Anisotropic fibrous scaffolds for articular cartilage regeneration

A Non-woven Path: Electrospun Poly(lactic acid) Scaffolds for Kidney Tissue Engineering

Chronic kidney disease is a major global health problem affecting millions of people; kidney tissue engineering provides an opportunity to better understand this disease, and has the capacity to provide a cure. Two-dimensional cell culture and decellularised tissue have been the main focus of this research thus far, but despite promising results these methods are not without their shortcomings. Polymer fabrication techniques such as electrospinning have the potential to provide a non-woven path for kidney tissue engineering. In this experiment we isolated rat primary kidney cells which were seeded on electrospun poly(lactic acid) scaffolds. Our results showed that the scaffolds were capable of sustaining a multi-population of kidney cells, determined by the presence of: aquaporin-1 (proximal tubules), aquaporin-2 (collecting ducts), synaptopodin (glomerular epithelia) and von Willebrand factor (glomerular endothelia cells), viability of cells appeared to be unaffected by fibre diameter. The ability of electrospun polymer scaffold to act as a conveyor for kidney cells makes them an ideal candidate within kidney tissue engineering; the non-woven path provides benefits over decellularised tissue by offering a high morphological control as well as providing superior mechanical properties with degradation over a tuneable time frame.

Exploiting Advanced Hydrogel Technologies to Address Key Challenges in Regenerative Medicine

Regenerative medicine aims to tackle a panoply of challenges from repairing focal damage to articular cartilage to preventing pathological tissue remodeling after myocardial infarction. Hydrogels are water-swollen networks formed from synthetic or naturally derived polymers and are emerging as important tools to address these challenges. Recent advances in hydrogel chemistries are enabling researchers to create hydrogels that can act as 3D ex vivo tissue models, allowing them to explore fundamental questions in cell biology by replicating tissues' dynamic and nonlinear physical properties. Enabled by cutting edge techniques such as 3D bioprinting, cell-laden hydrogels are also being developed with highly controlled tissue-specific architectures, vasculature, and biological functions that together can direct tissue repair. Moreover, advanced in situ forming and acellular hydrogels are increasingly finding use as delivery vehicles for bioactive compounds and in mediating host cell response. Here, advances in the design and fabrication of hydrogels for regenerative medicine are reviewed. It is also addressed how controlled chemistries are allowing for precise engineering of spatial and time-dependent properties in hydrogels with a look to how these materials will eventually translate to clinical applications.

Hierarchical Design of Tissue Regenerative Constructs

The worldwide shortage of organs fosters significant advancements in regenerative therapies. Tissue engineering and regeneration aim to supply or repair organs or tissues by combining material scaffolds, biochemical signals, and cells. The greatest challenge entails the creation of a suitable implantable or injectable 3D macroenvironment and microenvironment to allow for ex vivo or in vivo cell-induced tissue formation. This review gives an overview of the essential components of tissue regenerating scaffolds, ranging from the molecular to the macroscopic scale in a hierarchical manner. Further, this review elaborates about recent pivotal technologies, such as photopatterning, electrospinning, 3D bioprinting, or the assembly of micrometer-scale building blocks, which enable the incorporation of local heterogeneities, similar to most native extracellular matrices. These methods are applied to mimic a vast number of different tissues, including cartilage, bone, nerves, muscle, heart, and blood vessels. Despite the tremendous progress that has been made in the last decade, it remains a hurdle to build biomaterial constructs in vitro or in vivo with a native-like structure and architecture, including spatiotemporal control of biofunctional domains and mechanical properties. New chemistries and assembly methods in water will be crucial to develop therapies that are clinically translatable and can evolve into organized and functional tissues.

Biofunctionalized aligned microgels provide 3D cell guidance to mimic complex tissue matrices

Natural healing is based on highly orchestrated processes, in which the extracellular matrix plays a key role. To resemble the native cell environment, we introduce an artificial extracellular matrix (aECM) with the capability to template hierarchical and anisotropic structures in situ, allowing a minimally-invasive application via injection. Synthetic, magnetically responsive, rod-shaped microgels are locally aligned and fixed by a biocompatible surrounding hydrogel, creating a hybrid anisotropic hydrogel (Anisogel), of which the physical, mechanical, and chemical properties can be tailored. The microgels are rendered cell-adhesive with GRGDS and incorporated either inside a cell-adhesive fibrin or bioinert poly(ethylene glycol) hydrogel to strongly interact with fibroblasts. GRGDS-modified microgels inside a fibrin-based Anisogel enhance fibroblast alignment and lead to a reduction in fibronectin production, indicating successful replacement of structural proteins. In addition, YAP-translocation to the nucleus increases with the concentration of microgels, indicating cellular sensing of the overall anisotropic mechanical properties of the Anisogel. For bioinert surrounding PEG hydrogels, GRGDS-microgels are required to support cell proliferation and fibronectin production. In contrast to fibroblasts, primary nerve growth is not significantly affected by the biomodification of the microgels. In conclusion, this approach opens new opportunities towards advanced and complex aECMs for tissue regeneration.

Controlling the orientation of a cell-synthesized extracellular matrix by using engineered gelatin-based building blocks

Surface micropatterned gelatin building blocks clearly increment the alignment degree of collagen-based microtissues synthesized by human dermal fibroblasts.

Biomimetic multidirectional scaffolds for zonal osteochondral tissue engineering via a lyophilization bonding approach

The zonal organization of osteochondral tissue underlies its long term function. Despite this, tissue engineering strategies targeted for osteochondral repair commonly rely on the use of isotropic biomaterials for tissue reconstruction. There exists a need for a new class of highly biomimetic, anisotropic scaffolds that may allow for the engineering of new tissue with zonal properties. To address this need, we report the facile production of monolithic multidirectional collagen-based scaffolds that recapitulate the zonal structure and composition of osteochondral tissue. First, superficial and osseous zone-mimicking scaffolds were fabricated by unidirectional freeze casting collagen-hyaluronic acid and collagen-hydroxyapatite-containing suspensions, respectively. Following their production, a lyophilization bonding process was used to conjoin these scaffolds with a distinct collagen-hyaluronic acid suspension mimicking the composition of the transition zone. Resulting matrices contained a thin, highly aligned superficial zone that interfaced with a cellular transition zone and vertically oriented calcified cartilage and osseous zones. Confocal microscopy confirmed a zone-specific localization of hyaluronic acid, reflecting the depth-dependent increase of glycosaminoglycans in the native tissue. Poorly crystalline, carbonated hydroxyapatite was localized to the calcified cartilage and osseous zones and bordered the transition zone. Compressive testing of hydrated scaffold zones confirmed an increase of stiffness with scaffold depth, where compressive moduli of chondral and osseous zones fell within or near ranges conducive for chondrogenesis or osteogenesis of mesenchymal stem cells. With the combination of these biomimetic architectural and compositional cues, these multidirectional scaffolds hold great promise for the engineering of zonal osteochondral tissue. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 948-958, 2018.

The effect of electrospun polycaprolactone scaffold morphology on human kidney epithelial cells

There is a pressing need for further advancement in tissue engineering of functional organs with a view to providing a more clinically relevant model for drug development and reduce the dependence on organ donation. Polymer-based scaffolds, such as polycaprolactone (PCL), have been highlighted as a potential avenue for tissue engineered kidneys, but there is little investigation down this stream. Focus within kidney tissue engineering has been on two-dimensional cell culture and decellularised tissue. Electrospun polymer scaffolds can be created with a variety of fibre diameters and have shown a great potential in many areas. The variation in morphology of tissue engineering scaffold has been shown to effect the way cells behave and integrate. In this study we examined the cellular response to scaffold architecture of novel electrospun scaffold for kidney tissue engineering. Fibre diameters of 1.10 ± 0.16 μm and 4.49 ± 0.47 μm were used with three distinct scaffold architectures. Traditional random fibres were spun onto a mandrel rotating at 250 rpm, aligned at 1800 rpm with novel cryogenic fibres spun onto a mandrel loaded with dry ice rotating at 250 rpm. Human kidney epithelial cells were grown for 1 and 2 weeks. Fibre morphology had no effect of cell viability in scaffolds with a large fibre diameter but significant differences were seen in smaller fibres. Fibre diameter had a significant effect in aligned and cryogenic scaffold. Imaging detailed the differences in cell attachment due to scaffold differences. These results show that architecture of the scaffold has a profound effect on kidney cells; whether that is effects of fibre diameter on the cell attachment and viability or the effect of fibre arrangement on the distribution of cells and their alignment with fibres. Results demonstrate that PCL scaffolds have the capability to maintain kidney cells life and should be investigated further as a potential scaffold in kidney tissue engineering.

Silk fibroin-chondroitin sulfate scaffold with immuno-inhibition property for articular cartilage repair

The demand of favorable scaffolds has increased for the emerging cartilage tissue engineering. Chondroitin sulfate (CS) and silk fibroin have been investigated and reported with safety and excellent biocompatibility as tissue engineering scaffolds. However, the rapid degradation rate of pure CS scaffolds presents a challenge to effectively recreate neo-tissue similar to natural articular cartilage. Meanwhile the silk fibroin is well used as a structural constituent material because its remarkable mechanical properties, long-lasting in vivo stability and hypoimmunity. The application of composite silk fibroin and CS scaffolds for joint cartilage repair has not been well studied. Here we report that the combination of silk fibroin and CS could synergistically promote articular cartilage defect repair. The silk fibroin (silk) and silk fibroin/CS (silk-CS) scaffolds were fabricated with salt-leaching, freeze-drying and crosslinking methodologies. The biocompatibility of the scaffolds was investigated in vitro by cell adhesion, proliferation and migration with human articular chondrocytes. We found that silk-CS scaffold maintained better chondrocyte phenotype than silk scaffold; moreover, the silk-CS scaffolds reduced chondrocyte inflammatory response that was induced by interleukin (IL)-1β, which is in consistent with the well-documented anti-inflammatory activities of CS. The in vivo cartilage repair was evaluated with a rabbit osteochondral defect model. Silk-CS scaffold induced more neo-tissue formation and better structural restoration than silk scaffold after 6 and 12weeks of implantation in ICRS histological evaluations. In conclusion, we have developed a silk fibroin/ chondroitin sulfate scaffold for cartilage tissue engineering that exhibits immuno-inhibition property and can improve the self-repair capacity of cartilage.

STATEMENT OF SIGNIFICANCE

Severe cartilage defect such as osteoarthritis (OA) is difficult to self-repair because of its avascular, aneural and alymphatic nature. Current scaffolds often focus on providing sufficient mechanical support or bio-mimetic structure to promote cartilage repair. Thus, silk has been adopted and investigated broadly. However, inflammation is one of the most important factors in OA. But few scaffolds for cartilage repair reported anti-inflammation property. Meanwhile, chondroitin sulfate (CS) is a glycosaminoglycan present in the natural cartilage ECM, and has exhibited a number of useful biological properties including anti-inflammatory activity. Thus, we designed this silk-CS scaffold and proved that this scaffold exhibited good anti-inflammatory effects both in vitro and in vivo, promoted the repair of articular cartilage defect in animal model.

A Drug-Induced Hybrid Electrospun Poly-Capro-Lactone: Cell-Derived Extracellular Matrix Scaffold for Liver Tissue Engineering

Liver transplant is the only treatment option for patients with end-stage liver failure, however, there are too few donor livers available for transplant. Whole organ tissue engineering presents a potential solution to the problem of rapidly escalating donor liver shortages worldwide. A major challenge for liver tissue engineers is the creation of a hepatocyte microenvironment; a niche in which liver cells can survive and function optimally. While polymers and decellularized tissues pose an attractive option for scaffold manufacturing, neither alone has thus far proved sufficient. This study exploited cell's native extracellular matrix (ECM) producing capabilities using two different histone deacetylase inhibitors, and combined these with the customizability and reproducibility of electrospun polymer scaffolds to produce a "best of both worlds" niche microenvironment for hepatocytes. The resulting hybrid poly-capro-lactone (PCL)-ECM scaffolds were validated using HepG2 hepatocytes. The hybrid PCL-ECM scaffolds maintained hepatocyte growth and function, as evidenced by metabolic activity and DNA quantitation. Mechanical testing revealed little significant difference between scaffolds, indicating that cells were responding to a biochemical and topographical profile rather than mechanical changes. Immunohistochemistry showed that the biochemical profile of the drug-derived and nondrug-derived ECMs differed in ratio of Collagen I, Laminin, and Fibronectin. Furthermore, the hybrid PCL-ECM scaffolds influence the gene expression profile of the HepG2s drastically; with expression of Albumin, Cytochrome P450 Family 1 Subfamily A Polypeptide 1, Cytochrome P450 Family 1 Subfamily A Polypeptide 2, Cytochrome P450 Family 3 Subfamily A Polypeptide 4, Fibronectin, Collagen I, and Collagen IV undergoing significant changes. Our results demonstrate that drug-induced hybrid PCL-ECM scaffolds provide a viable, translatable platform for creating a niche microenvironment for hepatocytes, supporting in vivo phenotype and function. These scaffolds offer great potential for tissue engineering and regenerative medicine strategies for whole organ tissue engineering.

Monomeric, porous type II collagen scaffolds promote chondrogenic differentiation of human bone marrow mesenchymal stem cells in vitro

Osteoarthritis (OA) is a common cause of pain and disability and is often associated with the degeneration of articular cartilage. Lesions to the articular surface, which are thought to progress to OA, have the potential to be repaired using tissue engineering strategies; however, it remains challenging to instruct cell differentiation within a scaffold to produce tissue with appropriate structural, chemical and mechanical properties. We aimed to address this by driving progenitor cells to adopt a chondrogenic phenotype through the tailoring of scaffold composition and physical properties. Monomeric type-I and type-II collagen scaffolds, which avoid potential immunogenicity associated with fibrillar collagens, were fabricated with and without chondroitin sulfate (CS) and their ability to stimulate the chondrogenic differentiation of human bone marrow-derived mesenchymal stem cells was assessed. Immunohistochemical analyses showed that cells produced abundant collagen type-II on type-II scaffolds and collagen type-I on type-I scaffolds. Gene expression analyses indicated that the addition of CS - which was released from scaffolds quickly - significantly upregulated expression of type II collagen, compared to type-I and pure type-II scaffolds. We conclude that collagen type-II and CS can be used to promote a more chondrogenic phenotype in the absence of growth factors, potentially providing an eventual therapy to prevent OA.

Regenerative Engineering for Knee Osteoarthritis Treatment: Biomaterials and Cell-Based Technologies

Knee osteoarthritis (OA) is the most common form of arthritis worldwide. The incidence of this disease is rising and its treatment poses an economic burden. Two early targets of knee OA treatment include the predominant symptom of pain, and cartilage damage in the knee joint. Current treatments have been beneficial in treating the disease but none is as effective as total knee arthroplasty (TKA). However, while TKA is an end-stage solution of the disease, it is an invasive and expensive procedure. Therefore, innovative regenerative engineering strategies should be established as these could defer or annul the need for a TKA. Several biomaterial and cell-based therapies are currently in development and have shown early promise in both preclinical and clinical studies. The use of advanced biomaterials and stem cells independently or in conjunction to treat knee OA could potentially reduce pain and regenerate focal articular cartilage damage. In this review, we discuss the pathogenesis of pain and cartilage damage in knee OA and explore novel treatment options currently being studied, along with some of their limitations.

Raman Spectroscopy Reveals New Insights into the Zonal Organization of Native and Tissue-Engineered Articular Cartilage

Tissue architecture is intimately linked with its functions, and loss of tissue organization is often associated with pathologies. The intricate depth-dependent extracellular matrix (ECM) arrangement in articular cartilage is critical to its biomechanical functions. In this study, we developed a Raman spectroscopic imaging approach to gain new insight into the depth-dependent arrangement of native and tissue-engineered articular cartilage using bovine tissues and cells. Our results revealed previously unreported tissue complexity into at least six zones above the tidemark based on a principal component analysis and k-means clustering analysis of the distribution and orientation of the main ECM components. Correlation of nanoindentation and Raman spectroscopic data suggested that the biomechanics across the tissue depth are influenced by ECM microstructure rather than composition. Further, Raman spectroscopy together with multivariate analysis revealed changes in the collagen, glycosaminoglycan, and water distributions in tissue-engineered constructs over time. These changes were assessed using simple metrics that promise to instruct efforts toward the regeneration of a broad range of tissues with native zonal complexity and functional performance.

Harnessing the Versatility of Bacterial Collagen to Improve the Chondrogenic Potential of Porous Collagen Scaffolds

Collagen I foams are used in the clinic as scaffolds to promote articular cartilage repair as they provide a bioactive environment for cells with chondrogenic potential. However, collagen I as a base material does not allow for precise control over bioactivity. Alternatively, recombinant bacterial collagens can be used as "blank slate" collagen molecules to offer a versatile platform for incorporation of selected bioactive sequences and fabrication into 3D scaffolds. Here, we show the potential of Streptococcal collagen-like 2 (Scl2) protein foams modified with peptides designed to specifically and noncovalently bind hyaluronic acid and chondroitin sulfate to improve chondrogenesis of human mesenchymal stem cells (hMSCs) compared to collagen I foams. Specific compositions of functionalized Scl2 foams lead to improved chondrogenesis compared to both nonfunctionalized Scl2 and collagen I foams, as indicated by gene expression, extracellular matrix accumulation, and compression moduli. hMSCs cultured in functionalized Scl2 foams exhibit decreased collagens I and X gene and protein expression, suggesting an advantage over collagen I foams in promoting a chondrocytic phenotype. These highly modular foams can be further modified to improve specific aspects chondrogenesis. As such, these scaffolds also have the potential to be tailored for other regenerative medicine applications.

A Closed Chondromimetic Environment within Magnetic-Responsive Liquified Capsules Encapsulating Stem Cells and Collagen II/TGF-β3 Microparticles

TGF-β3 is enzymatically immobilized by transglutaminase-2 action to poly(l-lactic acid) microparticles coated with collagen II. Microparticles are then encapsulated with stem cells inside liquified spherical compartments enfolded with a permselective shell through layer-by-layer adsorption. Magnetic nanoparticles are electrostatically bound to the multilayered shell, conferring magnetic-response ability. The goal of this study is to engineer a closed environment inside which encapsulated stem cells would undergo a self-regulated chondrogenesis. To test this hypothesis, capsules are cultured in chondrogenic differentiation medium without TGF-β3. Their biological outcome is compared with capsules encapsulating microparticles without TGF-β3 immobilization and cultured in normal chondrogenic differentiation medium containing soluble TGF-β3. Glycosaminoglycans quantification demosntrates that similar chondrogenesis levels are achieved. Moreover, collagen fibrils resembling the native extracellular matrix of cartilage can be observed. Importantly, the genetic evaluation of characteristic cartilage markers confirms the successful chondrogenesis, while hypertrophic markers are downregulated. In summary, the engineered capsules are able to provide a suitable and stable chondrogenesis environment for stem cells without the need of TGF-β3 supplementation. This kind of self-regulated capsules with softness, robustness, and magnetic responsive characteristics is expected to provide injectability and in situ fixation, which is of great advantage for minimal invasive strategies to regenerate cartilage.

Creating biomaterials with spatially organized functionality

Biomaterials for tissue engineering provide scaffolds to support cells and guide tissue regeneration. Despite significant advances in biomaterials design and fabrication techniques, engineered tissue constructs remain functionally inferior to native tissues. This is largely due to the inability to recreate the complex and dynamic hierarchical organization of the extracellular matrix components, which is intimately linked to a tissue's biological function. This review discusses current state-of-the-art strategies to control the spatial presentation of physical and biochemical cues within a biomaterial to recapitulate native tissue organization and function.

Secreted Endothelial Cell Factors Immobilized on Collagen Scaffolds Enhance the Recipient Endothelial Cell Environment

Strategies to design novel vascular scaffolds are a continuing aim in tissue engineering and often such designs encompass the use of recombinant factors to enhance the performance of the scaffold. The established use of cell secretion utilized in feeder systems and conditioned media offer a source of paracrine factors, which has potential to be used in tissue-engineered (TE) scaffolds. Here we utilize this principle from endothelial cells (ECs), to create a novel TE scaffold by harnessing secreted factors and immobilizing these to collagen scaffolds. This research revealed increased cellular attachment and positive angiogenic gene upregulation responses in recipient ECs grown on these conditioned scaffolds. Also, the conditioning method did not affect the mechanical structural integrity of the scaffolds. These results may advocate the potential use of this system to improve vascular scaffolds' in vivo performance. In addition, this process may be a future method utilized to improve other tissue engineering scaffold therapies.

Arrays of 3D double-network hydrogels for the high-throughput discovery of materials with enhanced physical and biological properties

Synthetic hydrogels are attractive biomaterials due to their similarity to natural tissues and their chemical tunability, which can impart abilities to respond to environmental cues, e.g. temperature, pH and light. The mechanical properties of hydrogels can be enhanced by the generation of a double-network. Here, we report the development of an array platform that allows the macroscopic synthesis of up to 80 single- and double-network hydrogels on a single microscope slide. This new platform allows for the screening of hydrogels as 3D features in a high-throughput format with the added dimension of significant control over the compressive and tensile properties of the materials, thus widening their potential application. The platform is adaptable to allow different hydrogels to be generated, with the potential ability to tune and alter the first and second network, and represents an exciting tool in material and biomaterial discovery.

Engineering Tissues That Mimic the Zonal Nature of Articular Cartilage Using Decellularized Cartilage Explants Seeded with Adult Stem Cells

Articular cartilage (AC) possesses uniquely complex mechanical properties; for example its stiffness increases with depth through the tissue and it softens when compressed. These properties are integral to the function of AC and can be attributed to the tissue's collagen network and how it interacts with negatively charged proteoglycans. In this study, scaffolds containing arrays of channels were produced from decellularized AC explants derived from skeletally immature and mature pigs. These scaffolds were then repopulated with human infrapatellar fat pad derived stem cells (FPSCs). After 4 weeks in culture, FPSCs filled channels within the decellularized explants with a matrix rich in proteoglycans and collagen. Cellular and neo-matrix alignment within these scaffolds appeared to be influenced by the underlying collagen architecture of the decellularized cartilage. Repopulating scaffolds derived from decellularized skeletally mature cartilage with FPSCs led to the development of engineered cartilage with depth-dependent mechanical properties mimicking aspects of native tissue. Furthermore, these constructs displayed the characteristic strain softening behavior of AC. These findings highlight the importance of the collagen network to engineering mechanically functional cartilage grafts.

Analyzing Biological Performance of 3D-Printed, Cell-Impregnated Hybrid Constructs for Cartilage Tissue Engineering

Three-dimensional (3D) bioprinting of hybrid constructs is a promising biofabrication method for cartilage tissue engineering because a synthetic polymer framework and cell-impregnated hydrogel provide structural and biological features of cartilage, respectively. During bioprinting, impregnated cells may be subjected to high temperatures (caused by the adjacent melted polymer) and process-induced mechanical forces, potentially compromising cell function. This study addresses these biofabrication issues, evaluating the heat distribution of printed polycaprolactone (PCL) strands and the rheological property and structural stability of alginate hydrogels at various temperatures and concentrations. The biocompatibility of parameters from these studies was tested by culturing 3D hybrid constructs bioprinted with primary cells from embryonic chick cartilage. During initial two-dimensional culture expansion of these primary cells, two morphologically and molecularly distinct cell populations ("rounded" and "fibroblastic") were isolated. The biological performance of each population was evaluated in 3D hybrid constructs separately. The cell viability, proliferation, and cartilage differentiation were observed at high levels in hybrid constructs of both cell populations, confirming the validity of these 3D bioprinting parameters for effective cartilage tissue engineering. Statistically significant performance variations were observed, however, between the rounded and fibroblastic cell populations. Molecular and morphological data support the notion that such performance differences may be attributed to the relative differentiation state of rounded versus fibroblastic cells (i.e., differentiated chondrocytes vs. chondroprogenitors, respectively), which is a relevant issue for cell-based tissue engineering strategies. Taken together, our study demonstrates that bioprinting 3D hybrid constructs of PCL and cell-impregnated alginate hydrogel is a promising approach for cartilage tissue engineering.

Application of cell and biomaterial-based tissue engineering methods in the treatment of cartilage, menisci and ligament injuries

Over 20 years ago it was realized that the traditional methods of the treatment of injuries to joint components: cartilage, menisci and ligaments, did not give satisfactory results and so there is a need of employing novel, more effective therapeutic techniques. Recent advances in molecular biology, biotechnology and polymer science have led to both the experimental and clinical application of various cell types, adapting their culture conditions in order to ensure a directed differentiation of the cells into a desired cell type, and employing non-toxic and non-immunogenic biomaterial in the treatment of knee joint injuries. In the present review the current state of knowledge regarding novel cell sources, in vitro conditions of cell culture and major important biomaterials, both natural and synthetic, used in cartilage, meniscus and ligament repair by tissue engineering techniques are described, and the assets and drawbacks of their clinical application are critically evaluated.

A hydrogel/fiber composite scaffold for chondrocyte encapsulation in cartilage tissue regeneration

A composite was constructed by embedding fragmented electrospun PLA nanofibers into an alginate-graft-hyaluronate hydrogel to generate an ECM-mimicking environment for cartilage repair.

Multiscale Biofabrication of Articular Cartilage: Bioinspired and Biomimetic Approaches

Articular cartilage is the load-bearing tissue found inside all articulating joints of the body. It vastly reduces friction and allows for smooth gliding between contacting surfaces. The structure of articular cartilage matrix and cellular composition is zonal and is important for its mechanical properties. When cartilage becomes injured through trauma or disease, it has poor intrinsic healing capabilities. The spectrum of cartilage injury ranges from isolated areas of the joint to diffuse breakdown and the clinical appearance of osteoarthritis. Current clinical treatment options remain limited in their ability to restore cartilage to its normal functional state. This review focuses on the evolution of biomaterial scaffolds that have been used for functional cartilage tissue engineering. In particular, we highlight recent developments in multiscale biofabrication approaches attempting to recapitulate the complex 3D matrix of native articular cartilage tissue. Additionally, we focus on the application of these methods to engineering each zone of cartilage and engineering full-thickness osteochondral tissues for improved clinical implantation. These methods have shown the potential to control individual cell-to-scaffold interactions and drive progenitor cell differentiation into a chondrocyte lineage. The use of these bioinspired nanoengineered scaffolds hold promise for recreation of structure and function on the whole tissue level and may represent exciting new developments for future clinical applications for cartilage injury and restoration.

A developmentally inspired combined mechanical and biochemical signaling approach on zonal lineage commitment of mesenchymal stem cells in articular cartilage regeneration

Articular cartilage is organized into multiple zones including superficial, middle and calcified zones with distinct cellular and extracellular components to impart lubrication, compressive strength, and rigidity for load transmission to bone, respectively. During native cartilage tissue development, changes in biochemical, mechanical, and cellular factors direct the formation of stratified structure of articular cartilage. The objective of this work was to investigate the effect of combined gradients in cell density, matrix stiffness, and zone-specific growth factors on the zonal organization of articular cartilage. Human mesenchymal stem cells (hMSCs) were encapsulated in acrylate-functionalized lactide-chain-extended polyethylene glycol (SPELA) gels simulating cell density and stiffness of the superficial, middle and calcified zones. The cell-encapsulated gels were cultivated in a medium supplemented with growth factors specific to each zone and the expression of zone-specific markers was measured with incubation time. Encapsulation of 60 × 10(6) cells per mL hMSCs in a soft gel (80 kPa modulus) and cultivation with a combination of TGF-β1 (3 ng mL(-1)) and BMP-7 (100 ng mL(-1)) led to the expression of markers for the superficial zone. Conversely, encapsulation of 15 × 10(6) cells per mL hMSCs in a stiff gel (320 MPa modulus) and cultivation with a combination of TGF-β1 (30 ng mL(-1)) and hydroxyapatite (3%) led to the expression of markers for the calcified zone. Further, encapsulation of 20 × 10(6) cells per mL hMSCs in a gel with 2.1 MPa modulus and cultivation with a combination of TGF-β1 (30 ng mL(-1)) and IGF-1 (100 ng mL(-1)) led to up-regulation of the middle zone markers. Results demonstrate that a developmental approach with gradients in cell density, matrix stiffness, and zone-specific growth factors can potentially regenerate zonal structure of the articular cartilage.

Highly flexible silica/chitosan hybrid scaffolds with oriented pores for tissue regeneration

Inorganic/organic sol-gel hybrids have nanoscale co-networks of organic and inorganic components that give them the unique potential of tailored mechanical properties and controlled biodegradation in tissue engineering applications. Here, silica/chitosan hybrid scaffolds with oriented structures were fabricated through the sol-gel method with a unidirectional freeze casting process. 3-Glycidoxypropyl trimethoxysilane (GPTMS) was used to obtain covalent inorganic/organic coupling. Process variables were investigated such as cooling rate, GPTMS and inorganic content, which can be used to tailor the mechanical properties and hybrid chemical coupling. Structural characterization and dissolution tests confirmed the covalent cross-linking of the chitosan and the silica network in hybrids. The scaffolds had a directional lamellar structure along the freezing direction and a cellular morphology perpendicular to the freezing direction. Compression testing showed that the scaffolds with 60 wt% organic were flexible and elastomeric perpendicular to the freezing direction whilst behaving in an elastic-brittle fashion parallel to the freezing direction. The compressive strengths are about one order of magnitude higher in the latter direction reaching values of the order of 160 kPa. This behaviour provides potential for clinicians to be able to squeeze the materials to fit tissue defect sites while providing some mechanical support from the other direction.

Modular and Versatile Spatial Functionalization of Tissue Engineering Scaffolds through Fiber-Initiated Controlled Radical Polymerization

Native tissues are typically heterogeneous and hierarchically organized, and generating scaffolds that can mimic these properties is critical for tissue engineering applications. By uniquely combining controlled radical polymerization (CRP), end-functionalization of polymers, and advanced electrospinning techniques, a modular and versatile approach is introduced to generate scaffolds with spatially organized functionality. Poly-ε-caprolactone is end functionalized with either a polymerization-initiating group or a cell-binding peptide motif cyclic Arg-Gly-Asp-Ser (cRGDS), and are each sequentially electrospun to produce zonally discrete bilayers within a continuous fiber scaffold. The polymerization-initiating group is then used to graft an antifouling polymer bottlebrush based on poly(ethylene glycol) from the fiber surface using CRP exclusively within one bilayer of the scaffold. The ability to include additional multifunctionality during CRP is showcased by integrating a biotinylated monomer unit into the polymerization step allowing postmodification of the scaffold with streptavidin-coupled moieties. These combined processing techniques result in an effective bilayered and dual-functionality scaffold with a cell-adhesive surface and an opposing antifouling non-cell-adhesive surface in zonally specific regions across the thickness of the scaffold, demonstrated through fluorescent labelling and cell adhesion studies. This modular and versatile approach combines strategies to produce scaffolds with tailorable properties for many applications in tissue engineering and regenerative medicine.

Development and characterisation of a decellularised bovine osteochondral biomaterial for cartilage repair

It is proposed that an acellular natural osteochondral scaffold will provide a successful repair material for the early intervention treatment of cartilage lesions, to prevent or slow the progression of cartilage deterioration to osteoarthritis. Here, we investigated the efficacy of methods for the decellularisation of bovine osteochondral plugs. The plugs were subject to four freeze/thaw cycles followed by two cycles of washes in hypotonic solution and low concentration (0.1% w/v) sodium dodecyl sulphate with protease inhibitors. Plugs were treated with nuclease (DNase and RNase) treatment followed by sterilization in peracetic acid. Full tissue decellularisation was achieved as confirmed by histological analysis and DNA quantification, however the resultant acellular matrix had reduced glycosaminoglycan content which led to an increased percent deformation of cartilage. Furthermore, the acellular scaffold was not reproducibly biocompatible. Additional terminal washes were included in the process to improve biocompatibility, however, this led to visible structural damage to the cartilage. This damage was found to be minimised by reducing the cut edge to cartilage area ratio through decellularisation of larger cuts of osteochondral tissue.

Versatile wedge-based system for the construction of unidirectional collagen scaffolds by directional freezing: practical and theoretical considerations

Aligned unidirectional collagen scaffolds may aid regeneration of those tissues where alignment of cells and extracellular matrix is essential, as for instance in cartilage, nerve bundles, and skeletal muscle. Pores can be introduced by ice crystal formation followed by freeze-drying, the pore architecture reflecting the ice crystal morphology. In this study we developed a wedge-based system allowing the production of a wide range of collagen scaffolds with unidirectional pores by directional freezing. Insoluble type I collagen suspensions were frozen using a custom-made wedge system, facilitating the formation of a horizontal as well as a vertical temperature gradient and providing a controlled solidification area for ice dendrites. The system permitted the growth of aligned unidirectional ice crystals over a large distance (>2.5 cm), an insulator prolonging the freezing process and facilitating the construction of crack-free scaffolds. Unidirectional collagen scaffolds with tunable pore sizes and pore morphologies were constructed by varying freezing rates and suspension media. The versatility of the system was indicated by the construction of unidirectional scaffolds from albumin, poly(vinyl alcohol) (a synthetic polymer), and collagen-polymer blends producing hybrid scaffolds. Macroscopic observations, temperature measurements, and scanning electron microscopy indicated that directed horizontal ice dendrite formation, vertical ice crystal nucleation, and evolutionary selection were the basis of the aligned unidirectional ice crystal growth and, hence, the aligned unidirectional pore structure. In conclusion, a simple, highly adjustable freezing system has been developed allowing the construction of large (hybrid) bioscaffolds with tunable unidirectional pore architecture.

Collagen-mimetic peptide-modifiable hydrogels for articular cartilage regeneration

Regenerative medicine strategies for restoring articular cartilage face significant challenges to recreate the complex and dynamic biochemical and biomechanical functions of native tissues. As an approach to recapitulate the complexity of the extracellular matrix, collagen-mimetic proteins offer a modular template to incorporate bioactive and biodegradable moieties into a single construct. We modified a Streptococcal collagen-like 2 protein with hyaluronic acid (HA) or chondroitin sulfate (CS)-binding peptides and then cross-linked with a matrix metalloproteinase 7 (MMP7)-sensitive peptide to form biodegradable hydrogels. Human mesenchymal stem cells (hMSCs) encapsulated in these hydrogels exhibited improved viability and significantly enhanced chondrogenic differentiation compared to controls that were not functionalized with glycosaminoglycan-binding peptides. Hydrogels functionalized with CS-binding peptides also led to significantly higher MMP7 gene expression and activity while the HA-binding peptides significantly increased chondrogenic differentiation of the hMSCs. Our results highlight the potential of this novel biomaterial to modulate cell-mediated processes and create functional tissue engineered constructs for regenerative medicine applications.

Peptide-directed spatial organization of biomolecules in dynamic gradient scaffolds

Specific binding peptides are used to spatially organize biomolecule gradients within an electrospun fiber scaffold. Different biomolecule-binding peptide-polymer conjugates are sequentially co-electrospun with a fiber-forming host polymer to generate opposing gradients of peptide functionalization. The binding peptides specifically and non-covalently guide the spatial arrangement of biomolecules into dynamic gradients within the scaffold, mimicking biological gradients found in native tissues.

Enhanced chondrogenic differentiation of dental pulp stem cells using nanopatterned PEG-GelMA-HA hydrogels

We have examined the effects of surface nanotopography and hyaluronic acid (HA) on in vitro chondrogenesis of dental pulp stem cells (DPSCs). Ultraviolet-assisted capillary force lithography was employed to fabricate well-defined nanostructured scaffolds of composite PEG-GelMA-HA hydrogels that consist of poly(ethylene glycol) dimethacrylate (PEGDMA), methacrylated gelatin (GelMA), and HA. Using this microengineered platform, we first demonstrated that DPSCs formed three-dimensional spheroids, which provide an appropriate environment for in vitro chondrogenic differentiation. We also found that DPSCs cultured on nanopatterned PEG-GelMA-HA scaffolds showed a significant upregulation of the chondrogenic gene markers (Sox9, Alkaline phosphatase, Aggrecan, Procollagen type II, and Procollagen type X), while downregulating the pluripotent stem cell gene, Nanog, and epithelial-mesenchymal genes (Twist, Snail, Slug) compared with tissue culture polystyrene-cultured DPSCs. Immunocytochemistry showed more extensive deposition of collagen type II in DPSCs cultured on the nanopatterned PEG-GelMA-HA scaffolds. These findings suggest that nanotopography and HA provide important cues for promoting chondrogenic differentiation of DPSCs.

Combinatorial scaffold morphologies for zonal articular cartilage engineering

Articular cartilage lesions are a particular challenge for regenerative medicine strategies as cartilage function stems from a complex depth-dependent organization. Tissue engineering scaffolds that vary in morphology and function offer a template for zone-specific cartilage extracellular matrix (ECM) production and mechanical properties. We fabricated multi-zone cartilage scaffolds by the electrostatic deposition of polymer microfibres onto particulate-templated scaffolds produced with 0.03 or 1.0mm(3) porogens. The scaffolds allowed ample space for chondrocyte ECM production within the bulk while also mimicking the structural organization and functional interface of cartilage's superficial zone. Addition of aligned fibre membranes enhanced the mechanical and surface properties of particulate-templated scaffolds. Zonal analysis of scaffolds demonstrated region-specific variations in chondrocyte number, sulfated GAG-rich ECM, and chondrocytic gene expression. Specifically, smaller porogens (0.03mm(3)) yielded significantly higher sGAG accumulation and aggrecan gene expression. Our results demonstrate that bilayered scaffolds mimic some key structural characteristics of native cartilage, support in vitro cartilage formation, and have superior features to homogeneous particulate-templated scaffolds. We propose that these scaffolds offer promise for regenerative medicine strategies to repair articular cartilage lesions.

3D braid scaffolds for regeneration of articular cartilage

Regenerating articular cartilage in vivo from cultured chondrocytes requires that the cells be cultured and implanted within a biocompatible, biodegradable scaffold. Such scaffolds must be mechanically stable; otherwise chondrocytes would not be supported and patients would experience severe pain. Here we report a new 3D braid scaffold that matches the anisotropic (gradient) mechanical properties of natural articular cartilage and is permissive to cell cultivation. To design an optimal structure, the scaffold unit cell was mathematically modeled and imported into finite element analysis. Based on this analysis, a 3D braid structure with gradient axial yarn distribution was designed and manufactured using a custom-built braiding machine. The mechanical properties of the 3D braid scaffold were evaluated and compared with simulated results, demonstrating that a multi-scale approach consisting of unit cell modeling and continuum analysis facilitates design of scaffolds that meet the requirements for mechanical compatibility with tissues.

Electrospun cartilage-derived matrix scaffolds for cartilage tissue engineering

Macroscale scaffolds created from cartilage-derived matrix (CDM) demonstrate chondroinductive or chondro-inductive properties, but many fabrication methods do not allow for control of nanoscale architecture. In this regard, electrospun scaffolds have shown significant promise for cartilage tissue engineering. However, nanofibrous materials generally exhibit a relatively small pore size and require techniques such as multilayering or the inclusion of sacrificial fibers to enhance cellular infiltration. The objectives of this study were (1) to compare multilayer to single-layer electrospun poly(ɛ-caprolactone) (PCL) scaffolds for cartilage tissue engineering, and (2) to determine whether incorporation of CDM into the PCL fibers would enhance chondrogenesis by human adipose-derived stem cells (hASCs). PCL and PCL-CDM scaffolds were prepared by sequential collection of 60 electrospun layers from the surface of a grounded saline bath into a single scaffold, or by continuous electrospinning onto the surface of a grounded saline bath and harvest as a single-layer scaffold. Scaffolds were seeded with hASCs and evaluated over 28 days in culture. The predominant effects on hASCs of incorporation of CDM into scaffolds were to stimulate sulfated glycosaminoglycan synthesis and COL10A1 gene expression. Compared with single-layer scaffolds, multilayer scaffolds enhanced cell infiltration and ACAN gene expression. However, compared with single-layer constructs, multilayer PCL constructs had a much lower elastic modulus, and PCL-CDM constructs had an elastic modulus approximately 1% that of PCL constructs. These data suggest that multilayer electrospun constructs enhance homogeneous cell seeding, and that the inclusion of CDM stimulates chondrogenesis-related bioactivity.

Electrospun membranes: control of the structure and structure related applications in tissue regeneration and drug delivery

Multilevel structures of electrospun membranes can be controlled and the designed structures can strongly affect cell behavior and drug delivery.

The changing integrin expression and a role for integrin β8 in the chondrogenic differentiation of mesenchymal stem cells

Many cartilage tissue engineering approaches aim to differentiate human mesenchymal stem cells (hMSCs) into chondrocytes and develop cartilage in vitro by targeting cell-matrix interactions. We sought to better inform the design of cartilage tissue engineering scaffolds by understanding how integrin expression changes during chondrogenic differentiation. In three models of in vitro chondrogenesis, we studied the temporal change of cartilage phenotype markers and integrin subunits during the differentiation of hMSCs. We found that transcript expression of most subunits was conserved across the chondrogenesis models, but was significantly affected by the time-course of differentiation. In particular, ITGB8 was up-regulated and its importance in chondrogenesis was further established by a knockdown of integrin β8, which resulted in a non-hyaline cartilage phenotype, with no COL2A1 expression detected. In conclusion, we performed a systematic study of the temporal changes of integrin expression during chondrogenic differentiation in multiple chondrogenesis models, and revealed a role for integrin β8 in chondrogenesis. This work enhances our understanding of the changing adhesion requirements of hMSCs during chondrogenic differentiation and underlines the importance of integrins in establishing a cartilage phenotype.

Similar hyaline-like cartilage repair of osteochondral defects in rabbits using isotropic and anisotropic collagen scaffolds

Lesions in knee joint articular cartilage (AC) have limited repair capacity. Many clinically available treatments induce a fibrous-like cartilage repair instead of hyaline cartilage. To induce hyaline cartilage repair, we hypothesized that type I collagen scaffolds with fibers aligned perpendicular to the AC surface would result in qualitatively better tissue repair due to a guided cellular influx from the subchondral bone. By specific freezing protocols, type I collagen scaffolds with isotropic and anisotropic fiber architectures were produced. Rabbits were operated on bilaterally and two full thickness defects were created in each knee joint. The defects were filled with (1) an isotropic scaffold, (2) an anisotropic scaffold with pores parallel to the cartilage surface, and (3) an anisotropic scaffold with pores perpendicular to the cartilage surface. Empty defects served as controls. After 4 (n=13) and 12 (n=13) weeks, regeneration was scored qualitatively and quantitatively using histological analysis and a modified O'Driscoll score. After 4 weeks, all defects were completely filled with partially differentiated hyaline cartilage tissue. No differences in O'Driscoll scores were measured between empty defects and scaffold types. After 12 weeks, all treatments led to hyaline cartilage repair visualized by increased glycosaminoglycan staining. Total scores were significantly increased for parallel anisotropic and empty defects over time (p<0.05). The results indicate that collagen scaffolds allow the formation of hyaline-like cartilage repair. Fiber architecture had no effect on cartilage repair.

Factors influencing the long-term behavior of extracellular matrix-derived scaffolds for musculoskeletal soft tissue repair

Musculoskeletal connective tissues such as tendon, ligament, and cartilage possess a limited ability for self-repair. Tissue engineering seeks to use combinations of cells, bioactive molecules, and biomaterials to develop new treatment options for the repair or replacement of damaged tissues. The use of native extracellular matrix as scaffold material for tissue engineering has become increasingly attractive because such tissues can not only provide structural support, but also regulate cell behavior. Although demineralized bone matrix has long been recognized for its osteoinductive abilities, recent studies have identified the ability of cartilage and tendon extracellular matrices to stimulate the differentiation of mesenchymal or adipose-derived adult stem cells toward chondrogenic or tenogenic lineages, respectively. This review discusses the motivation for fabricating scaffolds from musculoskeletal tissues, the in vitro and in vivo efficacy of these tissue-derived scaffolds, and various processing techniques such as decellularization or cross-linking that can mitigate immunogenic responses, moderate the degradation profile, and enhance the mechanical properties of these constructs following long-term implantation in vivo.

Preparation of chitosan/mesoporous silica nanoparticle composite hydrogels for sustained co-delivery of biomacromolecules and small chemical drugs

We have developed composite hydrogels of chitosan (CS) and mesoporous silica nanoparticles (MSNs) in this study. The gelation rate, gel strength, drug delivery behavior and chondrocyte proliferation properties were investigated. The introduction of MSNs into CS accelerated the gelation process at body temperature and also increased the elastic modulus G' from 1000 to 1800 Pa. When we used gentamicin (GS) and bovine serum albumin (BSA) as model small chemical drugs and biomacromolecules, respectively, the CS/MSN hydrogels released GS and BSA in a sustained manner simultaneously, but the CS hydrogels only showed sustained BSA release. Furthermore, in vitro chondrocyte culture showed that the CS/MSN composite hydrogels indeed performed much better in supporting chondrocyte growth and maintaining chondrocytic phenotype compared to the CS hydrogels. Therefore, the results suggest that the CS/MSN composite hydrogels can be potentially very useful for cartilage regeneration.

Effects of fiber orientation on the frictional properties and damage of regenerative articular cartilage surfaces

Articular cartilage provides a low-friction, wear-resistant surface for diarthrodial joints. Due to overloading and overuse, articular cartilage is known to undergo significant wear and degeneration potentially resulting in osteoarthritis (OA). Regenerative medicine strategies offer a promising solution for the treatment of articular cartilage defects and potentially localized early OA. Such strategies rely on the development of materials to restore some aspects of cartilage. In this study, microfibrous poly(ɛ-caprolactone) scaffolds of varying fiber orientations (random and aligned) were cultured with bovine chondrocytes for 4 weeks in vitro, and the mechanical and frictional properties were evaluated. Mechanical properties were quantified using unconfined compression and tensile testing techniques. Frictional properties were investigated at physiological compressive strains occurring in native articular cartilage. Scaffolds were sheared along the fiber direction, perpendicular to the fiber direction and in random orientation. The evolution of damage as a result of shear was evaluated via white light interferometry and scanning electron microscopy. As expected, the fiber orientation strongly affected the tensile properties as well as the compressive modulus of the scaffolds. Fiber orientation did not significantly affect the equilibrium frictional coefficient, but it was, however, a key factor in dictating the evolution of surface damage on the surface. Scaffolds shear tested perpendicular to the fiber orientation displayed the highest surface damage. Our results suggest that the fiber orientation of the scaffold implanted in the joint could strongly affect its resistance to damage due to shear. Scaffold fiber orientation should thus be carefully considered when using microfibrous scaffolds.

The design and development of a high-throughput magneto-mechanostimulation device for cartilage tissue engineering

To recapitulate the in vivo environment and create neo-organoids that replace lost or damaged tissue requires the engineering of devices, which provide appropriate biophysical cues. To date, bioreactors for cartilage tissue engineering have focused primarily on biomechanical stimulation. There is a significant need for improved devices for articular cartilage tissue engineering capable of simultaneously applying multiple biophysical (electrokinetic and mechanical) stimuli. We have developed a novel high-throughput magneto-mechanostimulation bioreactor, capable of applying static and time-varying magnetic fields, as well as multiple and independently adjustable mechanical loading regimens. The device consists of an array of 18 individual stations, each of which uses contactless magnetic actuation and has an integrated Hall Effect sensing system, enabling the real-time measurements of applied field, force, and construct thickness, and hence, the indirect measurement of construct mechanical properties. Validation tests showed precise measurements of thickness, within 14 μm of gold standard calliper measurements; further, applied force was measured to be within 0.04 N of desired force over a half hour dynamic loading, which was repeatable over a 3-week test period. Finally, construct material properties measured using the bioreactor were not significantly different (p=0.97) from those measured using a standard materials testing machine. We present a new method for articular cartilage-specific bioreactor design, integrating combinatorial magneto-mechanostimulation, which is very attractive from functional and cost viewpoints.

Tissue engineering and regenerative medicine: recent innovations and the transition to translation

The field of tissue engineering and regenerative medicine (TERM) has exploded in the last decade. In this Year (or so) in Review, we highlight some of the high impact advances within the field over the past several years. Using the past as our guide and starting with an objective premise, we attempt so to identify recent "hot topics" and transformative publications within the field. Through this process, several key themes emerged: (1) tissue engineering: grafts and materials, (2) regenerative medicine: scaffolds and factors that control endogenous tissue formation, (3) clinical trials, and (4) novel cell sources: induced pluripotent stem cells. Within these focus areas, we summarize the highly impactful articles that emerged from our objective analysis and review additional recent publications to augment and expand upon these key themes. Finally, we discuss where the TERM field may be headed and how to monitor such a broad-based and ever-expanding community.