Kārlis A. Gross
University of Melbourne
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Featured researches published by Kārlis A. Gross.
Biomaterials | 2003
Luis M. Rodríguez-Lorenzo; Judy N. Hart; Kārlis A. Gross
Hydroxy-fluorapatites (OH-FAps) occur biologically in teeth and form the basis for application as biomaterials. This work aims to synthesize a series of fluoride substituted calcium hydroxyapatites (OHAps) to determine how fluoride influences the synthesis and the resulting characteristics of solid solutions. OH-FAPs powders were synthesized with a chemical composition of Ca(10)(PO(4))(6)(OH)(2-x) F(x), with x=0.0, 0.4, 0.8, 1.2, 1.6 and 2.0. The synthesis of partially substituted OHAp yields materials with lower crystallinity and higher specific surface area than OHAp or fluorapatite (FAp). The smallest crystal size of 263A, occurs at less than 50% hydroxyl substitution with fluoride at x=0.4, and the highest surface area of 132m(2)/g occurs at x=0.8. Reaction kinetics occur faster at higher fluoride content, producing the expected Ca/P ratio of 1.67 only for x=2.0. X-ray and IR studies show that OH-FAPs are homogeneous solid solutions instead of mixtures of OHAp and FAp. The presence of a high fluoride concentration increases the driving force for crystal growth during the calcination process.
Biomaterials | 2004
Kārlis A. Gross; Luis M. Rodríguez-Lorenzo
Fluoride substitution within hydroxyapatite is an important occurrence for biological apatites and is a promising approach for the chemical modification of synthetic hydroxyapatite. Limited information on the influence of fluoride substitution for hydroxyl groups on the mechanical properties has provided the rationale for this study. Hydroxyfluorapatites with 0%, 20%, 40%, 60%, 80% and 100% replacement of hydroxyl groups with fluoride ions were assessed for hardness, elastic modulus, fracture toughness and brittleness using microindentation of sintered pellets. The production of samples with a similar grain size and density allowed the influence of fluoride on mechanical properties to be determined. It was found that the hardness remains unaffected until 80% replacement of hydroxyl groups with fluoride, after which the hardness rapidly increases. The elastic modulus increases linearly with fluoride content. Fracture toughness is improved with fluoride incorporation into the lattice and reaches a peak of 1.8 for a 95% dense sintered pellet with a 60% fluoride replacement, followed by a rapid decrease at higher fluoride concentrations. The brittleness index is lowered to a minimum at 60%, after which a rapid increase occurs. High fluoride levels are unfavourable from a mechanical perspective, are not recommended for biomaterials, and can lead to a higher incidence of fracture where sodium fluoride, for treatment of osteoporosis, may produce a highly fluoridated hydroxyapatite.
Acta Biomaterialia | 2009
Saeed Saber-Samandari; Kārlis A. Gross
Knowledge of the intrinsic properties of hydroxyapatite (HAp) single crystals is important for the design of natural systems and will assist further improvements of manufactured biomaterials. Nanoindentation provides a useful tool for determining mechanical properties such as the hardness, elastic modulus and fracture toughness of small samples. A Berkovich indenter was placed on the side and basal planes of a natural single crystal of Durango HAp. The hardness and elastic modulus values obtained revealed higher values for the base (7.1 and 150.4GPa) compared to the side (6.4 and 143.6GPa). The cracking threshold, i.e., the load at which cracking initiates, revealed earlier crack formation on the base (at 8mN) compared to the side (at 11mN). Fracture toughness was measured as 0.45+/-0.09 and 0.35+/-0.06MPam(1/2) for the side and basal plane, respectively. These results suggest that crystals are less prone to cracking and resist microcrack events better on the side, which is useful in bone, while exposing the base, the hardest face, to minimize mass loss from abrasion with teeth.
Biomaterials | 2004
Kārlis A. Gross; Luis M. Rodríguez-Lorenzo
The presence of fluoride within apatites occurring naturally within the body provides the basis for investigating the sintering ability of fluoride containing hydroxyapatites for use as biomaterials. Hydroxyfluorapatites were synthesized and then calcined to produce a 0%, 20%, 40%, 60%, 80% and 100% replacement of the hydroxyl ions with fluoride in the hydroxyapatite structure. Fluoride ion occupancy within the apatites was found to be about 90% of the anticipated value. Pycnometry results revealed a constant true density for powders of low to medium fluoride content followed by a rapid increase to the fluorapatite composition. Powders were uniaxially pressed, cold-isostatically pressed and sintered at 1150 degrees C, 1200 degrees C and 1250 degrees C. All hydroxyfluorapatite powders displayed a comparable ability for densification except when hydroxyl groups and fluoride ions were present at a comparable concentration. The grain size revealed that this composition also exhibited the smallest grain size and displayed the highest activation energy for grain growth. The lower diffusion created by similar amounts of fluoride and hydroxide within the lattice decreases grain growth and densification.
Journal of Biomedical Materials Research Part B | 2010
Kārlis A. Gross; Saeed Saber-Samandari; Karina S. Heemann
The performance of biomedical implants relies on the ability to assess and refine the microstructure of biomaterials. Instrumented nanoindentation was applied to determine the mechanical properties of plasma sprayed hydroxyapatite-coated implants from different commercial vendors. All biomedical devices contained both amorphous and crystalline phases. Nanoindentation of the amorphous phase revealed a hardness of 1.5 + or - 0.3 GPa and an elastic modulus of 48 + or - 6 GPa. The crystalline phase revealed a range in hardness of 3.0-7.7 GPa. The large range is attributed to the presence of porosity, surrounding amorphous areas, and hydroxyapatite (unmolten particle cores and recrystallized hydroxyapatite). A selection of the powder type (spray-dried or sintered) leads to different mechanical properties within the coating. A spray-dried powder provides a lower hardness and elastic modulus when unmolten particle cores are included in the coating. Meanwhile recrystallized areas are intermediate in hardness. The combination of a polished cross-section and nanoindentation offers the ability to determine a range of quality control tests including hardness, elastic modulus, bond strength, fracture toughness, substrate and coating roughness, crystallinity, and coating thickness. Property maps determined from nanoindentation will provide a graphical representation of the mechanical property distribution within the coating and provide a basis for coating property refinements. The assessment of commercial coatings is used a basis for discussion of future developments for hydroxyapatite coatings.
Biomaterials | 2002
Kārlis A. Gross; M Babovic
The surface condition of hydroxyapatite thermal sprayed coatings can be measured in terms of smooth melted and unmelted regions. Both these areas collectively contribute to abrasion resistance of a coated implant. The purpose of this work was to determine the influence of abrasion on the surface characteristics of coatings. Coatings with three degrees of roughness (R(a) of 7, 10 and 24 microm) were produced by thermal spraying. These coatings were subjected to abrasion in a pin-on-disk test. It was found that the majority of weight loss occurs within the first minute of the abrasion. Raised areas from the coating are removed and the bone analogue pin spreads amongst the elevated areas to produce a smoother coating. Further, abrasion is dependent upon the applied load. Small loads produce a smaller or negligible increase in coating loss, however, a higher load is able to displace more material from the coating surface. It is expected that the coating loss from plasma sprayed coatings amounts to a value of less than 3 wt% during the insertion into bone. Modification of the coating surface from abrasion is dependent upon the surface roughness of the coating.
Journal of Materials Science: Materials in Medicine | 1998
Kārlis A. Gross; M. R. Phillips
The presence and distribution of the amorphous phase is a key factor in the performance and bone-bonding behavior of plasma-sprayed hydroxyapatite coatings. Microanalysis of coatings was conducted with microprobe Raman and scanning cathodoluminescence microscopy. It was confirmed that the darker regions in polished cross sections represent the amorphous phase. The more intense cathodoluminescence emission from the amorphous phase during electron-beam irradiation compared with the crystalline phase was used to detect the two structurally different areas within the sample. By selecting the peak of the emission at 450 nm it was possible to raster the surface with the electron beam and produce a map of the amorphous phase in polished sections, a fracture surface and an as-sprayed surface of the plasma-sprayed coating. Cathodoluminescence microscopy, based on the different light emission from the amorphous phase and hydroxyapatite, is a useful tool for identifying and mapping of the amorphous-phase constituent in plasma-sprayed coatings.
Biomaterials | 2010
Saeed Saber-Samandari; Kārlis A. Gross
The objective of this work was to characterize the deposits of calcium phosphate produced by thermal printing in terms of structure, topography and mechanical properties. Hydroxyapatite was molten and directed to (a) a titanium target in relative motion and (b) stationary titanium substrates preheated to 100 degrees C and 350 degrees C. Scanning electron microscopy showed round-like deposits, but high resolution profilometry measured the profile. Micro-Raman spectroscopy and X-ray diffraction characterized the surface for structure, while nanoindentation revealed the hardness and elastic modulus. A symmetrical hemispherical deposit was formed on a surface in slow relative motion, but an off-centre shape formed at a higher relative speed. Deposits on preheated surfaces (100 degrees C and 350 degrees C) were identified as amorphous calcium phosphate. Nanoindentation revealed no significant difference in hardness between the amorphous deposits (4.0-4.4+/-0.3 GPa), but the elastic modulus increased from 65+/-4 GPa (annealed calcium phosphate reference) to 88+/-3 GPa (100 degrees C surface) and then to 98+/-3 GPa (350 degrees C substrate). The large change in elastic modulus is thought to arise from the dehydroxylation during thermal printing. Production of functional materials through crystallization is discussed to extend the range of possible microstructures. The characterization and testing approach is useful for hemispherical deposits produced by printing, coatings (laser ablation, thermal spraying, simulated body fluid) and melt extrusion elements in scaffolds.
Key Engineering Materials | 2003
K.M. Spiers; J.D. Cashion; Kārlis A. Gross
Submicrometre particles of magnetite and maghemite were produce d for potential use for hyperthermic treatment of cancer. X-ray diffraction and Mössbauer eff ct measurements showed that the samples were of good crystallinity and SQUID magnetiz ation measurements showed that magnetite should produce the larger heating effect. Introduction Hyperthermia for treatment of tumours may be induced in tissue by a pplying an alternating magnetic field to a region implanted with magnetic materials. Small magnetic particles or spheres deposited in tissue will cause heating through hysteresis loss. This form of therapy requires a magnetic material with the optimum heat evolution. Little attention has been placed on the synthesis and characterizat ion of iron oxide-based magnetic particles for cancer therapy. A detailed knowledge of the magnetic particles is a key aspect in optimizing the magnetic properties for heat production. This work will detail the synthesis of magnetite and maghemite powders, discuss their differentiation using X-ray diffraction and Mössbauer spectroscopy and their heat generation characteristics using SQUID magnetometry. Methods To synthesize magnetite, 560 ml of 0.5 M FeSO 4·7H2O was dissolved in deionized water and heated to 90 °C. All deionized water used for the synthesis of magnetite wa s previously bubbled with N 2 to minimize any dissolved gases that may disrupt the synthesis reac tion. A mixture of 3.33 M KOH with 0.27 M KNO3, made to a volume of 240 ml, was placed in a stoppered separating funnel a bove the reaction vessel and added to the air-tight glass vessel contai ning the ferrous sulphate continuously purged with N 2. The resultant precipitate was washed in deionized water, in ethanol , and then dried at 80 °C in flowing N 2 [1] to produce submicrometre particles. Maghemite and haematite were produced by heating magnetite powder in flowing air in tube furnace for 6 h at different temperatures. X-ray diffraction (XRD) was performed on the powder samples using a CuKα radiation source within a 2 range of 15 to 65 degrees. Mössbauer spectroscopy was performed on magnetite powders heat treated at 220 °C, 250 °C and 280 °C. SQUID m agneto etry was conducted at 40 °C, as appropriate for in-vitro applications, on two sam ples pressed into cylindrical pellets at a pressure of approximately 700kPa. Samples included synthesized magnetite and maghemite produced from heat-treatment of magnetite in air at 280 °C. Results and Discussion Key Engineering Materials Online: 2003-12-15 ISSN: 1662-9795, Vols. 254-256, pp 213-216 doi:10.4028/www.scientific.net/KEM.254-256.213
Key Engineering Materials | 2006
Murat Aydın; L.S. Ozyegin; Faik N. Oktar; E.Z. Erkmen; O. Anzabi; Kārlis A. Gross
In order to overcome the fragility and to improve the physical stability of hydroxyapatite (HA) on the implant, 5, 10, and 15 % yttria stabilized zirconia (YTZP) was added to the starting plasma spraying HA powder Metco XPT-D-703. From the recent literature it is already known that HA coatings tend to dissolve in body fluid environment. To decrease the dissolution effect many additives like zirconia (Zr) could be added to HA powder. In this study, prepared HA composite powders were sprayed onto titanium (Ti) surfaces with a Metco plasma gun. As a control group, pure HA powder was sprayed onto other Ti samples. All samples were subjected to tensile tests according to the ASTM C-633-79. SEM images were taken using back-scattering from prepared cross-sections. X-ray diffraction images were taken from the surface. It was seen that with the increase of the Zr content, the tensile test values increased. Pure HA showed also that the addition of Zr had improved the tensile bond strenght (TBS) values.