT) into gelatin via dual-crosslinked network of both HP-based enzyme crosslinking and glycidyl methacrylate (GMA)-based photo-crosslinking reactions as the lower subchondral bone layer. The introduction of tidemark-like middle layer is conducive to the formation of well-defined cartilage-bone integrated architecture. The in vitro experiments demonstrated the significant mechanical difference of three layers, successful grafting of drugs, good cytocompatibility and tissue-specific induced function. The results of in vivo experiments also confirmed the mechanical difference of the trilayered bionic scaffold and the ability of inducing osteogenesis and chondrogenesis. Furthermore, the articular osteochondral defects were successfully repaired using the trilayered biomimetic hydrogel scaffolds by the activation of endogenous recovery, which offers a promising alternative for future clinical treatment.

Proinflammatory phenotype of iPS cell-derived JAK2 V617F megakaryocytes induces fibrosis in 3D in vitro bone marrow niche

The myeloproliferative disease polycythemia vera (PV) driven by the JAK2 V617F mutation can transform into myelofibrosis (post-PV-MF). It remains an open question how JAK2 V617F in hematopoietic stem cells induces MF. Megakaryocytes are major players in murine PV models but are difficult to study in the human setting. We generated induced pluripotent stem cells (iPSCs) from JAK2 V617F PV patients and differentiated them into megakaryocytes. In differentiation assays, JAK2 V617F iPSCs recapitulated the pathognomonic skewed megakaryocytic and erythroid differentiation. JAK2 V617F iPSCs had a TPO-independent and increased propensity to differentiate into megakaryocytes. RNA sequencing of JAK2 V617F iPSC-derived megakaryocytes reflected a proinflammatory, profibrotic phenotype and decreased ribosome biogenesis. In three-dimensional (3D) coculture, JAK2 V617F megakaryocytes induced a profibrotic phenotype through direct cell contact, which was reversed by the JAK2 inhibitor ruxolitinib. The 3D coculture system opens the perspective for further disease modeling and drug discovery.

Orthotopic equine study confirms the pivotal importance of structural reinforcement over the pre-culture of cartilage implants

In articular cartilage (AC), the collagen arcades provide the tissue with its extraordinary mechanical properties. As these structures cannot be restored once damaged, functional restoration of AC defects remains a major challenge. We report that the use of a converged bioprinted, osteochondral implant, based on a gelatin methacryloyl cartilage phase, reinforced with precisely patterned melt electrowritten polycaprolactone micrometer-scale fibers in a zonal fashion, inspired by native collagen architecture, can provide long-term mechanically stable neo-tissue in an orthotopic large animal model. The design of this novel implant was achieved via state-of-the-art converging of extrusion-based ceramic printing, melt electrowriting, and extrusion-based bioprinting. Interestingly, the cell-free implants, used as a control in this study, showed abundant cell ingrowth and similar favorable results as the cell-containing implants. Our findings underscore the hypothesis that mechanical stability is more determining for the successful survival of the implant than the presence of cells and pre-cultured extracellular matrix. This observation is of great translational importance and highlights the aptness of advanced 3D (bio)fabrication technologies for functional tissue restoration in the harsh articular joint mechanical environment.

Fabrication of Customizable and Reproducible 3D Chondrocyte-Laden Nanofibrous Architectures: Effect of Specific Fiber Alignments and Porosities on Chondrocyte Response under Cyclic Compression

Electrospinning has been widely employed to fabricate complex extracellular matrix-like microenvironments for tissue engineering due to its ability to replicate structurally biomimetic micro- and nanotopographic cues. Nevertheless, these nanofibrous structures are typically either confined to bidimensional systems or confined to three-dimensional ones that are unable to provide controlled multiscale patterns. Thus, an electrospinning modality was used in this work to fabricate chondrocyte-laden nanofibrous scaffolds with highly customizable three-dimensional (3D) architectures in an automated manner, with the ultimate goal of recreating a suitable 3D scaffold for articular cartilage tissue engineering. Three distinct architectures were designed and fabricated by combining multiple nanofibrous and chondrocyte-laden hydrogel layers and tested in vitro in a compression bioreactor system. Results demonstrated that it was possible to precisely control the placement and alignment of electrospun polycaprolactone and gelatin nanofibers, generating three unique architectures with distinctive macroscale porosity, water absorption capacity, and mechanical properties. The architecture organized in a lattice-like fashion was highly porous with substantial pore interconnectivity, resulting in a high-water absorption capacity but a poor compression modulus and relatively weaker energy dissipation capacity. The donut-like 3D geometry was the densest, with lower swelling, but the highest compression modulus and improved energy dissipation ability. The third architecture combined a lattice and donut-like fibrous arrangement, exhibiting intermediary behavior in terms of porosity, water absorption, compression modulus, and energy dissipation capacity. The properties of the donut-like 3D architecture demonstrated great potential for articular cartilage tissue engineering, as it mimicked key topographic, chemical, and mechanical characteristics of chondrocytes' surrounding environment. In fact, the combination of these architectural features with a dynamically compressive mechanical stimulus triggered the best in vitro results in terms of viability and biosynthetic production.

GelMA-catechol coated FeHAp nanorods functionalized nanofibrous reinforced bio-instructive and mechanically robust composite hydrogel scaffold for bone tissue engineering

Critical bone defects complicate tissue graft-based surgeries, raising healthcare expenditures and underscoring scaffold-based tissue-engineering strategies to support bone reconstruction. Our study highlighted that the phase-compatible combination of inorganic nanorods, nanofibers, and hydrogels is promising for developing biomimetic and cell-instructive scaffolds since the bone matrix is a porous organic/inorganic composite. In brief, methacrylated gelatin (GelMA) was reacted with dopamine to form catechol-modified GeLMA (GelMA-C). The GelMA-C was nanocoated onto an iron-doped hydroxyapatite (FeHAp) nanorod via metal-catechol network coordination. The modified nanorod (FeHAp@GelMA-C) was loaded onto GelMA-based nanofibers. The nanorods loaded pre-fibers were electrospun onto GelMA solution and photochemically crosslinked to fabricate a fiber-reinforced hydrogel. The structural, mechanical, physicochemical, biocompatibility, swelling properties, osteogenic potential, and bone remodelling potential (using rat femoral defect model) of modified nanorods, simple hydrogel, and nanorod-loaded fiber-reinforced hydrogel were studied. The results supported that the interface interaction between GelMA-C/nanorods, nanorods/nanofibers, nanorods/hydrogels, and nanofiber/hydrogels significantly improved the microstructural and mechanical properties of the scaffold. Compared to pristine hydrogel, the nanorod-loaded fiber-reinforced scaffold better supported cellular responses, osteogenic differentiation, matrix mineralization, and accelerated bone regeneration. The nanorod-loaded fiber-reinforced hydrogel proved more biomimetic and cell-instructive for guided bone reconstruction.

Microfiber-reinforced hydrogels prolong the release of human induced pluripotent stem cell-derived extracellular vesicles to promote endothelial migration

Extracellular vesicle (EV)-based approaches for promoting angiogenesis have shown promising results. Yet, further development is needed in vehicles that prolong EV exposure to target organs. Here, we hypothesized that microfiber-reinforced gelatin methacryloyl (GelMA) hydrogels could serve as sustained delivery platforms for human induced pluripotent stem cell (hiPSC)-derived EV. EV with 50-200 nm size and typical morphology were isolated from hiPSC-conditioned culture media and tested negative for common co-isolated contaminants. hiPSC-EV were then incorporated into GelMA hydrogels with or without a melt electrowritten reinforcing mesh. EV release was found to increase with GelMA concentration, as 12 % (w/v) GelMA hydrogels provided higher release rate and total release over 14 days in vitro, compared to lower hydrogel concentrations. Release profile modelling identified diffusion as a predominant release mechanism based on a Peppas-Sahlin model. To study the effect of reinforcement-dependent hydrogel mechanics on EV release, stress relaxation was assessed. Reinforcement with highly porous microfiber meshes delayed EV release by prolonging hydrogel stress relaxation and reducing the swelling ratio, thus decreasing the initial burst and overall extent of release. After release from photocrosslinked reinforced hydrogels, EV remained internalizable by human umbilical vein endothelial cells (HUVEC) over 14 days, and increased migration was observed in the first 4 h. EV and RNA cargo stability was investigated at physiological temperature in vitro, showing a sharp decrease in total RNA levels, but a stable level of endothelial migration-associated small noncoding RNAs over 14 days. Our data show that hydrogel formulation and microfiber reinforcement are superimposable approaches to modulate EV release from hydrogels, thus depicting fiber-reinforced GelMA hydrogels as tunable hiPSC-EV vehicles for controlled release systems that promote endothelial cell migration.

Microstructural differences in the osteochondral unit of terrestrial and aquatic mammals

During evolution, animals have returned from land to water, adapting with morphological modifications to life in an aquatic environment. We compared the osteochondral units of the humeral head of marine and terrestrial mammals across species spanning a wide range of body weights, focusing on microstructural organization and biomechanical performance. Aquatic mammals feature cartilage with essentially random collagen fiber configuration, lacking the depth-dependent, arcade-like organization characteristic of terrestrial mammalian species. They have a less stiff articular cartilage at equilibrium with a significantly lower peak modulus, and at the osteochondral interface do not have a calcified cartilage layer, displaying only a thin, highly porous subchondral bone plate. This totally different constitution of the osteochondral unit in aquatic mammals reflects that accommodation of loading is the primordial function of the osteochondral unit. Recognizing the crucial importance of the microarchitecture-function relationship is pivotal for understanding articular biology and, hence, for the development of durable functional regenerative approaches for treatment of joint damage, which are thus far lacking.

Fiber reinforced hydrated networks recapitulate the poroelastic mechanics of articular cartilage

The role of poroelasticity on the functional performance of articular cartilage has been established in the scientific literature since the 1960s. Despite the extensive knowledge on this topic there remain few attempts to design for poroelasticity and to our knowledge no demonstration of an engineered poroelastic material that approaches the physiological performance. In this paper, we report on the development of an engineered material that begins to approach physiological poroelasticity. We quantify poroelasticity using the fluid load fraction, apply mixture theory to model the material system, and determine cytocompatibility using primary human mesenchymal stem cells. The design approach is based on a fiber reinforced hydrated network and uses routine fabrication methods (electrohydrodynamic deposition) and materials (poly[ɛ-caprolactone] and gelatin) to develop the engineered poroelastic material. This composite material achieved a mean peak fluid load fraction of 68%, displayed consistency with mixture theory, and demonstrated cytocompatibility. This work creates a foundation for designing poroelastic cartilage implants and developing scaffold systems to study chondrocyte mechanobiology and tissue engineering. STATEMENT OF SIGNIFICANCE: Poroelasticity drives the functional mechanics of articular cartilage (load bearing and lubrication). In this work we develop the design rationale and approach to produce a poroelastic material, known as a fiber reinforced hydrated network (FiHy™), that begins to approach the native performance of articular cartilage. This is the first engineered material system capable of exceeding isotropic linear poroelastic theory. The framework developed here enables fundamental studies of poroelasticity and the development of translational materials for cartilage repair.

Growth of spiral ganglion neurons induced by graphene oxide/oxidized bacterial cellulose composite hydrogel

The damage or degeneration of spiral ganglion neurons (SGNs) can impair the auditory signals transduction from hair cells to the central auditory system, and cause significant hearing loss. Herein, a new form of bioactive hydrogel incorporating topological graphene oxide (GO) and TEMPO-oxidized bacterial cellulose (GO/TOBC hydrogel) was developed to provide a favorable microenvironment for SGN neurite outgrowth. As the network structure of lamellar interspersed fiber cross-linked by GO/TOBC hydrogels well simulated the structure and morphology of ECM, with the controllable hydrophilic property and appropriate Young's modulus well met those requirements of SGNs microenvironment, the GO/TOBC hybrid matrix exhibited great potential to promote the growth of SGNs. The quantitative real-time PCR result confirmed that the GO/TOBC hydrogel can significantly accelerate the development of growth cones and filopodia, by increasing the mRNA expression levels of diap3, fscn2, and integrin β1. These results suggest that GO/TOBC hydrogel scaffolds have the potential to be used to construct biomimetic nerve grafts for repairing or replacing nerve defects.

Reinforcement of Hydrogels with a 3D-Printed Polycaprolactone (PCL) Structure Enhances Cell Numbers and Cartilage ECM Production under Compression

Hydrogels show promise in cartilage tissue engineering (CTE) by supporting chondrocytes and maintaining their phenotype and extracellular matrix (ECM) production. Under prolonged mechanical forces, however, hydrogels can be structurally unstable, leading to cell and ECM loss. Furthermore, long periods of mechanical loading might alter the production of cartilage ECM molecules, including glycosaminoglycans (GAGs) and collagen type 2 (Col2), specifically with the negative effect of stimulating fibrocartilage, typified by collagen type 1 (Col1) secretion. Reinforcing hydrogels with 3D-printed Polycaprolactone (PCL) structures offer a solution to enhance the structural integrity and mechanical response of impregnated chondrocytes. This study aimed to assess the impact of compression duration and PCL reinforcement on the performance of chondrocytes impregnated with hydrogel. Results showed that shorter loading periods did not significantly affect cell numbers and ECM production in 3D-bioprinted hydrogels, but longer periods tended to reduce cell numbers and ECM compared to unloaded conditions. PCL reinforcement enhanced cell numbers under mechanical compression compared to unreinforced hydrogels. However, the reinforced constructs seemed to produce more fibrocartilage-like, Col1-positive ECM. These findings suggest that reinforced hydrogel constructs hold potential for in vivo cartilage regeneration and defect treatment by retaining higher cell numbers and ECM content. To further enhance hyaline cartilage ECM formation, future studies should focus on adjusting the mechanical properties of reinforced constructs and exploring mechanotransduction pathways.

Direct-Writing Electrospun Functionalized Scaffolds for Periodontal Regeneration: In Vitro Studies

Multiphasic scaffolds that combine different architectural, physical, and biological properties are the best option for the regeneration of complex tissues such as the periodontium. Current developed scaffolds generally lack architectural accuracy and rely on multistep manufacturing, which is difficult to implement for clinical applications. In this context, direct-writing electrospinning (DWE) represents a promising and rapid technique for developing thin 3D scaffolds with controlled architecture. The current study aimed to elaborate a biphasic scaffold using DWE based on two polycaprolactone solutions with interesting properties for bone and cement regeneration. One of the two scaffold parts contained hydroxyapatite nanoparticles (HAP) and the other contained the cementum protein 1 (CEMP1). After morphological characterizations, the elaborated scaffolds were assessed regarding periodontal ligament (PDL) cells in terms of cell proliferation, colonization, and mineralization ability. The results demonstrated that both HAP- and CEMP1-functionalized scaffolds were colonized by PDL cells and enhanced mineralization ability compared to unfunctionalized scaffolds, as revealed by alizarin red staining and OPN protein fluorescent expression. Taken together, the current data highlighted the potential of functional and organized scaffolds to stimulate bone and cementum regeneration. Moreover, DWE could be used to develop smart scaffolds with the ability to spatially control cellular orientation with suitable cellular activity at the micrometer scale, thereby enhancing periodontal and other complex tissue regeneration.

Safety and effectiveness of nano composite hydrogel stent implantation in the treatment of coronary cardiovascular disease: A preclinical study

With the improvement of people's quality of life, various cardiovascular diseases are the most common diseases. Therefore, the main site of disease atherosclerosis is blood vessels, so we can see that its flow rate has obvious changes. Through the analysis of coronary heart disease, this paper studies the relationship between coronary artery disease and cardiovascular disease, which is helpful to evaluate the risk of disease, and also provides the best prevention and treatment plan to overcome cardiovascular disease. As the material of artificial cartilage repair, nanocomposite hydrogel has excellent application value and attraction, because nanocomposite hydrogel has a structure similar to the extracellular matrix of natural chondrocytes. The patients in the experimental group were treated with nano composite hydrogel stent implantation. The other group of patients used the traditional way to carry out the comparative experiment. In the perfusion data of each ventricular wall in the coronary angiography and anterior wall perfusion group, the percentage of lateral wall in the normal proportion was the highest, 69.2%, 59.3% in the anterior wall, 39.5% in the inferior wall, and 19.7% in the apical value and interval. The percentage of LAD stenosis in anterior wall perfusion was O. The highest percentage in the lateral wall was 69.2%, and the lowest in the septum and apex was 19.7%. Nanocomposite hydrogel stent implantation can effectively treat coronary heart disease. The research shows that it is safe and effective in application.

Composite Graded Melt Electrowritten Scaffolds for Regeneration of the Periodontal Ligament-to-Bone Interface

Periodontitis is a ubiquitous chronic inflammatory, bacteria-triggered oral disease affecting the adult population. If left untreated, periodontitis can lead to severe tissue destruction, eventually resulting in tooth loss. Despite previous efforts in clinically managing the disease, therapeutic strategies are still lacking. Herein, melt electrowriting (MEW) is utilized to develop a compositionally and structurally tailored graded scaffold for regeneration of the periodontal ligament-to-bone interface. The composite scaffolds, consisting of fibers of polycaprolactone (PCL) and fibers of PCL-containing magnesium phosphate (MgP) were fabricated using MEW. To maximize the bond between bone (MgP) and ligament (PCL) regions, we evaluated two different fiber architectures in the interface area. These were a crosshatch pattern at a 0/90° angle and a random pattern. MgP fibrous scaffolds were able to promote in vitro bone formation even in culture media devoid of osteogenic supplements. Mechanical properties after MgP incorporation resulted in an increase of the elastic modulus and yield stress of the scaffolds, and fiber orientation in the interfacial zone affected the interfacial toughness. Composite graded MEW scaffolds enhanced bone fill when they were implanted in an in vivo periodontal fenestration defect model in rats. The presence of an interfacial zone allows coordinated regeneration of multitissues, as indicated by higher expression of bone, ligament, and cementoblastic markers compared to empty defects. Collectively, MEW-fabricated scaffolds having compositionally and structurally tailored zones exhibit a good mimicry of the periodontal complex, with excellent regenerative capacity and great potential as a defect-specific treatment strategy.

Bioprinting of structurally organized meniscal tissue within anisotropic melt electrowritten scaffolds

The meniscus is characterised by an anisotropic collagen fibre network which is integral to its biomechanical functionality. The engineering of structurally organized meniscal grafts that mimic the anisotropy of the native tissue remains a significant challenge. In this study, inkjet bioprinting was used to deposit a cell-laden bioink into additively manufactured scaffolds of differing architectures to engineer fibrocartilage grafts with user defined collagen architectures. Polymeric scaffolds consisting of guiding fibre networks with varying aspect ratios (1:1; 1:4; 1:16) were produced using either fused deposition modelling (FDM) or melt electrowriting (MEW), resulting in scaffolds with different internal architectures and fibre diameters. Scaffold architecture was found to influence the spatial organization of the collagen network laid down by the jetted cells, with higher aspect ratios (1:4 and 1:16) supporting the formation of structurally anisotropic tissues. The MEW scaffolds supported the development of a fibrocartilaginous tissue with compressive mechanical properties similar to that of native meniscus, while the anisotropic tensile properties of these constructs could be tuned by altering the fibre network aspect ratio. This MEW framework was then used to generate scaffolds with spatially distinct fibre patterns, which in turn supported the development of heterogenous tissues consisting of isotropic and anisotropic collagen networks. Such bioprinted tissues could potentially form the basis of new treatment options for damaged and diseased meniscal tissue. STATEMENT OF SIGNIFICANCE: This study describes a multiple tool biofabrication strategy which enables the engineering of spatially organized fibrocartilage tissues. The architecture of MEW scaffolds can be tailored to not only modulate the directionality of the collagen fibres laid down by cells, but also to tune the anisotropic tensile mechanical properties of the resulting constructs, thereby enabling the engineering of biomimetic meniscal-like tissues. Furthermore, the inherent flexibility of MEW enables the development of zonally defined and potentially patient-specific implants.

Unveiling the potential of melt electrowriting in regenerative dental medicine

For nearly three decades, tissue engineering strategies have been leveraged to devise effective therapeutics for dental, oral, and craniofacial (DOC) regenerative medicine and treat permanent deformities caused by many debilitating health conditions. In this regard, additive manufacturing (AM) allows the fabrication of personalized scaffolds that have the potential to recapitulate native tissue morphology and biomechanics through the utilization of several 3D printing techniques. Among these, melt electrowriting (MEW) is a versatile direct electrowriting process that permits the development of well-organized fibrous constructs with fiber resolutions ranging from micron to nanoscale. Indeed, MEW offers great prospects for the fabrication of scaffolds mimicking tissue specificity, healthy and pathophysiological microenvironments, personalized multi-scale transitions, and functional interfaces for tissue regeneration in medicine and dentistry. Excitingly, recent work has demonstrated the potential of converging MEW with other AM technologies and/or cell-laden scaffold fabrication (bioprinting) as a favorable route to overcome some of the limitations of MEW for DOC tissue regeneration. In particular, such convergency fabrication strategy has opened great promise in terms of supporting multi-tissue compartmentalization and predetermined cell commitment. In this review, we offer a critical appraisal on the latest advances in MEW and its convergence with other biofabrication technologies for DOC tissue regeneration. We first present the engineering principles of MEW and the most relevant design aspects for transition from flat to more anatomically relevant 3D structures while printing highly-ordered constructs. Secondly, we provide a thorough assessment of contemporary achievements using MEW scaffolds to study and guide soft and hard tissue regeneration, and draw a parallel on how to extrapolate proven concepts for applications in DOC tissue regeneration. Finally, we offer a combined engineering/clinical perspective on the fabrication of hierarchically organized MEW scaffold architectures and the future translational potential of site-specific, single-step scaffold fabrication to address tissue and tissue interfaces in dental, oral, and craniofacial regenerative medicine. STATEMENT OF SIGNIFICANCE: Melt electrowriting (MEW) techniques can further replicate the complexity of native tissues and could be the foundation for novel personalized (defect-specific) and tissue-specific clinical approaches in regenerative dental medicine. This work presents a unique perspective on how MEW has been translated towards the application of highly-ordered personalized multi-scale and functional interfaces for tissue regeneration, targeting the transition from flat to anatomically-relevant three-dimensional structures. Furthermore, we address the value of convergence of biofabrication technologies to overcome the traditional manufacturing limitations provided by multi-tissue complexity. Taken together, this work offers abundant engineering and clinical perspectives on the fabrication of hierarchically MEW architectures aiming towards site-specific implants to address complex tissue damage in regenerative dental medicine.

Preparation and properties of a photocrosslinked MCl n -doped PDMA-g-PSMA hydrogel

To treat damaged joint areas, photocrosslinked hydrophobically associating PDMA-g-PSMA hydrogels can act as mild and easily regulated materials due to their rich pore structure, which have been widely applied in articular cartilage replacement research. In this study, the effects of ADS-MCl _n (ADS-NaCl, ADS-MgCl₂ and ADS-CaCl₂) doping systems on the micro morphology, mechanical, self-healing, and friction properties and cytotoxicity of PDMA-g-PSMA hydrogels were studied. The results showed that the solubilization behavior of the ADS-MCl _n ionic micelles affected the hydrophobic association stability, thereby changing the toughness, self-healing and friction properties of the hydrogel. Ca²⁺-doping resulted in the crystallization and precipitation of the anionic surfactants, destroying the solubilization ability of the ionic micelles for the hydrophobic units, and thus hydrogels with high hardness, low toughness and no self-healing function were obtained. Doping with Na⁺ greatly improved the dissolving power of the ADS micelles for SMA, yielding PDMA-g-PSMA hydrogels with good mechanical strength and good self-healing ability. However, in this case, a drawback is that the Na⁺-doped system will lose its components during the swelling process, leading to the degradation of its self-healing performance. Interestingly, Mg²⁺ doping resulted in the formation of highly stable ADS micellar aggregates, and then PDMA-g-PSMA hydrogels with a lower friction coefficient (0.023), less wear (35.0 mg), higher elongation at break and 100% self-healing efficiency were obtained. The hydrogel products obtained from the three doping systems all exhibited good biocompatibility. Our research provides important guidelines for the design and preparation of anti-friction artificial articular cartilage.

The rise of mechanical metamaterials: Auxetic constructs for skin wound healing

Auxetic materials are known for their unique ability to expand/contract in multiple directions when stretched/compressed. In other words, they exhibit a negative Poisson's ratio, which is usually positive for most of materials. This behavior appears in some biological tissues such as human skin, where it promotes wound healing by providing an enhanced mechanical support and facilitating cell migration. Skin tissue engineering has been a growing research topic in recent years, largely thanks to the rapid development of 3D printing techniques and technologies. The combination of computational studies with rapid manufacturing and tailored designs presents a huge potential for the future of personalized medicine. Overall, this review article provides a comprehensive overview of the current state of research on auxetic constructs for skin healing applications, highlighting the potential of auxetics as a promising treatment option for skin wounds. The article also identifies gaps in the current knowledge and suggests areas for future research. In particular, we discuss the designs, materials, manufacturing techniques, and also the computational and experimental studies on this topic.

Mechanically conditioned cell sheets cultured on thermo-responsive surfaces promote bone regeneration

Cell sheet-based scaffold-free technology holds promise for tissue engineering applications and has been extensively explored during the past decades. However, efficient harvest and handling of cell sheets remain challenging, including insufficient extracellular matrix content and poor mechanical strength. Mechanical loading has been widely used to enhance extracellular matrix production in a variety of cell types. However, currently, there are no effective ways to apply mechanical loading to cell sheets. In this study, we prepared thermo-responsive elastomer substrates by grafting poly(N-isopropyl acrylamide) (PNIPAAm) to poly(dimethylsiloxane) (PDMS) surfaces. The effect of PNIPAAm grafting yields on cell behaviours was investigated to optimize surfaces suitable for cell sheet culturing and harvesting. Subsequently, MC3T3-E1 cells were cultured on the PDMS-g-PNIPAAm substrates under mechanical stimulation by cyclically stretching the substrates. Upon maturation, the cell sheets were harvested by lowering the temperature. We found that the extracellular matrix content and thickness of cell sheet were markedly elevated upon appropriate mechanical conditioning. Reverse transcription quantitative polymerase chain reaction and Western blot analyses further confirmed that the expression of osteogenic-specific genes and major matrix components were up-regulated. After implantation into the critical-sized calvarial defects of mice, the mechanically conditioned cell sheets significantly promoted new bone formation. Findings from this study reveal that thermo-responsive elastomer, together with mechanical conditioning, can potentially be applied to prepare high-quality cell sheets for bone tissue engineering.

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.

Within or Without You? A Perspective Comparing In Situ and Ex Situ Tissue Engineering Strategies for Articular Cartilage Repair

Human articular cartilage has a poor ability to self-repair, meaning small injuries often lead to osteoarthritis, a painful and debilitating condition which is a major contributor to the global burden of disease. Existing clinical strategies generally do not regenerate hyaline type cartilage, motivating research toward tissue engineering solutions. Prospective cartilage tissue engineering therapies can be placed into two broad categories: i) Ex situ strategies, where cartilage tissue constructs are engineered in the lab prior to implantation and ii) in situ strategies, where cells and/or a bioscaffold are delivered to the defect site to stimulate chondral repair directly. While commonalities exist between these two approaches, the core point of distinction-whether chondrogenesis primarily occurs "within" or "without" (outside) the body-can dictate many aspects of the treatment. This difference influences decisions around cell selection, the biomaterials formulation and the surgical implantation procedure, the processes of tissue integration and maturation, as well as, the prospects for regulatory clearance and clinical translation. Here, ex situ and in situ cartilage engineering strategies are compared: Highlighting their respective challenges, opportunities, and prospects on their translational pathways toward long term human cartilage repair.

Emerging polymeric material strategies for cartilage repair

Cartilage is found throughout the body, serving an array of essential functions. Owing to the limited healing capacity of cartilage, damage or degeneration is often permanent and so requires clinical intervention. Established surgical techniques generally rely on biological grafting. However, recent advances in polymeric materials provide an encouraging alternative to overcome limits of auto- and allografts. For regenerative engineering of cartilage, a polymeric scaffold ideally supports and instructs tissue regeneration while also providing mechanical integrity. Scaffolds direct regeneration via chemical and mechanical cues, as well as delivery and support of exogenous cells and bioactive factors. Advanced polymeric scaffolds aim to direct regeneration locally, replicating the heterogeneities of native tissues. Alternatively, new cartilage-mimetic hydrogels have potential to serve as synthetic cartilage replacements. Prepared as multi-network or composite hydrogels, the most promising candidates have simultaneously realized the hydration, mechanical, and tribological properties of native cartilage. Collectively, the recent rise in polymers for cartilage regeneration and replacement proposes a changing paradigm, with a new generation of materials paving the way for improved clinical outcomes.

Porous biomaterials for tissue engineering: a review

The field of biomaterials has grown rapidly over the past decades. Within this field, porous biomaterials have played a remarkable role in: (i) enabling the manufacture of complex three-dimensional structures; (ii) recreating mechanical properties close to those of the host tissues; (iii) facilitating interconnected structures for the transport of macromolecules and cells; and (iv) behaving as biocompatible inserts, tailored to either interact or not with the host body. This review outlines a brief history of the development of biomaterials, before discussing current materials proposed for use as porous biomaterials and exploring the state-of-the-art in their manufacture. The wide clinical applications of these materials are extensively discussed, drawing on specific examples of how the porous features of such biomaterials impact their behaviours, as well as the advantages and challenges faced, for each class of the materials.

Direct 3D printing of decellularized matrix embedded composite polycaprolactone scaffolds for cartilage regeneration

Treatment options for large osteochondral injuries (OCIs) are limited by donor tissue scarcity, morbidity, and anatomic mismatch. 3D printing technology can produce patient-specific scaffolds to address these large defects. Thermoplastics like polycaprolactone (PCL) offer necessary mechanical properties, but lack bioactivity. We fabricated 3D printed PCL scaffolds embedded with polylactic acid microspheres containing decellularized cartilage matrix (DM). DM incorporation within polylactic acid microspheres prevented its thermal degradation during the 3D printing process. The scaffolds replicated the mechanical properties of native cartilage and demonstrated controlled release of DM proteins. Human mesenchymal stem cells (hMSCs) seeded on the composite scaffolds with DM and cultured in basal media self-assembled into aggregates mimicking mesenchymal condensates during embryonic development. The DM composite scaffolds also induced higher expression of biochemical markers of cartilage development than controls, providing evidence for their translational application in the treatment of OCIs. The present study demonstrates the potential of direct incorporation of DM with thermoplastics for 3D printing of patient-specific scaffolds for osteochondral regeneration.

Advancing Engineered Heart Muscle Tissue Complexity with Hydrogel Composites

A heart attack results in the permanent loss of heart muscle and can lead to heart disease, which kills more than 7 million people worldwide each year. To date, outside of heart transplantation, current clinical treatments cannot regenerate lost heart muscle or restore full function to the damaged heart. There is a critical need to create engineered heart tissues with structural complexity and functional capacity needed to replace damaged heart muscle. The inextricable link between structure and function suggests that hydrogel composites hold tremendous promise as a biomaterial-guided strategy to advance heart muscle tissue engineering. Such composites provide biophysical cues and functionality as a provisional extracellular matrix that hydrogels cannot on their own. This review describes the latest advances in the characterization of these biomaterial systems and using them for heart muscle tissue engineering. The review integrates results across the field to provide new insights on critical features within hydrogel composites and perspectives on the next steps to harnessing these promising biomaterials to faithfully reproduce the complex structure and function of native heart muscle.

Pluripotent stem cells for skeletal tissue engineering

Here, we review the use of human pluripotent stem cells for skeletal tissue engineering. A number of approaches have been used for generating cartilage and bone from both human embryonic stem cells and induced pluripotent stem cells. These range from protocols relying on intrinsic cell interactions and signals from co-cultured cells to those attempting to recapitulate the series of steps occurring during mammalian skeletal development. The importance of generating authentic tissues rather than just differentiated cells is emphasized and enabling technologies for doing this are reported. We also review the different methods for characterization of skeletal cells and constructs at the tissue and single-cell level, and indicate newer resources not yet fully utilized in this field. There have been many challenges in this research area but the technologies to overcome these are beginning to appear, often adopted from related fields. This makes it more likely that cost-effective and efficacious human pluripotent stem cell-engineered constructs may become available for skeletal repair in the near future.

Creating a Functional Biomimetic Cartilage Implant Using Hydrogels Based on Methacrylated Chondroitin Sulfate and Hyaluronic Acid

The load-bearing function of articular cartilage tissue contrasts with the poor load-bearing capacity of most soft hydrogels used for its regeneration. The present study explores whether a hydrogel based on the methacrylated natural polymers chondroitin sulfate (CSMA) and hyaluronic acid (HAMA), injected into warp-knitted spacer fabrics, could be used to create a biomimetic construct with cartilage-like mechanical properties. The swelling ratio of the combined CSMA/HAMA hydrogels in the first 20 days was higher for hydrogels with a higher CSMA concentration, and these hydrogels also degraded quicker, whereas those with a 1.33 wt% of HAMA were stable for more than 120 days. When confined by a polyamide 6 (PA6) spacer fabric, the volumetric swelling of the combined CSMA/HAMA gels (10 wt%, 6.5 × CSMA:HAMA ratio) was reduced by ~53%. Both the apparent peak and the equilibrium modulus significantly increased in the PA6-restricted constructs compared to the free-swelling hydrogels after 28 days of swelling, and no significant differences in the moduli and time constant compared to native bovine cartilage were observed. Moreover, the cell viability in the CSMA/HAMA PA6 constructs was comparable to that in gelatin–methacrylamide (GelMA) PA6 constructs at one day after polymerization. These results suggest that using a HydroSpacer construct with an extracellular matrix (ECM)-like biopolymer-based hydrogel is a promising approach for mimicking the load-bearing properties of native cartilage.

Importance of Matrix Cues on Intervertebral Disc Development, Degeneration, and Regeneration

Back pain is one of the leading causes of disability worldwide and is frequently caused by degeneration of the intervertebral discs. The discs’ development, homeostasis, and degeneration are driven by a complex series of biochemical and physical extracellular matrix cues produced by and transmitted to native cells. Thus, understanding the roles of different cues is essential for designing effective cellular and regenerative therapies. Omics technologies have helped identify many new matrix cues; however, comparatively few matrix molecules have thus far been incorporated into tissue engineered models. These include collagen type I and type II, laminins, glycosaminoglycans, and their biomimetic analogues. Modern biofabrication techniques, such as 3D bioprinting, are also enabling the spatial patterning of matrix molecules and growth factors to direct regional effects. These techniques should now be applied to biochemically, physically, and structurally relevant disc models incorporating disc and stem cells to investigate the drivers of healthy cell phenotype and differentiation. Such research will inform the development of efficacious regenerative therapies and improved clinical outcomes.

Hybrid Stents Based on Magnetic Hydrogels for Biomedical Applications

Tremendous attention has been given to hydrogels due to their mechanical and physical properties. Hydrogels are promising biomaterials due to their high biocompatibility. Magnetic hydrogels, which are based on hydrogel incorporated magnetic nanoparticles, have been proposed in biomedical applications. The advantages of magnetic hydrogels are that they can easily respond to externally applied magnetic fields and prevent the leakage of magnetic nanoparticles in the surrounding area. Herein, a prototype hybrid stent of magnetic hydrogel was fabricated, characterized, and evaluated for magnetic hyperthermia treatment. First, magnetic hydrogel was produced by a solution of alginate with magnetic nanoparticles in a bath of calcium chloride (5-15 mg mL^-1) in order to achieve the external gelation and optimize the heating rate. The increased concentration (1-8 mg mL^-1) of magnetic nanoparticles inside the hydrogel resulted in almost zero leakage of iron oxide nanoparticles after 15 days, guaranteeing that they can be used safely in biomedical applications. Thus, magnetic hybrid stents, which are based on the magnetic hydrogels, were developed in a simple way and were evaluated both in an agarose phantom model and in an ex vivo tissue sample at 30 mT and 765 kHz magnetic hyperthermia conditions to examine the heating efficiency. In both cases, hyperthermia results indicate excellent heat generation from the hybrid stent and facile temperature control via tuning magnetic nanoparticle concentration (2-8 mg mL^-1). This study can be a promising method that promotes spatially thermal distribution in cancer treatment or restenosis treatment of hollow organs.

Valuable effect of Manuka Honey in increasing the printability and chondrogenic potential of a naturally derived bioink

Hydrogel-based bioinks are the main formulations used for Articular Cartilage (AC) regeneration due to their similarity to chondral tissue in terms of morphological and mechanical properties. However, the main challenge is to design and formulate bioinks able to allow reproducible additive manufacturing and fulfil the biological needs for the required tissue. In our work, we investigated an innovative Manuka honey (MH)-loaded photocurable gellan gum methacrylated (GGMA) bioink, encapsulating mesenchymal stem cells differentiated in chondrocytes (MSCs-C), to generate 3D bioprinted construct for AC studies. We demonstrated the beneficial effect of MH incorporation on the bioink printability, leading to the obtainment of a more homogenous filament extrusion and therefore a better printing resolution. Also, GGMA-MH formulation showed higher viscoelastic properties, presenting complex modulus G∗ values of ∼1042 ​Pa, compared to ∼730 ​Pa of GGMA. Finally, MH-enriched bioink induced a higher expression of chondrogenic markers col2a1 (14-fold), sox9 (3-fold) and acan (4-fold) and AC ECM main element production (proteoglycans and collagen).

Fabrication of bioinspired grid-crimp micropatterns by melt electrospinning writing for bone-ligament interface study

Many strategies have been adopted to engineer bone-ligament interface, which is of great value to both the tissue regeneration and the mechanism understanding underlying interface regeneration. However, how to recapitulate the complexity and heterogeneity of the native bone-ligament interface including the structural, cellular and mechanical gradients is still challenging. In this work, a bioinspired grid-crimp micropattern fabricated by melt electrospinning writing (MEW) was proposed to mimic the native structure of bone-ligament interface. The printing strategy of crimped fiber micropattern was developed and the processing parameters were optimized, which were used to mimic the crimp structure of the collagen fibrils in ligament. The guidance effect of the crimp angle and fiber spacing on the orientation of fibroblasts was studied, and both of them showed different levels of cell alignment effect.. MEW grid micropatterns with different fiber spacings were fabricated as bone region. Both the alkaling phosphatase activity and calcium mineralization results demonstrated the higher osteoinductive ability of the MEW grid structures, especially for that with smaller fiber spacing. The combined grid-crimp micropatterns were applied for the co-culture of fibroblasts and osteoblasts. The results showed that more cells were observed to migrate into the in-between interface region for the pattern with smaller fiber spacing, suggested the faster migration speed of cells. Finally, a cylindrical triphasic scaffold was successfully generated by rolling the grid-crimp micropatterns up, showing both structural and mechanical similarity to the native bone-ligament interface. In summary, the proposed strategy is reliable to fabricate grid-crimp triphasic micropatterns with controllable structural parameters to mimic the native bone-to-ligament structure, and the generated 3D scaffold shows great potential for the further bone-ligament interface tissue engineering.

Application and development of 3D bioprinting in cartilage tissue engineering

Bioprinting technology can build complex tissue structures and has the potential to fabricate engineered cartilage with bionic structures for achieving cartilage defect repair/regeneration.

Current Advances in the Regeneration of Degenerated Articular Cartilage: A Literature Review on Tissue Engineering and Its Recent Clinical Translation

Functional ability is the basis of healthy aging. Articular cartilage degeneration is amongst the most prevalent degenerative conditions that cause adverse impacts on the quality of life; moreover, it represents a key predisposing factor to osteoarthritis (OA). Both the poor capacity of articular cartilage for self-repair and the unsatisfactory outcomes of available clinical interventions make innovative tissue engineering a promising therapeutic strategy for articular cartilage repair. Significant progress was made in this field; however, a marked heterogeneity in the applied biomaterials, biofabrication, and assessments is nowadays evident by the huge number of research studies published to date. Accordingly, this literature review assimilates the most recent advances in cell-based and cell-free tissue engineering of articular cartilage and also focuses on the assessments performed via various in vitro studies, ex vivo models, preclinical in vivo animal models, and clinical studies in order to provide a broad overview of the latest findings and clinical translation in the context of degenerated articular cartilage and OA.

Fabrication of MSC-laden composites of hyaluronic acid hydrogels reinforced with MEW scaffolds for cartilage repair

Hydrogels are of interest in cartilage tissue engineering due to their ability to support the encapsulation and chondrogenesis of mesenchymal stromal cells (MSCs). However, features such as hydrogel crosslink density, which can influence nutrient transport, nascent matrix distribution, and the stability of constructs during and after implantation must be considered in hydrogel design. Here, we first demonstrate that more loosely crosslinked (i.e. softer, ∼2 kPa) norbornene-modified hyaluronic acid (NorHA) hydrogels support enhanced cartilage formation and maturation when compared to more densely crosslinked (i.e. stiffer, ∼6-60 kPa) hydrogels, with a >100-fold increase in compressive modulus after 56 d of culture. While soft NorHA hydrogels mature into neocartilage suitable for the repair of articular cartilage, their initial moduli are too low for handling and they do not exhibit the requisite stability needed to withstand the loading environments of articulating joints. To address this, we reinforced NorHA hydrogels with polycaprolactone (PCL) microfibers produced via melt-electrowriting (MEW). Importantly, composites fabricated with MEW meshes of 400µm spacing increased the moduli of soft NorHA hydrogels by ∼50-fold while preserving the chondrogenic potential of the hydrogels. There were minimal differences in chondrogenic gene expression and biochemical content (e.g. DNA, GAG, collagen) between hydrogels alone and composites, whereas the composites increased in compressive modulus to ∼350 kPa after 56 d of culture. Lastly, integration of composites with native tissue was assessedex vivo; MSC-laden composites implanted after 28 d of pre-culture exhibited increased integration strengths and contact areas compared to acellular composites. This approach has great potential towards the design of cell-laden implants that possess both initial mechanical integrity and the ability to support neocartilage formation and integration for cartilage repair.

Bioprinting of biomimetic self-organised cartilage with a supporting joint fixation device

Despite sustained efforts, engineering truly biomimetic articular cartilage (AC) via traditional top-down approaches remains challenging. Emerging biofabrication strategies, from 3D bioprinting to scaffold-free approaches that leverage principles of cellular self-organisation, are generating significant interest in the field of cartilage tissue engineering as a means of developing biomimetic tissue analoguesin vitro.Although such strategies have advanced the quality of engineered cartilage, recapitulation of many key structural features of native AC, in particular a collagen network mimicking the tissue's 'Benninghoff arcade', remains elusive. Additionally, a complete solution to fixating engineered cartilagesin situwithin damaged synovial joints has yet to be identified. This study sought to address both of these key challenges by engineering biomimetic AC within a device designed to anchor the tissue within a synovial joint defect. We first designed and fabricated a fixation device capable of anchoring engineered cartilage into the subchondral bone. Next, we developed a strategy for inkjet printing porcine mesenchymal stem/stromal cells (MSCs) into this supporting fixation device, which was also designed to provide instructive cues to direct the self-organisation of MSC condensations towards a stratified engineered AC. We found that a higher starting cell-density supported the development of a more zonally defined collagen network within the engineered tissue. Dynamic culture was implemented to further enhance the quality of this engineered tissue, resulting in an approximate 3 fold increase in glycosaminoglycan and collagen accumulation. Ultimately this strategy supported the development of AC that exhibited near-native levels of glycosaminoglycan accumulation (>5% WW), as well as a biomimetic collagen network organisation with a perpendicular to a parallel fibre arrangement (relative to the tissue surface) from the deep to superficial zones via arcading fibres within the middle zone of the engineered tissue. Collectively, this work demonstrates the successful convergence of novel biofabrication methods, bioprinting strategies and culture regimes to engineer a hybrid implant suited to resurfacing AC defects.

The Importance of Interfaces in Multi-Material Biofabricated Tissue Structures

Biofabrication exploits additive manufacturing techniques for creating 3D structures with a precise geometry that aim to mimic a physiological cellular environment and to develop the growth of native tissues. The most recent approaches of 3D biofabrication integrate multiple technologies into a single biofabrication platform combining different materials within different length scales to achieve improved construct functionality. However, the importance of interfaces between the different material phases, has not been adequately explored. This is known to determine material's interaction and ultimately mechanical and biological performance of biofabricated parts. In this review, this gap is bridged by critically examining the interface between different material phases in (bio)fabricated structures, with a particular focus on how interfacial interactions can compromise or define the mechanical (and biological) properties of the engineered structures. It is believed that the importance of interfacial properties between the different constituents of a composite material, deserves particular attention in its role in modulating the final characteristics of 3D tissue-like structures.

Spatial organization of biochemical cues in 3D-printed scaffolds to guide osteochondral tissue engineering

Functional repair of osteochondral (OC) tissue remains challenging because the transition from bone to cartilage presents gradients in biochemical and physical properties necessary for joint function. Osteochondral regeneration requires strategies that restore the spatial composition and organization found in the native tissue. Several biomaterial approaches have been developed to guide chondrogenic and osteogenic differentiation of human mesenchymal stem cells (hMSCs). These strategies can be combined with 3D printing, which has emerged as a useful tool to produce tunable, continuous scaffolds functionalized with bioactive cues. However, functionalization often includes one or more post-fabrication processing steps, which can lead to unwanted side effects and often produce biomaterials with homogeneously distributed chemistries. To address these challenges, surface functionalization can be achieved in a single step by solvent-cast 3D printing peptide-functionalized polymers. Peptide-poly(caprolactone) (PCL) conjugates were synthesized bearing hyaluronic acid (HA)-binding (HAbind-PCL) or mineralizing (E3-PCL) peptides, which have been shown to promote hMSC chondrogenesis or osteogenesis, respectively. This 3D printing strategy enables unprecedented control of surface peptide presentation and spatial organization within a continuous construct. Scaffolds presenting both cartilage-promoting and bone-promoting peptides had a synergistic effect that enhanced hMSC chondrogenic and osteogenic differentiation in the absence of differentiation factors compared to scaffolds without peptides or only one peptide. Furthermore, multi-peptide organization significantly influenced hMSC response. Scaffolds presenting HAbind and E3 peptides in discrete opposing zones promoted hMSC osteogenic behavior. In contrast, presenting both peptides homogeneously throughout the scaffolds drove hMSC differentiation towards a mixed population of articular and hypertrophic chondrocytes. These significant results indicated that hMSC behavior was driven by dual-peptide presentation and organization. The downstream potential of this platform is the ability to fabricate biomaterials with spatially controlled biochemical cues to guide functional tissue regeneration without the need for differentiation factors.

Shaping collagen for engineering hard tissues: Towards a printomics approach

Hard tissue engineering has evolved over the past decades, with multiple approaches being explored and developed. Despite the rapid development and success of advanced 3D cell culture, 3D printing technologies and material developments, a gold standard approach to engineering and regenerating hard tissue substitutes such as bone, dentin and cementum, has not yet been realised. One such strategy that differs from conventional regenerative medicine approach of other tissues, is the in vitro mineralisation of collagen templates in the absence of cells. Collagen is the most abundant protein within the human body and forms the basis of all hard tissues. Once mineralised, collagen provides important support and protection to humans, for example in the case of bone tissue. Multiple in vitro fabrication strategies and mineralisation approaches have been developed and their success in facilitating mineral deposition on collagen to achieve bone-like scaffolds evaluated. Critical to the success of such fabrication and biomineralisation approaches is the collagen template, and its chemical composition, organisation, and density. The key factors that influence such properties are the collagen processing and fabrication techniques utilised to create the template, and the mineralisation strategy employed to deposit mineral on and throughout the templates. However, despite its importance, relatively little attention has been placed on these two critical factors. Here, we critically examine the processing, fabrication and mineralisation strategies that have been used to mineralise collagen templates, and offer insights and perspectives on the most promising strategies for creating mineralised collagen scaffolds. STATEMENT OF SIGNIFICANCE: In this review, we highlight the critical need to fabricate collagen templates with advanced processing techniques, in a manner that achieves biomimicry of the hierarchical collagen structure, prior to utilising in vitro mineralisation strategies. To this end, we focus on the initial collagen that is selected, the extraction techniques used and the native fibril forming potential retained to create reconstituted collagen scaffolds. This review synthesises current best practises in material sourcing, processing, mineralisation strategies and fabrication techniques, and offers insights into how these can best be exploited in future studies to successfully mineralise collagen templates.

Innovations in Craniofacial Bone and Periodontal Tissue Engineering - From Electrospinning to Converged Biofabrication

From a materials perspective, the pillars for the development of clinically translatable scaffold-based strategies for craniomaxillofacial (CMF) bone and periodontal regeneration have included electrospinning and 3D printing (biofabrication) technologies. Here, we offer a detailed analysis of the latest innovations in 3D (bio)printing strategies for CMF bone and periodontal regeneration and provide future directions envisioning the development of advanced 3D architectures for successful clinical translation. First, the principles of electrospinning applied to the generation of biodegradable scaffolds are discussed. Next, we present on extrusion-based 3D printing technologies with a focus on creating scaffolds with improved regenerative capacity. In addition, we offer a critical appraisal on 3D (bio)printing and multitechnology convergence to enable the reconstruction of CMF bones and periodontal tissues. As a future outlook, we highlight future directions associated with the utilization of complementary biomaterials and (bio)fabrication technologies for effective translation of personalized and functional scaffolds into the clinics.

Effect of Pore Size on Cell Behavior Using Melt Electrowritten Scaffolds

Tissue engineering technology has made major advances with respect to the repair of injured tissues, for which scaffolds and cells are key factors. However, there are still some issues with respect to the relationship between scaffold and cell growth parameters, especially that between the pore size and cells. In this study, we prepared scaffolds with different pore sizes by melt electrowritten (MEW) and used bone marrow mensenchymal stem cells (BMSCs), chondrocytes (CCs), and tendon stem cells (TCs) to study the effect of the scaffold pore size on cell adhesion, proliferation, and differentiation. It was evident that different cells demonstrated different adhesion and proliferation rates on the scaffold. Furthermore, different cell types showed differential preferences for scaffold pore sizes, as evidenced by variations in cell viability. The pore size also affected the differentiation and gene expression pattern of cells. Among the tested cells, BMSCs exhibited the greatest viability on the 200-μm-pore-size scaffold, CCs on the 200- and 100-μm scaffold, and TCs on the 300-μm scaffold. The scaffolds with 100- and 200-μm pore sizes induced a significantly higher proliferation, chondrogenic gene expression, and cartilage-like matrix deposition after in vitro culture relative to the scaffolds with smaller or large pore sizes (especially 50 and 400 μm). Taken together, these results show that the architecture of 10 layers of MEW scaffolds for different tissues should be different and that the pore size is critical for the development of advanced tissue engineering strategies for tissue repair.

Thoughts on cartilage tissue engineering: A 21st century perspective

In mature individuals, hyaline cartilage demonstrates a poor intrinsic capacity for repair, thus even minor defects could result in progressive degeneration, impeding quality of life. Although numerous attempts have been made over the past years for the advancement of effective treatments, significant challenges still remain regarding the translation of in vitro cartilage engineering strategies from bench to bedside. This paper reviews the latest concepts on engineering cartilage tissue in view of biomaterial scaffolds, tissue biofabrication, mechanobiology, as well as preclinical studies in different animal models. The current work is not meant to provide a methodical review, rather a perspective of where the field is currently focusing and what are the requirements for bridging the gap between laboratory-based research and clinical applications, in light of the current state-of-the-art literature. While remarkable progress has been accomplished over the last 20 years, the current sophisticated strategies have reached their limit to further enhance healthcare outcomes. Considering a clinical aspect together with expertise in mechanobiology, biomaterial science and biofabrication methods, will aid to deal with the current challenges and will present a milestone for the furtherance of functional cartilage engineering.

Thermo-responsive injectable naringin-loaded hydrogel polymerised sodium alginate/bioglass delivery for articular cartilage

In the human body, joint cartilage is of great importance. It has long been a big therapeutic problem to fix joint cartilage lesions as it appears due to different conditions. Recent stories have shown that the cartilage replacement process must delay the extracellular (ECM) cartilage deterioration and modulate the host's inflammation response. For the reconstruction of the articular cartilage, drug-loaded injectable hydrogels were developed. This hydrogel could retain the chondrocyte phenotype, but the host's inflammatory reaction could also be controlled. The bioglass (BG)/sodium alginate (SA) injectable hydrogels was combined with agarose (AG)/Naringin hydrogel in injectable thermal response for articular cartilage regeneration with a non-chargeable hydrogel that contains both Naringin and BG (Naringin–BG hydrogels). The Naringin–BG hydrogel has an adequate swelling ratio that encourages the fusion of tissue formed with host tissue and enables the gradual release of Naringin bioavailabilities enhanced in situ. The Naringin–BG hydrogel can upgrade the typical chondrocyte phenotype by upregulating aggrecan, SRY-box 9, and collagen type II alpha one chain. It may also stimulate the polarization of M2 macrophage, lower inflammations, and prevent ECM degradations through the decrease of the expressions of the indictable metalloproteinase-13 matrix, nitric oxide synthase, and metalloproteinase-1 matrix. The formed tissues were identical to normal tissues and firmly incorporated with the surrounding tissue after administering the Naringin–BG hydrogels into the rat model articular cartilage defects. Then the injectable Naringin–BG hydrogel increases the bioavailable content of Naringin and retains the chondrocyte phenotype.

The development of natural polymer scaffold-based therapeutics for osteochondral repair

Due to the limited regenerative capacity of cartilage, untreated joint defects can advance to more extensive degenerative conditions such as osteoarthritis. While some biomaterial-based tissue-engineered scaffolds have shown promise in treating such defects, no scaffold has been widely accepted by clinicians to date. Multi-layered natural polymer scaffolds that mimic native osteochondral tissue and facilitate the regeneration of both articular cartilage (AC) and subchondral bone (SCB) in spatially distinct regions have recently entered clinical use, while the transient localized delivery of growth factors and even therapeutic genes has also been proposed to better regulate and promote new tissue formation. Furthermore, new manufacturing methods such as 3D bioprinting have made it possible to precisely tailor scaffold micro-architectures and/or to control the spatial deposition of cells in requisite layers of an implant. In this way, natural and synthetic polymers can be combined to yield bioactive, yet mechanically robust, cell-laden scaffolds suitable for the osteochondral environment. This mini-review discusses recent advances in scaffolds for osteochondral repair, with particular focus on the role of natural polymers in providing regenerative templates for treatment of both AC and SCB in articular joint defects.

Mesenchymal stem cells loaded on 3D-printed gradient poly(ε-caprolactone)/methacrylated alginate composite scaffolds for cartilage tissue engineering

Cartilage has limited self-repair ability due to its avascular, alymphatic and aneural features. The combination of three-dimensional (3D) printing and tissue engineering provides an up-and-coming approach to address this issue. Here, we designed and fabricated a tri-layered (superficial layer (SL), middle layer (ML) and deep layer (DL)) stratified scaffold, inspired by the architecture of collagen fibers in native cartilage tissue. The scaffold was composed of 3D printed depth-dependent gradient poly(ε-caprolactone) (PCL) impregnated with methacrylated alginate (ALMA), and its morphological analysis and mechanical properties were tested. To prove the feasibility of the composite scaffolds for cartilage regeneration, the viability, proliferation, collagen deposition and chondrogenic differentiation of embedded rat bone marrow mesenchymal stem cells (BMSCs) in the scaffolds were assessed by Live/dead assay, CCK-8, DNA content, cell morphology, immunofluorescence and real-time reverse transcription polymerase chain reaction. BMSCs-loaded gradient PCL/ALMA scaffolds showed excellent cell survival, cell proliferation, cell morphology, collagen II deposition and hopeful chondrogenic differentiation compared with three individual-layer scaffolds. Hence, our study demonstrates the potential use of the gradient PCL/ALMA construct for enhanced cartilage tissue engineering.

Biomimetic Tympanic Membrane Replacement Made by Melt Electrowriting

The tympanic membrane (TM) transfers sound waves from the air into mechanical motion for the ossicular chain. This requires a high sensitivity to small dynamic pressure changes and resistance to large quasi-static pressure differences. The TM achieves this by providing a layered structure of about 100µm in thickness, a low flexural stiffness, and a high tensile strength. Chronically infected middle ears require reconstruction of a large area of the TM. However, current clinical treatment can cause a reduction in hearing. With the novel additive manufacturing technique of melt electrowriting (MEW), it is for the first time possible to fabricate highly organized and biodegradable membranes within the dimensions of the TM. Scaffold designs of various fiber composition are analyzed mechanically and acoustically. It can be demonstrated that by customizing fiber orientation, fiber diameter, and number of layers the desired properties of the TM can be met. An applied thin collagen layer seals the micropores of the MEW-printed membrane while keeping the favorable mechanical and acoustical characteristics. The determined properties are beneficial for implantation, closely match those of the human TM, and support the growth of a neo-epithelial layer. This proves the possibilities to create a biomimimetic TM replacement using MEW.

Enhanced In Vitro and In Vivo Efficacy of Alginate/Silk Protein/Hyaluronic Acid with Polypeptide Microsphere Delivery for Tissue Regeneration of Articular Cartilage

Alginate/Silk fibroin/hyaluronic acid (ALG/SF/HA) nanocomposites were synthesised using blending, inter-linking, and lyophilization methods. We investigated the physicochemical properties of the resulting nanocomposites, including their water retention, weight loss, porosity and cytocompatibility. The optimum ratios of the ALG/SF/HA scaffolding were 3:6.5:0.5. Nanocomposites with optimum ratios were then prepared by integrating pilose antler polypeptides (PAPS) to poly(lactic-co-glycolic acid) (PLGA) microspheres, and the performance was investigated. PAPS-ALG/SF/HA nanocomposites exhibited desirable adhesions and proliferations. Rabbit cartilage deficiencies was developed by the animal model. The cartilage repair effects deficiencies were detected and analyzed between PAPS-SF/ALG/ALG/SF/HA, and control activity classes. The deficiencies were virtually fully remedied after 13 weeks in the presence of PAPS-ALG/SF/HA class, suggesting that the PAPS-ALG/SF/HA nanocomposites had a positive effects on joint cartilage repair.

Graphene Oxide-Doped Gellan Gum-PEGDA Bilayered Hydrogel Mimicking the Mechanical and Lubrication Properties of Articular Cartilage

Articular cartilage (AC) is a specialized connective tissue able to provide a low-friction gliding surface supporting shock-absorption, reducing stresses, and guaranteeing wear-resistance thanks to its structure and mechanical and lubrication properties. Being an avascular tissue, AC has a limited ability to heal defects. Nowadays, conventional strategies show several limitations, which results in ineffective restoration of chondral defects. Several tissue engineering approaches have been proposed to restore the AC's native properties without reproducing its mechanical and lubrication properties yet. This work reports the fabrication of a bilayered structure made of gellan gum (GG) and poly (ethylene glycol) diacrylate (PEGDA), able to mimic the mechanical and lubrication features of both AC superficial and deep zones. Through appropriate combinations of GG and PEGDA, cartilage Young's modulus is effectively mimicked for both zones. Graphene oxide is used as a dopant agent for the superficial hydrogel layer, demonstrating a lower friction than the nondoped counterpart. The bilayered hydrogel's antiwear properties are confirmed by using a knee simulator, following ISO 14243. Finally, in vitro tests with human chondrocytes confirm the absence of cytotoxicity effects. The results shown in this paper open the way to a multilayered synthetic injectable or surgically implantable filler for restoring AC defects.