Production and characterization of composites of hydroxyapatite reinforced with nano-yttrium-oxide

Abstract

Hydroxyapatite (HA) is one of the most widely used biomaterials for reconstructing skeleton. HA is mostly used in small devices that bear low mechanical load as well as in coatings since bulk HA-materials generally have poor mechanical properties. A significant effort has been spent towards reinforcing HA matrices with a variety of biocompatible elements and compounds. This study presents the experimental results of mechanical reinforcement of HA originated from biological materials (bovine femur bone, BHA) with nano-yttrium oxide (Y 2 O 3 , Sigma Aldrich™, particle size 99%). BHA- x wt% Y 2 O 3 ( x =5 and 10) powder blends were milled for 4 h in a planetary ball-mill with a 7:1 ball-to-powder weight ratio, using ZrO 2 milling vials and balls. The milled powder blends were consolidated via cold-pressing (350 MPa) and sintering at different temperatures (1000–1300 °C) for 4 h under air atmosphere. For comparison purposes, similarly produced composites with commercial synthetic HA (CSHA) were also tested. The experimental results of density measurements and SEM observations for both BHA-composites and CSHA-composites showed that sintering satisfactorily occurred at 1300 °C. XRD analyses showed that the reinforcing oxides stabilize the lattice of HA against transformation to tri-calcium phosphate (TCP) even at 1300 °C. However, CSHA was more prone to form glassy phase after sintering at 1300 °C than BHA in the presence of the reinforcing oxides. The highest values of microhardness and compression strength were observed in the samples sintered at 1300 °C. BHA-composites have better mechanical properties than CSHA-composites. In particular, BHA-composites have higher compression strength (for 5 wt% Y 2 O 3 reinforcement, 101.79 MPa) than CSHA (75.81 MPa). Similar results were obtained for 10 wt% Y 2 O 3 reinforcement. However, the best behavior is shown for 10 wt% Y 2 O 3 than 5 wt%. Probably, 10 wt% exceeds the capacity of HA lattice to accommodate the yttrium ions. The extensive formation of glassy phase in the CSHA-composites sintered at 1300 °C (443.57 HV for 5 wt% Y 2 O 3 ) resulted in harder materials than the BHA-composites sintered at the same temperatures (330.2 HV for 5 wt% Y 2 O 3 ). Nevertheless, hard biomaterials ruin rapidly the tissues around the implant. Consequently, the experimental results, in conjunction with the important role of Y 2 O 3 in bone forming and healing mechanism, qualify the produced BHA-composites for further in vitro and in vivo studies.