A perfusion bioreactor system efficiently generates cell-loaded bone substitute materials for addressing critical size bone defects

Perfusable Tissue Bioprinted into a 3D-Printed Tailored Bioreactor System

Bioprinting provides a powerful tool for regenerative medicine, as it allows tissue construction with a patient's specific geometry. However, tissue culture and maturation, commonly supported by dynamic bioreactors, are needed. We designed a workflow that creates an implant-specific bioreactor system, which is easily producible and customizable and supports cell cultivation and tissue maturation. First, a bioreactor was designed and different tissue geometries were simulated regarding shear stress and nutrient distribution to match cell culture requirements. These tissues were then directly bioprinted into the 3D-printed bioreactor. To prove the ability of cell maintenance, C2C12 cells in two bioinks were printed into the system and successfully cultured for two weeks. Next, human mesenchymal stem cells (hMSCs) were successfully differentiated toward an adipocyte lineage. As the last step of the presented strategy, we developed a prototype of an automated mobile docking station for the bioreactor. Overall, we present an open-source bioreactor system that is adaptable to a wound-specific geometry and allows cell culture and differentiation. This interdisciplinary roadmap is intended to close the gap between the lab and clinic and to integrate novel 3D-printing technologies for regenerative medicine.

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.

Translation of biophysical environment in bone into dynamic cell culture under flow for bone tissue engineering

Bone is a dynamic environment where osteocytes, osteoblasts, and mesenchymal stem/progenitor cells perceive mechanical cues and regulate bone metabolism accordingly. In particular, interstitial fluid flow in bone and bone marrow serves as a primary biophysical stimulus, which regulates the growth and fate of the cellular components of bone. The processes of mechano-sensory and -transduction towards bone formation have been well studied mainly in vivo as well as in two-dimensional (2D) dynamic cell culture platforms, which elucidated mechanically induced osteogenesis starting with anabolic responses, such as production of nitrogen oxide and prostaglandins followed by the activation of canonical Wnt signaling, upon mechanosensation. The knowledge has been now translated into regenerative medicine, particularly into the field of bone tissue engineering, where multipotent stem cells are combined with three-dimensional (3D) scaffolding biomaterials to produce transplantable constructs for bone regeneration. In the presence of 3D scaffolds, the importance of suitable dynamic cell culture platforms increases further not only to improve mass transfer inside the scaffolds but to provide appropriate biophysical cues to guide cell fate. In principle, the concept of dynamic cell culture platforms is rooted to bone mechanobiology. Therefore, this review primarily focuses on biophysical environment in bone and its translation into dynamic cell culture platforms commonly used for 2D and 3D cell expansion, including their advancement, challenges, and future perspectives. Additionally, it provides the literature review of recent empirical studies using 2D and 3D flow-based dynamic cell culture systems for bone tissue engineering.

Osteogenic differentiation of adipose-derived canine mesenchymal stem cells seeded in porous calcium-phosphate scaffolds

Introduction

Engineered bone graft substitutes are a promising alternative and supplement to autologous bone grafts as treatments for bone healing impairment. Advances in human medicine extend an invitation to pursue these biomimetic strategies in animal patients, substantiated by the theory that specialized scaffolds, multipotent cells, and biological cues may be combined into a bioactive implant intended for the enhancement of tissue regeneration.

Methods

This proof-of-concept study was designed to evaluate and validate the feasibility of beta-tricalcium phosphate foam scaffolds seeded with canine mesenchymal stem cells derived from adipose tissue. Cell-inoculated samples and sham controls were cultured statically for 72 hours in complete growth medium to evaluate seeding capacity, while a subset of loaded scaffolds was further induced with osteogenic culture medium for 21 days. Produced implants were characterized and validated with a combination of immunofluorescence and reflection confocal microscopy, scanning electron microscopy, and polymerase chain reaction to confirm osteogenic differentiation in tridimensional-induced samples.

Results

After 72 hours of culture, all inoculated scaffolds presented widespread yet heterogeneous surface seeding, distinctively congregating stem cells around pore openings. Furthermore, at 21 days of osteogenic culture conditions, robust osteoblastic differentiation of the seeded cells was confirmed by the change of cell morphology and evident deposition of extra-cellular matrix, accompanied by mineralization and scaffold remodeling; furthermore, all induced cell-loaded implants lost specific stemness immunophenotype expression and simultaneously upregulated genomic expression of osteogenic genes Osterix and Ostecalcin.

Conclusions

β-TCP bio-ceramic foam scaffolds proved to be suitable carriers and hosts of canine adipose-derived MSCs, promoting not only surface attachment and proliferation, but also demonstrating strong in-vitro osteogenic potential. Although this research provides satisfactory in-vitro validation for the conceptualization and feasibility of a canine bio-active bone implant, further testing such as patient safety, large-scale reproducibility, and quality assessment are needed for regulatory compliance in future commercial clinical applications.

Unique osteogenic profile of bone marrow stem cells stimulated in perfusion bioreactor is Rho-ROCK-mediated contractility dependent

The fate determination of bone marrow mesenchymal stem/stromal cells (BMSC) is tightly regulated by mechanical cues, including fluid shear stress. Knowledge of mechanobiology in 2D culture has allowed researchers in bone tissue engineering to develop 3D dynamic culture systems with the potential for clinical translation in which the fate and growth of BMSC are mechanically controlled. However, due to the complexity of 3D dynamic cell culture compared to the 2D counterpart, the mechanisms of cell regulation in the dynamic environment remain relatively undescribed. In the present study, we analyzed the cytoskeletal modulation and osteogenic profiles of BMSC under fluid stimuli in a 3D culture condition using a perfusion bioreactor. BMSC subjected to fluid shear stress (mean 1.56 mPa) showed increased actomyosin contractility, accompanied by the upregulation of mechanoreceptors, focal adhesions, and Rho GTPase-mediated signaling molecules. Osteogenic gene expression profiling revealed that fluid shear stress promoted the expression of osteogenic markers differently from chemically induced osteogenesis. Osteogenic marker mRNA expression, type 1 collagen formation, ALP activity, and mineralization were promoted in the dynamic condition, even in the absence of chemical supplementation. The inhibition of cell contractility under flow by Rhosin chloride, Y27632, MLCK inhibitor peptide-18, or Blebbistatin revealed that actomyosin contractility was required for maintaining the proliferative status and mechanically induced osteogenic differentiation in the dynamic culture. The study highlights the cytoskeletal response and unique osteogenic profile of BMSC in this type of dynamic cell culture, stepping toward the clinical translation of mechanically stimulated BMCS for bone regeneration.

Personalized Volumetric Tissue Generation by Enhancing Multiscale Mass Transport through 3D Printed Scaffolds in Perfused Bioreactors

Engineered tissues provide an alternative to graft material, circumventing the use of donor tissue such as autografts or allografts and non-physiological synthetic implants. However, their lack of vasculature limits the growth of volumetric tissue more than several millimeters thick which limits their success post-implantation. Perfused bioreactors enhance nutrient mass transport inside lab-grown tissue but remain poorly customizable to support the culture of personalized implants. Here, a multiscale framework of computational fluid dynamics (CFD), additive manufacturing, and a perfusion bioreactor system are presented to engineer personalized volumetric tissue in the laboratory. First, microscale 3D printed scaffold pore geometries are designed and 3D printed to characterize media perfusion through CFD and experimental fluid testing rigs. Then, perfusion bioreactors are custom-designed to combine 3D printed scaffolds with flow-focusing inserts in patient-specific shapes as simulated using macroscale CFD. Finally, these computationally optimized bioreactor-scaffold assemblies are additively manufactured and cultured with pre-osteoblast cells for 7, 20, and 24 days to achieve tissue growth in the shape of human calcaneus bones of 13 mL volume and 1 cm thickness. This framework enables an intelligent model-based design of 3D printed scaffolds and perfusion bioreactors which enhances nutrient transport for long-term volumetric tissue growth in personalized implant shapes. The novel methods described here are readily applicable for use with different cell types, biomaterials, and scaffold microstructures to research therapeutic solutions for a wide range of tissues.

Fully Synthetic 3D Fibrous Scaffolds for Stromal Tissues-Replacement of Animal-Derived Scaffold Materials Demonstrated by Multilayered Skin

The extracellular matrix (ECM) of soft tissues in vivo has remarkable biological and structural properties. Thereby, the ECM provides mechanical stability while it still can be rearranged via cellular remodeling during tissue maturation or healing processes. However, modern synthetic alternatives fail to provide these key features among basic properties. Synthetic matrices are usually completely degraded or are inert regarding cellular remodeling. Based on a refined electrospinning process, a method is developed to generate synthetic scaffolds with highly porous fibrous structures and enhanced fiber-to-fiber distances. Since this approach allows for cell migration, matrix remodeling, and ECM synthesis, the scaffold provides an ideal platform for the generation of soft tissue equivalents. Using this matrix, an electrospun-based multilayered skin equivalent composed of a stratified epidermis, a dermal compartment, and a subcutis is able to be generated without the use of animal matrix components. The extension of classical dense electrospun scaffolds with high porosities and motile fibers generates a fully synthetic and defined alternative to collagen-gel-based tissue models and is a promising system for the construction of tissue equivalents as in vitro models or in vivo implants.

Perfused Platforms to Mimic Bone Microenvironment at the Macro/Milli/Microscale: Pros and Cons

As life expectancy increases, the population experiences progressive ageing. Ageing, in turn, is connected to an increase in bone-related diseases (i.e., osteoporosis and increased risk of fractures). Hence, the search for new approaches to study the occurrence of bone-related diseases and to develop new drugs for their prevention and treatment becomes more pressing. However, to date, a reliable in vitro model that can fully recapitulate the characteristics of bone tissue, either in physiological or altered conditions, is not available. Indeed, current methods for modelling normal and pathological bone are poor predictors of treatment outcomes in humans, as they fail to mimic the in vivo cellular microenvironment and tissue complexity. Bone, in fact, is a dynamic network including differently specialized cells and the extracellular matrix, constantly subjected to external and internal stimuli. To this regard, perfused vascularized models are a novel field of investigation that can offer a new technological approach to overcome the limitations of traditional cell culture methods. It allows the combination of perfusion, mechanical and biochemical stimuli, biological cues, biomaterials (mimicking the extracellular matrix of bone), and multiple cell types. This review will discuss macro, milli, and microscale perfused devices designed to model bone structure and microenvironment, focusing on the role of perfusion and encompassing different degrees of complexity. These devices are a very first, though promising, step for the development of 3D in vitro platforms for preclinical screening of novel anabolic or anti-catabolic therapeutic approaches to improve bone health.

In Vivo Investigation of Polymer-Ceramic PCL/HA and PCL/β-TCP 3D Composite Scaffolds and Electrical Stimulation for Bone Regeneration

Critical bone defects are a major clinical challenge in reconstructive bone surgery. Polycaprolactone (PCL) mixed with bioceramics, such as hydroxyapatite (HA) and tricalcium phosphate (TCP), create composite scaffolds with improved biological recognition and bioactivity. Electrical stimulation (ES) aims to compensate the compromised endogenous electrical signals and to stimulate cell proliferation and differentiation. We investigated the effects of composite scaffolds (PCL with HA; and PCL with β-TCP) and the use of ES on critical bone defects in Wistar rats using eight experimental groups: untreated, ES, PCL, PCL/ES, HA, HA/ES, TCP, and TCP/ES. The investigation was based on histomorphometry, immunohistochemistry, and gene expression analysis. The vascular area was greater in the HA/ES group on days 30 and 60. Tissue mineralization was greater in the HA, HA/ES, and TCP groups at day 30, and TCP/ES at day 60. Bmp-2 gene expression was higher in the HA, TCP, and TCP/ES groups at day 30, and in the TCP/ES and PCL/ES groups at day 60. Runx-2, Osterix, and Osteopontin gene expression were also higher in the TCP/ES group at day 60. These results suggest that scaffolds printed with PCL and TCP, when paired with electrical therapy application, improve bone regeneration.

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.

Biomimetic Mineralization Promotes Viability and Differentiation of Human Mesenchymal Stem Cells in a Perfusion Bioreactor

In bone tissue engineering, the design of 3D systems capable of recreating composition, architecture and micromechanical environment of the native extracellular matrix (ECM) is still a challenge. While perfusion bioreactors have been proposed as potential tool to apply biomechanical stimuli, its use has been limited to a low number of biomaterials. In this work, we propose the culture of human mesenchymal stem cells (hMSC) in biomimetic mineralized recombinant collagen scaffolds with a perfusion bioreactor to simultaneously provide biochemical and biophysical cues guiding stem cell fate. The scaffolds were fabricated by mineralization of recombinant collagen in the presence of magnesium (RCP.MgAp). The organic matrix was homogeneously mineralized with apatite nanocrystals, similar in composition to those found in bone. X-Ray microtomography images revealed isotropic porous structure with optimum porosity for cell ingrowth. In fact, an optimal cell repopulation through the entire scaffolds was obtained after 1 day of dynamic seeding in the bioreactor. Remarkably, RCP.MgAp scaffolds exhibited higher cell viability and a clear trend of up-regulation of osteogenic genes than control (non-mineralized) scaffolds. Results demonstrate the potential of the combination of biomimetic mineralization of recombinant collagen in presence of magnesium and dynamic culture of hMSC as a promising strategy to closely mimic bone ECM.

Advanced 3D Cell Culture Techniques in Micro-Bioreactors, Part II: Systems and Applications

In this second part of our systematic review on the research area of 3D cell culture in micro-bioreactors we give a detailed description of the published work with regard to the existing micro-bioreactor types and their applications, and highlight important results gathered with the respective systems. As an interesting detail, we found that micro-bioreactors have already been used in SARS-CoV research prior to the SARS-CoV2 pandemic. As our literature research revealed a variety of 3D cell culture configurations in the examined bioreactor systems, we defined in review part one “complexity levels” by means of the corresponding 3D cell culture techniques applied in the systems. The definition of the complexity is thereby based on the knowledge that the spatial distribution of cell-extracellular matrix interactions and the spatial distribution of homologous and heterologous cell–cell contacts play an important role in modulating cell functions. Because at least one of these parameters can be assigned to the 3D cell culture techniques discussed in the present review, we structured the studies according to the complexity levels applied in the MBR systems.

Advanced 3D Cell Culture Techniques in Micro-Bioreactors, Part I: A Systematic Analysis of the Literature Published between 2000 and 2020

Bioreactors have proven useful for a vast amount of applications. Besides classical large-scale bioreactors and fermenters for prokaryotic and eukaryotic organisms, micro-bioreactors, as specialized bioreactor systems, have become an invaluable tool for mammalian 3D cell cultures. In this systematic review we analyze the literature in the field of eukaryotic 3D cell culture in micro-bioreactors within the last 20 years. For this, we define complexity levels with regard to the cellular 3D microenvironment concerning cell–matrix-contact, cell–cell-contact and the number of different cell types present at the same time. Moreover, we examine the data with regard to the micro-bioreactor design including mode of cell stimulation/nutrient supply and materials used for the micro-bioreactors, the corresponding 3D cell culture techniques and the related cellular microenvironment, the cell types and in vitro models used. As a data source we used the National Library of Medicine and analyzed the studies published from 2000 to 2020.

An "all-in-one" scaffold targeting macrophages to direct endogenous bone repair in situ

Scaffolds for tissue repair are designed in an increasingly complicated manner to meet multi-facet biological needs during the healing process. However, overly sophisticated design, especially the use of multiple components and delivery of exogenous cells, hampers the bench-to-bedside translation. Here, a multi-functional - yet mono-compositional - bioactive scaffold is devised to mediate the full-range, endogenous bone repair. Based on immunoactivity screening, a chemically-modified glucomannan polysaccharide is selected and processed into an anisotropic porous scaffold, which accurately stimulates macrophages to produce pro-regenerative cytokines. These cytokines effectively enhance the recruitment ("R") and induced osteogenesis ("IO") of the bone progenitor cells in situ. Meanwhile, the anisotropic porosity and carbohydrate signal of the scaffold facilitate differential adhesion ("A") and distribution ("D") of the macrophages and bone progenitor cells - enabling the former's accumulation at the surface while encouraging the latter's infiltration into the scaffold. Implanted in a rat calvarial defect model, this "RADIO" system effectively promotes healing over 12 weeks, with the obvious formation of hard callus through the scaffold. In summary, RADIO integrates multiple functions into one single scalable system ("all-in-one") to govern the dynamic bone-repair process, by harnessing the power of host macrophages. RADIO represents an open platform to solving the long-lasting complexity-versus-simplicity dilemma in biomaterials design. STATEMENT OF SIGNIFICANCE: Biomaterials as versatile tools for tissue repair are becoming increasingly complicated, yet overly sophisticated design - especially the use of multiple components, exogenous cells, and overdosed growth factors - hampers their clinical application. The pre-requisite for designing a successful integrative scaffold is to identify an inherent biological target responding to biomaterial signals, thereby efficiently and safely promoting tissue repair via the endogenous healing capability instead of extra multifarious biochemical components. For bone regeneration, the pivotal regulator is macrophages. Through activating host macrophages, our single-component scaffold system coordinates the entire bone regenerative cascade in situ and induces successful bone regeneration in a calvarial defect model. This scaffold represents a scalable and multi-functional approach to effectively simplify the sophisticated design in regenerative medicine.

In vivo safety assessment of a bio-inspired bone adhesive

A new class of materials, bone adhesives, could revolutionise the treatment of highly fragmented fractures. We present the first biological safety investigation of a bio-inspired bone adhesive. The formulation was based upon a modified calcium phosphate cement that included the amino acid phosphoserine. This material has recently been described as substantially stronger than other bioresorbable calcium phosphate cements. Four adhesive groups with the active substance (phosphoserine) and two control groups without phosphoserine were selected for in vitro and in vivo biocompatibility testing. The test groups were subject for cell viability assay and subcutaneous implantation in rats that was followed by gene expression analysis and histology assessment after 6 and 12 weeks. All adhesive groups supported the same rate of cell proliferation compared to the α-TCP control and had viability between 45-64% when compared to cell control. There was no evidence of an increased immune response or ectopic bone formation in vivo. To conclude, this bio-inspired bone adhesive has been proven to be safe, in the present study, without any harmful effects on the surrounding soft tissue.

Strategy for achieving standardized bone models

Reliably producing functional in vitro organ models, such as organ-on-chip systems, has the potential to considerably advance biology research, drug development time, and resource efficiency. However, despite the ongoing major progress in the field, three-dimensional bone tissue models remain elusive. In this review, we specifically investigate the control of perfusion flow effects as the missing link between isolated culture systems and scientifically exploitable bone models and propose a roadmap toward this goal.

Osteogenic induction of human mesenchymal stem cells in multilayered electrospun scaffolds at different flow rates and configurations in a perfusion bioreactor

Electrospun scaffolds are potentially interesting in bone tissue engineering due to a strong structural similarity to the natural bone matrix. To investigate the osteogenic behavior of cells on the scaffolds, dynamic culture of cells is essential to simulate the biological environment. In the present study, human mesenchymal stem cells (hMSCs) were cultured on multilayer nanohydroxyapatite-polycaprolactone electrospun scaffolds at different configurations (horizontal with or without pressure and parallel with the medium flow) and flow rates in a perfusion bioreactor. Alkaline phosphatase (ALP) activity, cell viability, Ca deposition and RUNX2 expression were determined in three different dynamic states, and compared with static culture after 1, 3, 7, and 14 days. Among dynamic groups, RUNX2 gene expression upregulated more in a horizontal state at a low flow rate without mechanical pressure (LF) and parallel flow (PF), than static group on day 7. At a high flow rate with mechanical pressure, Ca deposition and ALP activity increased 2.34 and 1.7 folds more than in static culture over 7 days, respectively. Furthermore, ALP activity, Ca deposition and RUNX2 gene expression increased in PF samples. PF provided longer culture time with higher cell differentiation. Therefore, high flow rate with mechanical pressure and PF are suggested for producing differentiated cell structure for bone tissue engineering.

Physical contact between mesenchymal stem cells and endothelial precursors induces distinct signatures with relevance to the very early phase of regeneration

Multipotent adult stem cells/precursor cells, especially of the mesenchymal and endothelial lineage, may have great potential for bone tissue engineering. Although their potential is highly recognized, not much is known about the underlying molecular mechanisms that initiate the regeneration process, connect osteogenesis, and angiogenesis and, finally, orchestrate renewal of bone tissue. Our study addressed these questions by generating two in vitro cell culture models to examine the changes in the global gene expression patterns of endothelial precursor cells and mesenchymal stem cells after 24 hours of either humoral (conditioned medium) or direct cell-cell interaction (co-culture). Endothelial precursor cells were isolated from human buffy coat and mesenchymal stem cells from the bone marrow of the femoral head. The comparison of the treated and control cells by microarray analyses revealed in total more than 1500 regulated genes, which were analyzed for their affiliation to angiogenesis and osteogenesis. Expression array analyses at the RNA and protein level revealed data with respect to regulated genes, pathways and targets that may represent a valid basis for further dissection of the systems biology of regeneration processes. It may also be helpful for the reconstitution of the natural composition of a regenerative microenvironment when targeting tissue regeneration both in vitro and in situ.

Improved vasculogenesis and bone matrix formation through coculture of endothelial cells and stem cells in tissue-specific methacryloyl gelatin-based hydrogels

The coculture of osteogenic and angiogenic cells and the resulting paracrine signaling via soluble factors are supposed to be crucial for successfully engineering vascularized bone tissue equivalents. In this study, a coculture system combining primary human adipose-derived stem cells (hASCs) and primary human dermal microvascular endothelial cells (HDMECs) within two types of hydrogels based on methacryloyl-modified gelatin (GM) as three-dimensional scaffolds was examined for its support of tissue specific cell functions. HDMECs, together with hASCs as supporting cells, were encapsulated in soft GM gels and were indirectly cocultured with hASCs encapsulated in stiffer GM hydrogels additionally containing methacrylate-modified hyaluronic acid and hydroxyapatite particles. After 14 days, the hASC in the stiffer gels (constituting the "bone gels") expressed matrix proteins like collagen type I and fibronectin, as well as bone-specific proteins osteopontin and alkaline phosphatase. After 14 days of coculture with HDMEC-laden hydrogels, the viscoelastic properties of the bone gels were significantly higher compared with the gels in monoculture. Within the soft vascularization gels, the formed capillary-like networks were significantly longer after 14 days of coculture than the structures in the control gels. In addition, the stability as well as the complexity of the vascular networks was significantly increased by coculture. We discussed and concluded that osteogenic and angiogenic signals from the culture media as well as from cocultured cell types, and tissue-specific hydrogel composition all contribute to stimulate the interplay between osteogenesis and angiogenesis in vitro and are a basis for engineering vascularized bone.

Numerical Evaluation and Prediction of Porous Implant Design and Flow Performance

Porous structure has been widely acknowledged as important factor for mass transfer and tissue regeneration. This study investigates effect of aimed-control design on mass transfer and tissue regeneration of porous implant with regular unit cell. Two shapes of unit cells (Octet truss, and Rhombic dodecahedron) were selected, which have similar symmetrical structure and are commonly used in practice. Through parametric design, porous scaffolds with the strut sizes of φ 0.5, 0.7, 0.9, and 1.1mm were created, respectively. Then using fluid flow simulation method, flow velocity, permeability, and shear stress which could reflect the properties of mass transfer and tissue regeneration were compared and evaluated, and the relationships between porous structure's physical parameters and flow performance were studied. Results demonstrated that unit cell shape and strut size greatly determine and influence other physical parameters and flow performances of porous implant. With the increasing of strut size, pore size and porosity linearly decrease, but the volume, surface area, and specific surface area increased. Importantly, implant with smaller strut size resulted in smaller flow velocity directly but greater permeability and more appropriate shear stress, which should be beneficial to cell attachment and proliferation. This study confirmed that porous implant with different unit cell shows different performances of mass transfer and tissue regeneration, and unit cell shape and strut size play vital roles in the control design. These findings could facilitate the quantitative assessment and optimization of the porous implant.

Journey into Bone Models: A Review

Bone is a complex tissue with a variety of functions, such as providing mechanical stability for locomotion, protection of the inner organs, mineral homeostasis and haematopoiesis. To fulfil these diverse roles in the human body, bone consists of a multitude of different cells and an extracellular matrix that is mechanically stable, yet flexible at the same time. Unlike most tissues, bone is under constant renewal facilitated by a coordinated interaction of bone-forming and bone-resorbing cells. It is thus challenging to recreate bone in its complexity in vitro and most current models rather focus on certain aspects of bone biology that are of relevance for the research question addressed. In addition, animal models are still regarded as the gold-standard in the context of bone biology and pathology, especially for the development of novel treatment strategies. However, species-specific differences impede the translation of findings from animal models to humans. The current review summarizes and discusses the latest developments in bone tissue engineering and organoid culture including suitable cell sources, extracellular matrices and microfluidic bioreactor systems. With available technology in mind, a best possible bone model will be hypothesized. Furthermore, the future need and application of such a complex model will be discussed.

Recent Advances in Tissue Engineering Strategies for the Treatment of Joint Damage

PURPOSE OF REVIEW

While the clinical potential of tissue engineering for treating joint damage has yet to be realized, research and commercialization efforts in the field are geared towards overcoming major obstacles to clinical translation, as well as towards achieving engineered grafts that recapitulate the unique structures, function, and physiology of the joint. In this review, we describe recent advances in technologies aimed at obtaining biomaterials, stem cells, and bioreactors that will enable the development of effective tissue-engineered treatments for repairing joint damage.

RECENT FINDINGS

3D printing of scaffolds is aimed at improving the mechanical structure and microenvironment necessary for bone regeneration within a damaged joint. Advances in our understanding of stem cell biology and cell manufacturing processes are informing translational strategies for the therapeutic use of allogeneic and autologous cells. Finally, bioreactors used in combination with cells and biomaterials are promising strategies for generating large tissue grafts for repairing damaged tissues in pre-clinical models. Together, these advances along with ongoing research directions are making tissue engineering increasingly viable for the treatment of joint damage.

Bioreactors and Microfluidics for Osteochondral Interface Maturation

The cell culture techniques are in the base of any biology-based science. The standard techniques are commonly static platforms as Petri dishes, tissue culture well plates, T-flasks, or well plates designed for spheroids formation. These systems faced a paradigm change from 2D to 3D over the current decade driven by the tissue engineering (TE) field. However, 3D static culture approaches usually suffer from several issues as poor homogenization of the formed tissues and development of a necrotic center which limits the size of in vitro tissues to hundreds of micrometers. Furthermore, for complex tissues as osteochondral (OC), more than recovering a 3D environment, an interface needs to be replicated. Although 3D cell culture is already the reality adopted by a newborn market, a technological revolution on cell culture devices needs a further step from static to dynamic already considering 3D interfaces with dramatic importance for broad fields such as biomedical, TE, and drug development. In this book chapter, we revised the existing approaches for dynamic 3D cell culture, focusing on bioreactors and microfluidic systems, and the future directions and challenges to be faced were discussed. Basic principles, advantages, and challenges of each technology were described. The reported systems for OC 3D TE were focused herein.

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

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

Deformation strain is the main physical driver for skeletal precursors to undergo osteogenesis in earlier stages of osteogenic cell maturation

Mesenchymal stem cells play a major role during bone remodelling and are thus of high interest for tissue engineering and regenerative medicine applications. Mechanical stimuli, that is, deformation strain and interstitial fluid-flow-induced shear stress, promote osteogenic lineage commitment. However, the predominant physical stimulus that drives early osteogenic cell maturation is not clearly identified. The evaluation of each stimulus is challenging, as deformation and fluid-flow-induced shear stress interdepend. In this study, we developed a bioreactor that was used to culture mesenchymal stem cells harbouring a strain-responsive AP-1 luciferase reporter construct, on porous scaffolds. In addition to the reporter, mineralization and vitality of the cells was investigated by alizarin red staining and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. Quantification of the expression of genes associated to bone regeneration and bone remodelling was used to confirm alizarin red measurements. Controlled perfusion and deformation of the 3-dimensional scaffold facilitated the alteration of the expression of osteogenic markers, luciferase activity, and calcification. To isolate the specific impact of scaffold deformation, a computational model was developed to derive a perfusion flow profile that results in dynamic shear stress conditions present in periodically loaded scaffolds. In comparison to actually deformed scaffolds, a lower expression of all measured readout parameters indicated that deformation strain is the predominant stimulus for skeletal precursors to undergo osteogenesis in earlier stages of osteogenic cell maturation.

3D models of the hematopoietic stem cell niche under steady-state and active conditions

Hematopoietic stem cells (HSCs) in the bone marrow are able to differentiate into all types of blood cells and supply the organism each day with billions of fresh cells. They are applied to cure hematological diseases such as leukemia. The clinical need for HSCs is high and there is a demand for being able to control and multiply HSCs in vitro. The hematopoietic system is highly proliferative and thus sensitive to anti-proliferative drugs such as chemotherapeutics. For many of these drugs suppression of the hematopoietic system is the dose-limiting toxicity. Therefore, biomimetic 3D models of the HSC niche that allow to control HSC behavior in vitro and to test drugs in a human setting are relevant for the clinics and pharmacology. Here, we describe a perfused 3D bone marrow analog that allows mimicking the HSC niche under steady-state and activated conditions that favor either HSC maintenance or differentiation, respectively, and allows for drug testing.

Hypoxic Three-Dimensional Scaffold-Free Aggregate Cultivation of Mesenchymal Stem Cells in a Stirred Tank Reactor

Extensive expansion of mesenchymal stem cells (MSCs) for cell-based therapies remains challenging since long-term cultivation and excessive passaging in two-dimensional conditions result in a loss of essential stem cell properties. Indeed, low survival rate of cells, alteration of surface marker profiles, and reduced differentiation capacity are observed after in vitro expansion and reduce therapeutic success in clinical studies. Remarkably, cultivation of MSCs in three-dimensional aggregates preserve stem cell properties. Hence, the large scale formation and cultivation of MSC aggregates is highly desirable. Besides other effects, MSCs cultivated under hypoxic conditions are known to display increased proliferation and genetic stability. Therefore, in this study we demonstrate cultivation of adipose derived human MSC aggregates in a stirred tank reactor under hypoxic conditions. Although aggregates were exposed to comparatively high average shear stress of 0.2 Pa as estimated by computational fluid dynamics, MSCs displayed a viability of 78-86% and maintained their surface marker profile and differentiation potential after cultivation. We postulate that cultivation of 3D MSC aggregates in stirred tank reactors is valuable for large-scale production of MSCs or their secreted compounds after further optimization of cultivation parameters.

Artificial Bone via Bone Tissue Engineering: Current Scenario and Challenges

Bone provides mechanical support, and flexibility to the body as a structural frame work along with mineral storage, homeostasis, and blood pH regulation. The repair and/or replacement of injured or defective bone with healthy bone or bone substitute is a critical problem in orthopedic treatment. Recent advances in tissue engineering have shown promising results in developing bone material capable of substituting the conventional autogenic or allogenic bone transplants. In the present review, we have discussed natural and synthetic scaffold materials such as metal and metal alloys, ceramics, polymers, etc. which are widely being used along with their cellular counterparts such as stem cells in bone tissue engineering with their pros and cons.

The use of rats and mice as animal models in ex vivo bone growth and development studies

In vivo animal experimentation has been one of the cornerstones of biological and biomedical research, particularly in the field of clinical medicine and pharmaceuticals. The conventional in vivo model system is invariably associated with high production costs and strict ethical considerations. These limitations led to the evolution of an ex vivo model system which partially or completely surmounted some of the constraints faced in an in vivo model system. The ex vivo rodent bone culture system has been used to elucidate the understanding of skeletal physiology and pathophysiology for more than 90 years. This review attempts to provide a brief summary of the historical evolution of the rodent bone culture system with emphasis on the strengths and limitations of the model. It encompasses the frequency of use of rats and mice for ex vivo bone studies, nutritional requirements in ex vivo bone growth and emerging developments and technologies. This compilation of information could assist researchers in the field of regenerative medicine and bone tissue engineering towards a better understanding of skeletal growth and development for application in general clinical medicine.Cite this article: A. A. Abubakar, M. M. Noordin, T. I. Azmi, U. Kaka, M. Y. Loqman. The use of rats and mice as animal models in ex vivo bone growth and development studies. Bone Joint Res 2016;5:610-618. DOI: 10.1302/2046-3758.512.BJR-2016-0102.R2.

Editorial: Biotechnology Journal brings more than biotechnology

Biotechnology Journal always brings the state-of-the-art biotechnologies to our readers. Different from other topical issues, this issue of Biotechnology Journal is complied with a series of exiting reviews and research articles from spontaneous submissions, again, addressing society's actual problems and needs. The progress is a real testimony how biotechnology contributes to achievements in healthcare, better utilization of resources, and a bio-based economy.

Beta-tricalcium phosphate granules improve osteogenesis in vitro and establish innovative osteo-regenerators for bone tissue engineering in vivo

The drawbacks of traditional bone-defect treatments have prompted the exploration of bone tissue engineering. This study aimed to explore suitable β-tricalcium phosphate (β-TCP) granules for bone regeneration and identify an efficient method to establish β-TCP-based osteo-regenerators. β-TCP granules with diameters of 1 mm and 1–2.5 mm were evaluated in vitro. The β-TCP granules with superior osteogenic properties were used to establish in vivo bioreactors, referred to as osteo-regenerators, which were fabricated using two different methods. Improved proliferation of bone mesenchymal stem cells (BMSCs), glucose consumption and ALP activity were observed for 1–2.5 mm β-TCP compared with 1-mm granules (P < 0.05). In addition, BMSCs incubated with 1–2.5 mm β-TCP expressed significantly higher levels of the genes for runt-related transcription factor-2, alkaline phosphatase, osteocalcin, osteopontin, and collagen type-1 and the osteogenesis-related proteins alkaline phosphatase, collagen type-1 and runt-related transcription factor-2 compared with BMSCs incubated with 1 mm β-TCP (P < 0.05). Fluorochrome labelling, micro-computed tomography and histological staining analyses indicated that the osteo-regenerator with two holes perforating the femur promoted significantly greater bone regeneration compared with the osteo-regenerator with a periosteum incision (P < 0.05). This study provides an alternative to biofunctionalized bioreactors that exhibits improved osteogenesis.

Early BMP, Wnt and Ca(2+)/PKC pathway activation predicts the bone forming capacity of periosteal cells in combination with calcium phosphates

The development of osteoinductive calcium phosphate- (CaP) based biomaterials has, and continues to be, a major focus in the field of bone tissue engineering. However, limited insight into the spatiotemporal activation of signalling pathways has hampered the optimisation of in vivo bone formation and subsequent clinical translation. To gain further knowledge regarding the early molecular events governing bone tissue formation, we combined human periosteum derived progenitor cells with three types of clinically used CaP-scaffolds, to obtain constructs with a distinct range of bone forming capacity in vivo. Protein phosphorylation together with gene expression for key ligands and target genes were investigated 24 hours after cell seeding in vitro, and 3 and 12 days post ectopic implantation in nude mice. A computational modelling approach was used to deduce critical factors for bone formation 8 weeks post implantation. The combined Ca(2+)-mediated activation of BMP-, Wnt- and PKC signalling pathways 3 days post implantation were able to discriminate the bone forming from the non-bone forming constructs. Subsequently, a mathematical model able to predict in vivo bone formation with 96% accuracy was developed. This study illustrates the importance of defining and understanding CaP-activated signalling pathways that are required and sufficient for in vivo bone formation. Furthermore, we demonstrate the reliability of mathematical modelling as a tool to analyse and deduce key factors within an empirical data set and highlight its relevance to the translation of regenerative medicine strategies.

Controlling Arteriogenesis and Mast Cells Are Central to Bioengineering Solutions for Critical Bone Defect Repair Using Allografts

Although most fractures heal, critical defects in bone fail due to aberrant differentiation of mesenchymal stem cells towards fibrosis rather than osteogenesis. While conventional bioengineering solutions to this problem have focused on enhancing angiogenesis, which is required for bone formation, recent studies have shown that fibrotic non-unions are associated with arteriogenesis in the center of the defect and accumulation of mast cells around large blood vessels. Recently, recombinant parathyroid hormone (rPTH; teriparatide; Forteo) therapy have shown to have anti-fibrotic effects on non-unions and critical bone defects due to inhibition of arteriogenesis and mast cell numbers within the healing bone. As this new direction holds great promise towards a solution for significant clinical hurdles in craniofacial reconstruction and limb salvage procedures, this work reviews the current state of the field, and provides insights as to how teriparatide therapy could be used as an adjuvant for healing critical defects in bone. Finally, as teriparatide therapy is contraindicated in the setting of cancer, which constitutes a large subset of these patients, we describe early findings of adjuvant therapies that may present future promise by directly inhibiting arteriogenesis and mast cell accumulation at the defect site.

Dynamic Bioreactor Culture of High Volume Engineered Bone Tissue

Within the field of tissue engineering and regenerative medicine, the fabrication of tissue grafts of any significant size--much less a whole organ or tissue--remains a major challenge. Currently, tissue-engineered constructs cultured in vitro have been restrained in size primarily due to the diffusion limit of oxygen and nutrients to the center of these grafts. Previously, we developed a novel tubular perfusion system (TPS) bioreactor, which allows the dynamic culture of bead-encapsulated cells and increases the supply of nutrients to the entire cell population. More interestingly, the versatility of TPS bioreactor allows a large range of engineered tissue volumes to be cultured, including large bone grafts. In this study, we utilized alginate-encapsulated human mesenchymal stem cells for the culture of a tissue-engineered bone construct in the size and shape of the superior half of an adult human femur (∼ 200 cm(3)), a 20-fold increase over previously reported volumes of in vitro engineered bone grafts. Dynamic culture in TPS bioreactor not only resulted in high cell viability throughout the femur graft, but also showed early signs of stem cell differentiation through increased expression of osteogenic genes and proteins, consistent with our previous models of smaller bone constructs. This first foray into full-scale bone engineering provides the foundation for future clinical applications of bioengineered bone grafts.

Potentiated Osteoinductivity via Cotransfection with BMP-2 and VEGF Genes in Microencapsulated C2C12 Cells

Microcapsules with entrapped cells hold great promise for repairing bone defects. Unfortunately, the osteoinductivity of microcapsules has been restricted by many factors, among which the deficiency of functional proteins is a significant priority. We potentiated the osteoinductivity of microencapsulated cells via cotransfection with BMP-2 and VEGF genes. Various tissue-derived mesenchymal stem cells and cell lines were compared for BMP-2 and VEGF cotransfection. Ethidium bromide (EB)/Calcein AM staining revealed that all of the cell categories could survive for 4 weeks after microencapsulation. An ELISA assay indicated that all microencapsulated BMP-2 or VEGF transfected cells could secrete gene products constitutively for 1 month. Particularly, the recombinant microencapsulated C2C12 cells released the most desirable level of BMP-2 and VEGF. Further experiments demonstrated that microencapsulated BMP-2 and VEGF cotransfected C2C12 cells generated both BMP-2 and VEGF for 4 weeks. Additionally, the cotransfection of BMP-2 and VEGF in microencapsulated C2C12 cells showed a stronger osteogenic induction against BMSCs than individual BMP-2-transfected microencapsulated C2C12 cells. These results demonstrated that the cotransfection of BMP-2 and VEGF into microencapsulated C2C12 cells is of potent utility for the potentiation of bone regeneration, which would provide a promising clinical strategy for cellular therapy in bone defects.