The repair of osteochondral defects requires a tissue engineering approach that aims at mimicking the physiological properties and structure of two different tissues (cartilage and bone) using specifically designed scaffold-cell constructs. Biphasic and triphasic approaches utilize two or three different architectures, materials, or composites to produce a multilayered construct. This article gives an overview of some of the current strategies in multiphasic/gradient-based scaffold architectures and compositions for tissue engineering of osteochondral defects. In addition, the application of finite element analysis (FEA) in scaffold design and simulation of in vitro and in vivo cell growth outcomes has been briefly covered. FEA-based approaches can potentially be coupled with computer-assisted fabrication systems for controlled deposition and additive manufacturing of the simulated patterns. Finally, a summary of the existing challenges associated with the repair of osteochondral defects as well as some recommendations for future directions have been brought up in the concluding section of this article.
This study reports the development of biological/synthetic scaffolds for bone tissue engineering (TE) via 3D bioplotting. These scaffolds were composed of poly(L-lactic-co-glycolic acid) (PLGA), type I collagen, and nano-hydroxyapatite (nHA) in an attempt to mimic the extracellular matrix of bone. The solvent used for processing the scaffolds was 1,1,1,3,3,3-hexafluoro-2-propanol. The produced scaffolds were characterized by scanning electron microscopy, microcomputed tomography, thermogravimetric analysis, and unconfined compression test. This study also sought to validate the use of finite-element optimization in COMSOL Multiphysics for scaffold design. Scaffold topology was simplified to three factors: nHA content, strand diameter, and strand spacing. These factors affect the ability of the scaffold to bear mechanical loads and how porous the structure can be. Twenty four scaffolds were constructed according to an I-optimal, split-plot designed experiment (DE) in order to generate experimental models of the factor-response relationships. Within the design region, the DE and COMSOL models agreed in their recommended optimal nHA (30%) and strand diameter (460 μm). However, the two methods disagreed by more than 30% in strand spacing (908 μm for DE; 601 μm for COMSOL). Seven scaffolds were 3D-bioplotted to validate the predictions of DE and COMSOL models (4.5-9.9 MPa measured moduli). The predictions for these scaffolds showed relative agreement for scaffold porosity (mean absolute percentage error of 4% for DE and 13% for COMSOL), but were substantially poorer for scaffold modulus (51% for DE; 21% for COMSOL), partly due to some simplifying assumptions made by the models. Expanding the design region in future experiments (e.g., higher nHA content and strand diameter), developing an efficient solvent evaporation method, and exerting a greater control over layer overlap could allow developing PLGA-nHA-collagen scaffolds to meet the mechanical requirements for bone TE.
Industrialization of stem-cell based therapies requires innovative solutions to close the gap between research and commercialization. Scalable cell production platforms are needed to reliably deliver the cell quantities needed during the various stages of development and commercial supply. Human pluripotent stem cells (hPSCs) are a key source material for generating therapeutic cell types. We have developed a closed, automated and scalable stirred tank bioreactor platform, capable of sustaining high fold expansion of hPSCs. Such a platform could facilitate the in-process monitoring and integration of online monitoring systems, leading to significantly reduced labor requirements and contamination risk. hPSCs are expanded in a controlled bioreactor using perfused xeno-free media. Cell harvest and concentration are performed in closed steps. The hPSCs can be cryopreserved to generate a bank of cells, or further processed as needed. Cryopreserved cells can be thawed into a two-dimensional (2D) tissue culture platform or a three-dimensional (3D) bioreactor to initiate a new expansion phase, or be differentiated to the clinically relevant cell type. The expanded hPSCs express hPSC-specific markers, have a normal karyotype and the ability to differentiate to the cells of the three germ layers. This end-to-end platform allows a large scale expansion of high quality hPSCs that can support the required cell demand for various clinical indications.Int. J. Mol. Sci. 2020, 21, 89 2 of 29 recapitulate in vivo conditions. To replace the number of cells lost during a myocardial infarction, for example, approximately 1 × 10 9 cells are required per patient dose [7].Given that 2D-based cell culture platforms are nonscalable with minimal capacity for expansion, achieving high cell densities in a 2D system would involve costly arrangements including extensive manual effort, laboratory space and personnel. These platforms also often do not possess adequate systems to control or monitor parameters, such as the production of key metabolites by hiPSCs in culture. Moreover, iPSC-derived cardiomyocytes remain phenotypically immature [8], despite a number of studies demonstrating enhanced maturation through the modulation of existing methodologies [9][10][11][12][13].Recent innovations in suspension culture systems provide robust, controlled and scalable platforms beyond conventional 2D approaches, which can be translated to current Good Manufacturing Practice (cGMP) compliant processes [14][15][16]. A number of studies have demonstrated the feasibility of hPSC expansion in suspension cultures using aggregate [14,16,17] and microcarrier (MC)-based [18][19][20] three dimensional (3D) culture systems. Aggregate-based 3D culture provides a more physiologically relevant microenvironment, but has been shown not only to require the small molecule, Y27632, for the survival of hPSCs [15], but also sequential passaging to achieve high fold expansion [21]. Not without its own advantages, microcarrier-based culture systems facilitate a larger surface...
Microcarriers, including crosslinked porous gelatin beads (Cultispher G) are widely used as cell carriers for cell therapy applications. Microcarriers can support a range of adherent cell types in stirred tank bioreactor culture, which is scalable up to several thousands of liters. Cultispher G in particular is advantageous for cell therapy applications because it can be dissolved enzymatically, and thus cells can be harvested without the need to perform a large‐scale cell‐bead filtration step. This enzymatic dissolution, however, is challenged by the slow degradation of the carriers in the presence of enzymes as new extracellular matrix is being deposited by the proliferating cells. This extended dissolution timelimits the yield of cell recovery while compromising cellular viability. We report herein the development of crosslinked porous gelatin beads that afford rapid, stimuli‐triggered dissolution for facile cell removal using human mesenchymal stem cells (hMSC) as a model system. We successfully fabricated redox‐sensitive beads (RS beads) and studied their cell growth, dissolution time and cell yield, compared to regular gelatin‐based beads (Reg beads). We have shown that RS beads allow for much faster dissolution compared to Reg beads, supporting better hMSC detachment and recovery following 8 days of culture in spinner flasks, or in 3L bioreactors. These newly synthesized RS beads show promise as cellular microcarriers and can be used for scale‐up manufacturing of different cell types while providing on‐demand degradation for facile cell retrieval.
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