Reduction In Pore Volume Research Articles

Microestrutura da fibra alimentar do albedo de laranja: um estudo por técnicas físicas e análise de imagens

The food fibers are consumed naturally in cereals, fruits and vegetables, and they are also added in a concentrated form in processed foods. Different fiber sources have different structures and chemical composition that will define their nutritional or technological purpose. This research aims the characterization of orange albedo dietary fiber submitted to two methods of drying (conventional and lyophilization) and split by granulometric intervals. For the density determinations, surface area, porosity and pore size distribution it was used classical techniques of the particulate systems tecnology and it was compared whit images of scanning electron microscopy. The results showed that the drying method influence more in the material pore volume reduction, this is also visualized in the microstructural characteristics reveled by image analysis. The granulometric interval of particulate fiber showed itself inversely proportional to the material density and material specific surface area. The orange albedo dietary fiber showed structural characteristics that allow the diversification of new food products with high nutritional and commercial value.

Improvement of the chloride ingress resistance of OPC mortars by using spent cracking catalyst

The influence of the incorporation of spent cracking catalyst (FC3R) on the chloride ingress resistance has been evaluated. Thermogravimetric analyses have shown that the pozzolanic reaction of FC3R yields higher contents of hydrated calcium aluminates and silicoaluminates, so chloride binding capacity of mortars was highly improved. Mercury intrusion porosimetry analyses demonstrated that FC3R produces a significant reduction of capillary pore volume. As a result non-steady-state and steady-state chloride diffusion coefficients were reduced, enhancing the chloride ingress resistance of mortars incorporating FC3R. Additionally, the corrosion behaviour of steel embedded in Portland cement mortars partially substituted by spent cracking catalyst (FC3R) under chloride attack has been studied. Results showed that the incorporation of FC3R decreased the corrosion rates of steels and increased chloride thresholds for corrosion. For this reason, FC3R is an interesting pozzolanic material that can be used in reinforced concrete for civil engineering applications exposed to the action of chlorides.

メタンハイドレートを胚胎する砂質堆積物の特徴とメタンハイドレート胚胎メカニズムへの影響

Methane hydrate-bearing sediments taken by a Pressure Temperature Core Sampler (PTCS), were recovered from the NE-Nankai Trough to clarify the accumulating mechanisms whereby methane hydrate accumulates in pore spaces. The sediments were of the sand and mud alternation layer, which were turbidities and hemi-pelagic mud, respectively. Their sediment features such as grain size, porosity, and methane hydrate saturation of pore spaces were analyzed systematically. Relationships among grain size distribution, Bouma's sequence and hydrocarbon gas volume produced by methane hydrate dissociation were confirmed, thus we understood the correspondence of the features of turbidities features and methane hydrate saturation; i.e., we understood that the part of high methane hydrate saturation had a porosity of 50-55%, median of 1.9-3.3φ, and grain sorting of from -1.4 to -0.4. These features indicate the lower section of turbidite called as Ta and/or Tb. Other sections of turbidite had small amounts of methane hydrate. From these results, we estimated that only permeable sections could maintain the accumulation of methane hydrate from first to last stage. If their pores had been filled with gas-saturated water and the pressure/temperature condition was sufficient for methane hydrate, the methane hydrate would precipitate homogeneously in sediments from its initial accumulation. However, methane hydrate precipitation reduces permeability because methane hydrate growth in pores causes a reduction of pore volume. During accumulation, it is inevitable that gas-rich fluid could not reach into low permeable layers, although it could still be supplied into high permeable layers. This mechanism does not need any cap rock or some trap system in sediments, and it is quite a different mechanism from conventional oil and gas accumulation.

Framework and grafted nickel ethylenediamine complexes in 2D hexagonal mesostructured templated silica

Well-ordered periodic mesoporous organosilicas (PMOs) of the MCM-41 type of structure containing framework Ni(II) complexes (Ni@PMOs) were synthesized via a one-pot synthesis route in mild conditions, using cheap and environmental friendly reactants. These materials were obtained using the N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPTMS) ligand in three different molar ratios of Ni, X = 1, 2, 3 and co-condensed with sodium silicate in the presence of cetyltrimethylammonium tosylate as a templating agent. The latter named OP-Ni-1, OP-Ni-2 and OP-Ni-3, respectively, were submitted to a treatment using a mixture of chlorotrimethylsilane (CTMS) and hexamethyldisilazane (HMDSA) that simultaneously removed the surfactant and capped the silanol groups leading to materials OP-Ni-1TA, OP-Ni-2TA and OP-Ni-3TA (∼2 wt% Ni content). For comparison, a series of mesoporous materials were prepared via a post-synthesis route to locate the Ni(II) complexes in the channel (∼2.4 wt% Ni). The latter were prepared using the “molecular stencil patterning” (MSP) technique based on sequential grafting of trimethylsilyl (TMS) groups and the same ligand cited above using AAPTMS/Ni molar ratios of 2 and 3. These steps lead to intermediate materials LUS to LUS-PSE and final materials LUS-MSP-Ni-2 and LUS-MSP-Ni-3. Both series of materials were thoroughly investigated in terms of (i) coordination state compared to molecular analogues, (ii) Ni(II) to Cu(II) exchange ability and (iii) coordination accessibility using ethylenediamine (en) or thiocyanate (SCN−) as probe ligands. All materials were investigated using XRD, TEM, N2 sorption isotherms, UV-visible, FT-IR and EPR spectroscopies. In particular, FT-IR and EPR data were treated quantitatively to monitor both the TMS loading and the concentration of isolated Cu(II) in conjunction with elemental analysis. Pore size decrease, pore volume reduction, nickel to copper exchangeability (> 90%) and ligand accessibility were observed for materials LUS-MSP-Ni-2 and LUS-MSP-Ni-3 as expected with grafted Ni(II) complexes located in the channel. By contrast, a clear opposite trend, observed for Ni@PMOs, OP-Ni-1, OP-Ni-2 and OP-Ni-3, was fully consistent with a complete insertion and captation of the Ni complexes in the siliceous pore walls despite the very small thickness (ca. 1.4 nm) of the wall. This is explained by the state of Ni(II) both in terms of coordination and cavity effect. Indeed, both absence of counterion coming from the solution (nitrate) and bathochromic shift of the d-d electronic transitions, are consistent with the presence of silanolate groups counterbalancing the electrical charges and mainly leading to neutral framework species of formula, [Ni(AAPS)2(SiO)2], [Ni(AAPS)(OL)2(SiO)2], with neutral ligand OL = H2O, SiOH, SiOSi and AAPS = N-(2-aminoethyl)-3-aminopropylsilyl moities.

Time‐dependent, anisotropic pore structure and soil strength in a 10‐year period after intensive tractor wheeling under conservation and conventional tillage

AbstractAny soil deformation induced by agricultural machinery is transmitted three‐dimensionally and the “kneading effect” of tractor wheeling further rearranges soil particles and aggregates anisotropically. In this work, we investigated how heterogeneous soil structure remained 10 y after a complete wheeling of fields in 1995 with a single pass of 2 × 2.5 Mg and of 6 × 5 Mg on a silty loam Luvisol derived from loess. Control plots received no tractor wheeling. We also analyzed how soil physical properties responded to the tractor wheeling under two management systems: continuous conservation tillage (chisel plow = CS) with mulch cover and conventional tillage (plowing to 25 cm depth annually = CT).We compared three sampling dates: done before wheeling in 1995, after wheeling in 1995, and in 2004. Results showed that applying tractor wheeling in 1995 not only reduced total soil‐pore volume but also increased soil strength as expressed by precompression stress. The reduction of total pore volume at 30 cm depth was more pronounced in CS than in CT. After 10 y of continuous use of the two tillage systems, the precompression stress of the wheeled soils was greater in the vertical direction than in the horizontal direction. This anisotropy of soil strength and its load dependency were also more pronounced in CS than in CT. The effect of wheeling on the fluxes of gas and water was covered up by the effects of biochannels, causing a prevailing vertical passage. From this study, we conclude that heavy, agricultural machinery causes soil degradation, which is more evident in CS than in CT.

Study on Hydrothermal Stability and Catalytic Activity of Nanosphere-Walled Mesoporous Silica

The Rh-embedded mesoporous silica (RMS) with a silica-nanosphere pore wall was successfully synthesized using silica nanospheres as a framework and citric acid as a nonsurfactant template. The RMS had a three-dimensionally interconnected and disordered wormhole-like mesostructure, and its pore size was easily controlled by simply changing the citric acid concentration or carrying out the aging treatment. In particular, we have first reported high hydrothermal stability and catalytic activity of the nanosphere-walled mesoporous silica in an ethanol steam reforming reactor at high temperature. As a result, the RMS showed 1.8% and 1.3% reduction of BET surface area and total pore volume after the ethanol steam reforming test for 5 h in the temperature range of 400 °C to 600 °C, whereas Rh-impregnated SBA-15 (RSBA15) showed 31.3% and 14.0% reduction of those. It is well-known that mesoporous materials have not been widely used as catalysts or catalyst supports in industry yet because of their poor hydrothermal stability. Accordingly, we have improved the hydrothermal stability of mesoporous silica through using silica nanospheres as a framework and have confirmed that the high hydrothermal stability of the RMS was attributed to the thermodynamically stable sphere-shaped pore wall, large intra-micropore volume of the silica-nanosphere wall, the thicker pore wall, and the highly branched structure of colloidal silica synthesized under base-catalyzed conditions. Moreover, compared to the RSBA15, the RMS gave a higher catalytic activity for the ethanol steam reforming because of its larger pore size and three-dimensionally interconnected pore structure, which lead to improvement in accessibility of reactants to the catalytic active sites.

Physico-chemical characterization of MCM-41 silica spheres made by the pseudomorphic route and grafted with octadecyl chains

This work reports on the first comprehensive characterization of octadecyl (C 18) modified MCM-41 silica spheres, prepared via the pseudomorphic route, followed by grafting with mono- or trifunctional octadecyl (C 18) alkyl chains and endcapping with hexamethyldisilazane. Small angle X-ray scattering (SAXS), nitrogen adsorption–desorption and scanning electron microscopy (SEM) measurements were performed to obtain information about the MCM-41 pore structure, surface properties and morphological features. The degree of grafting and cross-linking of the silanes were determined by 29Si magic angle spinning NMR spectroscopy, while FTIR and 13C NMR were employed to study the conformational behavior of the surface-immobilized alkyl chains. The SAXS pattern proved the existence of a hexagonal mesopore arrangement for both the ungrafted and the grafted MCM-41 silica spheres. In addition, there is evidence of some long-range distortion in the pore structure. SEM measurements revealed the same morphological features for the parent silica and the MCM-41 silica spheres before and after C 18 grafting. The achieved surface loading for the MCM-41 material is rather low. It was also shown that a substantial amount of the accessible surface silanol groups is endcapped by trimethylsilane which in turn results in a very low surface coverage due to the octadecyl chains. The nitrogen sorption studies provided values for the surface area, total pore volume and pore diameter which are very typical for mesoporous materials. The reduction in surface area and total pore volume upon surface grafting is related to the binding of trimethylsilane in the interior of the pores, while due to the spatial restrictions octadecyl chains are primarily attached near the pore entrance. The experimental FTIR and 13C NMR data point to a very low conformational order of the C 18 chains which is in accordance with the observed low surface coverage and the resulting spatial freedom for these surface-immobilized alkyl chains.

Paper coating properties as affected by pigment blending and calendering

To gain insight into how and why pigment blending affects coating pore and surface structures and how these structures are affected by calendering, we prepared coating colors with different blending ratios of ground calcium carbonate (GCC) and clay with a similar particle size distribution using a constant level of binder and co-binder. We applied the coatings on plastic films and calendered them using a paper sheet backing to account for the part of compression absorbed by the base paper. The coating layer structures were characterized by mercury porosimetry, atomic force microscopy, gloss, and spectrophotometric measurements. The results showed that synergy occurs for pore volume, pore size, and light-scattering coefficient when blending GCC and clay; that is, the property from a pigment blend can be higher than what is predicted from simple additivity. For example, the pore volume and pore size reached a maximum value of approximately 70% clay and 30% GCC. However, no synergy was observed for smoothness and gloss. Calendering significantly compressed the coating layers and reduced some of the synergy in pore volume and pore size created by the blending. Although the 100% clay coating was twice as much compressed by the calendering as the 100% GCC coating, the blending of clay and GCC at a wide range of ratios showed a similar extent of compression (calculated as reduction in pore volume). We proposed a mechanism to explain the synergies by blending and the compression caused by calendering.

Impact of Prior Physico-Chemical Treatment on the Clogging Process of Subsurface Flow Constructed Wetlands: Model-Based Evaluation

The objective of this study was to check the effect of the use of a physico-chemical treatment on the clogging process of horizontal subsurface flow constructed wetlands by means of dynamic modelling. The hydraulic submodel was based on series as well as parallel branched complete stirred tanks of equal volume. The model was validated with data obtained from 2 identical experimental wetlands, which had a surface area of 0.54 m2 and a water depth of 0.30 m, and that were monitored over a period of 5 months. One of the wetlands was fed with settled urban wastewater, whereas the other with the same wastewater, but previously treated with a physico-chemical treatment. In the model, pore volume reduction depends on the growth of bacteria and on solids retained. The effluent concentrations of COD and ammonium in both experimental wetlands were very similar in all the conditions tested, and therefore the physico-chemical treatment did not improve the removal efficiency. The model indicated that after 120 days of operation in some regions of the wetland fed with settled wastewater the porosity decreased in a 17%, whereas in the other wetlands it only decreased as much as 6%. The use of a prior physico-chemical treatment is a good alternative for avoiding an anticipated clogging of subsurface flow constructed wetlands.

Melt flow velocity in high pressure die casting: its effect on microstructure and mechanical properties in an Al–Si alloy

Analysis of as cast tensile specimens of Australian alloy CA313 (an equivalent of A380) made using the high pressure die casting process at three different melt flow velocities has revealed that increasing melt velocity resulted in a finer microstructure and a reduced pore volume fraction and, consequently, better mechanical properties. Mechanical properties are shown to increase more than proportionately with reducing pore volume fraction and the reason for this is demonstrated to be the contribution from the more refined microstructure. The gate used in the investigation was unusual in that its geometry encouraged and prolonged the shearing of the melt. It is proposed that the microstructure was refined by flow shear, the rate of which increased with melt velocity and consequently enhanced the fragmentation of the externally solidified α-Al grains and inclusions such as oxides, cold flakes and gas bubbles.

Global Optimization and Uncertainty Assessment for History Matching the Valhall Field

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 100946, "History Matching of the Valhall Field Using a Global- Optimization Method and Uncertainty Assessment," by B.R. Al-Shamma, SPE, Imperial College, London, and R. Teigland, Total E&P Norge, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September. A study was made of a global-optimization method to history match a complex reservoir model. Evolutionary algorithms (EAs) were applied, and methods for improving the convergence of the optimization cycle were used. Sensitivity parameters, correlations, and parameter trends were identified in a global search space, and an uncertainty assessment of the most recent history match was carried out with an experiment-design matrix. Introduction The focus of this study was to assess the Valhall field reservoir-simulation model by use of EAs and uncertainty assessments to improve the manual history match of the model. The Valhall field is complex with rock compaction as the main drive energy, which contributes to many challenges, including the reduction of porosity (Ø) and permeability (k) with time and continued change of reservoir thickness caused by the compaction. The compaction can affect different parts of the reservoir differently. For example, the reduction in the pore volume can vary between regions, resulting in problems with the history-matching process in which changing a particular parameter can have opposite effects on wells or regions. Fig. 1 shows the workflow used for uncertainty assessment. The Valhall field is an oversaturated Upper Cretaceous chalk reservoir in the North Sea approximately 290 km offshore Norway in 69 m of water. Discovered in 1975, oil production began in 1982. Oil originally in place was estimated at 4.3×108 m3. Total production at the time this paper was written was 7.9×107 m3 of oil equivalent. The primary reservoir is the Tor formation, with a secondary reservoir in the Hod formation. The thickness of the Tor formation varies, abruptly ranging from 0 to 80 m. Approximately 50% of the drive to date has been from rock compaction, observed by porosity reduction in infill wells. The porosity is greater than 50% in some places; some fractured permeability exists, but matrix permeability is low (less than 10 md). The average solution-gas/oil ratio (GOR) for the field is approximately 20 std m3/m3, with an increase in the field GOR after 20 years' depletion of only 35%. The main challenges experienced during drilling and production included chalk and solids production, well failures and stability problems, fracture closure during depletion, and a subsiding seafloor around the original production platform.

Impregnation of palm shell activated carbon with polyethyleneimine and its effects on Cd 2+ adsorption

Palm shell activated carbon (AC) was impregnated with low (LMW) and high molecular weight (HMW) polyethyleneimine (PEI) via batch adsorption. The maximum amount of LMW PEI adsorbed on AC was determined to be approximately 425 mg/g carbon while the maximum adsorbed HMW PEI was 2.8 mg/g carbon. The LMW PEI adsorption data fitted the Langmuir isotherm better than the Freundlich isotherm. The PEI impregnated AC were characterized via nitrogen adsorption, scanning electron microscopy, elemental analysis, thermogravimetric, Fourier transform infra red and pH drift analyses. Only LMW PEI could be successfully impregnated as monolayers on the surface of the micropores. This impregnation leads to drastic reduction of surface area and pore volume whereas impregnation of HMW PEI does not significantly affect the physical characteristics of the AC. LMW PEI impregnation appears to enhance Cd 2+ adsorption capacity of the AC while HMW PEI impregnation is found to have an opposite effect. This result shows that LMW PEI impregnation technique may have the potential to improve the batch adsorption capacity of AC for other transition metal ion adsorption as well.

Immediate and Time-Dependent Compression of Tire Derived Aggregate

This paper examines immediate and time-dependent compression of tire derived aggregate (TDA) and TDA-soil composites. To accommodate large particle sizes, modified experimental devices were developed and used to test tire chips and tire shreds. Immediate compression of TDA, which results almost entirely from the reduction of pore volume, increases with TDA content and tire particle size. The secant constrained modulus ( Msec ) of TDA defined over the stress range of 0–50 kPa varied from a low of 255 kPa (100% tire shreds) to a high of 1,320 kPa (50% tire chips). A characteristic relationship between strain and time exists for TDA and TDA composites under one-dimensional confined compression. Time-dependent deformation is well described by the modified secondary compression index ( Cαe ) , which ranged from 0.0010 (50% tire chips) to 0.0074 (100% tire chips). Time-dependent deformation was inversely proportional to sand content, with the most significant changes resulting from the addition of 15% sand. Bot...

Investigation of the undrained poroelastic response of sandstones to confining pressure via laboratory experiment, numerical simulation and analytical calculation

Abstract To describe the poroelastic behaviour of sandstones, two factors have to be considered: the grain structure and the pore volume included. Changes in these two factors through diagenetic processes, tectonic loading or other forces lead to different results. Often external stresses induce a compaction of the rock and, therefore, a reduction of pore volume and an increased fluid pressure. Under undrained conditions, the largest pore pressure response can be observed. Besides the Biot coefficient, the Skempton coefficient ( B ) is one of the most important variables of elastic rock deformation, as it describes the pore pressure change related to the acting stresses. This study shows three ways of determining the Skempton coefficient and gives evidence of its pressure dependence. First, the undrained poroelastic response of a Bentheimer sandstone sample to confining pressure change was measured. Second, a thin-section micrograph was transferred into a finite-element model, including a discretization of the grain structure and the pore space. Finally, the Skempton coefficient of a linear elastic hollow sphere was calculated to prove the laboratory experiment and the numerical simulation.

Ca 2+ Selectivity of a Chemically Modified OmpF with Reduced Pore Volume

We studied an E. coli OmpF mutant (LECE) containing both an EEEE-like locus, typical of Ca 2+ channels, and an accessible and reactive cysteine. After chemical modification with the cysteine-specific, negatively charged (−1 e) reagents MTSES or glutathione, this LECE mutant was tested for Ca 2+ versus alkali metal selectivity. Selectivity was measured by conductance and zero-current potential. Conductance measurements showed that glutathione-modified LECE had reduced conductance at Ca 2+ mole fractions <10 −3. MTSES-modified LECE did not. Apparently, the LECE protein is (somehow) a better Ca 2+ chelator after modification with the larger glutathione. Zero-current potential measurements revealed a Ca 2+ versus monovalent cation selectivity that was highest in the presence of Li + and lowest in the presence of Cs +. Our data clearly show that after the binding of Ca 2+ the LECE pore (even with the bulky glutathione present) is spacious enough to allow monovalent cations to pass. Theoretical computations based on density functional theory combined with Poisson-Nernst-Planck theory and a reduced pore model suggest a functional separation of ionic pathways in the pore, one that is specific for small and highly charged ions, and one that accepts preferentially large ions, such as Cs +.

Externally Tunable Dynamic Confinement Effect in Organosilica Sol−Gels

The feasibility of using the pores of organosilica sol-gels for dynamic confinement of a luminescent molecule is demonstrated in this paper. The porous organosilica sol-gels act as reversible thermoresponsive materials which exhibit reduced pore volume at higher temperature and enlarged pores at lower temperature, which causes dynamic alterations in the nanoenvironment of encapsulated entities. A particularly remarkable feature of this system is that the temperature-dependent confinement effect provided by the gels is reversible and can be efficiently modulated by simply changing the temperature.

Mechanical/thermal dewatering of lignite. Part 3: Physical properties and pore structure of MTE product coals

The mechanical thermal dewatering (MTE) process has been shown to effectively dewater high moisture content low rank coals via the application of mechanical force at elevated temperatures. Using mercury intrusion porosimetry (MIP) as an investigative tool, this study examines how MTE processing conditions, such as temperature and pressure, affect the compressibility, pore size distribution, apparent (skeletal) density and shrinkage behaviour of three low rank coals sourced from Australia, Greece and Germany. As both pore filling and sample compression occurred at high mercury intrusion pressures, all MIP data were corrected for compression effects by using compressibility values derived from mercury extrusion data. The MTE process is shown to produce a low porosity coal, which, depending upon the processing conditions used, undergoes further shrinkage upon oven drying at 105 °C. An increase in MTE temperature (above about 85 °C) led to an increase in mesopore volume, which is caused by a hardening of the coal structure, leading to pore volume retention and a consequent reduction in percent shrinkage on oven drying. The increase in measured mesopore volume is also associated with an increase in measured surface area. The reverse trend is seen with increasing MTE pressure, where both the macro and mesopore volume decrease with pressure, causing the percent shrinkage to increase accordingly. This effect may be due to an increase in capillary forces caused by a decrease in the average pore diameter. The percent shrinkage increased up to a pore volume of about 0.1 cm 3/g, beyond which no further reduction in pore volume was achieved. The decrease in mesopore volume is also associated with a decrease in measured surface area. Compressibility values derived from mercury extrusion data show that the MTE process has little impact on the network strength of the skeletal network structure of all three coals investigated. Likewise, the skeletal density remained relatively unchanged. The reduction in water content, pore volume and the changes in shrinkage behaviour under increasingly severe MTE conditions are suggestive of the physical changes that accompany increased coalification (rank) within the lignitic range.

Synthesis of tetradentate N 4 Schiff base dioxomolybdenum (VI) complex within MCM-41 as selective catalyst for epoxidation of olefins

Covalent grafting of MCM-41 with 3-chloropropyl trimethoxysilane and subsequent reactions respectively with pypr [ N,N′-bis(2-pyrrolmethylidenaminopropyl)amine] and complexation with MoO 2(acac) 2 afforded MoO 2pyprMCM-41. X-ray diffraction and nitrogen sorption analyses revealed the preservation of the textural properties of the support as well as accessibility of the channel system despite sequential reduction in surface area, pore volume and pore size. Elemental analyses showed nearly complete complexation of the supported ligands and the presence of 0.24 mmol molybdenum per gram of the catalyst. Epoxidation of cyclooctene, cyclohexene, 1-hexene and 1-octene in the presence of MoO 2pyprMCM-41 with tert-butyl hydroperoxide (TBHP) were carried out with 21–98% conversion under the mild reaction conditions.

The role of extra-framework chromium species for the catalytic activity of [Cr]APO-5-type materials

The properties of chromium-substituted aluminophosphate-5 ([Cr]APO-5) with and without extra-framework chromium generated during hydrothermal synthesis are compared with those of AlPO-5 and [Cr]APO-5 into which chromium species were partly or exclusively introduced by impregnation. The prepared materials were characterized by elemental analysis, XRD, UV–Vis spectroscopy and nitrogen adsorption. Extra-framework chromium species exhibit a slight activity for the acid-catalyzed gas-phase conversion of 2-propanol to propene. However, their major effect consists in a reduction of the specific pore volume and, thus, a decreased accessibility of acid sites in [Cr]APO-5 or AlPO-5. This pore volume reduction is more pronounced for materials with extra-framework chromium species from impregnation than from generation during synthesis.

Effects of Cross-linking Type II Collagen-GAG Scaffolds on Chondrogenesis In Vitro: Dynamic Pore Reduction Promotes Cartilage Formation

Articular cartilage tissue-engineering investigations often implement bioassays for chondrogenesis in vitro using articular chondrocytes or mesenchymal stem cells in cell pellets that contract with time in culture, suggesting an association between the processes of contraction of the cell pellet and cartilage formation. The objective of the present study was to investigate this relationship further using adult canine articular chondrocyte-seeded type II collagen-GAG scaffolds. The collagen-GAG scaffolds were chemically cross-linked to achieve a range of cross-link densities. Chondrocyte-seeded scaffolds of varying cross-link densities were then cultured for 2 weeks to evaluate the effect of crosslink density on scaffold contraction and chondrogenesis. Scaffolds with low cross-link densities experienced cell-mediated contraction, increased cell number densities, a greater degree of chondrogenesis (viz., chondrocytic morphology of cells, synthesis of type II collagen), and an apparent increase in the rate of degradation of the scaffold compared to more highly cross-linked scaffolds that resisted cellular contraction. The results of this study suggest the promise of "dynamic pore reduction" for scaffolds for articular cartilage tissue engineering. In this approach, scaffolds would have an initial pore diameter large enough to facilitate cell seeding and a mechanical stiffness low enough to allow for cell-mediated contraction to yield a reduced pore volume to favor chondrogenesis. This approach may provide a useful alternative to traditional means of increasing cell number density and retention of synthesized molecules that promote cartilage formation in tissue-engineered constructs.