Perfusion bioreactor system for human mesenchymal stem cell tissue engineering: Dynamic cell seeding and construct development

Human Bone-Marrow-Derived Stem-Cell-Seeded 3D Chitosan-Gelatin-Genipin Scaffolds Show Enhanced Extracellular Matrix Mineralization When Cultured under a Perfusion Flow in Osteogenic Medium

Tissue-engineered bone tissue grafts are a promising alternative to the more conventional use of natural donor bone grafts. However, choosing an appropriate biomaterial/scaffold to sustain cell survival, proliferation, and differentiation in a 3D environment remains one of the most critical issues in this domain. Recently, chitosan/gelatin/genipin (CGG) hybrid scaffolds have been proven as a more suitable environment to induce osteogenic commitment in undifferentiated cells when doped with graphene oxide (GO). Some concern is, however, raised towards the use of graphene and graphene-related material in medical applications. The purpose of this work was thus to check if the osteogenic potential of CGG scaffolds without added GO could be increased by improving the medium diffusion in a 3D culture of differentiating cells. To this aim, the level of extracellular matrix (ECM) mineralization was evaluated in human bone-marrow-derived stem cell (hBMSC)-seeded 3D CGG scaffolds upon culture under a perfusion flow in a dedicated custom-made bioreactor system. One week after initiating dynamic culture, histological/histochemical evaluations of CGG scaffolds were carried out to analyze the early osteogenic commitment of the culture. The analyses show the enhanced ECM mineralization of the 3D perfused culture compared to the static counterpart. The results of this investigation reveal a new perspective on more efficient clinical applications of CGG scaffolds without added GO.

Effects of cell deformability and adhesion strength on dynamic cell seeding: Cell-scale investigation via mesoscopic modeling

The flow of cell suspension through a porous scaffold is a common process in dynamic cell seeding, which determines the initial distribution of cells for constructing tissue-engineered grafts. Physical insights into the transport and adhesion behaviors of cells in this process are of great significance to the precise control of cell density and its distribution in the scaffold. Revealing of dynamic mechanisms underlying these cell behaviors through experiments is still difficult. The numerical approach therefore plays an important role in such studies. However, existing studies have mostly focused on external factors (e.g., flow conditions and scaffold architecture) but ignored the intrinsic biomechanical properties of cells as well as their associated effects. The present work utilized a well-established mesoscopic model to simulate the dynamic cell seeding within a porous scaffold, based on which a thorough investigation of the effects of cell deformability and cell-scaffold adhesion strength on the seeding process was carried out. The results show that the increase in either the stiffness or the bond strength of cells would augment the firm-adhesion rate and thus enhance seeding efficiency. In comparison to cell deformability, bond strength seems to play a more dominant role. Especially in the cases with weak bond strength, remarkable losses of seeding efficiency and distribution uniformity are observed. Noteworthily, it is found that both the firm-adhesion rate and the seeding efficiency are quantiatively related to the adhesion strength which is measured as the detachment force, suggesting a straightforward way to estimate the seeding outcome.

Triphasic 3D In Vitro Model of Bone-Tendon-Muscle Interfaces to Study Their Regeneration

The transition areas between different tissues, known as tissue interfaces, have limited ability to regenerate after damage, which can lead to incomplete healing. Previous studies focussed on single interfaces, most commonly bone-tendon and bone-cartilage interfaces. Herein, we develop a 3D in vitro model to study the regeneration of the bone-tendon-muscle interface. The 3D model was prepared from collagen and agarose, with different concentrations of hydroxyapatite to graduate the tissues from bones to muscles, resulting in a stiffness gradient. This graduated structure was fabricated using indirect 3D printing to provide biologically relevant surface topographies. MG-63, human dermal fibroblasts, and Sket.4U cells were found suitable cell models for bones, tendons, and muscles, respectively. The biphasic and triphasic hydrogels composing the 3D model were shown to be suitable for cell growth. Cells were co-cultured on the 3D model for over 21 days before assessing cell proliferation, metabolic activity, viability, cytotoxicity, tissue-specific markers, and matrix deposition to determine interface formations. The studies were conducted in a newly developed growth chamber that allowed cell communication while the cell culture media was compartmentalised. The 3D model promoted cell viability, tissue-specific marker expression, and new matrix deposition over 21 days, thereby showing promise for the development of new interfaces.

Current Progress in Vascular Engineering and Its Clinical Applications

Coronary heart disease (CHD) is caused by narrowing or blockage of coronary arteries due to atherosclerosis. Coronary artery bypass grafting (CABG) is widely used for the treatment of severe CHD cases. Although autologous vessels are a preferred choice, healthy autologous vessels are not always available; hence there is a demand for tissue engineered vascular grafts (TEVGs) to be used as alternatives. However, producing clinical grade implantable TEVGs that could healthily survive in the host with long-term patency is still a great challenge. There are additional difficulties in producing small diameter (<6 mm) vascular conduits. As a result, there have not been TEVGs that are commercially available. Properties of vascular scaffolds such as tensile strength, thrombogenicity and immunogenicity are key factors that determine the biocompatibility of TEVGs. The source of vascular cells employed to produce TEVGs is a limiting factor for large-scale productions. Advanced technologies including the combined use of natural and biodegradable synthetic materials for scaffolds in conjunction with the use of mesenchyme stem cells or induced pluripotent stem cells (iPSCs) provide promising solutions for vascular tissue engineering. The aim of this review is to provide an update on various aspects in this field and the current status of TEVG clinical applications.

Scale-up of a Composite Cultured Skin Using a Novel Bioreactor Device in a Porcine Wound Model

Extensive deep-burn management with a two-stage strategy can reduce reliance on skin autografts; a biodegradable polyurethane scaffold to actively temporize the wound and later an autologous composite cultured skin (CCS) for definitive closure. The materials fulfilling each stage have undergone in vitro and in vivo pretesting in "small" large animal wounds. For humans, producing multiple, large CCSs requires a specialized bioreactor. This article reports a system used to close large porcine wounds. Three Large White pigs were used, each with two wounds (24.5 cm × 12 cm) into which biodegradable dermal scaffolds were implanted. A sample from discarded tissue allowed isolation/culture of autologous fibroblasts and keratinocytes. CCS production began by presoaking a 1-mm-thick biodegradable polyurethane foam in autologous plasma. In the bioreactor cassette, fibroblasts were seeded into the matrix with thrombin until established, followed by keratinocytes. The CCSs were applied onto integrated dermal scaffolds on day 35, alongside a sheet skin graft (30% of one wound). Serial punch biopsies, trans-epidermal water loss readings (TEWL), and wound measurements indicated epithelialization. During dermal scaffold integration, negligible wound contraction was observed (average 4.5%). After CCS transplantation, the control skin grafts were "taken" by day 11 when visible islands of epithelium were clinically observed on 2/3 CCSs. Closure was confirmed histologically, with complete epithelialization by day 63 post-CCS transplantation (CCS TEWL ~ normal skin average 11.9 g/m2h). Four of six wounds demonstrated closure with robust, stratified epithelium. Generating large pieces of CCS capable of healing large wounds is thus possible using a specialized designed bioreactor.

Improved cell seeding efficiency and cell distribution in porous hydroxyapatite scaffolds by semi-dynamic method

Tissue engineering is a promising technique for the repair of bone defects. An efficient and homogeneous distribution of cell seeding into scaffold is a crucial but challenging step in the technique. Murine bone marrow mesenchymal stem cells were seeded into porous hydroxyapatite scaffolds of two morphologies by three methods: static seeding, semi-dynamic seeding, or dynamic perfusion seeding. Seeding efficiency, survival, distribution, and proliferation were quantitatively evaluated. To investigate the performance of the three seeding methods for larger/thicker scaffolds as well as batch seeding of numerous scaffolds, three scaffolds were stacked to form assemblies, and seeding efficiencies and cell distribution were analyzed. The semi-dynamic seeding and static seeding methods produced significantly higher seeding efficiencies, vitalities, and proliferation than did the dynamic perfusion seeding. On the other hand, the semi-dynamic seeding and dynamic perfusion seeding methods resulted in more homogeneous cell distribution than did the static seeding. For stacked scaffold assemblies, the semi-dynamic seeding method also created superior seeding efficiency and longitudinal cell distribution homogeneity. The semi-dynamic seeding method combines the high seeding efficiency of static seeding and satisfactory distribution homogeneity of dynamic seeding while circumventing their disadvantages. It may contribute to improved outcomes of bone tissue engineering.

A digital light processing 3D printed magnetic bioreactor system using silk magnetic bioink

Among various bioreactors used in the field of tissue engineering and regenerative medicine, a magnetic bioreactor is more capable of providing steady force to the cells while avoiding direct manipulation of the materials. However, most of them are complex and difficult to fabricate, with drawbacks in terms of consistency and biocompatibility. In this study, a magnetic bioreactor system and a magnetic hydrogel were manufactured by single-stage three-dimensional (3D) printing with digital light processing (DLP) technique for differentiation of myoblast cells. The hydrogel was composed of a magnetic part containing iron oxide and glycidyl-methacrylated silk fibroin, and a cellular part printed by adding mouse myoblast cell (C2C12) to gelatin glycidyl methacrylate, that was placed in the magnetic bioreactor system to stimulate the cells in the hydrogel. The composite hydrogel was steadily printed by a one-stage layering technique using a DLP printer. The magnetic bioreactor offered mechanical stretching of the cells in the hydrogel in 3D ways, so that the cellular differentiation could be executed in three dimensions just like the human environment. Cell viability, as well as gene expression using quantitative reverse transcription-polymerase chain reaction, were assessed after magneto-mechanical stimulation of the myoblast cell-embedded hydrogel in the magnetic bioreactor system. Comparison with the control group revealed that the magnetic bioreactor system accelerated differentiation of mouse myoblast cells in the hydrogel and increased myotube diameter and lengthin vitro. The DLP-printed magnetic bioreactor and the hydrogel were simply manufactured and easy-to-use, providing an efficient environment for applying noninvasive mechanical force via FDA-approved silk fibroin and iron oxide biocomposite hydrogel, to stimulate cells without any evidence of cytotoxicity, demonstrating the potential for application in muscle tissue engineering.

Current Advances on the Regeneration of Musculoskeletal Interfaces

The regeneration of the musculoskeletal system has been widely investigated. There is now detailed knowledge about the organs composing this system. Research has also investigated the zones between individual tissues where physical, mechanical, and biochemical properties transition. However, the understanding of the regeneration of musculoskeletal interfaces is still lacking behind. Numerous disorders and injuries can degrade or damage tissue interfaces. Their inability to regenerate can delay the tissue repair and regeneration process, leading to graft instability, high morbidity, and pain. Moreover, the knowledge of the mechanism of tissue interface development is not complete. This review presents an overview of the most recent approaches of the regeneration of musculoskeletal interfaces, including the latest in vitro, preclinical, and clinical studies. Impact statement Interfaces between soft and hard tissues are ubiquitous within the body. These transition zones are crucial for joint motion, stabilisation and load transfer between tissues, but do not seem to regenerate well after injury or deterioration. The knowledge about their biology is vast, but little is known about their development. Various musculoskeletal disorders in combination with risk factors including aging and unhealthy lifestyle, can lead to local imbalances, misalignments, inflammation, pain and restricted mobility. Our manuscript reviews the current approaches taken to promote the regeneration of musculoskeletal interfaces through in vitro, pre-clinical and clinical studies.

Preconditioning Human Adipose-Derived Stromal Cells on Decellularized Adipose Tissue Scaffolds Within a Perfusion Bioreactor Modulates Cell Phenotype and Promotes a Pro-regenerative Host Response

Cell-based therapies involving the delivery of adipose-derived stromal cells (ASCs) on decellularized adipose tissue (DAT) scaffolds are a promising approach for soft tissue augmentation and reconstruction. Our lab has recently shown that culturing human ASCs on DAT scaffolds within a perfusion bioreactor prior to implantation can enhance their capacity to stimulate in vivo adipose tissue regeneration. Building from this previous work, the current study investigated the effects of bioreactor preconditioning on the ASC phenotype and secretory profile in vitro, as well as host cell recruitment following implantation in an athymic nude mouse model. Immunohistochemical analyses indicated that culturing within the bioreactor increased the percentage of ASCs co-expressing inducible nitric oxide synthase (iNOS) and arginase-1 (Arg-1), as well as tumor necrosis factor-alpha (TNF-α) and interleukin-10 (IL-10), within the peripheral regions of the DAT relative to statically cultured controls. In addition, bioreactor culture altered the expression levels of a range of immunomodulatory factors in the ASC-seeded DAT. In vivo testing revealed that culturing the ASCs on the DAT within the perfusion bioreactor prior to implantation enhanced the infiltration of host CD31⁺ endothelial cells and CD26⁺ cells into the DAT implants, but did not alter CD45⁺F4/80⁺CD68⁺ macrophage recruitment. However, a higher fraction of the CD45⁺ cell population expressed the pro-regenerative macrophage marker CD163 in the bioreactor group, which may have contributed to enhanced remodeling of the scaffolds into host-derived adipose tissue. Overall, the findings support that bioreactor preconditioning can augment the capacity of human ASCs to stimulate regeneration through paracrine-mediated mechanisms.

Transparent PDMS Bioreactors for the Fabrication and Analysis of Multi-Layer Pre-vascularized Hydrogels Under Continuous Perfusion

Tissue engineering in combination with stem cell technology has the potential to revolutionize human healthcare. It aims at the generation of artificial tissues that can mimic the original with complex functions for medical applications. However, even the best current designs are limited in size, if the transport of nutrients and oxygen to the cells and the removal of cellular metabolites waste is mainly dependent on passive diffusion. Incorporation of functional biomimetic vasculature within tissue engineered constructs can overcome this shortcoming. Here, we developed a novel strategy using 3D printing and injection molding technology to customize multilayer hydrogel constructs with pre-vascularized structures in transparent Polydimethysiloxane (PDMS) bioreactors. These bioreactors can be directly connected to continuous perfusion systems without complicated construct assembling. Mimicking natural layer-structures of vascular walls, multilayer vessel constructs were fabricated with cell-laden fibrin and collagen gels, respectively. The multilayer design allows functional organization of multiple cell types, i.e., mesenchymal stem cells (MSCs) in outer layer, human umbilical vein endothelial cells (HUVECs) the inner layer and smooth muscle cells in between MSCs and HUVECs layers. Multiplex layers with different cell types showed clear boundaries and growth along the hydrogel layers. This work demonstrates a rapid, cost-effective, and practical method to fabricate customized 3D-multilayer vascular models. It allows precise design of parameters like length, thickness, diameter of lumens and the whole vessel constructs resembling the natural tissue in detail without the need of sophisticated skills or equipment. The ready-to-use bioreactor with hydrogel constructs could be used for biomedical applications including pre-vascularization for transplantable engineered tissue or studies of vascular biology.

Development of a Semi-Automated, Bulk Seeding Device for Large Animal Model Implantation of Tissue Engineered Vascular Grafts

Vascular tissue engineering is a field of regenerative medicine that restores tissue function to defective sections of the vascular network by bypass or replacement with a tubular, engineered graft. The tissue engineered vascular graft (TEVG) is comprised of a biodegradable scaffold, often combined with cells to prevent acute thrombosis and initiate scaffold remodeling. Cells are most effectively incorporated into scaffolds using bulk seeding techniques. While our group has been successful in uniform, rapid, bulk cell seeding of scaffolds for TEVG testing in small animals using our well-validated rotational vacuum technology, this approach was not directly translatable to large scaffolds, such as those required for large animal testing or human implants. The objective of this study was to develop and validate a semi-automated cell seeding device that allows for uniform, rapid, bulk seeding of large scaffolds for the fabrication of TEVGs appropriately sized for testing in large animals and eventual translation to humans. Validation of our device revealed successful seeding of cells throughout the length of our tubular scaffolds with homogenous longitudinal and circumferential cell distribution. To demonstrate the utility of this device, we implanted a cell seeded scaffold as a carotid interposition graft in a sheep model for 10 weeks. Graft remodeling was demonstrated upon explant analysis using histological staining and mechanical characterization. We conclude from this work that our semi-automated, rotational vacuum seeding device can successfully seed porous tubular scaffolds suitable for implantation in large animals and provides a platform that can be readily adapted for eventual human use.

A comprehensive comparison of cell seeding methods using highly porous melt electrowriting scaffolds

Cell seeding is challenging in the case of additively manufactured 3-dimensional scaffolds, as the open macroscopic pore network impedes the retention of the seeding solution. The present study aimed at comparing several seeding conditions (no fetal bovine serum, 10% or 100% serum) and methods (Static seeding in Tissue Culture Treated plate (CT), Static seeding of the MES in non-Culture Treated plate (nCT), Seeding in nCT plate placed on an orbital shaker at 20 rpm (nCTR), Static seeding of the MES previously incubated with 100% FBS for 1 h to allow for protein adsorption (FBS)) commonly utilised in tissue engineering using highly porous melt electrowritten scaffolds, assessing their seeding efficacy, cell distribution homogeneity and reproducibility. Firstly, we demonstrated that the incubation in 100% serum was superior to the 10% serum pre-incubation and that 1 h only was sufficient to obtain enhanced cell attachment. We further compared this technique to the other methods and demonstrated significant and beneficial impact of the 100% serum pre-incubation, which resulted in enhanced efficacy, homogeneous cell distribution and high reproducibility, leading to accelerated colonisation/maturation of the tissue engineered constructs. We further showed the superior performance of this method using 3D-printed scaffolds also made of different polymers, demonstrating its capacity for up-scaling. Therefore, the pre-incubation of the scaffold in 100% serum is a simple yet highly effective method for enhancing cell adhesion and ensuring seeding reproducibility. This is crucial for tissue engineering applications, especially when cell availability is scarce, and for product standardisation from a translational perspective.

A 3D computational model of perfusion seeding for investigating cell transport and adhesion within a porous scaffold

The process of cell seeding within a porous scaffold is an essential first step in the development of tissue-engineered bone grafts. Understanding the underlying mechanisms of cell distribution and adhesion is fundamental for the design and optimization of the seeding process. To that end, we present a numerical model to investigate the perfusion cell seeding process that incorporates cell mechanics, cell-fluid interaction, and cell-scaffold adhesion. The individual cells are modeled as deformable spherical capsules capable of adhering to the scaffold surface as well as to other cells with probabilistic bond formation and rupture. The mechanical deformation of the cell is calibrated with the stretching of mice mesenchymal stem cells induced by optical tweezers, while the predicted adhesive forces are consistent with the experimental data reported in the literature. A sub-domain is numerically reconstructed as the region of interest (ROI) which is representative of an actual scaffold. Through the simulations, the perfusion seeding kinetics within the ROI involving detailed transport and adhesion of cells over time is analyzed. The effects of the perfusion pressure and initial cell concentration on the seeding kinetics are studied in terms of adhesion rates, cell cluster formation, seeding uniformity, and efficiency, as well as scaffold permeability. The results highlight the importance of cell-fluid interaction and adhesion dynamics in modeling the dynamic seeding process. This bottom-up model provides a way to bridge detailed behaviors of individual cells to the seeding outcomes at the macroscopic scale, allowing for finding the best configuration to enhance cell seeding.

Biomaterial-assisted scalable cell production for cell therapy

Cell therapy, the treatment of diseases using living cells, offers a promising clinical approach to treating refractory diseases. The global market for cell therapy is growing rapidly, and there is an increasing demand for automated methods that can produce large quantities of high quality therapeutic cells. Biomaterials can be used during cell production to establish a biomimetic microenvironment that promotes cell adhesion and proliferation while maintaining target cell genotype and phenotype. Here we review recent progress and emerging techniques in biomaterial-assisted cell production. The increasing use of auxiliary biomaterials and automated production methods provides an opportunity to improve quality control and increase production efficiency using standardized GMP-compliant procedures.

An Image-Guided Intrascaffold Cell Assembly Technique for Accurate Printing of Heterogeneous Tissue Constructs

For tissue engineering and regenerative medicine, creating thick and heterogeneous scaffold-based tissue constructs requires deep and precise multicellular deposition. Traditional cell seeding strategies lack the ability to create multicellular tissue constructs with high cell penetration and distribution, while emerging strategies aim to simultaneously combine cell-laden tissue segments with scaffold fabrication. Here we describe a technique that allows for three-dimensional (3D) intrascaffold cell assembly in which scaffolds are prefabricated and pretreated, followed by accurate cell distribution within the scaffold using an image-guided technique. This two-step process yields less limitation in scaffold material choice as well as additional treatments, provides accurate cell distribution, and has less potential to harm cells. The image processing technique captures a 2D geometric image of the scaffold, followed by a series of processes, mainly including grayscale transformation, threshold segmentation, and boundary extraction, to ultimately locate scaffold macropore centroids. Coupled with camera calibration data, accurate 3D cell assembly pathway plans can be made. Intrascaffold assembly parameter optimization and complex intrascaffold gradient, multidirectional, and vascular structure assembly were studied. Demonstration was also made with path planning and cell assembly experiments using NIH3T3-cell-laden hydrogels and collagen-coated poly(lactic-co-glycolic acid) (PLGA) scaffolds. Experiments with CellTracker fluorescent monitoring, live/dead staining, and phalloidin-F-actin/DAPI immunostaining and comparison with two control groups (bioink manual injection and cell suspension static surface pipetting) showed accurate cell distribution and positioning and high cell viability (>93%). The PrestoBlue assay showed obvious cell proliferation over seven culture days in vitro. This technique provides an accurate method to aid simple and complex cell colonization with variant depth within 3D-scaffold-based constructs using multiple cells. The modular method can be used with any existing printing platform and shows potential in facilitating direct spatial organization and hierarchal 3D assembly of multiple cells and/or drugs within scaffolds for further tissue engineering studies and clinical applications.

Embossed Membranes with Vascular Patterns Guide Vascularization in a 3D Tissue Model

The vascularization of three-dimensional (3D) tissue constructs is necessary for transporting nutrients and oxygen to the component cells. In this study, a vacuum forming method was applied to emboss a vascular pattern on an electrospun membrane so that guided vascular structures could develop within the construct. Two- or six-layer constructs of electrospun membranes seeded with endothelial cells and pericytes were stacked and subcutaneously implanted into mice. Blood vessel formation in the implanted constructs with six alternating layers of flat membranes and membranes embossed with a blood vessel pattern was observed after two weeks of implantation. The formation of blood vessels was observed along the embossed blood vessel pattern in the structure of the embossed membrane laminated at four weeks and eight weeks. Vascular endothelial growth factor (VEGF) and angiopoietin 1 (Ang-1) were highly expressed in the vascularized structures. Therefore, we demonstrated that a structure capable of producing a desired blood vessel shape with electrospun membranes embossed with a blood vessel pattern can be manufactured, and that a variety of structures can be manufactured using electrospun membranes in the tissue engineering era.

Effect of scaffold architecture on cell seeding efficiency: A discrete phase model CFD analysis

Within perfusion cell culture systems, scaffold architecture is able to control important biological parameters such as permeability and fluid flow-induced shear stress. As well, one of the main factors affecting the final fate of this process as well as optimal cell differentiation and proliferation in these systems is initial adhesion of cells to scaffolds. In this study, the effect of scaffold architecture on the adhesion of the cells was computationally investigated. For this purpose, four scaffold models including double-diamond, gyroid, FR-D, and Schwarz-primitive were designed using triply periodic minimal surface (TPMS) geometry with a constant porosity of 80%. As well, the inlet velocity of zero to simulate static cell culture and three different inlet velocities for modeling the dynamic cell culture conditions were also selected. The results showed that cell culture efficiency of scaffolds could be changed up to seven times from architecture to architecture under the same conditions. The efficiency of cell culture in scaffolds with tortuous architecture was also reported higher than those with relatively straight microchannels. In terms of culture methods, unlike dynamic cell culture model in which almost a homogeneous cell distribution was observed in static cell culture simulation, more cells adhered, but they had agglomerated in the scaffold entrance regions and had failed to reach all regions. The results of this study shed more light on the selection and design of scaffold architecture for optimal cell culture in tissue engineering.

Assembly of Human Stem Cell-Derived Cortical Spheroids and Vascular Spheroids to Model 3-D Brain-like Tissues

Human cerebral organoids derived from induced pluripotent stem cells (iPSCs) provide novel tools for recapitulating the cytoarchitecture of human brain and for studying biological mechanisms of neurological disorders. However, the heterotypic interactions of neurovascular units, composed of neurons, pericytes, astrocytes, and brain microvascular endothelial cells, in brain-like tissues are less investigated. The objective of this study is to investigate the impacts of neural spheroids and vascular spheroids interactions on the regional brain-like tissue patterning in cortical spheroids derived from human iPSCs. Hybrid neurovascular spheroids were constructed by fusion of human iPSC-derived cortical neural progenitor cell (iNPC) spheroids, endothelial cell (iEC) spheroids, and the supporting human mesenchymal stem cells (MSCs). Single hybrid spheroids were constructed at different iNPC: iEC: MSC ratios of 4:2:0, 3:2:1 2:2:2, and 1:2:3 in low-attachment 96-well plates. The incorporation of MSCs upregulated the secretion levels of cytokines VEGF-A, PGE2, and TGF-β1 in hybrid spheroid system. In addition, tri-cultured spheroids had high levels of TBR1 (deep cortical layer VI) and Nkx2.1 (ventral cells), and matrix remodeling genes, MMP2 and MMP3, as well as Notch-1, indicating the crucial role of matrix remodeling and cell-cell communications on cortical spheroid and organoid patterning. Moreover, tri-culture system elevated blood-brain barrier gene expression (e.g., GLUT-1), CD31, and tight junction protein ZO1 expression. Treatment with AMD3100, a CXCR4 antagonist, showed the immobilization of MSCs during spheroid fusion, indicating a CXCR4-dependent manner of hMSC migration and homing. This forebrain-like model has potential applications in understanding heterotypic cell-cell interactions and novel drug screening in diseased human brain.

Commitment to Aerobic Glycolysis Sustains Immunosuppression of Human Mesenchymal Stem Cells

Human mesenchymal stem cells (hMSCs) promote endogenous tissue repair in part by coordinating multiple components of the host immune system in response to environmental stimuli. Recent studies have shown that hMSCs are metabolically heterogeneous and actively reconfigure metabolism to support the biochemical demands of tissue repair. However, how hMSCs regulate their energy metabolism to support their immunomodulatory properties is largely unknown. This study investigates hMSC metabolic reconfiguration during immune activation and provides evidence that the hMSC metabolic state significantly influences their immunomodulatory properties. Specifically, hMSC immune polarization by interferon‐gamma (IFN‐γ) treatment leads to remodeling of hMSC metabolic pathways toward glycolysis, which is required to sustain the secretion of immunosuppressive factors. IFN‐γ exposure also inhibited mitochondrial electron transport activity, and the accumulation of mitochondrial reactive oxygen species plays an important signaling role in this metabolic reconfiguration. The results also show that activation of the Akt/mTOR signaling pathway is required for metabolic reconfiguration during immune polarization and that interruption of these metabolic changes alters the immune response in IFN‐γ licensed hMSCs. The results demonstrate the potential of altering hMSC metabolism to enhance their immunomodulatory properties and therapeutic efficacy in various diseases. Stem Cells Translational Medicine2019;8:93–106

Mesenchymal stem cell cultivation in electrospun scaffolds: mechanistic modeling for tissue engineering

Tissue engineering is a multidisciplinary field of research in which the cells, biomaterials, and processes can be optimized to develop a tissue substitute. Three-dimensional (3D) architectural features from electrospun scaffolds, such as porosity, tortuosity, fiber diameter, pore size, and interconnectivity have a great impact on cell behavior. Regarding tissue development in vitro, culture conditions such as pH, osmolality, temperature, nutrient, and metabolite concentrations dictate cell viability inside the constructs. The effect of different electrospun scaffold properties, bioreactor designs, mesenchymal stem cell culture parameters, and seeding techniques on cell behavior can be studied individually or combined with phenomenological modeling techniques. This work reviews the main culture and scaffold factors that affect tissue development in vitro regarding the culture of cells inside 3D matrices. The mathematical modeling of the relationship between these factors and cell behavior inside 3D constructs has also been critically reviewed, focusing on mesenchymal stem cell culture in electrospun scaffolds.

From "Bench to Bedside": Current Advancement on Large-Scale Production of Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) are the primary cell source in cell therapy and regenerative medicine due to its extraordinary self-renewing capacity and multilineage differentiation potential. Clinical trials involving MSCs are being conducted in a range of human diseases and the number of registered cases is continuously increasing. However, a wide gap exists between the number of MSCs obtainable from the donor site and the number required for implantation to damage tissues, and also between MSC scalability and MSC phenotype stability. The clinical translation of MSCs necessitates a scalable expansion bioprocess for the biomanufacturing of therapeutically qualified cells. This review presents current achievements for expansion of MSCs. Issues involving culture condition modification, bioreactor systems, as well as microcarrier and scaffold platforms for optimal MSC systems are discussed. Most importantly, the gap between current MSC expansion and clinical application, as well as outbreak directions for the future are discussed. The present systemic review will bring new insights into future large-scale MSC expansion and clinical application.

A flow bioreactor system compatible with real-time two-photon fluorescence lifetime imaging microscopy

Bioreactors are essential cell and tissue culture tools that allow the introduction of biophysical signals into in vitro cultures. One major limitation is the need to interrupt experiments and sacrifice samples at certain time points for analyses. To address this issue, we designed a bioreactor that combines high-resolution contact-free imaging and continuous flow in a closed system that is compatible with various types of microscopes. The high throughput fluid flow bioreactor was combined with two-photon fluorescence lifetime imaging microscopy (2P-FLIM) and validated. The hydrodynamics of the bioreactor chamber were characterized using COMSOL. The simulation of shear stress indicated that the bioreactor system provides homogeneous and reproducible flow conditions. The designed bioreactor was used to investigate the effects of low shear stress on human umbilical vein endothelial cells (HUVECs). In a scratch assay, we observed decreased migration of HUVECs under shear stress conditions. Furthermore, metabolic activity shifts from glycolysis to oxidative phosphorylation-dependent mechanisms in HUVECs cultured under low shear stress conditions were detected using 2P-FLIM. Future applications for this bioreactor range from observing cell fate development in real-time to monitoring the environmental effects on cells or metabolic changes due to drug applications.

Neural Differentiation of Spheroids Derived from Human Induced Pluripotent Stem Cells-Mesenchymal Stem Cells Coculture

Organoids, the condensed three-dimensional (3D) tissues emerged at the early stage of organogenesis, are a promising approach to regenerate functional and vascularized organ mimics. While incorporation of heterotypic cell types, such as human mesenchymal stem cells (hMSCs) and human induced pluripotent stem cells (hiPSCs)-derived neural progenitors aid neural organ development, the interactions of secreted factors during neurogenesis have not been well understood. The objective of this study is to investigate the impact of the composition and structure of 3D hybrid spheroids of hiPSCs and hMSCs on dorsal cortical differentiation and the secretion of extracellular matrices and trophic factors in vitro. The hybrid spheroids were formed at different hiPSC:hMSC ratios (100:0, 75:25, 50:50, 25:75, 0:100) using direct mixing or pre-hiPSC aggregation method, which generated dynamic spheroid structure. The cellular organization, proliferation, neural marker expression, and the secretion of extracellular matrix proteins and the cytokines were characterized. The incorporation of MSCs upregulated Nestin and β-tubulin III expression (the dorsal cortical identity was shown by Pax6 and TBR1 expression), matrix remodeling proteins, and the secretion of transforming growth factor-β1 and prostaglandin E2. This study indicates that the appropriate composition and structure of hiPSC-MSC spheroids promote neural differentiation and trophic factor and matrix secretion due to the heterotypic cell-cell interactions.

Flow perfusion rate modulates cell deposition onto scaffold substrate during cell seeding

The combination of perfusion bioreactors with porous scaffolds is beneficial for the transport of cells during cell seeding. Nonetheless, the fact that cells penetrate into the scaffold pores does not necessarily imply the interception of cells with scaffold substrate and cell attachment. An in vitro perfusion system was built to relate the selected flow rate with seeding efficiency. However, the in vitro model does not elucidate how the flow rate affects the transport and deposition of cells onto the scaffold. Thus, a computational model was developed mimicking in vitro conditions to identify the mechanisms that bring cells to the scaffold from suspension flow. Static and dynamic cell seeding configurations were investigated. In static seeding, cells sediment due to gravity until they encounter the first obstacle. In dynamic seeding, 12, 120 and 600 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu \hbox {l/min}$$\end{document}μl/min flow rates were explored under the presence or the absence of gravity. Gravity and secondary flow were found to be key factors for cell deposition. In vitro and in silico seeding efficiencies are in the same order of magnitude and follow the same trend with the effect of fluid flow; static seeding results in higher efficiency than dynamic perfusion although irregular spatial distribution of cells was found. In dynamic seeding, 120 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu \hbox {l/min}$$\end{document}μl/min provided the best seeding results. Nevertheless, the perfusion approach reports low efficiencies for the scaffold used in this study which leads to cell waste and low density of cells inside the scaffold. This study suggests gravity and secondary flow as the driving mechanisms for cell-scaffold deposition. In addition, the present in silico model can help to optimize hydrodynamic-based seeding strategies prior to experiments and enhance cell seeding efficiency.

Human Mesenchymal Stem Cells Differentiation Regulated by Hydroxyapatite Content within Chitosan-Based Scaffolds under Perfusion Conditions

The extensive need for hard tissue substituent greatly motivates development of suitable allogeneic grafts for therapeutic recreation. Different calcium phosphate phases have been accepted as scaffold's components with positive influence on osteoinduction and differentiation of human mesenchymal stem cells, in terms of their higher fraction within the graft. Nevertheless, the creation of unlimited nutrients diffusion through newly formed grafts is of great importance. The media flow accomplished by perfusion forces can provide physicochemical, and also, biomechanical stimuli for three-dimensional bone-construct growth. In the present study, the influence of a different scaffold's composition on the human mesenchymal stem cells (hMSCs) differentiation performed in a U-CUP bioreactor under perfusion conditioning was investigated. The histological and immunohistochemical analysis of cultured bony tissues, and the evaluation of osteogenic genes' expression indicate that the lower fraction of in situ formed hydroxyapatite in the range of 10⁻30% within chitosan scaffold could be preferable for bone-construct development.

Differentiation of mesenchymal stem cells from human amniotic fluid to cardiomyocyte‑like cells

Ischemic heart disease (IHD) is a major factor influencing worldwide mortality rates. Furthermore, IHD has become a significant health problem among the Thai population. Stem cell therapy using mesenchymal stem cells (MSCs) is an alternative therapeutic method that has been applied to improve the quality of life of patients. Amniotic fluid (AF) contains a heterogeneous cell population, including MSCs, which are multipotent stem cells that have the capability to differentiate into mesenchymal lineages. The purpose of the present study was to evaluate the MSC characteristics of human (h)AF and determine its potency regarding cardiogenic differentiation. MSC characterization following flow cytometric analysis revealed that the cells expressed MSC markers, cluster of differentiation (CD)44, CD90, human leukocyte antigen-ABC and CD73. The results of the alamar blue assay demonstrated that cell proliferation rate continuously increased from the early cultivation phase up to 5-fold during days 1 to 5 of cell culturing. The highest rate of cell proliferation was observed on day 17 with a 30-fold increase compared with that on day 1. During the cardiogenic induction stage, morphological changes were observed between day 0 and day 21, and it was revealed that the hAF derived-MSCs in the cardiogenic-induced group exhibited myotube-like morphology after 7 days of cell culturing. Following cardiogenic induction, immunohistochemistry staining was performed on day 21, and reverse transcription-quantitative polymerase chain reaction on day 7 and 21. These steps were performed to detect the protein and gene expression levels of cardiac specific proteins (GATA4, cardiac troponin T, Nkx2.5 and Connexin43). The results of the present study indicated that hAF-MSCs possess the potential to differentiate into cardiomyocyte-like cells. Thus, it was concluded that hAF may be a suitable source of MSCs for stem cell therapy and tissue engineering.

Sensitivity of human pluripotent stem cells to insulin precipitation induced by peristaltic pump-based medium circulation: considerations on process development

Controlled large-scale production of human pluripotent stem cells (hPSCs) is indispensable for their envisioned clinical translation. Aiming at advanced process development in suspension culture, the sensitivity of hPSC media to continuous peristaltic pump-based circulation, a well-established technology extensively used in hydraulically-driven bioreactors, was investigated. Unexpectedly, conditioning of low protein media (i.e. E8 and TeSR-E8) in a peristaltic pump circuit induced severe viability loss of hPSCs cultured as aggregates in suspension. Optical, biochemical, and cytological analyses of the media revealed that the applied circulation mode resulted in the reduction of the growth hormone insulin by precipitation of micro-sized particles. Notably, in contrast to insulin depletion, individual withdrawal of other medium protein components (i.e. bFGF, TGFβ1 or transferrin) provoked minor reduction of hPSC viability, if any. Supplementation of the surfactant glycerol or the use of the insulin analogue Aspart did not overcome the issue of insulin precipitation. In contrast, the presence of bovine or human serum albumin (BSA or HSA, respectively) stabilized insulin rescuing its content, possibly by acting as molecular chaperone-like protein, ultimately supporting hPSC maintenance. This study highlights the potential and the requirement of media optimization for automated hPSC processing and has broad implications on media development and bioreactor-based technologies.

Development and Characterization of a Parallelizable Perfusion Bioreactor for 3D Cell Culture

The three dimensional (3D) cultivation of stem cells in dynamic bioreactor systems is essential in the context of regenerative medicine. Still, there is a lack of bioreactor systems that allow the cultivation of multiple independent samples under different conditions while ensuring comprehensive control over the mechanical environment. Therefore, we developed a miniaturized, parallelizable perfusion bioreactor system with two different bioreactor chambers. Pressure sensors were also implemented to determine the permeability of biomaterials which allows us to approximate the shear stress conditions. To characterize the flow velocity and shear stress profile of a porous scaffold in both bioreactor chambers, a computational fluid dynamics analysis was performed. Furthermore, the mixing behavior was characterized by acquisition of the residence time distributions. Finally, the effects of the different flow and shear stress profiles of the bioreactor chambers on osteogenic differentiation of human mesenchymal stem cells were evaluated in a proof of concept study. In conclusion, the data from computational fluid dynamics and shear stress calculations were found to be predictable for relative comparison of the bioreactor geometries, but not for final determination of the optimal flow rate. However, we suggest that the system is beneficial for parallel dynamic cultivation of multiple samples for 3D cell culture processes.

Review: bioreactor design towards generation of relevant engineered tissues: focus on clinical translation

In tissue engineering and regenerative medicine, studies that utilize 3D scaffolds for generating voluminous tissues are mostly confined in the realm of in vitro research and preclinical animal model testing. Bioreactors offer an excellent platform to grow and develop 3D tissues by providing conditions that mimic their native microenvironment. Aligning the bioreactor development process with a focus on patient care will aid in the faster translation of the bioreactor technology to clinics. In this review, we discuss the various factors involved in the design of clinically relevant bioreactors in relation to their respective applications. We explore the functional relevance of tissue grafts generated by bioreactors that have been designed to provide physiologically relevant mechanical cues on the growing tissue. The review discusses the recent trends in non-invasive sensing of the bioreactor culture conditions. It provides an insight to the current technological advancements that enable in situ, non-invasive, qualitative and quantitative evaluation of the tissue grafts grown in a bioreactor system. We summarize the emerging trends in commercial bioreactor design followed by a short discussion on the aspects that hamper the 'push' of bioreactor systems into the commercial market as well as 'pull' factors for stakeholders to embrace and adopt widespread utility of bioreactors in the clinical setting. Copyright © 2017 John Wiley & Sons, Ltd.

Silk as a Biomaterial to Support Long-Term Three-Dimensional Tissue Cultures

Tissue engineering has broad and diverse impacts on a variety of different applications from tissue regeneration to drug screening. While two-dimensional (2-D) cell culture platforms are suitable for tissue interfaces where planar surfaces are relevant, three dimensional (3-D) tissue models have enhanced relevance and sustainability over 2-D devices. The improvements between 2-D and 3-D functions and sustainability are related to the limitations of 2-D systems to support proper cellular morphology and signaling over time, resulting in cell overgrowth or changes in viability. For sustainable (long-term) cultures, 3-D silk protein scaffolds provide biocompatibility, porous features for transport, robust yet tunable mechanical properties, retain size and open porous structures for extended time frames due to slow proteolytic biodegradation, avoid specific cell signaling, and require no chemical cross-linking. Silk degradation can be extended for months to years without premature collapse of structures (that would result in necrosis) to support cell interactions during slow remodeling toward native tissue. Silk can also be fabricated into different material formats, such as hydrogels, tubes, sponges, composites, fibers, microspheres, and thin films, providing versatile platforms and interfaces for a variety of different applications. For sustainable tissue engineering applications, many formats have been used, including silk ionmer hydrogels that have been cultured for up to 8 weeks and porous silk scaffolds that have been cultured for up to 6 months. In this review, we highlight some of our tissue engineering work related to long-term in vitro cultures. While each tissue engineered system (adipose tissue, cortical brain tissue, intestine, kidney tissue, bone) is unique, they all use silk biomaterials as a base scaffolding material to achieve sustainable cultivation. Sustainability is important for studies that extend past a few weeks to study acute and chronic impacts of treatments, disease models, and other related applications in the field of tissue engineering.

The Use of Silk as a Scaffold for Mature, Sustainable Unilocular Adipose 3D Tissue Engineered Systems

There is a critical need for monitoring physiologically relevant, sustainable, human adipose tissues in vitro to gain new insights into metabolic diseases. To support long-term culture, a 3D silk scaffold assisted culture system is developed that maintains mature unilocular adipocytes ex vivo in coculture with preadipocytes, endothelial cells, and smooth muscle cells obtained from small volumes of liquefied adipose samples. Without the silk scaffold, adipose tissue explants cannot be sustained in long-term culture (3 months) due to their fragility. Adjustments to media components are used to tune lipid metabolism and proliferation, in addition to responsiveness to an inflammatory stimulus. Interestingly, patient specific responses to TNFα stimulation are observed, providing a proof-of-concept translational technique for patient specific disease modeling in the future. In summary, this novel 3D scaffold assisted approach is required for establishing physiologically relevant, sustainable, human adipose tissue systems from small volumes of lipoaspirate, making this methodology of great value to studies of metabolism, adipokine-driven diseases, and other diseases where the roles of adipocytes are only now becoming uncovered.

Numerical investigation of dynamic microorgan devices as drug screening platforms. Part II: Microscale modeling approach and validation

The authors have previously reported a rigorous macroscale modeling approach for an in vitro 3D dynamic microorgan device (DMD). This paper represents the second of a two-part model-based investigation where the effect of microscale (single liver cell-level) shear-mediated mechanotransduction on drug biotransformation is deconstructed. Herein, each cell is explicitly incorporated into the geometric model as single compartmentalized metabolic structures. Each cell's metabolic activity is coupled with the microscale hydrodynamic Wall Shear Stress (WSS) simulated around the cell boundary through a semi-empirical polynomial function as an additional reaction term in the mass transfer equations. Guided by the macroscale model-based hydrodynamics, only 9 cells in 3 representative DMD domains are explicitly modeled. Dynamic and reaction similarity rules based on non-dimensionalization are invoked to correlate the numerical and empirical models, accounting for the substrate time scales. The proposed modeling approach addresses the key challenge of computational cost towards modeling complex large-scale DMD-type system with prohibitively high cell densities. Transient simulations are implemented to extract the drug metabolite profile with the microscale modeling approach validated with an experimental drug flow study. The results from the author's study demonstrate the preferred implementation of the microscale modeling approach over that of its macroscale counterpart.

Density-Dependent Metabolic Heterogeneity in Human Mesenchymal Stem Cells

Human mesenchymal stem cells (hMSCs) are intrinsically heterogeneous and comprise subpopulations that differ in their proliferation, multi-potency, and functional properties, which are commonly demonstrated by culturing hMSCs at different plating densities. The objective of this study was to investigate the metabolic profiles of different subpopulations of hMSC by testing the hypothesis that the clonogenic hMSC subpopulation, which is selectively enriched in clonal density (CD) and low density (LD) culture (10 and 100 cells per square centimeter, respectively), possesses a metabolic phenotype that differs from that of hMSC in medium- or high-density (MD: 1,000 and HD: 3,000 cells per square centimeter, respectively). Cells at CD and LD conditions exhibited elevated expression of CD146 and colony forming unit-fibroblast compared with cells at MD- or HD. Global metabolic profiles revealed by gas chromatography-mass spectrometry of cell extracts showed clear distinction between LD and HD cultures, and density-dependent differences in coupling of glycolysis to the TCA cycle. Metabolic inhibitors revealed density-dependent differences in glycolysis versus oxidative phosphorylation (OXPHOS) for ATP generation, in glutamine metabolism, in the dependence on the pentose phosphate pathway for maintaining cellular redox state, and sensitivity to exogenous reactive oxygen species. We also show that active OXPHOS is not required for proliferation in LD culture but that OXPHOS activity increases senescence in HD culture. Together, the results revealed heterogeneity in hMSC culture exists at the level of primary metabolism. The unique metabolic characteristics of the clonogenic subpopulation suggest a novel approach for optimizing in vitro expansion of hMSCs.

Application of avidin-biotin technology to improve cell adhesion on nanofibrous matrices

BACKGROUND

Electrospinning is an easy and effective technique to produce submicron fibers possessing a range of attractive characteristics such as interconnected porous structures similar to natural ECM and good resilience to movement. Rapid and efficient cell attachment to nanofibrous matrices is a necessary prerequisite in tissue engineering. Thus, the aim of this study is to evaluate poly(ε-caprolactone-co-lactide)/Pluronic (PLCL/Pluronic) nanofibrous matrices with avidin-biotin technology for improving cell adhesion for the first time.

RESULTS

PLCL/Pluronic nanofibers had relatively homogeneous fibers and interconnected porous structures. Pluronic significantly modified the hydrophilicity of nanofibrous matrices and PLCL/Pluronic nanofibrous matrices had better performance on maintaining cell proliferation. Avidin-biotin technology had no negative effect on the hydrophilic property, mechanical property and cell proliferation. Meanwhile, the attachment and spreading of adipose-derived stem cells (ADSCs) onto PLCL/Pluronic nanofibrous matrices with avidin-biotin technology was promoted obviously.

CONCLUSIONS

PLCL/Pluronic nanofibrous matrices inheriting the excellent characteristics of both PLCL and Pluronic have the better cell adhesion ability through avidin-biotin technology, implying a promising application in skin care, tissue regeneration and other related area.

Strategies for improving the physiological relevance of human engineered tissues

This review examines important robust methods for sustained, steady-state, in vitro culture. To achieve 'physiologically relevant' tissues in vitro additional complexity must be introduced to provide suitable transport, cell signaling, and matrix support for cells in 3D environments to achieve stable readouts of tissue function. Most tissue engineering systems draw conclusions on tissue functions such as responses to toxins, nutrition, or drugs based on short-term outcomes with in vitro cultures (2-14 days). However, short-term cultures limit insight with physiological relevance because the cells and tissues have not reached a steady-state.

Seeding of mesenchymal stem cells into inner part of interconnected porous biodegradable scaffold by a new method with a filter paper

An appropriate physical support provided by scaffolds creates a supportive environment that directs proliferation and differentiation of stem cells. However, it is difficult to homogenously inoculate stem cells into the inner part of scaffolds at high cell densities. In this study, mesenchymal stem cells were seeded into a hydroxyapatite/poly (D, L-lactic-co-glycolic acid) (HAP/PLGA) scaffold that had enough mechanical strength and porous 3-D structure. With an aid of a filter paper placed under the bottom of a HAP/PLGA block, the cells suspended in a culture medium flowed from the top to the bottom through interconnected pores in the scaffold, and distributed almost homogenously, as compared to cell distribution near the surface of the block by the conventional method using centrifugation or reduced pressure. This simple method with a filter paper may be useful in preparation of cell-scaffold complexes for tissue engineering.