Andrew J. Ruys

Sintering effects on the strength of hydroxyapatite.

Mechanisms underlying temperature-strength interrelations for dense (> 95% dense, pores closed) hydroxyapatite (HAp) were investigated by comparative assessment of temperature effects on tensile strength, Weibull modulus, apparent density, decomposition (HAp:tricalcium phosphate ratio), dehydroxylation and microstructure. Significant dehydroxylation occurred above approximately 800 degrees C. Strength peaked at approximately 80 MPa just before the attainment of closed porosity (approximately 95% dense). For higher temperatures (closed porosity), the strength dropped sharply to approximately 60 MPa due to the closure of dehydroxylation pathways, and then stabilized at approximately 60 MPa. At very high temperatures (> 1350 degrees C), the strength dropped catastrophically to approximately 10 MPa corresponding to the decomposition of HAp to tricalcium phosphate and the associated sudden release of the remaining bonded water.

Electrophoretic deposition of hydroxyapatite coatings on metal substrates : A nanoparticulate dual-coating approach

Hydroxyapatite coatings can be readily deposited on metal substrates by electrophoretic deposition. However, subsequent sintering is highly problematic owing to the fact that temperatures in excess of 1100°C are required for commercial hydroxyapatite powders to achieve high density. Such temperatures damage the metal and induce metal-catalysed decomposition of the hydroxyapatite. Furthermore, the firing shrinkage of the hydroxyapatite coating on a constraining metal substrate leads to severe cracking. The present study has overcome these problems using a novel approach: the use of aged nanoparticulate hydroxyapatite sols (lower sintering temperature) and a dual coating strategy that overcomes the cracking problem. Dual layers of uncalcined hydroxyapatite (HAp) powder were electrophoretically coated on Ti, Ti6Al4V and 316L stainless steel metal substrates, sintered at 875–1000°C, and characterised by SEM and XRD, and interfacial shear strength measurement. Dual coatings on stainless steel had an average high bond strength (about 23 MPa), and dual coatings on titanium and titanium alloy had moderate strengths (about 14 and 11 MPa, respectively), in comparison with the measured shear strength of bone (35 MPa). SEM and XRD demonstrated that the second layer blended seamlessly with the first and filled the cracks in the first. The superior result on stainless steel is attributed to a more appropriate thermal expansion match with hydroxyapatite, the thinner oxide layer, or a combination of these factors.

Interfacial bond strength of electrophoretically deposited hydroxyapatite coatings on metals.

Hydroxyapatite (HAp) coatings were deposited onto substrates of metal biomaterials (Ti, Ti6Al4V, and 316L stainless steel) by electrophoretic deposition (EPD). Only ultra-high surface area HAp powder, prepared by the metathesis method 10Ca(NO3)2 + 6(NH4)2HPO4 + 8NH4OH), could produce dense coatings when sintered at 875–1000°C. Single EPD coatings cracked during sintering owing to the 15–18% sintering shrinkage, but the HAp did not decompose. The use of dual coatings (coat, sinter, coat, sinter) resolved the cracking problem. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) inspection revealed that the second coating filled in the “valleys” in the cracks of the first coating. The interfacial shear strength of the dual coatings was found, by ASTM F1044-87, to be ∼12 MPa on a titanium substrate and ∼22 MPa on 316L stainless steel, comparing quite favorably with the 34 MPa benchmark (the shear strength of bovine cortical bone was found to be 34 MPa). Stainless steel gave the better result since α-316L (20.5 μm mK-1) > α-HAp (∼14 μm mK-1), resulting in residual compressive stresses in the coating, whereas α-titanium (∼10.3 μm mK-1) < α-HAp, resulting in residual tensile stresses in the coating.

Synthesis of highly porous crosslinked elastin hydrogels and their interaction with fibroblasts in vitro

In this study the feasibility of using high pressure CO2 to produce porous alpha-elastin hydrogels was investigated. Alpha-elastin was chemically crosslinked with hexamethylene diisocyanate that can react with various functional groups in elastin such as lysine, cysteine, and histidine. High pressure CO2 substantially affected the characteristics of the fabricated hydrogels. The pore size of the hydrogels was enhanced 20-fold when the pressure was increased from 1 bar to 60 bar. The swelling ratio of the samples fabricated by high pressure CO2 was also higher than the gels produced under atmospheric pressure. The compression modulus of alpha-elastin hydrogels was increased as the applied strain magnitude was modified from 40% to 80%. The compression modulus of hydrogels produced under high pressure CO2 was 3-fold lower than the gels formed at atmospheric conditions due to the increased porosity of the gels produced by high pressure CO2. The fabrication of large pores within the 3D structures of these hydrogels substantially promoted cellular penetration and growth throughout the matrices. The highly porous alpha-elastin hydrogel structures fabricated in this study have potential for applications in tissue engineering.

Biodegradable poly(α‐hydroxy acid) polymer scaffolds for bone tissue engineering

Synthetic graft materials are emerging as a viable alternative to autogenous bone graft and bone allograft for the treatment of critical-sized bone defects. These materials can be osteoconductive but are rarely intrinsically osteogenic, although this can be greatly enhanced by the application of bone morphogenetic proteins (BMPs). This review will discuss the versatility of biodegradable poly(alpha-hydroxy acids) for the delivery of BMPs for bone tissue engineering. Poly(alpha-hydroxy acids) have a considerable potential for customization and adaptability via modification of design parameters, including scaffold architecture, composition, and biodegradability. Different fabrication techniques will also be discussed.

Functionally graded electrical/thermal ceramic systems

Abstract Ceramic–metal and ceramic–ceramic functionally graded materials (FGMs) show promise for hypothesised applications such as thermoelectric converters, graded solid oxide fuel cells, graded piezoelectrics, electrically insulating joints, heatsinks for fusion reactors, and thermal barrier skins for lightweight spaceplanes. Most research has focussed either on FGM films (microns across), or modelling of hypothetical FGMs. Large bulk-FGMs with continuous (not layered) gradients are ideal for the above listed applications, however existing bulk-FGM powder-processing technologies give little gradient control and slow processing throughputs. The authors have developed a new process, impeller-dry-blending, which offers the possibility of producing large bulk-FGMs of a wide range of controllable continuous gradients and compositions. In this, the first published study of the impeller-dry-blending process, Cu–SiC and stainless steel–SiC FGMs were fabricated. Electron microscopy and elemental analysis revealed linear compositional gradients across cross sections of several millimetres. Densification was by vacuum sintering and hydrostatic shock forming.

Hydroxyapatite-316L fibre composites prepared by vibration assisted slip casting

To prepare hydroxyapatite (HA, or HAp)-stainless steel 316L fibre composites with up to 30 vol% 316L fibres (∼1 mm long and 50 μm in diameter), slip casting assisted by vibration (frequency: ∼55 Hz; amplitude: ∼5 mm) was carried out, followed by both cold isostatic pressing (CIPing) and hot isostatic pressing (HIPing). With the addition of around 0.5 wt% sodium carboxymethylcellulose (Na-cmc), solids loadings up to 44 vol% were obtained in calcined HA powder-derived slips, which were castable only under the vibration. The slips were concentrated and viscous so that the preferential sedimentation of the dense and large 316L fibres could be avoided. Subsequent CIPing was able to increase the relative density of the cast and dried green compacts from 46% after casting to 60% after CIPing. With the dense and uniform green compacts of the HA-316L mixtures, final HIPing at 950 °C resulted in HA-316L fibre composites of 99% relative density. The HA-316L fibre composites had improved fracture toughness of 3.6 ± 0.3 MPa.m0.5, due to the bridging effect of the ductile 316L fibres. However, the mechanical strength of the composites was limited by the presence of residual thermal stresses and circumferential microcracks. The HA-316L fibre composites were biocompatible and exhibited favourable bone-bonding characteristics.

The nanoparticle-coating process: a potential sol-gel route to homogeneous nanocomposites

Abstract A sol-gel process was developed for fabricating inherently homogeneous nanocomposites involving: (a) precipitation of nanoparticles; (b) coating the nanoparticles with a film of matrix phase, and; (c) powder forming and sintering. Die-pressed ZrO 2 (nanoparticle)–glass(matrix) specimens were pressureless sintered at 1100°C. Scanning electron microscopy (SEM) of these partially sintered specimens revealed a uniform arrangement of nanopores (∼10 −8 m), suggesting homogeneous ZrO 2 nanoparticle distribution.

Distal tibial fracture repair in a neurofibromatosis type 1-deficient mouse treated with recombinant bone morphogenetic protein and a bisphosphonate

Congenital pseudarthrosis of the tibia is an uncommon manifestation of neurofibromatosis type 1 (NF1), but one that remains difficult to treat due to anabolic deficiency and catabolic excess. Bone grafting and more recently recombinant human bone morphogenetic proteins (rhBMPs) have been identified as pro-anabolic stimuli with the potential to improve the outcome after surgery. As an additional pharmaceutical intervention, we describe the combined use of rhBMP-2 and the bisphosphonate zoledronic acid in a mouse model of NF1-deficient fracture repair. Fractures were generated in the distal tibiae of neurofibromatosis type 1-deficient (Nf1(+/-)) mice and control mice. Fractures were open and featured periosteal stripping. All mice received 10 μg rhBMP-2 delivered in a carboxymethylcellulose carrier around the fracture as an anabolic stimulus. Bisphosphonate-treated mice also received five doses of 0.02 mg/kg zoledronic acid given by intraperitoneal injection. When only rhBMP but no zoledronic acid was used to promote repair, 75% of fractures in Nf1(+/-) mice remained ununited at three weeks compared with 7% of controls (p < 0.001). Systemic post-operative administration of zoledronic acid halved the rate of ununited fractures to 37.5% (p < 0.07). These data support the concept that preventing bone loss in combination with anabolic stimulation may improve the outcome following surgical treatment for children with congenital pseudarthosis of the tibia and NF1.

The effects of sintering atmosphere on the chemical compatibility of hydroxyapatite and particulate additives at 1200°C

According to Le Chateliers principle, dehydration and the associated decomposition of hydroxyapatite (HAP) to biodegradable unhydrated calcium phosphates during sintering may be suppressed under a moist sintering atmosphere (thermodynamic effect), or possibly under a pressurized sintering atmosphere (physical effect), by opposing the release of water. The present study explored this possibility. High-purity powdered additives were used to minimize impurity and morphological effects. Al2O3, C, SiC, SiO2, ZrO2, and 316L stainless steel were all trialled at an addition level of 20 vol%. Heat treatment was at 1200°C for 1 h under two experimental atmospheres and two corresponding control atmospheres: flowing H2O/O2 mix—ambient air as a control; pressurized (1 MPa) argon—ambient argon (0.1 MPa) as a control. Specimens were analysed for decomposition by X-ray diffraction (XRD), for densification by porosity measurement, and for microstructural uniformity by energy dispersive spectroscopy (EDS) and image analysis. Significant decomposition occurred under all atmospheres with the exception of flowing H2O/O2 which eliminated decomposition in the HAP-Al2O3, HAP-ZrO2, and HAP-316L systems, and reduced the decomposition levels from near completion to ∼50% in the HAP-SiC and HAP-SiO2 systems. Moistureless pressurization had little effect. Microstructural uniformity was confirmed. No generalized atmosphere-densification interrelationships were observed.