Hydroxyapatite nanorods as novel fillers for improving the properties of dental adhesives: Synthesis and application.

OBJECTIVES: This study evaluates the hypothesis that the incorporation of fibrous hydroxyapatite nanoparticles with high crystallinity and high aspect ratio, synthesized by hydrothermal method, into an experimental ethanol-based one-bottle dentin adhesive, improves the mechanical properties of the adhesive layer, and accordingly increases the bond strength to dentin.
METHODS: Hydroxyapatite nanorods were synthesized using a simple hydrothermal procedure. First, the HPO(4)(2)-containing solution was added drop-wise into the Ca(2+)-containing solution while the molar ratio of Ca/P was adjusted at 1.67. The HAp precursor was then treated hydrothermally at 200 degrees C for 60h. The resulting powder was characterized using XRD, FTIR, SEM, TEM, and EDXA. The synthesized HAp nanorods were added to an experimental one-bottle dentin adhesive followed by the characterization of the filled adhesive. The diametral tensile strength, flexural strength, flexural modulus, and the microshear bond strength to the dentin of human premolars of seven adhesive systems containing different nanorod contents were evaluated. The distribution of the filler was determined using EDX-mapping. The depth of cure was also evaluated using scraping technique. Moreover, after microshear testing, the fracture cross-section was observed using SEM to determine the mode of failure involved. The colloidal stability was studied using a separation analyzer and also zeta potential measurement. Data were analyzed using one-way analysis of variance followed by the Tukey test.
RESULTS: The results confirmed the high purity, high crystallinity, and high aspect ratio of synthesized HAp nanorods. The diametral tensile strength of nanorod containing adhesive system appeared to increase when 0.2-0.5wt.% HAp nanorods were incorporated (p<0.05). A similar trend was observed in the flexural test providing higher flexural strength at filler contents of 0.2-0.5wt.% while flexural modulus remained unchanged. The highest microshear bond strength was also obtained at 0.2wt.% filler content (p<0.05). The improved properties of the new adhesive system might be due to the high crystallinity and high aspect ratio of the nanorods. SEM observation of debonded surfaces revealed that most specimens showed an adhesive failure from the adhesive-dentin interface. Energy dispersive X-ray (EDX) mapping confirmed the uniform distribution of nanorods in the adhesive matrix. The colloidal stability studies indicated that synthesized hydroxyapatite nanorods have high colloidal stability in the dental adhesive solution. Indeed, the nanorods are well dispersed and protected from aggregation by their high surface charge confirmed by zeta potential measurement. SIGNIFICANCE: Hydroxyapatite-based composites have shown promising bioactivity. However, the knowledge about the influence of the nano-sized HAp on the properties of the dental materials, especially dentin bonding adhesives, is yet insufficient. The nanorod containing adhesive system presented here might be considered to have practical applications in dental clinics. 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.