Adhesion of Hydroxyapatite Nanoparticles to Dental Materials under Oral Conditions

Abstract

Hydroxyapatite nanoparticles (nano-HAP) are receiving considerable attention for dental applications, and their adhesion to enamel is well established. However, there are no reports concerning the effects of HAP on other dental materials, and most of the studies in this field are based on in vitro designs, neglecting the salivary pellicle-apatite interactions. Thus, this in situ pilot study aims to evaluate the effects of three hydroxyapatite-based solutions and their interactions with different dental material surfaces under oral conditions. Hence, two volunteers carried intraoral splints with mounted samples from enamel and from three dental materials: titanium, ceramics, and polymethyl-methacrylate (PMMA). Three HAP watery solutions (5%) were prepared with different shapes and sizes of nano-HAP (HAP I, HAP II, HAP III). After 3 min of pellicle formation, 10 ml rinse was performed during 30 sec. Rinsing with water served as control. Samples were accessed immediately after rinsing, 30 min and 2 h after rinsing. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the particles, and SEM evaluated the pellicle-HAP interactions. SEM and TEM results showed a high variation in the size range of the particles applied. A heterogeneous HAP layer was present after 2 h on enamel, titanium, ceramics, and PMMA surfaces under oral conditions. Bridge-like structures were visible between the nano-HAP and the pellicle formed on enamel, titanium, and PMMA surfaces. In conclusion, nano-HAP can adhere not only to enamel but also to artificial dental surfaces under oral conditions. The experiment showed that the acquired pellicle act as a bridge between the nano-HAP and the materials' surface.

Figure 1

Maxillary intraoral splints with mounted enamel slabs.

Figure 2

SEM figure at 20,000-fold (a), TEM figure at 68,000-fold (b), and EDX analysis at 5,000-fold (c) magnification from HAP I powder immersed in water solution. The images reveal the crystallite shape and the tendency for clusters formations.

Figure 3

SEM figure at 20,000-fold (a), TEM figure at 68,000-fold (b), and EDX analysis at 5,000-fold (c) magnification from HAP II powder immersed in water solution. The images reveal the crystallite shape and the tendency for clusters formations.

Figure 4

SEM figure at 20,000-fold (a), TEM figure at 68,000-fold (b), and EDX analysis at 5,000-fold (c) magnification from HAP III powder immersed in water solution. Individual particles can be easily detected.

Figure 5

SEM micrographs at 5,000-fold magnification of the pellicle and the nano-HAP particles on enamel samples. The pellicle formation and the hydroxyapatite particles are visible at three different time-points: immediately, 30 min and 2 h after mouthwash with 5% HAP I, HAP II, and HAP III. White arrows point to HAP particles accumulated onto the enamel surface.

Figure 6

SEM micrographs at 5,000-fold magnification of the pellicle and the nano-HAP on titanium samples. The pellicle formation and the hydroxyapatite particles are visible at three different time-points: immediately, 30 min and 2 h after mouthwash with 5% HAP I, HAP II, and HAP III. White arrows point to HAP particles accumulated onto the titanium surface.

Figure 7

SEM micrographs at 5,000-fold magnification of the pellicle and the nano-HAP on ceramic samples. The pellicle formation and the hydroxyapatite particles are visible at three different time-points: immediately, 30 min and 2 h after mouthwash with 5% HAP I, HAP II, and HAP III. White arrows point to HAP particles accumulated onto the ceramic surface. HAP III tend to accumulate on surface irregularities after 2 h (white asterisks).

Figure 8

SEM micrographs at 5,000-fold magnification of the pellicle and the nano-HAP on PMMA samples. The pellicle formation and the hydroxyapatite particles are visible at three different time-points: immediately, 30 min and 2 h after mouthwash with 5% HAP I, HAP II, and HAP III. White arrows point to HAP particles accumulated onto the PMMA surface.

Figure 9

TEM micrograph at 30,000-fold magnification revealing the presence of the 30 min pellicle on enamel sample rinsed with HAP II solution. The pellicle manifests itself as continuous electron-dense layer. The enamel was dissolved due to demineralization of the specimens and thus is no more visible in the TEM figure.

Figure 10

SEM micrographs at 20,000-fold magnification showing the 2-h pellicle on titanium (a) and enamel (c) control samples in comparison with titanium rinsed with HAP II (b) and enamel rinsed with HAP III (d). Higher magnification figures provide the visualization of connective structures between HAP II solution and titanium surface (b) and HAP III and enamel surface (d) 2 h after oral exposure to these HAP solutions. The HAP particles are in direct contact with globular structures from the acquired pellicle. Arrows point to connective bridges in-between the HAP particles and between them and the pellicle.