Lamellar Pore Structures Research Articles

Fabrication and enhanced degradation behavior of sinterless porous apatite scaffolds with centrosymmetric structure

The primary component of natural bone minerals, hydroxyapatite (HA), has been the subject of research on materials for bone implants. Sintered HA scaffolds, on the other hand, degrade slowly after implantation and exhibit a high degree of crystallization at high temperatures, making it challenging to match the rate at which new bone grows. In this study, the benefit of self-curing calcium phosphate bone cement was employed to create porous apatite scaffolds without sintering. The lamellar pore structure was created by directional freeze-casting, and the enhanced specific surface area aided the in-situ hydration process. The crystallinity of sinterless porous apatite scaffolds diminishes when the TTCP and DCPD molar ratios drop. When the molar ratio is adjusted to 1:2.25, the crystallinity of the fabricated scaffold is reduced to 63.99 %, and 10.21 % can be degraded in 30 days. The degradation of porous scaffolds in simulated body fluids mainly depends on the rapid dissolution and transformation of solid phase powders in the early hydration reaction and the slow diffusion of the apatite in the later stage. The compressive strength of the porous scaffold is 5.3 MPa and its elastic modulus is 0.68 GPa. After 14 days of degradation, the compressive strength was 4.0 MPa and the elastic modulus was 0.64 GPa, which was still within the applicable range of cancellous bone repair. It has a promising application prospect as a substitute scaffold for absorbable cancellous bone.

Densification behavior and attrition resistance of porous Al2O3beads prepared by dripping‐ice templating

AbstractPorous Al2O3ceramic beads with a small‐scale lamellar pore structure were prepared by the dripping‐ice templating method. The densification behavior of lamellar walls and the attrition resistance of porous Al2O3beads were investigated. For the two‐dimensional structural lamellar walls, the intrinsic pores shrunk, and the necking area grew during the sintering process, leading to the densification of lamellar walls and the shrinking of porous beads. In addition, the two‐dimensional lamellar walls and three‐dimensional Al2O3bulk are compared and possibilities of elaborating of the discrepancy in grain size and pore morphology are discussed. Attrition resistance was determined by a grinding test. The dominant attrition mechanism of porous Al2O3beads is abrasion. High sintering temperature led to a decrease in the attrition rate due to a wider necking area and denser pore walls.

Porous silicate cement membranes generated by the novel method combining freeze casting and heat-dry curing

Porous silicate cement membranes (PSCMs) fabricated by the freeze casting method show great potential to be utilized in seawater pretreatment, fermentation broth separation, and industrial wastewater treatment due to its merit of high-temperature resistance, low-cost, and hierarchically ordered porous structures, while the freeze casting method is complex and time-consuming. In this work, a combination of freeze casting and heat-dry curing was initially applied to generate PSCMs. The preparation periods of PSCMs could be shortened by simplifying preparation processes and reducing curing time. The resulting membranes presented double-layer structures, containing a nucleation zone (N-zone) with dense structures and a stability zone (S-zone) with lamellar pore structures. The X-ray diffraction pattern of membranes displayed the mixed hexagonal and rhombohedral structures. This novel method could save more than half of energy consumption compared with the traditional preparation technology of silicate cement samples. The membranes with a mesopore size of 3.794 nm showed high permeation performance with pure water flux reaching 207.23 L m−2 h−1 under 0.15 MPa and room temperature. The separation efficiency of oil-water was 78.05% under operating pressure of 0.05 MPa. Molecular dynamics simulation was applied to narrate the microscopic process of transformation during heat-dry curing, and obtained a good similarity of consequences between the computational method simulation and experimental operation.

Controlling the cell and surface architecture of cellulose nanofiber/PVA/Ti3C2TX MXene hybrid cryogels for optimized permittivity and EMI shielding performance

Anisotropic, nanoporous structures are promising materials for manipulating the propagation of electromagnetic waves at millimeter and sub-THz frequencies as well as for electromagnetic interference (EMI) shielding with rapidly evolving green electronics. In this work, cell and surface architecture of sustainable hybrid cryogels of cellulose nanofibers, polyvinyl alcohol (PVA) and Ti3C2TX MXene was controlled to adjust their GHz and THz dielectric permittivity and EMI shielding performance. Temperature gradient freeze-drying was used to obtain aligned honeycomb and lamellar pore structures with specific surface layer designs. The millimeter wave permittivity varied relative to thickness direction as the side of cryogels that was directly exposed to cold gradient had systematically a higher permittivity. This anisotropy was caused by a thin, smooth outermost surface layer covering the open core structure. The surface designs of all cryogels dominated signal permittivity, and the effects of higher MXene dosages could be offset by the surface layer. Cryogels with dense surface layer containing 70 and 50 wt % of MXene displayed very high average attenuation levels of 52.1 and 37.2 dB, respectively. Overall, the results show that the structural design of porous hybrid material can be used to adjust their EMI shielding performance at GHz and THz frequencies.

Utilization of Waste Amine-Oxime (WAO) Resin to Generate Carbon by Microwave and Its Removal of Pb(II) in Water.

Utilising waste amine-oxime (WAO) resin through microwave semi-carbonization, a carbon adsorbent (CA) was obtained to remove Pb(II). After microwave treatment, the pore size of the skeleton structure, three-dimensional porous network, and lamellar pore structure of WAO was improved. The distribution coefficient (Kd) of Pb(II) onto CA is 620 mL/g, and the maximum adsorption capacity of Pb(II) is 82.67 mg/g after 20 min of WAO microwave treatment. The adsorption kinetics and adsorption isotherms conform to the quasi-second-order kinetic equation and Langmuir adsorption isotherm model, respectively. The surface of MT-WAO is negatively charged and the adsorption mechanism is mainly electrostatic interaction. Pb(II) elution in hydrochloric acid solution is more than 98%, and its recovery is high at 318 K and for 1 h.

Programmable Local Orientation of Micropores by Mold-Assisted Ice Templating.

Pore geometry plays a crucial role in determining the properties and functions of porous materials. Various methods have been developed to prepare porous materials that have randomly distributed or well-aligned pores. However, a technique capable of fine regulation of local pore orientation is still highly desired but difficult to attain. A technique, termed mold-assisted ice templating (MIT), is reported to control and program the local orientation of micropores. MIT employs a copper mold of a particular shape (for instance a circle, square, hexagon, or star) and a cold finger to regulate the 3D orientation of a local temperature gradient, which directs the growth of ice crystals; this approach results in the formation of finely regulated patterns of lamellar pore structures. Moreover, the lamellar thickness and spacing can be tuned by controlling the solution concentration.

Fabrication of poly (1, 8-octanediol-co-Pluronic F127 citrate)/chitin nanofibril/bioactive glass (POFC/ChiNF/BG) porous scaffold via directional-freeze-casting

Abstract The citrate-based thermoset elastomer is a promising candidate for bone scaffold material, but the harsh curing condition made it difficult to fabricate porous structure. Recently, poly (1, 8-octanediol-co-Pluronic F127 citrate) (POFC) porous scaffold was creatively fabricated by chitin nanofibrils (ChiNFs) supported emulsion-freeze-casting. Thanks to the supporting role of ChiNFs, the lamellar pore structure formed by directional freeze-drying was maintained during the subsequent thermocuring. Herein, bioactive glass (BG) was introduced into the POFC porous scaffolds to improve bioactivity. It was found the complete replacement of ChiNF particles with BG particles could not form a stable porous structure; however, existing at least 15 wt% ChiNF could ensure the formation of lamellar pore, and the interlamellar distance increased with BG ratios. Thus, the BG granules did not contribute to the formation of pore structure like ChiNFs, however, they surely endowed the scaffolds with enhanced mechanical properties, improved osteogenesis bioactivity, better cytocompatibility as well as quick degradation rate. Reasonably adjusting BG ratios could balance the requirements of porous structure and bioactivity.

Permeable carbon nanotube-reinforced silicon oxycarbide via freeze casting with enhanced mechanical stability

Reinforcement strategies were applied to solution-based freeze-casting systems to form porous ceramic composites. Multi-walled carbon nanotubes (MWCNT) were used as the reinforcing fillers in a polysiloxane preceramic polymer freeze cast with dimethyl carbonate to produce silicon oxycarbide-based lamellar pore structures. Using a carefully designed dispersion procedure, CNT agglomerates were reduced both in suspension and in pore walls. Electrical conductivity increased by ten orders of magnitude over pure silicon oxycarbide, indicating the preservation of CNTs after pyrolysis. Permeability, compression, and diametral compression (Brazilian disk) tests were performed to demonstrate the reinforcing effect of CNTs without sacrificing the permeability of the final porous structures.

Fabrication, microstructure and physico-mechanical properties of highly porous acicular mullite ceramic foams by freeze casting method

In this article, highly porous mullite ceramics with whisker-interlocked microstructure were successfully prepared by freeze casting method. The ceramics possessed very high porosity (up to 94%) and high compressive strength (up to 3.65 MPa). It was found that polyvinyl alcohol (PVA) using as a binder could also affect the microstructure of ceramics significantly. Whisker-interlocked structure could be obtained when PVA concentration was high (4 wt%), while low PVA concentration (1%) led to the aligned lamellar pore structure. Moreover, the ceramic samples had small shrinkages of less than 3%, and solid loading in slurry was found to influence the porosity and compressive strength of the ceramics greatly.

Wicking into porous polymer-derived ceramic monoliths fabricated by freeze-casting

Silicon oxycarbide monoliths of different pore size distribution were fabricated by freeze-casting. The samples revealed a lamellar pore structure with an axial anisotropy. To evaluate the capillary transport abilities we performed wicking experiments. The sample weight measurement method was applied during the imbibition. The samples show deviations in permeability from 10% to 49% at different sample orientations that quantifies the impact of the anisotropy in the axial direction. The deviations were larger for the samples with smaller pore size. For these samples we also observed larger differences in the wicking behaviour. The samples with bigger pore size demonstrated higher permeability and faster wicking. Imbibition results at both sample orientations showed a good agreement with a prediction via the Lucas–Washburn equation with gravity effects. We demonstrate hereby, that our approach of macroscopic modelling predicts wicking behaviour in anisotropic structures reasonably well, providing a simple tool for further porous material investigations.

Freeze cast fabrication of porous ceramics using tert-butyl alcohol–water crystals as template

Tert-butyl alcohol (TBA)–water (H2O) crystals are used as template to fabricate aligned porous alumina with special pore structures. Alumina slurries were prepared using TBA–H2O mixtures, which could undergo continuous crystallization by unidirectional freezing. The pore shape of porous alumina constantly changed from lamellar to snowflake, dendritic, needle-like, and hexagonal with the increase in the TBA content in alumina slurries. The hexagonal porous structure obtained by pure TBA slurry exhibited the best mechanical properties, whereas the mixture structure with irregular pore walls showed the worst, exhibiting inferior properties to lamellar pore structures.

Fabrication of Barium Titanate/Hydroxyapatite with Aligned Pore Channels by Freeze Gelcasting

A novel and environmental-friendly route combining the conventional freeze casting and gel casting was proposed for the fabrication of aligned pore structures. The viscosity of the suspension increased with the increase of the addition of the Hydantion concentration. In comparison with the conventional freeze casting, the utilisation of a water-soluble Hydantion epoxy resin in the freeze casting promoted the gelation process and led to a higher compressive strength of the green body. After sintering, the freeze gelcast samples exhibited more than twice higher compressive strength and 11.5% higher porosity at the same time than those of conventional freeze cast sample. The improvements were attributed to the combination of gelation of Hydantion epoxy resin and the aligned ice growth, which led to the coexistence of aligned cylindrical and lamellar pore structures.

Bi-layered calcium phosphate cement-based composite scaffold mimicking natural bone structure

In this study, a core/shell bi-layered calcium phosphate cement (CPC)-based composite scaffold with adjustable compressive strength, which mimicked the structure of natural cortical/cancellous bone, was fabricated. The dense tubular CPC shell was prepared by isostatic pressing CPC powder with a specially designed mould. A porous CPC core with unidirectional lamellar pore structure was fabricated inside the cavity of dense tubular CPC shell by unidirectional freeze casting, followed by infiltration of poly(lactic-co-glycolic acid) and immobilization of collagen. The compressive strength of bi-layered CPC-based composite scaffold can be controlled by varying thickness ratio of dense layer to porous layer. Compared to the scaffold without dense shell, the pore interconnection of bi-layered scaffold was not obviously compromised because of its high unidirectional interconnectivity but poor three dimensional interconnectivity. The in vitro results showed that the rat bone marrow stromal cells attached and proliferated well on the bi-layered CPC-based composite scaffold. This novel bi-layered CPC-based composite scaffold is promising for bone repair.

Improvement of cell response of the poly(lactic-co-glycolic acid)/calcium phosphate cement composite scaffold with unidirectional pore structure by the surface immobilization of collagen via plasma treatment

In this study, calcium phosphate cement (CPC)-based scaffold with unidirectional lamellar pore structure was fabricated by unidirectional freeze casting. Poly(lactic-co-glycolic acid) (PLGA) was infiltrated into the CPC scaffold to improve its strength and toughness, which compromised the bioactivity and osteoconductivity of CPC. Collagen (Col) was immobilized on the pore surface of the PLGA/CPC scaffold to enhance the bioactivity of the scaffold using plasma treatment under the ammonia (NH3) atmosphere. The immobilization of collagen was characterized by infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Compared to the PLGA/CPC composite scaffold, the Col/PLGA/CPC composite scaffold had higher contact angle, porosity and water absorption, while the compressive strength of both scaffolds was comparable. Rat bone marrow mesenchymal stem cells (rMSCs) seeded on the Col/PLGA/CPC scaffold showed markedly improved cell seeding, attachment, proliferation and differentiation than those on the PLGA/CPC scaffold. These results suggest that the surface immobilization of collagen by plasma treatment can improve the bioactivity of the PLGA/CPC scaffold and the Col/PLGA/CPC composite scaffold is a promising candidate for bone tissue engineering.

In vitro degradation, biocompatibility, and in vivo osteogenesis of poly(lactic‐co‐glycolic acid)/calcium phosphate cement scaffold with unidirectional lamellar pore structure

The aim of this study was to investigate the in vitro degradation, cytocompatibility, and in vivo osteogenesis of poly(lactic-co-glycolic acid) (PLGA)/calcium phosphate cement (CPC) scaffold with unidirectional lamellar pore structure. CPC-based scaffold was fabricated by unidirectional freeze casting, and PLGA was used to improve the mechanical properties of the CPC-based scaffold, which covered the surface of the pore wall as coating. The in vitro degradation results demonstrated that the PLGA/CPC scaffold had good degradability. The degradation of PLGA film on the surface of the scaffold made the CPC matrix exposed, which facilitated cell response and osteogenesis. Rat bone mesenchymal stem cells (rMSCs) were seeded on the PLGA/CPC composite scaffold. Cell viability, proliferation, and differentiation on the PLGA/CPC composite scaffold were evaluated. The results showed that viable rMSCs attached on the surface of pore wall gradually penetrated into the internal pores of the scaffold as prolongation of culture time. In addition, the rMSCs seeded on the scaffold exhibited good proliferation and growing alkaline phosphatase activity. The scaffold was implanted in the defects in distal end of femora of New Zealand white rabbits. Histological evaluation indicated that the PLGA/CPC scaffold with unidirectional lamellar pore structure had good biocompatibility and effective osteogenesis. These results suggest PLGA/CPC composite scaffold with unidirectional lamellar pore structure is a promising scaffold for bone tissue engineering.

Freeze gelcasting of aqueous alumina suspensions for porous ceramics

A novel route combining the conventional freeze casting and gel casting was proposed for the fabrication of aligned pore structures. In comparison with the conventional freeze casting, the utilization of a water-soluble Hydantoin epoxy resin in the freeze casting promoted the gelation process and led to a higher compressive strength of the green body. After sintering, the freeze gelcast samples exhibited a twice higher compressive strength and 15.6% higher porosity at the same time than those of conventional freeze cast sample. The improvements were attributed to the combination of gelation of Hydantoin epoxy resin and the aligned ice growth, which led to the coexistence of aligned cylindrical and lamellar pore structures.

Aluminophosphate-based mesoporous molecular sieves: synthesis and characterization of TAPOs

Mesoporous Ti-substituted aluminophosphates (AlPOs) with a hexagonal, cubic and lamellar pore structure, characteristic of MCM-41, MCM-48, and MCM-50, respectively, were synthesized. The stability of these mesophases upon template removal was studied. The pore structures, surface properties, and local atom environments of Al, P, and Ti of the hexagonal and cubic Ti-containing mesoporous products were extensively characterized using X-ray diffraction, magic angle spinning nuclear magnetic resonance, AAS, XPS, ultraviolet–visible, and adsorption of nitrogen and water vapor techniques while the lamellar mesophase was not further characterized due to its very poor thermal stability. Ti-containing mesoporous AlPO materials show a reasonable thermal stability upon template removal, a hydrophilic surface property, and high porosity showing application potentials in catalytic oxidation of hydrocarbons.

ZrC strength and plasticity (II) ZrC deformation structure: A G Lanin, et al, (Research Technological Institute, Podolsk, USSR), Int J. Refractory Metals & Hard Materials, Vol 9, No 3, 1990, 139–142

Tert-butyl alcohol (TBA)–water (H2O) crystals are used as template to fabricate aligned porous alumina with special pore structures. Alumina slurries were prepared using TBA–H2O mixtures, which could undergo continuous crystallization by unidirectional freezing. The pore shape of porous alumina constantly changed from lamellar to snowflake, dendritic, needle-like, and hexagonal with the increase in the TBA content in alumina slurries. The hexagonal porous structure obtained by pure TBA slurry exhibited the best mechanical properties, whereas the mixture structure with irregular pore walls showed the worst, exhibiting inferior properties to lamellar pore structures.