Low intermittent flow promotes rat mesenchymal stem cell differentiation in logarithmic fluid shear device

High-viscosity driven modulation of biomechanical properties of human mesenchymal stem cells promotes osteogenic lineage

Biomechanical cues could effectively govern cell gene expression to direct the differentiation of specific stem cell lineage. Recently, the medium viscosity has emerged as a significant mechanical stimulator that regulates the cellular mechanical properties and various physiological functions. However, whether the medium viscosity can regulate the mechanical properties of human mesenchymal stem cells (hMSCs) to effectively trigger osteogenic differentiation remains uncertain. The mechanism by which cells sense and respond to changes in medium viscosity, and regulate cell mechanical properties to promote osteogenic lineage, remains elusive. In this study, we demonstrated that hMSCs, cultured in a high-viscosity medium, exhibited larger cell spreading area and higher intracellular tension, correlated with elevated formation of actin stress fibers and focal adhesion maturation. Furthermore, these changes observed in hMSCs were associated with activation of TRPV4 (transient receptor potential vanilloid sub-type 4) channels on the cell membrane. This feedback loop among TRPV4 activation, cell spreading and intracellular tension results in calcium influx, which subsequently promotes the nuclear localization of NFATc1 (nuclear factor of activated T cells 1). Concomitantly, the elevated intracellular tension induced nuclear deformation and promoted the nuclear localization of YAP (YES-associated protein). The concurrent activation of NFATc1 and YAP significantly enhanced alkaline phosphatase (ALP) for pre-osteogenic activity. Taken together, these findings provide a more comprehensive view of how viscosity-induced alterations in biomechanical properties of MSCs impact the expression of osteogenesis-related genes, and ultimately promote osteogenic lineage.

A tempo-spatial controllable microfluidic shear-stress generator for in-vitro mimicking of the thrombus

Pathological conditions linked to shear stress have been identified in hematological diseases, cardiovascular diseases, and cancer. These conditions often exhibit significantly elevated shear stress levels, surpassing 1000 dyn/cm2 in severely stenotic arteries. Heightened shear stress can induce mechanical harm to endothelial cells, potentially leading to bleeding and fatal consequences. However, current technology still grapples with limitations, including inadequate flexibility in simulating bodily shear stress environments, limited range of shear stress generation, and spatial and temporal adaptability. Consequently, a comprehensive understanding of the mechanisms underlying the impact of shear stress on physiological and pathological conditions, like thrombosis, remains inadequate. To address these limitations, this study presents a microfluidic-based shear stress generation chip as a proposed solution. The chip achieves a substantial 929-fold variation in shear stress solely by adjusting the degree of constriction in branch channels after PDMS fabrication. Experiments demonstrated that a rapid increase in shear stress up to 1000 dyn/cm2 significantly detached 88.2% cells from the substrate. Long-term exposure (24 h) to shear stress levels below 8.3 dyn/cm2 did not significantly impact cell growth. Furthermore, cells exposed to shear stress levels equal to or greater than 8.3 dyn/cm2 exhibited significant alterations in aspect ratio and orientation, following a normal distribution. This microfluidic chip provides a reliable tool for investigating cellular responses to the wide-ranging shear stress existing in both physiological and pathological flow conditions.

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

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

Osteogenesis and osteoclastogenesis on a chip: Engineering a self-assembling 3D coculture

Healthy bone is maintained by the process of bone remodeling. An unbalance in this process can lead to pathologies such as osteoporosis which are often studied with animal models. However, data from animals have limited power in predicting the results that will be obtained in human clinical trials. In search for alternatives to animal models, human in vitro models are emerging as they address the principle of reduction, refinement, and replacement of animal experiments (3Rs). At the moment, no complete in vitro model for bone-remodeling exists. Microfluidic chips offer great possibilities, particularly because of the dynamic culture options, which are crucial for in vitro bone formation. In this study, a scaffold free, fully human, 3D microfluidic coculture model of bone remodeling is presented. A bone-on-a-chip coculture system was developed in which human mesenchymal stromal cells differentiated into the osteoblastic lineage and self-assembled into scaffold free bone-like tissues with the shape and dimensions of human trabeculae. Human monocytes were able to attach to these tissues and to fuse into multinucleated osteoclast-like cells, establishing the coculture. Computational modeling was used to determine the fluid flow induced shear stress and strain in the formed tissue. Furthermore, a set-up was developed allowing for long-term (35 days) on-chip cell culture with benefits including continuous fluid-flow, low bubble formation risk, easy culture medium exchange inside the incubator and live cell imaging options. This on-chip coculture is a crucial advance towards developing in vitro bone remodeling models to facilitate drug testing.

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.

Silk fibroin films with embedded magnetic nanoparticles: evaluation of the magneto-mechanical stimulation effect on osteogenic differentiation of stem cells

We report about a biomaterial in the form of film ∼10 μm thick, consisting of a silk fibroin matrix with embedded iron oxide superparamagnetic nanoparticles, for prospective applications as bioactive coating in regenerative medicine. Films with different load of magnetic nanoparticles are produced (nanoparticles/silk fibroin nominal ratio = 5, 0.5 and 0 wt%) and the structural, mechanical and magnetic properties are studied. The nanoparticles form aggregates in the silk fibroin matrix and the film stiffness, as tested by nanoindentation, is spatially inhomogeneous, but the protein structure is not altered. In vitro biological tests are carried out on human bone marrow-derived mesenchymal stem cells cultured on the films up to 21 days, with and without an applied static uniform magnetic field. The sample with the highest nanoparticles/silk fibroin ratio shows the best performance in terms of cell proliferation and adhesion. Moreover, it promotes a faster and better osteogenic differentiation, particularly under magnetic field, as indicated by the gene expression level of typical osteogenic markers. These findings are explained in light of the results of the physical characterization, combined with numerical calculations. It is established that the applied magnetic field triggers a virtuous magneto-mechanical mechanism in which dipolar magnetic forces between the nanoparticle aggregates give rise to a spatial distribution of mechanical stresses in the silk fibroin matrix. The film with the largest nanoparticle load, under cell culture conditions (i.e. in aqueous environment), undergoes matrix deformations large enough to be sensed by the seeded cells as mechanical stimuli favoring the osteogenic differentiation.

How the mechanical microenvironment of stem cell growth affects their differentiation: a review

Stem cell differentiation is of great interest in medical research; however, specifically and effectively regulating stem cell differentiation is still a challenge. In addition to chemical factors, physical signals are an important component of the stem cell ecotone. The mechanical microenvironment of stem cells has a huge role in stem cell differentiation. Herein, we describe the knowledge accumulated to date on the mechanical environment in which stem cells exist, which consists of various factors, including the extracellular matrix and topology, substrate stiffness, shear stress, hydrostatic pressure, tension, and microgravity. We then detail the currently known signalling pathways that stem cells use to perceive the mechanical environment, including those involving nuclear factor-kB, the nicotinic acetylcholine receptor, the piezoelectric mechanosensitive ion channel, and hypoxia-inducible factor 1α. Using this information in clinical settings to treat diseases is the goal of this research, and we describe the progress that has been made. In this review, we examined the effects of mechanical factors in the stem cell growth microenvironment on stem cell differentiation, how mechanical signals are transmitted to and function within the cell, and the influence of mechanical factors on the use of stem cells in clinical applications.

Fluid shear stress in a logarithmic microfluidic device enhances cancer cell stemness marker expression

Fluid shear stress (FSS) is crucial in cancer cell survival and tumor development. Noteworthily, cancer cells are exposed to several degrees of FSS in the tumor microenvironment and during metastasis. Consequently, the stemness marker expression in cancer cells changes with the FSS signal, although it is unclear how it varies with different magnitudes and during metastasis. The current work explores the stemness and drug resistance characteristics of the cervical cancer cell line HeLa in a microfluidic device with a wide range of physiological FSS. Hence, the microfluidic device was designed to achieve a logarithmic flow distribution in four culture chambers, realizing four orders of biological shear stress on a single chip. The cell cycle analysis demonstrated altered cell proliferation and mitotic arrest after FSS treatment. In addition, EdU staining revealed increased cell proliferation with medium to low FSS, whereas high shear had a suppressing effect. FSS increased competence to withstand higher intracellular ROS and mitochondrial membrane potential in HeLa. Furthermore, stemness-related gene (Sox2, N-cadherin) and cell surface marker (CD44, CD33, CD117) expressions were enhanced by FSS mechanotransduction in a magnitude-dependent manner. In summary, these stemness-like properties were concurrent with the drug resistance capability of HeLa towards doxorubicin. Overall, our microfluidic device elucidates cancer cell survival and drug resistance mechanisms during metastasis and in cancer relapse patients.

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.

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.

Biophysical Stimuli as the Fourth Pillar of Bone Tissue Engineering

The repair of critical bone defects remains challenging worldwide. Three canonical pillars (biomaterial scaffolds, bioactive molecules, and stem cells) of bone tissue engineering have been widely used for bone regeneration in separate or combined strategies, but the delivery of bioactive molecules has several obvious drawbacks. Biophysical stimuli have great potential to become the fourth pillar of bone tissue engineering, which can be categorized into three groups depending on their physical properties: internal structural stimuli, external mechanical stimuli, and electromagnetic stimuli. In this review, distinctive biophysical stimuli coupled with their osteoinductive windows or parameters are initially presented to induce the osteogenesis of mesenchymal stem cells (MSCs). Then, osteoinductive mechanisms of biophysical transduction (a combination of mechanotransduction and electrocoupling) are reviewed to direct the osteogenic differentiation of MSCs. These mechanisms include biophysical sensing, transmission, and regulation. Furthermore, distinctive application strategies of biophysical stimuli are presented for bone tissue engineering, including predesigned biomaterials, tissue-engineered bone grafts, and postoperative biophysical stimuli loading strategies. Finally, ongoing challenges and future perspectives are discussed.

Induction of osteogenic differentiation of bone marrow stromal cells on 3D polyester-based scaffolds solely by subphysiological fluidic stimulation in a laminar flow bioreactor

The fatal determination of bone marrow mesenchymal stem/stromal cells (BMSC) is closely associated with mechano-environmental factors in addition to biochemical clues. The aim of this study was to induce osteogenesis in the absence of chemical stimuli using a custom-designed laminar flow bioreactor. BMSC were seeded onto synthetic microporous scaffolds and subjected to the subphysiological level of fluid flow for up to 21 days. During the perfusion, cell proliferation was significantly inhibited. There were also morphological changes, with F-actin polymerisation and upregulation of ROCK1. Notably, in BMSC subjected to flow, mRNA expression of osteogenic markers was significantly upregulated and RUNX2 was localised in the nuclei. Further, under perfusion, there was greater deposition of collagen type 1 and calcium onto the scaffolds. The results confirm that an appropriate level of fluid stimuli preconditions BMSC towards the osteoblastic lineage on 3D scaffolds in the absence of chemical stimulation, which highlights the utility of flow bioreactors in bone tissue engineering.