Improving mechanical properties of glass ionomer cements with
fluorhydroxyapatite nanoparticles
Kevin J. Roche and Kenneth T. Stanton
School of Mechanical and Materials Engineering, University College Dublin, Belfield, Dublin 4, Ireland.
Abstract
Glass ionomer cements show great potential as low cost, minimally invasive dental restorative
materials. However, their use is limited by relatively poor mechanical properties, especially
fracture toughness. One possibility for improving their fracture toughness is through the
addition of fluorhydroxyapatite nanoparticles, which resemble the crystals found in tooth
enamel. Hydroxyapatite and fluorhydroxyapatite nanoparticles with fluoride substitution
levels ranging from 0 to ∼95% and a small amount of AB-type carbonate substitution have
been prepared and added to commercially available Fuji IX glass ionomer cement to examine
the effect of the nanoparticles on the cement mechanical properties.
The fluoride substitution level and other nanoparticle properties were accurately determined using a combined approach of XRD, XPS, TEM and FTIR. The material was in
the form of agglomerated single crystal nanorods with volume average dimensions of approximately 30 × 30 × 60–80 nm. This combination of chemistry and morphology closely
resembles the crystals in enamel and makes these particles perfect candidates for producing
reinforced glass ionomer dental restorative materials.
Preliminary tests have been carried to assess the affect of these particles on the compressive strength, hardness, and fracture toughness of Fuji IX hand mixed cement. A significant
increase was found for hardness, but no significant change was found for fracture toughness or compressive strength, although there appeared to be a slight trend of increasing
compressive strength with wt% nanoparticle addition. The results indicate that the mixing procedure must be carefully controlled to obtain the desired mechanical effect without
degrading the working properties of the cement.
1
1
Introduction
Three quarters of Irish fifteen year olds suffer some level of tooth decay, and essentially
the entire population experiences it at some stage in their life [1]. Increasing life expectancy
and richer diets have made restorative dental materials more and more necessary throughout
the world. To meet this universal need restorative materials must be affordable, simple
to prepare without expensive equipment, long lasting, and suitable for a wide range of
restorations.
Atraumatic restorative treatment (ART) has been developed as a low cost restorative
technique that can be performed using only simple hand tools with minimal training and
minimal loss of healthy tooth material [2, 3]. It is primarily designed to make dental care
more accessible in poor communities, but is also advantageous in richer communities due
to reduced patient trauma [4]. ART involves simply scraping clean the cavity and filling
it with a self-adhesive material. The self-adhesive property of the material is crucial as it
removes the need to shape the surrounding tooth to mechanically retain the filling. This in
turn removes the need for power tools or local anesthetic in most cases, making it possible
to carry out the procedure with minimal resources.
Glass ionomer cements (GICs) are the preferred material for the ART approach [4, 5].
They are simple to prepare, chemically bond to enamel and dentine, and release fluoride
which has been shown to help reduce further tooth decay [6–11]. They are prepared by
mixing a solid aluminosilicate glass powder and a liquid mixture of mainly polyacrylic acid
(PAA) and water. Upon mixing, the acid attacks the glass, leading to the release of cations
that cross-link the PAA to form a gel which then hardens. Despite their advantages the type
and size of restorations that can be carried out with GICs are limited by their mechanical
properties, particularly fracture toughness. As a result, improving the mechanical properties of GICs has received a lot of attention. As well as optimising the glass and polyacid
properties [9, 12, 13], this can be done by adding a reinforcing filler material [11, 14–17].
Fluorhydroxyapatite (Ca10 (PO4 )6 (OH)2−x Fx , FHA)is an excellent candidate as a filler
material for GICs. The OH and F ions occupy the same space in the crystal structure, and
can be mixed in any ratio; 100% OH (x=0) is hydroxyapatite (HA) and 100% F (x=2) is
fluorapatite (FA). The mineral phase of teeth is essentially impure HA [18, 19] and makes
up approximately 98% of tooth enamel [20] by weight. The HA is found in a complex
hierarchical structure of HA nanoparticles and organic components [20–23] and contains a
range of impurities including fluoride [21, 23–30]. The close similarity between FHA and
natural enamel means that synthetic FHA nanoparticles can be expected to have excellent
biocompatibility and form a strong chemical bond with the GIC polyacid to maximise their
effect on the mechanical properties. The fluoride concentration can also be controlled to
2
help protect the enamel surrounding the restoration. These advantages have lead to a lot
of interest in HA and FHA as reinforcing phases for GICs, especially in nanoparticle form
[11, 14–16, 31].
By synthesising FHA nanoparticles with controlled fluoride substitution we can produce
particles tailored for improving the mechanical and cariostatic properties of GICs. The
precise level of fluoride substitution must be known, so several techniques have been used
to accurately measure the level of fluoride substitution in FHA particles produced in the
present study. Initial tests of the mixing and mechanical properties of FHA reinforced GICs
have also been carried out.
2
2.1
Experimental Method
HA and FHA Synthesis
HA and FHA particles were prepared by a wet precipitation process similar to those used
in previous studies [32–36]. There are a number of variations to this approach, and some
factors have been identified as critical to the final result; notably pH, temperature, reactant
concentration, mixing rate and aging time [37–40].
To produce HA, 5.55 g of CaCl2 (technical grade, Sigma-Aldrich, USA) and 4 g of
(NH4 )2 HPO4 (>97 %, BDH Chemicals, England) were dissolved separately in 100 ml
deionised water (DIW), giving a theoretical Ca/P ratio of 1.6375; lower than stoichiometric
HA (1.67) but closer to that found in enamel (1.63) [20]. The phosphate solution was placed
in a round bottom flask and heated to boiling point under reflux conditions and continuous
mechanical stirring. The pH of each solution was then adjusted to 10 by adding 0.8 M
NH4 OH and the calcium solution was added dropwise to the phosphate solution over 30
mins using a peristaltic pump. Once all the calcium solution was added the mixture was
allowed to age at boiling temperature for 1 hr, then the heat was removed and the mixture
allowed to age for a further 23 hrs. The precipitate was then washed 3 times in DIW using
a centrifuge. Half of each batch was dried overnight at 104◦ C and half was resuspended in
DIW.
The same process was used to produce FHA, except that various amount of NH4 F were
added to the phosphate solution to supply F− ions. Eight separate batches were prepared
with theoretical fluoride substitution rates ranging from 0 to 250 %. The theoretical fluoride
substitution level refers to the level of substitution if all the fluoride is assumed to be
incorporated into the FHA. Batches are identified by their theoretical substitution level, i.e.
‘F25’ refers to 25% theoretical fluoride substitution.
Carbonate impurities are common in HA synthesised in air as atmospheric CO2 is incorporated into the structure [24, 25, 34, 37]. The carbonate ion can substitute for either
3
hydroxyl (A-type), phosphate (B-type) or both (AB-type) [41–43]. All syntheses were carried out in air and no attempt was made to control the atmosphere, as it was thought that
a small amount of carbonate substitution might well be beneficial due to the similarity with
biological HA [20, 21].
2.2
Nanoparticle Characterisation
Synthesised nanoparticles were characterised by x-ray diffraction (XRD), transmission
electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS).
XRD was carried out using a Seimens D500 diffractometer (Munich, Germany) with
Bragg-Brentano geometry and a CuKα source. The angular range was 20-60◦ 2θ, the step
size was 0.02◦ and the time per step was 5 s. 45 mg of the sample were mixed by hand
with 5 mg of silicon as an internal reference for amorphous phase quantification, and the
mixture pressed flat onto a glass microscope slide. MAUD [44] Rietveld refinement software
was used to interpret the results. A separate scan of lanthanum hexaboride (LaB6 ) was used
to establish the instrument parameters.
TEM images were recorded with a FEI Tecnai G2 20 Twin microscope (Oregon, USA)
with a tungsten filament and CCD camera, at an accelerating voltage of 200 kV. Samples
were prepared by dropping very dilute suspensions of particles in DIW on formvar/carbon
copper TEM grids (Agar Scientific, England) and allowing them to dry in air.
XPS spectra were recorded in the range 50–1000 eV on a VG Microlab 310F (Sussex, UK)
at a resolution of 0.5 eV. High resolution scans (0.1 eV) of individual peaks were recorded
for quantitative analysis. Results were analysed using the peak fitting software ‘XPSPeak’
(Raymund Kwok).
FTIR spectra were recorded using a Bruker Vertex 70 spectrometer (Ettlingen, Germany)
in the range 400–4000 cm−1 . Samples were prepared by hand mixing 5 mg of the sample
with 50 mg KBr with a mortar and pestle and pressing the mixture into a 12 mm diameter
disk under a pressure of 50 MPa for 3 mins. Sixty four scans were recorded for each spectrum
and all spectra were baseline corrected with the ‘Opus 7’ (Bruker) software package using
the ‘rubber band’ technique.
2.3
GIC Preparation
Fuji IX GP (GC Corporation, Tokyo, Japan) was used as a reference cement and as a
basis for modified cement formulations as it has been used by several previous investigations
of GIC mechanical properties [16, 45, 46]. Modified cements were prepared by dispersing
HA in the liquid component of the cement prior to mixing the cement according to the
manufacturers specification. Only HA was used for initial testing as the results were expected
4
to be similar for HA and FHA. Dispersion was initially carried out using a combination of
hand mixing and ultrasonication in an ultrasonic bath (Decon FS300, Sussex, England) for 90
mins for compressive strength tests. After initial compressive strength tests it was decided to
switch from an ultrasonic bath to an ultrasonic probe (MSE Soniprep 150, London, England)
in an attempt to improve dispersion of the nanoparticles. Particles were dispersed for 90 s in
the PAA liquid at an amplitude of 10 µm. A maximum of 0.5 wt% HA was added, as larger
amounts required longer dispersion times and lead to excessive heating of the mixture.
2.4
GIC Characterisation
Compressive strength testing was carried out according to ISO 9917 [47]. Cylindrical
samples were prepared with height 6±0.1 mm and diameter 4±0.1 mm and stored in DIW
for 24 hrs. Testing was then carried out at a cross head speed of 1 mm/min using a Hounsfield
H50KS universal testing machine (Tinius Olsen, Surrey, UK).
The plane strain fracture toughness (KIc ) of cements with up to 0.5 wt% HA was measure
using the short rod chevron notch teqhnique [48]. Cylindrical specimens were prepared
according to ASTM E-1304 [49] and fracture under a tensile load in a universal testing
machine (Lloyd Instruments LRXPlus, Sussex, England). 12 samples were prepared for
each mixture, however many of these did not fracture in the chevron as required by the test
and had to be discarded. Chevron notch samples for each cement were mounted and gold
sputter coated for viewing by scanning electron microscopy (SEM).
Cylindrical samples were mounted, ground and polished to a flat surface for hardness
and indentation toughness testing. These samples were kept hydrated during polishing and
stored at 100% relative humidity at room temperature. Hardness was measured using the
Vickers hardness method with a diamond pyramid indenter with an angle of 136◦ and a load
of 2 kg. Three samples were tested for each cement mixture and the hardness was measured
at five points on each sample.
√
Indentation fracture toughness (KIc,idt (MPa m)) was also measured using the Vickers
method, according to Equation 1:
KIc,idt = ψb
P
3
!
(1)
c02
where ψb is the half angle of the Vickers indenter, P is the load (MN), and c0 is the radial
crack length (m). These measurements were taken from the same indents used for hardness
measurement.
The results of mechanical tests were analysed by single factor analysis of variance (ANOVA)
to test for statistical significance (P < 0.05).
5
3
3.1
Results and Discussion
XPS
XPS results show only a small amount of carbonate carbon impurity and no other species
(Table 1). Figure 1 show a representative spectrum from F100. The primary peaks are
labeled and other peaks are attributed to secondary orbitals of the same elements.
O
Intensity
Ca
F
C
P
0
200
400
600
800
1000
B.E. (eV)
Figure 1: XPS spectrum of F0 with primary peaks labeled. All samples showed similar spectra, with changes
only in the flourine (685-695 eV) and carbon (280-290 eV) regions.
XPS could not distinguish phase impurities detected using XRD. It was therefore necessary to break down the total atomic concentrations into contributions from each phase
based on XRD quantitative results (Table 1). Calcite (CaCO3 ) and fluorite (CaF2 ) were
assumed to be stoichiometric when calculating the relative contributions. The Ca/P ratio
Test
Total sample
Fsub (%)
Ca/P
P/C (CO3 )
C (CO3 ) wt%
F0
F50
F100
F150
0
1.63
20.9
0.47
60
1.69
15.1
0.92
89
1.79
20.2
0.73
112
2.00
33.4
0.36
FHA only
Fsub (%)
Ca/P
P/C (CO3 )
C (CO3 ) wt%
0
1.61
30.9
0.46
47
1.64
23.2
0.60
66
1.72
27.3
0.56
82
1.89
50.2
0.25
Table 1: XPS results showing total measure values and values calculated after calcite and fluorite contributions were removed. An Fsub value of 112 % is clearly impossible, so relying on XPS alone leads to errors.
Using phase composition results from XRD analysis to remove the F contribution of fluorite gives a more
accurate measurement for the F concentration in FHA.
6
increased significantly with F substitution from 1.61 in F0 to 1.89 in F150, with most of the
change occurring at high F levels. Fluorine and phosphorous can combine in several ways in
solution, so it seems likely that they form compounds such as fluorophosphoric acid, which
remove some of the available ions from the precipitation reaction [50, 51].
A large carbon peak is present in each sample due to adsorbed atmospheric hydrocarbons,
which is common in XPS studies and can be excluded from the sample analysis [52, 53].
Unfortunately this peak largely drowns out the carbonate signal and makes it more difficult
to estimate the level of carbonate substitution. Nevertheless a clear drop in carbonate
substitution was seen in F150, which was also visible in FTIR results.
3.2
XRD
Details of the XRD refinement are shown in Table 2. The small residual errors indicate
that the model is a good fit to the data, however the residual plot shows some areas of
imperfection (Figure 2), most likely due to inhomogeneous particle size (see Section 3.3)
and small amounts of carbonate impurities (see Sections 3.1 and 3.4). Small amounts of
fluorite (CaF2 ) and calcite (CaCO3 ) formed along with FHA; the amount of calcite was
fairly constant and ranged from approximately 1.2 to 1.5 wt% whereas the amount of fluorite
increased with fluoride concentration from 0 to 5.8 wt% (Figure 2). No other crystalline or
amorphous phases were detected.
Test
a (Å)
c (Å)
[h00] (nm)
[h00] (nm)
CaF2 (wt%)
CaCO3 (wt%)
Rw
Rexp
σ
F0
9.4525(3e-4)
6.89851(5e-5)
32.7(0.1)
79.4(2.8)
0
1.2328803
6.471
4.036
1.603
F25
9.4400
6.89972
33.6
80.4
0.60
1.1909174
6.509
4.070
1.599
F50
9.4282
6.90093
28.5
67.8
0.86
1.5507183
6.134
4.045
1.516
F75
9.4227
6.90150
25.5
56.6
1.34
1.3017212
5.761
4.010
1.437
F100
9.4151
6.90215
27.1
60.2
1.68
1.0286785
6.208
4.055
1.531
F150
9.4068(2e-4)
6.90356(2e-5)
32(0.1)
72.6(0.4)
2.61(0.06)
1.3685037
5.900
4.424
1.440
F200
9.4055
6.90332
32.7
71.5
4.66
1.3175359
6.365
4.740
1.554
F250
9.4048
6.90387
35.4
76.2
5.81
1.469094
6.175
4.025
1.534
Intensity1/2 [Count1/2]
Table 2: Details of Rietveld analysis for each test. Values in ( ) show 1 standard deviation. Rexp is the
residual experimental error, Rw is the total residual error, and σ is the goodness of fit.
60.0
40.0
20.0
Calcite
FHA
Fluorite
Silicon
30.0
40.0
50.0
60.0
2-Theta [degrees]
Figure 2: Representative XRD spectrum and residual plot of F100. Other spectra showed only minor peak
changes.
7
6.905
9.45
6.904
90
6.902
9.43
6.901
9.42
c (Å)
6.903
9.44
a (Å)
100
6.9
9.41
6.899
9.4
6.898
0
50
100
150
200
Calculated F substitution (%)
9.46
80
70
60
50
XRD a fit
40
30
XRD c fit
20
XPS data
10
XPS fit
0
0
250
100
200
Theoretical F substitution (%)
Theoretical F substitution (%)
(a)
(b)
Figure 3: (a) The effect of fluoride substitution on the unit cell parameters and the least squares fit to
the data. Standard deviations were estimated for F0 (a=3.2e-4, c=4.7e-5) and F150 a=2.3e-4, c=2.4e-5)
but were to small to be shown on the graph. (b) Calculated fluorine substitution level based on unit cell
dimensions and XPS data vs theoretical fluorine substitution level. There is some disagreement between the
two dimensions, but both are close to the XPS data.
3.2.1
Unit cell dimensions
Figure 3(a) shows the change in unit cell dimensions with increasing fluoride substitution.
In both cases, exponential functions of the form
y = y0 ± ∆y(1 − e
−f
A
)
(2)
where y is the unit cell dimension, y0 is the unit cell dimension for pure HA, ∆y is the
difference in unit cell dimension between HA and FA, f is the theoretical fluoride substitution
level, and A is a constant; provided a good fit to the data. Assuming the unit cell dimensions
change linearly with degree of fluoride substitution, as is generally expected [28, 54], the
fitted functions can be used to estimate the actual degree of fluoride substitution for a
given theoretical level (Figure 3(b)). However the unit cell parameters are also affected by
carbonate content [43] and this may cause errors in the calculated values if the carbonate
content changes between samples.
Although there is some difference between the values calculated from the a and c unit cell
dimensions, the XPS data lie between the two curves (Figure 3(b)). This shows that both
axes are slightly biased in opposite directions (Figure 3(b)) but that the overall agreement
is good. The difference can be explained by carbonate substitution; A-type substitution has
been found to cause an expansion of the a-axis and a contraction of the c-axis, while B-type
has the opposite affect [42, 43]. The effect of AB-type substitution is more variable as it
depends on the relative amount of A and B-type.
8
3.2.2
Crystallite Morphology
Figure 4(a) shows the volume averaged crystallite dimensions measured by XRD as
a function of fluoride concentration. All batches are clearly nanoscale rods with volume
average dimensions of approximately 30 × 30 × 60–80 nm with the long axis in the [00l]
direction. A slight linear decrease in aspect ratio with increasing fluoride concentration and
a minimum at F75–F100 are clear from Figure 4(b). The [00l] dimension shows a similar
pattern, while the [h00] dimension shows the minimum at F75 but no overall slope(Figure
4(a)).
90
2.45
Crystallite aspect ratio
80
Crystallite size (nm)
2.5
00l
h00
70
60
50
40
30
2.4
2.35
2.3
2.25
2.2
2.15
2.1
20
‐50
0
50 100 150 200
Theoretical F substitution (%)
250
‐50
300
0
50
100
150
200
250
300
Theoretical F substitution (%)
(a) Average crystallite dimensions as calculated by (b) Crystallite aspect ratio [00l]/[h00] showing a
Rietveld analysis, showing a minimum for F75.
general decrease with increasing fluoride substitution and a sharp minimum at F75–F100.
Figure 4: Effect of F substitution on average particle (a) size and (b) aspect ratio. Error bars show 1
standard deviation.
3.3
TEM
TEM images show hexagonal rod shaped particles of similar sizes to the crystal dimensions predicted by XRD, indicating that each particle is a single crystal. It is clear from
Figure 5 that particles are highly agglomerated when dried and there is significant variation
in particle size within each sample; a large number of small particles can be seen along with
a few much larger ones. At higher magnification (Figure 6) the hexagonal shape of some
particles is apparent and crystal planes are shown to be continuous within each particle,
confirming that the particles are single crystals.
3.4
FTIR
FTIR results are consistent with XRD and XPS, showing a small amount of carbonate
and no other impurities (Figure 7(a)). The two large peak groups at 550–650 and 1000–
−
−1
1150 cm−1 are mainly due to PO3−
in F0 that
4 with a contribution from OH at 633 cm
9
(a)
(b)
Figure 5: TEM of (a) F0 and (b) F150. Both show significant particle size distribution but F150 appears
to have slightly more uniform particles with lower aspect ratios.
Figure 6: TEM of F150 with high magnification inset highlighting hexagonal morphology and crystal planes.
10
F0
F50
F100
F200
F25
F75
F150
F250
F0
F25
F50
F75
F100
F150
F200
F250
OH‐OH
400
Absorbance (au)
Absorbance (au)
OH‐F‐HO
900
1400 1900 2400 2900 3400 3900
600
Wavenumber (cm‐1)
F‐OH‐F
650
700
750
800
Wavenumber (cm‐1)
(a)
(b)
Figure 7: (a) FTIR spectra of F0 and F100 showing characteristic peaks of FHA: phosphate at 550-650 cm−1
and 1000-1150 cm−1 ; hydroxyl at 3570 and 633 cm−1 . Small carbonate peaks appear at 1400-1550 and 870
cm−1 . (b) OH-F region of FTIR spectra showing the changing bond structure in the hydroxyl channels.
The hydroxyl peak shifts significantly as the composition changes from predominantly OH (633 cm−1 ) to
predominantly F (745 cm−1 ). At intermediate levels, OH ions are present in several different bond states
with either F ions or other OH ions, resulting in multiple peaks. The total intensity in this region decreases
as OH ions are substituted by F ions.
reduces intensity and shifts to higher wavelengths with increasing F substitution. These
groups, along with the OH− peak at 3570 and smaller phosphate peaks at 471 and 2000
cm−1 constitute the characteristic spectrum of HA [55, 56]. The peaks between 640 and
745 cm−1 in fluoride containing samples are attributed to the effect of OH-F interactions
on the OH bond [28, 54, 57]. The broad peaks at 1640 and 3450 cm−1 can be attributed
to absorbed or combined H2 O [39, 58] and small carbonate peaks are evident at 1400–
1550 cm−1 and 870 cm−1 [39, 41, 59]. These carbonate peaks are consistent with AB-type
carbonate substitution, and show a small reduction with increasing F substitution which is
most likely responsible for the slight variation between XRD and XPS results.
Figure 7(b) shows the 600–800 cm−1 region of the spectra. F0 contains a single OH−
peak at 633 cm−1 , which becomes several smaller peaks in the 620–650 cm−1 region in
intermediate samples, and finally a single small peak at ∼745 cm−1 in F150,F200, and F250.
Others have found that this single small peak suggests that these samples have greater than
75% F substitution; consistent with the present XRD and XPS results[28, 54, 57]. The
large peak shift and the formation of multiple intermediate peaks is as a result of the
formation of OH–F hydrogen bonds and the change in predominant bond type as the OH/F
ratio changes; from long chains of OH–OH, through to OH..OH-F–HO..HO, OH–F–OH–F,
and finally F..F–OH–F..F. This is disccussed in more detail in other work [54, 57]. For
11
220
Compressive Strength (MPa)
215
210
205
200
195
190
185
180
175
170
‐1
0
1
2
3
wt% HA
4
5
6
Figure 8: Compressive strength of cements with added HA nanoparticles. There is a general trend of
increasing compressive strength with wt% HA, although the standard deviation is high (p = 0.2). n=9 for
0 wt% and n=3 for other wt%.
the purpose of the present work it is most important to note that the difference between
consecutive spectra in Figure 7(b) decreases with increasing F substitution, in agreement
with the present XRD and XPS data.
3.5
Compressive Strength
Compressive samples with up to 5 wt% HA were initially prepared by adding HA
nanoparticles to the liquid component of Fuji IX and dispersing for 90 mins in an ultrasonic
bath. Figure 8 shows a general increase in compressive strength with wt% HA addition,
however the standard deviation is high and the sample number is low. Although the HA
appears to have improved the strength, it seriously degraded the working properties of the
cement so that consistency between samples was poor and some batches could not be mixed
at all. It is clear that although addition of HA can be beneficial, the preparation procedure
needs to be improved to obtain reliable results, even with low quantities of nanoparticle
addition.
3.6
Chevron Notch Fracture Toughness
No statistically significant change in KIc was found for any of the HA modified cements
however there appears to be a slight visible trend of increasing KIc (Figure 9). There was
significant scatter in the data due to the difficulty of preparing samples with perfect geometry
and the low fracture toughness of the material. The cement had a high tendency to chip
during cutting, causing uneven surfaces and uncontrollable surface cracks prior to testing.
Examining the fracture surfaces under SEM showed significant porosity in all samples
which may have added to the scatter in the data (Figure 10). HA agglomerates of several
12
0.45
0.4
KIc (Mpa√m)
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
control
.1 wt% HA .2 wt% HA .5 wt% HA
Figure 9: Plane strain fracture toughness of HA modified and unmodified GIC. No statistically significant
change was observed (p = 0.29), although there is a very slight visible upward trend.
microns in size were visible (Figure 10(b)) indicating that complete dispersion of the HA was
not achieved. The poor dispersion could explain why the addition of HA had no significant
effect on fracture toughness, as most of the material is essentially free of HA.
3.7
Vickers Hardness and Indentation Fracture Toughness
As with plane strain fracture toughness, there was no significant change in indentation
fracture toughness (Figure 11(a)). However, unlike KIc , the KIc,idt values actually appear
slightly lower with HA. The porosity of the cements caused significant inconsistency in
measuring the crack length for KIc,idt as the crack path was altered by pores.
Vickers hardness shows a small but significant increase from 70.1 HV2 to 76.3 HV2
with 0.5 wt% HA addition. This test was much more consistent than fracture toughness
tests, due to having a more robust test procedure, and shows that even small quantities of
nanoparticles can improve the mechanical properties of the cement.
4
Conclusion
Fluorhydroxyapatite nanoparticles have been synthesised and shown to have chemistry
and morphology similar to that of human enamel with additional fluoride. These particles
are ideal candidates for improving the mechanical properties of glass ionomer dental cements.
In-depth characterisation of these particles was carried out to accurately measure their size,
shape, and compostition, espescially fluoride content.
Initial mechanical testing of modified cements suggests that these particles can increase
compressive strength and hardness of the cement, but that the method of incorporating
the nanoparticles is crucial to realising the benefits and avoiding unwanted effects on the
13
(a)
(b)
Figure 10: SEM of (a) Control cement and (b) with 0.5 wt% HA (F0). Both show significant porosity and
have similar fracture patterns. HA nanoparticles are visible in (b) as agglomerates up to several microns in
size.
2
85
1.8
1.6
Vickers Hardness (HV2)
80
KIc,idt (MPa√m)
1.4
1.2
1
0.8
0.6
0.4
75
70
65
0.2
0
60
control
.1 wt% HA .2 wt% HA .5 wt% HA
control
(a)
.1 wt% HA .2 wt% HA .5 wt% HA
(b)
Figure 11: Vickers test results for (a) Indentation fracture toughness and (b) Hardness. Single factor ANOVA
showed no significant change for toughness (p = 0.16) but there was a significant increase in hardness (p =
9.5e−5 ) with HA addition.
14
working properties of the cement. No significant change in fracture toughness was detected
with up to 0.5 wt% hydroxyapatite nanoparticles. The high surface to mass ratio of the
nanoparticles makes them difficult to disperse, resulting in inconsistent mixing of the cement.
Further development of the mixing method will be required to produce a working cement
with significant improvements in mechanical properties.
Acknowledgments
This work was supported by the Irish Research Council for Science, Engineering and
Technology under the Embark Initiative and was carried out in collaboration with clinical
dentists and mechanical engineers at Queen’s University Belfast. We thank the UCD Conway
Institute for providing access to their TEM facilities.
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