Deciphering mechanical regulation of chondrogenesis in fibrin-polyurethane composite scaffolds enriched with human mesenchymal stem cells: a dual computational and experimental approach

A biomimetic three-layered fibrin gel/PLLA nanofibers composite as a potential scaffold for articular cartilage tissue engineering application

Developing an engineered scaffold inspired by structural features of healthy articular cartilage (AC) has attracted much attention. In this study, the design and fabrication of a three-layered fiber/hydrogel scaffold in which each layer replicates the organization of a pertinent layer of AC tissue is aimed. To this end, electrospun poly L-lactic acid (PLLA) nanofibers are prepared and fragmented into nano/micro cylinders via aminolysis. Three-layers of the scaffold in which continuous fibrous layer, fibrin gel incorporated by chopped fibers and fibrin gel embedded by cylindrical aligned fibrous mat perpendicular to articulating surface, respectively served as an upper, middle and bottom layers, are prepared. The layers' physicomechanical characteristics are comprehensively evaluated. Results show that optimized electrospinning set up results in the smallest fibers diameter of 367±317 nm and successful aminolysis provides amine-functionalized chopped nanofibers with a mean length of 1.65±1.2 µm. Static mechanical analysis of the layers demonstrates that Young tensile modulus of the upper layer is 152± 17 MPa while compressive moduli of the middle and bottom layers are 38±4 and 79± 6 KPa, respectively. Assessing mechanical parameters under dynamic loading also shows that adding fibrous part in the composite scaffold layers enhances viscoelastic behavior of fibrin gel. Also, incorporation of 0.25% chopped fibers into the fibrin matrix notably enhances the equilibrium water content; however, it increases in-vitro weigh loss rate from 6% to 10.5% during a seven-day period. cytocompatibility analysis confirms that all layers possess acceptable cytocompatibility. In a conclusion, the designed three-layered composite structure successfully mimics the physicomechanical as well as microstructural features of AC and could be suggested as a potential scaffold for this tissue regeneration.

Optimization of loading protocols for tissue engineering experiments

Tissue engineering (TE) combines cells and biomaterials to treat orthopedic pathologies. Maturation of de novo tissue is highly dependent on local mechanical environments. Mechanical stimulation influences stem cell differentiation, however, the role of various mechanical loads remains unclear. While bioreactors simplify the complexity of the human body, the potential combination of mechanical loads that can be applied make it difficult to assess how different factors interact. Human bone marrow-derived mesenchymal stromal cells were seeded within a fibrin-polyurethane scaffold and exposed to joint-mimicking motion. We applied a full factorial design of experiment to investigate the effect that the interaction between different mechanical loading parameters has on biological markers. Additionally, we employed planned contrasts to analyze differences between loading protocols and a linear mixed model with donor as random effect. Our approach enables screening of multiple mechanical loading combinations and identification of significant interactions that could not have been studied using classical mechanobiology studies. This is useful to screen the effect of various loading protocols and could also be used for TE experiments with small sample sizes and further combinatorial medication studies.

Guide to mechanical characterization of articular cartilage and hydrogel constructs based on a systematic in silico parameter sensitivity analysis

Osteoarthritis is a whole joint disease with cartilage degeneration being an important manifestation. Tissue engineering treatment is a solution for repairing cartilage defects by implantation of chondrocyte-laden hydrogel constructs within the defect. In silico models have recently been introduced to simulate and optimize the design of these constructs. These models require accurate knowledge on the mechanical properties of the hydrogel constructs and cartilage explants, which are challenging to obtain due to their anisotropic structure and time-dependent behaviour. We performed a systematic in silico parameter sensitivity analysis to find the most efficient unconfined compression testing protocols for mechanical characterization of hydrogel constructs and cartilage explants, with a minimum number of tests but maximum identifiability of the material parameters. The construct and explant were thereby modelled as porohyperelastic and fibril-reinforced poroelastic materials, respectively. Three commonly used loading regimes were simulated in Abaqus (ramp, relaxation and dynamic loading) with varying compressive strain magnitudes and rates. From these virtual experiments, the resulting material parameters were obtained for each combination using a numerical inverse identification scheme. For hydrogels, maximum sensitivity to the different material parameters was found when using a single step ramp loading (20% compression with 10%/s rate) followed by 15 min relaxation. For cartilage explants, a two-stepped ramp loading (10% compression with 10%/s rate and 10% compression with 1%/s rate), each step followed by 15 min relaxation, yielded the maximum sensitivity to the different material parameters. With these protocols, the material parameters could be retrieved with the lowest amount of uncertainty (hydrogel: < 2% and cartilage: < 6%). These specific results and the overall methodology can be used to optimize mechanical testing protocols to yield reliable material parameters for in silico models of cartilage and hydrogel constructs.

Complex mechanical behavior of human articular cartilage and hydrogels for cartilage repair

The mechanical behavior of cartilage tissue plays a crucial role in physiological mechanotransduction processes of chondrocytes and pathological changes like osteoarthritis. Therefore, intensive research activities focus on the identification of implant substitute materials that mechanically mimic the cartilage extracellular matrix. This, however, requires a thorough understanding of the complex mechanical behavior of both native cartilage and potential substitute materials to treat cartilage lesions. Here, we perform complex multi-modal mechanical analyses of human articular cartilage and two surrogate materials, commercially available ChondroFiller_liquid, and oxidized alginate-gelatin (ADA-GEL) hydrogels. We show that all materials exhibit nonlinearity and compression-tension asymmetry. However, while hyaline cartilage yields higher stresses in tension than in compression, ChondroFiller^liquid and ADA-GEL exhibit the opposite trend. These characteristics can be attributed to the materials' underlying microstructure: Both cartilage and ChondroFiller^liquid contain fibrillar components, but the latter constitutes a bi-phasic structure, where the 60% nonfibrillar hydrogel proportion dominates the mechanical response. Of all materials, ChondroFiller^liquid shows the most pronounced viscous effects. The present study provides important insights into the microstructure-property relationship of cartilage substitute materials, with vital implications for mechanically-driven material design in cartilage engineering. In addition, we provide a data set to create mechanical simulation models in the future.

Regenerative rehabilitation: The role of mechanotransduction in orthopaedic regenerative medicine

Regenerative rehabilitation is an emerging area of investigation that seeks to integrate regenerative medicine with rehabilitation medicine. It is based on the realization that combining these two areas of medicine at an early stage of treatment will produce a better clinical outcome than the traditional linear approach of first administering the elements of regeneration followed, after a delay, by rehabilitation. Indeed, in certain settings, a case can be made for initiating rehabilitation protocols before starting regenerative intervention. This review summarizes the contents of a workshop held during the 2018 annual meeting of the Orthopaedic Research Society. It introduced the concept of regenerative rehabilitation and then provided two orthopaedic examples drawn from the domains of cartilage repair and bone healing. Rehabilitation medicine can supply a variety of physical stimuli, including electrical stimulation, thermal stimulation and mechanical stimulation. Of these, mechanical stimulation has the most obvious relevance to orthopaedics. The mechano-responsiveness of cartilage and bone has been known for a long time, but is poorly understood and has led to only limited clinical application. Improved bioreactor designs that allow multi-axial loading enable new insights into the responsiveness of chondrocytes and chondroprogenitor cells to specific types of load, especially shear. Recent studies on the mechanobiology of bone healing show that modulating the mechanical environment of an experimental osseous lesion by a process of "Reverse Dynamization" soon after injury considerably enhances healing. Future studies are needed to probe the molecular mechanisms responsible for these phenomena and to translate these findings into clinical practice. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1263-1269, 2019.

Mechanically stimulated osteochondral organ culture for evaluation of biomaterials in cartilage repair studies

Surgical procedures such as microfracture or autologous chondrocyte implantation have been used to treat articular cartilage lesions; however, repair often fails in terms of matrix organization and mechanical behaviour. Advanced biomaterials and tissue engineered constructs have been developed to improve cartilage repair; nevertheless, their clinical translation has been hampered by the lack of reliable in vitro models suitable for pre-clinical screening of new implants and compounds. In this study, an osteochondral defect model in a bioreactor that mimics the multi-axial motion of an articulating joint, was developed. Osteochondral explants were obtained from bovine stifle joints, and cartilage defects of 4 mm diameter were created. The explants were used as an interface against a ceramic ball applying dynamic compressive and shear loading. Osteochondral defects were filled with chondrocytes-seeded fibrin-polyurethane constructs and subjected to mechanical stimulation. Cartilage viability, proteoglycan accumulation and gene expression of seeded chondrocytes were compared to free swelling controls. Cells within both cartilage and bone remained viable throughout the 10-day culture period. Loading did not wear the cartilage, as indicated by histological evaluation and glycosaminoglycan release. The gene expression of seeded chondrocytes indicated a chondrogenic response to the mechanical stimulation. Proteoglycan 4 and cartilage oligomeric matrix protein were markedly increased, while mRNA ratios of collagen type II to type I and aggrecan to versican were also enhanced. This mechanically stimulated osteochondral defect culture model provides a viable microenvironment and will be a useful pre-clinical tool to screen new biomaterials and biological regenerative therapies under relevant complex mechanical stimuli. STATEMENT OF SIGNIFICANCE: Articular cartilage lesions have a poor healing capacity and reflect one of the most challenging problems in orthopedic clinical practice. The aim of current research is to develop a testing system to assess biomaterials for implants, that can permanently replace damaged cartilage with the original hyaline structure and can withstand the mechanical forces long term. Here, we present an osteochondral ex vivo culture model within a cartilage bioreactor, which mimics the complex motion of an articulating joint in vivo. The implementation of mechanical forces is essential for pre-clinical testing of novel technologies in the field of cartilage repair, biomaterial engineering and regenerative medicine. Our model provides a unique opportunity to investigate healing of articular cartilage defects in a physiological joint-like environment.

Hyaluronan supplementation as a mechanical regulator of cartilage tissue development under joint-kinematic-mimicking loading

Articular cartilage plays an essential role in joint lubrication and impact absorption. Through this, the mechanical signals are coupled to the tissue's physiological response. Healthy synovial fluid has been shown to reduce and homogenize the shear stress acting on the cartilage surfaces due to its unique shear-thinning viscosity. As cartilage tissues are sensitive to mechanical changes in articulation, it was hypothesized that replacing the traditional culture medium with a healthy non-Newtonian lubricant could enhance tissue development in a cartilage engineering model, where joint-kinematic-mimicking mechanical loading is applied. Different amounts of hyaluronic acid were added to the culture medium to replicate the viscosities of synovial fluid at different health states. Hyaluronic acid supplementation, especially at a physiologically healthy concentration (2.0 mg ml^-1), promoted a better preservation of chondrocyte phenotype. The ratio of collagen II to collagen I mRNA was 4.5 times that of the control group, implying better tissue development (however, with no significant difference of measured collagen II content), with a good retention of collagen II and proteoglycan in the mechanically active region. Simulating synovial fluid properties by hyaluronic acid supplementation created a favourable mechanical environment for mechanically loaded constructs. These findings may help in understanding the influence of joint articulation on tissue homeostasis, and moreover, improve methods for functional cartilage tissue engineering.

Biomaterials for articular cartilage tissue engineering: Learning from biology

Articular cartilage is commonly described as a tissue that is made of up to 80% water, is devoid of blood vessels, nerves, and lymphatics, and is populated by only one cell type, the chondrocyte. At first glance, an easy tissue for clinicians to repair and for scientists to reproduce in a laboratory. Yet, chondral and osteochondral defects currently remain an open challenge in orthopedics and tissue engineering of the musculoskeletal system, without considering osteoarthritis. Why do we fail in repairing and regenerating articular cartilage? Behind its simple and homogenous appearance, articular cartilage hides a heterogeneous composition, a high level of organisation and specific biomechanical properties that, taken together, make articular cartilage a unique material that we are not yet able to repair or reproduce with high fidelity. This review highlights the available therapies for cartilage repair and retraces the research on different biomaterials developed for tissue engineering strategies. Their potential to recreate the structure, including composition and organisation, as well as the function of articular cartilage, intended as cell microenvironment and mechanically competent replacement, is described. A perspective of the limitations of the current research is given in the light of the emerging technologies supporting tissue engineering of articular cartilage.

STATEMENT OF SIGNIFICANCE

The mechanical properties of articular tissue reflect its functionally organised composition and the recreation of its structure challenges the success of in vitro and in vivo reproduction of the native cartilage. Tissue engineering and biomaterials science have revolutionised the way scientists approach the challenge of articular cartilage repair and regeneration by introducing the concept of the interdisciplinary approach. The clinical translation of the current approaches are not yet fully successful, but promising results are expected from the emerging and developing new generation technologies.

Advanced Therapy Medicinal Products: A Guide for Bone Marrow-derived MSC Application in Bone and Cartilage Tissue Engineering

Millions of people worldwide suffer from trauma- or age-related orthopedic diseases such as osteoarthritis, osteoporosis, or cancer. Tissue Engineering (TE) and Regenerative Medicine are multidisciplinary fields focusing on the development of artificial organs, biomimetic engineered tissues, and cells to restore or maintain tissue and organ function. While allogenic and future autologous transplantations are nowadays the gold standards for both cartilage and bone defect repair, they are both subject to important limitations such as availability of healthy tissue, donor site morbidity, and graft rejection. Tissue engineered bone and cartilage products represent a promising and alternative approach with the potential to overcome these limitations. Since the development of Advanced Therapy Medicinal Products (ATMPs) such as TE products requires the knowledge of diverse regulation and an extensive communication with the national/international authorities, the aim of this review is therefore to summarize the state of the art on the clinical applications of human bone marrow-derived stromal cells for cartilage and bone TE. In addition, this review provides an overview of the European legislation to facilitate the development and commercialization of new ATMPs.

Asymmetrical seeding of MSCs into fibrin-poly(ester-urethane) scaffolds and its effect on mechanically induced chondrogenesis

Mesenchymal stem cells (MSCs) are currently being investigated as candidate cells for regenerative medicine approaches for the repair of damaged articular cartilage. For these cells to be used clinically, it is important to understand how they will react to the complex loading environment of a joint in vivo. In addition to investigating alternative cell sources, it is also important for the structure of tissue‐engineered constructs and the organization of cells within them to be developed and, if possible, improved. A custom built bioreactor was used to expose human MSCs to a combination of shear and compression loading. The MSCs were either evenly distributed throughout fibrin‐poly(ester‐urethane) scaffolds or asymmetrically seeded with a small proportion seeded on the surface of the scaffold. The effect of cell distribution on the production and deposition of cartilage‐like matrix in response to mechanical load mimicking in vivo joint loading was then investigated. The results show that asymmetrically seeding the scaffold led to markedly improved tissue development based on histologically detectable matrix deposition. Consideration of cell location, therefore, is an important aspect in the development of regenerative medicine approaches for cartilage repair. This is particularly relevant when considering the natural biomechanical environment of the joint in vivo and patient rehabilitation protocols. © 2016 The Authors Journal of Tissue Engineering and Regenerative Medicine Published by John Wiley & Sons Ltd.

Critical review on the physical and mechanical factors involved in tissue engineering of cartilage

Articular cartilage defects often progress to osteoarthritis, which negatively impacts quality of life for millions of people worldwide and leads to high healthcare expenditures. Tissue engineering approaches to osteoarthritis have concentrated on proliferation and differentiation of stem cells by activation and suppression of signaling pathways, and by using a variety of scaffolding techniques. Recent studies indicate a key role of environmental factors in the differentiation of mesenchymal stem cells to mature cartilage-producing chondrocytes. Therapeutic approaches that consider environmental regulation could optimize chondrogenesis protocols for regeneration of articular cartilage. This review focuses on the effect of scaffold structure and composition, mechanical stress and hypoxia in modulating mesenchymal stem cell fate and the current use of these environmental factors in tissue engineering research.