Effects of flow shear stress and mass transport on the construction of a large-scale tissue-engineered bone in a perfusion bioreactor

3D bioprinted microfluidic based osteosarcoma-on-a chip model as a physiomimetic pre-clinical drug testing platform for anti-cancer drugs

Standard chemotherapeutic regimen for osteosarcoma (OS) treatment often leads to poor therapeutic outcome, primarily due to lack of an adequate representative model reflecting native OS structural and cellular complexity, posing a translational gap. Three-dimensional bioprinting (3D-BP) represents an efficient and advanced technique for precise recapitulation of the structural and cellular complexity of OS tumor microenvironment (TME). In the present study, we employed a dual extrusion-based 3D-BP method to develop an improved in vitro OS model consisting of both tumor and stromal components. Additionally, a human physiomimetic microfluidic bioreactor is introduced to mimic the dynamic TME and provide physiologically relevant mechanical stimulation to the cells. The model named TC-OS Dynamic model, demonstrated close resemblance to native OS-TME, validated by in vitro studies. Continuous media flow provided mechanical stimulation in the form of shear stress, positively influencing the growth and aggressiveness of OS. Further, drug screening with the model anticancer drugs (doxorubicin, cis-platin, sorafenib) demonstrated enhanced sensitivity in TC-OS Dynamic model as compared to TC-OS Static model, emphasizing enhanced mass transfer, availability and distribution of anticancer drug due to continuous media flow. Overall, TC-OS Dynamic model holds significant potential as a platform in future for high throughput pre-clinical screening of anticancer drugs.

Improvement in Active Cell Proliferation Area at Higher Permeability With Novel TPMS Lattice Structure

The utilization of lattice-based scaffolds emerging as an advance technique over conventional bio-implants in Bone Tissue Engineering. In this study, totally six lattice structures are considered for permeability and wall shear stress (WSS) investigation. Namely triply periodic minimal surfaces (TPMS)-based Gyroid, Schwarz-P, Schwarz-D, and two beam-based structure-Cubic and Fluorite are compared with the proposed new lattice structure at porosity level of 80%, 75%, and 70%. The proposed new lattice has combine characteristic of Gyroid and Schwarz-D TPMS lattice. The permeability is determined through Darcy's law, where the pressure drop across the lattice structure is calculated using a computational fluid dynamics (CFD) tool at flowrate between 0.2 and 10 ml/min. The Cubic and Schwarz-P lattice structures exhibited the highest permeability but at the cost of a lower active surface area for WSS, measuring below 155 mm2, means least cell proliferation occurs while the permeability value in New Lattice structure is in the ideal range with the enhanced active surface area for WSS (514 mm2). The complex internal curvatures of New Lattice promote the cell proliferation while the through-pore holes allow the efficient cell seeding.

Determination of the Optimum Architecture of Additively Manufactured Magnetic Bioactive Glass Scaffolds for Bone Tissue Engineering and Drug-Delivery Applications

For better bone regeneration, precise control over the architecture of the scaffolds is necessary. Because the shape of the pore may affect the bone regeneration, therefore, additive manufacturing has been used in this study to fabricate magnetic bioactive glass (MBG) scaffolds with three different architectures, namely, grid, gyroid, and Schwarz D surface with 15 × 15 × 15 mm3 dimensions and 70% porosity. These scaffolds have been fabricated using an in-house-developed material-extrusion-based additive manufacturing system. The composition of bioactive glass was selected as 45% SiO2, 20% Na2O, 23% CaO, 6% P2O5, 2.5% B2O3, 1% ZnO, 2% MgO, and 0.5% CaF2 (wt %), and additionally 0.4 wt % of iron carbide nanoparticles were incorporated. Afterward, MBG powder was mixed with a 25% (w/v) Pluronic F-127 solution to prepare a slurry for fabricating scaffolds at 23% relative humidity. The morphological characterization using microcomputed tomography revealed the appropriate pore size distribution and interconnectivity of the scaffolds. The compressive strengths of the fabricated grid, gyroid, and Schwarz D scaffolds were found to be 14.01 ± 1.01, 10.78 ± 1.5, and 12.57 ± 1.2 MPa, respectively. The in vitro study was done by immersing the MBG scaffolds in simulated body fluid for 1, 3, 7, and 14 days. Darcy's law, which describes the flow through porous media, was used to evaluate the permeability of the scaffolds. Furthermore, an anticancer drug (Mitomycin C) was loaded onto these scaffolds, wherein these scaffolds depicted good release behavior. Overall, gyroid-structured scaffolds were found to be the most suitable among the three scaffolds considered in this study for bone tissue engineering and drug-delivery applications.

Advances in Shear Stress Stimulation of Stem Cells: A Review of the Last Three Decades

Stem cells are widely used in scientific research because of their ability to self-renew and differentiate into a variety of specialized cell types needed for body functions. However, the self-renewal and differentiation of stem cells are regulated by various stimuli, with mechanical stimulation being particularly notable due to its ability to mimic the physical environment in the body. This study systematically collected 2638 research papers published between 1994 and 2024, employing tools such as VOSviewer, CiteSpace, and GraphPad Prism to uncover research hotspots, publication trends, and collaboration networks. The results indicate a yearly increase in global research on the shear stress stimulation of stem cells, with significant contributions from the United States and China in terms of research investment and output. Future research directions include a deeper understanding of the mechanisms underlying mechanical stimulation's effects on stem cell differentiation, the development of new materials and scaffold designs to better replicate the natural cellular environment, and advancements in regenerative medicine. Despite considerable progress, challenges remain in translating basic research findings into clinical applications.

Comparing the bone regeneration potential between a trabecular bone and a porous scaffold through osteoblast migration and differentiation: A multiscale approach

Both cell migration and osteogenic differentiation are critical for successful bone regeneration. Therefore, understanding the mechanobiological aspects that govern these two processes is essential in designing effective scaffolds that promote faster bone regeneration. Studying these two factors at different locations is necessary to manage bone regeneration in various sections of a scaffold. Hence, a multiscale computational model was used to observe the mechanical responses of osteoblasts placed in different positions of the trabecular bone and gyroid scaffold. Fluid shear stresses in scaffolds at cell seeded locations (representing osteogenic differentiation) and strain energy densities in cells at cell substrate interface (representing cell migration) were observed as mechanical response parameters in this study. Comparison of these responses, as two critical factors for bone regeneration, between the trabecular bone and gyroid scaffold at different locations, is the overall goal of the study. This study reveals that the gyroid scaffold exhibits higher osteogenic differentiation and cell migration potential compared to the trabecular bone. However, the responses in the gyroid only mimic the trabecular bone in two out of nine positions. These findings can guide us in predicting the ideal cell seeded sites within a scaffold for better bone regeneration and in replicating a replaced bone condition by altering the physical parameters of a scaffold.

Scaffold geometry and computational fluid dynamics simulation supporting osteogenic differentiation in dynamic culture

Geometry of porous scaffolds is critical to the success of cell adhesion, proliferation, and differentiation in bone tissue engineering. In this study, the effect of scaffold geometry on osteogenic differentiation of MC3T3-E1 pre-osteoblasts in a perfusion bioreactor was investigated. Three geometries of oligolactide-HA scaffolds, named Woodpile, LC-1000, and LC-1400, were fabricated with uniform pore size distribution and interconnectivity using stereolithography (SL) technique, and tested to evaluate for the most suitable scaffold geometry. Compressive tests revealed sufficiently high strength of all scaffolds to support new bone formation. The LC-1400 scaffold showed the highest cell proliferation in accordance with the highest level of osteoblast-specific gene expression after 21 days of dynamic culture in a perfusion bioreactor; however, it deposited less amount of calcium than the LC-1000 scaffold. Computational fluid dynamics (CFD) simulation was employed to predict and explain the effect of flow behavior on cell response under dynamic culture. The findings concluded that appropriate flow shear stress enhanced cell differentiation and mineralization in the scaffold, with the LC-1000 scaffold performing best due to its optimal balance between permeability and flow-induced shear stress.

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.

Evaluating the cell migration potential of TiO2 nanorods incorporated in a Ti6Al4V scaffold: A multiscale approach

Improvement of cell migration by the nano-topographical modification of implant surface can directly or indirectly accelerate wound healing and osseointegration between bone and implant. Therefore, modification of the implant surface was done with TiO2 nanorod (NR) arrays to develop a more osseointegration-friendly implant in this study. Modulating the migration of a cell, adhered to a scaffold, by the variations of NR diameter, density and tip diameter in vitro is the primary objective of the study. The fluid structure interaction method was used, followed by the submodelling technique in this multiscale analysis. After completing a simulation over a global model, fluid structure interaction data was applied to the sub-scaffold finite element model to predict the mechanical response over cells at the cell-substrate interface. Special focus was given to strain energy density at the cell interface as a response parameter due to its direct correlation with the migration of an adherent cell. The results showed a huge rise in strain energy density after the addition of NRs on the scaffold surface. It also highlighted that variation in NR density plays a more effective role than the variation in NR diameter to control cell migration over a substrate. However, the effect of NR diameter becomes insignificant when the NR tip was considered. The findings of this study could be used to determine the best nanostructure parameters for better osseointegration.

Design and 3D Printing of Personalized Hybrid and Gradient Structures for Critical Size Bone Defects

Treating critical-size bone defects with autografts, allografts, or standardized implants is challenging since the healing of the defect area necessitates patient-specific grafts with mechanically and physiologically relevant structures. Three-dimensional (3D) printing using computer-aided design (CAD) is a promising approach for bone tissue engineering applications by producing constructs with customized designs and biomechanical compositions. In this study, we propose 3D printing of personalized and implantable hybrid active scaffolds with a unique architecture and biomaterial composition for critical-size bone defects. The proposed 3D hybrid construct was designed to have a gradient cell-laden poly(ethylene glycol) (PEG) hydrogel, which was surrounded by a porous polycaprolactone (PCL) cage structure to recapitulate the anatomical structure of the defective area. The optimized PCL cage design not only provides improved mechanical properties but also allows the diffusion of nutrients and medium through the scaffold. Three different designs including zigzag, zigzag/spiral, and zigzag/spiral with shifting the zigzag layers were evaluated to find an optimal architecture from a mechanical point of view and permeability that can provide the necessary mechanical strength and oxygen/nutrient diffusion, respectively. Mechanical properties were investigated experimentally and analytically using finite element analysis (FEA), and computational fluid dynamics (CFD) simulation was used to determine the permeability of the structures. A hybrid scaffold was fabricated via 3D printing of the PCL cage structure and a PEG-based bioink comprising a varying number of human bone marrow mesenchymal stem cells (hBMSCs). The gradient bioink was deposited inside the PCL cage through a microcapillary extrusion to generate a mineralized gradient structure. The zigzag/spiral design for the PCL cage was found to be mechanically strong with sufficient and optimum nutrient/gas axial and radial diffusion while the PEG-based hydrogel provided a biocompatible environment for hBMSC viability, differentiation, and mineralization. This study promises the production of personalized constructs for critical-size bone defects by printing different biomaterials and gradient cells with a hybrid design depending on the need for a donor site for implantation.

Advances in In Vitro and In Vivo Bioreactor-Based Bone Generation for Craniofacial Tissue Engineering

Craniofacial reconstruction requires robust bone of specified geometry for the repair to be both functional and aesthetic. While native bone from elsewhere in the body can be harvested, shaped, and implanted within a defect, using either an in vitro or in vivo bioreactors eliminates donor site morbidity while increasing the customizability of the generated tissue. In vitro bioreactors utilize cells harvested from the patient, a scaffold, and a device to increase mass transfer of nutrients, oxygen, and waste, allowing for generation of larger viable tissues. In vivo bioreactors utilize the patient's own body as a source of cells and of nutrient transfer and involve the implantation of a scaffold with or without growth factors adjacent to vasculature, followed by the eventual transfer of vascularized, mineralized tissue to the defect site. Several different models of in vitro bioreactors exist, and several different implantation sites have been successfully utilized for in vivo tissue generation and defect repair in humans. In this review, we discuss the specifics of each bioreactor strategy, as well as the advantages and disadvantages of each and the future directions for the engineering of bony tissues for craniofacial defect repair.

Improvement of the cell viability of hepatocytes cultured in three-dimensional collagen gels using pump-free perfusion driven by water level difference

Cell-containing collagen gels are one of the materials employed in tissue engineering and drug testing. A collagen gel is a useful three-dimensional (3D) scaffold that improves various cell functions compared to traditional two-dimensional plastic substrates. However, owing to poor nutrient availability, cells are not viable in thick collagen gels. Perfusion is an effective method for supplying nutrients to the gel. In this study, we maintained hepatocytes embedded in a 3D collagen gel using a simple pump-free perfusion cell culture system with ordinary cell culture products. Flow was generated by the difference in water level in the culture medium. Hepatocytes were found to be viable in a collagen gel of thickness 3.26 (± 0.16 S.E.)-mm for 3 days. In addition, hepatocytes had improved proliferation and gene expression related to liver function in a 3D collagen gel compared to a 2D culture dish. These findings indicate that our perfusion method is useful for investigating the cellular functions of 3D hydrogels.

Culture of 3D Bioprinted Bone Constructs Requires an Increased Fluid Dynamic Stimulation

In vitro flow-induced mechanical stimulation of developing bone tissue constructs has been shown to favor mineral deposition in scaffolds seeded with cells directly exposed to the fluid flow. However, the effect of fluid dynamic parameters, such as shear stress (SS), within 3D bioprinted constructs is still unclear. Thus, this study aimed at correlating the SS levels and the mineral deposition in 3D bioprinted constructs, evaluating the possible dampening effect of the hydrogel. Human mesenchymal stem cells (hMSCs) were embedded in 3D bioprinted porous structures made of alginate and gelatin. 3D bioprinted constructs were cultured in an osteogenic medium assessing the influence of different flow rates (0, 0.7 and 7 ml/min) on calcium and collagen deposition through histology, and bone volume (BV) through micro-computed tomography. Uniform distribution of calcium and collagen was observed in all groups. Nevertheless, BV significantly increased in perfused groups as compared to static control, ranging from 0.35±0.28 mm³, 11.90±8.74 mm³ and 25.81±5.02 mm³ at week 3 to 2.28±0.78 mm³, 22.55±2.45 mm³ and 46.05±5.95 mm³ at week 6 in static, 0.7 and 7 ml/min groups, respectively. SS values on construct fibers in the range 10-100 mPa in 7 ml/min samples were twice as high as those in 0.7 ml/min samples showing the same trend of BV. The obtained results suggest that it is necessary to enhance the flow-induced mechanical stimulation of cell-embedding hydrogels to increase the amount of mineral deposited by hMSCs, compared to what is generally reported for the development of in vitro bone constructs. STATEMENT OF SIGNIFICANCE: : Culture of 3D Bioprinted Bone Constructs Requires an Increased Fluid Dynamic Stimulation, In this study, we evaluated for the first time how the hydrogel structure dampens the effect of flow-induced mechanical stimulation during the culture of 3D bioprinted bone tissue constructs. By combining computational and experimental techniques we demonstrated that those shear stress thresholds generally considered for culturing cells seeded on scaffold surface, are no longer applicable when cells are embedded in 3D bioprinted constructs. Significantly, more bone volume was formed in constructs exposed to shear stress values generally considered as detrimental than in constructs exposed shear stress values generally considered as beneficial after 3 weeks and 6 weeks of dynamic culture using a perfusion bioreactor.

Biomechanics and mechanobiology of the bone matrix

The bone matrix plays an indispensable role in the human body, and its unique biomechanical and mechanobiological properties have received much attention. The bone matrix has unique mechanical anisotropy and exhibits both strong toughness and high strength. These mechanical properties are closely associated with human life activities and correspond to the function of bone in the human body. None of the mechanical properties exhibited by the bone matrix is independent of its composition and structure. Studies on the biomechanics of the bone matrix can provide a reference for the preparation of more applicable bone substitute implants, bone biomimetic materials and scaffolds for bone tissue repair in humans, as well as for biomimetic applications in other fields. In providing mechanical support to the human body, bone is constantly exposed to mechanical stimuli. Through the study of the mechanobiology of the bone matrix, the response mechanism of the bone matrix to its surrounding mechanical environment can be elucidated and used for the health maintenance of bone tissue and defect regeneration. This paper summarizes the biomechanical properties of the bone matrix and their biological significance, discusses the compositional and structural basis by which the bone matrix is capable of exhibiting these mechanical properties, and studies the effects of mechanical stimuli, especially fluid shear stress, on the components of the bone matrix, cells and their interactions. The problems that occur with regard to the biomechanics and mechanobiology of the bone matrix and the corresponding challenges that may need to be faced in the future are also described.

3D printed scaffold design for bone defects with improved mechanical and biological properties

Bone defect treatment is still a challenge in clinics, and synthetic bone scaffolds with adequate mechanical and biological properties are highly needed. Adequate waste and nutrient exchange of the implanted scaffold with the surrounded tissue is a major concern. Moreover, the risk of mechanical instability in the defect area during regular activity increases as the defect size increases. Thus, scaffolds with better mass transportation and mechanical properties are desired. This study introduces 3D printed polymeric scaffolds with a continuous pattern, ZigZag-Spiral pattern, for bone defects treatments. This pattern has a uniform distribution of pore size, which leads to uniform distribution of wall shear stress which is crucial for uniform differentiation of cells attached to the scaffolds. The mechanical, mass transportation, and biological properties of the 3D printed scaffolds are evaluated. The results show that the presented scaffolds have permeability similar to natural bone and, with the same porosity level, have higher mechanical properties than scaffolds with conventional lay-down patterns 0-90° and 0-45°. Finally, human mesenchymal stem cells are seeded on the scaffolds to determine the effects of geometrical microstructure on cell attachment and morphology. The results show that cells in scaffold with ZigZag-Spiral pattern infilled pores gradually, while the other patterns need more time to fill the pores. Considering mechanical, transportation, and biological properties of the considered patterns, scaffolds with ZigZag-Spiral patterns can mimic the properties of cancellous bones and be a better choice for treatments of bone defects.

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.

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.

Mechanical Stimulation on Mesenchymal Stem Cells and Surrounding Microenvironments in Bone Regeneration: Regulations and Applications

Treatment of bone defects remains a challenge in the clinic. Artificial bone grafts are the most promising alternative to autologous bone grafting. However, one of the limiting factors of artificial bone grafts is the limited means of regulating stem cell differentiation during bone regeneration. As a weight-bearing organ, bone is in a continuous mechanical environment. External mechanical force, a type of biophysical stimulation, plays an essential role in bone regeneration. It is generally accepted that osteocytes are mechanosensitive cells in bone. However, recent studies have shown that mesenchymal stem cells (MSCs) can also respond to mechanical signals. This article reviews the mechanotransduction mechanisms of MSCs, the regulation of mechanical stimulation on microenvironments surrounding MSCs by modulating the immune response, angiogenesis and osteogenesis, and the application of mechanical stimulation of MSCs in bone regeneration. The review provides a deep and extensive understanding of mechanical stimulation mechanisms, and prospects feasible designs of biomaterials for bone regeneration and the potential clinical applications of mechanical stimulation.

Porous Geometry Guided Micro-mechanical Environment Within Scaffolds for Cell Mechanobiology Study in Bone Tissue Engineering

Mechanobiology research is for understanding the role of mechanics in cell physiology and pathology. It will have implications for studying bone physiology and pathology and to guide the strategy for regenerating both the structural and functional features of bone. Mechanobiological studies in vitro apply a dynamic micro-mechanical environment to cells via bioreactors. Porous scaffolds are commonly used for housing the cells in a three-dimensional (3D) culturing environment. Such scaffolds usually have different pore geometries (e.g. with different pore shapes, pore dimensions and porosities). These pore geometries can affect the internal micro-mechanical environment that the cells experience when loaded in the bioreactor. Therefore, to adjust the applied micro-mechanical environment on cells, researchers can tune either the applied load and/or the design of the scaffold pore geometries. This review will provide information on how the micro-mechanical environment (e.g. fluid-induced wall shear stress and mechanical strain) is affected by various scaffold pore geometries within different bioreactors. It shall allow researchers to estimate/quantify the micro-mechanical environment according to the already known pore geometry information, or to find a suitable pore geometry according to the desirable micro-mechanical environment to be applied. Finally, as future work, artificial intelligent - assisted techniques, which can achieve an automatic design of solid porous scaffold geometry for tuning/optimising the micro-mechanical environment are suggested.

Optimized Method to Improve Cell Activity in 3D Scaffolds Under a Dual Real-Time Dynamic Bioreactor System

Bioreactor systems that allow the simulation of in vivo variables in a controlled in vitro environment, were a great advance in the field of tissue engineering. Due to the dynamic-mechanical features that some tissues present, 3D-engineered constructs often do not exhibit the biomechanical properties of these native tissues. Thus, a successful approach must not only achieve tissue repair but also restore its function after injury. Here, we describe a method to improve cell activity in 3D scaffolds in a dynamic bioreactor system through the application of mechanical compression and fluid flow for tissue engineering approaches.

The effects of biophysical stimulation on osteogenic differentiation and the mechanisms from ncRNAs

Ample proof showed that non-coding RNAs (ncRNAs) play a crucial role in proliferation and differentiation of osteoblasts and bone marrow stromal cells (BMSCs). Varied forms of biophysical stimuli like mechanical strain, fluid shear stress (FSS), microgravity and vibration are verified to regulate ncRNAs expression in osteogenic differentiation and influence the expression of target genes associated with osteogenic differentiation and ultimately regulate bone formation. The consequences of biophysical stimulation on osteogenic differentiation validate the prospect of exercise for the prevention and treatment of osteoporosis. In this review, we tend to summarize the studies on regulation of osteogenic differentiation by ncRNAs beneath biophysical stimulation and facilitate to reveal the regulatory mechanism of biophysical stimulation on ncRNAs, and provide an update for the prevention of bone metabolism diseases by exercise.

Enhanced osteogenesis of 3D printed β-TCP scaffolds with Cissus Quadrangularis extract-loaded polydopamine coatings

Growing demand in bone tissue replacement has shifted treatment strategy from pursuing traditional autogenous and allogeneic grafts to tissue replacement with bioactive biomaterials. Constructs that exhibit the ability to support the bone structure while encouraging tissue regeneration, integration, and replacement represent the future of bone tissue engineering. The present study aimed to understand the osteogenic and mechanical effects of binder jet 3D printed, porous β-tricalcium phosphate scaffolds modified with a natural polymer/drug coating of polydopamine and Cissus Quadrangularis extract. Compression testing was used to determine the effect the polydopamine coating process had on the mechanical strength of the scaffolds. 3D printed scaffolds with and without polydopamine coatings fractured at 3.88 ± 0.51 MPa and 3.84 ± 1 MPa, respectively, suggesting no detrimental effect on strength due to the coating process. The osteogenic potential of the extract-loaded coating was tested in vitro, under static and dynamic flow conditions, and in vivo in a rat distal femur model. Static osteoblast cultures indicated polydopamine-coated samples with and without the extract exhibited greater proliferation after 3 days (p < 0.05). Similarly, polydopamine resulted in increased proliferation and alkaline phosphatase expression under dynamic flow, but alkaline phosphatase expression was significantly enhanced (p < 0.05) only in samples treated with the extract. Histological analysis of implanted scaffolds showed substantially more new bone growth throughout the implant pores at 4 weeks post-op in polydopamine and extract-loaded implants compared to pure β-tricalcium phosphate. These results indicated that implants coated with polydopamine and Cissus Quadrangularis extract facilitated osteoblast proliferation and alkaline phosphatase production and improved early bone formation and ingrowth while maintaining mechanical strength.

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.

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.

Changes in scaffold porosity during bone tissue engineering in perfusion bioreactors considerably affect cellular mechanical stimulation for mineralization

Bone tissue engineering (BTE) experiments in vitro have shown that fluid-induced wall shear stress (WSS) can stimulate cells to produce mineralized extracellular matrix (ECM). The application of WSS on seeded cells can be achieved through bioreactors that perfuse medium through porous scaffolds. In BTE experiments in vitro, commonly a constant flow rate is used. Previous studies have found that tissue growth within the scaffold will result in an increase of the WSS over time. To keep the WSS in a reported optimal range of 10–30 mPa, the applied external flow rate can be decreased over time. To investigate what reduction of the external flow rate during culturing is needed to keep the WSS in the optimal range, we here conducted a computational study, which simulated the formation of ECM, and in which we investigated the effect of constant fluid flow and different fluid flow reduction scenarios on the WSS. It was found that for both constant and reduced fluid flow scenarios, the WSS did not exceed a critical value, which was set to 60 mPa. However, the constant flow velocity resulted in a reduction of the cell/ECM surface being exposed to a WSS in the optimal range from 50% at the start of culture to 18.6% at day 21. Reducing the fluid flow over time could avoid much of this effect, leaving the WSS in the optimal range for 40.9% of the surface at 21 days. Therefore, for achieving more mineralized tissue, the conventional manner of loading the perfusion bioreactors (i.e. constant flow rate/velocity) should be changed to a decreasing flow over time in BTE experiments. This study provides an in silico tool for finding the best fluid flow reduction strategy.

Fluid flow-induced cell stimulation in bone tissue engineering changes due to interstitial tissue formation in vitro

In tissue engineering experiments in vitro, bioreactors have been used for applying wall shear stress (WSS) on cells to regulate cellular activities. To determine the loading conditions within bioreactors and to design tissue engineering products, in silico models are used. Previous in silico studies in bone tissue engineering (BTE) focused on quantifying the WSS on cells and the influence on appositional tissue growth. However, many BTE experiments also show interstitial tissue formation (i.e., tissue infiltrated in the pores rather than growing on the struts - appositional growth), which has not been considered in previous in silico studies. We hereby used a multiscale fluid-solid interaction model to quantify the WSS and mechanical strain on cells with interstitial tissue formation, taken from a reported BTE experiment. The WSS showed a high variation among different interstitial tissue morphologies. This is different to the situation under appositional tissue growth. It is found that a 35% filling of the pores results (by mineralised bone tissue) when the average WSS increases from 1.530 (day 0) to 5.735 mPa (day 28). Furthermore, the mechanical strain on cells caused by the fluid flow was extremely low (at the level of 10^-14 -10^-15 ), comparing to the threshold in a previous mechanobiological theory of osteogenesis (eg, 10^-2 ). The output from this study offers a significant insight of the WSS changes during interstitial tissue growth under a constant perfusion flow rate in a BTE experiment. It has paved the way for optimising the local micro-fluidic environment for interstitial tissue mineralisation.

Effects of Flow Rate on Mesenchymal Stem Cell Oxygen Consumption Rates in 3D Bone-Tissue-Engineered Constructs Cultured in Perfusion Bioreactor Systems

Bone grafts represent a multibillion-dollar industry, with over a million grafts occurring each year. Common graft types are associated with issues such as donor site morbidity in autologous grafts and immunological response in allogenic grafts. Bone-tissue-engineered constructs are a logical approach to combat the issues commonly encountered with these bone grafting techniques. When creating bone-tissue-engineered constructs, monitoring systems are required to determine construct characteristics, such as cellularity and cell type. This study aims to expand on the current predictive metrics for these characteristics, specifically analyzing the effects of media flow rate on oxygen uptake rates (OURs) of mesenchymal stem cells seeded on poly(L-lactic acid) (PLLA) scaffolds cultured in a flow perfusion bioreactor. To do this, oxygen consumption rates were measured for cell/scaffold constructs at varying flow rates ranging from 150 to 750 microliters per minute. Residence time analyses were performed for this bioreactor at these flow rates. Average observed oxygen uptake rates of stem cells in perfusion bioreactors were shown to increase with increased oxygen availability at higher flow rates. The residence time analysis helped identify potential pitfalls in current bioreactor designs, such as the presence of channeling. Furthermore, this analysis shows that oxygen uptake rates have a strong linear correlation with residence times of media in the bioreactor setup, where cells were seen to exhibit a maximum oxygen uptake rate of 3 picomoles O2/hr/cell.

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.

How Biophysical Forces Regulate Human B Cell Lymphomas

The role of microenvironment-mediated biophysical forces in human lymphomas remains elusive. Diffuse large B cell lymphomas (DLBCLs) are heterogeneous tumors, which originate from highly proliferative germinal center B cells. These tumors, their associated neo-vessels, and lymphatics presumably expose cells to particular fluid flow and survival signals. Here, we show that fluid flow enhances proliferation and modulates response of DLBCLs to specific therapeutic agents. Fluid flow upregulates surface expression of B cell receptors (BCRs) and integrin receptors in subsets of ABC-DLBCLs with either CD79A/B mutations or WT BCRs, similar to what is observed with xenografted human tumors in mice. Fluid flow differentially upregulates signaling targets, such as SYK and p70S6K, in ABC-DLBCLs. By selective knockdown of CD79B and inhibition of signaling targets, we provide mechanistic insights into how fluid flow mechanomodulates BCRs and integrins in ABC-DLBCLs. These findings redefine microenvironment factors that regulate lymphoma-drug interactions and will be critical for testing targeted therapies.

The Effects of Different Dynamic Culture Systems on Cell Proliferation and Osteogenic Differentiation in Human Mesenchymal Stem Cells

The culture environment plays an important role for stem cells' cultivation. Static or dynamic culture preserve differential potentials to affect human mesenchymal stem cells' (hMSCs) proliferation and differentiation. In this study, hMSCs were seeded on fiber disks and cultured in a bidirectional-flow bioreactor or spinner-flask bioreactor with a supplement of osteogenic medium. The hMSCs' proliferation, osteogenic differentiation, and extracellular matrix deposition of mineralization were demonstrated. The results showed that the spinner flask improved cell viability at the first two weeks while the bidirectional-flow reactor increased the cell proliferation of hMSCs through the four-week culture period. Despite the flow reactor having a higher cell number, a lower lactose/glucose ratio was noted, revealing that the bidirectional-flow bioreactor provides better oxygen accessibility to the cultured cells/disk construct. The changes of calcium ions in the medium, the depositions of Ca²⁺ in the cells/disk constructs, and alkaline phosphate/osteocalcin activities showed the static culture of hMSCs caused cells to mineralize faster than the other two bioreactors but without cell proliferation. Otherwise, cells were distributed uniformly with abundant extracellular matrix productions using the flow reactor. This reveals that the static and dynamic cultivations regulated the osteogenic process differently in hMSCs. The bidirectional-flow bioreactor can be used in the mass production and cultivation of hMSCs for applications in bone regenerative medicine.

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.

A multiscale computational fluid dynamics approach to simulate the micro-fluidic environment within a tissue engineering scaffold with highly irregular pore geometry

Mechanical stimulation can regulate cellular behavior, e.g., differentiation, proliferation, matrix production and mineralization. To apply fluid-induced wall shear stress (WSS) on cells, perfusion bioreactors have been commonly used in tissue engineering experiments. The WSS on cells depends on the nature of the micro-fluidic environment within scaffolds under medium perfusion. Simulating the fluidic environment within scaffolds will be important for gaining a better insight into the actual mechanical stimulation on cells in a tissue engineering experiment. However, biomaterial scaffolds used in tissue engineering experiments typically have highly irregular pore geometries. This complexity in scaffold geometry implies high computational costs for simulating the precise fluidic environment within the scaffolds. In this study, we propose a low-computational cost and feasible technique for quantifying the micro-fluidic environment within the scaffolds, which have highly irregular pore geometries. This technique is based on a multiscale computational fluid dynamics approach. It is demonstrated that this approach can capture the WSS distribution in most regions within the scaffold. Importantly, the central process unit time needed to run the model is considerably low.

Biomimetic delivery of signals for bone tissue engineering

Bone tissue engineering is an exciting approach to directly repair bone defects or engineer bone tissue for transplantation. Biomaterials play a pivotal role in providing a template and extracellular environment to support regenerative cells and promote tissue regeneration. A variety of signaling cues have been identified to regulate cellular activity, tissue development, and the healing process. Numerous studies and trials have shown the promise of tissue engineering, but successful translations of bone tissue engineering research into clinical applications have been limited, due in part to a lack of optimal delivery systems for these signals. Biomedical engineers are therefore highly motivated to develop biomimetic drug delivery systems, which benefit from mimicking signaling molecule release or presentation by the native extracellular matrix during development or the natural healing process. Engineered biomimetic drug delivery systems aim to provide control over the location, timing, and release kinetics of the signal molecules according to the drug's physiochemical properties and specific biological mechanisms. This article reviews biomimetic strategies in signaling delivery for bone tissue engineering, with a focus on delivery systems rather than specific molecules. Both fundamental considerations and specific design strategies are discussed with examples of recent research progress, demonstrating the significance and potential of biomimetic delivery systems for bone tissue engineering.

Modelling mesenchymal stromal cell growth in a packed bed bioreactor with a gas permeable wall

A mathematical model was developed for mesenchymal stromal cell (MSC) growth in a packed bed bioreactor that improves oxygen availability by allowing oxygen diffusion through a gas-permeable wall. The governing equations for oxygen, glucose and lactate, the inhibitory waste product, were developed assuming Michaelis-Menten kinetics, together with an equation for the medium flow based on Darcy's Law. The conservation law for the cells includes the effects of inhibition as the cells reach confluence, nutrient and waste product concentrations, and the assumption that the cells can migrate on the scaffold. The equations were solved using the finite element package, COMSOL. Previous experimental results collected using a packed bed bioreactor with gas permeable walls to expand MSCs produced a lower cell yield than was obtained using a traditional cell culture flask. This mathematical model suggests that the main contributors to the observed low cell yield were a non-uniform initial cell seeding profile and a potential lag phase as cells recovered from the initial seeding procedure. Lactate build-up was predicted to have only a small effect at lower flow rates. Thus, the most important parameters to optimise cell expansion in the proliferation of MSCs in a bioreactor with gas permeable wall are the initial cell seeding protocol and the handling of the cells during the seeding process. The mathematical model was then used to identify and characterise potential enhancements to the bioreactor design, including incorporating a central gas permeable capillary to further enhance oxygen availability to the cells. Finally, to evaluate the issues and limitations that might be encountered scale-up of the bioreactor, the mathematical model was used to investigate modifications to the bioreactor design geometry and packing density.

Flow rates in perfusion bioreactors to maximise mineralisation in bone tissue engineering in vitro

In bone tissue engineering experiments, fluid-induced shear stress is able to stimulate cells to produce mineralised extracellular matrix (ECM). The application of shear stress on seeded cells can for example be achieved through bioreactors that perfuse medium through porous scaffolds. The generated mechanical environment (i.e. wall shear stress: WSS) within the scaffolds is complex due to the complexity of scaffold geometry. This complexity has so far prevented setting an optimal loading (i.e. flow rate) of the bioreactor to achieve an optimal distribution of WSS for stimulating cells to produce mineralised ECM. In this study, we demonstrate an approach combining computational fluid dynamics (CFD) and mechano-regulation theory to optimise flow rates of a perfusion bioreactor and various scaffold geometries (i.e. pore shape, porosity and pore diameter) in order to maximise shear stress induced mineralisation. The optimal flow rates, under which the highest fraction of scaffold surface area is subjected to a wall shear stress that induces mineralisation, are mainly dependent on the scaffold geometries. Nevertheless, the variation range of such optimal flow rates are within 0.5-5 mL/min (or in terms of fluid velocity: 0.166-1.66 mm/s), among different scaffolds. This approach can facilitate the determination of scaffold-dependent flow rates for bone tissue engineering experiments in vitro, avoiding performing a series of trial and error experiments.

Numerical optimization of cell colonization modelling inside scaffold for perfusion bioreactor: A multiscale model

Part of clinically applicable bone graft substitutes are developed by using mechanical stimulation of flow-perfusion into cell-seeded scaffolds. The role of fluid flow is crucial in driving the nutrient to seeded cells and in stimulating cell colonization. A common numerical approach is to use a multiscale model to link some physical quantities (wall shear stress and inlet flow rate) that act at different scales. In this study, a multiscale model is developed in order to determine the optimal inlet flow rate to cultivate osteoblast-like cells seeded in a controlled macroporous biomaterial inside a perfusion bioreactor system. We focus particularly on the influence of Wall Shear Stress on cell colonization to predict cell colonization at the macroscale. Results obtained at the microscale are interpolated at the macroscale to determine the optimal flow rate. For a macroporous scaffold made of interconnected pores with pore diameters of above 350 μm and interconnection diameters of 150 μm, the model predicts a cell colonization of 325% after a 7-day-cell culture with a constant inlet flow rate of 0.69 mL·min^-1. Furthermore, the strength of this protocol is the possibility to adapt it to most porous biomaterials and dynamic cell culture systems.

In silico study of bone tissue regeneration in an idealised porous hydrogel scaffold using a mechano-regulation algorithm

Mechanical stimulation, in the form of fluid perfusion or mechanical strain, enhances osteogenic differentiation and overall bone tissue formation by mesenchymal stems cells cultured in biomaterial scaffolds for tissue engineering applications. In silico techniques can be used to predict the mechanical environment within biomaterial scaffolds, and also the relationship between bone tissue regeneration and mechanical stimulation, and thereby inform conditions for bone tissue engineering experiments. In this study, we investigated bone tissue regeneration in an idealised hydrogel scaffold using a mechano-regulation model capable of predicting tissue differentiation, and specifically compared five loading cases, based on known experimental bioreactor regimes. These models predicted that low levels of mechanical loading, i.e. compression (0.5% strain), pore pressure of 10 kPa and a combination of compression (0.5%) and pore pressure (10 kPa), could induce more osteogenic differentiation and lead to the formation of a higher bone tissue fraction. In contrast greater volumes of cartilage and fibrous tissue fractions were predicted under higher levels of mechanical loading (i.e. compression strain of 5.0% and pore pressure of 100 kPa). The findings in this study may provide important information regarding the appropriate mechanical stimulation for in vitro bone tissue engineering experiments.

Responses of MSCs to 3D Scaffold Matrix Mechanical Properties under Oscillatory Perfusion Culture

Both fluid shear stress and matrix stiffness are implicated in bone metabolism and functional adaptation, but the synergistic action of these mechanical cues on the biological behaviors of mesenchymal stem cells (MSCs) is still not well-known. In the present work, a homemade oscillatory flow device was applied to investigate the effects of matrix stiffness on MSCs survival, distribution, and osteogenic differentiation in three-dimensional (3D) conditions. Furthermore, the flow field and cell growth in this bioreactor were theoretically simulated. The results demonstrated that oscillatory shear stress significantly increased the viability and distribution uniformity of MSCs throughout the scaffold after culture for 3 weeks. Compared to static culture, oscillatory shear stress could promote the collagen secretion, mineral deposits, and osteogenic differentiation of MSCs. The findings obtained from this work indicate that the oscillatory perfusion not only provides a higher survival rate and a more uniform distribution of cells but also facilitates osteogenic differentiation of MSCs. Oscillating perfusion bioreactor culture of MSCs in 3D scaffold with optimal matrix stiffness could offer an easy-to-use but efficient bioreactor for bone tissue engineering.

Decellularized Bone Matrix Scaffold for Bone Regeneration

Decellularized bone matrix is gaining a lot of attention as implantable biomaterials and/or biological scaffolds for bone tissue repair, and shows good clinical performance. This chapter describes the processing techniques and characterization protocols of decellularized bone. For the applications of the decellularized bone scaffold in promoting bone repair and regeneration, we discuss some of the current advances, and highlight the advantages and disadvantages of these scaffolds. Fabrication and application of the hydrogel derived from decellularized bone for bone tissue engineering are also presented.

In Vitro Bone Cell Models: Impact of Fluid Shear Stress on Bone Formation

This review describes the role of bone cells and their surrounding matrix in maintaining bone strength through the process of bone remodeling. Subsequently, this work focusses on how bone formation is guided by mechanical forces and fluid shear stress in particular. It has been demonstrated that mechanical stimulation is an important regulator of bone metabolism. Shear stress generated by interstitial fluid flow in the lacunar-canalicular network influences maintenance and healing of bone tissue. Fluid flow is primarily caused by compressive loading of bone as a result of physical activity. Changes in loading, e.g., due to extended periods of bed rest or microgravity in space are associated with altered bone remodeling and formation in vivo. In vitro, it has been reported that bone cells respond to fluid shear stress by releasing osteogenic signaling factors, such as nitric oxide, and prostaglandins. This work focusses on the application of in vitro models to study the effects of fluid flow on bone cell signaling, collagen deposition, and matrix mineralization. Particular attention is given to in vitro set-ups, which allow long-term cell culture and the application of low fluid shear stress. In addition, this review explores what mechanisms influence the orientation of collagen fibers, which determine the anisotropic properties of bone. A better understanding of these mechanisms could facilitate the design of improved tissue-engineered bone implants or more effective bone disease models.

Stimulation of Bone Repair with Ultrasound

This chapter reviews the different options available for the use of ultrasound in the enhancement of fracture healing or in the reactivation of a failed healing process: LIPUS, shock waves and ultrasound-mediated delivery of bioactive molecules, such as growth factors or plasmids. The main emphasis is on LIPUS, or Low Intensity Pulsed Ultrasound, the most widespread and studied technique. LIPUS has pronounced bioeffects on tissue regeneration, while employing intensities within a diagnostic range. The biological response to LIPUS is complex as the response of numerous cell types to this stimulus involves several pathways. Known to-date mechanotransduction pathways involved in cell responses include MAPK and other kinases signaling pathways, gap-junctional intercellular communication, up-regulation and clustering of integrins, involvement of the COX-2/PGE2 and iNOS/NO pathways, and activation of the ATI mechanoreceptor. Mechanisms at the origin of LIPUS biological effects remain intriguing, and analysis is hampered by the diversity of experimental systems used in-vitro. Data point to clear evidence that bioeffects can be modulated by direct and indirect mechanical effects, like acoustic radiation force, acoustic streaming, propagation of surface waves, heat, fluid-flow induced circulation and redistribution of nutrients, oxygen and signaling molecules. One of the future engineering challenge is therefore the design of dedicated experimental set-ups allowing control of these different mechanical phenomena, and to relate them to biological responses. Then, the derivation of an 'acoustic dose' and the cross-calibration of the different experimental systems will be possible. Despite this imperfect knowledge of LIPUS biophysics, the clinical evidence, although most often of low quality, speaks in favor of the clinical use of LIPUS, when the economics of nonunion and the absence of toxicity of this ultrasound technology are taken into account.

A Focused Low-Intensity Pulsed Ultrasound (FLIPUS) System for Cell Stimulation: Physical and Biological Proof of Principle

Quantitative ultrasound (QUS) is a promising technique for bone tissue evaluation. Highly focused transducers used for QUS also have the capability to be applied for tissue-regenerative purposes and can provide spatially limited deposition of acoustic energy. We describe a focused low-intensity pulsed ultrasound (FLIPUS) system, which has been developed for the stimulation of cell monolayers in the defocused far field of the transducer through the bottom of the well plate. Tissue culture well plates, carrying the cells, were incubated in a special chamber, immersed in a temperature-controlled water tank. A stimulation frequency of 3.6 MHz provided an optimal sound transmission through the polystyrene well plate. The ultrasound was pulsed for 20 min daily at 100-Hz repetition frequency with 27.8% duty cycle. The calibrated output intensity corresponded to I(SATA) = 44.5 ± 7.1 mW/cm2, which is comparable to the most frequently reported nominal output levels in LIPUS studies. No temperature change by the ultrasound exposure was observed in the well plate. The system was used to stimulate rat mesenchymal stem cells (rMSCs). The applied intensity had no apoptotic effect and enhanced the expression of osteogenic markers, i.e., osteopontin (OPN), collagen 1 (Col-1), the osteoblast-specific transcription factor-Runx-2 and E11 protein, an early osteocyte marker, in stimulated cells on day 5. The proposed FLIPUS setup opens new perspectives for the evaluation of the mechanistic effects of LIPUS.

Packed Bed Bioreactor for the Isolation and Expansion of Placental-Derived Mesenchymal Stromal Cells

Large numbers of Mesenchymal stem/stromal cells (MSCs) are required for clinical relevant doses to treat a number of diseases. To economically manufacture these MSCs, an automated bioreactor system will be required. Herein we describe the development of a scalable closed-system, packed bed bioreactor suitable for large-scale MSCs expansion. The packed bed was formed from fused polystyrene pellets that were air plasma treated to endow them with a surface chemistry similar to traditional tissue culture plastic. The packed bed was encased within a gas permeable shell to decouple the medium nutrient supply and gas exchange. This enabled a significant reduction in medium flow rates, thus reducing shear and even facilitating single pass medium exchange. The system was optimised in a small-scale bioreactor format (160 cm²) with murine-derived green fluorescent protein-expressing MSCs, and then scaled-up to a 2800 cm² format. We demonstrated that placental derived MSCs could be isolated directly within the bioreactor and subsequently expanded. Our results demonstrate that the closed system large-scale packed bed bioreactor is an effective and scalable tool for large-scale isolation and expansion of MSCs.

Effect of osteogenic differentiation medium on proliferation and differentiation of human mesenchymal stem cells in three-dimensional culture with radial flow bioreactor

Human mesenchymal stem cells (hMSCs) are multipotent cells, and have been expanded and differentiated into several kinds of mesodermal tissue in vitro. In order to promote bone repair, enhancement of the proliferation and differentiation of hMSCs into osteoblasts in vitro is recommended prior to therapeutic delivery. However, for clinical applications, it is still unclear which method is more advanced for tissue engineering: to transplant undifferentiated cells or partially differentiated stem cells. Therefore, the present study aimed to investigate how osteogenic differentiation medium (ODM) affects hMSCs cultured in a 3D scaffold using a radial-flow bioreactor (RFB) besides cell growth medium (GM). To produce precultured sheets, the hMSCs were first seeded onto type 1 collagen sheets and incubated for 12 h, after which they were placed in the RFB for scaffold fabrication. The culture medium was circulated at 3 mL/min and the cells dynamically cultured for 1 week at 37 °C. Static cultivation in a culture dish was also carried out. Cell proliferations were evaluated by histological analysis and DNA-based cell count. Alkaline phosphatase (ALP) activity, immunocytochemical analysis with BMP-2, and osteopontin on the hMSCs in the collagen scaffold were performed. After 14 days of ODM culture, a significant increase in cell number and a higher density of cell distribution in the scaffold were observed after both static and dynamic cultivation compared to GM culture. A significant increase in ALP activity after 14 days of ODM was recognized in dynamic cultivation compared with that of static cultivation. Cells that BMP-2 expressed were frequently observed after 14 days in dynamic culture compared with other conditions, and the expression of osteopontin was confirmed in dynamic cultivation after both 7 days and 14 days. The results of this study revealed that both the proliferation and bone differentiation of hMSCs in 3D culture by RFB were accelerated by culture in osteogenic differentiation medium, suggesting an advantageous future clinical applications for RFB cell culture and cell transplantation for tissue engineering.

Cellular Nutrition in Complex Three-Dimensional Scaffolds: A Comparison between Experiments and Computer Simulations

Studies on bone cell ingrowth into synthetic, porous three-dimensional (3D) implants showed difficulties arising from impaired cellular proliferation and differentiation in the core region of these scaffolds with increasing scaffold volume in vitro. Therefore, we developed an in vitro perfusion cell culture module, which allows the analysis of cells in the interior of scaffolds under different medium flow rates. For each flow rate the cell viability was measured and compared with results from computer simulations that predict the local oxygen supply and shear stress inside the scaffold based on the finite element method. We found that the local cell viability correlates with the local oxygen concentration and the local shear stress. On the one hand the oxygen supply of the cells in the core becomes optimal with a higher perfusion flow. On the other hand shear stress caused by high flow rates impedes cell vitality, especially at the surface of the scaffold. Our results demonstrate that both parameters must be considered to derive an optimal nutrient flow rate.

Three-Dimensional Modelling inside a Differential Pressure Laminar Flow Bioreactor Filled with Porous Media

A three-dimensional computational fluid dynamics- (CFD-) model based on a differential pressure laminar flow bioreactor prototype was developed to further examine performance under changing culture conditions. Cell growth inside scaffolds was simulated by decreasing intrinsic permeability values and led to pressure build-up in the upper culture chamber. Pressure release by an integrated bypass system allowed continuation of culture. The specific shape of the bioreactor culture vessel supported a homogenous flow profile and mass flux at the scaffold level at various scaffold permeabilities. Experimental data showed an increase in oxygen concentration measured inside a collagen scaffold seeded with human mesenchymal stem cells when cultured in the perfusion bioreactor after 24 h compared to static culture in a Petri dish (dynamic: 11% O₂ versus static: 3% O₂). Computational fluid simulation can support design of bioreactor systems for tissue engineering application.