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 xyliu@mail.sic.ac.cn or xyliu@cityu.edu.hk

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

Licensed to City University of Hong Kong - Kowloon - Hong KongAll rights reserved. No part of the contents of this paper may be reproduced or transmitted in any form or by any means without thewritten permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 144.214.2.47-05/01/06,16:00:10)

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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