Biomedical subjects

Federica Boschetti

Publications and source records attributed to Federica Boschetti.

9 recordsLinked to original sources

PubMed2006

Effects of extracellular fiber architecture on cell membrane shear stress in a 3D fibrous matrix.

Interstitial fluid flow has been shown to affect the organization and behavior of cells in 3D environments in vivo and in vitro, yet the forces driving such responses are not clear. Due to the complex architecture of the extracellular matrix (ECM) and the difficulty of measuring fluid flow near cells embedded in it, the levels of shear stress experienced by cells in this environment are typically estimated using bulk-averaged matrix parameters such as hydraulic permeability. While this is useful for estimating average stresses, it cannot yield insight into how local matrix fiber architecture-which is cell-controlled in the immediate pericellular environment-affects the local stresses imposed on the cell surface. To address this, we used computational fluid dynamics to study flow through an idealized mesh constructed of a cubic lattice of fibers simulating a typical in vitro collagen gel. We found that, in such high porosity matrices, the fibers strongly affect the flow fields near the cell, with peak shear stresses up to five times higher than those predicted by the Brinkman equation. We also found that minor remodeling of the fibers near the cell surface had major effects on the shear stress profile on the cell. These findings demonstrate the importance of fiber architecture to the fluid forces on a cell embedded in a 3D matrix, and also show how small modifications in the local ECM can lead to large changes in the mechanical environment of the cell.

Cell Membrane↗

PubMed2006

Modeling evaluation of the fluid-dynamic microenvironment in tissue-engineered constructs: a micro-CT based model.

Natural cartilage remodels both in vivo and in vitro in response to mechanical stresses, hence mechanical stimulation is believed to be a potential tool to modulate extra-cellular matrix synthesis in tissue-engineered cartilage. Fluid-induced shear is known to enhance chondrogenesis in engineered cartilage constructs. The quantification of the hydrodynamic environment is a condition required to study the biochemical response to shear of 3D engineered cell systems. We developed a computational model of culture medium flow through the microstructure of a porous scaffold, during direct- perfused culture. The 3D solid model of the scaffold micro-geometry was reconstructed from 250 micro-computed tomography (micro-CT) images. The results of the fluid dynamic simulations were analyzed at the central portions of the fluid domain, to avoid boundary effects. The average, median and mode shear stress values calculated at the scaffold walls were 3.48, 2.90, and 2.45 mPa respectively, at a flow rate of 0.5 cm(3)/min, perfused through a 15 mm diameter scaffold, at an inlet fluid velocity of 53 microm/s. These results were compared to results estimated using a simplified micro-scale model and to results estimated using an analytical macro-scale porous model. The predictions given by the CT-based model are being used in conjunction with an experimental bioreactor model, in order to quantify the effects of fluid-dynamic shear on the growth modulation of tissue-engineered cartilage constructs, to potentially enhance tissue growth in vitro.

Bioreactors↗

PubMed2006

Prediction of the micro-fluid dynamic environment imposed to three-dimensional engineered cell systems in bioreactors.

Bioreactors allowing culture medium perfusion overcome diffusion limitations associated with static culturing and provide flow-mediated mechanical stimuli. The hydrodynamic stress imposed to cells will depend not only on the culture medium flow rate, but also on the scaffold three-dimensional (3D) micro-architecture. We developed a CFD model of the flow of culture medium through a 3D scaffold of homogeneous geometry, with the aim of predicting the shear stress acting on cells as a function of parameters that can be controlled during the scaffold fabrication process, such as the scaffold porosity and the pore size, and during the cell culture, such as the medium flow rate and the diameter of the perfused scaffold section. We built three groups of models corresponding to three pore sizes: 50, 100 and 150 microm. Each group was made of four models corresponding to 59%, 65%, 77%, and 89% porosity. A commercial finite-element code was used to set up and solve the problem and to analyze the results. The mode value of shear stress varied between 2 and 5 mPa, and was obtained for a circular scaffold of 15.5 mm diameter, perfused by a flow rate of 0.5 ml/min. The simulations showed that the pore size is a variable strongly influencing the predicted shear stress level, whereas the porosity is a variable strongly affecting the statistical distribution of the shear stresses, but not their magnitude. Our results provide a basis for the completion of more exhaustive quantitative studies to further assess the relationship between perfusion, at known micro-fluid dynamic conditions, and tissue growth in vitro.

Bioreactors↗

PubMed2005

Synergy between interstitial flow and VEGF directs capillary morphogenesis in vitro through a gradient amplification mechanism.

Cell organization is largely orchestrated by extracellular gradients of morphogenetic proteins. VEGF, an essential factor for capillary formation, is stored in the extracellular matrix, but the mechanisms by which it and other matrix-bound morphogens are mobilized to form spatial gradients are poorly understood. Here, we suggest an efficient mechanism for morphogen gradient generation by subtle biophysical forces in an in vitro model of capillary morphogenesis. Using a fibrin-bound VEGF variant that is released proteolytically to mimic the in vivo situation, we report that low levels of interstitial flow act synergistically with VEGF to drive endothelial organization, whereas each stimulus alone has very little effect. To help account for this synergy, we show how these slow flows can bias the distribution of cell-secreted proteases, which leads, interestingly, to the creation of an increasing VEGF gradient relative to the cell and skewed in the direction of flow. In contrast, diffusion alone can only account for symmetric, decreasing autocrine gradients. Indeed, branching of capillary structures was biased in the direction of flow only with the combination of VEGF and flow. This work thus demonstrates a general mechanism of morphogen gradient generation and amplification by small ubiquitous mechanical forces that are known to exist in vivo.

Capillaries↗

PubMed2004

Biomechanical properties of human articular cartilage under compressive loads.

The function of articular cartilage is to support and distribute loads and to provide lubrication in the diarthrodial joints. Cartilage function is described by proper mechanical and rheological properties, strain and depth-dependent, which are not completely assessed. Unconfined and confined compression are commonly used to evaluate the Young's modulus (E) and the aggregate modulus (H(A)), respectively. The Poisson's ratio (nu) can be calculated indirectly from the equilibrium compression data, or using the biphasic indentation technique; it has recently been optically evaluated by using video microscopy during unconfined compression. The transient response of articular cartilage during confined compression depends on its permeability k; a constant value of k can be easily identified by a simple analytical model of confined compression tests, whereas more complex models or direct measurements (permeation tests) are needed to study the permeability dependence on deformation. A poroelastic finite element model of articular cartilage was developed for this purpose. The elastic parameters (E,nu) of the model were evaluated performing unconfined compression creep tests on human articular cartilage disks, whereas k was identified from the confined test response. Our combined experimental and computational method can be used to identify the parameters that define the permeability dependence on deformation, as a function of depth from articular surface.

Adaptation, Physiological↗

PubMed2004

The effect of media perfusion on three-dimensional cultures of human chondrocytes: integration of experimental and computational approaches.

This work examines the effect of perfusion on human mature articular chondrocytes cultured on synthetic biodegradable scaffolds (DegraPol). Human chondrocytes were isolated, seeded on the scaffolds and subjected to perfused culture at a flow rate of 0.5 ml/min, corresponding to an average inlet fluid velocity of 44 microm/s, with flow inversion every 1 minute. The flow was imposed at the construct surface in some constructs, it was forced through the construct thickness in other constructs and was absent in the static controls. We compared cell viability and morphology and we evaluated material properties of the constructs at 1 month of culture. Thickness-perfused constructs showed significantly higher material properties and roughly a two-fold cell viability, when compared both to surface-perfused constructs and to static controls. Chondrocytes maintained a phenotypic morphology in all experiments, probably favoured by a limited cell-scaffold interaction. Biosynthetic activity could be demonstrated only in the bioreactor-cultured constructs. In this experimental model, a bi-directional flow of culture medium was applied to the cells at a macroscopic level and computational modelling was used to quantify the fluid-dynamic environment induced on the cells at a microscopic level. This method may be used to quantify the effects of fluid-dynamic shear on the growth modulation of tissue-engineered cartilage constructs, to potentially enhance tissue growth in vitro.

Biomechanical Phenomena↗

PubMedDate not supplied

Blood flow pulsatility effects upon oxygen transfer in artificial lungs.

This report discusses theoretical effects of blood flow pulsatility upon the rate of oxygen transfer in artificial lungs, demonstrates the effects with in vitro tests upon commercial oxygenators, and applies the theory to these oxygenators and to a thoracic artificial lung. Steady flow gas transfer theory is applied to pulsatile flow by using the instantaneous value of flow rate at each instant of time, that is, quasi-steady gas transfer. The theory suggests that the local rate of oxygen transfer for a given device and blood composition is proportional to the flow rate to a power less than unity and to the hemoglobin saturation level. It predicts, for some cases, overall reduced rates of gas transfer for pulsatile flow relative to those at steady flow for the same mean blood flow rates. In vitro bovine blood tests, using pediatric oxygenators, a pulsatile pump, and an adjustable compliance chamber, indicate a significant average 10% reduction of oxygen transfer for pulsatile flow relative to steady flow. The application of the theory to the oxygenators predicts gas transfer values that are in agreement with those measured during the experiments. The results have implications in the design of implantable thoracic artificial lungs, which should include a compliant section to dampen the cardiac pulse. A relatively small compliance (0.2 ml/mm Hg) at the thoracic artificial lung inlet is sufficient to obtain approximately 95% of steady flow oxygen transfer.

Artificial Organs↗

PubMedDate not supplied

The effect of hydrodynamic shear on 3D engineered chondrocyte systems subject to direct perfusion.

Bioreactors allowing direct-perfusion of culture medium through tissue-engineered constructs may overcome diffusion limitations associated with static culturing, and may provide flow-mediated mechanical stimuli. The hydrodynamic stress imposed on cells within scaffolds is directly dependent on scaffold microstructure and on bioreactor configuration. Aim of this study is to investigate optimal shear stress ranges and to quantitatively predict the levels of hydrodynamic shear imposed to cells during the experiments. Bovine articular chondrocytes were seeded on polyestherurethane foams and cultured for 2 weeks in a direct perfusion bioreactor designed to impose 4 different values of shear level at a single flow rate (0.5 ml/min). Computational fluid dynamics (CFD) simulations were carried out on reconstructions of the scaffold obtained from micro-computed tomography images. Biochemistry analyses for DNA and sGAG were performed, along with electron microscopy. The hydrodynamic shear induced on cells within constructs, as estimated by CFD simulations, ranged from 4.6 to 56 mPa. This 12-fold increase in the level of applied shear stress determined a 1.7-fold increase in the mean content in DNA and a 2.9-fold increase in the mean content in sGAG. In contrast, the mean sGAG/DNA ratio showed a tendency to decrease for increasing shear levels. Our results suggest that the optimal condition to favour sGAG synthesis in engineered constructs, at least at the beginning of culture, is direct perfusion at the lowest level of hydrodynamic shear. In conclusion, the presented results represent a first attempt to quantitatively correlate the imposed hydrodynamic shear level and the invoked biosynthetic response in 3D engineered chondrocyte systems.

Animals↗

PubMedDate not supplied

Poroelastic numerical modelling of natural and engineered cartilage based on in vitro tests.

The mechanisms underlying the ability of articular cartilage to withstand and distribute the loads applied across diarthrodial joints have been widely studied. Experimental tests have been done under several configurations to reveal the tissue response to mechanical stimuli, and theoretical models have been developed for the interpretation of the experimental results. The experiments demonstrated that the tissue is non-linear with strain, both in tension and in compression, non-linear with direction of stimulus, anisotropic in tension and compression, non-homogeneous with depth, resulting in depth dependent mechanical properties, and presents fluid dependent and fluid independent viscoelasticity. None of the models up to now developed is able to describe the whole set of responses of such a complex tissue. The purpose of this study was to develop a combined experimental-numerical approach for the proper description of the cartilage response under confined and unconfined compression. We defined a series of experimental tests to be performed on disks of natural and engineered cartilage and we developed a numerical model for cartilage, based on the biphasic theory, which potentially includes the tension-compression non-linearity, the strain non-linearity and the fluid independent viscoelasticity. The model successfully simulated the confined and unconfined compression experiments performed on disks of natural and engineered cartilage, and was also used to identify parameters of difficult experimental evaluation, such as the collagen stiffness and the permeability. In conclusion, the use of our model in combination with biomechanical experimental testing seems a valuable tool to analyze the mechanical properties of natural cartilage and the biofunctionality of tissue engineered cartilage.

Aged↗