In situ Mineralization of Hydroxyapatite for a Molecular Control of Mechanical Responses in Hydroxyapatite-Polymer Compo
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In situ Mineralization of Hydroxyapatite for a Molecular Control of Mechanical Responses in Hydroxyapatite-Polymer Composites for Bone Replacement Kalpana Katti, Praveen Gujjula, Arunprakash Ayyarsamy, Timothy Arens Department Civil Engineering, North Dakota State University, Fargo ND 58105 ABSTRACT In situ mineralization of hydroxyapatite (HAP) and the role of organics in initial nucleation and growth of HAP is critical for the resulting nano and microstructure of HAP. In situ mineralization of hydroxyapatite (HAP) in the presence of Ca binding polymers such as polyacrylic acid has shown some promise towards improvement of mechanical response of uniaxial compressed HAP/polymer composites to loading. This work represents fundamental studies on the nature of in situ HAP precipitation on resulting microstructure of the composite and bulk mechanical properties. Specifically, an experimental study, evaluating the role of initial stage mineralization of HAP on bulk mechanical responses is conducted. Fourier transform infrared (FT-IR) spectroscopic (with micro attenuated total reflectance) techniques are utilized to evaluate the association of polymer (polyacrylic acid) with HAP during mineralization of HAP. In situ HAP exhibits a faster mineralization as compared to the ex situ mineralization samples, This improved kinetics is responsible for altering the resulting micro and nanostructure of the HAP/polymer composite. Small spectral changes are detected in the absorbance spectra of in situ HAP as compared to ex situ samples. Changes in mechanical response to loading included improvement in strain-to-failure and resulting toughness characteristics of the in situ composite. The control and development of molecular-level associations of polymer with HAP is suggested to be critical for the resulting macro properties. Our results may have significant implications for design of nanocomposites for biomedical applications. INTRODUCTION At this turn of the century, it is becoming increasingly known that materials based on or mimicking biological systems exhibit more optimized properties in combination with their unique properties of hierarchy (structural order at all length scales) and adaptability [1-3]. In the field of biomedical implants, attempts to design suitable material systems have been made both from conventional engineering and tissue engineering directions. The effort to find substitutions for repair of seriously damaged human bones dates back centuries. Metals have been the primary materials in the past for this purpose due to their excellent mechanical properties [4] in spite of the dangerous ions that are released in vivo from these alloys. Attempts have been made to form high strength consolidated HAP bodies [5,6]. However, the poor mechanical properties of HAP, such as low strength and limited fatigue resistance restrict its applications. HAP coated implants have provided a superior mechanical implant anchorage and improved ceramic stability over other bioactive coatings such as tricalcium phosphate (TCP) [7]. So
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