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PubMed2004

Examination of Darcys law for flow in porous media with variable porosity.

Henry Darcy's experimental studies in 1856 of saturated water flowthrough a homogeneous porous medium contained in a vertical column have provided the basis for the quantitative description of fluid flow in a wide variety of both natural and engineered porous medium environmental systems. Extrapolation of Darcy's original observations and conclusions has led to several commonly applied equations used to model flow in porous media. This work examines this original experimental study, summarizes the appropriate mathematical expressions that ensue directly from the data, and indicates expressions in common use that are suggested, but not actually supported, by the data. The paradoxes that exist in the common approaches for the case of a porous medium with a spatiallyvariable porosity are illustrated. A modified form of Darcy's law, and also of the Hubbert potential, is derived based upon fundamental notions of averaging. The modified form of Darcy's law derived here reduces to the conventional form for a homogeneous porous medium.

Gravitation↗
PubMed1992

Modified polytetrafluoroethylene: indium 111-labeled platelet deposition on carbon-lined and high-porosity polytetrafluoroethylene grafts.

Platelet accumulation on carbon-lined (CL) and high-porosity (HP) polytetrafluoroethylene (PTFE) grafts was investigated in vivo. In experiment 1, 20 CL grafts and 20 control PTFE grafts, each 5 cm in length and 4 mm in diameter, were interposed into both carotid and femoral arteries of 10 dogs. In experiment 2, 12 HP grafts (90 microns mean internodal distance) and 12 control PTFE grafts were implanted in six dogs. Indium 111-labeled platelets were injected intravenously and the grafts were retrieved 48 hours later. Radioactivity of the grafts and a control segment of the carotid artery was counted. A graft platelet accumulation index (GPAI) was calculated as the ratio of emission from the graft compared to that from the control segment. The GPAI of the CL graft was significantly less than the GPAI of the control graft in both the carotid (control 29.7 +/- 5.46, CL 22.3 +/- 6.55; n = 9 [p < 0.05]) and the femoral arteries (control 30.7 +/- 9.65, CL 22.0 +/- 6.59; n = 9 [p < 0.05]). There was no significant difference in GPAI between the control and HP grafts in the carotid arteries (control 30.6 +/- 11.8, HP 31.5 +/- 9.71; n = 6) and in the femoral arteries (control 31.5 +/- 7.88, HP 34.0 +/- 4.97; n = 6). Carbon lining decreases platelet accumulation on PTFE grafts in the early postoperative period, and HP grafts do not exhibit increased platelet uptake.

Animals↗
PubMed2006

The influence of different plasticizers and polymers on the mechanical and thermal properties, porosity and drug permeability of free shellac films.

The effect of triethyl citrate (TEC) and different molecular weights and concentrations of polyethylene glycol (PEG), in addition to the effect of different water-soluble polymers and dispersions at different levels, hydroxypropyl methylcellulose (HPMC), methylcellulose (MC), carbomer 940, polyvinyl alcohol (PVA), ethyl cellulose (EC), on the mechanical and thermal properties, drug permeability, and porosity of free shellac films were investigated. Shellac films were cast from aqueous solutions, and their mechanical properties were studied by tensile test. Thermal analyses were performed using differential scanning calorimetry (DSC). The results showed that the addition of plasticizer caused a decrease in both elastic modulus and glass transition temperature (T(g)) and an increase in elongation at break of free shellac films. This effect was related to the concentrations of plasticizers. Different molecular weights of PEGs have different plasticization mechanisms.Moreover, the incorporation of different amounts of HPMC, MC, or carbomer in free shellac films caused an increase in the flexibility, decrease in T(g), and a marked increase in drug permeability of free shellac films, whereas the addition of PVA caused a decrease in flexibility and drug permeability and an increase in T(g). Addition of EC resulted in a slight decrease of the elasticity and a small decrease in drug permeability. However it does not show a considerable effect on the T(g). In addition, it was found that the drug permeability is directly related to the mechanical properties and T(g) of shellac films.

Calorimetry, Differential Scanning↗
PubMed2004

Polyurethane vascular grafts with thorough porosity: does an internal or an external membrane wrapping improve their in vivo blood compatibility and biofunctionality?

Using a freeze-drying multiple-step technique, porous polyurethane vascular grafts 1.5 mm in diameter with different internal and external surface characteristics were fabricated: Type A: with thorough porosity, Type B: with internal membrane wrapping and Type C: with external membrane wrapping. One centimeter long tubes of each three types of grafts were implanted into the infra-renal abdominal aorta of rats for periods of 2, 4, 8 weeks. All the grafts were patent at harvesting. At the end of 2 weeks postimplantation, the endothelial cells layer developed inside the grafts about 1.2 mm apart from either the distal anastomosis side or both the distal and proximal anastomoses. They spreaded to the middle part of the one cm-long graft at 4 weeks. The luminal surfaces were entirely covered by a layer of endothelial cells at 8 weeks. Smooth muscle cells also migrated underneath the endothelial cell layer through the inner surface from the anastomosis and grew into the porous wall to generate a biocomposite blood conduit. Type B grafts with an internal membrane wrapping, developed a thicker, easy to lift intima after all the durations of implantation because the internal capsule was poorly anchored to the device. It is therefore of paramount importance that a porous structure is exposed to the blood flow to permit an adequate healing together with a good anchorage of the internal capsule.

Animals↗
PubMed2000

The contributions of erosion, swelling, and porosity to theophylline release kinetics from Cissus populnea polymer matrices.

The variable factors of erosion rate, swelling rate, and porosity were used in studying the release patterns of theophylline from our Cissus populnea polymer (CPP) matrices under the different factor combinations given by a simple 2n factorial experimental design. The zero-order slopes and correlation coefficients representing release rate and linearity, respectively, as obtained from both the nonsteady state and steady state were statistically treated. It appears that the nonsteady-state analysis is more suitable for studying the effects of individual factors, while the steady-state analysis appears more suitable for studying interaction effects of the factors. The study also showed that erosion is the main mechanism by which theophylline is released from the matrices, while swelling is responsible for maintaining linearity in the zero-order release curves. A careful examination of the statistical results shows some functional relationships between the factors, which should be considered in designing more detailed factorial experiments to enable the establishment of equation models for predicting the release profile of theophylline from our CPP matrices under any given dissolution condition.

Bronchodilator Agents↗
PubMed2004

Controlled porosity osmotic pumps of highly aqueous soluble drug containing hydrophilic polymers as release retardants.

Controlled porosity osmotic pumps (CPOPs) are devoid of delivery orifice to release core contents and essentially possess pore-forming agent(s) in coating composition. When the pump comes in contact with aqueous media, pore-forming agent(s) generate pores through which core contents are delivered. Diltiazem hydrochloride (DLTZ) is a freely water-soluble drug and the release rates of DLTZ are higher from oral osmotic pumps including CPOPs, in which the drug release is controlled by concentration of pore-forming agents. The effect of appropriate concentration of hydroxypropyl methyl cellulose and sodium carboxy methyl cellulose mixture on the release of DLTZ from CPOPs was studied. In vitro drug release profiles were compared with that of different marketed controlled release formulations and statistically analysed to examine the suitability of CPOP for twice or once daily administration. Dissolution models were applied to drug release data in order to establish the mechanism of drug release and kinetics. Drug release from the CPOPs was effectively modified with the concentration of pore-forming agent in membrane and concentration of hydrophilic polymers in the core. CPOPs showed minimum 65% of consistent DLTZ release at 16 h. Statistical analysis confirmed that with an increase in the amount of hydrophilic polymers release rate decreased. Drug release from the systems follows Hixson-Crowell cube root model and mechanism of release follow non-Fickian diffusion.

Carboxymethylcellulose Sodium↗
PubMed2006

Multiphase modelling of cell behaviour on artificial scaffolds: effects of nutrient depletion and spatially nonuniform porosity.

This paper contains analysis of a recently formulated multiphase model for the growth of biological tissue that comprises motile cells and water inside a rigid scaffold material. The model is extended here to include a term describing cell proliferation which is mediated by the supply of a diffusible nutrient and to include the case where the scaffold porosity varies in space. Numerical solutions of the model equations are presented for different values of the parameters. Comparison is drawn between the different types of growth that arise when using static or dynamic methods for seeding the scaffold with cells. Analytical solutions are presented for the limiting cases in which the coefficient of drag between the cells and the scaffold is very large or zero. In the limit of large time, solutions reveal preferential tissue growth in the vicinity of the scaffold edge due to depletion of nutrient by the cells, consistent with experimental results. However, it is shown that reducing the coefficient of drag between the scaffold and the cells overcomes the effects of nutrient depletion by increasing cell mobility, thereby leading to improved uniformity of the cell distribution within the scaffold.

Algorithms↗
PubMed1990

Effects of material porosity on implant bonding strength in a craniofacial model.

Alloplastic craniofacial implantation is a developing surgical option. Several alloplastic materials are available with a porous feature. While porosity may permit bone ingrowth, it is unclear whether it offers improved implant fixation over that of nonporous bone substitutes in the craniofacial skeleton. Bilateral mandibular ramus defects were created and reconstructed with porous and nonporous block implants in the mature rabbit. At 3, 6, and 12 months postoperatively, the mandibles were removed and prepared so that only the bone-implant interface connected the proximal and distal segments. The anterior mandible was then loaded to destruction. The fatigue load (in kg) for each implant was obtained and statistically compared. Significant differences in fatigue loads between the implant types occurred at all postoperative periods with the porous block material consistently exhibiting the greater resistance to separation. Resistance to fatigue fracturing for either implant type did not increase after the sixth postoperative month.

Acrylic Resins↗
PubMed1998

Porosity reduction in acrylic cement is clinically irrelevant.

Evidence from studies on migration raises serious doubts regarding the part played by fatigue failure of acrylic bone cement in the genesis of aseptic loosening of the components of total hip arthroplasties. Moreover, the viscoelastic properties of cement raise serious doubts concerning the relevance of conventional laboratory fatigue studies to the in vivo behavior of acrylic cement. It is not surprising that there is considerable clinical evidence that reduction of porosity in acrylic cement is of no relevance to the long term outcomes of total hip arthroplasty. These matters remain controversial, and randomized prospective trials would be necessary to settle the issues. For various reasons, such trials are unlikely to be performed.

Arthroplasty, Replacement, Hip↗
PubMed2004

Limitations of titanium dioxide and aluminum oxide as ossicular replacement materials: an evaluation of the effects of porosity on ceramic prostheses.

BACKGROUND: Because the performance of titanium dioxide (TiO2) has not yet been assessed in the unique environment of the middle ear, its role as an ossicular replacement prototype in the form of a total ossicular replacement prosthesis (TORP) was tested and compared with aluminum oxide (Al2O3), once considered to be a suitable implant material. METHODS: Ossiculoplasty was performed by implanting TORPs into the tympanic cavities of rabbits. After an implantation period of 28, 84, or 300 days, the petrous bones were extracted, whereby the biocompatibility of the prostheses was examined using light microscopy and scanning electron microscopy to determine morphologic changes in situ. Proper implant placement and functionality was tested via manual manipulation. RESULTS: Mucosa was seen covering most of the implants by day 84. Inflammatory cells were not observed in any of the specimens examined. The macroporous TiO2 TORPs were subjected to osseous infiltration, material dissolution, and fragmentation, whereas the microporous TiO2 implants were subjected to an increasing frequency of fissure formations. The Al2O3 prostheses demonstrated signs of material dissolution by producing encapsulated aggregates during the experimental trial period. CONCLUSIONS: Neither the macroporous nor microporous oxide ceramics were able to withstand the oscillatory stress to which they were continually subjected. Although porosity allows for the rapid integration of an implant material into a biological environment, its properties are not suited to fulfill the requirements of strength and long-term stability, which are demanded of middle ear prostheses.

Aluminum Oxide↗
PubMed2003

Pit membrane porosity and water stress-induced cavitation in four co-existing dry rainforest tree species.

Aspects of xylem anatomy and vulnerability to water stress-induced embolism were examined in stems of two drought-deciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., and two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret., growing in a seasonally dry rainforest. The deciduous species were more vulnerable to water stress-induced xylem embolism. B. australis and C. gillivraei reached a 50% loss of hydraulic conductivity at -3.17 MPa and -1.44 MPa, respectively; a 50% loss of hydraulic conductivity occurred at -5.56 MPa in A. excelsa and -5.12 MPa in A. bidwillii. To determine whether pit membrane porosity was responsible for greater vulnerability to embolism (air seeding hypothesis), pit membrane structure was examined. Expected pore sizes were calculated from vulnerability curves; however, the predicted inter-specific variation in pore sizes was not detected using scanning electron microscopy (pores were not visible to a resolution of 20 nm). Suspensions of colloidal gold particles were then perfused through branch sections. These experiments indicated that pit membrane pores were between 5 and 20 nm in diameter in all four species. The results may be explained by three possibilities: (a) the pores of the expected size range were not present, (b) larger pores, within the size range to cause air seeding, were present but were rare enough to avoid detection, or (c) pore sizes in the expected range only develop while the membrane is under mechanical stress (during air seeding) due to stretching/flexing.

Algorithms↗
PubMedDate not supplied

Changes of flow characteristics by stenting in aneurysm models: influence of aneurysm geometry and stent porosity.

An endovascular technique using a stent has been developed and successfully applied in the treatment of wide neck aneurysms. A stent can facilitate thrombosis in the aneurysm pouch while maintaining biocompatible passage of the parent artery. Insertion of the stent changes the flow characteristics inside the aneurysm pouch, which can affect the intra-aneurysmal embolization process. The purpose of this study is to clarify the velocity and wall shear stress changes that are caused by stenting in fusiform and lateral aneurysm models. We used a flow visualization technique that incorporated a photochromic dye in order to observe the flow fields and measure the wall shear rates. The intra-aneurysmal flow motion was significantly reduced in the stented aneurysm models. Coherent inflow along the distal wall of the aneurysm was diminished and inflow was distributed along the pores of the stent wall in the stented models. Also, sluggish intra-aneurysmal vortex motion was well maintained in the stented aneurysm models during the deceleration phase. A less porous stent generally reduced the intraneurysmal fluid motion further, but the porosity effect was not significant. The magnitude and pulsatility of the wall shear rate were reduced by stenting, and the reductions were more significant in the lateral aneurysm models compared to the fusiform aneurysm models. The hemodynamic changes that were observed in our study can help explain the efficacy of in vivo thrombus formation caused by stenting.

Aneurysm↗
PubMed2004

An in vitro study of the ultrasonic axial transmission technique at the radius: 1-MHz velocity measurements are sensitive to both mineralization and intracortical porosity.

UNLABELLED: The ultrasonic axial transmission technique allows for investigating skeletal sites such as the cortical layer of long bones (radius, tibia, phalanges). Using synchrotron radiation microCT, we investigated, in vitro, the relationships between 1-MHz axial transmission SOS measurements at the radius and site-matched measurements of C.Th, POR, MIN, and vBMD. INTRODUCTION: The ultrasonic axial transmission technique allows for investigating skeletal sites such as the cortical layer of long bones (radius, tibia, phalanges). MATERIALS AND METHODS: Using synchrotron radiation microCT, we investigated, in vitro, the relationships between 1-MHz axial transmission speed of sound (SOS) measurements at the radius and site-matched measurements of cortical thickness (C.Th), intracortical porosity (POR), tissue mineralization (MIN), and volumetric BMD (vBMD). SOS measurements were based on bidirectional axial transmission and were performed with a 1-MHz proprietary probe on 39 excised human radii. RESULTS: The highest correlations between SOS values and bone parameters (R(2)(SOS/POR) = 0.28, p < 10(-3); R(2)(SOS/MIN) = 0.38, p < 10(-4); R(2)(SOS/vBMD) = 0.57, p < 10(-3)) were found for bone parameters assessed in a 1-mm-thick periosteal region of the cortex rather than throughout the whole cortex. The observed moderate correlation between SOS and C.Th values (R(2)(SOS/C.Th) = 0.20, p < 10(-2)) disappeared when controlled for other variables. The two best multilinear predictive models, including either BMD alone or the pair of dependent variables MIN and POR (all assessed in the periosteal cortex), were equally accurate in predicting SOS values (R(2)(SOS/(POR,MIN)) = 0.59, p < 10(-5); R(2)(SOS/vBMD) = 0.57, p < 10(-5)). CONCLUSION: For the first time, the respective adjusted contributions of POR (-24 m/s%(-1)) and tissue mineralization (+3.5 m/s/mg/cm(-3)) to SOS values were assessed. These results suggest potential sensitivity of axial transmission SOS values to changes in cortical bone status under different pathological conditions or treatments affecting POR and/or tissue mineralization.

Aged↗
PubMed1999

Fibrovascular ingrowth in porous ocular implants: the effect of material composition, porosity, growth factors, and coatings.

PURPOSE: Fibrovascular ingrowth into various porous ocular implants as a function of implant material composition, porosity, growth factors, and coatings was investigated in a pilot study in an animal model. METHODS: Eighty-one New Zealand white rabbits underwent unilateral enucleation and implantation with ocular implants composed of the following materials: coralline hydroxyapatite (HA) with 200-microm pores (HA200) or 500-microm pores (HA500), synthetic HA (synHA), and high-density porous polyethylene (PP). The HA200, HA500, and PP implants were implanted untreated or after treatment with recombinant human basic fibroblast growth factor (Rh-bFGF). Nine HA500 implants were implanted after coating with calcium sulfate (plaster of Paris) to provide a smooth outer surface. Implants were harvested at 1-, 2-, 4-, or 8-week intervals and were examined histologically. RESULTS: A significant difference was found between untreated HA500 and PP, with PP showing better ingrowth. There was no significant difference between untreated HA and PP, nor between untreated HA500 and synHA. Significant increases in ingrowth were found in HA200 compared with HA500, and in Rh-bFGF-treated implants compared with untreated controls. The calcium sulfate-coated implants showed less vascularization compared with the uncoated implants, although the difference was not significant. CONCLUSIONS: Fibrovascular ingrowth occurred earlier in HA200 implants than in HA500 implants, and was enhanced when implants were treated with Rh-bFGF.

Animals↗
PubMedDate not supplied

Porosity in manually and machine mixed resin-modified glass ionomer cements.

The powder and liquid components of resin-modified glass ionomer cements are available in manual and machine mixed forms. This study quantified the effect mixing methods have on the porosity and shear strength of a resin-modified glass ionomer cement (RMGIC). A RMGIC (Fuji II LC) was manually or machined mixed according to manufacturer's instructions. Thin, disc-shaped specimens (n = 5) were made by compressing the cement between glass platens to a thickness of approximately 76 microns. The specimens were light cured for 120 seconds. Digital images of the specimens were recorded using a measuring microscope and slide film scanner. Digital imaging software was used to determine the number and volume of the cement's pores. Shear test specimens of manual and machine mixed cements (n = 10) of each group (approximately 800 microns thickness) were made as previously described. Shear punch tests were conducted using a 3.75 mm diameter punch mounted on a universal testing machine. The mean number and total volume of pores in the manually mixed specimens was considerably greater than that of the machine mixed group (p < 0.05). The shear punch test results of the machine mixed group was significantly higher than the manual mixed group, (p < 0.05).

Drug Compounding↗
PubMed2002

Expulsion force, surface pH, and porosity of encapsulated glass-ionomer cements mixed with a Rotomix device.

The aim of this study was to evaluate the Rotomix mixing device, used for mixing glass ionomer cements, in comparison with conventional mixing procedures including both hand and amalgamator methods. The properties examined were extrusion force, surface pH (indicating homogeneity), and porosity. The results indicated that the Rotomix device produced cement mixtures without compromising any of the examined properties relative to other mixing methods.

Analysis of Variance↗