Bioactivity of gel– glass powders in the CaO-SiO
2
system:
A comparison with ternary (CaO-P
2
O
5
-SiO
2
) and
quaternary glasses (SiO
2
-CaO-P
2
O
5
-Na
2
O)
Priya Saravanapavan, Julian R. Jones, Russell S. Pryce, Larry L. Hench
Tissue Engineering Centre, Department of Materials, Imperial College of Science, Technology and Medicine, Prince
Consort Road, London SW7 2BP, United Kingdom
Received 22 March 2002; revised 18 July 2002; accepted 16 August 2002
Abstract: Bioactive glasses react chemically with body flu-
ids in a manner that is compatible with the repair processes
of the tissues. This results in the formation of an interfacial
bond between the glasses and living tissue. Bioactive glasses
also stimulate bone– cell proliferation. This behavior is de-
pendent on the chemical composition as well as the surface
texture of the glasses. It has been recently reported that
gel-derived monolith specimens in the binary SiO
2
™ CaO are
bioactive over a similar molar range of SiO
2
content as the
previously studied ternary CaO-P
2
O
5
-SiO
2
system. In this
report, the preparation and bioactivity of the binary gel–
glass powder with 70 mol % SiO
2
is discussed and its bio-
activity is compared with the melt-derived 45S5 (quaterna-
ry) Bioglass and sol– gel-derived 58S (ternary) bioactive
gel– glass compositions. Dissolution kinetic parameters K
1
and K
2
were also computed based on the silicon release for
all glass powders. It was shown that the simple two-com-
ponent SiO
2
-CaO gel– glass powder is bioactive with com-
parable dissolution rates as the clinically used melt-derived
45S5 Bioglass powder and extensively studied sol– gel-de-
rived 58S gel– glass powder. © 2003 Wiley Periodicals, Inc.
J Biomed Mater Res 66A: 110 –119, 2003
Key words: porosity; in vitro bioactivity; CaO™SiO
2
; gel–
glass powders; Bioglass
INTRODUCTION
A bioactive material is one that elicits a specific
biological response at the interface of the material,
which results in the formation of a bond between the
tissue and the material.
1
This concept is based upon
control of the surface chemistry of the material. A
bioactive implant reacts chemically with body fluids
in a manner that is compatible with the repair pro-
cesses of the tissues.
The first bioactive material reported was a four-
component glass composed of SiO
2
, CaO, Na
2
O, and
P
2
O
5
by Hench et al. in 1971.
2
The composition of
Bioglass 45S5, which has been in clinical use for the
last 15 years, is given in Table I. When implanted, the
low silica content and the presence of sodium ions in
the glass results in very rapid ion exchange with the
protons and hydronium ions of physiological solu-
tions.
3
The ion exchange creates an alkaline pH at the
implant interface with the body fluids, leading to nu-
cleation and crystallization of hydroxyl carbonate apa-
tite (HCA) bone mineral at the surface of the glass. The
growing bone mineral layer bonds to collagen, pro-
duced by the bone cells, and forms a strong interfacial
bond between the implant and the living tissues.
1,2
During the last 30 years, extensive research on bio-
active glasses and glass– ceramics has been conducted.
Many compositions containing SiO
2
, CaO, and P
2
O
5
are found to be biologically active. They include Cer-
avital,
4
Cerabone A/W glass ceramics,
5– 8
-tricalcium
phosphate,
9
sintered hydroxy apatite,
10
58S bioactive
gel– glasses,
11–14
modified 45S5 glass compositions,
15
and bioactive composites.
16
Interfacial reaction kinetics
The basis of the bone-bonding property of bioactive
glasses is the chemical reactivity of the glass in the
presence of body fluids. The surface reactions lead to
the formation of a HCA layer and as a result of this
sequence of reactions, bonding of implant to tissue
occurs. Hench and West
1
proposed a complex process
for the formation of apatite layer involving five reac-
tion stages (listed below). The literature for stages 1
and 2 is quite extensive,
17–19
whereas the study of
stages 3 to 5 is limited but conclusive.
Correspondence to: P. Saravanapavan; e-mail: p.pavan@
ic.ac.uk
© 2003 Wiley Periodicals, Inc.