Bone Tissue Engineering Bioreactors: A Role in the Clinic?

An automated 3D-printed perfusion bioreactor combinable with pulsed electromagnetic field stimulators for bone tissue investigations

In bone tissue engineering research, bioreactors designed for replicating the main features of the complex native environment represent powerful investigation tools. Moreover, when equipped with automation, their use allows reducing user intervention and dependence, increasing reproducibility and the overall quality of the culture process. In this study, an automated uni-/bi-directional perfusion bioreactor combinable with pulsed electromagnetic field (PEMF) stimulation for culturing 3D bone tissue models is proposed. A user-friendly control unit automates the perfusion, minimizing the user dependency. Computational fluid dynamics simulations supported the culture chamber design and allowed the estimation of the shear stress values within the construct. Electromagnetic field simulations demonstrated that, in case of combination with a PEMF stimulator, the construct can be exposed to uniform magnetic fields. Preliminary biological tests on 3D bone tissue models showed that perfusion promotes the release of the early differentiation marker alkaline phosphatase. The histological analysis confirmed that perfusion favors cells to deposit more extracellular matrix (ECM) with respect to the static culture and revealed that bi-directional perfusion better promotes ECM deposition across the construct with respect to uni-directional perfusion. Lastly, the Real-time PCR results of 3D bone tissue models cultured under bi-directional perfusion without and with PEMF stimulation revealed that the only perfusion induced a ~ 40-fold up-regulation of the expression of the osteogenic gene collagen type I with respect to the static control, while a ~ 80-fold up-regulation was measured when perfusion was combined with PEMF stimulation, indicating a positive synergic pro-osteogenic effect of combined physical stimulations.

Computational Modeling of Human Mesenchymal Stromal Cell Proliferation and Extra-Cellular Matrix Production in 3D Porous Scaffolds in a Perfusion Bioreactor: The Effect of Growth Factors

Stem cell expansion on 3D porous scaffolds cultured in bioreactor systems has been shown to be beneficial for maintenance of the original cell functionality in tissue engineering strategies (TE). However, the production of extracellular matrix (ECM) makes harvesting the progenitor cell population from 3D scaffolds a challenge. Medium composition plays a role in stimulating cell proliferation over extracellular matrix (ECM) production. In this regard, a computational model describing tissue growth inside 3D scaffolds can be a great tool in designing optimal experimental conditions. In this study, a computational model describing cell and ECM growth in a perfusion bioreactor is developed, including a description of the effect of a (generic) growth factor on the biological processes taking place inside the 3D scaffold. In the model, the speed of cell and ECM growth depends on the flow-induced shear stress, curvature and the concentrations of oxygen, glucose, lactate, and growth factor. The effect of the simulated growth factor is to differentially enhance cell proliferation over ECM production. After model calibration with historic in-house data, a multi-objective optimization procedure is executed aiming to minimize the total experimental cost whilst maximizing cell growth during culture. The obtained results indicate there are multiple optimum points for the medium refreshment regime and the initial growth factor concentration where a trade-off is made between the final amount of cells and the culture cost. Finally, the model is applied to experiments reported in the literature studying the effects of perfusion-based cell culture and/or growth factor supplementation on cell expansion. The qualitative similarities between the simulation and experimental results, even in the absence of proper model calibration, reinforces the generic character of the proposed modeling framework. The model proposed in this study can contribute to the cost efficient production of cell-based TE products, ultimately contributing to their affordability and accessibility.

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.

Defining hydrogel properties to instruct lineage- and cell-specific mesenchymal differentiation

The maintenance and direction of stem cell lineage after implantation remains challenging for clinical translation. Aggregation and encapsulation into instructive biomaterials after preconditioning can bolster retention of differentiated phenotypes. Since these procedures do not depend on cell type or lineage, we hypothesized we could use a common, tunable platform to engineer formulations that retain and enhance multiple lineages from different cell populations. To test this, we varied alginate stiffness and adhesive ligand content, then encapsulated spheroids of varying cellularity. We used Design-of-Experiments to determine the effect of these parameters and their interactions on phenotype retention. The combination of parameters leading to maximal differentiation varied with lineage and cell type, inducing a 2-4-fold increase over non-optimized levels. Phenotype was also retained for 4 weeks in a murine subcutaneous model. This widely applicable approach can facilitate translation of cell-based therapies by instructing phenotype in situ without prolonged induction or costly growth factors.

Orthopaedic regenerative tissue engineering en route to the holy grail: disequilibrium between the demand and the supply in the operating room

Orthopaedic disorders are very frequent, globally found and often partially unresolved despite the substantial advances in science and medicine. Their surgical intervention is multifarious and the most favourable treatment is chosen by the orthopaedic surgeon on a case-by-case basis depending on a number of factors related with the patient and the lesion. Numerous regenerative tissue engineering strategies have been developed and studied extensively in laboratory through in vitro experiments and preclinical in vivo trials with various established animal models, while a small proportion of them reached the operating room. However, based on the available literature, the current strategies have not yet achieved to fully solve the clinical problems. Thus, the gold standards, if existing, remain unchanged in the clinics, notwithstanding the known limitations and drawbacks. Herein, the involvement of regenerative tissue engineering in the clinical orthopaedics is reviewed. The current challenges are indicated and discussed in order to describe the current disequilibrium between the needs and solutions made available in the operating room. Regenerative tissue engineering is a very dynamic field that has a high growth rate and a great openness and ability to incorporate new technologies with passion to edge towards the Holy Grail that is functional tissue regeneration. Thus, the future of clinical solutions making use of regenerative tissue engineering principles for the management of orthopaedic disorders is firmly supported by the clinical need.

Establishment and characterization of a primary murine adipose tissue-chip

Better experimental models are needed to enhance our understanding of metabolic regulation which is seen in obesity and metabolic disorders, such as type 2 diabetes. In vitro models based on microfluidics enable physiological representations of tissues with several advantages over conventional culture systems, such as perfused flow to better mimic the physiological environment. Although cell lines such as 3T3-L1 have been incorporated in microfluidic devices, murine primary preadipocytes have not been differentiated and maintained for long-term monitoring in these culture systems. We describe the differentiation of these cells into white adipose depots on a perfused microfluidic chip. We compare the effects of shear flow on these cells, and show with a direct comparison of high/low shear conditions that direct shear is detrimental to the viability of preadipocytes. We further develop a dual-chamber microfluidic chip that enables perfusion while at the same time protects the cells from direct fluidic shear. We show that the dual-layer microfluidic device enables long-term culture of cells and allows stimulation of cells through perfusion-we can culture, differentiate, and maintain the differentiated adipose tissue for over multiple weeks in the device. Both triglycerides and lipolytic glycerol production increased significantly by several folds during differentiation. After successful differentiation, the adipocytes had upregulated expression of leptin and adiponectin, which are important makers of the final stage of adipogenic differentiation. In conclusion, the dual-layer microfluidic device incorporated with primary adipocytes improves the understanding of adipose differentiation under dynamic conditions and is positioned to serve as a disease model for studying obesity and other metabolic disorders.

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.

Engineering in-vitro stem cell-based vascularized bone models for drug screening and predictive toxicology

The production of veritable in-vitro models of bone tissue is essential to understand the biology of bone and its surrounding environment, to analyze the pathogenesis of bone diseases (e.g., osteoporosis, osteoarthritis, osteomyelitis, etc.), to develop effective therapeutic drug screening, and to test potential therapeutic strategies. Dysregulated interactions between vasculature and bone cells are often related to the aforementioned pathologies, underscoring the need for a bone model that contains engineered vasculature. Due to ethical restraints and limited prediction power of animal models, human stem cell-based tissue engineering has gained increasing relevance as a candidate approach to overcome the limitations of animals and to serve as preclinical models for drug testing. Since bone is a highly vascularized tissue, the concomitant development of vasculature and mineralized matrix requires a synergistic interaction between osteogenic and endothelial precursors. A number of experimental approaches have been used to achieve this goal, such as the combination of angiogenic factors and three-dimensional scaffolds, prevascularization strategies, and coculture systems. In this review, we present an overview of the current models and approaches to generate in-vitro stem cell-based vascularized bone, with emphasis on the main challenges of vasculature engineering. These challenges are related to the choice of biomaterials, scaffold fabrication techniques, and cells, as well as the type of culturing conditions required, and specifically the application of dynamic culture systems using bioreactors.

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.

A synergistic approach to the design, fabrication and evaluation of 3D printed micro and nano featured scaffolds for vascularized bone tissue repair

3D bioprinting has begun to show great promise in advancing the development of functional tissue/organ replacements. However, to realize the true potential of 3D bioprinted tissues for clinical use requires the fabrication of an interconnected and effective vascular network. Solving this challenge is critical, as human tissue relies on an adequate network of blood vessels to transport oxygen, nutrients, other chemicals, biological factors and waste, in and out of the tissue. Here, we have successfully designed and printed a series of novel 3D bone scaffolds with both bone formation supporting structures and highly interconnected 3D microvascular mimicking channels, for efficient and enhanced osteogenic bone regeneration as well as vascular cell growth. Using a chemical functionalization process, we have conjugated our samples with nano hydroxyapatite (nHA), for the creation of novel micro and nano featured devices for vascularized bone growth. We evaluated our scaffolds with mechanical testing, hydrodynamic measurements and in vitro human mesenchymal stem cell (hMSC) adhesion (4 h), proliferation (1, 3 and 5 d) and osteogenic differentiation (1, 2 and 3 weeks). These tests confirmed bone-like physical properties and vascular-like flow profiles, as well as demonstrated enhanced hMSC adhesion, proliferation and osteogenic differentiation. Additional in vitro experiments with human umbilical vein endothelial cells also demonstrated improved vascular cell growth, migration and organization on micro-nano featured scaffolds.

Bioreactor-Based Online Recovery of Human Progenitor Cells with Uncompromised Regenerative Potential: A Bone Tissue Engineering Perspective

The use of a 3D perfusion culture environment for stem cell expansion has been shown to be beneficial for maintenance of the original cell functionality but due to several system inherent characteristics such as the presence of extracellular matrix, the continued development and implementation of 3D perfusion bioreactor technologies is hampered. Therefore, this study developed a methodology for harvesting a progenitor cell population from a 3D open porous culture surface after expansion in a perfusion bioreactor and performed a functional characterization of the expanded cells. An initial screening showed collagenase to be the most interesting reagent to release the cells from the 3D culture surface as it resulted in high yields without compromising cell viability. Subsequently a Design of Experiment approach was used to obtain optimized 3D harvest conditions by assessing the interplay of flow rate, collagenase concentration and incubation time on the harvest efficiency, viability and single cell fraction. Cells that were recovered with the optimized harvest protocol, by perfusing a 880 U/ml collagenase solution for 7 hours at a flow rate of 4 ml/min, were thereafter functionally analyzed for their characteristics as expanded progenitor cell population. As both the in vitro tri-lineage differentiation capacity and the in vivo bone forming potential were maintained after 3D perfusion bioreactor expansion we concluded that the developed seeding, culture and harvest processes did not significantly compromise the viability and potency of the cells and can contribute to the future development of integrated bioprocesses for stem cell expansion.

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.

Engineering clinically relevant volumes of vascularized bone

Vascularization remains one of the most important challenges that must be overcome for tissue engineering to be consistently implemented for reconstruction of large volume bone defects. An extensive vascular network is needed for transport of nutrients, waste and progenitor cells required for remodelling and repair. A variety of tissue engineering strategies have been investigated in an attempt to vascularize tissues, including those applying cells, soluble factor delivery strategies, novel design and optimization of bio-active materials, vascular assembly pre-implantation and surgical techniques. However, many of these strategies face substantial barriers that must be overcome prior to their ultimate translation into clinical application. In this review recent progress in engineering vascularized bone will be presented with an emphasis on clinical feasibility.

Long term perfusion system supporting adipogenesis

Adipose tissue engineered models are needed to enhance our understanding of disease mechanisms and for soft tissue regenerative strategies. Perfusion systems generate more physiologically relevant and sustainable adipose tissue models, however adipocytes have unique properties that make culturing them in a perfusion environment challenging. In this paper we describe the methods involved in the development of two perfusion culture systems (2D and 3D) to test their applicability for long term in vitro adipogenic cultures. It was hypothesized that a silk protein biomaterial scaffold would provide a 3D framework, in combination with perfusion flow, to generate a more physiologically relevant sustainable adipose tissue engineered model than 2D cell culture. Consistent with other studies evaluating 2D and 3D culture systems for adipogenesis we found that both systems successfully model adipogenesis, however 3D culture systems were more robust, providing the mechanical structure required to contain the large, fragile adipocytes that were lost in 2D perfused culture systems. 3D perfusion also stimulated greater lipogenesis and lipolysis and resulted in decreased secretion of LDH compared to 2D perfusion. Regardless of culture configuration (2D or 3D) greater glycerol was secreted with the increased nutritional supply provided by perfusion of fresh media. These results are promising for adipose tissue engineering applications including long term cultures for studying disease mechanisms and regenerative approaches, where both acute (days to weeks) and chronic (weeks to months) cultivation are critical for useful insight.

Cell based advanced therapeutic medicinal products for bone repair: Keep it simple?

The development of cell based advanced therapeutic medicinal products (ATMPs) for bone repair has been expected to revolutionize the health care system for the clinical treatment of bone defects. Despite this great promise, the clinical outcomes of the few cell based ATMPs that have been translated into clinical treatments have been far from impressive. In part, the clinical outcomes have been hampered because of the simplicity of the first wave of products. In response the field has set-out and amassed a plethora of complexities to alleviate the simplicity induced limitations. Many of these potential second wave products have remained "stuck" in the development pipeline. This is due to a number of reasons including the lack of a regulatory framework that has been evolving in the last years and the shortage of enabling technologies for industrial manufacturing to deal with these novel complexities. In this review, we reflect on the current ATMPs and give special attention to novel approaches that are able to provide complexity to ATMPs in a straightforward manner. Moreover, we discuss the potential tools able to produce or predict 'goldilocks' ATMPs, which are neither too simple nor too complex.

Quantitative Validation of the Presto Blue Metabolic Assay for Online Monitoring of Cell Proliferation in a 3D Perfusion Bioreactor System

As the fields of tissue engineering and regenerative medicine mature toward clinical applications, the need for online monitoring both for quantitative and qualitative use becomes essential. Resazurin-based metabolic assays are frequently applied for determining cytotoxicity and have shown great potential for monitoring 3D bioreactor-facilitated cell culture. However, no quantitative correlation between the metabolic conversion rate of resazurin and cell number has been defined yet. In this work, we determined conversion rates of Presto Blue, a resazurin-based metabolic assay, for human periosteal cells during 2D and 3D static and 3D perfusion cultures. Our results showed that for the evaluated culture systems there is a quantitative correlation between the Presto Blue conversion rate and the cell number during the expansion phase with no influence of the perfusion-related parameters, that is, flow rate and shear stress. The correlation between the cell number and Presto Blue conversion subsequently enabled the definition of operating windows for optimal signal readouts. In conclusion, our data showed that the conversion of the resazurin-based Presto Blue metabolic assay can be used as a quantitative readout for online monitoring of cell proliferation in a 3D perfusion bioreactor system, although a system-specific validation is required.

Concise review: bridging the gap: bone regeneration using skeletal stem cell-based strategies - where are we now?

Skeletal stem cells confer to bone its innate capacity for regeneration and repair. Bone regeneration strategies seek to harness and enhance this regenerative capacity for the replacement of tissue damaged or lost through congenital defects, trauma, functional/esthetic problems, and a broad range of diseases associated with an increasingly aged population. This review describes the state of the field and current steps to translate and apply skeletal stem cell biology in the clinic and the problems therein. Challenges are described along with key strategies including the isolation and ex vivo expansion of multipotential populations, the targeting/delivery of regenerative populations to sites of repair, and their differentiation toward bone lineages. Finally, preclinical models of bone repair are discussed along with their implications for clinical translation and the opportunities to harness that knowledge for musculoskeletal regeneration.

Cellular and morphological aspects of fibrodysplasia ossificans progressiva. Lessons of formation, repair, and bone bioengineering

Fibrodysplasia ossificans progressiva (FOP) is a rare congenital disease that causes bone formation within the muscles, tendons, ligaments and connective tissues. There is no cure for this disorder and only treatment of the symptoms is available. The purpose of this study was to review the literature and describe the clinical, cellular and molecular aspects of FOP. The material used for the study was obtained by reviewing scientific articles published in various literature-indexed databases. In view of its rarity and of the lack of insightful information and the unpredictability of its course, FOP is a challenging disorder for professionals who are confronted by it. However, this rare disease raises a great deal of interest because understanding the mechanism of mature bone formation can encourage research lines related to bone regeneration and the prevention of heterotopic ossification.

Modulating the biochemical and biophysical culture environment to enhance osteogenic differentiation and maturation of human pluripotent stem cell-derived mesenchymal progenitors

Advances in the fields of stem cell biology, biomaterials, and tissue engineering over the last decades have brought the possibility of constructing tissue substitutes with a broad range of applications in regenerative medicine, disease modeling, and drug discovery. Different types of human stem cells have been used, each presenting a unique set of advantages and limitations with regard to the desired research goals. Whereas adult stem cells are at the frontier of research for tissue and organ regeneration, pluripotent stem cells represent a more challenging cell source for clinical translation. However, with their unlimited growth and wide differentiation potential, pluripotent stem cells represent an unprecedented resource for the construction of advanced human tissue models for biological studies and drug discovery. At the heart of these applications lies the challenge to reproducibly expand, differentiate, and organize stem cells into mature, stable tissue structures. In this review, we focus on the derivation of mesenchymal tissue progenitors from human pluripotent stem cells and the control of their osteogenic differentiation and maturation by modulation of the biophysical culture environment. Similarly to enhancing bone development, the described principles can be applied to the construction of other mesenchymal tissues for basic and applicative studies.

Human periosteal-derived cell expansion in a perfusion bioreactor system: proliferation, differentiation and extracellular matrix formation

Perfusion bioreactor systems have shown to be a valuable tool for the in vitro development of three-dimensional (3D) cell-carrier constructs. Their use for cell expansion, however, has been much less explored. Since maintenance of the initial cell phenotype is essential in this process, it is imperative to obtain insight into the bioreactor-related variables determining cell fate. Therefore, this study investigated the influence of fluid flow-induced shear stress on the proliferation, differentiation and matrix deposition of human periosteal-derived cells in the absence of additional differentiation-inducing stimuli; 120 000 cells were seeded on additive manufactured 3D Ti6Al4V scaffolds and cultured for up to 28 days at different flow rates in the range 0.04-6 ml/min. DNA measurements showed, on average, a three-fold increase in cell content for all perfused conditions in comparison to static controls, whereas the magnitude of the flow rate did not have an influence. Contrast-enhanced nanofocus X-ray computed tomography showed substantial formation of an engineered neotissue in all perfused conditions, resulting in a filling (up to 70%) of the total internal void volume, and no flow rate-dependent differences were observed. The expression of key osteogenic markers, such as RunX2, OCN, OPN and Col1, did not show any significant changes in comparison to static controls after 28 days of culture, with the exception of OSX at high flow rates. We therefore concluded that, in the absence of additional osteogenic stimuli, the investigated perfusion conditions increased cell proliferation but did not significantly enhance osteogenic differentiation, thus allowing for this process to be used for cell expansion. Copyright © 2014 John Wiley & Sons, Ltd.

Model-based cell number quantification using online single-oxygen sensor data for tissue engineering perfusion bioreactors

Online and non-invasive quantification of critical tissue engineering (TE) construct quality attributes in TE bioreactors is indispensable for the cost-effective up-scaling and automation of cellular construct manufacturing. However, appropriate monitoring techniques for cellular constructs in bioreactors are still lacking. This study presents a generic and robust approach to determine cell number and metabolic activity of cell-based TE constructs in perfusion bioreactors based on single oxygen sensor data in dynamic perfusion conditions. A data-based mechanistic modeling technique was used that is able to correlate the number of cells within the scaffold (R(2)  = 0.80) and the metabolic activity of the cells (R(2)  = 0.82) to the dynamics of the oxygen response to step changes in the perfusion rate. This generic non-destructive measurement technique is effective for a large range of cells, from as low as 1.0 × 10(5) cells to potentially multiple millions of cells, and can open-up new possibilities for effective bioprocess monitoring.

Spatial optimization in perfusion bioreactors improves bone tissue-engineered construct quality attributes

Perfusion bioreactors have shown great promise for tissue engineering applications providing a homogeneous and consistent distribution of nutrients and flow-induced shear stresses throughout tissue-engineered constructs. However, non-uniform fluid-flow profiles found in the perfusion chamber entrance region have been shown to affect tissue-engineered construct quality characteristics during culture. In this study a whole perfusion and construct, three dimensional (3D) computational fluid dynamics approach was used in order to optimize a critical design parameter such as the location of the regular pore scaffolds within the perfusion bioreactor chamber. Computational studies were coupled to bioreactor experiments for a case-study flow rate. Two cases were compared in the first instance seeded scaffolds were positioned immediately after the perfusion chamber inlet while a second group was positioned at the computationally determined optimum distance were a steady state flow profile had been reached. Experimental data showed that scaffold location affected significantly cell content and neo-tissue distribution, as determined and quantified by contrast enhanced nanoCT, within the constructs both at 14 and 21 days of culture. However, gene expression level of osteopontin and osteocalcin was not affected by the scaffold location. This study demonstrates that the bioreactor chamber environment, incorporating a scaffold and its location within it, affects the flow patterns within the pores throughout the scaffold requiring therefore dedicated optimization that can lead to bone tissue engineered constructs with improved quality attributes.

Bone tissue engineering: state of the union

The quest to surpass the clinical efficacy of the allogeneic bone graft has had limited success, an outcome that is symbolic of tissue engineering as a whole. In this 'State of the Union'-type review, we highlight recent advances in the design of bone regenerative therapeutics using the primary elements of stem cells, growth factors and scaffolds, and identify major obstacles in their paths to the clinic. We underscore the need for rigorous performance criteria in the design of holistic tissue regenerative therapeutics, and an increased emphasis on the product production, storage and handling issues that will ultimately influence clinical success.

Identification of a growth factor mimicking the synergistic effect of fetal bovine serum on BMP-9 cell response

The bone morphogenetic proteins (BMPs) are potent osteogenic molecules that are used for bone repair in delivery systems and in regenerative medicine. We studied the responses of murine MC3T3-E1 preosteoblasts to doses of recombinant human (rh)BMP-9 with and without fetal bovine serum (FBS). rhBMP-2 was used as a control since it is currently approved by the Food and Drug Administration for bone application. We analyzed the major cell signaling pathways and the expression of osteogenic markers. Without FBS, BMP-9 had a similar effect on MC3T3-E1 preosteoblast differentiation in comparison to BMP-2. In contrast, FBS reduced the EC50 of BMP-9 fourfold to sixfold, as determined by osterix gene expression and alkaline phosphatase (ALP) activity, while it had no influence on EC50 of BMP-2. As suggested by MAPK inhibitor assays, FBS could induce an intracellular signaling environment that favors cell response to BMP-9 by inhibiting ERK1/2 activation and increasing p38 phosphorylation. Finally, IGF-2 (100 ng/mL) could mimic the effect of FBS on BMP-9 cell response in terms of MAPK signaling and ALP activity. Thus, the action of BMP-9 on preosteoblast differentiation can be greatly improved by IGF-2. This finding may well be critical for developing optimal growth factor delivery systems and bone tissue engineering strategies.

Advances in regenerative orthopedics

Orthopedic injuries are common and a source of much misery and economic stress. Several relevant tissues, such as cartilage, meniscus, and intra-articular ligaments, do not heal. And even bone, which normally regenerates spontaneously, can fail to mend. The regeneration of orthopedic tissues requires 4 key components: cells, morphogenetic signals, scaffolds, and an appropriate mechanical environment. Although differentiated cells from the tissue in question can be used, most cellular research focuses on the use of mesenchymal stem cells. These can be retrieved from many different tissues, and one unresolved question is the degree to which the origin of the cells matters. Embryonic and induced pluripotent stem cells are also under investigation. Morphogenetic signals are most frequently supplied by individual recombinant growth factors or native mixtures provided by, for example, platelet-rich plasma; mesenchymal stem cells are also a rich source of trophic factors. Obstacles to the sustained delivery of individual growth factors can be addressed by gene transfer or smart scaffolds, but we still lack detailed, necessary information on which delivery profiles are needed. Scaffolds may be based on natural products, synthetic materials, or devitalized extracellular matrix. Strategies to combine these components to regenerate tissue can follow traditional tissue engineering practices, but these are costly, cumbersome, and not well suited to treating large numbers of individuals. More expeditious approaches make full use of intrinsic biological processes in vivo to avoid the need for ex vivo expansion of autologous cells and multiple procedures. Clinical translation remains a bottleneck.

Make no bones about it: cells could soon be reprogrammed to grow replacement bones?

Recent developments in nuclear reprogramming allow the generation of patient-matched stem cells with broad potential for applications in cell therapies, disease modeling and drug discovery. An increasing body of work is reporting the derivation of lineage-specific progenitors from human-induced pluripotent stem cells (hiPSCs), which could in the near future be used to engineer personalized tissue substitutes, including those for reconstructive therapies of bone. Although the potential clinical impact of such technology is not arguable, significant challenges remain to be addressed before hiPSC-derived progenitors can be employed to engineer bone substitutes of clinical relevance. The most important challenge is indeed the construction of personalized multicellular bone substitutes for the treatment of complex skeletal defects that integrate fast, are immune tolerated and display biofunctionality and long-term safety. As recent studies suggest, the merging of iPSC technology with advanced biomaterials and bioreactor technologies offers a way to generate bone substitutes in a controllable, automated manner with potential to meet the needs for scale-up and requirements for translation into clinical practice. It is only via the use of state-of-the-art cell culture technologies, process automation under GMP-compliant conditions, application of appropriate engineering strategies and compliance with regulatory policies that personalized lab-made bone grafts can start being used to treat human patients.

Engineering bone grafts with enhanced bone marrow and native scaffolds

The translation of tissue engineering approaches to the clinic has been hampered by the inability to find suitable multipotent cell sources requiring minimal in vitro expansion. Enhanced bone marrow (eBM), which is obtained by reaming long bone medullary canals and isolating the solid marrow putty, has large quantities of stem cells and demonstrates significant potential to regenerate bone tissues. eBM, however, cannot impart immediate load-bearing mechanical integrity or maintain the gross anatomical structure to guide bone healing. Yet, its putty-like consistency creates a challenge for obtaining the uniform seeding necessary to effectively combine it with porous scaffolds. In this study, we examined the potential for combining eBM with mechanically strong, osteoinductive trabecular bone scaffolds for bone regeneration by creating channels into scaffolds for seeding the eBM. eBM was extracted from the femurs of adult Yorkshire pigs using a Synthes reamer-irrigator-aspirator device, analyzed histologically, and digested to extract cells and characterize their differentiation potential. To evaluate bone tissue formation, eBM was seeded into the channels in collagen-coated or noncoated scaffolds, cultured in osteogenic conditions for 4 weeks, harvested and assessed for tissue distribution and bone formation. Our data demonstrates that eBM is a heterogenous tissue containing multipotent cell populations. Furthermore, coating scaffolds with a collagen hydrogel significantly enhanced cellular migration, promoted uniform tissue development and increased bone mineral deposition. These findings suggest the potential for generating customized autologous bone grafts for treating critical-sized bone defects by combining a readily available eBM cell source with decellularized trabecular bone scaffolds.

In vivo bone regeneration using tubular perfusion system bioreactor cultured nanofibrous scaffolds

The use of bioreactors for the in vitro culture of constructs for bone tissue engineering has become prevalent as these systems may improve the growth and differentiation of a cultured cell population. Here we utilize a tubular perfusion system (TPS) bioreactor for the in vitro culture of human mesenchymal stem cells (hMSCs) and implant the cultured constructs into rat femoral condyle defects. Using nanofibrous electrospun poly(lactic-co-glycolic acid)/poly(ε-caprolactone) scaffolds, hMSCs were cultured for 10 days in vitro in the TPS bioreactor with cellular and acellular scaffolds cultured statically for 10 days as a control. After 3 and 6 weeks of in vivo culture, explants were removed and subjected to histomorphometric analysis. Results indicated more rapid bone regeneration in defects implanted with bioreactor cultured scaffolds with a new bone area of 1.23 ± 0.35 mm(2) at 21 days compared to 0.99 ± 0.43 mm(2) and 0.50 ± 0.29 mm(2) in defects implanted with statically cultured scaffolds and acellular scaffolds, respectively. At the 21 day timepoint, statistical differences (p<0.05) were only observed between defects implanted with cell containing scaffolds and the acellular control. After 42 days, however, defects implanted with TPS cultured scaffolds had the greatest new bone area with 1.72 ± 0.40 mm(2). Defects implanted with statically cultured and acellular scaffolds had a new bone area of 1.26 ± 0.43 mm(2) and 1.19 ± 0.33 mm(2), respectively. The increase in bone growth observed in defects implanted with TPS cultured scaffolds was statistically significant (p<0.05) when compared to both the static and acellular groups at this timepoint. This study demonstrates the efficacy of the TPS bioreactor to improve bone tissue regeneration and highlights the benefits of utilizing perfusion bioreactor systems to culture MSCs for bone tissue engineering.

The key regulatory roles of the PI3K/Akt signaling pathway in the functionalities of mesenchymal stem cells and applications in tissue regeneration

Mesenchymal stem cells (MSCs) are multipotent cells that can differentiate into various cell types and have been widely used in tissue engineering application. In tissue engineering, a scaffold, MSCs and growth factors are used as essential components and their interactions have been regarded to be important for regeneration of tissues. A critical problem for MSCs in tissue engineering is their low survival ability and functionality. Most MSCs are going to be apoptotic after transplantation. Therefore, increasing MSC survival ability and functionalities is the key for potential applications of MSCs. Several approaches have been studied to increase MSC tissue forming capacity including application of growth factors, overexpression of stem cell regulatory genes, and improvement of biomaterials for scaffolds. The effects of these approaches on MSCs have been associated with activation of the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway. The pathway plays central regulatory roles in MSC survival, proliferation, migration, angiogenesis, cytokine production, and differentiation. In this review, we summarize and discuss the literatures related to the roles of the PI3K/Akt pathway in the functionalities of MSCs and the involvement of the pathway in biomaterials-increased MSC functionalities. Biomaterials have been modified in their properties and surface structure and loaded with growth factors to increase MSC functionalities. Several studies demonstrated that the biomaterials-increased MSC functionalities are mediated by the activation of the PI3K/Akt pathway.

Bone tissue engineering: recent advances and challenges

The worldwide incidence of bone disorders and conditions has trended steeply upward and is expected to double by 2020, especially in populations where aging is coupled with increased obesity and poor physical activity. Engineered bone tissue has been viewed as a potential alternative to the conventional use of bone grafts, due to their limitless supply and no disease transmission. However, bone tissue engineering practices have not proceeded to clinical practice due to several limitations or challenges. Bone tissue engineering aims to induce new functional bone regeneration via the synergistic combination of biomaterials, cells, and factor therapy. In this review, we discuss the fundamentals of bone tissue engineering, highlighting the current state of this field. Further, we review the recent advances of biomaterial and cell-based research, as well as approaches used to enhance bone regeneration. Specifically, we discuss widely investigated biomaterial scaffolds, micro- and nano-structural properties of these scaffolds, and the incorporation of biomimetic properties and/or growth factors. In addition, we examine various cellular approaches, including the use of mesenchymal stem cells (MSCs), embryonic stem cells (ESCs), adult stem cells, induced pluripotent stem cells (iPSCs), and platelet-rich plasma (PRP), and their clinical application strengths and limitations. We conclude by overviewing the challenges that face the bone tissue engineering field, such as the lack of sufficient vascularization at the defect site, and the research aimed at functional bone tissue engineering. These challenges will drive future research in the field.

Bioreactor engineering of stem cell environments

Stem cells hold promise to revolutionize modern medicine by the development of new therapies, disease models and drug screening systems. Standard cell culture systems have limited biological relevance because they do not recapitulate the complex 3-dimensional interactions and biophysical cues that characterize the in vivo environment. In this review, we discuss the current advances in engineering stem cell environments using novel biomaterials and bioreactor technologies. We also reflect on the challenges the field is currently facing with regard to the translation of stem cell based therapies into the clinic.

Numerical simulation of fluid field and in vitro three-dimensional fabrication of tissue-engineered bones in a rotating bioreactor and in vivo implantation for repairing segmental bone defects

In this paper, two-dimensional flow field simulation was conducted to determine shear stresses and velocity profiles for bone tissue engineering in a rotating wall vessel bioreactor (RWVB). In addition, in vitro three-dimensional fabrication of tissue-engineered bones was carried out in optimized bioreactor conditions, and in vivo implantation using fabricated bones was performed for segmental bone defects of Zelanian rabbits. The distribution of dynamic pressure, total pressure, shear stress, and velocity within the culture chamber was calculated for different scaffold locations. According to the simulation results, the dynamic pressure, velocity, and shear stress around the surface of cell-scaffold construction periodically changed at different locations of the RWVB, which could result in periodical stress stimulation for fabricated tissue constructs. However, overall shear stresses were relatively low, and the fluid velocities were uniform in the bioreactor. Our in vitro experiments showed that the number of cells cultured in the RWVB was five times higher than those cultured in a T-flask. The tissue-engineered bones grew very well in the RWVB. This study demonstrates that stress stimulation in an RWVB can be beneficial for cell/bio-derived bone constructs fabricated in an RWVB, with an application for repairing segmental bone defects.

Noninvasive real-time monitoring by alamarBlue(®) during in vitro culture of three-dimensional tissue-engineered bone constructs

Bone tissue engineering (TE) aims to develop reproducible and predictive three-dimensional (3D) TE constructs, defined as cell-seeded scaffolds produced by a controlled in vitro process, to heal or replace damaged and nonfunctional bone. To control and assure the quality of the bone TE constructs, a prerequisite for regulatory authorization, there is a need to develop noninvasive analysis techniques to evaluate TE constructs and to monitor their behavior in real time during in vitro culturing. Most analysis techniques, however, are limited to destructive end-point analyses. This study investigates the use of the nontoxic alamarBlue(®) (AB) reagent, which is an indicator for metabolic cell activity, for monitoring the cellularity of 3D TE constructs in vitro as part of a bioreactor culturing processes. Within the field of TE, bioreactors have a huge potential in the translation of TE concepts to the clinic. Hence, the use of the AB reagent was evaluated not only in static cultures, but also in dynamic cultures in a perfusion bioreactor setup. Hereto, the AB assay was successfully integrated in the bioreactor-driven TE construct culture process in a noninvasive way. The obtained results indicate a linear correlation between the overall metabolic activity and the total DNA content of a scaffold upon seeding as well as during the initial stages of cell proliferation. This makes the AB reagent a powerful tool to follow-up bone TE constructs in real-time during static as well as dynamic 3D cultures. Hence, the AB reagent can be successfully used to monitor and predict cell confluence in a growing 3D TE construct.

The design and features of apatite-coated chitosan microspheres as injectable scaffold for bone tissue engineering

In this paper we developed two types of chitosan-based microspheres with and without biomimetic apatite coatings and compared their potential as injectable scaffolds for bone regeneration. The microspheres were obtained by emulsion cross-linking (E0) and coacervate precipitation (C0), respectively. They were then biomimetically coated with apatite to become E1 and C1 microspheres. The physicochemical properties and biocompatibility of the microspheres were characterized. Both E0 and C0 microspheres presented favorable ranges of diameter, density and Rockwell hardness. However, there were differences in the degree of cross-linking, shape, morphology, degradation rate, swelling rate, pH value after PBS immersion and the biocompatibility between E0 and C0. The apatite coating was successfully prepared for both C0 and E0, which enhanced the attachment, proliferation and differentiation of MC3T3-E1 cells. In conclusion, our results suggest the feasibility of using chitosan microspheres as a potential injectable scaffold. Both the preparation method and the biomimetic apatite coating contribute to their biological properties.

Bioreactor design for tendon/ligament engineering

Tendon and ligament injury is a worldwide health problem, but the treatment options remain limited. Tendon and ligament engineering might provide an alternative tissue source for the surgical replacement of injured tendon. A bioreactor provides a controllable environment enabling the systematic study of specific biological, biochemical, and biomechanical requirements to design and manufacture engineered tendon/ligament tissue. Furthermore, the tendon/ligament bioreactor system can provide a suitable culture environment, which mimics the dynamics of the in vivo environment for tendon/ligament maturation. For clinical settings, bioreactors also have the advantages of less-contamination risk, high reproducibility of cell propagation by minimizing manual operation, and a consistent end product. In this review, we identify the key components, design preferences, and criteria that are required for the development of an ideal bioreactor for engineering tendons and ligaments.

Cystamine-terminated poly(beta-amino ester)s for siRNA delivery to human mesenchymal stem cells and enhancement of osteogenic differentiation

Enhancing human mesenchymal stem cell (hMSC) differentiation via RNA interference (RNAi) could provide an effective way of controlling cell fate for tissue engineering, but a safe and effective delivery vehicle must first be developed. Here, we evaluated an array of synthetic end-modified poly(beta-amino ester) (PBAE)-based nanoparticles to optimize siRNA delivery into hMSCs. In general, cystamine-terminated polymers caused the most knockdown, with the best polymer achieving 91% knockdown 20 days post-transfection. Binding studies revealed that the cystamine-terminated polymer bound siRNA tightly at lower weight ratios of polymer to siRNA but then efficiently released siRNA upon exposure to a reducing environment, suggesting that this class of PBAEs can form tight initial interactions with its cargo and then cause efficient, environmentally-triggered release in the cytoplasm. Finally, we tested a functional application of this system by transfecting hMSCs with siRNA against an inhibitor of osteogenesis, B-cell lymphoma (Bcl)-like protein 2 (BCL2L2). This resulted in enhanced osteogenesis over 4 weeks as evidenced by Alizarin Red S staining and calcium quantification. The bioreducible PBAE/siRNA nanoparticles developed here can provide a means of safe and effective control of hMSC differentiation for a wide variety of applications.

Bioreactor strategy in bone tissue engineering: pre-culture and osteogenic differentiation under two flow configurations

Since robust osteogenic differentiation and mineralization are integral to the engineering of bone constructs, understanding the impact of the cellular microenvironments on human mesenchymal stem cell (hMSCs) osteogenic differentiation is crucial to optimize bioreactor strategy. Two perfusion flow conditions were utilized in order to understand the impact of the flow configuration on hMSC construct development during both pre-culture (PC) in growth media and its subsequent osteogenic induction (OI). The media in the in-house perfusion bioreactor was controlled to perfuse either around (termed parallel flow [PF]) the construct surfaces or penetrate through the construct (termed transverse flow [TF]) for 7 days of the PC followed by 7 days of the OI. The flow configuration during the PC not only changed growth kinetics but also influenced cell distribution and potency of osteogenic differentiation and mineralization during the subsequent OI. While shear stress resulted from the TF stimulated cell proliferation during PC, the convective removal of de novo extracellular matrix (ECM) proteins and growth factors (GFs) reduced cell proliferation on OI. In contrast, the effective retention of de novo ECM proteins and GFs in the PC constructs under the PF maintained cell proliferation under the OI but resulted in localized cell aggregations, which influenced their osteogenic differentiation. The results revealed the contrasting roles of the convective flow as a mechanical stimulus, the redistribution of the cells and macromolecules in 3D constructs, and their divergent impacts on cellular events, leading to bone construct formation. The results suggest that the modulation of the flow configuration in the perfusion bioreactor is an effective strategy that regulates the construct properties and maximizes the functional outcome.

Perspective on the evolution of cell-based bone tissue engineering strategies

Despite the compelling clinical needs in enhancing bone regeneration and the potential offered by the field of tissue engineering, the adoption of cell-based bone graft substitutes in clinical practice is limited to date. In fact, no study has yet convincingly demonstrated reproducible clinical performance of tissue-engineered implants and at least equivalent cost-effectiveness compared to the current treatment standards. Here, we propose and discuss how tissue engineering strategies could be evolved towards more efficient solutions, depicting three different experimental paradigms: (i) bioreactor-based production; (ii) intraoperative manufacturing, and (iii) developmental engineering. The described approaches reflect the need to streamline graft manufacturing processes while maintaining the potency of osteoprogenitors and recapitulating the sequence of biological steps occurring during bone development, including vascularization. The need to combine the assessment of efficacy of the different strategies with the understanding of their mechanisms of action in the target regenerative processes is highlighted. This will be crucial to identify the necessary and sufficient set of signals that need to be delivered at the injury or defect site and should thus form the basis to define release criteria for reproducibly effective engineered bone graft substitutes.

Bioreactors to influence stem cell fate: augmentation of mesenchymal stem cell signaling pathways via dynamic culture systems

BACKGROUND

Mesenchymal stem cells (MSCs) are a promising cell source for bone and cartilage tissue engineering as they can be easily isolated from the body and differentiated into osteoblasts and chondrocytes. A cell based tissue engineering strategy using MSCs often involves the culture of these cells on three-dimensional scaffolds; however the size of these scaffolds and the cell population they can support can be restricted in traditional static culture. Thus dynamic culture in bioreactor systems provides a promising means to culture and differentiate MSCs in vitro.

SCOPE OF REVIEW

This review seeks to characterize key MSC differentiation signaling pathways and provides evidence as to how dynamic culture is augmenting these pathways. Following an overview of dynamic culture systems, discussion will be provided on how these systems can effectively modify and maintain important culture parameters including oxygen content and shear stress. Literature is reviewed for both a highlight of key signaling pathways and evidence for regulation of these signaling pathways via dynamic culture systems.

MAJOR CONCLUSIONS

The ability to understand how these culture systems are affecting MSC signaling pathways could lead to a shear or oxygen regime to direct stem cell differentiation. In this way the efficacy of in vitro culture and differentiation of MSCs on three-dimensional scaffolds could be greatly increased.

GENERAL SIGNIFICANCE

Bioreactor systems have the ability to control many key differentiation stimuli including mechanical stress and oxygen content. The further integration of cell signaling investigations within dynamic culture systems will lead to a quicker realization of the promise of tissue engineering and regenerative medicine. This article is part of a Special Issue entitled Biochemistry of Stem Cells.