Nutrient Channels Aid the Growth of Articular Surface-Sized Engineered Cartilage Constructs

Modeling Inelastic Responses Using Constrained Reactive Mixtures

This study reviews the progression of our research, from modeling growth theories for cartilage tissue engineering, to the formulation of constrained reactive mixture theories to model inelastic responses in any solid material, such as theories for damage mechanics, viscoelasticity, plasticity, and elasto-plastic damage. In this framework, multiple solid generations α can co-exist at any given time in the mixture. The oldest generation is denoted by α = s and is called the master generation, whose reference configuration X s is observable. The solid generations α are all constrained to share the same velocity v s , but may have distinct reference configurations X α . An important element of this formulation is that the time-invariant mapping F α s = ∂ X α / ∂ X s between these reference configurations is a function of state, whose mathematical formulation is postulated by constitutive assumption. Thus, reference configurations X α are not observable ( α ≠ s ) . This formulation employs only observable state variables, such as the deformation gradient F s of the master generation and the referential mass concentrations ρ r α of each generation, in contrast to classical formulations of inelastic responses which rely on internal state variable theory, requiring evolution equations for those hidden variables. In constrained reactive mixtures, the evolution of the mass concentrations is governed by the axiom of mass balance, using constitutive models for the mass supply densities ρ ˆ r α . Classical and constrained reactive mixture approaches share considerable mathematical analogies, as they both introduce a multiplicative decomposition of the deformation gradient, also requiring evolution equations to track some of the state variables. However, they also differ at a fundamental level, since one adopts only observable state variables while the other introduces hidden state variables. In summary, this review presents an alternative foundational approach to the modeling of inelastic responses in solids, grounded in the classical framework of mixture theory.

3D bioprinting of cartilaginous templates for large bone defect healing

Damaged or diseased bone can be treated using autografts or a range of different bone grafting biomaterials, however limitations with such approaches has motivated increased interest in developmentally inspired bone tissue engineering (BTE) strategies that seek to recapitulate the process of endochondral ossification (EO) as a means of regenerating critically sized defects. The clinical translation of such strategies will require the engineering of scaled-up, geometrically defined hypertrophic cartilage grafts that can be rapidly vascularised and remodelled into bone in mechanically challenging defect environments. The goal of this study was to 3D bioprint mechanically reinforced cartilaginous templates and to assess their capacity to regenerate critically sized femoral bone defects. Human mesenchymal stem/stromal cells (hMSCs) were incorporated into fibrin based bioinks and bioprinted into polycaprolactone (PCL) frameworks to produce mechanically reinforced constructs. Chondrogenic priming of such hMSC laden constructs was required to support robust vascularisation and graft mineralisation in vivo following their subcutaneous implantation into nude mice. With a view towards maximising their potential to support endochondral bone regeneration, we next explored different in vitro culture regimes to produce chondrogenic and early hypertrophic engineered grafts. Following their implantation into femoral bone defects within transiently immunosuppressed rats, such bioprinted constructs were rapidly remodelled into bone in vivo, with early hypertrophic constructs supporting higher levels of vascularisation and bone formation compared to the chondrogenic constructs. Such early hypertrophic bioprinted constructs also supported higher levels of vascularisation and spatially distinct patterns of new formation compared to BMP-2 loaded collagen scaffolds (here used as a positive control). In conclusion, this study demonstrates that fibrin based bioinks support chondrogenesis of hMSCs in vitro, which enables the bioprinting of mechanically reinforced hypertrophic cartilaginous templates capable of supporting large bone defect regeneration. These results support the use of 3D bioprinting as a strategy to scale-up the engineering of developmentally inspired templates for BTE. STATEMENT OF SIGNIFICANCE: Despite the promise of developmentally inspired tissue engineering strategies for bone regeneration, there are still challenges that need to be addressed to enable clinical translation. This work reports the development and assessment (in vitro and in vivo) of a 3D bioprinting strategy to engineer mechanically-reinforced cartilaginous templates for large bone defect regeneration using human MSCs. Using distinct in vitro priming protocols, it was possible to generate cartilage grafts with altered phenotypes. More hypertrophic grafts, engineered in vitro using TGF-β3 and BMP-2, supported higher levels of blood vessel infiltration and accelerated bone regeneration in vivo. This study also identifies some of the advantages and disadvantages of such endochondral bone TE strategies over the direct delivery of BMP-2 from collagen-based scaffolds.

Fabrication and maturation of integrated biphasic anatomic mesenchymal stromal cell-laden composite scaffolds for osteochondral repair and joint resurfacing

Articular cartilage injury can lead to joint-wide erosion and the early onset of osteoarthritis. To address this, we recently developed a rapid fabrication method to produce patient-specific engineered cartilage tissues to replace an entire articular surface. Here, we extended that work by coupling a mesenchymal stromal cell-laden hydrogel (methacrylated hyaluronic acid) with the porous polycaprolactone (PCL) bone integrating phase and assessed the composition and mechanical performance of these constructs over time. To improve initial construct stability, PCL/hydrogel interface parameters were first optimized by varying PCL pretreatment (with sodium hydroxide before ethanol) before hydrogel infusion. Next, cylindrical osteochondral constructs were formed and cultured in media containing transforming growth factor β3 for up to 8 weeks, with constructs evaluated for viability, histological features, and biochemical content. Mechanical properties were also assessed in axial compression and via an interface shear strength assay. Results showed that the fabrication process was compatible with cell viability, and that construct biochemical content and mechanical properties increased with time. Interestingly, compressive properties peaked at 5 weeks, while interfacial shear properties continued to improve beyond this time point. Finally, these fabrication methods were combined with a custom mold developed from limb-specific computed tomography imaging data to create an anatomic implantable cell-seeded biologic joint surface, which showedmaturation similar to the osteochondral cylinders. Future work will apply these advances in large animal models of critically sized osteochondral defects to study repair and whole joint resurfacing.

A synthetic mechanogenetic gene circuit for autonomous drug delivery in engineered tissues

Mechanobiologic signals regulate cellular responses under physiologic and pathologic conditions. Using synthetic biology and tissue engineering, we developed a mechanically responsive bioartificial tissue that responds to mechanical loading to produce a preprogrammed therapeutic biologic drug. By deconstructing the signaling networks induced by activation of the mechanically sensitive ion channel transient receptor potential vanilloid 4 (TRPV4), we created synthetic TRPV4-responsive genetic circuits in chondrocytes. We engineered these cells into living tissues that respond to mechanical loading by producing the anti-inflammatory biologic drug interleukin-1 receptor antagonist. Chondrocyte TRPV4 is activated by osmotic loading and not by direct cellular deformation, suggesting that tissue loading is transduced into an osmotic signal that activates TRPV4. Either osmotic or mechanical loading of tissues transduced with TRPV4-responsive circuits protected constructs from inflammatory degradation by interleukin-1α. This synthetic mechanobiology approach was used to develop a mechanogenetic system to enable long-term, autonomously regulated drug delivery driven by physiologically relevant loading.

The use of antifreeze proteins to modify pore structure in directionally frozen alginate sponges for cartilage tissue engineering

It is thought that osteoarthritis is one of the world's leading causes of disability, with over 8.75 million people in the UK alone seeking medical treatment in 2013. Although a number of treatments are currently in use, a new wave of tissue engineered structures are being investigated as potential solutions for early intervention. One of the key challenges seen in cartilage tissue engineering is producing constructs that can support the formation of articular cartilage, rather than mechanically inferior fibrocartilage. Some research has suggested that mimicking structural properties of the natural cartilage can be used to enhance this response. Herein directional freezing was used to fabricate scaffolds with directionally aligned pores mimicking the mid-region of cartilage, anti-freeze proteins were used to modify the porous structure, which in turn effected the mechanical properties. Pore areas at the tops of the scaffolds were 180.46 ± 44.17 μm2 and 65.66 ± 36.20 μm² for the AFP free and the AFP scaffolds respectively, and for the bases of the scaffolds were 91.22 ± 19.05 μm² and 69.41 ± 21.94 μm² respectively. Scaffolds were seeded with primary bovine chondrocytes, with viability maintained over the course of the study, and regulation of key genes was observed.

Treatment of Focal Cartilage Defects in Minipigs with Zonal Chondrocyte/Mesenchymal Progenitor Cell Constructs

Despite advances in cartilage repair strategies, treatment of focal chondral lesions remains an important challenge to prevent osteoarthritis. Articular cartilage is organized into several layers and lack of zonal organization of current grafts is held responsible for insufficient biomechanical and biochemical quality of repair-tissue. The aim was to develop a zonal approach for cartilage regeneration to determine whether the outcome can be improved compared to a non-zonal strategy. Hydrogel-filled polycaprolactone (PCL)-constructs with a chondrocyte-seeded upper-layer deemed to induce hyaline cartilage and a mesenchymal stromal cell (MSC)-containing bottom-layer deemed to induce calcified cartilage were compared to chondrocyte-based non-zonal grafts in a minipig model. Grafts showed comparable hardness at implantation and did not cause visible signs of inflammation. After 6 months, X-ray microtomography (µCT)-analysis revealed significant bone-loss in both treatment groups compared to empty controls. PCL-enforcement and some hydrogel-remnants were retained in all defects, but most implants were pressed into the subchondral bone. Despite important heterogeneities, both treatments reached a significantly lower modified O'Driscoll-score compared to empty controls. Thus, PCL may have induced bone-erosion during joint loading and misplacement of grafts in vivo precluding adequate permanent orientation of zones compared to surrounding native cartilage.

Dynamics of Intrinsic Glucose Uptake Kinetics in Human Mesenchymal Stem Cells During Chondrogenesis

Chondrogenesis of human mesenchymal stem cells (hMSCs) is an important biological process in many applications including cartilage tissue engineering. We investigated the glucose uptake characteristics of aggregates of hMSCs undergoing chondrogenesis over a 3-week period both experimentally and by using a mathematical model. Initial concentrations of glucose in the medium were varied from 1 to 4.5 g/L to mimic limiting conditions and glucose uptake profiles were obtained. A reaction-diffusion mathematical model was implemented and solved to estimate kinetic parameters. Experimental glucose uptake rates increased with culture time for aggregates treated with higher initial glucose concentrations (3 and 4.5 g/L), whereas they decreased or remained constant for those treated with lower initial glucose concentrations (1 and 2 g/L). Lactate production rate increased by as much as 40% for aggregates treated with higher initial glucose concentrations (2, 3 and 4.5 g/L), whereas it remained constant for those treated with 1 g/L initial glucose concentration. The estimated DNA-normalized maximum glucose uptake rate decreased by a factor of 9 from day 0-2 (12.5 mmol/s/g DNA) to day 6-8 (1.5 mmol/s/g DNA), after which it started to increase. On day 18-20, its value (17.5 mmol/s/g DNA) was about 11 times greater than its lowest value. Further, the extracellular matrix levels of aggregates at day 14 and day 21 correlated with their overall glucose uptake and lactate production. The results suggest that during chondrogenesis, for optimal results, cells require increasing amounts of glucose. Our results also suggest that diffusion limitations play an important role in glucose uptake even in the smaller size aggregate model of chondrogenesis. Further, the results indicate that glucose uptake or lactate production can be a tool for predicting the end quality of tissue during the process of chondrogenesis. The estimated kinetic parameters can be used to model glucose requirements in cartilage tissue engineering applications.

3D printed microchannel networks to direct vascularisation during endochondral bone repair

Bone tissue engineering strategies that recapitulate the developmental process of endochondral ossification offer a promising route to bone repair. Clinical translation of such endochondral tissue engineering strategies will require overcoming a number of challenges, including the engineering of large and often anatomically complex cartilage grafts, as well as the persistence of core regions of avascular cartilage following their implantation into large bone defects. Here 3D printing technology is utilized to develop a versatile and scalable approach to guide vascularisation during endochondral bone repair. First, a sacrificial pluronic ink was used to 3D print interconnected microchannel networks in a mesenchymal stem cell (MSC) laden gelatin-methacryloyl (GelMA) hydrogel. These constructs (with and without microchannels) were next chondrogenically primed in vitro and then implanted into critically sized femoral bone defects in rats. The solid and microchanneled cartilage templates enhanced bone repair compared to untreated controls, with the solid cartilage templates (without microchannels) supporting the highest levels of total bone formation. However, the inclusion of 3D printed microchannels was found to promote osteoclast/immune cell invasion, hydrogel degradation, and vascularisation following implantation. In addition, the endochondral bone tissue engineering strategy was found to support comparable levels of bone healing to BMP-2 delivery, whilst promoting lower levels of heterotopic bone formation, with the microchanneled templates supporting the lowest levels of heterotopic bone formation. Taken together, these results demonstrate that 3D printed hypertrophic cartilage grafts represent a promising approach for the repair of complex bone fractures, particularly for larger defects where vascularisation will be a key challenge.

Engineering large cartilage tissues using dynamic bioreactor culture at defined oxygen conditions

Mesenchymal stem cells maintained in appropriate culture conditions are capable of producing robust cartilage tissue. However, gradients in nutrient availability that arise during three-dimensional culture can result in the development of spatially inhomogeneous cartilage tissues with core regions devoid of matrix. Previous attempts at developing dynamic culture systems to overcome these limitations have reported suppression of mesenchymal stem cell chondrogenesis compared to static conditions. We hypothesize that by modulating oxygen availability during bioreactor culture, it is possible to engineer cartilage tissues of scale. The objective of this study was to determine whether dynamic bioreactor culture, at defined oxygen conditions, could facilitate the development of large, spatially homogeneous cartilage tissues using mesenchymal stem cell laden hydrogels. A dynamic culture regime was directly compared to static conditions for its capacity to support chondrogenesis of mesenchymal stem cells in both small and large alginate hydrogels. The influence of external oxygen tension on the response to the dynamic culture conditions was explored by performing the experiment at 20% O₂ and 3% O₂. At 20% O₂, dynamic culture significantly suppressed chondrogenesis in engineered tissues of all sizes. In contrast, at 3% O₂ dynamic culture significantly enhanced the distribution and amount of cartilage matrix components (sulphated glycosaminoglycan and collagen II) in larger constructs compared to static conditions. Taken together, these results demonstrate that dynamic culture regimes that provide adequate nutrient availability and a low oxygen environment can be employed to engineer large homogeneous cartilage tissues. Such culture systems could facilitate the scaling up of cartilage tissue engineering strategies towards clinically relevant dimensions.

A modular approach to creating large engineered cartilage surfaces

Native articular cartilage has limited capacity to repair itself from focal defects or osteoarthritis. Tissue engineering has provided a promising biological treatment strategy that is currently being evaluated in clinical trials. However, current approaches in translating these techniques to developing large engineered tissues remains a significant challenge. In this study, we present a method for developing large-scale engineered cartilage surfaces through modular fabrication. Modular Engineered Tissue Surfaces (METS) uses the well-known, but largely under-utilized self-adhesion properties of de novo tissue to create large scaffolds with nutrient channels. Compressive mechanical properties were evaluated throughout METS specimens, and the tensile mechanical strength of the bonds between attached constructs was evaluated over time. Raman spectroscopy, biochemical assays, and histology were performed to investigate matrix distribution. Results showed that by Day 14, stable connections had formed between the constructs in the METS samples. By Day 21, bonds were robust enough to form a rigid sheet and continued to increase in size and strength over time. Compressive mechanical properties and glycosaminoglycan (GAG) content of METS and individual constructs increased significantly over time. The METS technique builds on established tissue engineering accomplishments of developing constructs with GAG composition and compressive properties approaching native cartilage. This study demonstrated that modular fabrication is a viable technique for creating large-scale engineered cartilage, which can be broadly applied to many tissue engineering applications and construct geometries.

Enhanced nutrient transport improves the depth-dependent properties of tri-layered engineered cartilage constructs with zonal co-culture of chondrocytes and MSCs

Biomimetic design in cartilage tissue engineering is a challenge given the complexity of the native tissue. While numerous studies have generated constructs with near-native bulk properties, recapitulating the depth-dependent features of native tissue remains a challenge. Furthermore, limitations in nutrient transport and matrix accumulation in engineered constructs hinders maturation within the central core of large constructs. To overcome these limitations, we fabricated tri-layered constructs that recapitulate the depth-dependent cellular organization and functional properties of native tissue using zonally derived chondrocytes co-cultured with MSCs. We also introduced porous hollow fibers (HFs) and HFs/cotton threads to enhance nutrient transport. Our results showed that tri-layered constructs with depth-dependent organization and properties could be fabricated. The addition of HFs or HFs/threads improved matrix accumulation in the central core region. With HF/threads, the local modulus in the deep region of tri-layered constructs nearly matched that of native tissue, though the properties in the central regions remained lower. These constructs reproduced the zonal organization and depth-dependent properties of native tissue, and demonstrate that a layer-by-layer fabrication scheme holds promise for the biomimetic repair of focal cartilage defects.

STATEMENT OF SIGNIFICANCE

Articular cartilage is a highly organized tissue driven by zonal heterogeneity of cells, extracellular matrix proteins and fibril orientations, resulting in depth-dependent mechanical properties. Therefore, the recapitulation of the functional properties of native cartilage in a tissue engineered construct requires such a biomimetic design of the morphological organization, and this has remained a challenge in cartilage tissue engineering. This study demonstrates that a layer-by-layer fabrication scheme, including co-cultures of zone-specific articular CHs and MSCs, can reproduce the depth-dependent characteristics and mechanical properties of native cartilage while minimizing the need for large numbers of chondrocytes. In addition, introduction of a porous hollow fiber (combined with a cotton thread) enhanced nutrient transport and depth-dependent properties of the tri-layered construct. Such a tri-layered construct may provide critical advantages for focal cartilage repair. These constructs hold promise for restoring native tissue structure and function, and may be beneficial in terms of zone-to-zone integration with adjacent host tissue and providing more appropriate strain transfer after implantation.

Co-culture of infrapatellar fat pad-derived mesenchymal stromal cells and articular chondrocytes in plasma clot for cartilage tissue engineering

BACKGROUND

Cell source plays a deterministic role in defining the outcome of a cell-based cartilage regenerative therapy and its clinical translational ability. Recent efforts in the direction of co-culture of two or more cell types attempt to combine the advantages of constituent cell types and negate their demerits.

METHODS

We examined the potential of co-culture of infrapatellar fat pad-derived mesenchymal stromal cells (IFP MSCs) and articular chondrocytes (ACs) in plasma clots in terms of their ratios and culture formats for cartilage tissue engineering.

RESULTS AND DISCUSSION

It was observed that IFP MSCs and ACs interact positively to produce a better quality hyaline cartilage-like matrix. While a supra-additive deposition of sulfated Glycosaminoglycans (sGAG), collagen type II, aggrecan and link protein was observed, deposition of collagen type I and X was sub-additive. (Immuno)-histologically similar cartilage was generated in vitro in IFP MSC:AC ratio of 50:50 and pure AC groups thus yielding a hyaline cartilage with 50% reduced requirement of ACs. Subsequently, we investigated if this response could be improved further by enabling better cell-cell interactions using scaffold-free systems such as self-assembled cartilage or by encapsulating cellular micro-aggregates in plasma clot. However, it was inferred that while self-assembly may have enabled better cell-cell interaction, poor cell survival negated its overall beneficial role, whereas the micro-aggregate group demonstrated highly heterogeneous matrix deposition within the construct, thus diminishing its translational utility. Overall, it was concluded that co-culture of IFP MSCs and ACs at a ratio of 50:50 within plasma clots demonstrated potential for cell-based cartilage regenerative therapy.

* Constrained Cage Culture Improves Engineered Cartilage Functional Properties by Enhancing Collagen Network Stability

When cultured with sufficient nutrient supply, engineered cartilage synthesizes proteoglycans rapidly, producing an osmotic swelling pressure that destabilizes immature collagen and prevents the development of a robust collagen framework, a hallmark of native cartilage. We hypothesized that mechanically constraining the proteoglycan-induced tissue swelling would enhance construct functional properties through the development of a more stable collagen framework. To test this hypothesis, we developed a novel "cage" growth system to mechanically prevent tissue constructs from swelling while ensuring adequate nutrient supply to the growing construct. The effectiveness of constrained culture was examined by testing constructs embedded within two different scaffolds: agarose and cartilage-derived matrix hydrogel (CDMH). Constructs were seeded with immature bovine chondrocytes and cultured under free swelling (FS) conditions for 14 days with transforming growth factor-β before being placed into a constraining cage for the remainder of culture. Controls were cultured under FS conditions throughout. Agarose constructs cultured in cages did not expand after the day 14 caging while FS constructs expanded to 8 × their day 0 weight after 112 days of culture. In addition to the physical differences in growth, by day 56, caged constructs had higher equilibrium (agarose: 639 ± 179 kPa and CDMH: 608 ± 257 kPa) and dynamic compressive moduli (agarose: 3.4 ± 1.0 MPa and CDMH 2.8 ± 1.0 MPa) than FS constructs (agarose: 193 ± 74 kPa and 1.1 ± 0.5 MPa and CDMH: 317 ± 93 kPa and 1.8 ± 1.0 MPa for equilibrium and dynamic properties, respectively). Interestingly, when normalized to final day wet weight, cage and FS constructs did not exhibit differences in proteoglycan or collagen content. However, caged culture enhanced collagen maturation through the increased formation of pyridinoline crosslinks and improved collagen matrix stability as measured by α-chymotrypsin solubility. These findings demonstrate that physically constrained culture of engineered cartilage constructs improves functional properties through improved collagen network maturity and stability. We anticipate that constrained culture may benefit other reported engineered cartilage systems that exhibit a mismatch in proteoglycan and collagen synthesis.