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Showan N Nazhat - One of the best experts on this subject based on the ideXlab platform.
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Hydraulic Permeability of multilayered collagen gel scaffolds under plastic compression induced unidirectional fluid flow
Acta Biomaterialia, 2013Co-Authors: Vahid Serpooshan, Thomas M Quinn, Naser Muja, Showan N NazhatAbstract:Abstract Under conditions of free fluid flow, highly hydrated fibrillar collagen gels expel fluid and undergo gravity driven consolidation (self-compression; SC). This process can be accelerated by the application of a compressive stress (plastic compression; PC) in order to generate dense collagen scaffolds for tissue engineering. To define the microstructural evolution of collagen gels under PC, this study applied a two-layer micromechanical model that was previously developed to measure Hydraulic Permeability (k) under SC. Radially confined PC resulted in unidirectional fluid flow through the gel and the formation of a dense lamella at the fluid expulsion boundary which was confirmed by confocal microscopy of collagen immunoreactivity. Gel mass loss due to PC and subsequent SC were measured and applied to Darcy’s law to calculate the thickness of the lamella and hydrated layer, as well as their relative permeabilities. Increasing PC level resulted in a significant increase in mass loss fraction and lamellar thickness, while the thickness of the hydrated layer dramatically decreased. Permeability of lamella also decreased from 1.8 × 10−15 to 1.0 × 10−15 m2 in response to an increase in PC level. Ongoing SC, following PC, resulted in a uniform decrease in mass loss and k with increasing PC level and as a function SC time. Experimental k data were in close agreement with those estimated by the Happel model. Calculation of average k values for various two-layer microstructures indicated that they each approached 10−15–10−14 m2 at equilibrium. In summary, the two-layer micromechanical model can be used to define the microstructure and Permeability of multi-layered biomimetic scaffolds generated by PC.
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fibroblast contractility and growth in plastic compressed collagen gel scaffolds with microstructures correlated with Hydraulic Permeability
Journal of Biomedical Materials Research Part A, 2011Co-Authors: Vahid Serpooshan, Naser Muja, Benedetto Marelli, Showan N NazhatAbstract:Scaffold microstructure is hypothesized to influence physical and mechanical properties of collagen gels, as well as cell function within the matrix. Plastic compression under increasing load was conducted to produce scaffolds with increasing collagen fibrillar densities ranging from 0.3 to above 4.1 wt % with corresponding Hydraulic Permeability (k) values that ranged from 1.05 to 0.03 μm2, as determined using the Happel model. Scanning electron microscopy revealed that increasing the level of collagen gel compression yielded a concomitant reduction in pore size distribution and a slight increase in average fibril bundle diameter. Decreasing k delayed the onset of contraction and significantly reduced both the total extent and the maximum rate of contraction induced by NIH3T3 fibroblasts seeded at a density of either 6.0 × 104 or 1.5 × 105 cells mL−1. At the higher cell density, however, the effect of k reduction on collagen gel contraction was overcome by an accelerated onset of contraction which led to an increase in both the total extent and the maximum rate of contraction. AlamarBlue™ measurements indicated that the metabolic activity of fibroblasts within collagen gels increased as k decreased. Moreover, increasing seeded cell density from 2.0 × 104 to 1.5 × 105 cells mL−1 significantly increased NIH3T3 proliferation. In conclusion, fibroblast–matrix interactions can be optimized by defining the microstructural properties of collagen scaffolds through k adjustment which in turn, is dependent on the cell seeding density. © 2011 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 2011.
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characterization and modelling of a dense lamella formed during self compression of fibrillar collagen gels implications for biomimetic scaffolds
Soft Matter, 2011Co-Authors: Vahid Serpooshan, Thomas M Quinn, Naser Muja, Showan N NazhatAbstract:The three dimensional microstructure and fluid conductivity of hydrogels are major determinants of their physical and mechanical properties. Under free fluid flow conditions, highly hydrated fibrillar collagen gels expel fluid and undergo a gravity driven consolidation process (self-compression). Within minutes of the initiation of self-compression, collagen scaffolds with fibrillar densities resembling those of native tissues are produced. However, the microstructural and mechanical processes responsible for collagen gel consolidation have not been fully investigated. During self-compression of collagen gels, a thin, high density lamella of collagen forms at the fluid expulsion boundary thereby generating a two-layer structure. By applying Darcy's law to model fluid flow in the two-layer structure, a novel method was developed to measure the Hydraulic Permeability of hydrated collagen gels as a function of gel mass loss. Experimentally measured Permeability values of the lamella ranged from 4.3 × 10−15 to 1.2 × 10−14 m2 which were 100 to 1000-fold less than those of the hydrated collagen layer. These experimental data were in close agreement with Permeability values estimated by the Happel model. Scanning electron and confocal laser scanning microscopy each confirmed the presence of a two-layer structure beyond three minutes of self-compression. Therefore, the formation of a dense lamella characterized by a significantly reduced Hydraulic Permeability modulates the kinetics of consolidation, as well as the microstructure of fibrillar collagen gels. This provides important implications and functional significance in the processing of multi-layered biomimetic tissue equivalent collagen based scaffolds and drug delivery systems.
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Reduced Hydraulic Permeability of three-dimensional collagen scaffolds attenuates gel contraction and promotes the growth and differentiation of mesenchymal stem cells
Acta Biomaterialia, 2010Co-Authors: Vahid Serpooshan, Naser Muja, Marion Julien, Oliver Nguyen, Huifen Wang, Ailian Li, Janet E. Henderson, Showan N NazhatAbstract:Abstract Optimal scaffold characteristics are essential for the therapeutic application of engineered tissues. Hydraulic Permeability ( k ) affects many properties of collagen gels, such as mechanical properties, cell–scaffold interactions within three dimensions (3D), oxygen flow and nutrient diffusion. However, the cellular response to 3D gel scaffolds of defined k values has not been investigated. In this study, unconfined plastic compression under increasing load was used to produce collagen gels with increasing solid volume fractions. The Happel model was used to calculate the resulting Permeability values in order to study the interaction of k with gel mechanical properties and mesenchymal stem cell (MSC)-induced gel contraction, metabolism and differentiation in both non-osteogenic (basal medium) and osteogenic medium for up to 3 weeks. Collagen gels of fibrillar densities ranging from 0.3 to >4.1 wt.% gave corresponding k values that ranged from 1.00 to 0.03 μm 2 . Mechanical testing under compression showed that the collagen scaffold modulus increased with collagen fibrillar density and a decrease in k value. MSC-induced gel contraction decreased as a direct function of decreasing k value. Relative to osteogenic conditions, non-osteogenic MSC cultures exhibited a more than 2-fold increase in gel contraction. MSC metabolic activity increased similarly under both osteogenic and non-osteogenic culture conditions for all levels of plastic compression. Under osteogenic conditions MSC differentiation and mineralization, as indicated by alkaline phosphatase activity and von Kossa staining, respectively, increased in response to an elevation in collagen fibrillar density and decreased gel Permeability. In this study, gel scaffolds with higher collagen fibrillar densities and corresponding lower k values provided a greater potential for MSC differentiation and appear most promising for bone grafting purposes. Thus, cell–scaffold interactions can be optimized by defining the 3D properties of collagen scaffolds through k adjustment.
Thomas M Quinn - One of the best experts on this subject based on the ideXlab platform.
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Hydraulic Permeability of multilayered collagen gel scaffolds under plastic compression induced unidirectional fluid flow
Acta Biomaterialia, 2013Co-Authors: Vahid Serpooshan, Thomas M Quinn, Naser Muja, Showan N NazhatAbstract:Abstract Under conditions of free fluid flow, highly hydrated fibrillar collagen gels expel fluid and undergo gravity driven consolidation (self-compression; SC). This process can be accelerated by the application of a compressive stress (plastic compression; PC) in order to generate dense collagen scaffolds for tissue engineering. To define the microstructural evolution of collagen gels under PC, this study applied a two-layer micromechanical model that was previously developed to measure Hydraulic Permeability (k) under SC. Radially confined PC resulted in unidirectional fluid flow through the gel and the formation of a dense lamella at the fluid expulsion boundary which was confirmed by confocal microscopy of collagen immunoreactivity. Gel mass loss due to PC and subsequent SC were measured and applied to Darcy’s law to calculate the thickness of the lamella and hydrated layer, as well as their relative permeabilities. Increasing PC level resulted in a significant increase in mass loss fraction and lamellar thickness, while the thickness of the hydrated layer dramatically decreased. Permeability of lamella also decreased from 1.8 × 10−15 to 1.0 × 10−15 m2 in response to an increase in PC level. Ongoing SC, following PC, resulted in a uniform decrease in mass loss and k with increasing PC level and as a function SC time. Experimental k data were in close agreement with those estimated by the Happel model. Calculation of average k values for various two-layer microstructures indicated that they each approached 10−15–10−14 m2 at equilibrium. In summary, the two-layer micromechanical model can be used to define the microstructure and Permeability of multi-layered biomimetic scaffolds generated by PC.
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characterization and modelling of a dense lamella formed during self compression of fibrillar collagen gels implications for biomimetic scaffolds
Soft Matter, 2011Co-Authors: Vahid Serpooshan, Thomas M Quinn, Naser Muja, Showan N NazhatAbstract:The three dimensional microstructure and fluid conductivity of hydrogels are major determinants of their physical and mechanical properties. Under free fluid flow conditions, highly hydrated fibrillar collagen gels expel fluid and undergo a gravity driven consolidation process (self-compression). Within minutes of the initiation of self-compression, collagen scaffolds with fibrillar densities resembling those of native tissues are produced. However, the microstructural and mechanical processes responsible for collagen gel consolidation have not been fully investigated. During self-compression of collagen gels, a thin, high density lamella of collagen forms at the fluid expulsion boundary thereby generating a two-layer structure. By applying Darcy's law to model fluid flow in the two-layer structure, a novel method was developed to measure the Hydraulic Permeability of hydrated collagen gels as a function of gel mass loss. Experimentally measured Permeability values of the lamella ranged from 4.3 × 10−15 to 1.2 × 10−14 m2 which were 100 to 1000-fold less than those of the hydrated collagen layer. These experimental data were in close agreement with Permeability values estimated by the Happel model. Scanning electron and confocal laser scanning microscopy each confirmed the presence of a two-layer structure beyond three minutes of self-compression. Therefore, the formation of a dense lamella characterized by a significantly reduced Hydraulic Permeability modulates the kinetics of consolidation, as well as the microstructure of fibrillar collagen gels. This provides important implications and functional significance in the processing of multi-layered biomimetic tissue equivalent collagen based scaffolds and drug delivery systems.
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reinforcement with cellulose nanocrystals of poly vinyl alcohol hydrogels prepared by cyclic freezing and thawing
Soft Matter, 2011Co-Authors: Tiffany Abitbol, Thomas M Quinn, Timothy C Johnstone, Derek G GrayAbstract:Cellulose nanocrystals (CNCs) were incorporated into polyvinyl alcohol (PVA) hydrogels prepared by repeated freeze–thaw processing. The CNC-loaded hydrogels had improved structural stabilities and distinct microstructures, characterized by ordered domains of CNCs. The water sorption of the gels increased with CNC content due to the hydrophilic nature of the cellulose and the decrease in PVA crystallinity. A reinforcement effect was observed in the CNC-loaded samples upon the application of uniaxial, confined compression, with the elastic moduli of the PVA–CNC samples increased relative to pure PVA hydrogels. Hydraulic Permeability values were derived from the stress transients: at strains of ∼15 to 20% and greater, the Permeability of all samples approached a plateau value reflective of the hindered flow in soft gels which have been compressed, densified and dehydrated.
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anisotropic Hydraulic Permeability in compressed articular cartilage
Journal of Biomechanics, 2006Co-Authors: Boris Reynaud, Thomas M QuinnAbstract:The extent to which articular cartilage Hydraulic Permeability is anisotropic is largely unknown, despite its importance for understanding mechanisms of joint lubrication, load bearing, transport phenomena, and mechanotransduction. We developed and applied new techniques for the direct measurement of Hydraulic Permeability within statically compressed adult bovine cartilage explant disks, dissected such that disk axes were perpendicular to the articular surface. Applied pressure gradients were kept small to minimize flow-induced matrix compaction, and fluid outflows were measured by observation of a meniscus in a glass capillary under a microscope. Explant disk geometry under radially unconfined axial compression was measured by direct microscopic observation. Pressure, flow, and geometry data were input to a finite element model where Hydraulic permeabilities in the disk axial and radial directions were determined. At less than 10% static compression, near free-swelling conditions, Hydraulic Permeability was nearly isotropic, with values corresponding to those of previous studies. With increasing static compression, Hydraulic Permeability decreased, but the radially directed Permeability decreased more dramatically than the axially directed Permeability such that strong anisotropy (a 10-fold difference between axial and radial directions) in the Hydraulic Permeability tensor was evident for static compression of 20-40%. Results correspond well with predictions of a previous microstructurally-based model for effects of tissue mechanical deformations on glycosaminoglycan architecture and cartilage Hydraulic Permeability. Findings inform understanding of structure-function relationships in cartilage matrix, and suggest several biomechanical roles for compression-induced anisotropic Hydraulic Permeability in articular cartilage.
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glycosaminoglycan network geometry may contribute to anisotropic Hydraulic Permeability in cartilage under compression
Journal of Biomechanics, 2001Co-Authors: P Dierickx, Thomas M Quinn, Alan J GrodzinskyAbstract:Resistance to fluid flow within cartilage extracellular matrix is provided primarily by a dense network of rod-like glycosaminoglycans (GAGs). If the geometrical organization of this network is random, the Hydraulic Permeability tensor of cartilage is expected to be isotropic. However, experimental data have suggested that Hydraulic Permeability may become anisotropic when the matrix is mechanically compressed, contributing to cartilage biomechanical functions such as lubrication. We hypothesized that this may be due to preferred GAG rod orientations and directionally-dependent reduction of inter-GAG spacings which reflect molecular responses to tissue deformations. To examine this hypothesis, we developed a model for effects of compression which allows the GAG rod network to deform consistently with tissue-scale deformations but while still respecting limitations imposed by molecular structure. This network deformation model was combined with a perturbation analysis of a classical analytical model for Hydraulic Permeability based on molecular structure. Finite element analyses were undertaken to ensure that this approach exhibited results similar to those emerging from more exact calculations. Model predictions for effects of uniaxial confined compression on the Hydraulic Permeability tensor were consistent with previous experimental results. Permeability decreased more rapidly in the direction perpendicular to compression than in the parallel direction, for matrix solid volume fractions associated with fluid transport in articular cartilage. GAG network deformations may therefore introduce anisotropy to the Permeability (and other GAG-associated matrix properties) as physiological compression is applied, and play an important role in cartilage lubrication and other biomechanical functions.
Ulrich Tallarek - One of the best experts on this subject based on the ideXlab platform.
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influence of the particle size distribution on Hydraulic Permeability and eddy dispersion in bulk packings
Analytical Chemistry, 2011Co-Authors: Anton Daneyko, Alexandra Holtzel, Siarhei Khirevich, Ulrich TallarekAbstract:The narrow particle size distribution (PSD) of certain packing materials has been linked to a reduced eddy dispersion contribution to band broadening in chromatographic columns. It is unclear if the influence of the PSD acts mostly on the stage of the packing process or if a narrow PSD provides an additional, intrinsic advantage to the column performance. To investigate the latter proposition, we created narrow-PSD and wide-PSD random packings based on the experimental PSDs of sub-3 μm core–shell and sub-2 μm fully porous particles, respectively, as determined by scanning electron microscopy. Unconfined packings were computer-generated with a fixed packing protocol at bed porosities from random-close to random-loose packing to simulate fluid flow and advective–diffusive mass transport in the packings’ interparticle void space. The comparison of wide-PSD, narrow-PSD, and monodisperse packings revealed no systematic differences in Hydraulic Permeability and only small differences in hydrodynamic dispersion,...
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influence of the particle size distribution on Hydraulic Permeability and eddy dispersion in bulk packings
Analytical Chemistry, 2011Co-Authors: Anton Daneyko, Alexandra Holtzel, Siarhei Khirevich, Ulrich TallarekAbstract:The narrow particle size distribution (PSD) of certain packing materials has been linked to a reduced eddy dispersion contribution to band broadening in chromatographic columns. It is unclear if th...
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packing density Permeability and separation efficiency of packed microchips at different particle aspect ratios
Journal of Chromatography A, 2009Co-Authors: Stephanie Jung, Steffen Ehlert, Joseangel Mora, Karsten Kraiczek, Monika Dittmann, Gerard P Rozing, Ulrich TallarekAbstract:Abstract HPLC microchips are investigated experimentally with respect to packing density, pressure drop–flow rate relation, Hydraulic Permeability, and separation efficiency. The prototype microchips provide minimal dead volume, on-chip UV detection, and a 75 mm long separation channel with a ca. 50 μm × 75 μm trapezoidal cross-section. A custom-built stainless-steel holder allowed to adopt optimized packing conditions. Separation channels were slurry-packed with 3, 5, and 10 μm-sized spherical, porous C8-silica particles. Differences in interparticle porosity, Permeability, and plate height data are analyzed and consistently explained by different microchannel-to-particle size (particle-aspect) ratios and particle size distributions.
Yoram Cohen - One of the best experts on this subject based on the ideXlab platform.
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tuning the Hydraulic Permeability and molecular weight cutoff mwco of surface nano structured ultrafiltration membranes
Journal of Membrane Science, 2021Co-Authors: Yian Chen, Soomin Kim, Anditya Rahardianto, Yoram CohenAbstract:Abstract Hydraulic Permeability and molecular weight cutoff (MWCO) performance tuning of polysulfone (PSf) ultrafiltration (UF) membrane was attained via surface modification with a tethered layer of poly(acrylic acid) (PAA). Surface nano-structured (SNS) PAA layer was synthesized onto base PSf UF membranes by atmospheric pressure plasma (APP) surface activation followed by graft polymerization (GP) of acrylic acid (AA). Through a systematic study, it was shown that effective SNS-PAA-PSf UF membrane performance tuning was feasible by adjustments of APP and graft polymerization conditions. It was shown, for the first time, that SNS-PAA-PSf membranes can be synthesized with a range of Hydraulic Permeability (spanning a factor of 1.1–2.6 in magnitude) for a given MWCO, or a range of MWCO (spanning a factor of 1.5–2.3 in magnitude) for a given Hydraulic Permeability, thereby overcoming the Hydraulic Permeability-MWCO tradeoff.
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fouling and rejection behavior of ceramic and polymer modified ceramic membranes for ultrafiltration of oil in water emulsions and microemulsions
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2001Co-Authors: Ron S Faibish, Yoram CohenAbstract:Abstract The effectiveness of poly(vinylpyrrolidone) (PVP)-modification of a zirconia-based ultrafiltration membrane was investigated for the treatment of oil-in-water (o/w) emulsions. Fouling, Hydraulic Permeability, flux decline, and solute rejection for modified and native membranes were evaluated using a diagnostic o/w emulsion as well as an emulsion prepared using a commercial cutting oil. The native membrane was irreversibly fouled by both the o/w microemulsion and the commercial cutting oil emulsion. In contrast, irreversible fouling was not observed for the PVP-modified membrane. The main fouling agent for the diagnostic o/w microemulsion was identified as the anionic surfactant octanoate, which lead to an irreversible decline of initial membrane Hydraulic Permeability of up to 20%. Adsorption of surfactant species onto the native membrane surface were attributed to charge screening due to high ionic strength environment and presence of hydroxyl groups on the zirconia surface. Analysis of permeate flux decline and membrane resistance behavior suggests that membrane fouling was mainly due to a solute cake (gel) layer buildup at the membrane surface. However, the possibility of some internal pore plugging, due to deposition of small surfactant molecules, micelles or oil droplets, during the initial few minutes of filtration runs, could not be ruled out. Improved oil rejection (two-fold for the microemulsion and over 20% for the cutting oil emulsion) with the modified membrane compared to the native membrane was attributed to repair (or narrowing) of defects (or ‘pin-holes’) of the native membrane upon polymer grafting.
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fouling resistant ceramic supported polymer membranes for ultrafiltration of oil in water microemulsions
Journal of Membrane Science, 2001Co-Authors: Ron S Faibish, Yoram CohenAbstract:Abstract Fouling-resistant ultrafiltration ceramic-supported polymer (CSP) ultrafiltration membrane was developed for the treatment of oil-in-water (o/w) microemulsions. The CSP zirconia-based membrane was prepared via free-radical graft polymerization of vinylpyrrolidone onto the membrane surface. Pore reduction of about 25–28% was encountered upon modification as revealed by Hydraulic Permeability measurements. Oil rejection was higher for grafted membranes with smaller resultant pore size. Atomic force microscopy imaging of polymer-modified surfaces showed complete surface coverage by the polymer chains. The native zirconia membrane was irreversibly fouled after treating the o/w microemulsion for a brief period, while the CSP membranes maintained its pre-filtration Hydraulic Permeability even after many filtration runs. The present surface modification was found to be effective in preventing irreversible membrane fouling despite significant membrane surface roughness. Relative to the native membrane, oil rejection of the CSP membrane increased more than two-fold for the range of studied oil droplet size (18–66 nm), while surfactant rejection remained low for both the native and modified membranes.
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on the loss of Hydraulic Permeability in ceramic membranes
Journal of Colloid and Interface Science, 1996Co-Authors: Josep Font, Robert Castro, Yoram CohenAbstract:The properties of inorganic membranes are dependent on the surface chemistry of the material, which in turn are affected by parameters such as temperature, pH, and chemisorption of molecules. For both silica and γ-alumina membranes, a significant reduction in water Hydraulic Permeability was observed after heat treatment or exposure to organic solvents. For silica membranes, the water Permeability reduction due to heating at 140°C has been attributed to the removal of physically adsorbed water, which exposes hydrophobic surface patches covered by isolated hydroxyl groups. In the case of γ-alumina membranes strong adsorption of a wide range of organic compounds can result in significant Permeability loss.
Vahid Serpooshan - One of the best experts on this subject based on the ideXlab platform.
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Hydraulic Permeability of multilayered collagen gel scaffolds under plastic compression induced unidirectional fluid flow
Acta Biomaterialia, 2013Co-Authors: Vahid Serpooshan, Thomas M Quinn, Naser Muja, Showan N NazhatAbstract:Abstract Under conditions of free fluid flow, highly hydrated fibrillar collagen gels expel fluid and undergo gravity driven consolidation (self-compression; SC). This process can be accelerated by the application of a compressive stress (plastic compression; PC) in order to generate dense collagen scaffolds for tissue engineering. To define the microstructural evolution of collagen gels under PC, this study applied a two-layer micromechanical model that was previously developed to measure Hydraulic Permeability (k) under SC. Radially confined PC resulted in unidirectional fluid flow through the gel and the formation of a dense lamella at the fluid expulsion boundary which was confirmed by confocal microscopy of collagen immunoreactivity. Gel mass loss due to PC and subsequent SC were measured and applied to Darcy’s law to calculate the thickness of the lamella and hydrated layer, as well as their relative permeabilities. Increasing PC level resulted in a significant increase in mass loss fraction and lamellar thickness, while the thickness of the hydrated layer dramatically decreased. Permeability of lamella also decreased from 1.8 × 10−15 to 1.0 × 10−15 m2 in response to an increase in PC level. Ongoing SC, following PC, resulted in a uniform decrease in mass loss and k with increasing PC level and as a function SC time. Experimental k data were in close agreement with those estimated by the Happel model. Calculation of average k values for various two-layer microstructures indicated that they each approached 10−15–10−14 m2 at equilibrium. In summary, the two-layer micromechanical model can be used to define the microstructure and Permeability of multi-layered biomimetic scaffolds generated by PC.
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fibroblast contractility and growth in plastic compressed collagen gel scaffolds with microstructures correlated with Hydraulic Permeability
Journal of Biomedical Materials Research Part A, 2011Co-Authors: Vahid Serpooshan, Naser Muja, Benedetto Marelli, Showan N NazhatAbstract:Scaffold microstructure is hypothesized to influence physical and mechanical properties of collagen gels, as well as cell function within the matrix. Plastic compression under increasing load was conducted to produce scaffolds with increasing collagen fibrillar densities ranging from 0.3 to above 4.1 wt % with corresponding Hydraulic Permeability (k) values that ranged from 1.05 to 0.03 μm2, as determined using the Happel model. Scanning electron microscopy revealed that increasing the level of collagen gel compression yielded a concomitant reduction in pore size distribution and a slight increase in average fibril bundle diameter. Decreasing k delayed the onset of contraction and significantly reduced both the total extent and the maximum rate of contraction induced by NIH3T3 fibroblasts seeded at a density of either 6.0 × 104 or 1.5 × 105 cells mL−1. At the higher cell density, however, the effect of k reduction on collagen gel contraction was overcome by an accelerated onset of contraction which led to an increase in both the total extent and the maximum rate of contraction. AlamarBlue™ measurements indicated that the metabolic activity of fibroblasts within collagen gels increased as k decreased. Moreover, increasing seeded cell density from 2.0 × 104 to 1.5 × 105 cells mL−1 significantly increased NIH3T3 proliferation. In conclusion, fibroblast–matrix interactions can be optimized by defining the microstructural properties of collagen scaffolds through k adjustment which in turn, is dependent on the cell seeding density. © 2011 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 2011.
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characterization and modelling of a dense lamella formed during self compression of fibrillar collagen gels implications for biomimetic scaffolds
Soft Matter, 2011Co-Authors: Vahid Serpooshan, Thomas M Quinn, Naser Muja, Showan N NazhatAbstract:The three dimensional microstructure and fluid conductivity of hydrogels are major determinants of their physical and mechanical properties. Under free fluid flow conditions, highly hydrated fibrillar collagen gels expel fluid and undergo a gravity driven consolidation process (self-compression). Within minutes of the initiation of self-compression, collagen scaffolds with fibrillar densities resembling those of native tissues are produced. However, the microstructural and mechanical processes responsible for collagen gel consolidation have not been fully investigated. During self-compression of collagen gels, a thin, high density lamella of collagen forms at the fluid expulsion boundary thereby generating a two-layer structure. By applying Darcy's law to model fluid flow in the two-layer structure, a novel method was developed to measure the Hydraulic Permeability of hydrated collagen gels as a function of gel mass loss. Experimentally measured Permeability values of the lamella ranged from 4.3 × 10−15 to 1.2 × 10−14 m2 which were 100 to 1000-fold less than those of the hydrated collagen layer. These experimental data were in close agreement with Permeability values estimated by the Happel model. Scanning electron and confocal laser scanning microscopy each confirmed the presence of a two-layer structure beyond three minutes of self-compression. Therefore, the formation of a dense lamella characterized by a significantly reduced Hydraulic Permeability modulates the kinetics of consolidation, as well as the microstructure of fibrillar collagen gels. This provides important implications and functional significance in the processing of multi-layered biomimetic tissue equivalent collagen based scaffolds and drug delivery systems.
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Reduced Hydraulic Permeability of three-dimensional collagen scaffolds attenuates gel contraction and promotes the growth and differentiation of mesenchymal stem cells
Acta Biomaterialia, 2010Co-Authors: Vahid Serpooshan, Naser Muja, Marion Julien, Oliver Nguyen, Huifen Wang, Ailian Li, Janet E. Henderson, Showan N NazhatAbstract:Abstract Optimal scaffold characteristics are essential for the therapeutic application of engineered tissues. Hydraulic Permeability ( k ) affects many properties of collagen gels, such as mechanical properties, cell–scaffold interactions within three dimensions (3D), oxygen flow and nutrient diffusion. However, the cellular response to 3D gel scaffolds of defined k values has not been investigated. In this study, unconfined plastic compression under increasing load was used to produce collagen gels with increasing solid volume fractions. The Happel model was used to calculate the resulting Permeability values in order to study the interaction of k with gel mechanical properties and mesenchymal stem cell (MSC)-induced gel contraction, metabolism and differentiation in both non-osteogenic (basal medium) and osteogenic medium for up to 3 weeks. Collagen gels of fibrillar densities ranging from 0.3 to >4.1 wt.% gave corresponding k values that ranged from 1.00 to 0.03 μm 2 . Mechanical testing under compression showed that the collagen scaffold modulus increased with collagen fibrillar density and a decrease in k value. MSC-induced gel contraction decreased as a direct function of decreasing k value. Relative to osteogenic conditions, non-osteogenic MSC cultures exhibited a more than 2-fold increase in gel contraction. MSC metabolic activity increased similarly under both osteogenic and non-osteogenic culture conditions for all levels of plastic compression. Under osteogenic conditions MSC differentiation and mineralization, as indicated by alkaline phosphatase activity and von Kossa staining, respectively, increased in response to an elevation in collagen fibrillar density and decreased gel Permeability. In this study, gel scaffolds with higher collagen fibrillar densities and corresponding lower k values provided a greater potential for MSC differentiation and appear most promising for bone grafting purposes. Thus, cell–scaffold interactions can be optimized by defining the 3D properties of collagen scaffolds through k adjustment.
