骨组织工程用多孔HA生物材料的制备

用PVB、NH4HCO3和(NH4)2CO3粒子作造孔剂,制备了骨组织工程用多孔HA生物材料.讨论了烧结工艺和造孔剂含量等对材料结构的影响.研究表明,较佳的烧结工艺为1200℃烧结4h,烧结后样品主要是HA相.造孔剂PVB、(NH4)2CO3、NH4HCO3含量分别为10vol%、15vol%和20vol%时,多孔HA陶瓷拥有大于100μm和5～50 μm的贯通孔,具有较好的孔连通性与孔结构,有利于细胞和组织的生长以及营养输送;其最大孔隙率为50.3%,抗压强度为6.33MPa.     

Understanding granular media: from fundamentals and simulations to industrial application

Academic and industrial research on the behaviour of granular material is increasingly supported by numerical simulations. Such simulation environments are not only a vital tool in design and analysis, but also a key link between physicists and engineers. This Topical Collection includes a range of papers that use simulation to study granular matter from the micro- and macroscopic behaviour of particles to the behaviour of large industrial or geomechanical systems. These contributions enhance our understanding of basic physical effects and processes such as heat transfer, agglomeration and screening but also give essential parameters for the simulation of industrial systems, or, at least, guide the strategies required to provide realistic simulation results. The collection focuses on the presentation of (a) new coupled simulation methods to consider the interaction of granular bodies with structural or fluid systems (b) new findings for the consideration of particle shape and particle size distributions within simulations, (c) acoustic wake agglomeration, and (d) gravitational flow in geomechanics.     

Role of hydroxyapatite crystal shape in nanoscale mechanical behavior of model tropocollagen–hydroxyapatite hard biomaterials

Shape of mineral (e.g. hydroxyapatite (HAP) or aragonite) crystals can be a strong determinant of the nanoscale strength of hard biological materials such as bone, dentin, and nacre. This work presents an understanding of the effect of HAP shape variation on nanoscale strength of model TC-HAP biomaterials. For this purpose, 3-dimensional molecular dynamics analyses of direction dependent tensile deformation in two structurally distinct TC-HAP cells with HAP crystals in needle shaped configuration and plate shaped configuration are performed. Analyses point out that the peak interfacial strength for failure is the highest for supercells with plate shaped HAP crystals. In addition, the plate shaped HAP crystals result in the localization of peak stress over a larger length scale indicating higher fracture strength. Peak strength during transverse loading is always found to be lower than that during the longitudinal loading. However, interfacial strength shows a reverse trend. Overall, analyses point out that HAP crystal shape along with the optimal direction of applied loading with respect to the TC-HAP orientation strongly influence biomaterial strength at the nanoscale.     

Thermophysics of the phenomena of gradientless heat transfer in porous solid cryogens: 1. Theoretical fundamentals for metallic conductivity of porous solid cryogens

Earlier we found anomalously high thermal conductivity for certain porous solid cryogens, which was close to metallic thermal conductivity. In this paper the theory which accounts for this phenomenon is proposed. It is based on the concept of a new sublimation-filtration-condensation mechanism of heat transfer in porous structures with high saturated vapour pressure. The λ-like character of the variation in effective thermal conductivity with its maximum near the temperature corresponding to the saturated vapour pressures 0.25-0.3 of the triple point pressure, is predicted for such systems.     

Improvement of compressive strength of porous hydroxyapatite scaffolds by adding polystyrene to camphene-based slurries

The compressive strength of porous hydroxyapatite (HA) scaffolds was enhanced by adding polystyrene (PS) polymer as a binder to hydroxyapatite (HA)/camphene slurries. As the PS content was increased from 0 to 20 vol.% in relation to the HA content, the compressive strength was significantly increased from 1.1 ± 0.2 to 2.3 ± 0.5 MPa, while the pore size was decreased from 277 ± 47 to 170 ± 29 µm. The improvement in the compressive strength was mainly attributed to both the suppression of the cracking of the green sample during freeze drying and the mitigation of the formation of micro-pores in the HA walls.     

Development of porous spherical hydroxyapatite granules: application towards protein delivery

A new method for the preparation of porous spherical hydroxyapatite granules is reported. It may be clinically applied towards orthopaedic or maxillofacial surgery as fillers or packing materials, and as biological chromatography supports. Its application towards delivery of macromolecules or protein drugs is discussed utilizing human serum albumin (HSA) as a model protein. © 1999 Kluwer Academic Publishers     

Effect of changes in tropocollagen residue sequence and hydroxyapatite mineral texture on the strength of ideal nanoscale tropocollagen-hydroxyapatite biomaterials

Changes in mineral texture (e.g. hydroxyapatite (HAP) or aragonite) and polypeptide (e.g. tropocollagen (TC)) residue sequence are characteristic features of a disease known as osteogenesis imperfecta (OI). In OI, different possibilities of changes in polypeptide residue sequence as well as changes in polypeptide helix replacement (e.g. 3 α1 chains instead of 2 α1 and 1 α2 chain in OI murine) exist. The cross section of the HAP crystals could be needle like or plate like. Such texture and residue sequence related changes can significantly affect the material strength at the nanoscale. In this work, a mechanistic understanding of such factors in determining strength of nanoscale TC–HAP biomaterials is presented using three dimensional molecular dynamics (MD) simulations. Analyses point out that the peak interfacial strength for failure is the highest for supercells with plate shaped HAP crystals. TC molecules with higher number of side chain functional groups impart higher strength to the TC–HAP biomaterials at the nanoscale. Overall, HAP crystal shape variation, the direction of applied loading with respect to the relative TC–HAP orientation, and the number of side chain functional groups in TC molecules are the factor that affect TC–HAP biomaterial strength in a significant manner.     

The effect of solid conversion on travelling combustion waves in porous media

A system of nonlinear partial differential equations, which describe the combustion of a gas passing through a porous medium, is examined. This model provides a bridge between recent porous-medium combustion studies and more classical combustion models. In particular, the effect of solid conversion on the downstream temperature is determined for travelling-wave solutions to the system. In cases for which the solid matrix is a perfect catalyst, solely enhancing the exothermic chemical reaction, with zero mass exchange, it is proved that the downstream temperature must always exceed the upstream temperature. However, when the solid is allowed to react with the gas, travelling-wave solutions for which the up- and down-stream temperatures are equal may be realised. A key result of this work is then the investigation of physical processes and related parameter ranges that give rise to travelling wave solutions with equal up- and down-stream temperatures. Specifically, two critical wavespeeds c_i with 0 < c₁ < c₂ are identified. For c < c₁ the downstream temperature always exceeds that upstream, whilst for c > c₂ no bounded travelling-wave solutions exist. Behaviour in the region c₁ < c < c₂ is new: here solutions having equal up- and down-stream temperatures are realised. The two factors upon which this result depends are the inclusion into the model of solid conversion effects and the distinction between solid and gas temperatures. Thus the inclusion of a new physical process, that of heat storage in the solid varying as the reaction proceeds, allows the existence of a new type of travelling-wave solution. Further phenomena studied include the appearance of nonunique solutions (which are of a type that may be related to ignition processes) and degenerate solutions which terminate precisely when all the solid and gaseous fuel is used up. Parameter regimes for the existence of these various types of solutions are found and the stability of such solutions discussed.     

Structure reinforcement of porous hydroxyapatite pellets using sodium carbonate as sintering aid: Microstructure,secondary phases and mechanical properties

Porous HAP pellets suitable for loading therapeutic agents were prepared using microcrystalline cellulose (MCC) as pore former and sodium carbonate as sintering aid (SAID). The effect of sintering temperature on the microstructure, mechanical properties and disintegration of pellets prepared at different SAID content was studied. Pellets were characterized by SEM, image analysis, porosimetry and surface area. Secondary phases were identified by PXRD, ATR-FTIR and Raman spectroscopy. Increasing the sintering temperature decreased the diameter, porosity, surface area and friability of the pellets but increased the pore size, tensile strength and disintegration time. The effect of SAID was dependent on sintering temperature. With 5% SAID, a secondary β-tricalcium phosphate (β-TCP) phase was formed, indicated by FTIR peak at 980 cm^?1 and characteristic PXRD reflections, whereas with 10%, a secondary B-type carbonated hydroxyapatite phase (CHA) formed, indicated by FTIR peaks at 878 and 1450 cm^?1, a broad Raman peak in the region 1020 to 1050 cm^?1 and PXRD reflections. Pellets prepared with SAID showed high strength and also porosity. The biphasic HAP/β-TCP pellets exhibited remarkably great strength (4.39 MPa) at the high sintering temperature, while still retaining 43.9% porosity. Relationships were established between the mechanical properties or disintegration time of the porous pellets and the microstructural parameters.     

Heat and mass transport in random velocity fields with application to dispersion in porous media

The effective equations describing the transport of a Brownian passive tracer in a random velocity field are derived, assuming that the lengthscales and timescales on which the transport process takes place are much larger than the scales of variations in the velocity field. The effective equations are obtained by applying the method of homogenization, that is a multiple-scale perturbative analysis in terms of the small ratio between the characteristic micro- and macro-lengthscales. After expanding the dependent variable and both space and time gradients in terms of , equating coefficients of like powers of yields expressions to determine the dependent variable up to any order of approximation. Finally, a Fickian constitutive relation is determined, where the effective transport coefficients are expressed in terms of the ensemble properties of the velocity field. Our results are applied to the transport of passive tracers in the stationary flow field generated in dilute fixed beds of randomly distributed spheroids, finding the effective diffusivity as a function of the spheroid eccentricity. Our result generalizes the expression of Koch and Brady (1985), who considered spherical inclusions, and is readily applied to the cases of random beds of slender fibers and flat disks.     

The effect of grinding direction on flaw character and strength of single crystal and polycrystalline ceramics

The effect upon the room-temperature strengths and fractures of flexure bars caused by grinding them either parallel or perpendicular to their tensile axis was investigated for selected single- and polycrystalline ceramics. Particular attention was given to the character of the flaws from which failure initiated. It was shown that grinding introduces two basic sets of flaws: one set forms at an average angle of 0°, and the other at an average angle of 90°, to the grinding direction. The angles of these flaws varied somewhat due to their formation on preferred fracture planes in single-crystal or larger-grain polycrystalline bodies as well as due to statistical effects. However, overall, the difference in these two sets of flaws was a major factor in the effect of grinding direction on strength. Flaws that formed approximately parallel with the grinding direction were typically more severe, and hence lead to lower strengths for grinding perpendicular to the bar axis, i.e. when the stress was perpendicular, or nearly so, to them.     

Micromechanics of spatially uniform heterogeneous media: A critical review and emerging approaches

Outside of the classical microstructural detail-free estimates of effective moduli, micromechanical analyses of macroscopically uniform heterogeneous media may be grouped into two categories based on different geometric representations of material microstructure. Analysis of periodic materials is based on the repeating unit cell (RUC) concept and the associated periodic boundary conditions. This contrasts with analysis of statistically homogeneous materials based on the representative volume element (RVE) concept and the associated homogeneous boundary conditions. In this paper, using the above classification framework we provide a critical review of the various micromechanical approaches that had evolved along different paths, and outline recent emerging trends. We begin with the basic framework for the solution of micromechanics problems independent of microstructural representation, and then clarify the often confused RVE and RUC concepts. Next, we describe classical models, including the available RVE-based models, and critically examine their limitations. This is followed by discussion of models based on the concept of microstructural periodicity. In the final part, two recent unit cell-based models, which continue to evolve, are outlined. First, a homogenization technique called finite-volume direct averaging micromechanics theory is presented as a viable and easily implemented alternative to the mainstream finite-element based asymptotic homogenization of unit cells. The recent incorporation of parametric mapping into this approach has made it competitive with the finite-element method. Then, the latest work based on locally-exact solutions of unit cell problems is described. In this approach, the interior unit cell problem is solved exactly using the elasticity approach. The exterior problem is tackled with a new variational principle that successfully overcomes the non-separable nature of the overall unit cell problem.     

Equilibrium of partially dried porous media influenced by dissolved species and the development of new interfaces

The equilibrium state of partially dried porous media is modeled using continuum thermodynamics as a framework for identifying the equilibrium state. Entropy of the interfaces within the porous material are taken into account in the thermodynamic analysis of the system. Additionally, the effect on the equilibrium condition of increasing concentration of dissolved species in the fluid within the pores caused by moisture loss to the surroundings is examined. Both the presence of dissolved species in the fluid within the pores and the development of new interfaces during the drying process suppress internal vapor pressure, indicating that the equilibrium internal vapor pressure within the porous media will be lower than that of the boundary at equilibrium.     

Characterization and osteoblast-like cell compatibility of porous scaffolds: bovine hydroxyapatite and novel hydroxyapatite artificial bone

Three different porous scaffolds were tested. The first two were prepared by sintering bovine bone. The third scaffold was prepared using three-dimensional gel-lamination, a new rapid prototyping method, and was named as hydroxyapatite artificial bone. X-ray diffraction and Fourier transform infrared spectroscopy analysis confirmed that the samples were mainly highly crystalline hydroxyapatite ceramics. Scanning electron microscopy and mercury intrusion porosimetry measurement showed that the pores were interconnected and pore sizes ranged from several microns to hundreds of microns. Mouse osteoblast-like cells grown on the three scaffolds retained their characteristic morphology. Cell proliferation and differentiation, analyzed by methylthiazol tetrazolium (MTT) and alkaline phosphatase activity assays, were significantly higher on the hydroxyapatite artificial bone than on the other two scaffolds tested. All the scaffolds provided good attachment, proliferation and differentiation of bone cells. These results indicate that the scaffolds have a favorable interaction with cells, they support cell growth and functions, and therefore these scaffolds may have great potential as bone substitutes. The three-dimensional gel-lamination method is proven to be an attractive process to design and fabricate bone scaffolds with favorable properties, and therefore, has promising potential for bone repair applications.     

Preparation of porous CdIn2S4 photocatalyst films by hydrothermal crystal growth at solid/liquid/gas interfaces

A chemical method has been used to prepare highly crystalline porous CdIn₂S₄ photocatalyst films at low temperature on a glass substrate. The conversion of cadmium and indium salts with thiourea on CdIn₂S₄ powder particles into CdIn₂S₄ has been achieved via hydrothermal reaction, which acts as a “glue” to connect the original CdIn₂S₄ particles chemically thus forming a mechanically stable photocatalyst film.     

Creep mapping in a polycrystalline ceramic: application to magnesium oxide and magnesiowustite

The construction of deformation mechanism maps for a polycrystalline ionic solid in which anion and cation transport are coupled has been demonstrated. Because of anioncation ambipolar coupling, two regimes of Coble creep are possible in systems where anion grain boundary transport is rapid: (1) rate-controlled at low temperatures and small grain sizes by cation grain-boundary diffusion, and (2) rate-limited at high temperatures and large grain sizes by anion grain-boundary diffusion. A new type of deformation mechanism map was introduced in which the temperature and grain size were primary variables. This map was shown to be particularly useful for materials which deform primarily by diffusional creep mechanisms. Ambipolar diffusional creep theory was used to construct several deformation mechanism maps for polycrystalline MgO and magnesiowustite over wide ranges of stress, grain size, temperature and composition.