[Bone tissue porosity in individuals from various age groups (author's transl)].
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Adolescent↗
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Certain porosity aspects of membrane filters: their pore-distributions and anisotropy.
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Filtration↗
Very small-diameter polyurethane vascular prostheses with rapid endothelialization for coronary artery bypass grafting.
Two types of spongy polyurethane-polydimethylsiloxane blend (Cardiothane 51, Kontron Instruments, Inc., Everett, Mass.) vascular grafts with an internal diameter of 1.5 mm were fabricated by a spray, phase-inversion technique. Low-porosity grafts with hydraulic permeability of 2.7 +/- 0.4 ml/min per square centimeter and medium-porosity grafts with hydraulic permeability of 39 +/- 8 ml/min per square centimeter displayed good handling properties and suturability. Twelve straight low-porosity grafts, 17 straight medium-porosity grafts (1.5 to 2.0 cm in length), and one loop medium-porosity graft (10 cm in length) were implanted by the same surgeon end to end in the infrarenal aorta of 30 male Sprague-Dawley rats. Three months after implantation, patency was 8% for low-porosity grafts (1/12) and 76% for straight medium-porosity grafts (13/17). The loop medium-porosity graft was also patent. The sole patent low-porosity graft showed neointimal hyperplasia and incomplete endothelialization. All but one of the patent straight medium-porosity grafts showed a glistening and transparent neointima with complete endothelialization and no anastomotic hyperplasia. The loop medium-porosity graft displayed endothelialization from each anastomosis and in many islands in the middle portion of the graft, totalling 47% of the luminal surface by morphometric analysis. Thick mural thrombus, anastomotic hyperplasia, or aneurysm formation were not observed in any patent medium-porosity graft. These data indicate that in the rat aortic replacement model it is possible to achieve patency and a high degree of endothelialization in very small-diameter prostheses of appropriate porosity.
Animals↗
Modulation of the tabletting behaviour of microcrystalline cellulose pellets by the incorporation of polyethylene glycol.
This study investigated the effect of incorporating a soft material (polyethylene glycol; PEG) into pellets of microcrystalline cellulose (MCC) on the compression behaviour and compactability of the pellets. Low and high porosity MCC pellets were formed. Pellets were also formed from a 1:1 (w/w) mixture of MCC and PEG 6000 to have a porosity equal to that of the low porosity MCC pellets. The degree of compression of the pellets and the porosity, permeability to air and tensile strength of the resulting tablets were determined. Some of the tablets were also deaggregated, and the thickness and porosity of the retrieved pellets were determined. The lowest total tablet porosity was seen with tablets made from pellets containing PEG. The intergranular porosity and the permeability of these tablets were similar to those of tablets made of the high porosity MCC pellets. However, the thickness and porosity of retrieved MCC/PEG pellets were similar to those of the low porosity MCC pellets. The degree of compression levelled off at a lower pressure for the MCC/PEG pellets than for the MCC pellets. Tablet strength values for the MCC/PEG pellets was intermediate between the MCC pellet types and not greatly affected by the applied pressure. It was concluded that the deformation propensity of the pellets was, in general terms, increased due to the presence of the soft material. However, the character of the deformation behaviour changed towards an increased tendency for local deformation during compression, i.e., the ability of adjacent pellet surfaces to conform to each other increased, without marked changes in the main dimensions and porosity of the pellets.
Cellulose↗
Velocity measurements in natural porous rocks.
In the petroleum industry, there is a general belief that there must be a correlation between porosity and permeability in oil field cores. This has been demonstrated using porosity and permeability values measured on large cores from the same oil field. Now, with MRI techniques for non-invasively measuring fluid flow velocities and porosities inside oil cores, it is possible to develop quantitative correlations between local porosity and fluid flow velocity inside single core samples. Since velocity and permeability are related, these results could eventually be extended to give empirical equations relating porosity and permeability within a particular core sample. MRI was used to obtain local values of the porosity and fluid flow velocity in Casper sandstone during water injection. The correlation between local fluid velocity and local porosity is shown graphically and indicates that the average velocity increases with porosity, with no velocity for porosity equal to zero, as expected. In addition, the average flow rate of the fluid through the cross-section, as measured by MRI, is calculated and compared to the actual flow rate through the core sample. The results indicate bypassing of fluid around the perimeter of the core, and this conclusion is supported by 1D projections of the flow velocities at different overburden pressures.
Fourier Analysis↗
Estimates of the peak pressures in bone pore water.
The maximum pore fluid pressures due to uniaxial compression are determined for both the vascular porosity (Haversian and Volkmann's canals) and the lacunar-canalicular porosity of live cortical bone. It is estimated that the peak pore water pressure will be 19 percent of the applied axial stress in the vascular porosity and 12 percent of the applied axial stress in the lacunar-canalicular porosity for an impulsive step loading. However, the estimated relaxation time for the vascular porosity (1.36 microseconds) is three orders of magnitude faster than that estimated for the lacunar-canalicular porosity (4.9 ms). Thus, under physiological loading, which has a stress rise time generally larger than 1 ms, pressures higher than the vascular pressure cannot be sustained in the vascular porosity due to the swift pressure relaxation in this porosity (unless the fluid drainage through the boundary is obstructed). The model also predicts a slight hydraulic stiffening of the bulk modulus due to longer draining time of the lacunar-canalicular porosity. The undrained bulk modulus is 6 percent higher than the drained bulk modulus in this case.
Cell Membrane Permeability↗
Granule deformation and densification during compression of binary mixtures of granules.
The purpose of this study was to investigate whether the deformation and densification during compression of one type of granules are affected by adjacent granules of a different porosity, corresponding to different mechanical strength. Three mixtures were prepared, each consisting of two types of microcrystalline cellulose pellets (intermediate porosity study pellets plus low, intermediate or high porosity surrounding pellets) in the proportion 1:7. The mixtures were compressed and the study pellets were retrieved and analysed in terms of porosity, thickness, surface area and shape. It was shown that the study pellets were compressed by deformation and densification. The degree of densification (decrease in porosity) of the study pellets was independent of the porosity of the surrounding pellets but the deformability (changes in the thickness, surface area and shape) of the study pellets was linked with the porosity of the surrounding pellets. It is concluded that the mode of deformation of the study pellets was regulated by the porosity of the surrounding granules; in a mixture containing granules with a low porosity, compression resulted in irregular study granules with regularly positioned indentations caused by the surrounding granules. The compression properties of the surrounding granules affected the flattening of the study granules to a lesser degree.
Cellulose↗
Sinusoidal ultrastructure evaluated during the revascularization of regenerating rat liver.
Sinusoidal endothelial cell (SEC) porosities were compared between the periportal (zone 1) and pericentral (zone 3) regions of the rat liver during regeneration following partial hepatectomy (PHx). SEC porosities and fenestration diameters were measured in control livers, as well as at 5 minutes, 24, 48, 72, 96, 120 hours, and 14 days following PHx. Bimodal maximums in both porosity and fenestration diameters were observed in both zones at 5 minutes and 5 days following PHx. SEC porosities increased significantly in both zones 1 and 3 within 5 minutes following PHx, but the increase was maintained only in zone 1 at 24 hours after resection. Following the initial rise, both zones displayed a gradual decrease to less than half their porosity values at 72 hr post-PHx. After 72 hours, porosities increased to over control levels and remained elevated until 14 days after PHx. The decrease in porosity at 72 hr post-PHx is accompanied by ultrastructural changes within the sinusoid at this time. Vascular corrosion casting and transmission electron microscopy (TEM) show sinusoid compression resulting from increased hepatic plate widths due to hepatocyte proliferation in the absence of SEC proliferation. Also at this time, we observed many SEC completely enveloped by stellate cells. The zonal variations observed for porosities throughout regeneration did not correlate with changes in laminin, collagen I and IV, or fibronectin deposition within the space of Disse. Taken together, the data reveal that SEC are dynamic regulators of porosity that respond rapidly and locally to environmental zonal stimuli during liver regeneration.
Animals↗
Microporosity enhances bioactivity of synthetic bone graft substitutes.
This paper describes an investigation into the influence of microporosity on early osseointegration and final bone volume within porous hydroxyapatite (HA) bone graft substitutes (BGS). Four paired grades of BGS were studied, two (HA70-1 and HA70-2) with a nominal total porosity of 70% and two (HA80-1 and HA80-2) with a total-porosity of 80%. Within each of the total-porosity paired grades the nominal volume fraction of microporosity within the HA struts was varied such that the strut porosity of HA70-1 and HA80-1 was 10% while the strut-porosity of HA70-2 and HA80-2 was 20%. Cylindrical specimens, 4.5 mm diameter x 6.5 mm length, were implanted in the femoral condyle of 6 month New Zealand White rabbits and retrieved for histological, histomorphometric, and mechanical analysis at 1, 3, 12 and 24 weeks. Histological observations demonstrated variation in the degree of capillary penetration at 1 week and bone morphology within scaffolds 3-24 weeks. Moreover, histomorphometry demonstrated a significant increase in bone volume within 20% strut-porosity scaffolds at 3 weeks and that the mineral apposition rate within these scaffolds over the 1-2 week period was significantly higher. However, an elevated level of bone volume was only maintained at 24 weeks in HA80-2 and there was no significant difference in bone volume at either 12 or 24 weeks for 70% total-porosity scaffolds. The results of mechanical testing suggested that this disparity in behaviour between 70 and 80% total-porosity scaffolds may have reflected variations in scaffold mechanics and the degree of reinforcement conferred to the bone-BGS composite once fully integrated. Together these results indicate that manipulation of the levels of microporosity within a BGS can be used to accelerate osseointegration and elevate the equilibrium volume of bone.
Animals↗
Expanded polytetrafluoroethylene grafts for portal vein replacement: use of omentum wrap to promote graft healing.
This study was designed to reexamine the healing process of expanded polytetrafluoroethylene (EPTFE) grafts with standard porosity (30 microm) and high porosity (60 microm) in portal vein replacement, and to evaluate the effect of an omentum wrap, which has certain functions that promote healing, on graft healing. These grafts, either wrapped by the omentum or not, were placed as portal vein replacements in 24 mongrel dogs. After 1 month, the grafts were retrieved and examined for patency, thrombus-free areas, thickness of the pseudointima, and the total number of cells growing into the graft wall. There were no statistical differences in the patency rates. The high-porosity grafts had a significantly larger thrombus-free area, a thicker pseudointima, and a larger growth of cells than the standard-porosity grafts. The omentum wrap significantly increased the thrombus-free area and stimulated a larger growth of cells in both grafts. The high-porosity grafts plus omentum demonstrated a thrombus-free area of 82.2% vs 27.3% in the standard-porosity grafts. In addition, the migration of fibroblasts and macrophages was most evident in the high-porosity grafts wrapped by the omentum. In conclusion, graft healing enhancement was observed in the high-porosity EPTFE grafts wrapped by the omentum. It is thus suggested that transmural cellular migration plays an important role in the process of graft healing.
Animals↗
Development of a bioartificial nerve graft. II. Nerve regeneration in vitro.
A promising alternative for the repair of peripheral nerve injuries is the bioartificial nerve graft, or BNG, comprised of a tubular conduit preseeded with Schwann cells, which are an effective substrate for enhancing nerve regeneration. The physical properties of the conduit, porosity and wall thickness, as well as the Schwann cell seeding density, were tested for their effect on axon growth using rat dorsal root ganglia. These parameters can influence the amount of nutrients and growth factors made available to the neural tissue. Results show that a greater wall thickness and lower porosities have a detrimental effect on the growth of the axons. Over a four week period, axons extended 3.2 mm for the optimum case (DeltaR = 0.82 mm, epsilon = 0.75) compared to 1.8 and 1.6 mm for a lower porosity (0.55) and a greater wall thickness (1.4 mm), respectively. A maximum in the growth rate occurs at a porosity of 75% for Schwann cell seeded conduits but not for unseeded ones. When compared to mass transfer predictions, the results suggest that, at higher porosities, more growth factors diffuse out of the conduit, while at low porosities there is competition for nutrients. Increasing the Schwann cell seeding density enhances growth but also leads to an increase in the number of axons along the length of the conduit. This is indicative of branching of the axons, which requires additional resources to maintain and can lead to painful neuroma formation. Wall thickness and porosity were found not to have any significant effect on the axon number sprouting from the dorsal root ganglia and the mean diameter (p > 0.05). Considerations need to be made, not just on the polymer used, but also on its porosity, wall thickness, and Schwann cell seeding density. These parameters can be adjusted to create a bioartificial nerve graft that provides the optimal environment for nerve growth.
Analysis of Variance↗
Particle and pore size distributions of investments.
The particle-size distributions of investment powders and the pore-size distributions of set-fired investments were investigated by electrozone size analysis, and by mercury intrusion scanning porosimetry, respectively, for a number of different investments, representing all three types available, namely: the gypsum-, phosphate- and silicate-bonded. The pore volume (V), the pore surface area (S), and their first derivatives with respect to radius, r, dV/dr and dS/dr, were obtained with pressures up to 414 MPa corresponding to spherical pores of 0.0018 micron radius if the Washburn equation is to apply. Results distinguished pores with a range of different sizes. Besides the porosity due to the matrix binder, porosity was also attributable to the refractory components. This latter type of porosity occurred with pore radii below about 0.03 micron, whereas matrix porosity occurred above this pore size. Matrix porosity for the phosphate materials was smallest at about 0.03 to 0.5 micron in radius, followed by the gypsum materials at about 0.3 to 0.6 micron, and by the silicate materials at about 1 to 10 micron. The particle-size distribution of the investment powders may affect both types of porosities, whereas the setting chemistries of the particular investment types may affect matrix porosity, by the generation of gaseous by-products. No major distinctions in pore size and distributions occurred between air-set and hygroscopic-set materials. This study emphasizes that at present, knowledge of the total pore volume only is inadequate for characterizing porosity distribution and hence investment permeability.
Calcium Sulfate↗
Matrix diffusion-derived plume attenuation in fractured bedrock.
Matrix diffusion can attenuate the rate of plume migration in fractured bedrock relative to the rate of ground water flow for both conservative and nonconservative solutes of interest. In a system of parallel, equally spaced constant aperture fractures subject to steady-state ground water flow and an infinite source width, the degree of plume attenuation increases with time and travel distance, eventually reaching an asymptotic level. The asymptotic degree of plume attenuation in the absence of degradation can be predicted by a plume attenuation factor, beta, which is readily estimated as R' (phi(m)/phi(f)), where R' is the retardation factor in the matrix, phi(m) is the matrix porosity, and phi(f) is the fracture porosity. This dual-porosity relationship can also be thought of as the ratio of primary to secondary porosity. Beta represents the rate of ground water flow in fractures relative to the rate of plume advance. For the conditions examined in this study, beta increases with greater matrix porosity, greater matrix fraction organic carbon, larger fracture spacing, and smaller fracture aperture. These concepts are illustrated using a case study where dense nonaqueous phase liquid in fractured sandstone produced a dissolved-phase trichloroethylene (TCE) plume approximately 300 m in length. Transport parameters such as matrix porosity, fracture porosity, hydraulic gradient, and the matrix retardation factor were characterized at the site through field investigations. In the fractured sandstone bedrock examined in this study, the asymptotic plume attenuation factors (beta values) for conservative and nonconservative solutes (i.e., chloride and TCE) were predicted to be approximately 800 and 12,210, respectively. Quantitative analyses demonstrate that a porous media (single-porosity) solute transport model is not appropriate for simulating contaminant transport in fractured sandstone where matrix diffusion occurs. Rather, simulations need to be conducted with either a discrete fracture model that explicitly incorporates matrix diffusion, or a dual-continuum model that accounts for mass transfer between mobile and immobile zones. Simulations also demonstrate that back diffusion from the matrix to fractures will likely be the time-limiting factor in reaching ground water cleanup goals in some fractured bedrock environments.
Connecticut↗
The influence of deposition method on the packing uniformity of powder beds.
A number of methods of preparing powder packings have been assessed in terms of the resulting variability of local porosity. Measurement of local porosity was carried out by a gamma-ray attenuation technique. It was found that overall porosity and the distribution of local porosity were highly dependent on particle properties and deposition method. Some deposition methods tended to give rise to the alternate formation and collapse of temporary structures within a growing bed, and in such cases, relatively high variability of local porosity was detected. More uniform packings could be formed by deposition of powder in even layers. In many cases, a significant relationship between local porosity and position within a packing was established, the central areas generally being of lowest porosity. This was true of packings both of high and low overall variability of porosity.
Analysis of Variance↗
Resorbable defect analog PLGA scaffolds using CO2 as solvent: structural characterization.
After tooth extraction, the immediate wound treatment by implanting an exact copy of the root could prevent alveolar bone atrophy. The implant should have an interconnected porosity in order to promote tissue in-growth. This communication reports a novel method to realize such net-shaped porous scaffolds fabricated within a few minutes. Porosity and micro-architecture are evaluated by Hg-porosimetry and by image analysis of electron and light microscopy as well as by computed micro-tomography. The total porosity of the scaffold corresponds to (63 +/- 3)%, mainly related to open interconnected porosity. Micro-tomography, as a noninvasive 3D method, is best suited to uncover pores of about 100 microm, a diameter especially important for tissue in-growth. The differentiation between open and closed porosity, however, depends on the method chosen. This effect is attributed to the spherical pores with an orifice only detected in the 3D analysis. Consequently, the closed porosity is overestimated by 8% evaluating 2D images. Finally, the mean pore diameter is found to be 106 and 100 microm for 2D and 3D analysis, respectively. Although the porosity of the scaffold needs to be further optimized for clinical applications, the procedure proposed is a promising route in manufacturing open porous implants without the use of any organic solvent.
Alveolar Bone Loss↗
Experimental and theoretical investigation of directional permeability of human vertebral cancellous bone for cement infiltration.
The use of acrylic polymers in infiltrating the porous bone structure is an emerging procedure for the augmentation of osteoporotic vertebrae. Although this procedure is employed frequently, it is performed based on empirical knowledge, and therefore, does not take into consideration the porosity-dependent permeability of human vertebral cancellous bone. The purpose of this study was to: (a). experimentally and theoretically investigate interdependence of the vertebral cancellous bone permeability and porosity, and (b). examine if the bone permeability of spinal cancellous bone can be predicted using bone mineral density measurements. If these relations can be established, they can be useful in optimizing the injection conditions for predicable cement infiltration. To determine the porosity-dependent and directional permeability, 34 bone cores-20 samples in the superior-inferior (SI) direction and 14 in the anterior-posterior (AP) direction-were cut from 20 lumbar vertebrae and infiltrated with silicone oil with a viscosity matching that of PMMA. The permeability of the cores was determined based on Darcy's law. The mean permeability of SI and AP cores was 4.45+/-1.72 x 10(-8) and 3.44+/-1.26 x 10(-8)m(2), respectively. An interesting finding of this study was that the permeability of the AP cores was approximately 78% of that of SI cores, though the porosity of the SI and AP cores taken from the same vertebra was approximately equal. In addition, we provided a theoretical model for the porosity-dependent permeability that accurately described non-linear dependency of the bone permeability and porosity in both directions. Although the relation of the bone permeability and porosity was established, bone mineral density was a weak predictor of the bone permeability. The experimental and theoretical results of this study can be used to understand polymer flow in cement infiltration procedures.
Adsorption↗
Controlled release of macromolecules from PLA microspheres: using porous structure topology.
Release of hydrophilic macromolecules (FITC-dextran, M(w)=71 and 2000 kDa) from porous poly (D,L-lactic acid), PLA, microspheres was studied by applying percolation theory. Microspheres were prepared by the double emulsion method using high molecular weight PLA. The microspheres showed a percolation threshold, rho(C), at porosity 0. 34. From this parameter, the effective diffusion coefficients, D(eff), and the accessible porosity (total releasable active ingredient), rho(A), of the microspheres were calculated using the Bethe lattice model with coordination number 4. Decreasing porosity of the microspheres decreased the release rate of the active ingredients and a long-term release was observed for the microspheres with porosity close to rho(C). The calculated rho(A) agreed with the experimental data and also the calculated D(eff) for the microspheres with larger porosity (>0.4) was a good estimation to predict the experimentally determined release profiles by applying the continuum structure model (CSM). For microspheres with porosity larger than 0.4 which contained FITC-dextran M(w)=71 kDa, a lag time was observed, which was attributed to delayed saturation of the microspheres with release medium. For microspheres with porosity close to rho(C), the limited number of exit holes on the exterior surface changed the mechanism of release and controlled the release rate rather than the tortuosity of the porous structure of the microspheres.
Algorithms↗
Tissue engineering human placenta trophoblast cells in 3-D fibrous matrix: spatial effects on cell proliferation and function.
Nonwoven polyethylene teraphathalate (PET) fabrics with different porosities and knitted fabric were used as support matrixes to grow human trophoblast cells to study the spatial effects of fibrous matrix on cell adhesion, spatial organization, proliferation, and metabolic functions. In general, cells grown on 2-D surface and knitted fabric had faster metabolic rates and also showed higher proliferation activities as detected by cyclin B assay. For nonwoven PET fibers, matrix porosity had profound effects on cell morphology, spatial organization, and proliferation. Cells grown in a low-porosity fibrous matrix formed small aggregates ( approximately 100 cells per aggregate), whereas cells grown in high-porosity matrix formed big aggregates ( approximately 1000 cells per aggregate). This was attributed to the difference in pore volume or averaged fiber distance, which dictated a cell's ability to cross over and form a bridge between adjacent fibers. The high-porosity matrix had a relatively poor surface accessibility for cells to attach and spread, which are essential for cell proliferation. Dual staining with PI and BrdU showed that 60% of cells in the small aggregates found in the low-porosity matrix were proliferating, while only 18% of cells in the large aggregates found in the high-porosity matrix were proliferating. These results suggest that spatial characteristics of fibrous matrix are important to cell proliferation and function and should be considered in tissue-engineering human cells.
Biocompatible Materials↗