Structure control of hydroxyapatite ceramics via an electric field assisted freeze casting method

Using H₂O₂ aqueous solution as pore-forming agent, hydroxyapatite (HA) porous scaffolds with both lamellar and spherical pores were fabricated by a freeze casting method. The highest porosity was obtained in HA scaffolds prepared using 5 vol% H₂O₂ aqueous solution. The relationship between the electric field intensity and the properties of HA scaffolds was investigated. Results showed that when the electric field intensity was increased from 0 to 90 kV/m, the average diameters of lamellar and spherical pores of HA scaffold were increased from 460 μm to 810 μm, and from 320 μm to 420 μm, respectively. Vitro cellular assay indicated that HA scaffold with both the lamellar and the spherical pores has a better biocompatibility, compared with that with single pores.     

In vitro assessment of degradation and bioactivity of robocast bioactive glass scaffolds in simulated body fluid

In this study porous three-dimensional scaffolds of borate (13-93B3) bioactive glass were prepared by robocasting and in vitro degradation and bioactivity was evaluated. Grid like scaffolds with interconnected pores was assembled using robotic deposition technique which is a direct ink writing method. After binder burnout, the constructs were sintered for 1 h at 560 °C to produce scaffolds (porosity≈60%) consisting of dense glass struts (300±20 μm in diameter) and interconnected pores of width 580±20 μm. Hydroxyapatite formation on borate bioactive glass scaffolds was investigated in simulated body fluid (SBF) using three different scaffold/SBF (S/S) ratios (1, 2 and 10 mg/ml) at 37 °C. When immersed in SBF, degradation rate of the scaffolds and conversion to a calcium phosphate material showed a strong dependence to the S/S ratio. At high solid concentration (10 mg/ml) surface of the glass scaffolds converted to the calcium rich amorphous calcium phosphate after 30 days. At lower solid concentrations (2 and 1 mg/ml) an amorphous calcium phosphate layer formation was observed followed by the conversion to hydroxyapatite.     

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.     

Three dimensional polyurethane/ hydroxyapatite bioactive scaffolds: The role of hydroxyapatite on pore generation

Fabrication of a suitable scaffold with highly interconnected and well-distributed pores for cell proliferation and growth in the field of bone tissue engineering is of high importance. In this study, three-dimensional porous polyurethane (PU) scaffolds, with 0, 15, 25, and 32 wt% hydroxyapatite (HA), were fabricated. In this regard, HA was incorporated into PU constituents prior to starting in-situ polymerization of PU. Porosity and density measurement of the scaffolds revealed that higher amount of HA in the scaffolds led to increasing the former and decreasing the latter quantity. The field emission scanning electron microscopy (FESEM) images revealed that by increasing HA content, the pore size showed a descending trend while the number of pores increased. This would be attributed to the type of interactions between HA and PU, and the role of HA in pore formation. Mechanical test revealed that Young's Modulus of the samples was reduced by increasing scaffold porosity caused by the increase of HA content. Bioactivity tests in the simulated body fluid (SBF) showed the ability of scaffolds forming apatite precipitates. MTT assay showed that by increasing HA content, MG63 osteoblast cell viability increased and FESEM images revealed proper attachment of the cells to the scaffold surface.     

Gelatin-layered and multi-sized porous β-tricalcium phosphate for tissue engineering scaffold

The multi-sized porous β-tricalcium phosphate scaffolds were fabricated by freeze drying followed by slurry coating using a multi-sized porous sponge as a template. Then, gelatin was dip coated on the multi-sized porous β-tricalcium phosphate scaffolds under vacuum. The mechanical and biological properties of the fabricated scaffolds were evaluated and compared to the uniformly sized porous scaffolds and scaffolds that were not coated by gelatin. The compressive strength was tested by a universal testing machine, and the cell viability and differentiation behavior were measured using a cell counting kit and alkaline phosphatase activity using the MC3T3-E1 cells. In comparison, the gelatin-coated multi-sized porous β-tricalcium phosphate scaffold showed enhanced compressive strength. After 14 days, the multi-sized pores were shown to affect cell differentiation, and gelatin coatings were shown to affect the cell viability and differentiation. The results of this study demonstrated that the multi-sized porous β-tricalcium phosphate scaffold coated by gelatin enhanced the mechanical and biological strengths.     

Development of porous ultra high molecular weight polyethylene scaffolds for the fabrication of orbital implant

Porous implants having interconnecting channels allow ingrowth of host connective tissue. Complete implant vascularization reduces the risk of infection, extrusion, and other complications associated with nonintegrated implants. Attempts were made to develop 60% and 70% porous ultra high molecular weight polyethylene (UHMWPE) scaffolds and blocks using sodium chloride as channeling agent, which when dissolved in boiling water leaves behind the interconnecting channels. The average diameter of the pores of 60% and 70% porous scaffolds was found to be approx. 170 μm and 210 μm, respectively. Mechanical characterizations of the scaffolds indicated sufficient strength to be used for orbital implant fabracation. Surface roughness of the scaffolds indicated increase in surface roughness with the increase in porosity. The scaffolds developed were found to be hemocompatible with the human blood. Subsequently, the 70% porous scaffold was dip coated with a solution mixture of sodium carboxy methyl cellulose (SCMC)/polyvinyl alcohol (PVA)/hydroxyapatite (HA) which also showed hemocompatibility. Ciprofloxacin release pattern from the membrane was determined. Finally an orbital implant was fabricated from the 70% porous scaffold.     

Porous Hydroxyapatite Scaffolds Coated With Bioactive Apatite–Wollastonite Glass–Ceramics

The objective of this study was to fabricate porous hydroxyapatite (HA) scaffolds coated with bioactive A/W glass–ceramics and to examine their mechanical and biological properties. Firstly, the HA scaffolds were prepared by the polymeric sponge replication method, and then A/W glasses were coated on the surface of the struts. All of the scaffolds had a highly porous structure with well-interconnected pores. It was observed that the bioactive glass coating markedly increased the strength of the HA scaffolds. This enhancement was attributed to the formation of a dense and strong coating layer on the weak HA struts. The in vitro bioactivities of the scaffolds were markedly improved by the coatings. When the coated scaffolds were soaked in a simulated body fluid (SBF), the bone-like apatite crystals were well mineralized on their surfaces. Osteoblast-like cells (MC3T3) adhered, spread, and grew well on the porous scaffolds. The cells placed on the glass-coated HA scaffold showed a higher proliferation rate and alkaline phosphatase (ALP) activity than those on the pure HA scaffold. These results demonstrate that the bioactive glass coating is effective in improving the strength and bioactivity of the porous HA scaffolds.     

The effect of macro/micro combination pore structure of biphasic calcium phosphate scaffold on bioactivity

Cellular responses such as adhesion, spreading, growth and proliferation can be altered by macro and micro pores as well as surface patterns on its surface. In this study, the scaffold was prepared with interconnected macro pores by the polymeric sponge method and with interconnected micro pores by BCP slurry coating with different particle sizes on sintered porous body and necking sintering process. Two particle sizes for micro pores, 200–400 nm and 600–800 nm, were coated on a pre-sintered porous body. A combined macro and micro pore structure of the BCP scaffold can improve osteoblast cellular activity. In vitro study of MG63 cells suggest that the modified scaffolds improved cell response such as cell spreading, proliferation and differentiation. The scaffold that has a uniform distribution of micro pores ranging 1~5 µm in diameter on its surface yields the highest rate of cellular response.     

Manufacturing of highly porous calcium phosphate bioceramics via gel-casting using agarose

There is a technological need for highly porous bioceramics to be produced in an environmentally friendly manner. Gel-casting of highly porous HAp-(α-TCP) (CaP) foams using agarose as a gelling agent was investigated. Foaming of gel-cast suspension was performed at the temperature of 60 °C followed by transformation of the foams from a liquid state to a gelled state by cooling them to 15 °C. The sintered (1250 °C, 2 h soaking time) foams were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ adsorption isoterm and Hg porosimetry. XRD study revealed that additives used in the gel-casting process did not influence the phase composition of the investigated materials. The macroporous microstructure of HAp-(α-TCP) foams was typically composed of approximately spherical pores (cells) interconnected by circular windows. The foams exhibited a broad pore size distribution with cells and windows ranging from 250 to 900 μm, and from 25 to 250 μm, respectively. The mode for spherical pore size was approximately 500 μm while the above value for window was ∼100 μm. Additionally, the small amount of wall microporosity in the range of 0.2–0.9 μm was confirmed by SEM and Hg porosimetry. The obtained porous (P = 90%) HAp-(α-TCP) scaffolds with interesting two types of macropores and a small amount of micropores seem to be a promising bone substitution material.     

Fabrication of porous SiC ceramics having pores shaped with Si grain templates

SiC porous ceramics were prepared by heating mixtures of Si powder and carbon black at 900 °C for 24 h in Na vapor. The grains of the Si powder were not only the source of Si for SiC but also served as templates for the pores in the SiC porous ceramics. Angular-shaped pores with sizes of 2-10, 10-150 and 50-150 μm were formed by angular Si grains with sizes of ≤10, ≤50 and ≤150 μm, respectively. The porosity of the SiC porous ceramics was around 55-59%. Spherical pores were also formed when spherical Si grains were used. A bending strength of 14 MPa was measured for the SiC porous ceramics prepared with the Si grains (≤50 μm).     

Synthesis of silicate glass/poly(l-lactide) composite scaffolds by freeze-extraction technique: Characterization and in vitro bioactivity evaluation

The main objectives of the present study were to fabricate the silicate glass/poly(l-lactide) composite scaffolds for bone engineering applications, by using the freeze-extraction technique, and to evaluate the possibility for optimizing their degradation rate by changing their glass content. The scaffolds characterized by SEM-EDXA, FT-IR, TGA and XRD. Examination of the SEM microphotographs revealed that the pore size of the scaffolds decreased as the glass content increased. The neat polymer scaffold (PLA) had a highly interconnected porous structure with a maximum pore size of 200 μm. For the composite scaffold containing glass content up to 25 wt% (SP25) and up to 50 wt% (SP50), the maximum pore size was 40 μm and 20 μm respectively. The apparent porosity was 56.56%, 52.49% and 48.74% for PLA, SP25 and SP50, respectively. The results of the degradation study showed that the water absorption of the scaffolds decreased by increasing their glass content, It reached finally to 48.71%and 30.93% for SP25 and SP50, respectively. It revealed that also the weight loss of the scaffolds increased by increasing the glass content. The final weight loss was around 5.44%, 9.31% and 26.17% for the PLA, SP25 and SP50, respectively, indicating that it was possible to modulate the degradation rate of the scaffolds by varying their glass content. In addition, the pH measurement of incubation medium indicated that the glass could compensate the acidic degradation products of the polymer. In vitro bioactivity evaluation showed that the composite scaffolds were able to induce the formation of hydroxyapaptite layer on their surfaces, demonstrating their potential application in bone engineering.     

A novel approach for the fabrication of carbon nanofibre/ceramic porous structures

This paper describes the fabrication of hybrid ceramic/carbon scaffolds in which carbon nanofibres and multi-walled carbon nanotubes fully cover the internal walls of a microporous ceramic structure that provides mechanical stability. Freeze casting is used to fabricate a porous, lamellar ceramic (Al₂O₃) structure with aligned pores whose width can be controlled between 10 and 90 μm. Subsequently, a two step chemical vapour deposition process that uses iron as a catalyst is used to grow the carbon nanostructures inside the scaffold. This catalyst remains in the scaffold after the growth process. The formation of the alumina scaffold and the influence of its structure on the growth of nanofibres and tubes are investigated. A set of growth conditions is determined to produce a dense covering of the internal walls of the porous ceramic with the carbon nanostructures. The limiting pore size for this process is located around 25 μm.     

Biocompatibility and strengthening of porous hydroxyapatite scaffolds using poly(l-lactic acid) coating

Hydroxyapatite (HA) is a well-known biocompatible bone substitute. Porous HA is more resorbable and osteoconductive compared with non-porous HA, and has been studied both experimentally and clinically. However, the mechanical strength of porous HA scaffolds is known to be weak. In this study, we developed a porous HA scaffold coated with a synthetic biodegradable polymer, poly(l-lactic acid) (PLLA), to strengthen the scaffold. PLLA-coated HA pellets were used to investigate the in vitro proliferation and alkaline phosphatase (ALP) activity of osteoblasts. PLLA-coated porous HA scaffolds were observed using scanning electron microscopy to investigate surface characteristics, porosity, and mechanical strength. PLLA coating concentration varied from 2 to 10 wt%. Osteoblast proliferation was higher in HA samples coated with PLLA compared with non-coated. ALP activity was highest on 8 wt% PLLA-coating after 3 days and on 4 wt% and 6 wt% PLLA after 9 and 12 days. Porous HA scaffolds with higher concentrations of PLLA were found to have a smoother, flatter surface. This enhanced proliferation and attachment of osteoblasts onto the porous HA scaffold. PLLA solution at a concentration of 10 wt% decreased scaffold porosity to half that of HA scaffolds with no PLLA coating. Scaffold mechanical strength was increased two-fold with a PLLA concentration of 2 wt%. Based on in vitro experimentation, it can be concluded that PLLA-coating on porous HA scaffolds enhances both the biocompatibility and the mechanical strength.     

Evaluation of mechanical and biocompatibility properties of hydroxyapatite/manganese dioxide nanocomposite scaffolds for bone tissue engineering application

The aim of this research was to evaluate the mechanical properties, biocompatibility, and degradation behavior of scaffolds made of pure hydroxyapatite (HA) and HA-modified by MnO₂ for bone tissue engineering applications. HA and MnO₂ were developed using sol-gel and precipitation methods, respectively. The scaffolds properties were characterized using X-ray diffraction (XRD), Fourier transform spectroscopy (FTIR), scanning electron microcopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). The interaction of scaffold with cells was assessed using in vitro cell proliferation and alkaline phosphatase (ALP) assays. The obtained results indicate that the HA/MnO₂ scaffolds possess higher compressive strength, toughness, hardness, and density when compared to the pure HA scaffolds. After immersing the scaffold in the SBF solution, more deposited apatite appeared on the HA/MnO₂, which results in the rougher surface on this scaffold compared to the pure HA scaffold. Finally, the in vitro biological analysis using human osteoblast cells reveals that scaffolds are biocompatible with adequate ALP activity.     

Production, mechanical properties and in vitro biocompatibility of highly aligned porous poly(ε-caprolactone) (PCL)/hydroxyapatite (HA) scaffolds

We produced highly aligned porous poly(ε-caprolactone) (PCL)/hydroxyapatite (HA) scaffolds by unidirectionally freezing PCL/HA solutions with various HA contents (0, 5, 10 and 20 wt% in relation to the PCL polymer) and evaluated their mechanical properties and in vitro biocompatibility to examine their potential applications in bone tissue engineering. All the prepared scaffolds had a highly aligned porous structure, in which the HA particles were uniformly dispersed in the PCL walls. The elastic modulus of the PCL/HA scaffolds significantly increased from 0.12 ± 0.02 to 2.65 ± 0.05 MPa with increasing initial HA content from 0 to 20 wt%, whereas the pore size decreased from 9.2 ± 0.7 to 4.2 ± 0.8 μm. In addition, the PCL/HA scaffolds showed considerably enhanced in vitro cellular responses that were assessed in terms of cell attachment, proliferation and osteoblastic differentiation.     

Effects of phase separation on stress development in polymeric coatings

A cantilever deflection technique was used to monitor stress in situ during drying of cellulose acetate coatings. Porosity was introduced in some coatings using dry-cast phase separation. Stress and weight loss profiles for dense coatings, a coating that contained small (∼1 μm) pores, and a coating that contained small (∼1 μm) pores and macrovoids (∼200 μm) are compared. In-plane tensile stress after drying ranged from 30 MPa (dense coatings) to 5 MPa (macrovoid-containing coating). The stress profiles for dense coatings feature a period of rapidly and then slowly increasing stress due to constrained shrinkage. For a coating that formed small pores, drying and stress development are delayed, stress rises and then drops a small amount due to capillary pressure relief. The stress profiles for the small pore and macrovoid-containing coatings are similar, except for a stress plateau at early stages of drying, which may be caused by macrovoid growth.     

Synthesis of functional gradient BCP/ZrO2 bone substitutes using ZrO2 and BCP nanopowders

BCP/BCP-ZrO₂/ZrO₂ scaffold with a functionally gradient layered structure (FG BCP/ZrO₂) was fabricated by the polymeric sponge replica method and subsequent dipping process. To enhance the compressive strength and bioactive properties of monolithic ZrO₂ scaffold, ZrO₂ and BCP phases were selected as a main frame and surface layer, respectively. The formation of microcracks was significantly decreased by incorporating an intermediate layer consisting of BCP-ZrO₂ phase. The thicknesses of the monolithic ZrO₂, BCP-ZrO₂, and BCP layer were around 10-30 μm, 3-5 μm, and 2-3 μm, respectively. The FG BCP/ZrO₂ scaffold showed highly interconnected pores as well as good material properties, which were 68% porosity and 7.2 MPa of compressive strength. Average pore size of FG BCP/ZrO₂ scaffold was about 220 μm in diameter. From MTT assay and SEM observation of osteoblast-like MG-63 cells, FG BCP/ZrO₂ scaffold showed good cell viability and faster proliferation behavior.     

Mechanical and Biological Performance of Calcium Phosphate Coatings on Porous Bone Scaffold

Composite coatings, consisting of calcium phosphate (CaP) ceramics and phosphate-based glass (P-glass), were obtained on a strong ZrO₂ porous scaffold to improve biocompatibility by combining mechanical properties and biological activity. Powder mixtures of hydroxyapatite (HA) and P-glass in varying composition and content were dip-coated on a ZrO₂ porous scaffold and heat-treated above 800°C for 2 h in air. During thermal treatment, substantial reaction and crystallization occurred, resulting in coating phases of HA, tricalcium phosphate (TCP), dicalcium phosphate (DCP), and surrounding glass. The CaP-glass coating layer was highly dense and uniform and adhered firmly to the ZrO₂ scaffold. The adhesion strength of the coating layer as tested on a nonporous disk increased with increasing glass addition and decreasing CaO content in glass. The highest strength was about 40 MPa, an improvement of twice as high as that of pure HA coating. The osteoblastic cells grew and spread actively through the coated scaffolds. The differentiation of cells on the CaP coatings was much higher than that on ZrO₂ substrate and comparable to or slightly higher than that on pure HA coating.     

Surface characteristics of titanium anodized in the four different types of electrolyte

This study evaluated the surface characteristics of titanium modified by anodic spark oxidation and a subsequent hydrothermal treatment. The electrolytic compositions of the experimental groups are as follows: GA: 0.015 M dl-α-glycerophosphate disodium salt hydrate (dl-α-GP) and 0.2 M calcium acetate (CA), GB: anodized in 0.015 M β-GP (glycerophosphate disodium salt) and 0.2 M CA, GC: anodized in 0.015 M GP (glycerophosphate disodium salt) and 0.2 M CA, and GD: anodized in 0.015 M GP-Ca (glycerophosphate calcium salt) and 0.2 M CA. Anodic spark oxidation was carried out at 30 mA/cm² to 290 V. In addition, the anodized samples were treated hydrothermally at 300 °C for 2 h in an autoclave system. Regardless of the electrolytic composition, the anodic oxide films on the titanium surface contained pores ∼5 μm in size and the diameter was larger at the protrusion parts than that at the lower parts. The phase of the anodic oxide layer consisted mainly of anatase with a small amount of rutile. HA crystals precipitated on the porous titanium oxide layer after a hydrothermal treatment. Moreover, the morphology of the HA crystals was a dense fine needle shape, which changed according to the electrolytic composition. The mean surface roughness (R_a) was highest in group GB at 0.437 μm. The R_a values of the hydrothermally treated group was approximately 0.14-0.2 μm higher than the anodized groups. Anodic spark oxidation and the hydrothermal treatment resulted in increasing corrosion potential and decreasing corrosion current density, which means an improvement in the corrosion resistance. The surface activity of the specimens in Hanks’ solution was GD > GA > GB > GC.     

Novel Layered Hydroxyapatite/Tri‐Calcium Phosphate–Zirconia Scaffold Composite with High Bending Strength for Load‐Bearing Bone Implant Application

The integration of biological and mechanical requirements remains a challenge in developing porous hydroxyapatite (HA) and tri‐calcium phosphate (TCP) scaffolds for load‐bearing bone implant application. With the newly developed slip‐deposition and coating‐substrate co‐sintering technique, a strong layered HA/TCP‐zirconia scaffold composite structure was successfully fabricated. The bending strength (321 MPa) of this composite can match upper strength limit of the natural compact bone. The HA‐based scaffold coating has multiple scale porous structures with pore size ranging 1–10 and 20–50 μm. The zirconia‐based substrate is also porous with submicropores. Focus ion beam micrographs show most of the micropores in the coating are interconnected. Microindentation and primarily adhesive strength tests demonstrate that the scaffold coating strongly bonds with the zirconia based substrate. In vitro cell culture study indicates that the coatings have no cytotoxicity. It is evident that the strong layered HA–zirconia scaffold composite offers new implant options for bone repairs requiring immediate load bearing capacity.