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.

Computational simulation of applying mechanical vibration to mesenchymal stem cell for mechanical modulation toward bone tissue engineering.

Evaluation of cell response to mechanical stimuli at in vitro conditions is known as one of the important issues for modulating cell behavior. Mechanical stimuli, including mechanical vibration and oscillatory fluid flow, act as important biophysical signals for the mechanical modulation of stem cells. In the present study, mesenchymal stem cell (MSC) consists of cytoplasm, nucleus, actin, and microtubule. Also, integrin and primary cilium were considered as mechanoreceptors. In this study, the combined effect of vibration and oscillatory fluid flow on the cell and its components were investigated using numerical modeling. The results of the FEM and FSI model showed that the cell response (stress and strain values) at the frequency of 30Hz mechanical vibration has the highest value. The achieved results on shear stress caused by the fluid flow on the cell showed that the cell experiences shear stress in the range of 0.1-10Pa. Mechanoreceptors that bind separately to the cell surface, can be highly stimulated by hydrodynamic pressure and, therefore, can play a role in the mechanical modulation of MSCs at in vitro conditions. The results of this research can be effective in future studies to optimize the conditions of mechanical stimuli applied to the cell culture medium and to determine the mechanisms involved in mechanotransduction.

A Fluid-Structure Interaction Analysis of Blood Clot Motion in a Branch of Pulmonary Arteries.

INTRODUCTION: Pulmonary embolism (PE) is one of the most prevalent diseases amid hospitalized patients taking many people's lives annually. This phenomenon, however, has not been investigated via numerical simulations. METHODS: In this study, an image-based model of pulmonary arteries has been constructed from a 44-year-old man's computed tomography images. The fluid-structure interaction method was used to simulate the motion of the blood clot. In this regard, Navier-Stokes equations, as the governing equations, have been solved in an arbitrary Lagrangian-Eulerian (ALE) formulation. RESULTS: According to our results, the velocity of visco-hyperelastic model of the emboli was relatively higher than the emboli with hyperelastic model, despite their similar behavioral pattern. The stresses on the clot were also investigated and showed that the blood clot continuously sustained stresses greater than 165 Pa over an about 0.01 s period, which can cause platelets to leak and make the clot grow or tear apart. CONCLUSIONS: It could be concluded that in silico analysis of the cardiovascular diseases initiated from clot motion in blood flow is a valuable tool for a better understanding of these phenomena.

In silico analysis of a hierarchical microfluidic vascular network: Detecting the location of angiogenic sprouting.

Lack of oxygen is one of the leading causes of failure in engineered tissue. Therefore, angiogenesis will be necessary for the survival of larger tissues in vivo. In addition, a proper lymphatic system that plays an essential role in relieving inflammation and maintaining tissue homeostasis is of great importance for tissue regeneration and repair. Many biomechanical parameters are involved in controlling angiogenesis and capillary network generation, which are challenging to study and control in experimental studies or in vitro. In the present study, using numerical modeling, the effect of various geometric and biomechanical parameters in creating suitable conditions for angiogenesis was investigated. Furthermore, sprouting points were predicted using flow dynamics. For this purpose, a porous scaffold, flow channels with parametric geometry that followed Murray's law, and a drainage channel were considered. Results suggested that the geometry of the microfluidic channels and the characteristics of the vessel wall and scaffold plays a complementary role in determining the transmural pressure. It was found that a twofold increase in the vascular hydraulic conductivity can reduce the minimum transmural pressure by up to 28% and increase the drainage flow rate by 44%. In addition, the minimum magnitude of transmural pressure tends to zero for scaffold's hydraulic conductivity values smaller than 10-11  m3 s kg-1 . The results of this study can be used in optimizing the design of the relevant microfluidic systems to induce angiogenesis and avoid leakage in the constructed implantable tissue.

Finite element study of stem cells under fluid flow for mechanoregulation toward osteochondral cells.

Investigating the effects of mechanical stimuli on stem cells under in vitro and in vivo conditions is a very important issue to reach better control on cellular responses like growth, proliferation, and differentiation. In this regard, studying the effects of scaffold geometry, steady, and transient fluid flow, as well as influence of different locations of the cells lodged on the scaffold on effective mechanical stimulations of the stem cells are of the main goals of this study. For this purpose, collagen-based scaffolds and implicit surfaces of the pore architecture was used. In this study, computational fluid dynamics and fluid-structure interaction method was used for the computational simulation. The results showed that the scaffold microstructure and the pore architecture had an essential effect on accessibility of the fluid to different portions of the scaffold. This leads to the optimization of shear stress and hydrodynamic pressure in different surfaces of the scaffold for better transportation of oxygen and growth factors as well as for optimized mechanoregulative responses of cell-scaffold interactions. Furthermore, the results indicated that the HP scaffold provides more optimizer surfaces to culture stem cells rather than Gyroid and IWP scaffolds. The results of exerting oscillatory fluid flow into the HP scaffold showed that the whole surface of the HP scaffold expose to the shear stress between 0.1 and 40 mPa and hydrodynamics factors on the scaffold was uniform. The results of this study could be used as an aid for experimentalists to choose optimist fluid flow conditions and suitable situation for cell culture.

A computational analysis of the effect of supporting organs on predicted vesical pressure in stress urinary incontinence.

Stress urinary incontinence (SUI) or urine leakage from urethra occurs due to an increase in abdominal pressure resulting from stress like a cough or jumping height. SUI is more frequent among post-menopausal women. In the absence of bladder contraction, vesical pressure exceeds urethral pressure leading to urine leakage. The main aim of this study is to utilize fluid-structure interaction techniques to model bladder and urethra computationally under an external pressure like sneezing. Both models have been developed with linear elastic properties for the bladder wall while the patient model has also been simulated utilizing the Mooney-Rivlin solid model. The results show a good agreement between the clinical data and the predicted values of the computational models, specifically the pressure at the center of the bladder. There is 1.3% difference between the predicted vesical pressure and the vesical pressure obtained from urodynamic tests. It can be concluded that the accuracy of the predicted pressure in the center of the bladder is significantly higher for the simulation assuming nonlinear material property (hyperelastic) for the bladder in comparison to the accuracy of the linear elastic model. The model is beneficial for exploring treatment solutions for SUI disorder. Graphical abstract 3D processing of bladder deformation during abdominal pressure of a the physiological model and b the pathological model (starting from left to right and up to down, consecutively).

Computational analysis of adhesion between a cancer cell and a white blood cell in a bifurcated microvessel.

BACKGROUND AND OBJECTIVE: Cancer is one of the diseases caused by irregular and uncontrolled growth of cells and their propagation into various parts of the body. The motion and adhesion of cancer cells in a blood vessel is a critical step in tumor progression that depends on some vascular parameters such as vessel branching. In this study, effect of microvessel branching on the bonds between a cancer cell and a white blood has been investigated as compared to an analogous problem in a straight vessel. METHODS: The analysis is performed using finite elements and fluid-structure interaction methods. Moreover, the equations for adhesion of the cancer cell to white blood cell are coded in MATLAB for calculating forces between them and the code is coupled directly and simultaneously with the COMSOL software. For fluid-structure interaction analysis, it is assumed that the properties of the blood and the cells are homogeneous and the fluid is incompressible and Newtonian. Cancer cell is modeled as a rigid body and white blood cell is assumed as linear elastic. RESULTS: The results show that although the geometry of the vessel does not affect on the separation distance of cancer cell considerably, but at the area near a bifurcation, high fluctuations in cancer cell velocity is occurred due to increasing in number of bond formation between the cancer cell and the white blood cell. Accordingly, it can be predicted that higher concentration of adhered particles occurs near the bifurcations. Moreover, shear stress at the point of contact of the cancer cell and the white blood cell in the branched vessel is greater than that in the straight path. In addition to, the probability of breaking of the bond between the cancer cell and the white blood cell increases in the branched vessel. CONCLUSIONS: Through consideration in the adhesion charts of this study along with observations from medical interventions such as drug delivery to cancer patients, considerable developments on the treatment or prevention of cancer metastasis may be achieved.

An investigation into osteogenic differentiation effects of silk fibroin-nettle (Urtica dioica L.) nanofibers.

The purpose of this study was to investigate physical, mechanical, and osteogenic properties of silk fibroin (SF) nanofibers containing Urtica dioica L. (nettle) extract at different concentrations. In this respect, the successful incorporation of nettle in SF nanofibers was analyzed and then confirmed through Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The mean fiber diameter, water uptake, breaking strain, cellular attachment, and proliferation of the given nanofibers also increased as the nettle content was added, while this trend was opposite in terms of tensile strength and modulus. The in vitro release studies correspondingly demonstrated that the nettle release had been controlled according to Fickian diffusion and it was faster in the samples including more nettle. Furthermore, both ARS staining and real-time RT-PCR results suggested that nettle had enhanced the expression of both early and late markers of osteoblast differentiation in a dose-dependent manner.

A computational simulation of cyclic stretch of an individual stem cell using a nonlinear model.

Many experiments have shown that mechanical stimuli like cyclic strains might be helpful in stem cell differentiation. To maximize such differentiations efficiency, it is imperative to detect the cellular mechanical responses to these stimuli. The purpose of this research was to show that a newly presented hyper-viscoelastic model could correctly predict the level of stresses required to obtain a different response from a single mesenchymal stem cell cultured in a fibrin hydrogel block under a 10% cyclic strain at a frequency of 1 Hz, employing finite element method. One of the novelties of the research was the use of a model based on Simo's model. Another important feature of the research was the proposition of a multiscale model considering a layer of integrins. It was concluded that the forces exerted on the biological molecules had the maximum values of 24, 45, and 15 pN for the circumferential, radial, and shear forces, respectively. According to the results, the exerted forces within the cytoskeleton can lead to a different cellular response. These results might be a premise for interpreting events that lead to differentiation of stem cells into fibrochondrocytes. In addition, they can be beneficial in effective design of biological experiments as regards to this issue.

Engineering mesenchymal stem cell spheroids by incorporation of mechanoregulator microparticles.

Mechanical forces throughout human mesenchymal stem cell (hMSC) spheroids (mesenspheres) play a predominant role in determining cellular functions of cell growth, proliferation, and differentiation through mechanotransductional mechanisms. Here, we introduce microparticle (MP) incorporation as a mechanical intervention method to alter tensional homeostasis of the mesensphere and explore MSC differentiation in response to MP stiffness. The microparticulate mechanoregulators with different elastic modulus (34 kPa, 0.6 MPa, and 2.2 MPa) were prepared by controlled crosslinking cell-sized microdroplets of polydimethylsiloxane (PDMS). Preparation of MP-MSC composite spheroids enabled us to study the possible effects of MPs through experimental and computational assays. Our results showed that MP incorporation selectively primed MSCs toward osteogenesis, yet hindered adipogenesis. Interestingly, this behavior depended on MP mechanics, as the spheroids that contained MPs with intermediate stiffness behaved similar to control MP-free mesenspheres with more tendencies toward chondrogenesis. However, by using the soft or stiff MPs, the MP-mesenspheres significantly showed signs of osteogenesis. This could be explained by the complex of forces which acted in the cell spheroid and, totally, provided a homeostasis situation. Incorporation of cell-sized polymer MPs as mechanoregulators of cell spheroids could be utilized as a new engineering toolkit for multicellular organoids in disease modeling and tissue engineering applications.

Computational simulation of static/cyclic cell stimulations to investigate mechanical modulation of an individual mesenchymal stem cell using confocal microscopy.

It has been found that cells react to mechanical stimuli, while the type and magnitude of these cells are different in various physiological and pathological conditions. These stimuli may affect cell behaviors via mechanotransduction mechanisms. The aim of this study is to evaluate mechanical responses of a mesenchymal stem cell (MSC) to a pressure loading using finite elements method (FEM) to clarify procedures of MSC mechanotransduction. The model is constructed based on an experimental set up in which statics and cyclic compressive loads are implemented on a model constructed from a confocal microscopy 3D image of a stem cell. Both of the applied compressive loads are considered in the physiological loading regimes. Moreover, a viscohyperelastic material model was assumed for the cell through which the finite elements simulation anticipates cell behavior based on strain and stress distributions in its components. As a result, high strain and stress values were captured from the viscohyperelastic model because of fluidic behavior of cytosol when compared with the obtained results through the hyperelastic models. It can be concluded that the generated strain produced by cyclic pressure is almost 8% higher than that caused by the static load and the von Mises stress distribution is significantly increased to about 150kPa through the cyclic loading. In total, the results does not only trace the efficacy of an individual 3D model of MSC using biomechanical experiments of cell modulation, but these results provide knowledge in interpretations from cell geometry. The current study was performed to determine a realistic aspect of cell behavior.

Analysis of mechanical parameters on the thromboembolism using a patient-specific computational model.

Ischemic stroke is a major cause of death and long-term disabilities worldwide. In this paper, we aim to represent a comprehensive simulation of the motion of emboli through cerebrovascular network within patient-specific computational model. The model consists of major arteries of the circle of Willis reconstructed from magnetic resonance angiography images, pulsatile flow and emboli with different sizes and material properties. Here, the fluid-structure interactions method was used to simulate the motion of deformable and rigid emboli through cerebral arteries. Hemodynamic changes in the circle of Willis due to the entrance of embolus are observed. The effect of material properties on the distribution ratio and dynamics of motion of the emboli in the cerebral arterial network is also analyzed. Our results reveal that as the rigidity of emboli increases, higher proportion of them tend to enter to the larger arteries (e.g., middle cerebral artery). Scrutinizing the amount of stress acting on the emboli represented in this paper can broaden our understanding of the rheological phenomenon (e.g., lysis or growth of emboli during embolism). The approach of considering different material properties of the thrombus in a patient-specific computational model not only enable us to better understand the roll of biomechanical parameters causing the embolism, but also lead to a better clinical decision making to manage patients with stroke.

Fluid-Structure Interactions Analysis of Shear-Induced Modulation of a Mesenchymal Stem Cell: An Image-Based Study.

Although effects of biochemical modulation of stem cells have been widely investigated, only recent advances have been made in the identification of mechanical conditioning on cell signaling pathways. Experimental investigations quantifying the micromechanical environment of mesenchymal stem cells (MSCs) are challenging while computational approaches can predict their behavior due to in vitro stimulations. This study introduces a 3D cell-specific finite element model simulating large deformations of MSCs. Here emphasizing cell mechanical modulation which represents the most challenging multiphysics phenomena in sub-cellular level, we focused on an approach attempting to elicit unique responses of a cell under fluid flow. Fluorescent staining of MSCs was performed in order to visualize the MSC morphology and develop a geometrically accurate model of it based on a confocal 3D image. We developed a 3D model of a cell fixed in a microchannel under fluid flow and then solved the numerical model by fluid-structure interactions method. By imposing flow characteristics representative of vigorous in vitro conditions, the model predicts that the employed external flow induces significant localized effective stress in the nucleo-cytoplasmic interface and average cell deformation of about 40%. Moreover, it can be concluded that a lower strain level is made in the cell by the oscillatory flow as compared with steady flow, while same ranges of effective stress are recorded inside the cell in both conditions. The deeper understanding provided by this study is beneficial for better design of single cell in vitro studies.

Large deforming buoyant embolus passing through a stenotic common carotid artery: a computational simulation.

Arterial embolism is responsible for the death of lots of people who suffers from heart diseases. The major risk of embolism in upper limbs is that the ruptured particles are brought into the brain, thus stimulating neurological symptoms or causing the stroke. We presented a computational model using fluid-structure interactions (FSI) to investigate the physical motion of a blood clot inside the human common carotid artery. We simulated transportation of a buoyant embolus in an unsteady flow within a finite length tube having stenosis. Effects of stenosis severity and embolus size on arterial hemodynamics were investigated. To fulfill realistic nonlinear property of a blood clot, a rubber/foam model was used. The arbitrary Lagrangian-Eulerian formulation (ALE) and adaptive mesh method were used inside fluid domain to capture the large structural interfacial movements. The problem was solved by simultaneous solution of the fluid and the structure equations. Stress distribution and deformation of the clot were analyzed and hence, the regions of the embolus prone to lysis were localized. The maximum magnitude of arterial wall shear stress during embolism occurred at a short distance proximal to the throat of the stenosis. Through embolism, arterial maximum wall shear stress is more sensitive to stenosis severity than the embolus size whereas role of embolus size is more significant than the effect of stenosis severity on spatial and temporal gradients of wall shear stress downstream of the stenosis and on probability of clot lysis due to clot stresses while passing through the stenosis.

A biomechanical simulation of ureteral flow during peristalsis using intraluminal morphometric data.

Reflux nephropathy and vesicoureteral reflux are two of the most important abnormalities in the upper urinary system in which toxins and bacteria from the bladder infect the ureter and the kidney and initiate renal scar formation. A quantitative analysis that characterizes urine flow will further help our understanding of the ureter and also assist in the design of flow aided devices such as valves and stents to correct reflux situations. Here, A numerical simulation with fluid-structure interactions (FSI) using arbitrary Lagrangian-Eulerian (ALE) formulation and adaptive mesh procedure was introduced and solved to perform ureteral flow analysis. Incompressible Navier-Stokes equations were utilized as the governing equations of fluid domain. Ureteral in-vivo morphometric data during peristalsis were used to construct the presented model. A nonlinear material model was used to exhibit ureteral wall mechanical properties. Direct coupling method was used to solve the solid, fluid and interface equations simultaneously. Results showed that recirculation regions formed against the jet flow, neighboring the bolus peak. Through wave propagation, separation occurred behind the moving bolus on the wall and ureteropelvic reflux began from that location and extended upstream to the ureteral inlet. The maximum luminal pressure consistently occurred behind the urine bolus during peristalsis. The measured magnitude of maximum volumetric flow rate resulted from isolated bolus transportation was 0.92 ml/min. In conclusion; due to presence of fluid inertial forces during peristalsis, the function of ureteropelvic junction in prevention of reflux is significant, especially at the beginning of peristaltic wave propagation. Moreover, modeling of ureteral function using imaging data will be valuable and it may help physicians to diagnose and cure the abnormalities.

A mathematical simulation of the ureter: effects of the model parameters on ureteral pressure/flow relations.

Ureteral peristaltic mechanism facilitates urine transport from the kidney to the bladder. Numerical analysis of the peristaltic flow in the ureter aims to further our understanding of the reflux phenomenon and other ureteral abnormalities. Fluid-structure interaction (FSI) plays an important role in accuracy of this approach and the arbitrary Lagrangian-Eulerian (ALE) formulation is a strong method to analyze the coupled fluid-structure interaction between the compliant wall and the surrounding fluid. This formulation, however, was not used in previous studies of peristalsis in living organisms. In the present investigation, a numerical simulation is introduced and solved through ALE formulation to perform the ureteral flow and stress analysis. The incompressible Navier-Stokes equations are used as the governing equations for the fluid, and a linear elastic model is utilized for the compliant wall. The wall stimulation is modeled by nonlinear contact analysis using a rigid contact surface since an appropriate model for simulation of ureteral peristalsis needs to contain cell-to-cell wall stimulation. In contrast to previous studies, the wall displacements are not predetermined in the presented model of this finite-length compliant tube, neither the peristalsis needs to be periodic. Moreover, the temporal changes of ureteral wall intraluminal shear stress during peristalsis are included in our study. Iterative computing of two-way coupling is used to solve the governing equations. Two phases of nonperistaltic and peristaltic transport of urine in the ureter are discussed. Results are obtained following an analysis of the effects of the ureteral wall compliance, the pressure difference between the ureteral inlet and outlet, the maximum height of the contraction wave, the contraction wave velocity, and the number of contraction waves on the ureteral outlet flow. The results indicate that the proximal part of the ureter is prone to a higher shear stress during peristalsis compared with its middle and distal parts. It is also shown that the peristalsis is more efficient as the maximum height of the contraction wave increases. Finally, it is concluded that improper function of ureteropelvic junction results in the passage of part of urine back flow even in the case of slow start-up of the peristaltic contraction wave.

A numerical simulation of peristaltic motion in the ureter using fluid structure interactions.

An axisymmetric model with fluid-structure interactions (FSI) is introduced and solved to perform ureter flow and stress analysis. The Navier-Stokes equations are solved for the fluid and a linear elastic model for ureter is used. The finite element equations for both the structure and the fluid were solved by the Newton-Raphson iterative method. Our results indicated that shear stresses were high around the throat of moving contracted wall. The pressure gradient magnitude along the ureter wall and the symmetry line had the maximum value around the throat of moving contracted wall which decreased as the peristalsis propagates toward the bladder. The flow rate at the ureter outlet at the end of the peristaltic motion was about 650 mm3/s. During propagation of the peristalsis toward the bladder, the inlet backward flow region was limited to the areas near symmetry line but the inner ureter backward flow regions extended to the whole ureter contraction part. The backward flow was vanished after 1.5 seconds of peristalsis propagation start up and after that time the urine flow was forward in the whole ureter length, so reflux is more probable to be present at the beginning of the wall peristaltic motion.