OLI Conference 2010
Think Simulation!
Harnessing the Power of the OLI Engine
OLI Engine
Applications of Hydroxyapatite Thermochemistry to Biomaterials Synthesis
Richard E. Riman, Rutgers University
Thermodynamic computations have been used in our laboratory to design methods for the preparation of
hydroxyapatite materials that show promise as biomedical implants for hard tissue replacement. This paper will
discuss the utility of using thermodynamic computations as both a process design tool as well as one that provides
mechanistic insight into hydrothermal crystallization. Detailed phase equilibria studies used to search and select
chemical precursor systems that result in phase pure hydroxyapatite under mild reaction conditions will be presented.
Three commonly employed precursor systems for hydroxyapatite synthesis will be compared, namely, CaCl2-Na3PO4,
CaCl2-K2HPO4, and Ca(OH)2-H3PO4 aqueous systems. The resulting equilibrium species and chemical equilibria
were explored utilizing a wide range of reactant conditions. We discovered that all of these systems could be
described with a universal stability diagram. However, yield diagram computations were found to differ greatly,
illustrating the importance of using yield diagrams instead of stability diagrams for process design. More importantly,
these various precursor systems show contrasting differences in processing conditions promoting the stability of
hydroxyapatite, however some precursor systems were far more robust than others. We will systematically illustrate
this by examining the role of input concentrations of calcium and phosphate, pH, Ca/P ratio and temperature on
phase stability. Our simulation work with these systems led us to consider a precursor system not previously
evaluated in the literature for hydroxyapatite, namely the Ca(C2H3O2)2-K3PO4 aqueous system. This new system
proved to be the most robust system both conceptually and experimentally, demonstrating a wide range of reaction
conditions leading to phase pure hydroxyapatite at room temperature. The powders that result from this new
precursor system are on the nanometer scale and show unusual phase equilibria owing to their fine size. Our
thermochemical research has also examined the phase equilibrium of aqueous systems at elevated temperature,
particularly those systems where nucleation, growth and ageing can be controlled with temperature to make powders
or coatings. Focusing on the coatings, our work has explored controlled release of cations and anions at elevated
temperature (25˚C≤T≤200˚C) by utilizing Ca-EDTA and controlled hydrolysis of triethyl phosphate to deposit
hydroxyapatite on titanium as well as other alloys. Phase equilibrium diagrams detailing the formation of these films
on titanium, stainless steel and cobalt chrome will be presented. The thermodynamic description of the process
provides detailed insight into the origin of the nanostructure and chemistry of the film. In particular, for titanium and
its alloys our modeling work explains how the release of calcium and phosphate is phase-sequenced. This process
enables the growth of a buffer layer of calcium titanate followed by deposition of hydroxyapatite, and results in the
formation of a non-porous and highly adherent film. Experimental results are compared with apriori thermodynamic
computations to illustrate these findings.
Prof. Richard Riman, Rutgers University
Professor Richard Riman received his B.S. in Ceramic Engineering from Rutgers University. He obtained his Ph.D.
from the Massachusetts Institute of Technology in Materials Science and Engineering. He has been at Rutgers for 24
years and is currently a tenured distinguished professor teaching at both the undergraduate and graduate levels. He
is a 2009 recipient of a Rutgers University Board of Trustees Excellence in Research Award.
Professor Riman has been recognized as an ONR Young Investigator and has received distinguished research
initiation awards from organizations such as NIH, NSF, Alcoa Science Foundation, DuPont, and Johnson & Johnson.
He is an active member of Keramos, Tau Beta Pi and Sigma Xi honor societies, and professional societies, including
the Materials Research Society, American Association of Crystal Growth and the American Chemical Society. The
American Ceramic Society honored and elevated him to Fellow status in 2000. He has authored over 172
publications, 11 patents, and delivered over 416 presentations. His research interests focus on low temperature
solvothermal synthesis and processing of ceramic coatings for structural, optical, electronic, and biomedical
applications.
American Enterprise Park • 108 American Road • Morris Plains, NJ 07950 • main 973-539-4996 • sales 973-998-0240
Website: www.olisystems.com • FAX: 973-539-5922 • Email: [email protected]