Comparison in In-Vitro Evaluation of Wollastonite and Dicalcium Silicate
Coatings
Xuanyong Liu 1,2,a, Chuanxian Ding 1 and Paul K. Chu 2
1 Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai
200050, China 2 Dept. of Physics & Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon,
Hong Kong a [email protected] or [email protected]
Keywords: wollastonite, dicalcium silicate, plasma sprayed, bioactivity
Abstract
In-vitro evaluation of plasma sprayed wollastonite and dicalcium silicate coatings was carried
out by SBF soaking test and osteoblasts seeding test. Apatite layers were formed on the surfaces of
the two coatings indicative of the excellent bioactivity. The formation rate of apatite is higher on
dicalcium silicate than on wollastonite. The cause is believed to be the presence of orthosilicate
species in the dicalcium silicate coating promoting easier leaching by exchanging H3O+ ions from
the solution with calcium ions concentrated in the orthosilicate positions. At the same time, loss of
soluble silicon occurs, and it is supposed to enhance the repolymerization of the silica gel layer and
provide the active sites for the nucleation of apatite. The outcome is that apatite forms faster on
dicalcium silicate than on wollastonite. The data obtained from the osteoblasts seeding test
indicate that the wollastonite and dicalcium silicate coatings promote the proliferation of osteoblasts
and possess excellent biocompatibility.
Introduction
Since Hench et al. [1] discovered a group of special glasses based on the 45S5 Bioglass to bond
with bones in 1971, many kinds of CaO·SiO2 based glasses and ceramics have been developed to
adhere directly to bone tissues. In 1981, Kokubo et al. working with glasses of the
SiO2-CaO-P2O5-MgO system developed the A-W.G glass and an apatite- and
wollastonite-containing glass ceramic (A-W.GC) [2]. Ono et al. [3] reported that A-W.GC had
higher bioactivity than sintered hydroxyapatite (HA). It has also been reported that the presence of
crystalline apatite is not necessary for implants into bones and living tissues. The same applies to
the presence of phosphorus in the composition. Kokubo and Ohtsuki et al. [4-6] showed that
CaO-SiO2 based glasses including P2O5-free glasses formed a carbonate-containing hydroxyapatite
layer on their surface in vivo as well as in vitro, whereas non-bioactive CaO-P2O5 based glasses did
not form it, thereby concluding that the CaO-SiO2 components contributed mainly to the bioactivity
of those materials. Thence, some CaO-SiO2 based ceramics have been developed as bioactive
materials. In practice, CaO·SiO2 based bioactive glasses and ceramics have not been widely used
in load-bearing implants because of inadequate strength. One solution is to combine the glasses
and ceramics with a fracture tough phase such as a metal or polymer to produce a composite.
Plasma spraying is most popular deposition technique due to its process feasibility as well as
Key Engineering Materials Vols. 288-289 (2005) pp. 359-362online at http://www.scientific.net© 2005 Trans Tech Publications, Switzerland
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reasonably high coating bond strength and mechanical property. In previous experiments,
wollastonite and dicalcium silicate were deposited to Ti-6Al-4V substrate by plasma spray
technology [7-9]. In this work reported here, a comparison between plasma sprayed wollastonite
and dicalcium silicate (Ca2SiO4) coatings is investigated utilizing SBF and osteoblasts seeding tests.
Experimental procedures
Wollastonite and dicalcium silicate coatings were deposited onto Ti-6Al-4V substrates with
dimensions of 10mm×10mm×2mm by an atmosphere plasma spray (APS) system (Sulzer Metco,
Switzerland). After ultrasonically washed in acetone and rinsed in deionized water, the specimens
were soaked in a SBF solution whose ion concentrations are nearly equal to those of the human
body blood plasma. SEM were used to examine the morphologies and the composition of coatings
before and after immersion. The surface structural changes after immersion were analyzed by
thin-film XRD.
Osteoblasts were obtained from the calvaria of 1 day old fetal rats. The sterile specimens were
each placed in 6-well culture dishes. Samples for morphological studies were seeded with a cell
density of approximately 2×104 cells/m. After 4 and 7 days culture, the samples were fixed in
2.5% glutaraldehyde in a 0.1 M sodium cacodylate buffer (pH=7.4) for 1 h, rinsed with PBS (3×10
min), and dehydrated in a grade ethanol series. Critical point drying of the samples was followed
by gold sputtering and the samples were examined using a 20kV electron beam in a Hitachi 2460N
SEM.
Results
The SEM micrographs acquired from the cross-sectioned
wollastonite and dicalcium silicate coatings after soaking in
SBF solution for 2 days are depicted in Fig. 1. Two layers
having different compositions can be observed. The top layer
is Ca-P rich and the bottom layer is silica rich. The thickness
of the Ca-P-rich layer formed on dicalcium silicate is about
10µm whereas that on wollastonite is only about 1µm. It
indicates that the formation rate of the Ca-P-rich layer on
dicalcium silicate is faster. The thickness of the silica-rich
layer formed on wollastonite and dicalcium silicate is about
2µm and 15µm, respectively.
Fig. 2 shows the thin film-XRD patterns of the surface of
the wollastonite and dicalcium silicate coatings soaked in SBF
solution for 2 days. The crystalline apatite peak can be
observed in the XRD patterns acquired from both coatings,
indicating that the Ca-P-rich layer formed on the coating
surface has an apatite structure. The wollastonite peaks can
also be observed in the XRD spectrum in Fig. 2a, suggesting
that the apatite layer formed on wollastonite is quite thin. On
the other hand, the apatite layer formed on dicalcium silicate is
thicker. The apatite peaks in the XRD spectrum obtained
from the dicalcium silicate coating soaked in SBF for 2 days
(a)
(b)
Fig. 1. SEM views of the
cross-sectioned coatings after
soaking in SBF solution for 2
days: (a) wollastonite and (b)
dicalcium silicate.
Advanced Biomaterials VI360
are obviously higher and sharper than those acquired
from the wollastonite coating, implying that the
apatite formed on dicalcium silicate has higher
crystallinity.
After 4 days, the cells cultured on wollastonite
and dicalcium silicate are compact and exhibit dorsal
ruffles and filapodia (with a fibre like attachment to
the coating surface) (Fig. 3a and Fig. 4a). At 7 days,
the wollastonite and dicalcium silicate coatings are
completely covered by the cells and extracellular
matrix. On top of the monolayer, individual cells as
well as clustered cells having a compact structure
with many dorsal ruffles can be observed (Fig. 3b and
Fig. 4b). It indicates that the wollastonite and
dicalcium silicate coatings can promote the
proliferation of osteoblasts and possess excellent biocompatibility.
Fig. 3. Surface views of wollastonite coatings seeded osteoblasts: (a) 4 days and (b) 7 days
Fig. 4. Surface views of dicalcium silicate coatings seeded osteoblasts: (a) 4 days and (b) 7 days
Discussion
Wollastonite is a chain silicate including two bridging oxygen atoms, whereas dicalcium silicate
is a isolated silicate without bridging oxygen atoms. Some reports [10] in the literature indicate
that orthosilicates hydrolyze more rapidly than other silicate species (e.g. disilicate, chain silicate)
because the bridging oxygen atoms are much more resistant to attack than non-bridging oxygen
atoms. Hence, the presence of orthosilicate species will promote an easier glass leaching by
exchanging H3O+ ions from the solution with alkaline earth ions concentrated in the orthosilicate
(a) (b)
(a) (b)
Fig. 2. XRD spectra obtained from the
surface of coatings soaked in SBF
solution for 2 days: (a) wollastonite and
(b) dicalcium silicate.
Key Engineering Materials Vols. 288-289 361
positions. At the same time, loss of soluble silicon occurs, and it is supposed to enhance the
repolymerization of the silica gel layer resulting in the formation of a silica-rich layer on dicalcium
silicate that is thicker than that formed on wollastonite. Hayakawa et al. [11] reported that
condensation between the Si-OH units formed at a glass surface and dissolved Si-OH could be the
dominant mechanism by which the apatite layer formed on CaO-SiO2-based glasses. Therefore,
the formation rate of apatite on dicalcium silicate in SBF is higher than that on wollastonite. The
reason why osteoblasts can survive and proliferate on wollastonite and dicalcium silicate is believed
to be the local chemical environment that is suitable for the proliferation of osteoblasts due to the
dissolution of the wollastonite and dicalcium silicate coatings. The dissolution products from the
wollastonite and dicalcium silicate coatings in the culture medium are composed of hydrated silicon
and calcium ions.
Conclusion
Plasma sprayed wollastonite and dicalcium silicate coatings were soaked in simulated body
fluids to investigate their bioactivity. Apatite layers were formed on the coating surfaces
suggesting that both wollastonite and dicalcium silicate coatings possess excellent bioactivity. A
silica-rich layer was also found under the apatite layer. The apatite layer formed on the dicalcium
silicate coating was thicker than that formed on the wollastonite coating after the same soaking time,
showing that the formation rate of apatite on dicalcium silicate was higher than that in wollastonite.
Our cell seeding experiments demonstrate that osteoblasts can survive and proliferate on both the
wollastonite and dicalcium silicate coatings, confirming that the materials promote the proliferation
of osteoblasts and possess excellent biocompatibility.
Acknowledgments
This work was jointly supported by Shanghai Science and Technology R&D Fund under grant
02QE14052 and 03JC14074, and National Basic Research Fund under grant G1999064701 and National
Natural Science Foundation of China 50102008, and City University of Hong Kong Strategic Research
Grants (SRG) 3 7001447.
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