Fabrication and characterization of porous β-tricalcium phosphate scaffolds coated with alginate

All porous materials have a common limitation which is lack of strength due to the porosity. In this study, two different methods have been used to produce porous β-tricalcium phosphate (β-TCP) scaffolds: liquid-nitrogen freeze casting and a combination of the direct-foaming and sacrificial-template methods. Among these two methods, porous β-TCP scaffolds with acceptable pore size and compressive strength and defined pore-channel interconnectivity were successfully fabricated by the combined direct-foaming and sacrificial-template method. The average pore size of the scaffolds was in the range of 100–150 µm and the porosity was around 70%. Coating with 4 wt% alginate on porous β-TCP scaffolds led to higher compressive strength and low porosity. In order to make a chemical link between the β-TCP scaffolds and the alginate coating, silane coupling agent was used. Treated β-TCP scaffold showed improvements in compressive strength of up to 38% compared to the pure β-TCP scaffold and 11% compared to coated β-TCP scaffold.     

Porous hydroxyapatite ceramics by ice templating: Freezing characteristics and mechanical properties

Ice templating produces porous hydroxyapatite (HA) scaffolds with a lamellar morphology and aligned channels when using aqueous HA slurries. We investigated the freezing characteristics of HA slurries with regard to the pore structures of the porous HA scaffolds. We found that by increasing the cooling rate, the lamellar spacing decreased. The average lamellar spacing is about 785.7 μm at a cooling rate of 1.3 °C/min. The porous geometry changes from lamella and well aligned channels to a partial dendrite and partially aligned cavities with a decrease in the initial nucleation temperature and an increase in the degree of supercooling. Additionally, we determined the relationship between compressive strength and porosity. The compressive strength of the porous HA scaffolds reach 6.7 MPa at a porosity of 64% and the lamellar spacing is about 124 μm.     

Effects of oxide addition on the microstructure and mechanical properties of lamellar SiC scaffolds and Al–Si–Mg/SiC composites prepared by freeze casting and pressureless infiltration

Silicate-bonded porous SiC scaffolds with lamellar structures were prepared by freeze casting and liquid-phase sintering. It was found that the viscosity and solidification velocity of SiC water-based slurries with 30 vol% solid loading decreased with increasing Al₂O₃–MgO (AM) addition. As the AM content increased from 10 to 30 wt%, the lamellae of the sintered scaffolds became denser and the porosity decreased from 69±0.5% to 62±0.5%, while the compressive strength improved from 25±2 to 51±2 MPa. The dynamics of pressureless infiltration for an Al–12 Si–10 Mg alloy on the SiC porous scaffold was measured and the composites with lamellar-interpenetrated structures were successfully produced. Both the compressive strength and the elastic modulus of the composites increased with increasing AM content. The maximum strength reached 952±24 MPa and the highest elastic modulus about 156 GPa, respectively, in a longitudinal direction, increasing about 32% and 11% as compared with those of the composites without AM.     

The Tunable Porous Structure of Gelatin–Bioglass Nanocomposite Scaffolds for Bone Tissue Engineering Applications: Physicochemical,Mechanical, and In Vitro Properties

Unidirectional freeze‐casting method is used to fabricate gelatin–bioglass nanoparticles (BGNPs) scaffolds. Transmission electron microscopy (TEM) images show that sol–gel prepared BGNPs are distributed throughout the scaffold with diameters of less than 10 nm. Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetric are used to evaluate the physicochemical properties of BGNPs. Scanning electron microscopy (SEM) micrographs present an oriented porous structure and a homogeneous distribution of BGNPs in the gelatin matrix. The lamellar‐type structure indicates an improvement of mechanical strength and absorption capacity of the scaffolds. Increasing the concentration of BGNPs from 0 to 50 wt% have no noticeable effect on pore orientation, but decreases porosity and pore size distribution. Increase in BGNPs content improves the compressive strength. The absorption and biodegradation rate reduces with augmentation in BGNPs concentration. Bioactivity is evaluated through apatite formation after immersion of the nanocomposites in simulated body fluid and is verified by SEM–energy‐dispersive X‐ray spectroscopy (EDS), an element map analysis, X‐ray powder diffractometer, and FTIR spectrum. SEM images and methyl thiazolyl tetrazolium assay confirm the biocompatibility of scaffolds and the supportive behavior of nanocomposites in cellular spreading. The results show that gelatin–(30 wt%)bioglass nanocomposites have incipient physicochemical and biological properties.     

Preparation and properties of high-porosity ZrB2-SiC ceramics by water-based freeze casting

The development of novel cermet composites based on porous ceramics with high porosity, interconnected pore structure and good mechanical property has attracted considerable attention in engineering application. In this work, water-based freeze casting process was employed to fabricate ZrB₂-SiC porous ceramic with aligned lamellar-channels structure using PAA-NH₄ as the dispersant. The results revealed that the well-dispersed suspension with best rheological behavior was obtained using 1.0 wt% PAA-NH₄ at pH 9. The crack-free porous ceramic exhibited small volume shrinkage ranging from 2.59 % to 1.87 %. By varying the solid loading, the fabricated samples displayed a tailored porosity ranging from 76.12% to 59.37% and an excellent compressive strength of 7 MPa to 78 MPa. After oxidation, the samples displayed a decreased porosity and an increased compressive strength. The ZrB₂­SiC porous ceramic fabricated in this work will be a promising candidate for the framework of cermet composite.     

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.     

Porous mullite ceramics derived from coal fly ash using a freeze-gel casting/polymer sponge technique

The objective of this study was to prepare highly porous mullite ceramics with relatively large-sized pores and improved compressive strength using a freeze/gel casting route combined with polymer sponge for recycling of coal fly ash into high value-added ceramics. In this work, a tertiary-butyl alcohol /coal fly ash slurry system with an appropriate addition of Al₂O₃ was used. A reticulated structure with large pore size of 220–300 μm, which formed on burnout of polyurethane was obtained; then, the skeletons consisted mainly of more dense crystalline phases together with a few fine pores (<3 μm). The rod-shaped mullite crystals with an aspect ratio of >3.7 (~4 μm in diameter) seen to have grown within the silicate melts existed. The compressive strength of the sintered porous materials increased in the reverse order of the degree of porosity, i.e. low porosity gave a high compressive strength. The porous materials with an average porosity of 61.6 %, sintered at 1600 °C with 70 wt.% solid loading showed the maximum average compressive strength (~45 MPa).     

Effect of particle size and freezing conditions on freeze-casted scaffold

Freeze casting is one of the emerging and novel manufacturing routes to fabricate porous scaffolds for various applications including orthopedic implants, drug delivery, energy storing devices etc. Thus, it becomes important to understand this process in a deeper sense. Present work was focused to study the effect/influence of basic parameters, particle sizes, and freezing conditions on the mechanical properties and microstructures of porous scaffold fabricated by freeze casting. β-tricalcium phosphate (β-TCP) and hydroxyapatite (HAp) powder with particle sizes of 10?μm and 20?nm were used. Prepared slurries were freeze casted at constant freezing temperature (5?°C) and constant freezing rate (1.86?°C/min) to study the effect of freezing conditions on mechanical and microstructural properties of the porous scaffold. It was observed that porous scaffold fabricated by nanoparticles has given better porosity (63.22–76.16%), than scaffold fabricated by microparticles (13–43.05%) at given solid loading of both freezing conditions. Although, the range of pore size of the scaffold fabricated by nanoparticles (CFR: 2.60–0.84?μm; CFT: 1.66–0.46?μm) was lower than that of scaffold fabricated by microparticles (CFR: 9.45–4.83?μm; CFT: 4.72–2.84?μm). The compressive strength of scaffolds prepared by nanoparticles was in the range of trabecular bone. Moreover, the results of present work will pave the way for the fabrication of porous scaffold with desired pore size and porosity for various implants, energy, and drug delivery applications.     

Preparation of high strength lamellar porous calcium silicate ceramics by directional freeze casting

In this study, porous calcium silicate (CS) ceramics with oriented arrangement of lamellar macropore structure were prepared by directional freeze casting method. The lamellar macropores were connected by the micropores on the pore wall, which had good pore interconnectivity. The effects of solid loading of the slurry, freezing temperature, sintering additive content, and sintering temperature on the microstructures and compressive strength of the synthesized porous materials were investigated systematically. The results showed that with the increase of solid loading (≤20 vol%) and sintering additive content, the sizes of lamellar pores and pore walls increased gradually, the open porosity decreased and the compressive strength increased. The sintering temperature had little effect on the pore size of the ceramics, but increasing the sintering temperature (≤1050 °C) promoted the densification of the pore wall, reduced the porosity, and improved the strength. The decrease of freezing temperature had little effect on porosity, but it reduced the size of lamellar pore and pore wall, so as to improve the strength. Finally, porous CS ceramics with lamellar macropores of about 300–600 μm and 2–10 μm micropores on the pore wall were obtained. The porous CS ceramics had high pore interconnectivity, an open porosity of 66.25% and a compressive strength of 5.47 MPa, which was expected to be used in bone tissue engineering.     

TBA-based freeze/gel casting of porous hydroxyapatite scaffolds

Porous ceramic scaffolds with a controlled “designer” pore structure have been prepared by the freeze/gel casting route using a TBA-based hydroxyapatite slurry system with 20–40 wt.% solid content. The products were characterized in terms of sintered microstructure, together with physical and mechanical properties. After sintering at 1050–1250 °C, the advantages of freeze casting and gel casting appeared in the pore structure and compressive strength of the ceramics, i.e., unidirectional aligned macro-pore channels developed by controlling the solidification direction of the TBA solvent used in the freeze casting together with small diameter (micron sized) isolated pores formed in the dense outer walls of the pore channels when processed by gel casting. The sintered porosity and pore size generally resulted in a high solid loading giving low porosity and small pore size, this leading to higher compressive strengths. The scaffolds obtained exhibited an average porosity and compressive strength in the range 41.9–79.3% and 35.1–2.7 MPa, respectively, depending on the processing conditions used.     

Collagen‐based scaffolds reinforced by chitosan fibres for bone tissue engineering

In this paper, porous bone scaffolds reinforced with chitosan fibres were prepared. The porosity and pore size of the reinforced scaffolds were both satisfactory. The reinforced scaffolds resembled natural bone in both components and crystal size. Only if the length of the fibres was no shorter than the critical length, could the fibres reinforce the material. We have proposed an empirical formula to calculate the critical length of the fibres for the porous materials and determined the modifying factor (F_l) for the porous bone scaffold investigated in this work. Along with the increase of the fibres' volume content, the compressive strength of the scaffold also increased. We have proposed a further empirical formula for calculating the compressive strength of the porous reinforced materials and determined the modifying factor (F_σ) for the porous reinforced bone scaffold examined in these studies. Along with the degradation in vitro, the decrease in strength of the reinforced scaffold was less than that of the unreinforced scaffold. The growth rate of osteoblast cells on the reinforced scaffold was higher than that on the unreinforced scaffold. These results suggest that the reinforced scaffold may be a promising candidate matrix for repairing large bone defects. Copyright © 2005 Society of Chemical Industry     

Porous SiC using polycarbosilane/camphene solutions: Roles of freeze casting parameters

In order to control the pore characteristics and macroscopical performance of porous ceramics, roles of the freeze casting parameters are the key points. Herein, aligned dendritic porous SiC was fabricated by freeze casting of PCS-camphene solutions with different solid loading, freeze front velocity, temperature gradient, and freezing temperature. Influence of these parameters on the microstructure and compressive strength of porous SiC was investigated. With increasing the PCS content, freeze temperature, freeze front velocity or temperature gradient, degree of undercooling of the camphene was increased, resulting in the formation of smaller pore size, decreased porosity and increased compressive strength. Compared to variables of freeze temperature and temperature gradient, increased freeze front velocity was more efficiency in improving the compressive strength of porous SiC, owing to the formation of smaller pore size and longer secondary dendritic crystals. Promising micron-sized porous SiC with high porosity (79.93 vol%) and satisfactory strength (15.84 MPa) was achieved for 10% PCS-camphene solution under optimized freezing conditions.     

Microstructure,phase compositions and in vitro evaluation of freeze casting hydroxyapatite-silica scaffolds

Hydroxyapatite-silica (HA-SiO₂) scaffolds with different SiO₂ content (0, 2, 5 and 10 wt% SiO₂) were fabricated by freeze casting. After sintering, the scaffolds maintained the interconnected unidirectional pore channels by removing the frozen ice crystals via sublimation. X-ray diffraction (XRD) analysis indicated that SiO₂ promoted the decomposition of HA to tricalcium phosphate (TCP), comprised of α-TCP and β-TCP, which became more apparent with the increase of SiO₂ content. The microstructure observation of scanning electron microscope (SEM) showed that the scaffolds surface feature had great changes in terms of grain size and grain boundary with the addition of SiO₂. Moreover, the addition of SiO₂ could increase the porosity and pore size of the scaffolds, even allowing it to reach a maximum as the SiO₂ content increased to 5 wt%. Compression tests investigated the variation in the compressive strength of the scaffolds with the increase in the SiO₂ content, which showed first decreasing and then increasing behavior. In vitro evaluation results in simulated body fluid (1.5×SBF) revealed that the introduction of SiO₂ enhanced the growth rate of bone-like layer, especially the scaffold with 5 wt% SiO₂, which exhibited faster growth rate of bone-like layer than the other scaffolds. The XRD and fourier transformed infrared spectroscopy (FT-IR) characterization confirmed that the bone-like layer formed on the scaffold surface was a carbonate-containing hydroxyapatite bone-like layer.     

Physical and mechanical properties of the fully interconnected chitosan ice‐templated scaffolds

Porous chitosan scaffolds were prepared with a freeze‐casting technique with different concentrations, 1.5 and 3 wt %, and also different cooling rates, 1 and 4°C/min. The pore morphology, porosity, pore size, mechanical properties, and water absorption characteristics of the scaffolds were studied. Scanning electron microscopy images showed that the freeze‐cast scaffolds were fully interconnected because of the existence of pores on the chitosan walls in addition to many unidirectionally elongated pores. Increases in the chitosan concentration and freezing rate led to elevations in the thickness of the chitosan walls and reductions in the pores size, respectively. These two results led to the enhancement of the compressive strength from 34 to 110 kPa for the scaffolds that had 96–98% porosity. Also, augmentation of the chitosan concentration and decreases in the freezing rate led to the reduction of the number of pores on the chitosan walls. Furthermore, the volume of water absorption increased with a reduction in the chitosan concentration and cooling rate from 690 to 1020%. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41476.     

Microstructure and mechanical properties of poly(L‐lactide) scaffolds fabricated by gelatin particle leaching method

Biodegradable poly(L ‐lactide) (PLLA) scaffolds with well‐controlled interconnected irregular pores were fabricated by a porogen leaching technique using gelatin particles as the porogen. The gelatin particles (280–450 μm) were bonded together through a treatment in a saturated water vapor condition at 70°C to form a 3‐dimensional assembly in a mold. PLLA was dissolved in dioxane and was cast onto the gelatin assembly. The mixtures were then freeze‐dried or dried at room temperature, followed by removal of the gelatin particles to yield the porous scaffolds. The microstructure of the scaffolds was characterized by scanning electron microscopy with respect to the pore shape, interpore connectivity, and pore wall morphology. Compression measurements revealed that scaffolds fabricated by freeze‐drying exhibited better mechanical performance than those by room temperature dying. Along with the increase of the polymer concentration, the porosity of the scaffolds decreased whereas the compressive modulus increased. When the scaffolds were in a hydrated state, the compressive modulus decreased dramatically. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1373–1379, 2005     

Evaluation of the pore morphologies for piezoelectric energy harvesting application

Piezoelectric energy harvesting has attracted significant attention in recent years due to their high-power density and potential applications for self-powered sensor networks. In comparison to dense piezoelectric ceramics, porous piezoelectric ceramics exhibit superiority due to an enhancement of piezoelectric energy harvesting figure of merit. This paper provides a detailed examination of the effect of pore morphology on the piezoelectric energy harvesting performance of porous barium calcium zirconate titanate 0.5Ba(Zr_0.2 Ti_0.8)O₃-0.5(Ba_0.7Ca_0.3)TiO₃ (BCZT) ceramics. Three different pore morphologies of spherical, elliptical, and aligned lamellar pores were created via the burnt-out polymer spheres method and freeze casting. The relative permittivity decreased with increasing porosity volume fraction for all porous BCZT ceramics. Both experimental and simulation results demonstrate that porous BCZT ceramics with aligned lamellar pores exhibit a higher remanent polarization. The longitudinal d₃₃ piezoelectric charge coefficient decreased with increasing porosity volume fraction for the porous ceramics with three different pore morphologies; however, the rate of decrease in d₃₃ with porosity is slower for aligned lamellar pores, leading to the highest piezoelectric energy harvesting figure of merit. Moreover, the peak power density of porous BCZT ceramics with aligned lamellar pores is shown to reach up to 38 μW cm^-2 when used as an energy harvester, which is significantly higher than that of porous BCZT ceramics with spherical or elliptical pores. This work is beneficial for the design and manufacture of porous ferroelectric materials in devices for piezoelectric energy harvesting applications.     

Bi-directional freeze casting of porous alumina ceramics: A study of the effects of different processing parameters on microstructure

Highly aligned lamellar ceramic scaffolds were produced using a bi-directional freeze casting technique. A specially designed, sloped copper mould was covered with a polymer to modulate the temperature field. Effects of different processing parameters (cooling rate, mould slope angle, ceramic solid loading and binder concentration) on lamellar orientation were systematically studied. The results showed that freezing under a dual temperature gradient produced highly aligned ceramic scaffolds. Increasing both the cooling rate and the mould slope angle increased the size of the ordered ceramic region. Using different alumina solid loadings in the initial suspension had little effect on the aligned lamellar structure. Increasing the binder concentration affected ice crystal growth in a highly aligned direction. Therefore, freeze casting using a dual temperature gradient can be used to fabricate highly aligned porous materials.     

Chitosan/gelatin porous bone scaffolds made by crosslinking treatment and freeze‐drying technology: Effects of crosslinking durations on the porous structure,compressive strength,and in vitro cytotoxicity

In this study, freezing was used to separate a solute (polymer) and solvent (deionized water). The polymer in the ice crystals was then crosslinked with solvents, and this diminished the linear pores to form a porous structure. Gelatin and chitosan were blended and frozen, after which crosslinking agents were added, and the whole was frozen again and then freeze‐dried to form chitosan/gelatin porous bone scaffolds. Stereomicroscopy, scanning electron microscopy, compressive strength testing, porosity testing, in vitro biocompatibility, and cytotoxicity were used to evaluate the properties of the bone scaffolds. The test results show that both crosslinking agents, glutaraldehyde (GA) and 1‐ethyl‐3‐(3‐dimethylaminopropyl) carbodiimide, were able to form a porous structure. In addition, the compressive strength increased as a result of the increased crosslinking time. However, the porosity and cell viability were not correlated with the crosslinking times. The optimal porous and interconnected pore structure occurred when the bone scaffolds were crosslinked with GA for 20 min. It was proven that crosslinking the frozen polymers successfully resulted in a division of the linear pores, and this resulted in interconnected multiple pores and a compressively strong structure. The 48‐h cytotoxicity did not affect the cell viability. This study successfully produced chitosan/gelatin porous materials for biomaterials application. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41851.     

Controlling anisotropy of porous B4C structures through magnetic field-assisted freeze-casting

Anisotropic porous boron carbide (B₄C) structures were successfully produced, for the first time, using the magnetic field-assisted freeze casting method. The effect of the magnetic field on the structure and mechanical strength of the formed porous B₄C was compared for two different magnetic field directions that were either aligned with ice growth (vertical), or perpendicular to the ice growth direction (horizontal). It was shown that applying even a weak horizontal magnetic field of 0.1–0.3 T noticeably affected the alignment of mineral bridges between lamellar walls. Both the porosity and the channel widths decreased with increasing horizontal magnetic field strength. In the case of a vertical magnetic field, a larger strength of 0.4 T was required for highly aligned lamellar walls and larger channel widths. Compression strength tests indicated that the application of magnetic fields led to more homogeneously aligned channels, which resulted in increased compression strength in the longitudinal (parallel to the ice growth) direction. Applying a vertical magnetic field of 0.4 T with a cooling rate of 2 °C/min during the freezing step of the magnetic field-assisted freeze-casting method was found to result in the best conditions for producing highly anisotropic structures with large channel widths and fewer mineral bridges, which led to an increase in the mechanical strength.     

In situ growth of hydroxyapatite on lamellar alumina scaffolds with aligned pore channels

We prepared hydroxyapatite/alumina (HA/Al₂O₃) porous composite materials by the in situ growth of HA on lamellar Al₂O₃ ceramics. The procedure comprises two steps. First, lamellar Al₂O₃ ceramics with aligned pore channels were obtained by freeze casting Al₂O₃ slurry. Second, as-prepared monoliths were used as scaffolds and subjected to hydrothermal treatment in the HA precursor solution. We characterized the microstructure, chemical composition, and mechanical properties of synthesized HA/Al₂O₃. Experiment results show that the addition of H₂O₂ into the reaction solution plays an important role in controlling the microstructure of the cultivated HA. The formation of HA has no obvious influence on the lamellar architecture of Al₂O₃ ceramics. The prepared composite materials exhibit superior compressive strength. Combination of the high strength of lamellar Al₂O₃ with the excellent biological properties of HA increases the potential of obtained HA/Al₂O₃ for load-bearing biological applications.