Delivery of TGF-β1 and chondrocytes via injectable, biodegradable hydrogels for cartilage tissue engineering applications

Evaluation of cell-laden polyelectrolyte hydrogels incorporating poly(L-Lysine) for applications in cartilage tissue engineering

To address the lack of reliable long-term solutions for cartilage injuries, strategies in tissue engineering are beginning to leverage developmental processes to spur tissue regeneration. This study focuses on the use of poly(L-lysine) (PLL), previously shown to up-regulate mesenchymal condensation during developmental skeletogenesis in vitro, as an early chondrogenic stimulant of mesenchymal stem cells (MSCs). We characterized the effect of PLL incorporation on the swelling and degradation of oligo(poly(ethylene) glycol) fumarate) (OPF)-based hydrogels as functions of PLL molecular weight and dosage. Furthermore, we investigated the effect of PLL incorporation on the chondrogenic gene expression of hydrogel-encapsulated MSCs. The incorporation of PLL resulted in early enhancements of type II collagen and aggrecan gene expression and type II/type I collagen expression ratios when compared to blank controls. The presentation of PLL to MSCs encapsulated in OPF hydrogels also enhanced N-cadherin gene expression under certain culture conditions, suggesting that PLL may induce the expression of condensation markers in synthetic hydrogel systems. In summary, PLL can function as an inductive factor that primes the cellular microenvironment for early chondrogenic gene expression but may require additional biochemical factors for the generation of fully functional chondrocytes.

Strategies to Maximize the Potential of Marine Biomaterials as a Platform for Cell Therapy

Marine biopolymers have been explored as a promising cell therapy system for efficient cell delivery and tissue engineering. However, the marine biomaterial-based systems themselves have exhibited limited performance in terms of maintenance of cell viability and functions, promotion of cell proliferation and differentiation as well as cell delivery efficiency. Thus, numerous novel strategies have been devised to improve cell therapy outcomes. The strategies include optimization of physical and biochemical properties, provision of stimuli-responsive functions, and design of platforms for efficient cell delivery and tissue engineering. These approaches have demonstrated substantial improvement of therapeutic outcomes in a variety of research settings. In this review, therefore, research progress made with marine biomaterials as a platform for cell therapy is reported along with current research directions to further advance cell therapies as a tool to cure incurable diseases.

Clinical translation of controlled protein delivery systems for tissue engineering

Strategies that utilize controlled release of drugs and proteins for tissue engineering have enormous potential to regenerate damaged organs and tissues. The multiple advantages of controlled release strategies merit overcoming the significant challenges to translation, including high costs and long, difficult regulatory pathways. This review highlights the potential of controlled release of proteins for tissue engineering and regenerative medicine. We specifically discuss treatment modalities that have reached preclinical and clinical trials, with emphasis on controlled release systems for bone tissue engineering, the most advanced application with several products already in clinic. Possible strategies to address translational and regulatory concerns are also discussed.

Effects of Nanoscale Spatial Arrangement of Arginine-Glycine-Aspartate Peptides on Dedifferentiation of Chondrocytes

Cell dedifferentiation is of much importance in many cases such as the classic problem of dedifferentiation of chondrocytes during in vitro culture in cartilage tissue engineering. While cell differentiation has been much investigated, studies of cell dedifferentiation are limited, and the nanocues of cell dedifferentiation have little been reported. Herein, we prepared nanopatterns and micro/nanopatterns of cell-adhesive arginine-glycine-aspartate (RGD) peptides on nonfouling poly(ethylene glycol) (PEG) hydrogels to examine the effects of RGD nanospacing on adhesion and dedifferentiation of chondrocytes. The relatively larger RGD nanospacing above 70 nm was found to enhance the maintainence of the chondrocyte phenotype in two-dimensional culture, albeit not beneficial for adhesion of chondrocytes. A unique micro/nanopattern was employed to decouple cell spreading, cell shape, and cell-cell contact from RGD nanospacing. Under given spreading size and shape of single cells, the large RGD nanospacing was still in favor of preserving the normal phenotype of chondrocytes. Hence, the nanoscale spatial arrangement of cell-adhesive ligands affords a new independent regulator of cell dedifferentiation, which should be taken into consideration in biomaterial design for regenerative medicine.

Spatially organized differentiation of mesenchymal stem cells within biphasic microparticle-incorporated high cell density osteochondral tissues

Giving rise to both bone and cartilage during development, bone marrow-derived mesenchymal stem cells (hMSC) have the unique capacity to generate the complex tissues of the osteochondral interface. Utilizing a scaffold-free hMSC system, biphasic osteochondral constructs are incorporated with two types of growth factor-releasing microparticles to enable spatially organized differentiation. Gelatin microspheres (GM) releasing transforming growth factor-β1 (TGF-β1) combined with hMSC form the chondrogenic phase. The osteogenic phase contains hMSC only, mineral-coated hydroxyapatite microparticles (MCM), or MCM loaded with bone morphogenetic protein-2 (BMP-2), cultured in medium with or without BMP-2. After 4 weeks, TGF-β1 release from GM within the cartilage phase promotes formation of a glycosaminoglycan- and type II collagen-rich matrix, and has a local inhibitory effect on osteogenesis. In the osteogenic phase, type X collagen and osteopontin are produced in all conditions. However, calcification occurs on the outer edges of the chondrogenic phase in some constructs cultured in media containing BMP-2, and alkaline phosphatase levels are elevated, indicating that BMP-2 releasing MCM provides better control over region-specific differentiation. The production of complex, stem cell-derived osteochondral tissues via incorporated microparticles could enable earlier implantation, potentially improving outcomes in the treatment of osteochondral defects.

Strategies for the chemical and biological functionalization of scaffolds for cardiac tissue engineering: a review

The development of biomaterials for cardiac tissue engineering (CTE) is challenging, primarily owing to the requirement of achieving a surface with favourable characteristics that enhances cell attachment and maturation. The biomaterial surface plays a crucial role as it forms the interface between the scaffold (or cardiac patch) and the cells. In the field of CTE, synthetic polymers (polyglycerol sebacate, polyethylene glycol, polyglycolic acid, poly-l-lactide, polyvinyl alcohol, polycaprolactone, polyurethanes and poly(N-isopropylacrylamide)) have been proven to exhibit suitable biodegradable and mechanical properties. Despite the fact that they show the required biocompatible behaviour, most synthetic polymers exhibit poor cell attachment capability. These synthetic polymers are mostly hydrophobic and lack cell recognition sites, limiting their application. Therefore, biofunctionalization of these biomaterials to enhance cell attachment and cell material interaction is being widely investigated. There are numerous approaches for functionalizing a material, which can be classified as mechanical, physical, chemical and biological. In this review, recent studies reported in the literature to functionalize scaffolds in the context of CTE, are discussed. Surface, morphological, chemical and biological modifications are introduced and the results of novel promising strategies and techniques are discussed.

Incorporation of TGF-beta 3 within collagen-hyaluronic acid scaffolds improves their chondrogenic potential

Incorporation of therapeutics in the form of growth factors within biomaterials can enhance their biofunctionality. Two methods of incorporating transforming growth factor-beta 3 within collagen-hyaluronic acid scaffolds are described, markedly improving mesenchymal stem cell-mediated chondrogenic differentiation and matrix production. Such scaffolds offer control over the release of therapeutics, demonstrating their potential for repair of complex chondral defects requiring additional stimuli.

A novel tissue-engineered trachea with a mechanical behavior similar to native trachea

A novel tissue-engineered trachea was developed with appropriate mechanical behavior and substantial regeneration of tracheal cartilage. We designed hollow bellows scaffold as a framework of a tissue-engineered trachea and demonstrated a reliable method for three-dimensional (3D) printing of monolithic bellows scaffold. We also functionalized gelatin sponge to allow sustained release of TGF-β1 for stimulating tracheal cartilage regeneration and confirmed that functionalized gelatin sponge induces cartilaginous tissue formation in vitro. A tissue-engineered trachea was then created by assembling chondrocytes-seeded functionalized gelatin sponges into the grooves of bellows scaffold and it showed very similar mechanical behavior to that of native trachea along with substantial regeneration of tracheal cartilage in vivo. The tissue-engineered trachea developed here represents a novel concept of tracheal substitute with appropriate mechanical behavior similar to native trachea for use in reconstruction of tracheal stenosis.

An injectable, in situ forming type II collagen/hyaluronic acid hydrogel vehicle for chondrocyte delivery in cartilage tissue engineering

In this study, chondrocytes were encapsulated into an injectable, in situ forming type II collagen/hyaluronic acid (HA) hydrogel cross-linked with poly(ethylene glycol) ether tetrasuccinimidyl glutarate (4SPEG) and supplemented with the transforming growth factor β1 (TGFβ1). The chondrocyte-hydrogel constructs were cultured in vitro for 7 days and studied for cell viability and proliferation, morphology, glycosaminoglycan production, and gene expression. Type II collagen/HA/4SPEG formed a strong and stable hydrogel, and the chondrocytes remained viable during the encapsulation process and for the 7-day culture period. In addition, the encapsulated cells showed spherical morphology characteristic for chondrocytic phenotype. The cells were able to produce glycosaminoglycans into their extracellular matrix, and the gene expression of type II collagen and aggrecan, genes specific for differentiated chondrocytes, increased over time. The results indicate that the studied composite hydrogel with incorporated chondrogenic growth factor TGFβ1 is able to maintain chondrocyte viability and characteristics, and thus, it can be regarded as potential injectable cell delivery vehicle for cartilage tissue engineering.

Fabrication of cell-laden macroporous biodegradable hydrogels with tunable porosities and pore sizes

In this work, we investigated a cytocompatible particulate leaching method for the fabrication of cell-laden macroporous hydrogels. We used dehydrated and uncrosslinked gelatin microspheres as leachable porogens to create macroporous oligo(poly(ethylene glycol) fumarate) hydrogels. Varying gelatin content and size resulted in a wide range of porosities and pore sizes, respectively. Encapsulated mesenchymal stem cells (MSCs) exhibited high viability immediately following the fabrication process, and culture of cell-laden hydrogels revealed improved cell viability with increasing porosity. Additionally, the osteogenic potential of the encapsulated MSCs was evaluated over 16 days. Overall, this study presents a robust method for the preparation of cell-laden macroporous hydrogels with desired porosity and pore size for tissue engineering applications.

BMSCs laden injectable amino-diethoxypropane modified alginate-chitosan hydrogel for hyaline cartilage reconstruction

We developed an injectable hydrogel composed of amino-diethoxypropane modified alginate and chitosan, and also investigated bone marrow mesenchy + mal stromal cells (BMSCs) laden hydrogel for cartilage reconstruction in vitro and in vivo.

Injectable scaffold as minimally invasive technique for cartilage tissue engineering: in vitro and in vivo preliminary study

Cartilage is a tissue with limited repair capacity and also sparse population of cells entrapped within a dense extracellular matrix, therefore, delivery of the cells to site of damaged cartilage can improve its healing potential. Synthetic biomaterials such as poly (d,l-lactide-co-glycolide) (PLGA) have been used as both preformed or injectable scaffolds in tissue engineering in order to carry and keep cells in the site of injury with minimal side effects. The injectable biocompatible polymeric scaffolds can reach to effected area via minimally invasive injection without need to open the joint, less painful approach and also having possibility to fill complicated shape defects. In this study, it was hypothesized that PLGA solved in n-methyl pyrrolidine (NMP) may act as a proper carrier for cell delivery to the site of the damage and also supports their growth. The results of in vitro assays including both live/dead (AO/PI) and MTT showed the majority of the cells were remained alive between 3 up to 21 days, respectively. The amount of resealed GAG from the mesenchymal stem cells (MSCs) which were in contact with both PLGA and alginate constructs (used as control) indicated that for day 7 MSCs in contact with alginate secreted more GAG (3.45 ± 0.453 µg/mL for alginate and 2.36 ± 0.422 µg/mL for PLGA matrices), but at longer times (21 days) cells in contact with PLGA elicited more GAG (6.26 ± 0.968 µg/mL for alginate and 8.47 ± 0.871 µg/mL for the PLGA matrices). Sol–gel systems comprising PLGA, NMP, and cells as well as alginate/cells were subcutaneously injected into four nude mice (each mouse had three injection sites). PLGA/NMP was solidify immediately and formed an interconnecting 3-D porous structure that allowed body fluid to penetrate through them. In vivo evaluation showed that PLGA/NMP scaffolds could support injected cells as a fibrocartilage tissue was formed after 6 months of injection. We found that PLGA/NMP system might be a proper minimally invasive therapeutics option for cartilage repair.

Regulation and Role of TGFβ Signaling Pathway in Aging and Osteoarthritis Joints

Transforming growth factor beta (TGFβ) is a major signalling pathway in joints. This superfamilly is involved in numerous cellular processes in cartilage. Usually, they are considered to favor chondrocyte differentiation and cartilage repair. However, other studies show also deleterious effects of TGFβ which may induce hypertrophy. This may be explained at least in part by alteration of TGFβ signaling pathways in aging chondrocytes. This review focuses on the functions of TGFβ in joints and the regulation of its signaling mediators (receptors, Smads) during aging and osteoarthritis.

Effect of compatibilizers on in vitro biocompatibility of PLA–HA bioscaffold

This paper exclusively describes the biocompatibility evaluation of biodegradable PLA–HA-based composites as temporary bone scaffolds for bone tissue engineering in orthopaedic applications. For that purpose, a set of composites were prepared using 3D melt-deposition method that comprises a biopolymer namely polylactic acid (PLA), and a bioceramic filler, namely hydroxyapatite (HA) 10 wt%, and compatibilizers, namely poly acrylic acid (PAA) 2 wt% and maleic anhydirde (MAH) 2 wt%. The composite samples were evaluated by in vitro assays and biodegradability tests were conducted in phosphate-buffered saline (PBS). For the in vitro analysis, osteogenic-induced stem cells were seeded onto the composite scaffold. An inverted optical microscope with computerised image analysis system was used to obtain data regarding cell attachment and contact characteristics after seeding for 48 h. Results showed that the PLA–HA-based composites did not induce adverse reactions from the cells, which in addition to their bone-matching mechanical properties makes them promising materials for bone scaffold applications.

Pre-clinical characterization of tissue engineering constructs for bone and cartilage regeneration

Pre-clinical animal models play a crucial role in the translation of biomedical technologies from the bench top to the bedside. However, there is a need for improved techniques to evaluate implanted biomaterials within the host, including consideration of the care and ethics associated with animal studies, as well as the evaluation of host tissue repair in a clinically relevant manner. This review discusses non-invasive, quantitative, and real-time techniques for evaluating host-materials interactions, quality and rate of neotissue formation, and functional outcomes of implanted biomaterials for bone and cartilage tissue engineering. Specifically, a comparison will be presented for pre-clinical animal models, histological scoring systems, and non-invasive imaging modalities. Additionally, novel technologies to track delivered cells and growth factors will be discussed, including methods to directly correlate their release with tissue growth.

Tissue-engineered cartilage: the crossroads of biomaterials, cells and stimulating factors

Damage to cartilage represents one of the most challenging tasks of musculoskeletal therapeutics due to its limited propensity for healing and regenerative capabilities. Lack of current treatments to restore cartilage tissue function has prompted research in this rapidly emerging field of tissue regeneration of functional cartilage tissue substitutes. The development of cartilaginous tissue largely depends on the combination of appropriate biomaterials, cell source, and stimulating factors. Over the years, various biomaterials have been utilized for cartilage repair, but outcomes are far from achieving native cartilage architecture and function. This highlights the need for exploration of suitable biomaterials and stimulating factors for cartilage regeneration. With these perspectives, we aim to present an overview of cartilage tissue engineering with recent progress, development, and major steps taken toward the generation of functional cartilage tissue. In this review, we have discussed the advances and problems in tissue engineering of cartilage with strong emphasis on the utilization of natural polymeric biomaterials, various cell sources, and stimulating factors such as biophysical stimuli, mechanical stimuli, dynamic culture, and growth factors used so far in cartilage regeneration. Finally, we have focused on clinical trials, recent innovations, and future prospects related to cartilage engineering.

Controlled release of transforming growth factor-β3 from cartilage-extra-cellular-matrix-derived scaffolds to promote chondrogenesis of human-joint-tissue-derived stem cells

The objective of this study was to develop a scaffold derived from cartilaginous extracellular matrix (ECM) that could be used as a growth factor delivery system to promote chondrogenesis of stem cells. Dehydrothermal crosslinked scaffolds were fabricated using a slurry of homogenized porcine articular cartilage, which was then seeded with human infrapatellar-fat-pad-derived stem cells (FPSCs). It was found that these ECM-derived scaffolds promoted superior chondrogenesis of FPSCs when the constructs were additionally stimulated with transforming growth factor (TGF)-β3. Cell-mediated contraction of the scaffold was observed, which could be limited by the additional use of 1-ethyl-3-3dimethyl aminopropyl carbodiimide (EDAC) crosslinking without suppressing cartilage-specific matrix accumulation within the construct. To further validate the utility of the ECM-derived scaffold, we next compared its chondro-permissive properties to a biomimetic collagen-hyaluronic acid (HA) scaffold optimized for cartilage tissue engineering (TE) applications. The cartilage-ECM-derived scaffold supported at least comparable chondrogenesis to the collagen-HA scaffold, underwent less contraction and retained a greater proportion of synthesized sulfated glycosaminoglycans. Having developed a promising scaffold for TE, with superior chondrogenesis observed in the presence of exogenously supplied TGF-β3, the final phase of the study explored whether this scaffold could be used as a TGF-β3 delivery system to promote chondrogenesis of FPSCs. It was found that the majority of TGF-β3 that was loaded onto the scaffold was released in a controlled manner over the first 10days of culture, with comparable long-term chondrogenesis observed in these TGF-β3-loaded constructs compared to scaffolds where the TGF-β3 was continuously added to the media. The results of this study support the use of cartilage-ECM-derived scaffolds as a growth factor delivery system for use in articular cartilage regeneration.

Effect of hydroxyapatite-containing microspheres embedded into three-dimensional magnesium phosphate scaffolds on the controlled release of lysozyme and in vitro biodegradation

The functionality of porous three-dimensional (3D) magnesium phosphate (MgP) scaffold was investigated for the development of a novel protein delivery system and biomimetic bone tissue engineering scaffold. This enhancement can be achieved by incorporation of hydroxyapatite (HA)-containing polymeric microspheres (MSs) into a bulk MgP matrix, and a paste-extruding deposition (PED) system. In this work, the amount of MS and HA was precisely controlled when manufacturing MS-embedded MgP (MS/MgP) composite scaffolds. The main influence was researched in terms of in vitro lysozyme-release, in vitro biodegradation, mechanical properties, and in vitro calcification. The controlled release of lysozyme was indicated, while showing graded release patterns according to HA content. The composite scaffolds degraded gradually with MS content and degradation time. Due to the effect of HA inclusion, the higher HA-containing MS/MgP scaffolds could, not only delay the biodegradation process but also, compensate for the possible loss of mechanical properties. In this regard, it is reasonable to confirm the inverse relationship between biodegradation and corresponding compressive properties. In order to encourage bioactivity and osteoconductivity, the MS/MgP composite scaffolds were subjected to simulated body fluid treatment. Calcium deposition was, in turn, improved with increasing MS and HA content over time. This quantitative result was also proved using morphological and elemental analysis. In summary, a significant transformation of a monolithic MgP scaffold was directed toward a multifunctional bone tissue engineering scaffold equipped with controlled protein delivery, biodegradability, and bioactivity.

Healing improvement after rotator cuff repair using gelatin-grafted poly(L-lactide) electrospun fibrous membranes

BACKGROUND

Rotator cuff tears (RCTs) are a common cause of shoulder pain and disability in middle and older age. Despite improvements in the understanding of this disease process and advances in surgical treatment, rotator cuff (RC) repair failure rates remain high. Insufficient healing capacity is likely the main factor for failure of reconstruction.

MATERIALS AND METHODS

We fabricated implantable biodegradable gelatin-grafted poly(L-lactide) (PLLA) fibrous membranes using electrospinning technology and evaluated them using in vitro cell proliferation assays. Then, we established chronic rat RCT models and randomly assigned rats into one of three groups. In group 1 (n = 48), the detached supraspinatus tendon was repaired to its anatomic footprint (transosseous repair). In groups 2 and 3, the rats underwent transosseous repair and were implanted with either pure PLLA membranes (n = 48) or gelatin-PLLA membranes (n = 48) to augment the repairs. The animals were killed at 2, 4, and 8 wk postoperatively, which was followed by histomorphometric and biomechanical evaluation.

RESULTS

Histologic observations revealed that gelatin-PLLA membranes have excellent biocompatibility and biodegradability. At 2, 4, and 8 wk postoperatively, the gelatin-PLLA membranes significantly increased the area of glycosaminoglycan staining at the tendon-bone interface compared with the control group (P < 0.05) and significantly improved collagen organization, as measured by birefringence under polarized light at the healing enthesis compared with the control and PLLA groups (P < 0.05). Biomechanical testing revealed that the gelatin-PLLA group had a greater ultimate load to failure and stiffness than the control group at 4 and 8 wk (P < 0.05). The gelatin-PLLA membranes had the highest stress of the healing enthesis.

CONCLUSIONS

Local application of gelatin-PLLA fibrous membranes to the healing tendon-bone interface after RC repair in a rat chronic RCT model was found to strengthen the healing enthesis, increase the area of fibrocartilage, and improve collagen organization compared with repair alone. Augmentation with gelatin-grafted PLLA may enhance healing after RC repair and might eventually lead to improvement of clinical surgical outcomes.

Mathematical modelling of glycosaminoglycan production by stem cell aggregates incorporated with growth factor-releasing polymer microspheres

Systems composed of high density cells incorporated with growth factor-releasing polymer microspheres have recently been shown to promote chondrogenic differentiation and cartilage formation. Within these systems, the effects of spatial and temporal patterning of growth factor release on hyaline cartilage-specific extracellular matrix production have been examined. However, at present, it is unclear which microsphere densities and growth factor delivery profiles are optimal for inducing human mesenchymal stem cell differentiation and glycosaminoglycan production. A mathematical model to describe glycosaminoglycan production as a function of initial microsphere loading and microsphere degradation rate over a period of 3 weeks is presented. Based on predictions generated by this model, it may be feasible to design a bioactive microsphere system with specific spatiotemporal growth factor presentation characteristics to promote glycosaminoglycan production at controllable rates. Copyright © 2014 John Wiley & Sons, Ltd.

Dual growth factor delivery from bilayered, biodegradable hydrogel composites for spatially-guided osteochondral tissue repair

The present work investigated the use of biodegradable hydrogel composite scaffolds, based on the macromer oligo(poly(ethylene glycol) fumarate) (OPF), to deliver growth factors for the repair of osteochondral tissue in a rabbit model. In particular, bilayered OPF composites were used to mimic the structural layers of the osteochondral unit, and insulin-like growth factor-1 (IGF-1) and bone morphogenetic protein-2 (BMP-2) were loaded into gelatin microparticles and embedded within the OPF hydrogel matrix in a spatially controlled manner. Three different scaffold formulations were implanted in a medial femoral condyle osteochondral defect: 1) IGF-1 in the chondral layer, 2) BMP-2 in the subchondral layer, and 3) IGF-1 and BMP-2 in their respective separate layers. The quantity and quality of osteochondral repair was evaluated at 6 and 12 weeks with histological scoring and micro-computed tomography (micro-CT). While histological scoring results at 6 weeks showed no differences between experimental groups, micro-CT analysis revealed that the delivery of BMP-2 alone increased the number of bony trabecular islets formed, an indication of early bone formation, over that of IGF-1 delivery alone. At 12 weeks post-implantation, minimal differences were detected between the three groups for cartilage repair. However, the dual delivery of IGF-1 and BMP-2 had a higher proportion of subchondral bone repair, greater bone growth at the defect margins, and lower bone specific surface than the single delivery of IGF-1. These results suggest that the delivery of BMP-2 enhances subchondral bone formation and that, while the dual delivery of IGF-1 and BMP-2 in separate layers does not improve cartilage repair under the conditions studied, they may synergistically enhance the degree of subchondral bone formation. Overall, bilayered OPF hydrogel composites demonstrate potential as spatially-guided, multiple growth factor release vehicles for osteochondral tissue repair.

Direct and indirect co-culture of chondrocytes and mesenchymal stem cells for the generation of polymer/extracellular matrix hybrid constructs

In this work, the influence of direct cell-cell contact in co-cultures of mesenchymal stem cells (MSCs) and chondrocytes for the improved deposition of cartilage-like extracellular matrix (ECM) within nonwoven fibrous poly(∊-caprolactone) (PCL) scaffolds was examined. To this end, chondrocytes and MSCs were either co-cultured in direct contact by mixing on a single PCL scaffold or produced via indirect co-culture, whereby the two cell types were seeded on separate scaffolds which were then cultured together in the same system either statically or under media perfusion in a bioreactor. In static cultures, the chondrocyte scaffold of an indirectly co-cultured group generated significantly greater amounts of glycosaminoglycan and collagen than the direct co-culture group initially seeded with the same number of chondrocytes. Furthermore, improved ECM production was linked to greater cellular proliferation and distribution throughout the scaffold in static culture. In perfusion cultures, flow had a significant effect on the proliferation of the chondrocytes. The ECM contents within the chondrocyte-containing scaffolds of the indirect co-culture groups either approximated or surpassed the amounts generated within the direct co-culture group. Additionally, within bioreactor culture there were indications that chondrocytes had an influence on the chondrogenesis of MSCs as evidenced by increases in cartilaginous ECM synthetic capacity. This work demonstrates that it is possible to generate PCL/ECM hybrid scaffolds for cartilage regeneration by utilizing the factors secreted by two different cell types, chondrocytes and MSCs, even in the absence of juxtacrine signaling.

Degradable poly(ethylene glycol)-based hydrogels: Synthesis, physico-chemical properties and in vitro characterization

Designing degradable hydrogels is complicated by the structural and temporal complexities of the gel and evolving tissue. A major challenge is to create scaffolds with sufficient mechanical properties to restore initial function while simultaneously controlling temporal changes in the gel structure to facilitate tissue formation. Poly(ethylene glycol) was used in this work, to form biodegradable poly(ethylene glycol)-based hydrogels with hydrolyzable poly-l-lactide segments in the backbone. Non-degradable poly(ethylene glycol) was also introduced in the formulation to obtain control of the degradation profile that encompasses cell growth and new tissue formation. The dependence on polymer composition was observed by higher degradation profiles and decreased mechanical properties as the content of degradable segments was increased in the formulation. Based on in vitro tests, no toxicity of extracts or biomaterial in direct contact with human adipose tissue stem cells was observed, and the ultraviolet light treatment did not affect the proliferation capacity of the cells.

Cartilage tissue engineering: molecular control of chondrocyte differentiation for proper cartilage matrix reconstruction

BACKGROUND

Articular cartilage defects are a veritable therapeutic problem because therapeutic options are very scarce. Due to the poor self-regeneration capacity of cartilage, minor cartilage defects often lead to osteoarthritis. Several surgical strategies have been developed to repair damaged cartilage. Autologous chondrocyte implantation (ACI) gives encouraging results, but this cell-based therapy involves a step of chondrocyte expansion in a monolayer, which results in the loss in the differentiated phenotype. Thus, despite improvement in the quality of life for patients, reconstructed cartilage is in fact fibrocartilage. Successful ACI, according to the particular physiology of chondrocytes in vitro, requires active and phenotypically stabilized chondrocytes.

SCOPE OF REVIEW

This review describes the unique physiology of cartilage, with the factors involved in its formation, stabilization and degradation. Then, we focus on some of the most recent advances in cell therapy and tissue engineering that open up interesting perspectives for maintaining or obtaining the chondrogenic character of cells in order to treat cartilage lesions.

MAJOR CONCLUSIONS

Current research involves the use of chondrocytes or progenitor stem cells, associated with "smart" biomaterials and growth factors. Other influential factors, such as cell sources, oxygen pressure and mechanical strain are considered, as are recent developments in gene therapy to control the chondrocyte differentiation/dedifferentiation process.

GENERAL SIGNIFICANCE

This review provides new information on the mechanisms regulating the state of differentiation of chondrocytes and the chondrogenesis of mesenchymal stem cells that will lead to the development of new restorative cell therapy approaches in humans. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.

Injectable foams for regenerative medicine

The design of injectable biomaterials has attracted considerable attention in recent years. Many injectable biomaterials, such as hydrogels and calcium phosphate cements (CPCs), have nanoscale pores that limit the rate of cellular migration and proliferation. While introduction of macroporosity has been suggested to increase cellular infiltration and tissue healing, many conventional methods for generating macropores often require harsh processing conditions that preclude their use in injectable foams. In recent years, processes such as porogen leaching, gas foaming, and emulsion-templating have been adapted to generate macroporosity in injectable CPCs, hydrogels, and hydrophobic polymers. While some of the more mature injectable foam technologies have been evaluated in clinical trials, there are challenges remaining to be addressed, such as the biocompatibility and ultimate fate of the sacrificial phase used to generate pores within the foam after it sets in situ. Furthermore, while implantable scaffolds can be washed extensively to remove undesirable impurities, all of the components required to synthesize injectable foams must be injected into the defect. Thus, every compound in the foam must be biocompatible and noncytotoxic at the concentrations utilized. As future research addresses these critical challenges, injectable macroporous foams are anticipated to have an increasingly significant impact on improving patient outcomes for a number of clinical procedures.

Generation of osteochondral tissue constructs with chondrogenically and osteogenically predifferentiated mesenchymal stem cells encapsulated in bilayered hydrogels

This study investigated the ability of chondrogenic and osteogenic predifferentiation of mesenchymal stem cells (MSCs) to play a role in the development of osteochondral tissue constructs using injectable bilayered oligo(poly(ethylene glycol) fumarate) (OPF) hydrogel composites. We hypothesized that the combinatorial approach of encapsulating cell populations of both chondrogenic and osteogenic lineages in a spatially controlled manner within bilayered constructs would enable these cells to maintain their respective phenotypes via the exchange of biochemical factors even without the influence of external growth factors. During monolayer expansion prior to hydrogel encapsulation, it was found that 7 (CG7) and 14 (CG14) days of MSC exposure to TGF-β3 allowed for the generation of distinct cell populations with corresponding chondrogenic maturities as indicated by increasing aggrecan and type II collagen/type I collagen expression. Chondrogenic and osteogenic cells were then encapsulated within their respective (chondral/subchondral) layers in bilayered hydrogel composites to include four experimental groups. Encapsulated CG7 cells within the chondral layer exhibited enhanced chondrogenic phenotype when compared to other cell populations based on stronger type II collagen and aggrecan gene expression and higher glycosaminoglycan-to-hydroxyproline ratios. Osteogenic cells that were co-cultured with chondrogenic cells (in the chondral layer) showed higher cellularity over time, suggesting that chondrogenic cells stimulated the proliferation of osteogenic cells. Groups with osteogenic cells displayed mineralization in the subchondral layer, confirming the effect of osteogenic predifferentiation. In summary, it was found that MSCs that underwent 7 days, but not 14 days, of chondrogenic predifferentiation most closely resembled the phenotype of native hyaline cartilage when combined with osteogenic cells in a bilayered OPF hydrogel composite, indicating that the duration of chondrogenic preconditioning is an important factor to control. Furthermore, the respective chondrogenic and osteogenic phenotypes were maintained for 28 days in vitro without the need for external growth factors, demonstrating the exciting potential of this novel strategy for the generation of osteochondral tissue constructs for cartilage engineering applications.

Covalently tethered TGF-β1 with encapsulated chondrocytes in a PEG hydrogel system enhances extracellular matrix production

Healing articular cartilage defects remains a significant clinical challenge because of its limited capacity for self-repair. While delivery of autologous chondrocytes to cartilage defects has received growing interest, combining cell-based therapies with growth factor delivery that can locally signal cells and promote their function is often advantageous. We have previously shown that PEG thiol-ene hydrogels permit covalent attachment of growth factors. However, it is not well known if embedded chondrocytes respond to tethered signals over a long period. Here, chondrocytes were encapsulated in PEG hydrogels functionalized with transforming growth factor-beta 1 (TGF-β1) with the goal of increasing proliferation and matrix production. Tethered TGF-β1 was found to be distributed homogenously throughout the gel, and its bioactivity was confirmed with a TGF-β1 responsive reporter cell line. Relative to solubly delivered TGF-β1, chondrocytes presented with immobilized TGF-β1 showed significantly increased DNA content, and GAG and collagen production over 28 days, while maintaining markers of articular cartilage. These results indicate the potential of thiol-ene chemistry to covalently conjugate TGF-β1 to PEG to locally influence chondrocyte function over 4 weeks. Scaffolds with other or multiple tethered growth factors may prove broadly useful in the design of chondrocyte delivery vehicles for cartilage tissue engineering applications.

Bioengineered osteochondral precursor for treatment of osteochondritis dissecans in a Thoroughbred filly

CASE REPORT

A 13-month-old Thoroughbred filly was diagnosed with osteochondritis dissecans (OCD) of the medial tibial malleolus. A sponge impregnated with platelet-rich plasma, bone morphogenetic protein-2, mesenchymal stem cells and gelatin β-tricalcium phosphate was applied to the OCD site following arthroscopy and debridement. Postoperative radiography (every week for 16 weeks), computed tomography (CT) (16 weeks postoperatively), arthroscopy (16 weeks postoperatively) and biopsy of the regenerated tissue (16 weeks postoperatively) were performed to evaluate the outcome. Radiographically, the defect began to diminish 3 weeks postoperatively and had disappeared by 12 weeks. CT images showed that the debrided site was filled with ossified tissue and arthroscopy showed that the regenerated tissue was covered with smooth tissue, which a biopsy showed was fibrocartilage.

CONCLUSIONS

Placing the impregnated sponge in the OCD lesion facilitated satisfactory regeneration of tissue in the debrided area, but the regenerated cartilage was fibrocartilage. This method may be a viable option for the treatment of cases of equine OCD, but further work to determine how to induce hyaline cartilage regeneration is required.

Porous chitosan scaffolds with embedded hyaluronic acid/chitosan/plasmid-DNA nanoparticles encoding TGF-β1 induce DNA controlled release, transfected chondrocytes, and promoted cell proliferation

Cartilage defects resulting from traumatic injury or degenerative diseases have very limited spontaneous healing ability. Recent progress in tissue engineering and local therapeutic gene delivery systems has led to promising new strategies for successful regeneration of hyaline cartilage. In the present study, tissue engineering and local therapeutic gene delivery systems are combined with the design of a novel gene-activated matrix (GAM) embedded with hybrid hyaluronic acid(HA)/chitosan(CS)/plasmid-DNA nanoparticles encoding transforming growth factor (TGF)-β1. A chitosan scaffold functioned as the three-dimensional carrier for the nanoparticles. Results demonstrated that scaffold-entrapped plasmid DNA was released in a sustained and steady manner over 120 days, and was effectively protected in the HA/CS/pDNA nanoparticles. Culture results demonstrated that chondrocytes grown in the novel GAM were highly proliferative and capable of filling scaffold micropores with cells and extracellular matrix. Confocal laser scanning microscopy indicated that chondrocytes seeded in the GAM expressed exogenous transgenes labeled with green fluorescent protein. ELISA results demonstrated detectable TGF-β1 expression in the supernatant of GAM cultures, which peaked at the sixth day of culture and afterwards showed a moderate decline. Histological results and biochemical assays confirmed promotion of chondrocyte proliferation. Cell culture indicated no affects on phenotypic expression of ECM molecules, such as GAG. The results of this study indicate the suitability of this novel GAM for enhanced in vitro cartilage tissue engineering.

Microstructure and characteristic properties of gelatin/chitosan scaffold prepared by a combined freeze-drying/leaching method

A combined freeze-drying and particulate leaching method for scaffold synthesis showed an improvement in the horizontal microstructure of the gelatin/chitosan scaffolds. Type and concentration of the cross-linking agent, freezing temperature, concentration of the polymeric solution and gelatin/chitosan weight ratio were the variables affecting the scaffold properties. Assessment of the tensile properties of the scaffolds revealed that for a scaffold with 50% chitosan, glutaraldehyde, as a cross-linking agent, created much tighter polymeric network compared to N,N-(3-dimethylaminopropyl)-N'-ethyl carbodiimide (EDC). However, in the case of gelatin scaffolds, EDC was identified as the stronger cross-linker. Compressive behavior of the scaffolds satisfied formulations obtained from the theoretical modeling of the low-density, elastomeric foams. The investigation of the scaffold degradation indicated that the increase in the mechanical strength of the scaffolds would not always reduce their degradation rate.

Oligo[poly(ethylene glycol)fumarate] hydrogel enhances osteochondral repair in porcine femoral condyle defects

BACKGROUND

Management of osteochondritis dissecans remains a challenge. Use of oligo[poly(ethylene glycol)fumarate] (OPF) hydrogel scaffold alone has been reported in osteochondral defect repair in small animal models. However, preclinical evaluation of usage of this scaffold alone as a treatment strategy is limited.

QUESTIONS/PURPOSES

We therefore (1) determined in vitro pore size and mechanical stiffness of freeze-dried and rehydrated freeze-dried OPF hydrogels, respectively; (2) assessed in vivo gross defect filling percentage and histologic findings in defects implanted with rehydrated freeze-dried hydrogels for 2 and 4 months in a porcine model; (3) analyzed highly magnified histologic sections for different types of cartilage repair tissues, subchondral bone, and scaffold; and (4) assessed neotissue filling percentage, cartilage phenotype, and Wakitani scores.

METHODS

We measured pore size of freeze-dried OPF hydrogel scaffolds and mechanical stiffness of fresh and rehydrated forms. Twenty-four osteochondral defects from 12 eight-month-old micropigs were equally divided into scaffold and control (no scaffold) groups. Gross and histologic examination, one-way ANOVA, and one-way Mann-Whitney U test were performed at 2 and 4 months postoperatively.

RESULTS

Pore sizes ranged from 20 to 433 μm in diameter. Rehydrated freeze-dried scaffolds had mechanical stiffness of 1 MPa. The scaffold itself increased percentage of neotissue filling at both 2 and 4 months to 58% and 54%, respectively, with hyaline cartilage making up 39% of neotissue at 4 months.

CONCLUSIONS

Rehydrated freeze-dried OPF hydrogel can enhance formation of hyaline-fibrocartilaginous mixed repair tissue of osteochondral defects in a porcine model.

CLINICAL RELEVANCE

Rehydrated freeze-dried OPF hydrogel alone implanted into cartilage defects is insufficient to generate a homogeneously hyaline cartilage repair tissue, but its spacer effect can be enhanced by other tissue-regenerating mediators.

Osteochondral tissue regeneration using a bilayered composite hydrogel with modulating dual growth factor release kinetics in a rabbit model

Biodegradable oligo(poly(ethylene glycol) fumarate) (OPF) composite hydrogels have been investigated for the delivery of growth factors (GFs) with the aid of gelatin microparticles (GMPs) and stem cell populations for osteochondral tissue regeneration. In this study, a bilayered OPF composite hydrogel that mimics the distinctive hierarchical structure of native osteochondral tissue was utilized to investigate the effect of transforming growth factor-β3 (TGF-β3) with varying release kinetics and/or insulin-like growth factor-1 (IGF-1) on osteochondral tissue regeneration in a rabbit full-thickness osteochondral defect model. The four groups investigated included (i) a blank control (no GFs), (ii) GMP-loaded IGF-1 alone, (iii) GMP-loaded IGF-1 and gel-loaded TGF-β3, and (iv) GMP-loaded IGF-1 and GMP-loaded TGF-β3 in OPF composite hydrogels. The results of an in vitro release study demonstrated that TGF-β3 release kinetics could be modulated by the GF incorporation method. At 12weeks post-implantation, the quality of tissue repair in both chondral and subchondral layers was analyzed based on quantitative histological scoring. All groups incorporating GFs resulted in a significant improvement in cartilage morphology compared to the control. Single delivery of IGF-1 showed higher scores in subchondral bone morphology as well as chondrocyte and glycosaminoglycan amount in adjacent cartilage tissue when compared to a dual delivery of IGF-1 and TGF-β3, independent of the TGF-β3 release kinetics. The results suggest that although the dual delivery of TGF-β3 and IGF-1 may not synergistically enhance the quality of engineered tissue, the delivery of IGF-1 alone from bilayered composite hydrogels positively affects osteochondral tissue repair and holds promise for osteochondral tissue engineering applications.

Subcutaneous tissue response and osteogenic performance of calcium phosphate nanoparticle-enriched hydrogels in the tibial medullary cavity of guinea pigs

In the current study, oligo(poly(ethylene glycol) fumarate) (OPF)-based hydrogels were tested for the first time as injectable bone substitute materials. The primary feature of the material design was the incorporation of calcium phosphate (CaP) nanoparticles within the polymeric matrix in order to compare the soft tissue response and bone-forming capacity of plain OPF hydrogels with CaP-enriched OPF hydrogel composites. To that end, pre-set scaffolds were implanted subcutaneously, whereas flowable polymeric precursor solutions were injected in a tibial ablation model in guinea pigs. After 8 weeks of implantation, histological and histomorphometrical evaluation of the subcutaneous scaffolds confirmed the biocompatibility of both types of hydrogels. Nevertheless, OPF hydrogels presented a loose structure, massive cellular infiltration and extensive material degradation compared to OPF-CaP hydrogels that were more compact. Microcomputed tomography and histological and histomorphometrical analyses showed comparable amounts of new trabecular bone in all tibias and some material remnants in the medial and distal regions. Particularly, highly calcified areas were observed in the distal region of OPF-CaP-treated tibias, which indicate a heterogeneous distribution of the mineral phase throughout the hydrogel matrix. This phenomenon can be attributed to either hindered gelation under highly perfused in vivo conditions or a faster degradation rate of the polymeric hydrogel matrix compared to the nanostructured mineral phase, resulting in loss of entrapment of the CaP nanoparticles and subsequent sedimentation.

Degradable natural polymer hydrogels for articular cartilage tissue engineering

Articular cartilage has poor ability to heal once damaged. Tissue engineering with scaffolds of polymer hydrogels is promising for cartilage regeneration and repair. Polymer hydrogels composed of highly hydrated crosslinked networks mimic the collagen networks of the cartilage extracellular matrix and thus are employed as inserts at cartilage defects not only to temporarily relieve the pain but also to support chondrocyte proliferation and neocartilage regeneration. The biocompatibility, biofunctionality, mechanical properties, and degradation of the polymer hydrogels are the most important parameters for hydrogel‐based cartilage tissue engineering. Degradable biopolymers with natural origin have been widely used as biomaterials for tissue engineering because of their outstanding biocompatibility, low immunological response, low cytotoxicity, and excellent capability to promote cell adhesion, proliferation, and regeneration of new tissues. This review covers several important natural proteins (collagen, gelatin, fibroin, and fibrin) and polysaccharides (chitosan, hyaluronan, alginate and agarose) widely used as hydrogels for articular cartilage tissue engineering. The mechanical properties, structures, modification, and structure–performance relationship of these hydrogels are discussed since the chemical structures and physical properties dictate the in vivo performance and applications of polymer hydrogels for articular cartilage regeneration and repair. © 2012 Society of Chemical Industry

Gelatin microparticles aggregates as three-dimensional scaffolding system in cartilage engineering

A three-dimensional (3D) scaffolding system for chondrocytes culture has been produced by agglomeration of cells and gelatin microparticles with a mild centrifuging process. The diameter of the microparticles, around 10 μ, was selected to be in the order of magnitude of the chondrocytes. No gel was used to stabilize the construct that maintained consistency just because of cell and extracellular matrix (ECM) adhesion to the substrate. In one series of samples the microparticles were charged with transforming growth factor, TGF-β1. The kinetics of growth factor delivery was assessed. The initial delivery was approximately 48 % of the total amount delivered up to day 14. Chondrocytes that had been previously expanded in monolayer culture, and thus dedifferentiated, adopted in this 3D environment a round morphology, both with presence or absence of growth factor delivery, with production of ECM that intermingles with gelatin particles. The pellet was stable from the first day of culture. Cell viability was assessed by MTS assay, showing higher absorption values in the cell/unloaded gelatin microparticle pellets than in cell pellets up to day 7. Nevertheless the absorption drops in the following culture times. On the contrary the cell viability of cell/TGF-β1 loaded gelatin microparticle pellets was constant during the 21 days of culture. The formation of actin stress fibres in the cytoskeleton and type I collagen expression was significantly reduced in both cell/gelatin microparticle pellets (with and without TGF-β1) with respect to cell pellet controls. Total type II collagen and sulphated glycosaminoglycans quantification show an enhancement of the production of ECM when TGF-β1 is delivered, as expected because this growth factor stimulate the chondrocyte proliferation and improve the functionality of the tissue.

Controlled release strategies for bone, cartilage, and osteochondral engineering--Part II: challenges on the evolution from single to multiple bioactive factor delivery

The development of controlled release systems for the regeneration of bone, cartilage, and osteochondral interface is one of the hot topics in the field of tissue engineering and regenerative medicine. However, the majority of the developed systems consider only the release of a single growth factor, which is a limiting step for the success of the therapy. More recent studies have been focused on the design and tailoring of appropriate combinations of bioactive factors to match the desired goals regarding tissue regeneration. In fact, considering the complexity of extracellular matrix and the diversity of growth factors and cytokines involved in each biological response, it is expected that an appropriate combination of bioactive factors could lead to more successful outcomes in tissue regeneration. In this review, the evolution on the development of dual and multiple bioactive factor release systems for bone, cartilage, and osteochondral interface is overviewed, specifically the relevance of parameters such as dosage and spatiotemporal distribution of bioactive factors. A comprehensive collection of studies focused on the delivery of bioactive factors is also presented while highlighting the increasing impact of platelet-rich plasma as an autologous source of multiple growth factors.

Perspectives on the role of nanotechnology in bone tissue engineering

OBJECTIVE

This review surveys new developments in bone tissue engineering, specifically focusing on the promising role of nanotechnology and describes future avenues of research.

METHODS

The review first reinforces the need to fabricate scaffolds with multi-dimensional hierarchies for improved mechanical integrity. Next, new advances to promote bioactivity by manipulating the nanolevel internal surfaces of scaffolds are examined followed by an evaluation of techniques using scaffolds as a vehicle for local drug delivery to promote bone regeneration/integration and methods of seeding cells into the scaffold.

RESULTS

Through a review of the state of the field, critical questions are posed to guide future research toward producing materials and therapies to bring state-of-the-art technology to clinical settings.

SIGNIFICANCE

The development of scaffolds for bone regeneration requires a material able to promote rapid bone formation while possessing sufficient strength to prevent fracture under physiological loads. Success in simultaneously achieving mechanical integrity and sufficient bioactivity with a single material has been limited. However, the use of new tools to manipulate and characterize matter down to the nano-scale may enable a new generation of bone scaffolds that will surpass the performance of autologous bone implants.

Particle assemblies: toward new tools for regenerative medicine

Regenerative medicine is a demanding field in terms of design and elaboration of materials able to meet the specifications that this application imposes. The regeneration of tissue is a multiscale issue, from the signaling molecule through cell expansion and finally tissue growth requiring a large variety of cues that should be delivered in place and time. Hence, the materials should be able to accommodate cells with respect to their phenotypes, to allow cell division to the right tissue, to maintain the integrity of the surrounding sane tissue, and eventually use their signaling machinery to serve the development of the appropriate neo-tissue. They should also present the ability to deliver growth factors and regulate tissue development, to be degraded into safe products, in order not to impede tissue development, and finally be easily implanted/injected into the patients. In this context, colloid-based materials represent a very promising family of products because one can take advantage of their high specific area, their capability to carry/deliver bio-active molecules, and their capacity of assembling (eventually in vivo) into materials featuring other mechanical, rheological, physicochemical properties. Other benefits of great interest would be their ease of production even via high through-put processes and their potential manufacturing from safe, biodegradable and biocompatible parent raw material. This review describes the state-of-the-art of processes leading to complex materials from the assembly of colloids meeting, at least partially, the above-described specifications for tissue engineering and regenerative medicine.