T.S. Sampath Kumar

Development of hydroxyapatite derived from Indian coral

A simple method of converting the calcium carbonate skeleton of the corals available in the Indian coast into hydroxyapatite granules has been developed. By heating the coral to 900 degrees C, the organic materials were eliminated. Powder X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were employed to characterize the coral and to optimize the processing parameters as well as to confirm the hydroxyapatite formation. The coral used exhibits the presence of both aragonite and calcite phases (dimorphism). At a temperature of 900 degrees C the coral was found to decompose all the carbonate phases. The pre-heated coral is converted into hydroxyapatite by a chemical exchange reaction with di-ammonium phosphate under hydrothermal conditions. The hydroxyapatite obtained was in powder form and does not contain any impurities. The in vitro solubility test of the apatite granules performed in Gomoris, Michalelis, Sorensens, Ringers and phosphate buffer of pH 7.2 and de-ionized water indicated the stability of the coralline hydroxyapatite.

Synthesis of carbonated calcium phosphate ceramics using microwave irradiation

Carbonated hydroxyapatites (CHA) were synthesized by the substitution of calcium carbonate for calcium hydroxide during the reaction with diammonium phosphate under microwave irradiation. The X-ray powder diffraction analysis indicates the decrease of alpha-axis up to 20 mol% of carbonate substitution confirming the formation of the B-type CHA. Further increase of carbonate content shows the presence of tricalcium phosphate (TCP) in addition to CHA. Reaction of substituted magnesium carbonate instead of calcium carbonate in the above process results in the formation of biphasic calcium phosphate (BCP) ceramics consisting of both CHA and TCP phases. Fourier transform infrared spectroscopic study also confirmed the carbonate substitution in HA and BCP formation. The in vitro solubility study in phosphate buffer of pH 7.2 at 37 degrees C showed the resorbable nature of the BCP samples. The present study thus indicates the feasibility of in situ formation of BCP ceramics using microwave irradiation.

Design of biocomposite materials for bone tissue regeneration

Several synthetic scaffolds are being developed using polymers, ceramics and their composites to overcome the limitations of auto- and allografts. Polymer-ceramic composites appear to be the most promising bone graft substitute since the natural bone itself is a composite of collagen and hydroxyapatite. Ceramics provide strength and osteoconductivity to the scaffold while polymers impart flexibility and resorbability. Natural polymers have an edge over synthetic polymers because of their biocompatibility and biological recognition property. But, very few natural polymer-ceramic composites are available as commercial products, and those few are predominantly based on type I collagen. Disadvantages of using collagen include allergic reactions and pathogen transmission. The commercial products also lack sufficient mechanical properties. This review summarizes the recent developments of biocomposite materials as bone scaffolds to overcome these drawbacks. Their characteristics, in vitro and in vivo performance are discussed with emphasis on their mechanical properties and ways to improve their performance.

Heat-deproteinated xenogeneic bone from slaughterhouse waste: Physico-chemical properties

Xenogeneic bone procured from the slaughterhouse waste was deproteinated by heat treatment method intended for use as a bone substitute. The effect of heat treatment was investigated by thermal analysis and by physico-chemical methods such as X-ray powder diffraction (XRD) and Fourier transformed infrared (FTIR) spectroscopy. The heat treatment temperatures for the bovine bone samples were predetermined by thermogravimetric (TG) analysis. The XRD results revealed that the process of heat treatment promoted the crystallinity of bone samples, particularly at 700 and 900†C. There was no secondary phase transformation detected for heat- deproteinated bone except the presence of the hydroxyapatite (HA) phase, which indicated its phase purity even at a higher temperature. The FTIR spectra of raw bone and bone heated at 300†C indicated the presence of organic macromolecules whereas these disappeared in the samples heated at 500, 700 and 900†C, which suggested the removal of antigenic organic matters around 500†C. The same results were also confirmed quantitatively by calculating the amount of collagen using hydroxyproline estimation. There was no significant change in the TG-thermogram of bone heated at 500, 700 and 900†C, which indicated their thermal stability. These findings implied that the heat treated bone at 500†C had properties similar to carbonated HA with low crystallinity, while 700 and 900†C samples had the same with higher crystallinity. As low temperature treatment does not alter morphological and structural properties, we propose that the 500†C heat treated xenogeneic bone may act as an excellent osteogenic bone substitute.

Fluorinated bovine hydroxyapatite: preparation and characterization

Chemically and thermally treated bovine bone has been proposed as an alternative to bone grafts and synthetic bone substitutes. Fluorinated bovine hydroxyapatite (FBHA) was prepared by two different methods at low and high temperatures. The effect of reaction temperature and fluorine concentration on the crystal morphology, phase purity and functional groups were observed by X-ray diffraction (XRD) method and FT-IR spectroscopy. The XRD results indicted that [3 0 0] reflection peak was shifted towards higher angle due to fluorination. The a-axis dimensions of both the FBHA samples were found to be decreased with the fluorination. The FT-IR spectra of FBHA samples confirm partial fluorination into the hydroxyl group of bovine hydroxyapatite. The in vitro physiological stability study was performed in phosphate buffered saline (PBS) of pH 7.4 at 37 °C and the samples were found to be quite stable for about a month. These results suggest the tremendous potential of fluorinated bovine hydroxyapatite as a bone substitute due to their bioactive nature.

In situ synthesized TiB-TiN reinforced Ti6Al4V alloy composite coatings: microstructure, tribological and in-vitro biocompatibility.

Wear resistant TiB-TiN reinforced Ti6Al4V alloy composite coatings were deposited on Ti substrate using laser based additive manufacturing technology. Ti6Al4V alloy powder premixed with 5wt% and 15wt% of boron nitride (BN) powder was used to synthesize TiB-TiN reinforcements in situ during laser deposition. Influences of laser power, scanning speed and concentration of BN on the microstructure, mechanical, in vitro tribological and biological properties of the coatings were investigated. Microstructural analysis of the composite coatings showed that the high temperature generated due to laser interaction with Ti6Al4V alloy and BN results in situ formation of TiB and TiN phases. With increasing BN concentration, from 5wt% to 15wt%, the Youngs modulus of the composite coatings, measured by nanoindentation, increased from 170±5GPa to 204±14GPa. In vitro tribological tests showed significant increase in the wear resistance with increasing BN concentration. Under identical test conditions TiB-TiN composite coatings with 15wt% BN exhibited an order of magnitude less wear rate than CoCrMo alloy-a common material for articulating surfaces of orthopedic implants. Average top surface hardness of the composite coatings increased from 543±21HV to 877±75HV with increase in the BN concentration. In vitro biocompatibility and flow cytometry study showed that these composite coatings were non-toxic, exhibit similar cell-materials interactions and biocompatibility as that of commercially pure titanium (CP-Ti) samples. In summary, excellent in vitro wear resistance, high stiffness and suitable biocompatibility make these composite coatings as a potential material for load-bearing articulating surfaces towards orthopaedic implants.

Effect of TiO2–Ag2O additives on the formation of calcium phosphate based functionally graded bioceramics

The combined effect of titanium dioxide and silver oxide on the in situ formation of biphasic calcium phosphate ceramics was investigated. Titania (5-20 mol%) was mixed with pure hydroxyapatite (HA) or HA containing Ag2O (10-20 mol%) and was heated to 900 degrees C for 12 h. The sintered samples were found to contain tricalcium phosphate (beta-TCP) and other phases along with HA depending upon the amount as well as the type of the additives used as evidenced by X-ray powder diffraction (XRD) and fourier transform infrared (FT-IR) spectroscopic studies. Enhanced TCP formation with reduced impurity phases was observed with TiO2-Ag2O addition. In vitro solubility study in phosphate buffer at physiological conditions shows the resorbable nature of these materials. A functionally graded material (FGM) structure was formed by spreading TiO2-Ag2O mixture on the surface of the HA green compact and heating at 900 degrees C. The FGM shows gradient structure of TCP and HA from the surface to the interior of the pellet in addition to titania and silver phases.

Friction stir processing of magnesium-nanohydroxyapatite composites with controlled in vitro degradation behavior.

Nano-hydroxyapatite (nHA) reinforced magnesium composite (Mg-nHA) was fabricated by friction stir processing (FSP). The effect of smaller grain size and the presence of nHA particles on controlling the degradation of magnesium were investigated. Grain refinement from 1500μm to ≈3.5μm was observed after FSP. In vitro bioactivity studies by immersing the samples in supersaturated simulated body fluid (SBF 5×) indicate that the increased hydrophilicity and pronounced biomineralization are due to grain refinement and the presence of nHA in the composite respectively. Electrochemical test to assess the corrosion behavior also clearly showed the improved corrosion resistance due to grain refinement and enhanced biomineralization. Using MTT colorimetric assay, cytotoxicity study of the samples with rat skeletal muscle (L6) cells indicate marginal increase in cell viability of the FSP-Mg-nHA sample. The composite also showed good cell adhesion.

Synthesis and characterization of nanocrystalline apatites from eggshells at different Ca/P ratios.

Nanocrystalline apatites with different Ca/P ratios were synthesized using eggshell as a calcium source by microwave processing. The apatites were found to have a minor amount of Mg, Sr, Si and Na ions inherited from the eggshells. The presence of several foreign ions results in a perturbed lattice structure indicated by an increase in lattice constants and shift in vibrational frequencies of the functional groups. The apatites were heat treated to investigate the influence of foreign ions on thermal stability. The minor amounts of ions do not affect the thermal stability. The differences in thermal behaviour of these apatites were due to the presence of HPO(2-)(4) ions only and not due to other ions because of their low content.

Role of biomineralization on the degradation of fine grained AZ31 magnesium alloy processed by groove pressing

Groove pressing (GP) has been successfully adopted to achieve fine grain size up to 7 μm in AZ31 magnesium alloy with an initial grain size of 55 μm. The effect of microstructural evolution and surface features on wettability, corrosion resistance, bioactivity and cell adhesion were investigated with an emphasis to study the influence of deposited phases when the samples were immersed in simulated body fluid (SBF 5×). The role of microstructure was also evaluated without any surface treatments or coatings on the material. GPed samples exhibit improved hydrophilicity compared to the annealed sample. After immersion in SBF, specimens were characterized using scanning electron microscopy (SEM), energy dispersive X-ray (EDAX) analysis and X-ray diffraction (XRD) methods. More amount of white precipitates composed of hydroxyapatite and magnesium phosphate along with magnesium hydroxide was observed on the surfaces of groove pressed specimens as compared to the annealed specimens with an increase in immersion time in SBF. Corrosion behavior of the samples estimated using potentiodynamic polarization curves indicate good corrosion resistance for GPed samples before and after immersion in SBF. The MTT assay using rat skeletal muscle (L6) cells revealed that both the processed and unprocessed samples are nontoxic and cell adhesion was promising for GPed sample.