Effects of medium perfusion rate on cell-seeded three-dimensional bone constructs in vitro

Evaluation of rabbit adipose derived stem cells fate in perfused multilayered silk fibroin composite scaffold for Osteochondral repair

Development of osteochondral tissue engineering approaches using scaffolds seeded with stem cells in association with mechanical stimulations has been recently considered as a promising technique for the repair of this tissue. In this study, an integrated and biomimetic trilayered silk fibroin (SF) scaffold containing SF nanofibers in each layer was fabricated. The osteogenesis and chondrogenesis of stem cells seeded on the fabricated scaffolds were investigated under a perfusion flow. 3-Dimethylthiazol-2,5-diphenyltetrazolium bromide assay showed that the perfusion flow significantly enhanced cell viability and proliferation. Analysis of gene expression by stem cells revealed that perfusion flow had significantly upregulated the expression of osteogenic and chondrogenic genes in the bone and cartilage layers and downregulated the hypertrophic gene expression in the intermediate layer of the scaffold. In conclusion, applying flow perfusion on the prepared integrated trilayered SF-based scaffold can support osteogenic and chondrogenic differentiation for repairing osteochondral defects.

Design of a Haversian system-like gradient porous scaffold based on triply periodic minimal surfaces for promoting bone regeneration

INTRODUCTION

The bone ingrowth depth in the porous scaffolds is greatly affected by the structural design, notably the pore size, pore geometry, and the pore distribution. To enhance the bone regeneration capability of scaffolds, the bionic design can be regarded as a potential solution.

OBJECTIVES

We proposed a Haversian system-like gradient structure based on the triply periodic minimal surface architectures with pore size varying from the edge to the center. And its effects in promoting bone regeneration were evaluated in the study.

METHODS

The gradient scaffold was designed using the triply periodic minimal surface architectures. The mechanical properties were analyzed by the finite element simulation and confirmed using the universal machine. The fluid characteristics were calculated by the computational fluid dynamics analysis. The bone regeneration process was simulated using a in silico computational model containing the main biological, physical, and chemical variation during the bone growth process. Finally, the in vitro and in vivo studies were carried out to verify the actual osteogenic effect.

RESULTS

Compared to the uniform scaffold, the biomimetic gradient scaffold demonstrated better performance in stress conduction and reduced stress shielding effects. The fluid features were appropriate for cell migration and flow diffusion, and the permeability was in the same order of magnitude with the natural bone. The bone ingrowth simulation exhibited improved angiogenesis and bone regeneration. Higher expression of the osteogenesis-related genes, higher alkaline phosphatase activity, and increased mineralization could be observed on the gradient scaffold in the in vitro study. The 12-week in vivo study proved that the gradient scaffold had deeper bone inserting depth and a more stable bone-scaffold interface.

CONCLUSION

The Haversian system-like gradient structure can effectively promote the bone regeneration. This structural design can be used as a new solution for the clinical application of prosthesis design.

Cell-Based Modeling of Tissue Developing in the Scaffold Pores of Varying Cross-Sections

In this work, we present a mathematical model of cell growth in the pores of a perfusion bioreactor through which a nutrient solution is pumped. We have developed a 2-D vertex model that allows us to reproduce the microscopic dynamics of the microenvironment of cells and describe the occupation of the pore space with cells. In this model, each cell is represented by a polygon; the number of vertices and shapes may change over time. The model includes mitotic cell division and intercalation. We study the impact of two factors on cell growth. On the one hand, we consider a channel of variable cross-section, which models a scaffold with a porosity gradient. On the other hand, a cluster of cells grows under the influence of a nutrient solution flow, which establishes a non-uniform distribution of shear stresses in the pore space. We present the results of numerical simulation of the tissue growth in a wavy channel. The model allows us to obtain complete microscopic information that includes the dynamics of intracellular pressure, the local elastic energy, and the characteristics of cell populations. As we showed, in a functional-graded scaffold, the distribution of the shear stresses in the pore space has a complicated structure, which implies the possibility of controlling the growth zones by varying the pore geometry.

From Free Tissue Transfer to Hydrogels: A Brief Review of the Application of the Periosteum in Bone Regeneration

The periosteum is a thin layer of connective tissue covering bone. It is an essential component for bone development and fracture healing. There has been considerable research exploring the application of the periosteum in bone regeneration since the 19th century. An increasing number of studies are focusing on periosteal progenitor cells found within the periosteum and the use of hydrogels as scaffold materials for periosteum engineering and guided bone development. Here, we provide an overview of the research investigating the use of the periosteum for bone repair, with consideration given to the anatomy and function of the periosteum, the importance of the cambium layer, the culture of periosteal progenitor cells, periosteum-induced ossification, periosteal perfusion, periosteum engineering, scaffold vascularization, and hydrogel-based synthetic periostea.

Ex Vivo Preservation of Ovine Periosteum Using a Perfusion Bioreactor System

Periosteum is a highly vascularized membrane lining the surface of bones. It plays essential roles in bone repair following injury and reconstruction following invasive surgeries. To broaden the use of periosteum, including for augmenting in vitro bone engineering and/or in vivo bone repair, we have developed an ex vivo perfusion bioreactor system to maintain the cellular viability and metabolism of surgically resected periosteal flaps. Each specimen was placed in a 3D printed bioreactor connected to a peristaltic pump designed for the optimal flow rates of tissue perfusate. Nutrients and oxygen were perfused via the periosteal arteries to mimic physiological conditions. Biochemical assays and histological staining indicate component cell viability after perfusion for almost 4 weeks. Our work provides the proof-of-concept of ex vivo periosteum perfusion for long-term tissue preservation, paving the way for innovative bone engineering approaches that use autotransplanted periosteum to enhance in vivo bone repair.

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.

Assessing the effect of manufacturing defects and non-Newtonian blood model on flow behaviors of additively manufactured Gyroid TPMS structures

In the field of medical engineering, Triply Periodic Minimal Surfaces (TPMS) structures have been studied widely owing to their physical attributes similar to those of human bones. Computational Fluid Dynamics (CFD) is often used to reveal the interaction between structural architectures and flow fields. Nevertheless, a comprehensive study on the effect of manufacturing defects and non-Newtonian behavior on the fluid responses in TPMS scaffolds is still lacking. Therefore, the present study fabricated Gyroid TPMS with four relative densities from 0.1 to 0.4. Non-destructive techniques were used to examine surface roughness and geometric deviation. We found that the manufacturing defects had a minor effect on fluid responses. The pressure drop comparison between defect-containing and defect-free models could be differed up to 7%. The same comparison for the average shear stress showed a difference up to 23%, in which greater deviation between both models was observed at higher relative density. On the contrary, the viscosity model played a significant role in flow prediction. By comparing the Newtonian model with Carreau-Yasuda non-Newtonian model, the resulting pressure drop and average wall shear stress from non-Newtonian viscosity could be higher than those of the Newtonian model by more than a factor of two. In addition, we matched the fluid-induced shear stress from both viscosity models with desirable ranges of shear stresses for tissue growth obtained from the literature. Up to 70% from the Newtonian model fell within the desirable range while the matching stress reduced to lower than 8% for the non-Newtonian results. Furthermore, by correlating geometric features with physical outputs, the geometric deviation was seen associated with surface curvature while the local shear stress revealed a strong correlation with inclination angle. Overall, the present work emphasized the importance of the viscosity model for CFD analysis of the scaffolds, especially when resulting fluid-induced wall shear stress is of interest. In addition, the geometric correlation has introduced the alternative consideration of structural architectures from local perspectives, which could assist the further comparison and optimization among different porous scaffolds in the future.

In vitromagnetohydrodynamics system for modulating cell migration

Fluid shear stress (FSS) is an important parameter that regulates various cell functions such as proliferation and migration. While there are a number of techniques to generate FSSin vitro, many of them require physical deformation or movement of solid objects to generate the fluid shear, making it difficult to decouple the effects of FSS and mechanical strains. This work describes the development of a non-mechanical means to generate fluid flow and FSS in a 2Din vitrosetting. This was accomplished with a magnetohydrodynamic (MHD) pump, which creates liquid flow by generating a Lorentz force through the interaction between an electric field and an orthogonal magnetic field. The MHD pump system presented here consisted of trapezoidal prism-shaped magnets, a pair of platinum electrodes, and a modified petri dish. The system was validated and tested on anin vitrowound model, which is based on analyzing the migration of fibroblast cells through an artificially created scratch on a confluent cell culture surface. Experiments were performed to a control group, an electric field only group, and a group that was subject to fluid flow with the application of both electric field and magnetic field. Results show that fibroblast cells that experienced fluid shear have higher wound closure rate compared to the control group and the electric field only group. The data shows that the MHD pump can be a great tool to study FSSin vitro. Furthermore, due to its fluid flow generation without mechanical force, this system can be adapted and implemented to study the role of FSS and electric field on wound healingin vivo.

The Influence of Fluid Shear Stress on Bone and Cancer Cells Proliferation and Distribution

We investigated the potential correlation between the fluid shear stress and the proliferation of bone prostate cancer cells on the surface of nanoclay-based scaffolds in a perfusion bioreactor. Human mesenchymal stem cells (hMSCs) were seeded on the scaffolds to initiate bone growth. After 23 days, prostate cancer cells (MDAPCa2b) were cultured on top of the osteogenically differentiated hMSCs. The scaffolds were separated into two groups subjected to two distinct conditions: (i) static (no flow); and (ii) dynamic (with flow) conditions to recapitulate bone metastasis of prostate cancer. Based on measured data, Computational Fluid Dynamics (CFD) models were constructed to determine the velocity and shear stress distributions on the scaffold surface. Our experimental results show distinct differences in the growth pattern of hMSCs and MDAPCa2b cells between the static and dynamic conditions. Our computational results further suggest that the dynamic flow leads to drastic change in cell morphology and tumorous distribution. Our work points to a strong correlation between tumor growth and local interstitial flows in bones.

Real-time luminescence enables continuous drug-response analysis in adherent and suspension cell lines

The drug-induced proliferation (DIP) rate is a metric of in vitro drug response that avoids inherent biases in commonly used metrics such as 72 h viability. However, DIP rate measurements rely on direct cell counting over time, a laborious task that is subject to numerous challenges, including the need to fluorescently label cells and automatically segment nuclei. Moreover, it is incredibly difficult to directly count cells and accurately measure DIP rates for cell populations in suspension. As an alternative, we use real-time luminescence measurements derived from the cellular activity of NAD(P)H oxidoreductase to efficiently estimate drug response in both adherent and suspension cell populations to a panel of known anticancer agents. For the adherent cell lines, we collect both luminescence reads and direct cell counts over time simultaneously to assess their congruency. Our results demonstrate that the proposed approach significantly speeds up data collection, avoids the need for cellular labels and image segmentation, and opens the door to significant advances in high-throughput screening of anticancer drugs.

Pancreatic Lineage Cell Differentiation of Bone Marrow Mesenchymal Stromal Cells on Acellular Pancreatic Bioscaffold

OBJECTIVES

We evaluated the potential differentiation ability of bone mesenchymal stromal cells (BMSCs) into pancreatic lineage cells on a rat acellular pancreatic bioscaffold (APB) and the effect of differentiated BMSCs in vivo.

METHODS

The BMSCs were dynamically or statically cultured with or without growth factor in both culture systems. We assessed the cytological behavior and differentiation. We also evaluated the pancreatic fibrosis and pathological scores.

RESULTS

The proliferation rates of BMSCs were significantly higher in the APB groups. The APB induced BMSCs to express mRNA markers at higher levels. All tested pancreatic functional proteins were also expressed at higher levels in the APB group. The secretion of metabolic enzymes was higher in the APB system. The ultrastructure of BMSCs in the APB group further revealed the morphological characteristics of pancreatic-like cells. For the in vivo study, the pancreatic fibrosis and pathological scores were significantly lower in the differentiated BMSCs group. In addition, in both the in vitro and the in vivo study, growth factor significantly improved proliferation, differentiation, and pancreatic cell therapy.

CONCLUSIONS

The APB can promote BMSC differentiation toward pancreatic lineage and pancreatic-like phenotypes, giving it the potential for use in pancreatic cell therapies and tissue engineering.

Generation and evaluation of input values for computational analysis of transport processes within tissue cultures

Techniques for tissue culture have seen significant advances during the last decades and novel 3D cell culture systems have become available. To control their high complexity, experimental techniques and their Digital Twins (modelling and computational tools) are combined to link different variables to process conditions and critical process parameters. This allows a rapid evaluation of the expected product quality. However, the use of mathematical simulation and Digital Twins is critically dependent on the precise description of the problem and correct input parameters. Errors here can lead to dramatically wrong conclusions. The intention of this review is to provide an overview of the state-of-the-art and remaining challenges with respect to generating input values for computational analysis of mass and momentum transport processes within tissue cultures. It gives an overview on relevant aspects of transport processes in tissue cultures as well as modelling and computational tools to tackle these problems. Further focus is on techniques used for the determination of cell-specific parameters and characterization of culture systems, including sensors for on-line determination of relevant parameters. In conclusion, tissue culture techniques are well-established, and modelling tools are technically mature. New sensor technologies are on the way, especially for organ chips. The greatest remaining challenge seems to be the proper addressing and handling of input parameters required for mathematical models. Following Good Modelling Practice approaches when setting up and validating computational models is, therefore, essential to get to better estimations of the interesting complex processes inside organotypic tissue cultures in the future.

In vivo evaluation of osseointegration ability of sintered bionic trabecular porous titanium alloy as artificial hip prosthesis

Hydroxyapatite (HA) coatings have been widely used for improving the bone-implant interface (BII) bonding of the artificial joint prostheses. However, the incidence of prosthetic revisions due to aseptic loosening remains high. Porous materials, including three-dimensional (3D) printing, can reduce the elastic modulus and improve osseointegration at the BII. In our previous study, we identified a porous material with a sintered bionic trabecular structure with in vitro and in vivo bio-safety as well as in vivo mechanical safety. This study aimed to compare the difference in osseointegration ability of the different porous materials and HA-coated titanium alloy in the BII. We fabricated sintered bionic trabecular porous titanium acetabular cups, 3D-printed porous titanium acetabular cups, and HA-coated titanium alloy acetabular cups for producing a hip prosthesis suitable for beagle dogs. Subsequently, the imaging and histomorphological analysis of the three materials under mechanical loading in animals was performed (at months 1, 3, and 6). The results suggested that both sintered bionic porous titanium alloy and 3D-printed titanium alloy exhibited superior performances in promoting osseointegration at the BII than the HA-coated titanium alloy. In particular, the sintered bionic porous titanium alloy exhibited a favorable bone ingrowth performance at an early stage (month 1). A comparison of the two porous titanium alloys suggested that the sintered bionic porous titanium alloys exhibit superior bone in growth properties and osseointegration ability. Overall, our findings provide an experimental basis for the clinical application of sintered bionic trabecular porous titanium alloys.

Design considerations of benchtop fluid flow bioreactors for bio-engineered tissue equivalents in vitro

One of the major aims of bio-engineering tissue equivalents in vitro is to create physiologically relevant culture conditions to accurately recreate the cellular microenvironment. This often includes incorporation of factors such as the extracellular matrix, co-culture of multiple cell types and three-dimensional culture techniques. These advanced techniques can recapitulate some of the properties of tissue in vivo, however fluid flow is a key aspect that is often absent. Fluid flow can be introduced into cell and tissue culture using bioreactors, which are becoming increasingly common as we seek to produce increasingly accurate tissue models. Bespoke technology is continuously being developed to tailor systems for specific applications and to allow compatibility with a range of culture techniques. For effective perfusion of a tissue culture many parameters can be controlled, ranging from impacts of the fluid flow such as increased shear stress and mass transport, to potentially unwanted side effects such as temperature fluctuations. A thorough understanding of these properties and their implications on the culture model can aid with a more accurate interpretation of results. Improved and more complete characterisation of bioreactor properties will also lead to greater accuracy when reporting culture conditions in protocols, aiding experimental reproducibility, and allowing more precise comparison of results between different systems. In this review we provide an analysis of the different factors involved in the development of benchtop flow bioreactors and their potential biological impacts across a range of applications.

Osteoblasts in a Perfusion Flow Bioreactor-Tissue Engineered Constructs of TiO2 Scaffolds and Cells for Improved Clinical Performance

Combining biomaterial scaffolds with cells serves as a promising strategy for engineering critical size defects; however, homogenous cellular growth within large scaffolds is challenging. Mechanical stimuli can enhance bone regeneration by modulating cellular growth and differentiation. Here, we compare dynamic seeding in a perfusion flow bioreactor with static seeding for a synthetic bone scaffold for up to 21 days using the cell line MC3T3-E1 and primary human osteoblast, confocal laser scanning microscopy, and real-time reverse transcriptase-polymerase chain reaction. The secretion of bone-related proteins was quantified using multiplex immunoassays. Dynamic culture improved cellular distribution through the TiO2 scaffold and induced a five-fold increase in cell number after 21 days. The relative mRNA expression of osteopontin of MC3T3-E1 was 40-fold enhanced after 7 and 21 days at a flow rate of 0.08 mL/min, and that of collagen type I alpha I expression was 18-fold after 21 days. A flow rate of 0.16 mL/min was 10-fold less effective. Dynamic culture increased the levels of dickkopf-related protein 1 (60-fold), osteoprotegrin (29-fold), interleukin-6 (23-fold), interleukin-8 (36-fold), monocyte chemoattractant protein 1 (28-fold) and vascular endothelial growth factor (6-fold) in the medium of primary human osteoblasts after 21 days compared to static seeding. The proposed method may have clinical potential for bone tissue engineering.

Impact of Fluid Dynamics on the Viability and Differentiation Capacity of 3D-Cultured Jaw Periosteal Cells

Perfused bioreactor systems are considered to be a promising approach for the 3D culturing of stem cells by improving the quality of the tissue-engineered grafts in terms of better cell proliferation and deeper penetration of used scaffold materials. Our study aims to establish an optimal perfusion culture system for jaw periosteal cell (JPC)-seeded scaffolds. For this purpose, we used beta-tricalcium phosphate (β-TCP) scaffolds as a three-dimensional structure for cell growth and osteogenic differentiation. Experimental set-ups of tangential and sigmoidal fluid configurations with medium flow rates of 100 and 200 µL/min were applied within the perfusion system. Cell metabolic activities of 3D-cultured JPCs under dynamic conditions with flow rates of 100 and 200 µL/min were increased in the tendency after 1, and 3 days of culture, and were significantly increased after 5 days. Significantly higher cell densities were detected under the four perfused conditions compared to the static condition at day 5. However, cell metabolic and proliferation activity under dynamic conditions showed flow rate independency in our study. In this study, dynamic conditions increased the expression of osteogenic markers (ALPL, COL1A1, RUNX2, and OCN) compared to static conditions and the tangential configuration showed a stronger osteogenic effect than the sigmoidal flow configuration.

Optimization and Validation of a Custom-Designed Perfusion Bioreactor for Bone Tissue Engineering: Flow Assessment and Optimal Culture Environmental Conditions

Various perfusion bioreactor systems have been designed to improve cell culture with three-dimensional porous scaffolds, and there is some evidence that fluid force improves the osteogenic commitment of the progenitors. However, because of the unique design concept and operational configuration of each study, the experimental setups of perfusion bioreactor systems are not always compatible with other systems. To reconcile results from different systems, the thorough optimization and validation of experimental configuration are required in each system. In this study, optimal experimental conditions for a perfusion bioreactor were explored in three steps. First, an in silico modeling was performed using a scaffold geometry obtained by microCT and an expedient geometry parameterized with porosity and permeability to assess the accuracy of calculated fluid shear stress and computational time. Then, environmental factors for cell culture were optimized, including the volume of the medium, bubble suppression, and medium evaporation. Further, by combining the findings, it was possible to determine the optimal flow rate at which cell growth was supported while osteogenic differentiation was triggered. Here, we demonstrated that fluid shear stress up to 15 mPa was sufficient to induce osteogenesis, but cell growth was severely impacted by the volume of perfused medium, the presence of air bubbles, and medium evaporation, all of which are common concerns in perfusion bioreactor systems. This study emphasizes the necessity of optimization of experimental variables, which may often be underreported or overlooked, and indicates steps which can be taken to address issues common to perfusion bioreactors for bone tissue engineering.

Measuring the density and viscosity of culture media for optimized computational fluid dynamics analysis of in vitro devices

Culture medium is frequently modelled as water in computational fluid dynamics (CFD) analysis of in vitro culture systems involving flow, such as bioreactors and organ-on-chips. However, culture medium can be expected to have different properties to water due to its higher solute content. Furthermore, cellular activities such as metabolism and secretion of ECM proteins alter the composition of culture medium and therefore its properties during culture. As these properties directly determine the hydromechanical stimuli exerted on cells in vitro, these, along with any changes during culture must be known for CFD modelling accuracy and meaningful interpretation of cellular responses. In this study, the density and dynamic viscosity of DMEM and RPMI-1640 media supplemented with typical concentrations of foetal bovine serum (0, 5, 10 and 20% v/v) were measured to serve as a reference for computational design analysis. Any changes in the properties of medium during culture were also investigated with NCI-H460 and HN6 cell lines. The density and dynamic viscosity of the media increased proportional to the % volume of added foetal bovine serum (FBS). Importantly, the viscosity of 5% FBS-supplemented RPMI-1640 was found to increase significantly after 3 days of culture of NCI-H460 and HN6 cell lines, with distinct differences between magnitude of change for each cell line. Finally, these experimentally-derived values were applied in CFD analysis of a simple microfluidic device, which demonstrated clear differences in maximum wall shear stress and pressure between fluid models. Overall, these results highlight the importance of characterizing model-specific properties for CFD design analysis of cell culture systems.

Numerical modelling of osteocyte growth on different bone tissue scaffolds

The most common solution for the regeneration or replacement of damaged bones is the implantation of prostheses comprising ceramic or metallic materials. However, these implants are known to cause problems such as post-operative infections, collapse of the prosthesis, and lack of osseointegration. Consequently, bone tissue engineering was established because of the limitations of such implants. Osteogenic implants offer promising solutions for bone regeneration; however, three-dimensional scaffolds should be used as supportive structures. It is challenging to correctly design these structures and their compositions or properties to provide a microenvironment that promotes tissue regeneration and expedites bone formation. Computational fluid dynamics can be used to model the main phenomena that occur in bioreactors, such as cell metabolism, nutrient transport, and cell culture growth, or to model the influence of several key mechanisms related to the fluid medium, in particular, the wall shear stress. In this work, a new numerical bone cell growth model was developed, which considered the oxygen and nutrient consumption as well as the wall shear stress effect on cell proliferation. The model was implemented using 35 three-dimensional scaffolds of different porosities, and the effect of the main geometrical parameters involved in each scaffold type was analysed. The porosity plays an important role, however, a similar porosity did not guarantee similar shear stress or cell growth among the scaffolds. Randomised trabecular scaffolds, that more closely resembled trabecular bone, showed the highest cell growth values, so these are the best candidates for cell growth in a bioreactor.

A microfluidics-based method for culturing osteoblasts on biomimetic hydroxyapatite

The reliability of conventional cell culture studies to evaluate biomaterials is often questioned, as in vitro outcomes may contradict results obtained through in vivo assays. Microfluidics technology has the potential to reproduce complex physiological conditions by allowing for fine control of microscale features such as cell confinement and flow rate. Having a continuous flow during cell culture is especially advantageous for bioactive biomaterials such as calcium-deficient hydroxyapatite (HA), which may otherwise alter medium composition and jeopardize cell viability, potentially producing false negative results in vitro. In this work, HA was integrated into a microfluidics-based platform (HA-on-chip) and the effect of varied flow rates (2, 8 and 14 µl/min, corresponding to 0.002, 0.008 and 0.014 dyn/cm², respectively) was evaluated. A HA sample placed in a well plate (HA-static) was included as a control. While substantial calcium depletion and phosphate release occurred in static conditions, the concentration of ions in HA-on-chip samples remained similar to those of fresh medium, particularly at higher flow rates. Pre-osteoblast-like cells (MC3T3-E1) exhibited a significantly higher degree of proliferation on HA-on-chip (8 μl/min flow rate) as compared to HA-static. However, cell differentiation, analysed by alkaline phosphatase (ALP) activity, showed low values in both conditions. This study indicates that cells respond differently when cultured on HA under flow compared to static conditions, which indicates the need for more physiologically relevant methods to increase the predictive value of in vitro studies used to evaluate biomaterials. STATEMENT OF SIGNIFICANCE: There is a lack of correlation between the results obtained when testing some biomaterials under cell culture as opposed to animal models. To address this issue, a cell culture method with slightly enhanced physiological relevance was developed by incorporating a biomaterial, known to regenerate bone, inside of a microfluidic platform that enabled a continuous supply of cell culture medium. Since the utilized biomaterial interacts with surrounding ions, the perfusion of medium allowed for shielding of these changes similarly as would happen in the body. The experimental outcomes observed in the dynamic platform were different than those obtained with standard static cell culture systems, proving the key role of the platform in the assessment of biomaterials.

Designing and validation of an automated ex-vivo bioreactor system for long term culture of bone

Several different bioreactors have been developed to study bone biology. Keeping a bone viable for long-term studies is still a challenge. We have developed an ex-vivo bone bioreactor that can keep the ex-vivo live bone viable for more than 4 weeks. Keeping a bone viable for over a month can be used as an alternative model for in-vivo experiments in animals. We hypothesize that the perfusion flow and mechanical load on the bone provide a real-time environment for the bone to survive. Cancellous bones were harvested from the bovine metatarsals and were placed in the dynamic culture with cyclic loading at regular intervals. After a period of week 4, the bone cores were retrieved from the bioreactor and tested for viability using calcein-AM and ethidium homodimer -1 fluorescent dyes and were compared with the cores that were placed in static culture with and without any loads on them and Day 0 bone core that acted as a positive control. The bone blocks were then fixed in 10% formalin, and bone mineral density was evaluated using a DXA scanner before staining them for H&E to study the morphological changes. Results revealed that the bone cultured in the bioreactor was viable as compared to the one in the static culture with and without constant load. Bone cores cultured in our ex-vivo bioreactor system also maintained their morphology and no statistical difference was found in the bone mineral density compared to positive controls and the statistical difference was found when compared with the cores cultured in static culture. This tool can be used to study bone biology for various applications such as bone ingrowth studies, to study the effect of drugs, hormones, or any growth factors, and much more.

Multiphysics simulation of a compression-perfusion combined bioreactor to predict the mechanical microenvironment during bone metastatic breast cancer loading experiments

Incurable breast cancer bone metastasis causes widespread bone loss, resulting in fragility, pain, increased fracture risk, and ultimately increased patient mortality. Increased mechanical signals in the skeleton are anabolic and protect against bone loss, and they may also do so during osteolytic bone metastasis. Skeletal mechanical signals include interdependent tissue deformations and interstitial fluid flow, but how metastatic tumor cells respond to each of these individual signals remains underinvestigated, a barrier to translation to the clinic. To delineate their respective roles, we report computed estimates of the internal mechanical field of a bone mimetic scaffold undergoing combinations of high and low compression and perfusion using multiphysics simulations. Simulations were conducted in advance of multimodal loading bioreactor experiments with bone metastatic breast cancer cells to ensure that mechanical stimuli occurring internally were physiological and anabolic. Our results show that mechanical stimuli throughout the scaffold were within the anabolic range of bone cells in all loading configurations, were homogenously distributed throughout, and that combined high magnitude compression and perfusion synergized to produce the largest wall shear stresses within the scaffold. These simulations, when combined with experiments, will shed light on how increased mechanical loading in the skeleton may confer anti-tumorigenic effects during metastasis.

Numerical Analysis of the Influence of Porosity and Pore Geometry on Functionality of Scaffolds Designated for Orthopedic Regenerative Medicine

BACKGROUND

Scaffolds are vital for orthopedic regenerative medicine. Therefore, comprehensive studies evaluating their functionality with consideration of variable parameters are needed. The research aim was to evaluate pore geometry and scaffold porosity influence on first, cell culture efficiency in a perfusion bioreactor and second, osteogenic cell diffusion after its implantation.

METHODS

For the studies, five pore geometries were selected (triangular prism with a rounded and a flat profile, cube, octagonal prism, sphere) and seven porosities (up to 80%), on the basis of which 70 models were created for finite element analyses. First, scaffolds were placed inside a flow channel to estimate growth medium velocity and wall shear stress. Secondly, scaffolds were placed in a bone to evaluate osteogenic cell diffusion.

RESULTS

In terms of fluid minimal velocity (0.005 m/s) and maximal wall shear stress (100 mPa), only cubic and octagonal pores with 30% porosity and spherical pores with 20% porosity fulfilled the requirements. Spherical pores had the highest osteogenic cell diffusion efficiency for porosities up to 30%. For higher porosities, the octagonal prism's pores gave the best results up to 80%, where no differences were noted.

CONCLUSIONS

The data obtained allows for the appropriate selection of pore geometry and scaffold porosity for orthopedic regenerative medicine.

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

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

Controlled release of soy isoflavones from multifunctional 3D printed bone tissue engineering scaffolds

Recent challenges in post-surgical bone tumor management have elucidated the need for a multifunctional scaffold, which can be used for residual tumor-cell suppression, defect repair, and simultaneous bone regeneration. In this perspective, 3D printing allows to create a wide variety of patient-specific implant with complex porous architecture and compatible mechanical strength to that of cancellous bone. Here, a multifunctional bone graft substitute is designed by incorporating the three primary soy isoflavones: genistein, daidzein, and glycitein onto a 3D printed (3DP) tricalcium phosphate (TCP) scaffolds with designed pores, endowing them with in vitro chemopreventive, bone-cell proliferating and immune-modulatory potential. The interconnected porosity and biodegradability of 3DP TCP ceramics have allowed controlled release kinetics of genistein, daidzein and glycitein in acidic and physiological buffer medium for 16 days, which is fitted with Korsmeyer-Peppas model. Presence of genistein, a well-known natural biomolecule shows a 90% reduction in vitro osteosarcoma (MG-63) cell viability and proliferation after 11 days. Meanwhile, daidzein, the other primary isoflavone, promotes in vitro cellular attachment and enhances viability and proliferation of human fetal osteoblast cell (hFOB). Furthermore, controlled release of genistein, daidzein, and glycitein from 3DP TCP scaffold demonstrates improved hFOB cell proliferation, viability, and differentiation in a dynamic flow-perfusion bioreactor, which is utilized to better simulate the clinical microenvironment. Finally, in vivo H&E staining confirms controlled co-delivery of genistein-daidzein-glycitein from 3DP scaffold carefully modulated neutrophil recruitment to the surgery site after 24 h of implantation in a rat distal femur model. These results advance our understanding towards multipronged therapeutic approaches utilizing synthetic bone graft substitutes as a drug delivery vehicle, and more importantly, demonstrate the feasibility of localized tumor cell suppression and bone cell proliferation for post-surgical defect repair application. STATEMENT OF SIGNIFICANCE: Designed multimodal porosity of 3D printed TCP scaffold allows a controlled and sustained release of soy isoflavones, genistein, daidzein and glycitein in both physiological and acidic pH. Presence of genistein shows 90% reduction in vitro bone cancer cell viability and proliferation. Meanwhile, controlled release of genistein, daidzein, and glycitein from 3DP TCP scaffolds demonstrate improved osteoblast cell proliferation, viability, and differentiation in static and dynamic flow-perfusion bioreactor. Furthermore, H&E staining at 24 h post-surgical specimens from rat distal femur model shows neutrophil recruitment at the surgery site is significantly decreased, suggesting the anti-inflammatory property of soy isoflavones. This work provides deeper understanding on the design of a multifunctional 3D printed patient-specific scaffold with enhanced in vitro chemopreventive, osteogenic and in vivo anti-inflammatory ability.

A simple in vitro biomimetic perfusion system for mechanotransduction study

In mechanotransduction studies, flow-induced shear stress (FSS) is often applied to two-dimensional (2D) cultured cells with a parallel-plate flow chamber (PPFC) due to its simple FSS estimation. However, cells behave differently under FSS inside a 3D scaffold (e.g. 10 mPa FSS was shown to induce osteogenesis of human mesenchymal stem cells (hMSC) in 3D but over 900 mPa was needed for 2D culture). Here, a simple in vitro biomimetic perfusion system using borosilicate glass capillary tubes has been developed to study the cellular behaviour under low-level FSS that mimics 3D culture. It has been shown that, compared to cells in the PPFC, hMSC in the capillary tubes had upregulated Runx-2 expression and osteogenic cytoskeleton actin network under 10 mPa FSS for 24 h. Also, an image analysis method based on Haralick texture measurement has been used to identify osteogenic actin network. The biomimetic perfusion system can be a valuable tool to study mechanotransduction in 3D for more clinical relevant tissue-engineering applications.

Design and Evaluation of an Osteogenesis-on-a-Chip Microfluidic Device Incorporating 3D Cell Culture

Microfluidic-based tissue-on-a-chip devices have generated significant research interest for biomedical applications, such as pharmaceutical development, as they can be used for small volume, high throughput studies on the effects of therapeutics on tissue-mimics. Tissue-on-a-chip devices are evolving from basic 2D cell cultures incorporated into microfluidic devices to complex 3D approaches, with modern designs aimed at recapitulating the dynamic and mechanical environment of the native tissue. Thus far, most tissue-on-a-chip research has concentrated on organs involved with drug uptake, metabolism and removal (e.g., lung, skin, liver, and kidney); however, models of the drug metabolite target organs will be essential to provide information on therapeutic efficacy. Here, we develop an osteogenesis-on-a-chip device that comprises a 3D environment and fluid shear stresses, both important features of bone. This inexpensive, easy-to-fabricate system based on a polymerized High Internal Phase Emulsion (polyHIPE) supports proliferation, differentiation and extracellular matrix production of human embryonic stem cell-derived mesenchymal progenitor cells (hES-MPs) over extended time periods (up to 21 days). Cells respond positively to both chemical and mechanical stimulation of osteogenesis, with an intermittent flow profile containing rest periods strongly promoting differentiation and matrix formation in comparison to static and continuous flow. Flow and shear stresses were modeled using computational fluid dynamics. Primary cilia were detectable on cells within the device channels demonstrating that this mechanosensory organelle is present in the complex 3D culture environment. In summary, this device aids the development of ‘next-generation’ tools for investigating novel therapeutics for bone in comparison with standard laboratory and animal testing.

Influence of Titanium Alloy Scaffolds on Enzymatic Defense against Oxidative Stress and Bone Marrow Cell Differentiation

Studies have been directed towards the production of new titanium alloys, aiming for the replacement of Ti-6 Aluminium-4 Vanadium (TiAlV) alloy in the future. Many mechanisms related to biocompatibility and chemical characteristics have been studied in the field of implantology, but enzymatic defenses against oxidative stress remain underexplored. Bone marrow stromal cells have been explored as source of cells, which have the potential to differentiate into osteoblasts and therefore could be used as cells-based therapy. The objective of this study was to evaluate the activity of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT) in porous scaffolds of Ti-6 Aluminium-4 Vanadium (TiAlV), Ti-35 Niobium (TiNb), and Ti-35 Niobium-7 Zirconium-5 Tantalum (TiNbZrTa) on mouse bone marrow stromal cells. Porous titanium alloy scaffolds were prepared by powder metallurgy. After 24 hours, cells plated on the scaffolds were analyzed by scanning electron microscopy (SEM). The antioxidant enzyme activity was measured 72 hours after cell plating. Quantitative real time PCR (qRT-PCR) was performed after 3, 7, and 14 days, and Runx2 (Runt-related transcription factor2) expression was evaluated. The SEM images showed the presence of interconnected pores and growth, adhesion, and cell spreading in the 3 scaffolds. Although differences were noted for SOD and CAT activity for all scaffolds analyzed, no statistical differences were observed (p > 0.05). The osteogenic gene Runx2 presented high expression levels for TiNbZrTa at day 7, compared to the control group (TiAlV day 3). At day 14, all scaffolds had more than 2-fold induction for Runx2 mRNA levels, with statistically significant differences compared to the control group. Even though we were not able to confirm statistically significant differences to justify the replacement of TiAlV regarding antioxidant enzymes, TiNbZrTa was able to induce faster bone formation at early time points, making it a good choice for biomedical and tissue bioengineering applications.

Enhancement of cartilage extracellular matrix synthesis in Poly(PCL-TMC)urethane scaffolds: a study of oriented dynamic flow in bioreactor

The development of new technologies to produce three-dimensional and biocompatible scaffolds associated with high-end cell culture techniques have shown to be promising for the regeneration of tissues and organs. Some biomedical devices, as meniscus prosthesis, require high flexibility and tenacity and such features are found in polyurethanes which represent a promising alternative. The Poly(PCL-TMC)urethane here presented, combines the mechanical properties of PCL with the elasticity attributed by TMC and presents great potential as a cellular carrier in cartilage repair. Scanning electron microscopy showed the presence of interconnected pores in the three-dimensional structure of the material. The scaffolds were submitted to proliferation and cell differentiation assays by culturing mesenchymal stem cells in bioreactor. The tests were performed in dynamic flow mode at the rate of 0.4 mL/min. Laser scanning confocal microscopy analysis showed that the flow rate promoted cell growth and cartilage ECM synthesis of aggrecan and type II collagen within the Poly(PCL-TMC)urethane scaffolds. This study demonstrated the applicability of the polymer as a cellular carrier in tissue engineering, as well as the ECM was incremented only when under oriented flow rate stimuli. Therefore, our results may also provide data on how oriented flow rate in dynamic bioreactors culture can influence cell activity towards cartilage ECM synthesis even when specific molecular stimuli are not present. This work addresses new perspectives for future clinical applications in cartilage tissue engineering when the molecular factors resources could be scarce for assorted reasons.

Inlet flow rate of perfusion bioreactors affects fluid flow dynamics, but not oxygen concentration in 3D-printed scaffolds for bone tissue engineering: Computational analysis and experimental validation

Fluid flow dynamics and oxygen-concentration in 3D-printed scaffolds within perfusion bioreactors are sensitive to controllable bioreactor parameters such as inlet flow rate. Here we aimed to determine fluid flow dynamics, oxygen-concentration, and cell proliferation and distribution in 3D-printed scaffolds as a result of different inlet flow rates of perfusion bioreactors using experiments and finite element modeling. Pre-osteoblasts were treated with 1 h pulsating fluid flow with low (0.8 Pa; PFF^low) or high peak shear stress (6.5 Pa; PFF^high), and nitric oxide (NO) production was measured to validate shear stress sensitivity. Computational analysis was performed to determine fluid flow between 3D-scaffold-strands at three inlet flow rates (0.02, 0.1, 0.5 ml/min) during 5 days. MC3T3-E1 pre-osteoblast proliferation, matrix production, and oxygen-consumption in response to fluid flow in 3D-printed scaffolds inside a perfusion bioreactor were experimentally assessed. PFF^high more strongly stimulated NO production by pre-osteoblasts than PFF^low. 3D-simulation demonstrated that dependent on inlet flow rate, fluid velocity reached a maximum (50-1200 μm/s) between scaffold-strands, and fluid shear stress (0.5-4 mPa) and wall shear stress (0.5-20 mPa) on scaffold-strands surfaces. At all inlet flow rates, gauge fluid pressure and oxygen-concentration were similar. The simulated cell proliferation and distribution, and oxygen-concentration data were in good agreement with the experimental results. In conclusion, varying a perfusion bioreactor's inlet flow rate locally affects fluid velocity, fluid shear stress, and wall shear stress inside 3D-printed scaffolds, but not gauge fluid pressure, and oxygen-concentration, which seems crucial for optimized bone tissue engineering strategies using bioreactors, scaffolds, and cells.

Computational modeling of media flow through perfusion-based bioreactors for bone tissue engineering

Bioreactor system has been used in bone tissue engineering in order to simulate dynamic nature of bone tissue environments. Perfusion bioreactors have been reported as the most efficient types of shear-loading bioreactor. Also, combination of forces, such as rotation plus perfusion, has been reported to enhance cell growth and osteogenic differentiation. Mathematical modeling using sophisticated infrastructure processes could be helpful and streamline the development of functional grafts by estimating and defining an effective range of bioreactor settings for better augmentation of tissue engineering. This study is aimed to conduct computational modeling for newly designed bioreactors in order to alleviate the time and material consuming for evaluating bioreactor parameters and effect of fluid flow hydrodynamics (various amounts of shear stress) on osteogenesis. Also, biological assessments were performed in order to validate similar parameters under implementing the perfusion or rotating and perfusion fluid motions in bioreactors' prototype. Finite element method was used to investigate the effect of hydrodynamic of fluid flow inside the bioreactors. The equations used in the simulation to calculate the velocity values and consequently the shear stress values include Navier-Stokes and Brinkman equations. It has been shown that rotational fluid motion in rotating and perfusion bioreactor produces more velocity and shear stress compared with perfusion bioreactor. Moreover, implementing the perfusion together with rotational force in rotating and perfusion bioreactors has been shown to have more cell proliferation and higher activity of alkaline phosphatase enzyme as well as formation of extra cellular matrix sheet, as an indicator of bone-like tissue formation.

Engineering functional microvessels in synthetic polyurethane random-pore scaffolds by harnessing perfusion flow

Recently reported biomaterial-based approaches toward prevascularizing tissue constructs rely on biologically or structurally complex scaffolds that are complicated to manufacture and sterilize, and challenging to customize for clinical applications. In the current work, a prevascularization method for soft tissue engineering that uses a non-patterned and non-biological scaffold is proposed. Human fibroblasts and HUVECs were seeded on an ionomeric polyurethane-based hydrogel and cultured for 14 days under medium perfusion. A flow rate of 0.05 mL/min resulted in a greater lumen density in the constructs relative to 0.005 and 0.5 mL/min, indicating the critical importance of flow magnitude in establishing microvessels. Constructs generated at 0.05 mL/min perfusion flow were implanted in a mouse subcutaneous model and intravital imaging was used to characterize host blood perfusion through the construct after 2 weeks. Engineered microvessels were functional (i.e. perfused with host blood and non-leaky) and neovascularization of the construct by host vessels was enhanced relative to non-prevascularized constructs. We report on the first strategy toward engineering functional microvessels in a tissue construct using non-bioactive, non-patterned synthetic polyurethane materials.

Perfusion-based co-culture model system for bone tissue engineering

In this work, we report on a perfusion-based co-culture system that could be used for bone tissue engineering applications. The model system is created using a combination of Primary Human Umbilical Vein Endothelial Cells (HUVECs) and osteoblast-like Saos-2 cells encapsulated within a Gelatin Methacrylate (GelMA)-collagen hydrogel blend contained within 3D printed, perfusable constructs. The constructs contain dual channels, within a custom-built bioreactor, that were perfused with osteogenic media for up to two weeks in order to induce mineral deposition. Mineral deposition in constructs containing only HUVECs, only Saos-2 cells, or a combination thereof was quantified by microCT to determine if the combination of endothelial cells and bone-like cells increased mineral deposition. Histological and fluorescent staining was used to verify mineral deposition and cellular function both along and between the perfused channels. While there was not a quantifiable difference in the amount of mineral deposited in Saos-2 only versus Saos-2 plus HUVEC samples, the location of the deposited mineral differed dramatically between the groups and indicated that the addition of HUVECs within the GelMA matrix allowed Saos-2 cells, in diffusion limited regions of the construct, to deposit bone mineral. This work serves as a model on how to create perfusable bone tissue engineering constructs using a combination of 3D printing and cellular co-cultures.

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.

Osteochondral tissue coculture: An in vitro and in silico approach

Osteochondral tissue engineering aims to regenerate functional tissue‐mimicking physiological properties of injured cartilage and its subchondral bone. Given the distinct structural and biochemical difference between bone and cartilage, bilayered scaffolds, and bioreactors are commonly employed. We present an osteochondral culture system which cocultured ATDC5 and MC3T3‐E1 cells on an additive manufactured bilayered scaffold in a dual‐chamber perfusion bioreactor. Also, finite element models (FEM) based on the microcomputed tomography image of the manufactured scaffold as well as on the computer‐aided design (CAD) were constructed; the microenvironment inside the two FEM was studied and compared. In vitro results showed that the coculture system supported osteochondral tissue growth in terms of cell viability, proliferation, distribution, and attachment. In silico results showed that the CAD and the actual manufactured scaffold had significant differences in the flow velocity, differentiation media mixing in the bioreactor and fluid‐induced shear stress experienced by the cells. This system was shown to have the desired microenvironment for osteochondral tissue engineering and it can potentially be used as an inexpensive tool for testing newly developed pharmaceutical products for osteochondral defects.

Assessment of the interplay between scaffold geometry, induced shear stresses, and cell proliferation within a packed bed perfusion bioreactor

By favoring cell proliferation and differentiation, perfusion bioreactors proved efficient at optimizing cell culture. The aim of this study was to quantify cell proliferation within a perfusion bioreactor and correlate it to the wall shear stress (WSS) distribution by combining 3-D imaging and computational fluid dynamics simulations.NIH-3T3 fibroblasts were cultured onto a scaffold model made of impermeable polyacetal spheres or Polydimethylsiloxane cubes. After 1, 2, and 3 weeks of culture, constructs were analyzed by micro-computed tomography (μCT) and quantification of cell proliferation was assessed. After 3 weeks, the volume of cells was found four times higher in the stacking of spheres than in the stacking of cube.3D-μCT reconstruction of bioreactors was used as input for the numerical simulations. Using a lattice-Boltzmann method, we simulated the fluid flow within the bioreactors. We retrieved the WSS distribution (PDF) on the scaffolds surface at the beginning of cultivation and correlated this distribution to the local presence of cells after 3 weeks of cultivation. We found that the WSS distributions strongly differ between spheres and cubes even if the porosity and the specific wetted area of the stackings were very similar. The PDF is narrower and the mean WSS is lower for cubes (11 mPa) than for spheres (20 mPa). For the stacking of spheres, the relative occupancy of the surface sites by cells is maximal when WSS is greater than 20 mPa. For cubes, the relative occupancy is maximal when the WSS is lower than 10 mPa. The discrepancies between spheres and cubes are attributed to the more numerous sites in stacking of spheres that may induce 3-D (multi-layered) proliferation.

Bioreactors Design, Types, Influencing Factors and Potential Application in Dentistry. A Literature Review

Objectives:

A variety of bioreactors and related approaches have been applied to dental tissues as their use has become more essential in the field of regenerative dentistry and dental tissue engineering. The review discusses the various types of bioreactors and their potential application in dentistry.

Methods:

Review of the literature was conducted using keywords (and MeSH) like Bioreactor, Regenerative Dentistry, Fourth Factor, Stem Cells, etc., from the journals published in English. All the searched abstracts, published in indexed journals were read and reviewed to further refine the list of included articles. Based on the relevance of abstracts pertaining to the manuscript, full-text articles were assessed.

Results:

Bioreactors provide a prerequisite platform to create, test, and validate the biomaterials and techniques proposed for dental tissue regeneration. Flow perfusion, rotational, spinner-flask, strain and customize-combined bioreactors have been applied for the regeneration of bone, periodontal ligament, gingiva, cementum, oral mucosa, temporomandibular joint and vascular tissues. Customized bioreactors can support cellular/biofilm growth as well as apply cyclic loading. Center of disease control & dip-flow biofilm-reactors and micro-bioreactor have been used to evaluate the biological properties of dental biomaterials, their performance assessment and interaction with biofilms. Few case reports have also applied the concept of in vivo bioreactor for the repair of musculoskeletal defects and used customdesigned bioreactor (Aastrom) to repair the defects of cleft-palate.

Conclusions:

Bioreactors provide a sterile simulated environment to support cellular differentiation for oro-dental regenerative applications. Also, bioreactors like, customized bioreactors for cyclic loading, biofilm reactors (CDC & drip-flow), and micro-bioreactor, can assess biological responses of dental biomaterials by simultaneously supporting cellular or biofilm growth and application of cyclic stresses.

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

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

Evaluation and Prediction of Mass Transport Properties for Porous Implant with Different Unit Cells: A Numerical Study

Efficient exchange of nutrients and wastes required for cell proliferation and differentiation plays a pivotal role in improving the service life of porous implants. In this study, mass transport properties for porous implant with different unit cells were evaluated and predicted when the porosities are kept the same. To this end, three typical unit cells (diamond (DO), rhombic dodecahedron (RD), and octet truss (OT)) were selected, in which DO displayed diagonal-symmetrical shape, while RD and OT share midline-symmetrical structure. Then, single unit cells were designed quantitatively, and its shape parameters were measured and calculated. Moreover, corresponding porous scaffolds with same outline size were created, respectively. Furthermore, using computational fluid dynamics (CFD) methodology, flow performances with Dulbecco's Modified Eagle's Medium (DMEM) in vitro were simulated for three different porous implants, and flow trajectory, velocity, and wall shear stress which could reflect the properties of mass transfer and tissue regeneration were compared and predicted numerically. Results demonstrated that different unit cell could directly lead to different mass transport properties for porous implant, in spite of same porosity, scaffold size, and service environment. Additionally, by the results, DO displayed greater tortuosity, more appropriate areas, and smoother shear stress distribution than RD and OT, which would provide better surroundings for implant fixation and tissue regeneration. However, RD and OT showed better mass transport properties because of bigger maximum velocity (5.177 mm/s, 4.381 mm/s) than DO (3.941 mm/s). This study would provide great helps for unit cell selection and biological performance optimization for 3D printed bone implants.

Histological Method to Study the Effect of Shear Stress on Cell Proliferation and Tissue Morphology in a Bioreactor

Background

Tissue engineering represents a promising approach for the production of bone substitutes. The use of perfusion bioreactors for the culture of bone-forming cells on a three-dimensional porous scaffold resolves mass transport limitations and provides mechanical stimuli. Despite the recent and important development of bioreactors for tissue engineering, the underlying mechanisms leading to the production of bone substitutes remain poorly understood.

Methods

In order to study cell proliferation in a perfusion bioreactor, we propose a simplified experimental set-up using an impermeable scaffold model made of 2 mm diameter glass beads on which mechanosensitive cells, NIH-3T3 fibroblasts are cultured for up to 3 weeks under 10 mL/min culture medium flow. A methodology combining histological procedure, image analysis and analytical calculations allows the description and quantification of cell proliferation and tissue production in relation to the mean wall shear stress within the bioreactor.

Results

Results show a massive expansion of the cell phase after 3 weeks in bioreactor compared to static control. A scenario of cell proliferation within the three-dimensional bioreactor porosity over the 3 weeks of culture is proposed pointing out the essential role of the contact points between adjacent beads. Calculations indicate that the mean wall shear stress experienced by the cells changes with culture time, from about 50 mPa at the beginning of the experiment to about 100 mPa after 3 weeks.

Conclusion

We anticipate that our results will help the development and calibration of predictive models, which rely on estimates and morphological description of cell proliferation under shear stress.

Bone Tissue Engineering in a Perfusion Bioreactor Using Dexamethasone-Loaded Peptide Hydrogel

The main goal of this study was the formation of bone tissue using dexamethasone (DEX)-loaded [COCH₃]-RADARADARADARADA-[CONH₂] (RADA 16-I) scaffold that has the ability to release optimal DEX concentration under perfusion force. Bone-marrow samples were collected from three patients during a hip arthroplasty. Human mesenchymal stem cells (hMSCs) were isolated and propagated in vitro in order to be seeded on scaffolds made of DEX-loaded RADA 16-I hydrogel in a perfusion bioreactor. DEX concentrations were as follows: 4 × 10^-3, 4 × 10^-4 and 4 × 10^-5 M. After 21 days in a perfusion bioreactor, tissue was analyzed by scanning electron microscopy (SEM) and histology. Markers of osteogenic differentiation were quantified by real-time polymerase chain reaction (RT-PCR) and immunocytochemistry. Minerals were quantified and detected by the von Kossa method. In addition, DEX release from the scaffold in a perfusion bioreactor was assessed. The osteoblast differentiation was confirmed by the expression analysis of osteoblast-related genes (alkaline phosphatase (ALP), collagen I (COL1A1) and osteocalcin (OC). The hematoxylin/eosin staining confirmed the presence of cells and connective tissue, while SEM revealed morphological characteristics of cells, extracellular matrix and minerals-three main components of mature bone tissue. Immunocytochemical detection of collagen I is in concordance with given results, supporting the conclusion that scaffold with DEX concentration of 4 × 10^-4 M has the optimal engineered tissue morphology. The best-engineered bone tissue is produced on scaffold loaded with 4 × 10^-4 M DEX with a perfusion rate of 0.1 mL/min for 21 days. Differentiation of hMSCs on DEX-loaded RADA 16-I scaffold under perfusion force has a high potential for application in regenerative orthopedics.

A parameterised mathematical model to elucidate osteoblast cell growth in a phosphate-glass microcarrier culture

Tissue engineering has the potential to augment bone grafting. Employing microcarriers as cell-expansion vehicles is a promising bottom-up bone tissue engineering strategy. Here we propose a collaborative approach between experimental work and mathematical modelling to develop protocols for growing microcarrier-based engineered constructs of clinically relevant size. Experiments in 96-well plates characterise cell growth with the model human cell line MG-63 using four phosphate glass microcarrier materials. Three of the materials are doped with 5 mol% TiO₂ and contain 0%, 2% or 5% CoO, and the fourth material is doped only with 7% TiO₂ (0% CoO). A mathematical model of cell growth is parameterised by finding material-specific growth coefficients through data-fitting against these experiments. The parameterised mathematical model offers more insight into the material performance by comparing culture outcome against clinically relevant criteria: maximising final cell number starting with the lowest cell number in the shortest time frame. Based on this analysis, material 7% TiO₂ is identified as the most promising.

Effect of cyclic compression on bone marrow mesenchymal stromal cells in tissue engineered cartilage scaffold

In this current study, a novel multilayer porous composite scaffold was fabricated with chitosan (CS), silk fibrin (SF) and nano-hydroxyapatite (n-HA). Scanning electron microscope was utilized to detect the characteristics of the composed scaffold. Rat bone marrow stromal cells (rBMSC) were loaded onto the CS/SF/n-HA scaffold and cultured in a bioreactor under an on-off dynamic compression (10% compressive strain, 0.5 Hz, [2 h action + 4 h pause]/cycle, 4 cycles/day). Metabolism of the loaded rBMSC was assessed through CCK-8 test. Qualitative polymerase chain reaction and western blot were applied to assess the chondrogenic differentiation of the seeded cells. Compressive modulus of the cell/scaffold constructs was analyzed. Additionally, a pig model was employed to evaluate the effect of the tissue-engineered cartilage on repairing of cartilage defect. Results showed that the four layers within the scaffold were tightly connected without gaps between porous interfaces of the layers. Scaffold porosity was 92.20% ± 1.30%. The cyclic compression upregulated chondrogenesis markers (Aggrecan, Sox-9, and collagen II). Increased compressive modulus of the cell/scaffold complex was detected after dynamic compression. The pig bone marrow stromal cells/scaffold complex exposed to cyclic compression presented most favorable reparative effect on the mini pig femoral condyle cartilage defects. Our study suggested that the on-off dynamic compression might be a promising approach to fabricate tissue-engineered cartilage in vitro. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1294-1302, 2019.

Mechanical behaviours and mass transport properties of bone-mimicking scaffolds consisted of gyroid structures manufactured using selective laser melting

Bone scaffolds created in porous structures manufactured using selective laser melting (SLM) are widely used in tissue engineering, since the elastic moduli of the scaffolds are easily adjusted according to the moduli of the tissues, and the large surfaces the scaffolds provide are beneficial to cell growth. SLM-built gyroid structures composed of 316L stainless steel have demonstrated superior properties such as good corrosion resistance, strong biocompatibility, self-supported performance, and excellent mechanical properties. In this study, gyroid structures of different volume fraction were modelled and manufactured using SLM; the mechanical properties of the structures were then investigated under quasi-static compression loads. The elastic moduli and yield stresses of the structures were calculated from stress-strain diagrams, which were developed by conducting quasi-static compression tests. In order to estimate the discrepancies between the designed and as-produced gyroid structures, optical microscopy and micro-CT scanner were used to observe the structures' micromorphology. Since good fluidness is conducive to the transport of nutrients, computational fluid dynamics (CFD) values were used to investigate the pressure and flow velocity of the channel of the three kinds of gyroid structures. The results show that the sizes of the as-produced structures were larger than their computer aided design (CAD) sizes, but the manufacturing errors are within a relatively stable range. The elastic moduli and yield stresses of the structures improved as their volume fractions increased. Gyroid structure can match the mechanical properties of human bone by changing the porosity of scaffold. The process of compression failure showed that 316L gyroid structures manufactured using SLM demonstrated high degrees of toughness. The results obtained from CFD simulation showed that gyroid structures have good fluidity, which has an accelerated effect on the fluid in the middle of the channel, and it is suitable for transport nutrients. Therefore, we could predict the scaffold's permeability by conducting CFD simulation to ensure an appropriate permeability before the scaffold being manufactured. SLM-built gyroid structures that composed of 316L stainless steel were suitable to be designed as bone scaffolds in terms of mechanical properties and mass-transport properties, and had significant promise.

Architectural Design of 3D Printed Scaffolds Controls the Volume and Functionality of Newly Formed Bone

The successful regeneration of functional bone tissue in critical-size defects remains a significant clinical challenge. To address this challenge, synthetic bone scaffolds are widely developed, but remarkably few are translated to the clinic due to poor performance in vivo. Here, it is demonstrated how architectural design of 3D printed scaffolds can improve in vivo outcomes. Ceramic scaffolds with different pore sizes and permeabilities, but with similar porosity and interconnectivity, are implanted in rabbit calvaria for 12 weeks, and then the explants are harvested for microcomputed tomography evaluation of the volume and functionality of newly formed bone. The results indicate that scaffold pores should be larger than 390 µm with an upper limit of 590 µm to enhance bone formation. It is also demonstrated that a bimodal pore topology-alternating large and small pores-enhances the volume and functionality of new bone substantially. Moreover, bone formation results indicate that stiffness of new bone is highly influenced by the scaffold's permeability in the direction concerned. This study demonstrates that manipulating pore size and permeability in a 3D printed scaffold architecture provides a useful strategy for enhancing bone regeneration outcomes.

Mathematical and computational models for bone tissue engineering in bioreactor systems

Research into cellular engineered bone grafts offers a promising solution to problems associated with the currently used auto- and allografts. Bioreactor systems can facilitate the development of functional cellular bone grafts by augmenting mass transport through media convection and shear flow-induced mechanical stimulation. Developing successful and reproducible protocols for growing bone tissue in vitro is dependent on tuning the bioreactor operating conditions to the specific cell type and graft design. This process, largely reliant on a trial-and-error approach, is challenging, time-consuming and expensive. Modelling can streamline the process by providing further insight into the effect of the bioreactor environment on the cell culture, and by identifying a beneficial range of operational settings to stimulate tissue production. Models can explore the impact of changing flow speeds, scaffold properties, and nutrient and growth factor concentrations. Aiming to act as an introductory reference for bone tissue engineers looking to direct their experimental work, this article presents a comprehensive framework of mathematical models on various aspects of bioreactor bone cultures and overviews modelling case studies from literature.

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

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