Mechanical Properties of Biomimetic Composites for Bone Tissue Engineering
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Mechanical Properties of Biomimetic Composites for Bone Tissue Engineering Devendra Verma, Kalpana S. Katti and Bedabibhas Mohanty Civil Engineering, North Dakota State University, Fargo, ND 58105 ABSTRACT A biomimetic process involving in situ mineralization of hydroxyapatite (HAP) is used to design new composite biomaterials for bone tissue engineering. Surface and bulk properties of HAP composites have been studied for hydroxyapatite mineralized in absence (ex situ) of polyacrylic acid (PAAc) and in presence (in situ) of PAAc. XRD studies show existence of structural disorder within in situ HAP. It has been observed that PAAc increases the rate of crystallization. FTIR studies indicate calcium deficiency in structure of both in situ and ex situ HAP. PAAc provides favorable sites for nucleation of HAP. During crystallization of HAP, PAAc dissociates to form carboxylate ion, which binds to HAP. Porous and solid composites of in situ and ex situ HAP with polycaprolactone (PCL) in 50:50 ratio have been made to evaluate their applicability as bone scaffold. Mechanical tests on solid samples indicate ex situ HAP/PCL composites have higher elastic modulus (1.16 GPa) than in situ HAP/PCL composites (0.82 GPa). However, in case of porous composites, in situ HAP/PCL composites are found to have higher elastic modulus (29.5 MPa) than ex situ HAP/PCL composites (10.4 MPa). Nanoindentation tests were also performed at different loads to evaluate mechanical properties of the composites. In situ HAP mineralized using non-degradable polymers has thus been shown to improve mechanical response in porous composites. INTRODUCTION Mimicking of biomineralization processes opens the possibility to design new materials which 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 [4]. 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. It has been reported that organic additives affect nucleation and growth of HAP crystallization [7,8] and could be used to control the sizes, orientation and morphology of the formed HAP crystals effectively [9]. In particular, polyacrylic acid accelerates the nucleation of HAP from highly concentrated calcium, and phosphate solution [10]. In our previous study, in situ HAP/PAAc composites have shown to have improved recovery and smaller plastic strain than the ex situ HAP/PAAc composites [11]. Simulated body fluid (SBF) soaked in situ HAP/PAAc composites have also shown better mechanical response than ex situ HAP/PAAc composite. In the present study we have investigated the effect of PAAc on mechanical properties of composites of in situ HAP with a biodegradable polymer, polycaprolacton
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