Porous Block Research Articles

A New Approach for Fabricating Collagen/ECM-Based Bioinks Using Preosteoblasts and Human Adipose Stem Cells.

Cell-printing methods have been used widely in tissue regeneration because they enable fabricating biomimetic 3D structures laden with various cells. To achieve a cell-matrix block, various natural hydrogels that are nontoxic, biocompatible, and printable have been combined to obtain "bioinks." Unfortunately, most bioinks, including those with alginates, show low cell-activating properties. Here, a strategy for obtaining highly bioactive ink, which consisted of collagen/extracellular matrix (ECM) and alginate, for printing 3D porous cell blocks is developed. An in vitro assessment of the 3D porous structures laden with preosteoblasts and human adipose stem cells (hASCs) demonstrates that the cells in the bioinks are viable. Osteogenic activities with the designed bioinks show much higher levels than with the "conventional" alginate-based bioink. Furthermore, the hepatogenic differentiation ability of hASCs with the bioink is evaluated using the liver-specific genes, albumin, and TDO2, under hepatogenic differentiation conditions. The genes are activated within the 3D cell block fabricated using the new bioink. These results demonstrate that the 3D cell-laden structure fabricated using collagen/ECM-based bioinks can provide a novel platform for various tissue engineering applications.

Unified fluid flow model for pressure transient analysis in naturally fractured media

Naturally fractured reservoirs present special challenges for flow modeling with regards to their internal geometrical structure. The shape and distribution of matrix porous blocks and the geometry of fractures play key roles in the formulation of transient interporosity flow models. Although these models have been formulated for several typical geometries of the fracture networks, they appeared to be very dissimilar for different shapes of matrix blocks, and their analysis presents many technical challenges. The aim of this paper is to derive and analyze a unified approach to transient interporosity flow models for slightly compressible fluids that can be used for any matrix geometry and fracture network. A unified fractional differential transient interporosity flow model is derived using asymptotic analysis for singularly perturbed problems with small parameters arising from the assumption of a much smaller permeability of the matrix blocks compared to that of the fractures. This methodology allowed us to unify existing transient interporosity flow models formulated for different shapes of matrix blocks including bounded matrix blocks, unbounded matrix cylinders with any orthogonal crossection, and matrix slabs. The model is formulated using a fractional order diffusion equation for fluid pressure that involves Caputo derivative of order 1/2 with respect to time. Analysis of the unified fractional derivative model revealed that the surface area-to-volume ratio is the key parameter in the description of the flow through naturally fractured media. Expressions of this parameter are presented for matrix blocks of the same geometrical shape as well as combinations of different shapes with constant and random sizes. Numerical comparisons between the predictions of the unified model and those obtained from existing transient interporosity ones for matrix blocks in the form of slabs, spheres and cylinders are presented for linear, radial and spherical flow types for both fixed boundary pressure and rate.

Response of stem cells from different origins to biphasic calcium phosphate bioceramics.

Biphasic calcium phosphate (BCP) bioceramics have been successfully applied in a broad variety of presentation forms and with different ratios of hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP). BCPs have been loaded with stem cells from different origins for bone tissue engineering purposes, but evidence of stem cell behavior on different compositions (various HA/β-TCP ratios) and physical features of BCPs is limited. We compared the adhesion, proliferation, viability and osteogenic potential of human mesenchymal stem cells (MSCs) on granular BCPs with equal HA/β-TCP ratio of diverse particle sizes and on porous blocks which had different chemical compositions. In addition, the osteogenic differentiation of MSCs was compared to adipose-derived (ADSC) and dental pulp (DPSC) stem cells, as well as to pre-osteoblasts on a particulate BCP. MSCs growing on granular BCPs demonstrated increased number as compared to MSCs growing on blocks. Cells proliferated to a greater extent on small granular BCPs, while large granular BCPs and blocks promoted cell differentiation. Surprisingly, the expression of genes involved in osteogenesis was upregulated in MSCs on bioceramics in basal medium which indicates that BCPs may have osteoinductive potential. This was confirmed with the upregulation of osteochondrogenic markers, at different time points, when stem cells from various tissues were grown on the BCP. This study demonstrates that BCPs, depending on their physical features and chemical composition, modulate stem cell behavior, and that stem cells from different origins are inherently distinct in their gene expression profile and can be triggered toward osteochondrogenic fate by BCPs.

Fabrication of porous calcite using chopped nylon fiber and its evaluation using rats.

Although porous calcite has attracted attention as bone substitutes, limited studies have been made so far. In the present study, porous calcite block was fabricated by introducing chopped nylon fiber as porogen. Ca(OH)2 powder containing 10 wt% chopped nylon fiber was compacted at 150 MPa, and sintered to burn out the fiber and to carbonate the Ca(OH)2 under stream of 1:2 O2-CO2. Sintering of Ca(OH)2 at 750 °C or lower temperature resulted in incomplete burning out of the fiber whereas sintering at 800 °C or higher temperature resulted in the formation of CaO due to the thermal decomposition of Ca(OH)2. However, sintering at 770 °C resulted in complete burning out of the fiber and complete carbonation of Ca(OH)2 to calcite without forming CaO. Macro- and micro-porosities of the porous calcite were approximately 23 and 16%, respectively. Diameter of the macropores was approximately 100 μm which is suitable for bone tissue penetration. Porous calcite block fabricated by this method exhibited good tissue response when implanted in the bone defect in femur of 12-weeks-old rat. Four weeks after implantation, bone bonded on the surface of calcite. Furthermore, bone tissue penetrated interior to the macropore at 8 weeks. These results demonstrated the good potential value of porous calcite as artificial bone substitutes.

Entropy generation due flow passing over the porous block in the cavity

Flow over porous structures finds applications in heat transferring devices due to attainment of high heat transfer rates. Thermodynamic irreversibility can be used as a measure of quality of the porous body in the thermal system. In the present study, flow over a porous block situated in a square open–ends cavity and rate of entropy generation due to heat transfer and fluid friction are considered. The effects of the block aspect ratio and porosity on volumetric entropy generation rate are examined numerically. In the simulations, four aspect ratios and three porosities of the block are accommodated. The total area of the block is kept constant for all the aspect ratios considered in the simulations. A uniform heat generation (105 W/m²) is considered in the block resembling the heat generating device, such as encapsulated electronic device, and air is used as working fluid in the cavity. It is found that volumetric entropy generation rate in the cavity reduces with increasing aspect ratio and porosity of the block. Entropy generation rate due to heat transfer is considerably higher than its counter part due to fluid friction because of low Reynolds number flow at cavity inlet and small natural convection current generated in the cavity.

AN OPTIMIZATION STUDY OF HEAT TRANSFER ENHANCEMENT DUE TO JET IMPINGEMENT OVER POROUS HEAT SINKS USING THE LATTICE BOLTZMANN METHOD

In this paper, an optimization study involving five variable parameters that are critical in evaluating the thermal performance of the air jet flow impingement over porous heat sinks in a mixed convection regime is presented. The variable parameters include the porous block height, channel height, jet width, porosity, and Darcy number. The lattice Boltzmann method for porous media is used as the numerical tool to simulate the objective functions in terms of the Nusselt number and pressure drop. The Kriging method combined with a genetic algorithm is used to generate the optimal Pareto plot. Three optimal cases from the Pareto plot showing the maximum, minimum, and optimum Nusselt numbers and the pressure drop are considered. The optimized results show that the porosity and Darcy number are relatively invariant along the Pareto surface. Thus, the geometry of the channel and porous block are the most significant parameters governing the Nusselt number and pressure drop values. The results from optimization corresponding to the three optimal cases are analyzed in detail. The local variation in the Nusselt number for these cases is also plotted to understand the effect of the channel and porous heat sink parameters on heat transfer.

Controlled synthesis and gas sensing properties of porous Fe2O3/NiO hierarchical nanostructures

Porous Fe2O3/NiO hierarchical nanostructures with cuboid-like and flower-like architectures are prepared by directly annealing the coordination polymer (CP) precursor Fe(3-Clpy)2[Ni(CN)4]. Both of the hierarchical architectures are constructed by porous sheet-like building blocks, which are further composed of nanocrystals with a size smaller than 6 nm. Fe2O3 and NiO components display highly uniform distribution in the composites. An interesting monocrystal-like electron diffraction pattern is demonstrated in the cuboid-like Fe2O3/NiO composite. It is revealed that the crystal structure of the CP precursor can exert an obvious effect on the morphology and microstructure of the calcined products. The potential application of the Fe2O3/NiO composites in gas sensors is investigated towards various organic vapors. It is found that the cuboid-like Fe2O3/NiO shows a much higher gas sensing performance than the flower-like counterpart, demonstrating that the different architectures of hierarchical nanostructures have an important influence on the gas sensing performance. The facile and effective CP precursor approach presented here can be extended to synthesize a wide range of metal oxide composites with unique architectures, highly tailored compositions and improved functions.

多孔質体を用いた液体メタノールからの合成ガスのパッシブ生成

Passive production of synthesis gas using a porous material block containing liquid methanol has been investigated. A penetrating hole prepared in the block, around which the catalyst is supported, is heated using a wire coil heater that is set up so as to contact with the hole-surface. Evaporation, catalytic action, and liquid supply due to capillary suction are induced due to the heat transfer into the porous structure, resulting in the successive production of the decomposed gases. However, the capillary suction suppresses the extension of a dried region and the temperature increase in a catalyst-supported region to the reaction level. Theoretical analysis of the reacting gas flow in the catalyst-supported region suggests that the yield is significantly dependent on the catalyst temperature, which increases the reaction rate exponentially, and on the thickness of the catalyst-supported region. Separation of the catalyst-supported region from the liquid containing region by a small gap suppresses excessive evaporation and raises temperature over the catalyst-supported region, resulting in the significant increases in the yield and the response at smaller heating rates.

Spiral and Mesoporous Block Polymer Nanofibers Generated in Confined Nanochannels

Spiral-like or various porous polymer nanofibers have great applications in biosensor, bioengineering, and template-fabrication of functional inorganic materials. However, the fabrication of polymer nanostructures with controllable porous or spiral morphology in one process is a big challenge. Here we first demonstrated a general and easy method to generate spiral or porous block copolymer (BCP) nanofibers by using geometric confinement of nanochannels to disturb the self-assembly of BCP while nonsolvent is induced into BCP solution. Continuous spiral polymer nanofibers and polymer nanofibers with hierarchical porous nanostructures can be easily generated within channels of anodic aluminum oxide (AAO) membranes by tuning the composition and concentration of BCP. This study first reports the influence of cylinder confinement to the arrangement of BCP micelles. These spiral and porous BCP nanostructures are not only good templates to generate functional inorganic nanostructures, but also promising candidates to create biosensors or to load catalyst because their enlarged surface area enables high guest concentrations.

The roles of geometry and topology structures of graphite fillers on thermal conductivity of the graphite/aluminum composites

Various graphite fillers, such as graphite particles, graphite fibers, graphite flakes and porous graphite blocks, have been successfully incorporated into an Al alloy by squeeze casting in order to fabricate graphite/Al composites with enhanced thermal conductivity (TC). Microstructural characterization by X-ray diffraction and scanning electron microscopy has revealed a tightly-adhered, clean and Al4C3-free interface between the graphite fillers and the Al matrix in all the as-fabricated composites. Taking the microstructural features into account, we generalized the corresponding predictive models for the TCs of these composites with the effective medium approximation and the Maxwell mean-field scheme, which both show good agreement with the experimental data. The roles of geometry and topology structures of graphite fillers on the TCs of the composites were further discussed.

Forced Convection Analysis of Discrete Heated Porous Convergent Channel

This study investigates numerically forced convection heat transfer and flow analyses of a passive heat exchanger for nonporous and partially filled porous channels with varying exit height (1, 0.5, and 0.25). Four discrete heat sources with uniform heat flux are simulated on the channel bottom wall. The partially filled porous channels are tested at two different porous block heights (0.5 and 1). The flow field and thermal analyses inside the channels are investigated across a wide range of Reynolds and Darcy numbers for Prandtl number of 0.71. The results reveal that the porous block and the exit height affect substantially the flow and heat transfer characteristics inside the tested channels. The Nusselt number is enhanced by 20–40% for the partially filled porous convergent channel (exit height = 0.25 and porous block height = 1) compared to the nonporous channel. Consequently, the heat exchanger size can be reduced by 37.5%. Moreover, the overall heat transfer performance parameter is enhanced with further increase in Darcy number at low Reynolds number. As a result, compact heat exchangers that provide superior heat transfer coefficients lead to development of macro- and microelectronic devices.

Modeling solute/contaminant transport in heterogeneous aquifers.

A fissured aquifer may be considered as a dense network of fissures separated by low permeability matrix blocks. A conceptual modeling of such a system consists of an infinite number of parallel fractures separated by constant width matrix slabs. While the fissures are assumed to be main flow conduits, the fluid in the porous matrix blocks are considered to be virtually immobile. The mathematical model of the transport of a solute and/or contaminant which assumes a purely convective flow in fissures and diffusion into the matrix blocks consists of two coupled differential equations. An analytical solution of this model for the case of solute entering into the system at a constant concentration has been presented by Skopp and Warrick in Soil Sci Soc Am Proc 38:545-550, 1974. Note however, Skopp and Warrick (Soil Sci Soc Am Proc 38:545-550, 1974) have not considered the additional processes of adsorption and radioactive decay. Unfortunately, their solution had computational limitations as it involved numerical integration of a quite complex expression. Therefore, one had to turn to employing numerical Laplace transform inverters to compute the solutions. This work presents simple real space analytical solutions for the contaminant transport model described above including the adsorption and radioactive decay. The real space solutions have been developed using the method of double Laplace transform and binomial series approximation. An accurate approximate solution has also been presented which converges to the exact solution only after computing three terms in the series full solution. The developed model has been used for 1) assessment of the efficiency of numerical Laplace transform algorithms and 2) investigation of the degree and scale of contamination, and 3) designing remediation schemes for the already contaminated aquifers.

Preliminary study of airflow and heat transfer in a cold room filled with apple pallets: Comparison between two modelling approaches and experimental results

Two CFD modelling approaches of transient heat transfer by forced convection in a cold room filled with four apple pallets have been performed and compared with experimental data. For the first approach, each apple pallet is considered as one porous medium block. For the second, each pallet is considered as 8 solid blocks representing the apple bins. The numerical results obtained by these two approaches show a good agreement with the experimental results regarding air velocity and product temperature evolution during cooling process. A better prediction of product temperature evolution is obtained with the solid block approach. The porous medium approach gives a fairly good result with a much smaller mesh numbers (nearly 4 times) than the solid block approach. Four turbulence models (standard k–ɛ, RNG k–ɛ, realizable k–ɛ, and shear stress transport SST k–ω) and two heat transfer models for porous medium (local thermal equilibrium and local thermal non-equilibrium model) are tested to determine the most appropriate ones for the studied configuration.

Bone Induction in Porous HAp Block Modified by Partial Dissolution-Precipitation Technique with Supersonic Treatment in Rat Scalp

Microcracks and trabecular fractures can be observed in physiological bone. Biomimetic hydroxyapatite (HAp) scaffolds have been strongly needed in bone regenerative medicine. We have been developing the combination method of the partial dissolution-precipitation techniques involving the stirring-supersonic treatment in 1.7×10-2 N HNO3 solution containing Ca2＋and PO43- ions to improve the surface and the bulk of commercially available synthetic HAp block (82.5% in porosity, 50-300µm in macropore). The modified HAp was named as a partially dissolved and precipitated HAp (PDP-HAp). The aims of this study are to characterize the PDP-HAp and to observe cell response for the ceramics in rat scalp tissue. The PDP-HAp exhibited the macropore sizes of 50-200µm, the porosities of 85-90%, and the specific surface areas of 1.0-2.0 m2･g-1, with many micro-cracks. Twenty rats were divided into 2 groups. At 9 months, bone induction occurred inside the many pores in the PDP-HAp group, while bone and cartilage were not found in the HAp group. We believe that osteoinduction by the PDP-HAp is different from the process of BMP-loaded HAp-induced bone formation. The PDP-HAp might be applied as potential ceramics with osteoinductive properity and excellent biocompatibility in difficult bone regenerative cases.

Reinfiltration through liquid bridges formed between two matrix blocks in fractured rocks

Summary Liquid reinfiltration is a significant process, which can considerably retard and slowdown the transport of oil, water and contaminants in fractured subsurface formations. However, accurate modeling of the reinfiltration via liquid bridges formed in a horizontal fracture or space between two rock porous blocks remains a controversial topic. In an attempt to improve an understanding of the problem, the reinfiltration from upper to lower matrix blocks through formation of liquid bridges is theoretically modeled by using generalization of the Lucas–Washburn theory for a porous medium, which takes into account pressure differences due to matrix capillary, gravity, inertia, viscous, and fracture capillary forces. The developed model results in a second-order nonlinear ordinary differential equation (ODE), which is solved numerically to obtain depth and rate of the reinfiltrated liquid versus time. The results showed that three reinfiltration regimes: including Early Time Regime (ETR) ( z ∼ t , q = constant ), Middle Time Regime (MTR) ( z ∼ t , q ∼ 1 / t ) and Late Time Regime (LTR) ( z ∼ t , q = constant ) can be observed, where the inertia, viscous, and gravity forces are dominant, respectively. The results also indicated that by increasing the permeability of the porous medium, the durations of the ETR ( z ∼ t , q = constant ) and the LTR ( z ∼ t , q = constant ) are prolonged while the duration of the MTR ( z ∼ t , q ∼ 1 / t ) is reduced. Moreover, the results revealed that by increasing the liquid viscosity, the durations of the ETR ( z ∼ t , q = constant ) and LTR ( z ∼ t , q = constant ) are reduced whereas the duration of the MTR ( z ∼ t , q ∼ 1 / t ) is prolonged. In addition, the results showed that in the case of high permeability of the porous medium when the fracture capillary pressure is strong enough only the LTR ( z ∼ t , q = constant ) can be observed. The MTR ( z ∼ t , q ∼ 1 / t ) and the LTR ( z ∼ t , q = constant ) scalings are of practical significance since the liquid reinfiltration in fractured rocks associated with gas–liquid drainage mechanism is a very slow process. These findings advance the understanding of the two-phase flow in fractured porous media.

A 3D printed TCP/HA structure as a new osteoconductive scaffold for vertical bone augmentation.

OsteoFlux(®) (OF) is a 3D printed porous block of layered strands of tricalcium phosphate (TCP) and hydroxyapatite. Its porosity and interconnectivity are defined, and it can be readily shaped to conform the bone bed's morphology. We investigated the performance of OF as a scaffold to promote the vertical growth of cortical bone in a sheep calvarial model. Six titanium hemispheres were filled with OF, Bio-Oss (particulate bovine bone, BO), or Ceros (particulate TCP, CO) and placed onto the calvaria of 12 adult sheep (6 hemispheres/sheep). Histomorphometric analyses were performed after 8 and 16weeks. OF led to substantial vertical bone growth by 8weeks and outperformed BO and CO by a factor 2 yielding OF 22%±2.1; BO 11.5%±1.9; and CO 12.9%±2.1 total new bone. 3mm away from the bony bed, OF led to a fourfold increase in new bone relative to BO and CO (n=8, P<0.002). At 16weeks, OF, BO, and CO behaved similarly and showed marked new bone synthesis. A moderate degradation was observed at 16weeks for all bone substitutes. When compared to existing bone substitutes, OF enhances vertical bone growth during the first 2months after implantation in a sheep calvarial model. The controlled porous structure translated in a high osteoconductivity and resulted in a bone mass 3mm above the bony bed that was four times greater than that obtained with standard substitutes. These results are promising but must be confirmed in clinical tests.

Design of a Porous Annular Refractory Injection Block for the Tundish Refining of Steel

A new annular injection block containing pores of the optimum size has been developed. Use of the block maximizes the removal of nonmetallic inclusions from the tundish bath. The efficiency of the block has been confirmed by theoretical calculations, data obtained from physical modeling, and factory tests.

Generation of an rhBMP-2-loaded beta-tricalcium phosphate/hydrogel composite and evaluation of its efficacy on peri-implant bone formation

Dental implant insertion on a site with low bone quality or bone defect should be preceded by a bone graft or artificial bone graft insertion to heal the defect. We generated a beta-tricalcium phosphate (β-TCP) and poloxamer 407-based hydrogel composite and penetration of the β-TCP/hydrogel composite into the peri-implant area of bone was evaluated by porous bone block experiments. The maximum penetration depth for porous bone blocks and dense bone blocks were 524 μm and 464 μm, respectively. We report the in-vivo performance of a composite of β-TCP/hydrogel composite as a carrier of recombinant human bone morphogenetic protein (rhBMP-2), implanted into a rabbit tibial defect model. Three holes drilled into each tibia of eight male rabbits were (1) grafted with dental implant fixtures; (2) filled with β-TCP/hydrogel composite (containing 5 μg of rhBMP-2), followed by grafting of the dental implant fixtures. Four weeks later, bone-implant contact ratio and peri-implant bone formation were analyzed by radiography, micro-CT and histology of undecalcified specimens. The micro-CT results showed a significantly higher level of trabecular thickness and new bone and peri-implant new bone formation in the experimental treatment compared to the control treatment. Histomorphometry revealed a significantly higher bone-implant contact ratio and peri-implant bone formation with the experimental treatment. The use of β-TCP/poloxamer 407 hydrogel composite as a carrier of rhBMP-2 significantly promoted new bone formation around the dental implant fixture and it also improved the quality of the new bone formed in the tibial marrow space.

Flow visualization of fluid flow through a U-bend channel with periodical high-permeability porous blocks

The issue about the heat transfer enhancement in a 180-deg turned channel has attracted many attentions. The heat transfer capacity is influenced strongly by the behavior of the fluid flow. This work experimentally investigated the fluid flow characteristics in a 180-deg round turned channel with discrete aluminum-foam blocks. The air was used as coolant. Several aluminum-foam blocks (0.90 porosity and 10PPI pore density) were installed discretely in the 180-deg round turned channel with the square cross section. Four kinds of test section were employed to perform the flow visualizations. The results indicate that the arrangement of aluminum-foam blocks impacts on the flow pattern remarkably. The present results can be used to explain the pressure drop characteristics of such aluminum-foam channel, and be benchmarking by other investigators who would like to solve this problem numerically.

Blockage effects in wave and current: 2D planar simulations of combined regular oscillations and steady flow through porous blocks

This paper provides numerical evidence for reduced fluid loading on space-frame structures exposed to ocean waves and in-line current. Comparisons are made between the current blockage model presented previously (Taylor et al., 2013), Computational Fluid Dynamics (CFD) simulation and experimental data. Three different flow models are considered: steady flow, time-averaged mean flow and fully unsteady flow both for regular oscillations with an in-line steady flow. A porous block is used to model an obstacle array of cylinders. This is appropriate because of the global nature of current blockage, which has significant effects over distances of the order of the frontal width of the obstacle array. We find good agreement among the numerical simulation, the experimental data and the previously published current blockage model, which lends support to the validity and applicability of the theoretical model in predicting the blockage effects. We also demonstrate that, in general, the 2D porous block model simulates the reduced flow better than the simple 1D analytical current blockage model.