Chondrogenic differentiation of human ASCs by stiffness control in 3D fibrin hydrogel

Pump-Less Platform Enables Long-Term Recirculating Perfusion of 3D Printed Tubular Tissues

The direction and pattern of fluid flow affect vascular structure and function, in which vessel-lining endothelial cells exhibit variable cellular morphologies and vessel remodeling by mechanosensing. To recapitulate this microenvironment, some approaches have been reported to successfully apply unidirectional flow on endothelial cells in organ-on-a-chip systems. However, these platforms encounter drawbacks such as the dependency on pumps or confinement to closed microfluidic channels. These constraints impede their synergy with advanced biofabrication techniques like 3D bioprinting, thereby curtailing the potential to introduce greater complexity into engineered tissues. Herein, a pumpless recirculating platform (UniPlate) that enables unidirectional media recirculation through 3D printed tubular tissues, is demonstrated.The device is made of polystyrene via injection molding in combination with 3D printed sacrifical gelatin templates. Tubular blood vessels with unidirectional perfusion are firstly engineered. Then the design is expanded to incorporate duo-recirculating flow for culturing vascularized renal proximal tubules with glucose reabsorption function. In addition to media recirculation, human monocyte recirculation in engineered blood vessels is also demonstrated for over 24 h, with minimal loss of cells, cell viability, and inflammatory activation. UniPlate can be a valuable tool to more precisely control the cellular microenvironment of organ-on-a-chip systems for drug discovery.

Immune cell extravasation in an organ-on-chip to model lung imflammation

Acute respiratory distress syndrome (ARDS) is a severe lung condition with high mortality and various causes, including lung infection. No specific treatment is currently available and more research aimed at better understanding the pathophysiology of ARDS is needed. Most lung-on-chip models that aim at mimicking the air-blood barrier are designed with a horizontal barrier through which immune cells can migrate vertically, making it challenging to visualize and investigate their migration. In addition, these models often lack a barrier of natural protein-derived extracellular matrix (ECM) suitable for live cell imaging to investigate ECM-dependent migration of immune cells as seen in ARDS. This study reports a novel inflammation-on-chip model with live cell imaging of immune cell extravasation and migration during lung inflammation. The three-channel perfusable inflammation-on-chip system mimics the lung endothelial barrier, the ECM environment and the (inflamed) lung epithelial barrier. A chemotactic gradient was established across the ECM hydrogel, leading to the migration of immune cells through the endothelial barrier. We found that immune cell extravasation depends on the presence of an endothelial barrier, on the ECM density and stiffness, and on the flow profile. In particular, bidirectional flow, broadly used in association with rocking platforms, was found to importantly delay extravasation of immune cells in contrast to unidirectional flow. Extravasation was increased in the presence of lung epithelial tissue. This model is currently used to study inflammation-induced immune cell migration but can be used to study infection-induced immune cell migration under different conditions, such as ECM composition, density and stiffness, type of infectious agents used, and the presence of organ-specific cell types.

Additive Manufacturing and Physicomechanical Characteristics of PEGDA Hydrogels: Recent Advances and Perspective for Tissue Engineering

In this brief review, we discuss the recent advancements in using poly(ethylene glycol) diacrylate (PEGDA) hydrogels for tissue engineering applications. PEGDA hydrogels are highly attractive in biomedical and biotechnology fields due to their soft and hydrated properties that can replicate living tissues. These hydrogels can be manipulated using light, heat, and cross-linkers to achieve desirable functionalities. Unlike previous reviews that focused solely on material design and fabrication of bioactive hydrogels and their cell viability and interactions with the extracellular matrix (ECM), we compare the traditional bulk photo-crosslinking method with the latest three-dimensional (3D) printing of PEGDA hydrogels. We present detailed evidence combining the physical, chemical, bulk, and localized mechanical characteristics, including their composition, fabrication methods, experimental conditions, and reported mechanical properties of bulk and 3D printed PEGDA hydrogels. Furthermore, we highlight the current state of biomedical applications of 3D PEGDA hydrogels in tissue engineering and organ-on-chip devices over the last 20 years. Finally, we delve into the current obstacles and future possibilities in the field of engineering 3D layer-by-layer (LbL) PEGDA hydrogels for tissue engineering and organ-on-chip devices.

An assessment of the response of human MSCs to hydrostatic pressure in environments supportive of differential chondrogenesis

Mechanical stimulation can modulate the chondrogenic differentiation of stem/progenitor cells and potentially benefit tissue engineering (TE) of functional articular cartilage (AC). Mechanical cues like hydrostatic pressure (HP) are often applied to cell-laden scaffolds, with little optimization of other key parameters (e.g. cell density, biomaterial properties) known to effect lineage commitment. In this study, we first sought to establish cell seeding densities and fibrin concentrations supportive of robust chondrogenesis of human mesenchymal stem cells (hMSCs). High cell densities (15*10⁶ cells/ml) were more supportive of sGAG deposition on a per cell basis, while collagen deposition was higher at lower seeding densities (5*10⁶ cells/ml). Employment of lower fibrin (2.5 %) concentration hydrogels supported more robust chondrogenesis of hMSCs, with higher collagen type II and lower collagen type X deposition compared to 5 % hydrogels. The application of HP to hMSCs maintained in identified chondro-inductive culture conditions had little effect on overall levels of cartilage-specific matrix production. However, if hMSCs were first temporally primed with TGF-β3 before its withdrawal, they responded to HP by increased sGAG production. The response to HP in higher cell density cultures was also associated with a metabolic shift towards glycolysis, which has been linked with a mature chondrocyte-like phenotype. These results suggest that mechanical stimulation may not be necessary to engineer functional AC grafts using hMSCs if other culture conditions have been optimised. However, such bioreactor systems can potentially be employed to better understand how engineered tissues respond to mechanical loading in vivo once removed from in vitro culture environments.

Mechanical Behavior of 3D Printed Poly(ethylene glycol) Diacrylate Hydrogels in Hydrated Conditions Investigated Using Atomic Force Microscopy

Three-dimensional (3D) printed hydrogels fabricated using light processing techniques are poised to replace conventional processing methods used in tissue engineering and organ-on-chip devices. An intrinsic potential problem remains related to structural heterogeneity translated in the degree of cross-linking of the printed layers. Poly(ethylene glycol) diacrylate (PEGDA) hydrogels were used to fabricate both 3D printed multilayer and control monolithic samples, which were then analyzed using atomic force microscopy (AFM) to assess their nanomechanical properties. The fabrication of the hydrogel samples involved layer-by-layer (LbL) projection lithography and bulk cross-linking processes. We evaluated the nanomechanical properties of both hydrogel types in a hydrated environment using the elastic modulus (E) as a measure to gain insight into their mechanical properties. We observed that E increases by 4-fold from 2.8 to 11.9 kPa transitioning from bottom to the top of a single printed layer in a multilayer sample. Such variations could not be seen in control monolithic sample. The variation within the printed layers is ascribed to heterogeneities caused by the photo-cross-linking process. This behavior was rationalized by spatial variation of the polymer cross-link density related to variations of light absorption within the layers attributed to spatial decay of light intensity during the photo-cross-linking process. More importantly, we observed a significant 44% increase in E, from 9.1 to 13.1 kPa, as the indentation advanced from the bottom to the top of the multilayer sample. This finding implies that mechanical heterogeneity is present throughout the entire structure, rather than being limited to each layer individually. These findings are critical for design, fabrication, and application engineers intending to use 3D printed multilayer PEGDA hydrogels for in vitro tissue engineering and organ-on-chip devices.

Wetting of Cell Aggregates on Microdisk Topography Structures Achieved by Maskless Optical Projection Lithography

Cell aggregates as a 3D culture model can effectively mimic the physiological processes such as embryonic development, immune response, and tissue renewal in vivo. Researches show that the topography of biomaterials plays an important role in regulating cell proliferation, adhesion, and differentiation. It is of great significance to understand how cell aggregates respond to surface topography. Herein, microdisk array structures with the optimized size are used to investigate the wetting of cell aggregates. Cell aggregates exhibit complete wetting with distinct wetting velocities on the microdisk array structures of different diameters. The wetting velocity of cell aggregates reaches a maximum of 293 µm h^-1 on microdisk structures with a diameter of 2 µm and is a minimum of 247 µm h^-1 on microdisk structures of 20 µm diameter, which suggests that the cell-substrates adhesion energy on the latter is smaller. Actin stress fibers, focal adhesions (FAs), and cell morphology are analyzed to reveal the mechanisms of variation of wetting velocity. Furthermore, it is demonstrated that cell aggregates adopt climb and detour wetting modes on small and large-sized microdisk structures, respectively. This work reveals the response of cell aggregates to micro-scale topography, providing guidance for better understanding of tissue infiltration.

Scaffolds for Cartilage Tissue Engineering from a Blend of Polyethersulfone and Polyurethane Polymers

In recent years, one of the main goals of cartilage tissue engineering has been to find appropriate scaffolds for hyaline cartilage regeneration, which could serve as a matrix for chondrocytes or stem cell cultures. The study presents three types of scaffolds obtained from a blend of polyethersulfone (PES) and polyurethane (PUR) by a combination of wet-phase inversion and salt-leaching methods. The nonwovens made of gelatin and sodium chloride (NaCl) were used as precursors of macropores. Thus, obtained membranes were characterized by a suitable structure. The top layers were perforated, with pores over 20 µm, which allows cells to enter the membrane. The use of a nonwoven made it possible to develop a three-dimensional network of interconnected macropores that is required for cell activity and mobility. Examination of wettability (contact angle, swelling ratio) showed a hydrophilic nature of scaffolds. The mechanical test showed that the scaffolds were suitable for knee joint applications (stress above 10 MPa). Next, the scaffolds underwent a degradation study in simulated body fluid (SBF). Weight loss after four weeks and changes in structure were assessed using scanning electron microscopy (SEM) and MeMoExplorer Software, a program that estimates the size of pores. The porosity measurements after degradation confirmed an increase in pore size, as expected. Hydrolysis was confirmed by Fourier-transform infrared spectroscopy (FT-IR) analysis, where the disappearance of ester bonds at about 1730 cm−1 wavelength is noticeable after degradation. The obtained results showed that the scaffolds meet the requirements for cartilage tissue engineering membranes and should undergo further testing on an animal model.

Nanocomposite Hydrogels as Functional Extracellular Matrices

Over recent years, nano-engineered materials have become an important component of artificial extracellular matrices. On one hand, these materials enable static enhancement of the bulk properties of cell scaffolds, for instance, they can alter mechanical properties or electrical conductivity, in order to better mimic the in vivo cell environment. Yet, many nanomaterials also exhibit dynamic, remotely tunable optical, electrical, magnetic, or acoustic properties, and therefore, can be used to non-invasively deliver localized, dynamic stimuli to cells cultured in artificial ECMs in three dimensions. Vice versa, the same, functional nanomaterials, can also report changing environmental conditions-whether or not, as a result of a dynamically applied stimulus-and as such provide means for wireless, long-term monitoring of the cell status inside the culture. In this review article, we present an overview of the technological advances regarding the incorporation of functional nanomaterials in artificial extracellular matrices, highlighting both passive and dynamically tunable nano-engineered components.

Nanoindentation Response of 3D Printed PEGDA Hydrogels in a Hydrated Environment

Hydrogels are commonly used materials in tissue engineering and organ-on-chip devices. This study investigated the nanomechanical properties of monolithic and multilayered poly(ethylene glycol) diacrylate (PEGDA) hydrogels manufactured using bulk polymerization and layer-by-layer projection lithography processes, respectively. An increase in the number of layers (or reduction in layer thickness) from 1 to 8 and further to 60 results in a reduction in the elastic modulus from 5.53 to 1.69 and further to 0.67 MPa, respectively. It was found that a decrease in the number of layers induces a lower creep index (CIT) in three-dimensional (3D) printed PEGDA hydrogels. This reduction is attributed to mesoscale imperfections that appear as pockets of voids at the interfaces of the multilayered hydrogels attributed to localized regions of unreacted prepolymers, resulting in variations in defect density in the samples examined. An increase in the degree of cross-linking introduced by a higher dosage of ultraviolet (UV) exposure leads to a higher elastic modulus. This implies that the elastic modulus and creep behavior of hydrogels are governed and influenced by the degree of cross-linking and defect density of the layers and interfaces. These findings can guide an optimal manufacturing pathway to obtain the desirable nanomechanical properties in 3D printed PEGDA hydrogels, critical for the performance of living cells and tissues, which can be engineered through control of the fabrication parameters.

Technological advances in fibrin for tissue engineering

Fibrin is a promising natural polymer that is widely used for diverse applications, such as hemostatic glue, carrier for drug and cell delivery, and matrix for tissue engineering. Despite the significant advances in the use of fibrin for bioengineering and biomedical applications, some of its characteristics must be improved for suitability for general use. For example, fibrin hydrogels tend to shrink and degrade quickly after polymerization, particularly when they contain embedded cells. In addition, their poor mechanical properties and batch-to-batch variability affect their handling, long-term stability, standardization, and reliability. One of the most widely used approaches to improve their properties has been modification of the structure and composition of fibrin hydrogels. In this review, recent advances in composite fibrin scaffolds, chemically modified fibrin hydrogels, interpenetrated polymer network (IPN) hydrogels composed of fibrin and other synthetic or natural polymers are critically reviewed, focusing on their use for tissue engineering.

Mesenchymal Stromal Cells Laden in Hydrogels for Osteoarthritis Cartilage Regeneration: A Systematic Review from In Vitro Studies to Clinical Applications

This systematic review is focused on the main characteristics of the hydrogels used for embedding the mesenchymal stromal cells (MSCs) in in vitro/ex vivo studies, in vivo OA models and clinical trials for favoring cartilage regeneration in osteoarthritis (OA). PubMED and Embase databases were used to select the papers that were submitted to a public reference manager Rayyan Systematic Review Screening Software. A total of 42 studies were considered eligible: 25 articles concerned in vitro studies, 2 in vitro and ex vivo ones, 5 in vitro and in vivo ones, 8 in vivo ones and 2 clinical trials. Some in vitro studies evidenced a rheological characterization of the hydrogels and description of the crosslinking methods. Only 37.5% of the studies considered at the same time chondrogenic, fibrotic and hypertrophic markers. Ex vivo studies focused on hydrogel adhesion properties and the modification of MSC-laden hydrogels subjected to compression tests. In vivo studies evidenced the effect of cell-laden hydrogels in OA animal models or defined the chondrogenic potentiality of the cells in subcutaneous implantation models. Clinical studies confirmed the positive impact of these treatments on patients with OA. To speed the translation to the clinical use of cell-laden hydrogels, further studies on hydrogel characteristics, injection modalities, chemo-attractant properties and adhesion strength are needed.

Delivering Mechanical Stimulation to Cells: State of the Art in Materials and Devices Design

Biochemical signals, such as growth factors, cytokines, and transcription factors are known to play a crucial role in regulating a variety of cellular activities as well as maintaining the normal function of different tissues and organs. If the biochemical signals are assumed to be one side of the coin, the other side comprises biophysical cues. There is growing evidence showing that biophysical signals, and in particular mechanical cues, also play an important role in different stages of human life ranging from morphogenesis during embryonic development to maturation and maintenance of tissue and organ function throughout life. In order to investigate how mechanical signals influence cell and tissue function, tremendous efforts have been devoted to fabricating various materials and devices for delivering mechanical stimuli to cells and tissues. Here, an overview of the current state of the art in the design and development of such materials and devices is provided, with a focus on their design principles, and challenges and perspectives for future research directions are highlighted.

Identification of thrombin as a key regulator of chondrocyte catabolic activity through RNA-Seq and experimental verification

The present study was designed to explore the relationship between thrombin and catabolic activity in chondrocytes. Primary rat chondrocytes were cultured for 24 h with rat serum (RS), rat plasma (RP), or rat plasma supplemented with thrombin (RPT). RNA-sequencing was then performed. Cell proliferation was analyzed by EdU uptake, CCK-8 assays and protein-protein interaction (PPI) network of proliferation-related genes. Heatmaps were used to visualize differences in gene expression. Gene Ontology (GO) enrichment analyses of up- and down-regulated differentially expressed genes were conducted. Molecular probes were used to label the endoplasmic reticulum in chondrocytes from three treatment groups. Immunofluorescence and Safranin O staining were used to assess type II collagen (Col2a1) expression and proteoglycan synthesis, whereas Lox expression was assessed by immunocytochemistry. The expression of enzymes involved in the synthesis and maturation of extracellular matrix (ECM) components and chemokines were measured by qPCR while matrix metalloproteinases (MMPs) levels were evaluated by Western blotting. Relevant nodules were selected through further PPI network analyses. A total of 727 and 1162 genes were up- and down-regulated based on the Venn diagrams comparison among groups. Thrombin was thus able to promote chondrocyte proliferation and a shift towards fibrotic morphology, while upregulating MMPs and chemokines linked to ECM degradation. In addition, thrombin decreased the enzyme expression involved in the synthesis and maturation of ECM.

Hydrostatic pressure promotes chondrogenic differentiation and microvesicle release from human embryonic and bone marrow stem cells

Mechanical stimulation plays in an important role in regulating stem cell differentiation and their release of extracellular vesicles (EVs). In this study, effects of low magnitude hydrostatic pressure (HP) on the chondrogenic differentiation and microvesicle release from human embryonic stem cells (hESCs) and human bone marrow stem cells (hBMSCs) are examined. hESCs were differentiated into chondroprogenitors and then embedded in fibrin gels and subjected to HP (270 kPa, 1 Hz, 5 days per week). hBMSC pellets were differentiated in chondrogenic media and subjected to the same regime. HP significantly enhanced ACAN expression in hESCs. It also led to a significant increase in DNA content, sGAG content and total sGAG/DNA level in hBMSCs. Furthermore, HP significantly increased microvesicle protein content released from both cell types. These results highlight the benefit of HP bioreactor in promoting chondrogenesis and EV production for cartilage tissue engineering.

Soft substrates direct stem cell differentiation into the chondrogenic lineage without the use of growth factors

Mesenchymal stem cells (MSCs) hold great promise for the treatment of cartilage related injuries. However, selectively promoting stem cell differentiation in vivo is still challenging. Chondrogenic differentiation of MSCs usually requires the use of growth factors that lead to the overexpression of hypertrophic markers. In this study, for the first time the effect of stiffness on MSC differentiation has been tested without the use of growth factors. Three-dimensional collagen and alginate scaffolds were developed and characterised. Stiffness significantly affected gene expression and ECM deposition. While, all hydrogels supported chondrogenic differentiation and allowed deposition of collagen type II and aggrecan, the 5.75 kPa hydrogel showed limited production of collagen type I compared to the other two formulations. These findings demonstrated for the first time that stiffness can guide MSCs differentiation without the use of growth factors within a tissue engineering scaffold suitable for the treatment of cartilage defects.

Bioorthogonal hydroxyethyl cellulose-based scaffold crosslinked via click chemistry for cartilage tissue engineering applications

In this study, azide and alkyne moieties were introduced to the structure of citric acid-modified hydroxyethyl cellulose (HEC) and then through a bioorthogonal click chemistry method: Strain-promoted azide-alkyne cycloaddition, a novel crosslinked HEC scaffold (click sample) was obtained. Chemical modifications and successful crosslinking of the samples were assessed with FTIR and ¹H NMR spectroscopy. Lyophilized samples exhibited a porous interconnected microarchitecture with desirable features for commensurate cartilage tissue engineering applications. As the stability of scaffolds improved upon crosslinking, considerable water uptake and swelling degree of ~650% could still be measured for the click sample. Offering Young's modulus of ~10 MPa and tensile strength of ~0.43 MPa, the mechanical characteristics of click sample were comparable with those of normal cartilage tissue. Various in vitro biological assays, including MTT analysis, cellular attachment, histological staining with safranin O, and real-time PCR decisively approved significant biocompatibility, chondrogenic ability, and bioorthogonal features of click sample.

Porcine fibrin sealant combined with autologous chondrocytes successfully promotes full-thickness cartilage regeneration in a rabbit model

Xenogeneic porcine fibrin sealant (PFS), derived from porcine blood, was used as a scaffold for cartilage tissue engineering. PFS has a porous microstructure, biocompatibility and degradation, and it provides a perfect extracellular matrix environment for the adhesion and proliferation of chondrocytes. Recently, PFS in combination with autologous chondrocytes (ACs) were used to study the microstructure of PFS scaffolds and promotion effect on the proliferation and migration of ACs. In this study, we investigated the effects of PFS in combination with ACs on the healing of cartilage defects in rabbits. A full‐thickness cartilage defect was made in the femoral trochlear in rabbits, subsequently, three surgical procedures were used to repair the defect, namely: the defect was treated with microfracture (MF group); the defect was filled with PFS alone (PFS group) or in combination with ACs (PFS + ACs group); the unrepaired cartilage defects served as the control group (CD group). Three and 6 months after the operation, the reparative effect was evaluated using medical imaging, gross scoring, pathological staining, biomechanical testing and biochemical examination. The PFS group showed a limited effect on defect repair, this result was significantly worse than the MF group. The best reparative effect was observed in the PFS + ACs group. These results hinted that PFS in combination with autologous chondrocytes has broad prospects for clinical applications in cartilage tissue engineering.

Applications of Biomaterials in 3D Cell Culture and Contributions of 3D Cell Culture to Drug Development and Basic Biomedical Research

The process of evaluating the efficacy and toxicity of drugs is important in the production of new drugs to treat diseases. Testing in humans is the most accurate method, but there are technical and ethical limitations. To overcome these limitations, various models have been developed in which responses to various external stimuli can be observed to help guide future trials. In particular, three-dimensional (3D) cell culture has a great advantage in simulating the physical and biological functions of tissues in the human body. This article reviews the biomaterials currently used to improve cellular functions in 3D culture and the contributions of 3D culture to cancer research, stem cell culture and drug and toxicity screening.

ChondroGELesis: Hydrogels to harness the chondrogenic potential of stem cells

The extracellular matrix is a highly complex microenvironment, whose various components converge to regulate cell fate. Hydrogels, as water-swollen polymer networks composed by synthetic or natural materials, are ideal candidates to create biologically active substrates that mimic these matrices and target cell behaviour for a desired tissue engineering application. Indeed, the ability to tune their mechanical, structural, and biochemical properties provides a framework to recapitulate native tissues. This review explores how hydrogels have been engineered to harness the chondrogenic response of stem cells for the repair of damaged cartilage tissue. The signalling processes involved in hydrogel-driven chondrogenesis are also discussed, identifying critical pathways that should be taken into account during hydrogel design.

Composition and Mechanism of Three-Dimensional Hydrogel System in Regulating Stem Cell Fate

Three-dimensional (3D) hydrogel systems integrating different types of stem cells and scaffolding biomaterials have an important application in tissue engineering. The biomimetic hydrogels that pattern cell suspensions within 3D configurations of biomaterial networks allow for the transport of bioactive factors and mimic the stem cell niche in vivo, thereby supporting the proliferation and differentiation of stem cells. The composition of a 3D hydrogel system determines the physical and chemical characteristics that regulate stem cell function through a biological mechanism. Here, we discuss the natural and synthetic hydrogel compositions that have been employed in 3D scaffolding, focusing on their characteristics, fabrication, biocompatibility, and regulatory effects on stem cell proliferation and differentiation. We also discuss the regulatory mechanisms of cell-matrix interaction and cell-cell interaction in stem cell activities in various types of 3D hydrogel systems. Understanding hydrogel compositions and their cellular mechanisms can yield insights into how scaffolding biomaterials and stem cells interact and can lead to the development of novel hydrogel systems of stem cells in tissue engineering and stem cell-based regenerative medicine. Impact statement Three-dimensional hydrogel system of stem cell mimicking the stemcell niche holds significant promise in tissue engineering and regenerative medicine. Exactly how hydrogel composition regulates stem cell fate is not well understood. This review focuses on the composition of hydrogel, and how the hydrogel composition and its properties regulate the stem cell adhesion, growth, and differentiation. We propose that cell-matrix interaction and cell-cell interaction are important regulatory mechanisms in stem cell activities. Our review provides key insights into how the hydrogel composition regulates the stem cell fate, untangling the engineering of three-dimensional hydrogel systems for stem cells.

Review of Synthetic and Hybrid Scaffolds in Cartilage Tissue Engineering

Cartilage tissue is under extensive investigation in tissue engineering and regenerative medicine studies because of its limited regenerative potential. Currently, many scaffolds are undergoing scientific and clinical research. A key for appropriate scaffolding is the assurance of a temporary cellular environment that allows the cells to function as in native tissue. These scaffolds should meet the relevant requirements, including appropriate architecture and physicochemical and biological properties. This is necessary for proper cell growth, which is associated with the adequate regeneration of cartilage. This paper presents a review of the development of scaffolds from synthetic polymers and hybrid materials employed for the engineering of cartilage tissue and regenerative medicine. Initially, general information on articular cartilage and an overview of the clinical strategies for the treatment of cartilage defects are presented. Then, the requirements for scaffolds in regenerative medicine, materials intended for membranes, and methods for obtaining them are briefly described. We also describe the hybrid materials that combine the advantages of both synthetic and natural polymers, which provide better properties for the scaffold. The last part of the article is focused on scaffolds in cartilage tissue engineering that have been confirmed by undergoing preclinical and clinical tests.

Perfusion promotes endothelialized pore formation in high concentration fibrin gels otherwise unsuitable for tube development

Vascularization of tissue engineered implants is crucial for their survival and integration in the recipient's body. Pre-vascularized, fibrin-based implants offer a solution since low concentration fibrin hydrogels (1 mg/mL) have been shown to promote tube formation of endothelial cells in co-culture with adipogenic stem cells. However, higher fibrinogen concentrations (> 20 mg/mL) enabling the fabrication of stable implants are necessary.We here characterized fibrin gels of 1-30 mg/mL for their rheological properties and whether they support tube formation of endothelial cell-adipogenic stem cell co-cultures for up to 7 days. Moreover, 20 mg/mL gels containing preformed channels and endothelial cell-adipogenic stem cell co-culture were perfused continuously in a customized flow chamber with 3.9 dyn/cm² for 12 days and analyzed for capillary formation.Rheology of fibrin gels showed increasing stability proportional to fibrinogen concentration with 20 mg/mL gels having a storage module of 465 Pa. Complex tube networks stable for 7 days were observed at 1-5 mg/mL gels whereas higher concentrations showed initial sprouting only. However, perfusion of 20 mg/mL fibrin gels resulted in endothelialized pore formation in several layers of the gel with endothelial cell-adipogenic stem cell co-culture.Thus, perfusion supports the formation of capillary-like structures in fibrin gels that are too dense for spontaneous tube formation under static conditions. Future studies are necessary to further increase pore density and to investigate proper nutrition of tissue-specific target cells in the scaffold.