Tissue mimetic hyaluronan bioink containing collagen fibers with controlled orientation modulating cell migration and alignment

Composite Alginate Dialdehyde-Gelatin (ADA-GEL) Hydrogel Containing Short Ribbon-Shaped Fillers for Skeletal Muscle Tissue Biofabrication

Skeletal muscle tissue can be severely damaged by disease or trauma beyond its ability to self-repair, necessitating the further development of biofabrication and tissue-engineering tools for reconstructive processes. Hence, in this study, a composite bioink of oxidized alginate (ADA) and gelatin (GEL) including cell-laden ribbon-shaped fillers is used for enhancing cell alignment and the formation of an anisotropic structure. Different plasma treatments combined with protein coatings were evaluated for the improvement of cell adhesion to poly(lactic-co-glycolic acid) (PLGA) ribbon surfaces. Oxygen plasma activation of 30 W for 5 min showed high immobilization of fibronectin as a protein coating on the PLGA ribbon surface, which resulted in enhanced cell adhesion and differentiation of muscle cells. Furthermore, the effect of various concentrations of CaCl2 solution, used for ionic cross-linking of ADA, on ADA-GEL physical and mechanical properties as well as encapsulated C2C12 cell viability and proliferation behavior was investigated. The pore area was measured via two approaches, cryofixation and lyophilization, which, in accordance with degradation tests and mechanical analysis, showed that 60 mM CaCl2 concentration is the optimum range for cross-linking of the formulation of ADA 2.5%w/v-GEL 3.75%w/v. These cross-linked hydrogels showed a compression modulus of 11.5 kPa (similar to the native skeletal muscle tissue), a high viability of C2C12 muscle cells (>80%), and a high proliferation rate during 7 days of culture. Rheological characterization of the ADA-GEL composite hydrogel containing short fillers (100 μm long) showed its suitability as a bioink with shear-thinning and flow behavior compared to ADA-GEL.

A biological guide to glycosaminoglycans: current perspectives and pending questions

Mammalian glycosaminoglycans (GAGs), except hyaluronan (HA), are sulfated polysaccharides that are covalently attached to core proteins to form proteoglycans (PGs). This article summarizes key biological findings for the most widespread GAGs, namely HA, chondroitin sulfate/dermatan sulfate (CS/DS), keratan sulfate (KS), and heparan sulfate (HS). It focuses on the major processes that remain to be deciphered to get a comprehensive view of the mechanisms mediating GAG biological functions. They include the regulation of GAG biosynthesis and postsynthetic modifications in heparin (HP) and HS, the composition, heterogeneity, and function of the tetrasaccharide linkage region and its role in disease, the functional characterization of the new PGs recently identified by glycoproteomics, the selectivity of interactions mediated by GAG chains, the display of GAG chains and PGs at the cell surface and their impact on the availability and activity of soluble ligands, and on their move through the glycocalyx layer to reach their receptors, the human GAG profile in health and disease, the roles of GAGs and particular PGs (syndecans, decorin, and biglycan) involved in cancer, inflammation, and fibrosis, the possible use of GAGs and PGs as disease biomarkers, and the design of inhibitors targeting GAG biosynthetic enzymes and GAG-protein interactions to develop novel therapeutic approaches.

Light from Afield: Fast, High-Resolution, and Layer-Free Deep Vat 3D Printing

Harnessing light for cross-linking of photoresponsive materials has revolutionized the field of 3D printing. A wide variety of techniques leveraging broad-spectrum light shaping have been introduced as a way to achieve fast and high-resolution printing, with applications ranging from simple prototypes to biomimetic engineered tissues for regenerative medicine. Conventional light-based printing techniques use cross-linking of material in a layer-by-layer fashion to produce complex parts. Only recently, new techniques have emerged which deploy multidirection, tomographic, light-sheet or filamented light-based image projections deep into the volume of resin-filled vat for photoinitiation and cross-linking. These Deep Vat printing (DVP) approaches alleviate the need for layer-wise printing and enable unprecedented fabrication speeds (within a few seconds) with high resolution (>10 μm). Here, we elucidate the physics and chemistry of these processes, their commonalities and differences, as well as their emerging applications in biomedical and non-biomedical fields. Importantly, we highlight their limitations, and future scope of research that will improve the scalability and applicability of these DVP techniques in a wide variety of engineering and regenerative medicine applications.

In-Bath 3D Printing of Anisotropic Shape-Memory Cryogels Functionalized with Bone-Bioactive Nanoparticles

Cryogels exhibit unique shape memory with full recovery and structural stability features after multiple injections. These constructs also possess enhanced cell permeability and nutrient diffusion when compared to typical bulk hydrogels. Volumetric processing of cryogels functionalized with nanosized units has potential to widen their biomedical applications, however this has remained challenging and relatively underexplored. In this study, we report a novel methodology that combines suspension 3D printing with directional freezing for the fabrication of nanocomposite cryogels with configurable anisotropy. When compared to conventional bulk or freeze-dried hydrogels, nanocomposite cryogel formulations exhibit excellent shape recovery (>95%) and higher pore connectivity. Suspension printing, assisted with a prechilled metal grid, was optimized to induce anisotropy. The addition of calcium- and phosphate-doped mesoporous silica nanoparticles into the cryogel matrix enhanced bioactivity toward orthopedic applications without hindering the printing process. Notably, the nanocomposite 3D printed cryogels exhibit injectable shape memory while also featuring a lamellar topography. The fabrication of these constructs was highly reproducible and exhibited potential for a cell-delivery injectable cryogel with no cytotoxicity to human-derived adipose stem cells. Hence, in this work, it was possible to combine a gravity defying 3D printed methodology with injectable and controlled anisotropic macroporous structures containing bioactive nanoparticles. This methodology ameliorates highly tunable injectable 3D printed anisotropic nanocomposite cryogels with a user-programmable degree of structural complexity.

4-Axis 3D-Printed Tubular Biomaterials Imitating the Anisotropic Nanofiber Orientation of Porcine Aortae

Many of the peculiar properties of the vasculature are related to the arrangement of anisotropic proteinaceous fibers in vessel walls. Understanding and imitating these arrangements can potentially lead to new therapies for cardiovascular diseases. These can be pre-surgical planning, for which patient-specific ex vivo anatomical models for endograft testing are of interest. Alternatively, therapies can be based on tissue engineering, for which degradable in vitro cell growth substrates are used to culture replacement parts. In both cases, materials are desirable that imitate the biophysical properties of vessels, including their tubular shapes and compliance. This work contributes to these demands by offering methods for the manufacturing of anisotropic 3D-printed nanofibrous tubular structures that have similar biophysical properties as porcine aortae, that are biocompatible, and that allow for controlled nutrient diffusion. Tubes of various sizes with axial, radial, or alternating nanofiber orientation along the blood flow direction are manufactured by a customized method. Blood pressure-resistant, compliant, stable, and cell culture-compatible structures are obtained, that can be degraded in vitro on demand. It is suggested that these healthcare materials can contribute to the next generation of cardiovascular therapies of ex vivo pre-surgical planning or in vitro cell culture.

Human Chondrocytes, Metabolism of Articular Cartilage, and Strategies for Application to Tissue Engineering

Hyaline cartilage, which is characterized by the absence of vascularization and innervation, has minimal self-repair potential in case of damage and defect formation in the chondral layer. Chondrocytes are specialized cells that ensure the synthesis of extracellular matrix components, namely type II collagen and aggregen. On their surface, they express integrins CD44, α1β1, α3β1, α5β1, α10β1, αVβ1, αVβ3, and αVβ5, which are also collagen-binding components of the extracellular matrix. This article aims to contribute to solving the problem of the possible repair of chondral defects through unique methods of tissue engineering, as well as the process of pathological events in articular cartilage. In vitro cell culture models used for hyaline cartilage repair could bring about advanced possibilities. Currently, there are several variants of the combination of natural and synthetic polymers and chondrocytes. In a three-dimensional environment, chondrocytes retain their production capacity. In the case of mesenchymal stromal cells, their favorable ability is to differentiate into a chondrogenic lineage in a three-dimensional culture.

Collagen-Based Hydrogels for Cartilage Regeneration

Cartilage regeneration remains difficult due to a lack of blood vessels. Degradation of the extracellular matrix (ECM) causes cartilage defects, and the ECM provides the natural environment and nutrition for cartilage regeneration. Until now, collagen hydrogels are considered to be excellent material for cartilage regeneration due to the similar structure to ECM and good biocompatibility. However, collagen hydrogels also have several drawbacks, such as low mechanical strength, limited ability to induce stem cell differentiation, and rapid degradation. Thus, there is a demanding need to optimize collagen hydrogels for cartilage regeneration. In this review, we will first briefly introduce the structure of articular cartilage and cartilage defect classification and collagen, then provide an overview of the progress made in research on collagen hydrogels with chondrocytes or stem cells, comprehensively expound the research progress and clinical applications of collagen-based hydrogels that integrate inorganic or organic materials, and finally present challenges for further clinical translation.

Optimizing the physical properties of collagen/hyaluronan hydrogels by inhibition of polyionic complexes formation at pH close to the collagen isoelectric point

Collagen/hyaluronan hydrogels with physical properties well suited for biomedical applications are challenging to synthesize due to the formation of polyionic complexes (PICs). A systematic physicochemical study was thus performed to determine novel conditions to inhibit the formation of collagen/hyaluronan PICs and obtain composite hydrogels with high physical properties. Using a range of pH from 1 to 5.5 and the addition of NaCl, type I collagen and tyramine-substituted hyaluronic acid (THA) solutions were mixed and analyzed by cryo-scanning electron microscopy and ATR-FTIR. PIC formation was inhibited at pH 1 without salt and at pH 2.5 and 5.5 in the presence of 400 mM NaCl. Interestingly, collagen fibrils were observed in solution at pH 5.5 before mixing with THA. After collagen gelling by pH increase, a homogeneous hydrogel consisting of collagen fibrils was only observed when PICs were inhibited. Then, the THA gelling performed by photo-crosslinking increased the rheological properties by four when hydrogels were formed with collagen/THA mixtures at pH 1 or 5.5 with salt. Taken together, these results show that a pH of 5.5, close to the collagen isoelectric point, enables the formation of collagen fibrils in solution, inhibits the PICs formation, and allows the formation of homogenous collagen/THA composite hydrogels compatible with cell survival.

Polyvinyl-alcohol, chitosan and graphene-oxide composed conductive hydrogel for electrically controlled fluorescein sodium transdermal release

Accurate and controlled release of drug molecules is crucial for transdermal drug delivery. Electricity, as an adjustable parameter, offers the potential for precise and controllable drug delivery. However, challenges exist in selecting the appropriate drug carrier, electrical parameters, and release model to achieve controlled electronic drug release. To overcome these challenges, this study designed a functional hydrogel using polyvinyl alcohol, chitosan, and graphene oxide as components that can conduct electricity, and constructed a drug transdermal release model using fluorescein sodium salt with proper electrical parameters. The results demonstrated that the hydrogel system exhibited low cytotoxicity, good conductivity, and desirable drug delivery characteristics. The study also integrated the effects of drug release and tissue repair promotion under electrical stimulation. Cell growth was enhanced under low voltage direct current pulses, promoting cell migration and the release of VEGF and FGF. Furthermore, the permeability of fluorescein sodium salt in the hydrogel increased with direct current stimulation. These findings suggest that the carbohydrate polymers hydrogel could serve as a drug carrier for controlled release, and electrical stimulation offers new possibilities for functional drug delivery and transdermal therapy.

Development of a hyaluronic acid-collagen bioink for shear-induced fibers and cells alignment

Human tissues are characterized by complex composition and cellular and extracellular matrix (ECM) organization at microscopic level. In most of human tissues, cells and ECM show an anisotropic arrangement, which confers them specific properties.In vitro, the ability to closely mimic this complexity is limited. However, in the last years, extrusion bioprinting showed a certain potential for aligning cells and biomolecules, due to the application of shear stress during the bio-fabrication process. In this work, we propose a strategy to combine collagen (col) with tyramine-modified hyaluronic acid (THA) to obtain a printable col-THA bioink for extrusion bioprinting, solely-based on natural-derived components. Collagen fibers formation within the hybrid hydrogel, as well as collagen distribution and spatial organization before and after printing, were studied. For the validation of the biological outcome, fibroblasts were selected as cellular model and embedded in the col-THA matrix. Cell metabolic activity and cell viability, as well as cell distribution and alignment, were studied in the bioink before and after bioprinting. Results demonstrated successful collagen fibers formation within the bioink, as well as collagen anisotropic alignment along the printing direction. Furthermore, results revealed suitable biological properties, with a slightly reduced metabolic activity at day 1, fully recovered within the first 3 d post-cell embedding. Finally, results showed fibroblasts elongation and alignment along the bioprinting direction. Altogether, results validated the potential to obtain collagen-based bioprinted constructs, with both cellular and ECM anisotropy, without detrimental effects of the fabrication process on the biological outcome. This bioink can be potentially used for a wide range of applications in tissue engineering and regenerative medicine in which anisotropy is required.

Recent advances in defined hydrogels in organoid research

Organoids are in vitro model systems that mimic the complexity of organs with multicellular structures and functions, which provide great potential for biomedical and tissue engineering. However, their current formation heavily relies on using complex animal-derived extracellular matrices (ECM), such as Matrigel. These matrices are often poorly defined in chemical components and exhibit limited tunability and reproducibility. Recently, the biochemical and biophysical properties of defined hydrogels can be precisely tuned, offering broader opportunities to support the development and maturation of organoids. In this review, the fundamental properties of ECM in vivo and critical strategies to design matrices for organoid culture are summarized. Two typically defined hydrogels derived from natural and synthetic polymers for their applicability to improve organoids formation are presented. The representative applications of incorporating organoids into defined hydrogels are highlighted. Finally, some challenges and future perspectives are also discussed in developing defined hydrogels and advanced technologies toward supporting organoid research.

Suspension bath bioprinting and maturation of anisotropic meniscal constructs

Due to limited intrinsic healing capacity of the meniscus, meniscal injuries pose a significant clinical challenge. The most common method for treatment of damaged meniscal tissues, meniscectomy, leads to improper loading within the knee joint, which can increase the risk of osteoarthritis. Thus, there is a clinical need for the development of constructs for meniscal repair that better replicate meniscal tissue organization to improve load distributions and function over time. Advanced three-dimensional bioprinting technologies such as suspension bath bioprinting provide some key advantages, such as the ability to support the fabrication of complex structures using non-viscous bioinks. In this work, the suspension bath printing process is utilized to print anisotropic constructs with a unique bioink that contains embedded hydrogel fibers that align via shear stresses during printing. Constructs with and without fibers are printed and then cultured for up to 56 din vitroin a custom clamping system. Printed constructs with fibers demonstrate increased cell and collagen alignment, as well as enhanced tensile moduli when compared to constructs printed without fibers. This work advances the use of biofabrication to develop anisotropic constructs that can be utilized for the repair of meniscal tissue.

Fucoidan and topography modification improved in situ endothelialization on acellular synthetic vascular grafts

Thrombogenesis remains the primary failure of synthetic vascular grafts. Endothelial coverage is crucial to provide an antithrombogenic surface. However, most synthetic materials do not support cell adhesion, and transanastomotic endothelial migration is limited. Here, a surface modification strategy using fucoidan and topography was developed to enable fast in situ endothelialization of polyvinyl alcohol, which is not endothelial cell-adhesive. Among three different immobilization approaches compared, conjugation of aminated-fucoidan promoted endothelial monolayer formation while minimizing thrombogenicity in both in vitro platelet rich plasma testing and ex vivo non-human primate shunt assay. Screening of six topographical patterns showed that 2 μm gratings increased endothelial cell migration without inducing inflammation responses of endothelial cells. Mechanistic studies demonstrated that fucoidan could attract fibronectin, enabling integrin binding and focal adhesion formation and activating focal adhesion kinase (FAK) signaling, and 2 μm gratings further enhanced FAK-mediated cell migration. In a clinically relevant rabbit carotid artery end-to-side anastomosis model, 60% in situ endothelialization was observed throughout the entire lumen of 1.7 mm inner diameter modified grafts, compared to 0% of unmodified graft, and the four-week graft patency also increased. This work presents a promising strategy to stimulate in situ endothelialization on synthetic materials for improving long-term performance.

Is 3D Printing Promising for Osteochondral Tissue Regeneration?

Osteochondral tissue regeneration is quite difficult to achieve due to the complexity of its organization. In the design of these complex multilayer structures, a fabrication method, 3D printing, started to be employed, especially by using extrusion, stereolithography and inkjet printing approaches. In this paper, the designs are discussed including biphasic, triphasic, and gradient structures which aim to mimic the cartilage and the calcified cartilage and the whole osteochondral tissue closely. In the first section of the review paper, 3D printing of hydrogels including gelatin methacryloyl (GelMa), alginate, and polyethylene glycol diacrylate (PEGDA) are discussed. However, their physical and biological properties need to be augmented, and this generally is achieved by blending the hydrogel with other, more durable, less hydrophilic, polymers. These scaffolds are very suitable to carry growth factors, such as TGF-β1, to further stimulate chondrogenesis. The bone layer is mimicked by printing calcium phosphates (CaPs) or bioactive glasses together with the hydrogels or as a component of another polymer layer. The current research findings indicate that polyester (i.e. polycaprolactone (PCL), polylactic acid (PLA) and poly(lactide-co-glycolide) (PLGA)) reinforced hydrogels may more successfully mimic the complex structure of osteochondral tissue. Moreover, more recent printing methods such as melt electrowriting (MEW), are being used to integrate polyester fibers to enhance the mechanical properties of hydrogels. Additionally, polyester scaffolds that are 3D printed without hydrogels are discussed after the hydrogel-based scaffolds. In this review paper, the relevant studies are analyzed and discussed, and future work is recommended with support of tables of designed scaffolds. The outcome of the survey of the field is that 3D printing has significant potential to contribute to osteochondral tissue repair.

Swelling-Dependent Shape-Based Transformation of a Human Mesenchymal Stromal Cells-Laden 4D Bioprinted Construct for Cartilage Tissue Engineering

3D bioprinting is usually implemented on flat surfaces, posing serious limitations in the fabrication of multilayered curved constructs. 4D bioprinting, combining 3D bioprinting with time-dependent stimuli-induced transformation, enables the fabrication of shape-changing constructs. Here, a 4D biofabrication method is reported for cartilage engineering based on the differential swelling of a smart multi-material system made from two hydrogel-based materials: hyaluronan and alginate. Two ink formulations are used: tyramine-functionalized hyaluronan (HAT, high-swelling) and alginate with HAT (AHAT, low-swelling). Both inks have similar elastic, shear-thinning, and printability behavior. The inks are 3D printed into a bilayered scaffold before triggering the shape-change by using liquid immersion as stimulus. In time (4D), the differential swelling between the two zones leads to the scaffold's self-bending. Different designs are made to tune the radius of curvature and shape. A bioprinted formulation of AHAT and human bone marrow cells demonstrates high cell viability. After 28 days in chondrogenic medium, the curvature is clearly present while cartilage-like matrix production is visible on histology. A proof-of-concept of the recently emerged technology of 4D bioprinting with a specific application for the design of curved structures potentially mimicking the curvature and multilayer cellular nature of native cartilage is demonstrated.

Fabrication of Collagen-Hyaluronic Acid Cryogels by Directional Freezing Mimicking Cartilage Arcade-like Structure

The internal architecture of tissue-like constructs is fundamental to their structural and biological functions. Here, we introduce a simple and robust method to fabricate cryogels based on derivatized extracellular matrix (ECM) macromolecules with porosity arranged according to the typical Benninghoff zonal architecture of articular cartilage. To obtain this arcade-like structure, the technique used the growth of ice crystals from copper pins at cryogenic temperatures. The directional cryogel formation enabled the organized growth of ice crystals over a large distance (>4 mm). The compositional properties were achieved by forming double networks (DNs) of hyaluronic acid and collagen derivatives (MeHA and CollGTA, respectively), which also served to improve the mechanical properties of the otherwise weak collagen scaffolds. Compositionally biomimetic and more resilient MeHA-CollGTA DNs (Young's modulus ≈ 200 kilopascals) were therefore produced. The technique presented expands the fabrication methods available for providing ECM macromolecules with architectural elements mimicking cartilage complexity.

Emerging Magnetic Fabrication Technologies Provide Controllable Hierarchically-Structured Biomaterials and Stimulus Response for Biomedical Applications

Multifunctional nanocomposites which exhibit well-defined physical properties and encode spatiotemporally-controlled responses are emerging as components for advanced responsive systems. For biomedical applications magnetic nanocomposite materials have attracted significant attention due to their ability to respond to spatially and temporally varying magnetic fields. The current state-of-the-art in development and fabrication of magnetic hydrogels toward biomedical applications is described. There is accelerating progress in the field due to advances in manufacturing capabilities. Three categories can be identified: i) Magnetic hydrogelation, DC magnetic fields are used during solidification/gelation for aligning particles; ii) additive manufacturing of magnetic materials, 3D printing technologies are used to develop spatially-encoded magnetic properties, and more recently; iii) magnetic additive manufacturing, magnetic responses are applied during the printing process to develop increasingly complex structural arrangement that may recapitulate anisotropic tissue structure and function. The magnetic responsiveness of conventionally and additively manufactured magnetic hydrogels are described along with recent advances in soft magnetic robotics, and the categorization is related to final architecture and emergent properties. Future challenges and opportunities, including the anticipated role of combinatorial approaches in developing 4D-responsive functional materials for tackling long-standing problems in biomedicine including production of 3D-specified responsive cell scaffolds are discussed.

Filamented Light (FLight) Biofabrication of Highly Aligned Tissue-Engineered Constructs

Cell-laden hydrogels used in tissue engineering generally lack sufficient 3D topographical guidance for cells to mature into aligned tissues. A new strategy called filamented light (FLight) biofabrication rapidly creates hydrogels composed of unidirectional microfilament networks, with diameters on the length scale of single cells. Due to optical modulation instability, a light beam is divided optically into FLight beams. Local polymerization of a photoactive resin is triggered, leading to local increase in refractive index, which itself creates self-focusing waveguides and further polymerization of photoresin into long hydrogel microfilaments. Diameter and spacing of the microfilaments can be tuned from 2 to 30 µm by changing the coherence length of the light beam. Microfilaments show outstanding cell instructive properties with fibroblasts, tenocytes, endothelial cells, and myoblasts, influencing cell alignment, nuclear deformation, and extracellular matrix deposition. FLight is compatible with multiple types of photoresins and allows for biofabrication of centimeter-scale hydrogel constructs with excellent cell viability within seconds (<10 s per construct). Multidirectional microfilaments are achievable within a single hydrogel construct by changing the direction of FLight projection, and complex multimaterial/multicellular tissue-engineered constructs are possible by sequentially exchanging the cell-laden photoresin. FLight offers a transformational approach to developing anisotropic tissues using photo-crosslinkable biomaterials.

Design of a New 3D Gelatin-Alginate Scaffold Loaded with Cannabis sativa Oil

There is an increasing medical need for the development of new materials that could replace damaged organs, improve healing of critical wounds or provide the environment required for the formation of a new healthy tissue. The three-dimensional (3D) printing approach has emerged to overcome several of the major deficiencies of tissue engineering. The use of Cannabis sativa as a therapy for some diseases has spread throughout the world thanks to its benefits for patients. In this work, we developed a bioink made with gelatin and alginate that was able to be printed using an extrusion 3D bioprinter. The scaffolds obtained were lyophilized, characterized and the swelling was assessed. In addition, the scaffolds were loaded with Cannabis sativa oil extract. The presence of the extract provided antimicrobial and antioxidant activity to the 3D scaffolds. Altogether, our results suggest that the new biocompatible material printed with 3D technology and with the addition of Cannabis sativa oil could become an attractive alternative to common treatments of soft-tissue infections and wound repair.

Systematic review on the application of 3D-bioprinting technology in orthoregeneration: current achievements and open challenges

BACKGROUND

Joint degeneration and large or complex bone defects are a significant source of morbidity and diminished quality of life worldwide. There is an unmet need for a functional implant with near-native biomechanical properties. The potential for their generation using 3D bioprinting (3DBP)-based tissue engineering methods was assessed. We systematically reviewed the current state of 3DBP in orthoregeneration.

METHODS

This review was performed using PubMed and Web of Science. Primary research articles reporting 3DBP of cartilage, bone, vasculature, and their osteochondral and vascular bone composites were considered. Full text English articles were analyzed.

RESULTS

Over 1300 studies were retrieved, after removing duplicates, 1046 studies remained. After inclusion and exclusion criteria were applied, 114 articles were analyzed fully. Bioink material types and combinations were tallied. Cell types and testing methods were also analyzed. Nearly all papers determined the effect of 3DBP on cell survival. Bioink material physical characterization using gelation and rheology, and construct biomechanics were performed. In vitro testing methods assessed biochemistry, markers of extracellular matrix production and/or cell differentiation into respective lineages. In vivo proof-of-concept studies included full-thickness bone and joint defects as well as subcutaneous implantation in rodents followed by histological and µCT analyses to demonstrate implant growth and integration into surrounding native tissues.

CONCLUSIONS

Despite its relative infancy, 3DBP is making an impact in joint and bone engineering. Several groups have demonstrated preclinical efficacy of mechanically robust constructs which integrate into articular joint defects in small animals. However, notable obstacles remain. Notably, researchers encountered pitfalls in scaling up constructs and establishing implant function and viability in long term animal models. Further, to translate from the laboratory to the clinic, standardized quality control metrics such as construct stiffness and graft integration metrics should be established with investigator consensus. While there is much work to be done, 3DBP implants have great potential to treat degenerative joint diseases and provide benefit to patients globally.

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.

Programming Cellular Alignment in Engineered Cardiac Tissue via Bioprinting Anisotropic Organ Building Blocks

The ability to replicate the 3D myocardial architecture found in human hearts is a grand challenge. Here, the fabrication of aligned cardiac tissues via bioprinting anisotropic organ building blocks (aOBBs) composed of human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) is reported. A bioink composed of contractile cardiac aOBBs is first generated and aligned cardiac tissue sheets with linear, spiral, and chevron features are printed. Next, aligned cardiac macrofilaments are printed, whose contractile force and conduction velocity increase over time and exceed the performance of spheroid-based cardiac tissues. Finally, the ability to spatially control the magnitude and direction of contractile force by printing cardiac sheets with different aOBB alignment is highlighted. This research opens new avenues to generating functional cardiac tissue with high cell density and complex cellular alignment.

Articulation inspired by nature: a review of biomimetic and biologically active 3D printed scaffolds for cartilage tissue engineering

In the human body, articular cartilage facilitates the frictionless movement of synovial joints. However, due to its avascular and aneural nature, it has a limited ability to self-repair when damaged due to injury or wear and tear over time. Current surgical treatment options for cartilage defects often lead to the formation of fibrous, non-durable tissue and thus a new solution is required. Nature is the best innovator and so recent advances in the field of tissue engineering have aimed to recreate the microenvironment of native articular cartilage using biomaterial scaffolds. However, the inability to mirror the complexity of native tissue has hindered the clinical translation of many products thus far. Fortunately, the advent of 3D printing has provided a potential solution. 3D printed scaffolds, fabricated using biomimetic biomaterials, can be designed to mimic the complex zonal architecture and composition of articular cartilage. The bioinks used to fabricate these scaffolds can also be further functionalised with cells and/or bioactive factors or gene therapeutics to mirror the cellular composition of the native tissue. Thus, this review investigates how the architecture and composition of native articular cartilage is inspiring the design of biomimetic bioinks for 3D printing of scaffolds for cartilage repair. Subsequently, we discuss how these 3D printed scaffolds can be further functionalised with cells and bioactive factors, as well as looking at future prospects in this field.

Pre-culture of human mesenchymal stromal cells in spheroids facilitates chondrogenesis at a low total cell count upon embedding in biomaterials to generate cartilage microtissues

Material-assisted cartilage tissue engineering has limited application in cartilage treatment due to hypertrophic tissue formation and high cell counts required. This study aimed at investigating the potential of human mesenchymal stromal cell (hMSC) spheroids embedded in biomaterials to study the effect of biomaterial composition on cell differentiation. Pre-cultured (3 days, chondrogenic differentiation media) spheroids (250 cells/spheroid) were embedded in tyramine-modified hyaluronic acid (THA) and collagen type I (Col) composite hydrogels (four combinations of THA (12.5 vs 16.7 mg/ml) and Col (2.5 vs 1.7 mg/ml) content) at a cell density of 5 × 10⁶ cells/ml (2 × 10⁴ spheroids/ml). Macropellets derived from single hMSCs (2.5 × 10⁵ cells, ScMP) or hMSC spheroids (2.5 × 10⁵ cells, 10³ spheroids, SpMP) served as control. hMSC differentiation was analyzed using glycosaminoglycan (GAG) quantification, gene expression analysis and (immuno-)histology. Embedding of hMSC spheroids in THA-Col induced chondrogenic differentiation marked by upregulation of aggrecan (ACAN) and COL2A1, and the production of GAGs . Lower THA led to more pronounced chondrogenic phenotype compared to higher THA content. Col content had no significant influence on hMSC chondrogenesis. Pellet cultures showed an upregulation in chondrogenic-associated genes and production of GAGs with less upregulation of hypertrophic-associated genes in SpMP culture compared to ScMP group. This study presents hMSC pre-culture in spheroids as promising approach to study chondrogenic differentiation after biomaterial encapsulation at low total cell count (5 × 10⁶/ml) without compromising chondrogenic matrix production. This approach can be applied to assemble microtissues in biomaterials to generate large cartilage construct. STATEMENT OF SIGNIFICANCE: In vitro studies investigating the chondrogenic potential of biomaterials are limited due to the low cell-cell contact of encapsulated single cells. Here, we introduce the use of pre-cultured hMSC spheroids to study chondrogenesis upon encapsulation in a biomaterial. The use of spheroids takes advantage of the high cell-cell contact within each spheroid being critical in the early chondrogenesis of hMSCs. At a low seeding density of 5·10⁶ cells/ml (2 × 10⁴ spheroids/ml) we demonstrated hMSC chondrogenesis and cartilaginous matrix deposition. Our results indicate that the pre-culture might have a beneficial effect on hypertrophic gene expression without compromising chondrogenic differentiation. This approach has shown potential to assemble microtissues (here spheroids) in biomaterials to generate large cartilage constructs and to study the effect of biomaterial composition on cell alignment and migration.

Recent strategies of collagen-based biomaterials for cartilage repair: from structure cognition to function endowment

Collagen, characteristic in biomimetic composition and hierarchical structure, boasts a huge potential in repairing cartilage defect due to its extraordinary bioactivities and regulated physicochemical properties, such as low immunogenicity, biocompatibility and controllable degradation, which promotes the cell adhesion, migration and proliferation. Therefore, collagen-based biomaterial has been explored as porous scaffolds or functional coatings in cell-free scaffold and tissue engineering strategy for cartilage repairing. Among those forming technologies, freeze-dry is frequently used with special modifications while 3D-printing and electrospinning serve as the structure-controller in a more precise way. Besides, appropriate cross-linking treatment and incorporation with bioactive substance generally help the collagen-based biomaterials to meet the physicochemical requirement in the defect site and strengthen the repairing performance. Furthermore, comprehensive evaluations on the repair effects of biomaterials are sorted out in terms of in vitro, in vivo and clinical assessments, focusing on the morphology observation, characteristic production and critical gene expression. Finally, the challenge of biomaterial-based therapy for cartilage defect repairing was summarized, which is, the adaption to the highly complex structure and functional difference of cartilage.

Graphical abstract

Natural Hydrogel-Based Bio-Inks for 3D Bioprinting in Tissue Engineering: A Review

Three-dimensional (3D) printing is well acknowledged to constitute an important technology in tissue engineering, largely due to the increasing global demand for organ replacement and tissue regeneration. In 3D bioprinting, which is a step ahead of 3D biomaterial printing, the ink employed is impregnated with cells, without compromising ink printability. This allows for immediate scaffold cellularization and generation of complex structures. The use of cell-laden inks or bio-inks provides the opportunity for enhanced cell differentiation for organ fabrication and regeneration. Recognizing the importance of such bio-inks, the current study comprehensively explores the state of the art of the utilization of bio-inks based on natural polymers (biopolymers), such as cellulose, agarose, alginate, decellularized matrix, in 3D bioprinting. Discussions regarding progress in bioprinting, techniques and approaches employed in the bioprinting of natural polymers, and limitations and prospects concerning future trends in human-scale tissue and organ fabrication are also presented.

The 3D Bioprinted Scaffolds for Wound Healing

Skin tissue engineering and regeneration aim at repairing defective skin injuries and progress in wound healing. Until now, even though several developments are made in this field, it is still challenging to face the complexity of the tissue with current methods of fabrication. In this review, short, state-of-the-art on developments made in skin tissue engineering using 3D bioprinting as a new tool are described. The current bioprinting methods and a summary of bioink formulations, parameters, and properties are discussed. Finally, a representative number of examples and advances made in the field together with limitations and future needs are provided.

Recent Developments in Hyaluronic Acid-Based Hydrogels for Cartilage Tissue Engineering Applications

Articular cartilage lesions resulting from injurious impact, recurring loading, joint malalignment, etc., are very common and encompass the risk of evolving to serious cartilage diseases such as osteoarthritis. To date, cartilage injuries are typically treated via operative procedures such as autologous chondrocyte implantation (ACI), matrix-associated autologous chondrocyte implantation (MACI) and microfracture, which are characterized by low patient compliance. Accordingly, cartilage tissue engineering (CTE) has received a lot of interest. Cell-laden hydrogels are favorable candidates for cartilage repair since they resemble the native tissue environment and promote the formation of extracellular matrix. Various types of hydrogels have been developed so far for CTE applications based on both natural and synthetic biomaterials. Among these materials, hyaluronic acid (HA), a principal component of the cartilage tissue which can be easily modified and biofunctionalized, has been favored for the development of hydrogels since it interacts with cell surface receptors, supports the growth of chondrocytes and promotes the differentiation of mesenchymal stem cells to chondrocytes. The present work reviews the various types of HA-based hydrogels (e.g., in situ forming hydrogels, cryogels, microgels and three-dimensional (3D)-bioprinted hydrogel constructs) that have been used for cartilage repair, specially focusing on the results of their preclinical and clinical assessment.

Effect of Chitosan Deacetylation on Its Affinity to Type III Collagen: A Molecular Dynamics Study

The ability to form strong intermolecular interactions by linear glucosamine polysaccharides with collagen is strictly related to their nonlinear dynamic behavior and hence bio-lubricating features. Type III collagen plays a crucial role in tissue regeneration, and its presence in the articular cartilage affects its bio-technical features. In this study, the molecular dynamics methodology was applied to evaluate the effect of deacetylation degree on the chitosan affinity to type III collagen. The computational procedure employed docking and geometry optimizations of different chitosan structures characterized by randomly distributed deacetylated groups. The eight different degrees of deacetylation from 12.5% to 100% were taken into account. We found an increasing linear trend (R2 = 0.97) between deacetylation degree and the collagen-chitosan interaction energy. This can be explained by replacing weak hydrophobic contacts with more stable hydrogen bonds involving amino groups in N-deacetylated chitosan moieties. In this study, the properties of chitosan were compared with hyaluronic acid, which is a natural component of synovial fluid and cartilage. As we found, when the degree of deacetylation of chitosan was greater than 0.4, it exhibited a higher affinity for collagen than in the case of hyaluronic acid.

Recent Advances in Synthetic and Natural Biomaterials-Based Therapy for Bone Defects

Synthetic and natural biomaterials are a promising alternative for the treatment of critical-sized bone defects. Several parameters such as their porosity, surface, and mechanical properties are extensively pointed out as key points to recapitulate the bone microenvironment. Many biomaterials with this pursuit are employed to provide a matrix, which can supply the specific environment and architecture for an adequate bone growth. Nevertheless, some queries remain unanswered. This review discusses the recent advances achieved by some synthetic and natural biomaterials to mimic the native structure of bone and the manufacturing technology applied to obtain biomaterial candidates. The focus of this review is placed in the recent advances in the development of biomaterial-based therapy for bone defects in different types of bone. In this context, this review gives an overview of the potentialities of synthetic and natural biomaterials: polyurethanes, polyesters, hyaluronic acid, collagen, titanium, and silica as successful candidates for the treatment of bone defects.

3D models of dilated cardiomyopathy: Shaping the chemical, physical and topographical properties of biomaterials to mimic the cardiac extracellular matrix

The pathophysiology of dilated cardiomyopathy (DCM), one major cause of heart failure, is characterized by the dilation of the heart but remains poorly understood because of the lack of adequate in vitro models. Current 2D models do not allow for the 3D organotypic organization of cardiomyocytes and do not reproduce the ECM perturbations. In this review, the different strategies to mimic the chemical, physical and topographical properties of the cardiac tissue affected by DCM are presented. The advantages and drawbacks of techniques generating anisotropy required for the cardiomyocytes alignment are discussed. In addition, the different methods creating macroporosity and favoring organotypic organization are compared. Besides, the advances in the induced pluripotent stem cells technology to generate cardiac cells from healthy or DCM patients will be described. Thanks to the biomaterial design, some features of the DCM extracellular matrix such as stiffness, porosity, topography or chemical changes can impact the cardiomyocytes function in vitro and increase their maturation. By mimicking the affected heart, both at the cellular and at the tissue level, 3D models will enable a better understanding of the pathology and favor the discovery of novel therapies.

Emerging Technologies in Multi-Material Bioprinting

Bioprinting, within the emerging field of biofabrication, aims at the fabrication of functional biomimetic constructs. Different 3D bioprinting techniques have been adapted to bioprint cell-laden bioinks. However, single-material bioprinting techniques oftentimes fail to reproduce the complex compositions and diversity of native tissues. Multi-material bioprinting as an emerging approach enables the fabrication of heterogeneous multi-cellular constructs that replicate their host microenvironments better than single-material approaches. Here, bioprinting modalities are reviewed, their being adapted to multi-material bioprinting is discussed, and their advantages and challenges, encompassing both custom-designed and commercially available technologies are analyzed. A perspective of how multi-material bioprinting opens up new opportunities for tissue engineering, tissue model engineering, therapeutics development, and personalized medicine is offered.

Functional hydrogels as therapeutic tools for spinal cord injury: New perspectives on immunopharmacological interventions

Spinal cord injury (SCI) is a complex medical and psychological challenge for which there is no curative therapy currently available. Despite major progress in pharmacological and surgical approaches, clinical trials for SCI patients have been uniformly disappointing thus far as there are many practical and biological issues yet to be resolved. Neuroinflammation is a critical event of the secondary injury phase after SCI, and recent research strategies have focused on modulating the immune response after injury to provide a more favorable recovery environment. Biomaterials can serve this purpose by providing physical and trophic support to the injured spinal cord after SCI. Of all potential biomaterials, functional hydrogels are emerging as a key component in novel treatment strategies for SCI, including controlled and localized delivery of immunomodulatory therapies to drive polarization of immune cells towards a pro-regenerative phenotype. Here, we extensively review recent developments in the use of functional hydrogels as immunomodulatory therapies for SCI. We briefly describe physicochemical properties of hydrogels and demonstrate how advanced fabrication methods lead to the required heterogeneity and hierarchical arrangements that increasingly mimic complex spinal cord tissue. We then summarize potential SCI therapeutic modalities including: (i) hydrogels alone; (ii) hydrogels as cellular or (iii) bioactive molecule delivery vehicles, and; (iv) combinatorial approaches. By linking the structural properties of hydrogels to their functions in treatment with particular focus on immunopharmacological stimuli, this may accelerate further development of functional hydrogels for SCI, and indeed next-generation central nervous system regenerative therapies.

Collagen Hydrogels Loaded with Silver Nanoparticles and Cannabis Sativa Oil

Wounds represent a major healthcare problem especially in hospital-associated infections where multi-drug resistant strains are often involved. Nowadays, biomaterials with therapeutic molecules play an active role in wound healing and infection prevention. In this work, the development of collagen hydrogels loaded with silver nanoparticles and Cannabis sativa oil extract is described. The presence of the silver nanoparticles gives interesting feature to the biomaterial such as improved mechanical properties or resistance to collagenase degradation but most important is the long-lasting antimicrobial effect. Cannabis sativa oil, which is known for its anti-inflammatory and analgesic effects, possesses antioxidant activity and successfully improved the biocompatibility and also enhances the antimicrobial activity of the nanocomposite. Altogether, these results suggest that this novel nanocomposite biomaterial is a promising alternative to common treatments of wound infections and wound healing.

Recent Advances on Stimuli-Responsive Hydrogels Based on Tissue-Derived ECMs and Their Components: Towards Improving Functionality for Tissue Engineering and Controlled Drug Delivery

Due to their highly hydrophilic nature and compositional versatility, hydrogels have assumed a protagonic role in the development of physiologically relevant tissues for several biomedical applications, such as in vivo tissue replacement or regeneration and in vitro disease modeling. By forming interconnected polymeric networks, hydrogels can be loaded with therapeutic agents, small molecules, or cells to deliver them locally to specific tissues or act as scaffolds for hosting cellular development. Hydrogels derived from decellularized extracellular matrices (dECMs), in particular, have gained significant attention in the fields of tissue engineering and regenerative medicine due to their inherently high biomimetic capabilities and endowment of a wide variety of bioactive cues capable of directing cellular behavior. However, these hydrogels often exhibit poor mechanical stability, and their biological properties alone are not enough to direct the development of tissue constructs with functional phenotypes. This review highlights the different ways in which external stimuli (e.g., light, thermal, mechanical, electric, magnetic, and acoustic) have been employed to improve the performance of dECM-based hydrogels for tissue engineering and regenerative medicine applications. Specifically, we outline how these stimuli have been implemented to improve their mechanical stability, tune their microarchitectural characteristics, facilitate tissue morphogenesis and enable precise control of drug release profiles. The strategic coupling of the bioactive features of dECM-based hydrogels with these stimulation schemes grants considerable advances in the development of functional hydrogels for a wide variety of applications within these fields.

A biofabrication method to align cells within bioprinted photocrosslinkable and cell-degradable hydrogel constructs via embedded fibers

The extracellular matrix (ECM) is composed of biochemical and biophysical cues that control cell behaviors and bulk mechanical properties. For example, anisotropy of the ECM and cell alignment are essential in the directional properties of tissues such as myocardium, tendon, and the knee meniscus. Technologies are needed to introduce anisotropic behavior into biomaterial constructs that can be used for the engineering of tissues as models and towards translational therapies. To address this, we developed an approach to align hydrogel fibers within cell-degradable bioink filaments with extrusion printing, where shear stresses during printing align fibers and photocrosslinking stabilizes the fiber orientation. Suspensions of hydrogel fibers were produced through the mechanical fragmentation of electrospun scaffolds of norbornene-modified hyaluronic acid, which were then encapsulated with meniscal fibrochondrocytes, mesenchymal stromal cells, or cardiac fibroblasts within gelatin-methacrylamide bioinks during extrusion printing into agarose suspension baths. Bioprinting parameters such as the needle diameter and the bioink flow rate influenced shear profiles, whereas the suspension bath properties and needle translation speed influenced filament diameters and uniformity. When optimized, filaments were formed with high levels of fiber alignment, which resulted in directional cell spreading during culture over one week. Controls that included bioprinted filaments without fibers or non-printed hydrogels of the same compositions either with or without fibers resulted in random cell spreading during culture. Further, constructs were printed with variable fiber and resulting cell alignment by varying print direction or using multi-material printing with and without fibers. This biofabrication technology advances our ability to fabricate constructs containing aligned cells towards tissue repair and the development of physiological tissue models.

Colonization versus encapsulation in cell-laden materials design: porosity and process biocompatibility determine cellularization pathways

Seeding materials with living cells has been-and still is-one of the most promising approaches to reproduce the complexity and the functionality of living matter. The strategies to associate living cells with materials are limited to cell encapsulation and colonization, however, the requirements for these two approaches have been seldom discussed systematically. Here we propose a simple two-dimensional map based on materials' pore size and the cytocompatibility of their fabrication process to draw, for the first time, a guide to building cellularized materials. We believe this approach may serve as a straightforward guideline to design new, more relevant materials, able to seize the complexity and the function of biological materials. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Biofabrication Strategies for Musculoskeletal Disorders: Evolution towards Clinical Applications

Biofabrication has emerged as an attractive strategy to personalise medical care and provide new treatments for common organ damage or diseases. While it has made impactful headway in e.g., skin grafting, drug testing and cancer research purposes, its application to treat musculoskeletal tissue disorders in a clinical setting remains scarce. Albeit with several in vitro breakthroughs over the past decade, standard musculoskeletal treatments are still limited to palliative care or surgical interventions with limited long-term effects and biological functionality. To better understand this lack of translation, it is important to study connections between basic science challenges and developments with translational hurdles and evolving frameworks for this fully disruptive technology that is biofabrication. This review paper thus looks closely at the processing stage of biofabrication, specifically at the bioinks suitable for musculoskeletal tissue fabrication and their trends of usage. This includes underlying composite bioink strategies to address the shortfalls of sole biomaterials. We also review recent advances made to overcome long-standing challenges in the field of biofabrication, namely bioprinting of low-viscosity bioinks, controlled delivery of growth factors, and the fabrication of spatially graded biological and structural scaffolds to help biofabricate more clinically relevant constructs. We further explore the clinical application of biofabricated musculoskeletal structures, regulatory pathways, and challenges for clinical translation, while identifying the opportunities that currently lie closest to clinical translation. In this article, we consider the next era of biofabrication and the overarching challenges that need to be addressed to reach clinical relevance.

Current Trends on Protein Driven Bioinks for 3D Printing

In the last decade, three-dimensional (3D) extrusion bioprinting has been on the top trend for innovative technologies in the field of biomedical engineering. In particular, protein-based bioinks such as collagen, gelatin, silk fibroin, elastic, fibrin and protein complexes based on decellularized extracellular matrix (dECM) are receiving increasing attention. This current interest is the result of protein's tunable properties, biocompatibility, environmentally friendly nature and possibility to provide cells with the adequate cues, mimicking the extracellular matrix's function. In this review we describe the most relevant stages of the development of a protein-driven bioink. The most popular formulations, molecular weights and extraction methods are covered. The different crosslinking methods used in protein bioinks, the formulation with other polymeric systems or molecules of interest as well as the bioprinting settings are herein highlighted. The cell embedding procedures, the in vitro, in vivo, in situ studies and final applications are also discussed. Finally, we approach the development and optimization of bioinks from a sequential perspective, discussing the relevance of each parameter during the pre-processing, processing, and post-processing stages of technological development. Through this approach the present review expects to provide, in a sequential manner, helpful methodological guidelines for the development of novel bioinks.

Double network laminarin-boronic/alginate dynamic bioink for 3D bioprinting cell-laden constructs

The design of dynamically crosslinked hydrogel bioinks for three-dimensional (3D) bioprinting is emerging as a valuable strategy to advance the fabrication of mechanically tuneable cell-laden constructs for 3Din vitrodisease modelling and tissue engineering applications. Herein, a dynamic bioink comprising boronic acid-functionalised laminarin and alginate is explored for bioprinting 3D constructs under physiologically relevant conditions. The formulated bioink takes advantage of a double crosslinked network that combines covalent but reversible boronate ester bonds and ionic gelation via divalent cations. Moreover, it exhibits suitable rheological properties and improved mechanical features owing to its modular crosslinking chemistry, yielding stable constructs with user-programmable architecture. We explored such dynamic bioink as a supporting matrix for different cell classes, namely osteoblast precursors, fibroblasts and breast cancer cells. The resulting cell-laden bioprinted hydrogels display a homogeneous cell distribution post-printing and exceptional cell viability (>90%) that can be maintained for prolonged time periods in culture (14 days) for all cell lines. This simple and chemically versatile approach is envisaged to accelerate the development of multifunctional bioinks and contribute towards the fabrication of biomimetic 3D scaffolds with applicability in a wide range of predictive or exploratory biomedical platforms.

Stem Cells and Extrusion 3D Printing for Hyaline Cartilage Engineering

Hyaline cartilage is deficient in self-healing properties. The early treatment of focal cartilage lesions is a public health challenge to prevent long-term degradation and the occurrence of osteoarthritis. Cartilage tissue engineering represents a promising alternative to the current insufficient surgical solutions. 3D printing is a thriving technology and offers new possibilities for personalized regenerative medicine. Extrusion-based processes permit the deposition of cell-seeded bioinks, in a layer-by-layer manner, allowing mimicry of the native zonal organization of hyaline cartilage. Mesenchymal stem cells (MSCs) are a promising cell source for cartilage tissue engineering. Originally isolated from bone marrow, they can now be derived from many different cell sources (e.g., synovium, dental pulp, Wharton’s jelly). Their proliferation and differentiation potential are well characterized, and they possess good chondrogenic potential, making them appropriate candidates for cartilage reconstruction. This review summarizes the different sources, origins, and densities of MSCs used in extrusion-based bioprinting (EBB) processes, as alternatives to chondrocytes. The different bioink constituents and their advantages for producing substitutes mimicking healthy hyaline cartilage is also discussed.

The Influence of Printing Parameters and Cell Density on Bioink Printing Outcomes

Bioink printability persists as a limiting factor toward many bioprinting applications. Printing parameter selection is largely user-dependent, and the effect of cell density on printability has not been thoroughly investigated. Recently, methods have been developed to give greater insight into printing outcomes. This study aims to further advance those methods and apply them to study the effect of printing parameters (feedrate and flowrate) and cell density on printability. Two printed structures, a crosshatch and five-layer tube, were established as printing standards and utilized to determine the printing outcomes. Acellular bioinks were printed using a testing matrix of feedrates of 37.5, 75, 150, 300, and 600 mm/min and flowrates of 21, 42, 84, 168, and 336 mm³/min. Structures were also printed with cell densities of 5, 10, 20, and 40 × 10⁶ cell/mL at 150 mm/min and 84 mm³/min. Only speed ratios (defined as flowrate divided by feedrate) from 0.07 to 2.24 mm² were suitable for analysis. Increasing speed ratio dramatically increased the height, width, and wall thickness of tubular structures, but did not influence radial accuracy. For crosshatch structures, the area of pores and the frequency of broken filaments were decreased without impacting pore shape (Pr). Within speed ratios, feedrate and flowrate had negligible, inconsistent effects. Cell density did not affect any printing outcomes despite slight rheological changes. Printing outcomes were dominated by the speed ratio, with feedrate, flowrate, and cell density having little impact on printing outcomes when controlling for speed ratio within the ranges tested. The relevance of these results to other bioinks and printing conditions requires continued investigation by the bioprinting community, as well as highlight speed ratio as a key variable to report and suggest that rheology is a more sensitive measure than printing outcomes.

Extracellular Matrix Mimics Using Hyaluronan-Based Biomaterials

Hyaluronan (HA) is a critical element of the extracellular matrix (ECM). The regulated synthesis and degradation of HA modulates the ECM chemical and physical properties that, in turn, influence cellular behavior. HA triggers signaling pathways associated with the adhesion, proliferation, migration, and differentiation of cells, mediated by its interaction with specific cellular receptors or by tuning the mechanical properties of the ECM. This review summarizes the recent advances on strategies used to mimic the HA present in the ECM to study healthy or pathological cellular behavior. This includes the development of HA-based 2D and 3D in vitro tissue models for the seeding and encapsulation of cells, respectively, and HA particles as carriers for the targeted delivery of therapeutic agents.