Composites with up to 40 wt% of fluorapatite could be made using dual asymmetric centrifugal mixing.
At high filler loads, fluorapatite rods introduced additional toughening mechanisms.
Degree of conversion was not changed by replacing glass for fluorapatite filler.
Hardness and wear were not changed by replacing glass for fluorapatite filler.
Under acidic challenge fluorapatite crystals dissolved releasing fluoride ions.
To develop dental composites incorporating fluorapatite (FA) crystals as a secondary filler and to characterise degree of conversion, key mechanical properties and fluoride release.
FA rod-like crystals and bundles were hydrothermally synthesised and characterised by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD) and 19 F MAS-NMR. Composites were formulated containing BisGMA/TEGDMA/BisEMA and barium-aluminium-silicate glass (0FA). FA crystals were incorporated at 10 (10FA), 20 (20FA), 30 (30FA) and 40 wt% (40FA) maintaining a filler content of 80 wt% (63–67 vol%). Degree of conversion (DC), flexural strength (FS), flexural modulus (FM), fracture toughness ( K 1 C ), Vickers hardness (HV) and 2-body wear were measured. Fluoride release was measured in neutral and acidic buffers.
XRD and 19 F MAS-NMR confirmed that only FA was formed, whilst SEM revealed the presence of single rods and bundles of nano-rods. DC ranged between 56–60% ( p > 0.05). FA composites showed lower FM and lower FS ( p < 0.05), but comparable wear resistance and HV ( p > 0.05) to 0FA. 30FA and 40FA showed similar K 1 C to 0FA ( p > 0.05), with SEM showing evidence of toughening mechanisms, whereas 10FA and 20FA showed lower K 1 C ( p < 0.05). FA containing composites released fluoride that was proportional to the amount of FA incorporated ( p < 0.05) but only under acidic conditions.
The addition of FA to the experimental composites reduced strength and stiffness but not the DC, hardness or wear rate. 30FA and 40FA had a higher K 1 C compared to other FA groups. Fluoride release occurred under an accelerated acidic regime, suggesting potential as a bioactive ‘smart’ composite.
Resin composites are considered to have superior aesthetics and allow more conservative cavity preparation when compared to amalgam . This has led to them becoming increasingly popular for use as direct restorative materials, with around 800 million composite resin restorations placed worldwide in 2015 . Of these restorations approximately 80% were placed in the posterior region, with composite use exceeding amalgam use in several countries . With the call for the phase down in the use of mercury containing products, and hence amalgam, due to the Minimata convention, it likely that the use of resin composites will increase worldwide. Current composite formulations have an average life span of just under 10 years after which clinical intervention may be required . Restoration fracture and recurrent caries remain the primary reasons for clinical failures of composite restorations . Therefore, it is essential to develop new innovative composite formulations with novel chemistries to enhance their physical and mechanical properties further and that exhibit effective bioactive properties against recurrent caries.
One strategy to enhance the physical properties is by incorporation of hydroxyapatite (HA) particles, rods or whiskers as a filler phase. Given enamel essentially consists of rod-like hydroxyapatite crystals with a high degree of anisotropy, many authors have produced composites containing HA fillers on the basis that the resultant materials would be biocompatible and bioactive . These vary from HA whiskers with an aspect ratio >100 to rods with an aspect ratio of ∼5 to particles and materials containing HA as the sole filler or in combination with glass . These fillers may be nano- or micro- scale and may be surface treated or silane coupled or not . With such a range of experimental parameters and outcomes measured, it is not surprising that the effect of HA reinforcement on many physical properties is somewhat ambiguous.
As well as introducing HA as a filler, a further strategy to produce bioactivity is to introduce bioglass filler particles that are able to release specific ions that will potentially enable the composite to stimulate remineralisation. Bioactive glasses (BAGs) have been used in experimental resin composites . BAG has been suggested as a promising bioactive material that can interact with the surrounding environment to precipitate a biologically active hydroxycarbonate layer on their surfaces when they are exposed to bodily fluids. Bioactive glass (BAG) fillers were also reported to increase the fracture toughness of experimental dental composites . Moreover experimental composite materials containing chlorhexidine salts and reactive calcium phosphates as well as glass fillers have been developed; these materials promoted surface hydroxyapatite/CHX co-precipitation , however the strength of the materials decreased linearly upon raising CaP levels .
Finely ground fluorapatite (FA) Ca 5 (PO 4 ) 3 F has also been investigated as a potential filler for experimental bioactive dental restoratives . FA is chemically stable but releases fluoride in an acidic environment as the crystals dissolve. Since enamel demineralisation starts when the pH drops below 5.5, FA crystals could be a suitable and effective chemically stable anti-caries material that could mimic the natural caries resistance properties of enamel. A recent study showed that powdered FA crystals (0.6–1 μm) could be a potentially suitable filler when incorporated with conventional resin (BisGMA/TEGDMA) forming novel resin composites . It was found that bacterial biofilm mass and colony formation were significantly reduced by the addition of FA to all composites. However using this physical form of FA resulted in a significant reduction in the mechanical properties of the materials.
This study aimed to investigate fillers which could both have an effect on the mechanical properties and have a bioactive role. Whilst BAG fillers do have a potential bioactive function, they tend to be a similar size and isotropic morphology to conventional fillers. Particle anisotropy potentially offers additional toughening mechanisms for fillers in composites that are difficult to obtain simply by size-tuning of spherical particles. Accordingly, the objectives of this study were to develop novel resin composites with micro-, rod like fluorapatite crystals incorporated as a secondary filler and to characterise, as a function of the FA filler content, their degree of conversion, key clinically relevant mechanical properties and fluoride ion release.
Materials and methods
Fluorapatite (FA) crystal synthesis
Fluorapatite crystals were synthesised using a hydrothermal method . Briefly, 9.36 g of ethylenediamine tetra acetic acid calcium disodium salt (EDTA-Ca-Na 2 ), (Sigma-Aldrich) and 2.07 g of NaH 2 PO 4 ·H 2 O (Sigma-Aldrich) were mixed with 90 ml of distilled water. The suspension was stirred continuously, and pH adjusted to 6.0 using NaOH. To this solution was added 0.21 g of NaF (Sigma-Aldrich) dissolved in 10 ml water. FA crystal growth was achieved by autoclaving the EDTA-Ca-Na 2 /NaH 2 PO 4 /NaF mixture at 121 °C at 2.4 × 10 5 Pa for 10 h. The powder was then washed five times by adding 100 ml of distilled water and manually stirred for 2 min to separate the agglomerates. The powder was then collected and stored in an airtight vial at room temperature.
Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) (Hitachi-S-3400N, London, UK) was used to assess the morphology and elemental composition of the powder. XRD analysis (X pert Philips PW3050, Malvern Panalytical, UK) was also performed on the powder; scans were taken between 20°–60°, with a step size of 0.05° and dwell of 1 s. The data collected were analysed using HighScore diffraction software (Malvern Panalytical, UK). Search and match was performed against ICDD 2014 . Crystallography unit cell search and refinements were performed, and crystallographic parameters were calculated . Samples from three different batches of FA were collected, dried and ground to fine powders for solid-state 19 F MAS-NMR analysis (600 MHz (14.1 T) spectrometer (Bruker, Germany). A low fluorine background probe was used to acquire the spectra, in a single-pulse experiment of 30 s recycle duration. The 19 F chemical shift scale was referenced using the −120 ppm peak of 1 M NaF solution, with a secondary reference of CFCl 3 . Spectra were acquired over a period of 10–24 h depending on the fluoride level and were an accumulation of between 600 and 1440 scans.
Preparation of dental composite formulations
Experimental composites were formulated with monomer:filler ratio of 20:80 wt%. The resin phase was composed of 70%BisGMA:10%TEGDMA:20%BisEMA (ESSTECH Inc., PA, USA) to which 1 wt% CQ (camphorquinone, Sigma-Aldrich) and 1 wt% DMAEMA (dimethylamino ethyl methacrylate, Sigma-Aldrich) were added as photoinitiator and activator and mixed for 60 min at 60 °C using a magnetic stirrer. The primary filler was silanised barium aluminium silicate glass particles ( D 50 = 0.7 μm, First Scientific Dental GmbH, Elmshorn, Germany) which was replaced systematically by FA, namely 10 wt% (10FA), 20 wt% (20FA), 30 wt% (30FA) and 40 wt% (40FA), maintaining an overall filler content of 80 wt% (63–66 vol%). A group containing no FA (0FA) was produced as an 80 wt% (67 vol %) glass filled control. The resin and filler phases were mechanically mixed for 5 min at 3000 rpm in a dual asymmetric centrifugal mixer (SpeedMixerTM DAC 150.1 FVZ, Hauschild Engineering and Co. KG, Hamm, Germany). All specimens were stored in lightproof containers at 4 °C and tested within 4 weeks of manufacture.
Degree of conversion (DC)
Unpolymerised composite was packed manually into the centre of stainless steel washers (diameter = 4 mm, thickness = 0.8 mm, A2, metric BS4320, RS Components, UK) and then pressed between two glass microscope slides. Representative specimens of each composite were set aside ( n = 5 per composite) and then the remainder were light-activated polymerised for 5, 10, 20, 30, 40 and 60 s respectively. All specimens were exposed to the same light emitting diode (LED) light curing unit (LCU) (Demi Plus, Kerr, Orange Co., CA, USA) at ambient room temperature (23 ± 1 °C) with a spectral range of 450–470 nm and an irradiance of 1200 mW/cm 2 (checkMARK Bluelight Analytics Inc., Halifax, Canada).
All DC measurements were made using an ATR-FTIR Spectrometer (Spectrum 100, PerkinElmer, UK) between 4000–650 cm −1 with 32 co-added scans at 4 cm −1 spectral resolution. DC at each time point was determined by calculating the ratio of the absorbance intensities of the aliphatic carbon–carbon (C=C) double bond peak at 1640 cm −1 and aromatic carbon–carbon (C=C) double bond peak at 1607 cm −1 and comparing it to this same ratio for the uncured material.
Flexural strength (FS) and flexural modulus (FM)
Flexural modulus and flexural strength were determined following the ISO 4049 using a universal testing machine (Instron 3365, USA) equipped with a three-point bending apparatus. Bar shaped specimens (25 × 2 × 2 mm, n = 10) were prepared using a custom-made split steel mould. The specimens were then stored in distilled water at 37 ± 1 °C for seven days before testing. Prior to testing, specimen thickness and width were measured using digital callipers (±0.01 mm) (Mitutoyo, Japan) and then the specimens were loaded on a 20 mm support span with knife-edge geometry and tested at 0.75 mm/min cross head speed. The maximum load exerted on the specimen at the point of fracture was recorded and flexural modulus ( E ) and flexural strength were calculated using 1 and 2 respectively.
where F is the maximum load (N) exerted on the specimen, l is the distance (mm) between the supports, b is the width (mm) at the centre of the specimen, h is the height (mm) at the centre of the specimen and δ is the slope of a force/deformation curve in the elastic region (N/mm).
Fracture toughness ( K 1 C )
The sharp single edge notch beam (SENB) method was used to determine the materials’ fracture toughness ( K 1 C ) following the ASTM (E399-83) standards . Bar shaped composite specimens (25 × 6 × 3 mm, n = 10) were made according to ISO 4049 using a custom-made PMMA split mould. After light activated polymerisation, specimens were removed from the mould and a sharp notch (3.0 ± 0.1 mm length × 0.3 ± 0.1 mm width) was cut into each specimen using a custom-made jig containing a diamond disc mounted on a dental hand piece, so that a 2.8 ± 0.1 mm long notch was cut in the centre of the sample. Next, a razor blade mounted on a custom made jig was then passed through the notch to create a very sharp notch (0.2 ± 0.01 mm length). Specimens were polished using P400 silicon carbide abrasive papers (Struers, Denmark) and stored in distilled water at 37 ± 1 °C for seven days prior to testing.
The notched composite specimens were tested in a three-point bending test with a crosshead speed of 0.5 mm/min in a universal testing machine (Instron 3365, USA). Calculations of the fracture toughness values were determined using the following equations: