Chemical Preparation of Calcium Hydroxyapatite Bioceramic Powders
in Synthetic Body Fluids
at 37°C (pH=7.4) and
Its Use in (In Situ) Coating of
Ti-6Al-4V and Stainless Steel (316L) Surfaces



    * F. A. Simsek, M.Sc. Thesis, Middle East Technical University (METU), Ankara, Turkey, July 1997.

     * F. A. Simsek, H. Onder Pamuk and A. Cuneyt TAS, "Chemical Preparation of Biomimetic Calcium Hydroxyapatite Powders at 37 C in Synthetic Body Fluids and Its Use in Coating Ti-Alloy and Stainless Steel Strips," Journal of American Ceramic Society, Submitted for Publication, August 1997, Manuscript No: 190775P. (Rejected for publication, 1998)

    * F. A. Simsek, H. Onder Pamuk and A. Cuneyt TAS, "Synthesis of Calcium Hydroxyapatite Bioceramic Powders at 37°C by Using Synthetic Body Fluids," XI. National Chemistry Conference (KIMYA-97), June 16-20, 1997, Van.

    * F. A. Simsek, H. Onder Pamuk and A. Cuneyt TAS, "Synthesis of Calcium Hydroxyapatite Bioceramic Powders at 37°C by Using Synthetic Body Fluids and Its Use in Coating Titanium Alloy and Stainless Steel Surfaces," 99th Annual Meeting of the American Ceramic Society, May 4-7, 1997, Cincinnati, OH, USA.
 
 

The most important inorganic phase of synthetic bone applications, calcium hydroxyapatite (HA: Ca10(PO4)6(OH)2) was prepared as a nano-sized, chemically homogeneous and high-purity ceramic powder, from calcium nitrate tetrahydrate and di-ammonium hydrogen phosphate salts dissolved in aqueous synthetic body fluid (SBF) solutions, by a patented chemical precipitation technique. It was observed that the synthesized precursors have easily reached a compound purity above 99% after 6 hours of calcination, in a dry air atmosphere, at 900°C, following the oven drying at 80°C. There was observed, surprisingly, no decomposition of HA into the undesired beta-TCP phase even after heating at 1600°C, in an air atmosphere, for 6 hours. This observation evidenced the superior "high-temperature stability" of such "biomimetic" HA powders, as compared to the ones reported in previous studies. These powders were also found to contain trace amounts of Na and Mg impurities in them, originated from the use of SBF solutions during their synthesis instead of pure water.
 
 

SBF solutions have been prepared in our laboratory, for the first time, by using the following chemicals:

NaCl, NaHCO3, KCl, Na2HPO4.2H2O, MgCl2.6H2O, CaCl2.2H2O, Na2SO4 and (CH2OH)3CNH2.

On the other hand, Kokubo et al have prepared their SBF solutions by using the following chemicals:

NaCl, NaHCO3, KCl, K2HPO4.3H2O, MgCl2.6H2O, CaCl2, Na2SO4 and (CH2OH)3CNH2.

One of the most striking and unique parts of our study is the “realization” by us of the fact that when one initially changes the “potassium hydrogen phosphate” (of Kokubo, et al) to “sodium hydrogen phosphate” (of Tas, et al) used in SBF preparation, then the ion concentrations in the resultant SBF solutions would better match those of the human plasma.

Our SBF solutions contained 125 mM of Cl- ions as opposed to the recipe of Kokubo group (i.e., 147.8 mM). Our SBF solutions did therefore contain reduced levels of Cl- (with a better resemblance to the human plasma value = 103 mM).

This reduction in Cl- concentration can only be achieved by the use of Na2HPO4.2H2O in SBF preparation instead of using K2HPO4.3H2O. This is another “unique” feature of our SBF solutions.
 

Our SBF solutions contained 27 mM of HCO3- ions as opposed to the recipe of Kokubo group (i.e., only 4.2 mM). Our SBF solutions did thus contain increased levels of HCO3- (in the form of a “perfect match” with the human plasma value = 27 mM).

This increase in HCO3- concentration can only be achieved by the use of Na2HPO4.2H2O in SBF preparation instead of using K2HPO4.3H2O, while all the other starting chemicals have been kept the same. This is another “unique” feature of our SBF solutions.
 

The crystal structure of the "SBF-synthesized (at 37°C and pH=7.4) calcium hydroxyapatite powders" (calcined at 1200°C) is shown below (after a Rietveld Analysis on the powder XRD data):                                       Click on the figures to display the pics:


Drawn by  Dr. A. Cuneyt Tas  (June 17, 1997)

The produced HA ceramic powders were observed to be spherical in particle shape, though agglomerated, and to have an average particle size of 35 to 50 nm. To our knowledge, such a small particle size has never been reported or achieved before for synthetic HA bioceramic powders manufactured by using the calcium nitrate/di-ammonium hydrogen phosphate route. The most significant findings in the relevant literature on HA powders, referred to be produced by other chemical powder synthesis techniques, were their lack of high purity, and the possession of lower usable structural and chemical stability thresholds (below 1200°C) to decomposition at high-temperatures (-----> download pdf: hasbf37.pdf).

The pellets prepared from the HA powders, synthesized at 37°C by using our SBF solutions (which do not resemble to those of Kokubo, et al. in their compositions), were observed to spontaneously gain weights by allowing the deposition and growth of fresh spherical HA particles on their surfaces, when immersed in SBF solutions kept constant at the human body temperature of 37°C.

The below diagram shows the "weight gain" data of the HA pellets soaked in our "modified" SBF solutions at 37°C and pH=7.4.

Titanium and stainless steel strips were found to be coated in situ with a thin layer of HA when immersed in SBF solutions kept at the human body temperature of 37°C. The below SEM micrographs show the advance of in situ HA deposition process as a function of soaking time in the SBF solution; (left): 3 weeks in SBF, (right): 3 days in SBF...


BODY FLUID CREATES THE BONE MINERAL (i.e., HYDROXYAPATITE)
BY ITSELF
ON ANY SUITABLE SURFACE IT FINDS,
WHETHER THAT SURFACE IS A CERAMIC, A METAL OR A POLYMER,
IT WOULDN'T MATTER!.
 


The above SEM photomicrograph shows the (initially smooth) surface
of an HA pellet soaked in our SBF solutions for 6 days.. The "beads" of average  diameter of 3 to 5 microns deposited on the surface belonged to a phase of carbonated, not-well-crystallized HA...





A typical XRD pattern of the "SBF-synthesized (at 37°C and pH=7.4) calcium hydroxyapatite powders" (calcined in air at 1200°C for 6 hours) is reproduced below:

The materials characterization and chemical analysis of the synthesized powders, pellets and coatings were performed by powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDXS), Fourier-transformed infrared spectroscopy (FT-IR), scan ning electron microscopy (SEM), and inductively-coupled plasma atomic emission spectroscopy (ICP-AES).

The high-temperature behavior of the "SBF-synthesized (at 37°C and pH=7.4) calcium hydroxyapatite powders" is depicted in the below XRD diagrams:
 
 


Our HA powders (precipitated from our SBF solutions at 37°C and pH value of 7.4) displayed superior, unprecedented
"high-temperature stability,"
owing to their "carbonated" and "Na and Mg-containing" chemical compositions,
as compared to HA powders synthesized
by just using distilled or de-ionized water.

1