Biomedical Engineering Reference
In-Depth Information
and lack of strength are still major problems for GICs. Despite major improvements since their
invention, significant advances are still needed, e.g., chemistries that increase the degree of the
cross-linking and polysalt bridge formation improve mechanical properties, making the material a
suitable choice for both posterior tooth restorations and bone grafting material in stress-bearing
areas.
GICs contain ion leachable calcium fluoroaluminosilicate (FAS) glass to which other compo-
nents, such as lanthanum, strontium, barium, and zinc oxide, have been added that can react with
water-soluble acids such as polyacrylic acid and tartaric acid. The setting reaction is based on
acid
base reaction between the FAS glass and homo- and copolymers of polyacrylic acid
[4]
. The
glass ionomer powder comprises silica (SiO
2
), alumina (Al
2
O
3
), and calcium fluoride (CaF
2
) as flux
and typically sodium fluoride (NaF), cryolite (Na
3
AlF
6
) and aluminum phosphate (AlPO
4
), which
are soluble in acids. Phosphate and fluoride ions are used in the basic glass to modify the setting char-
acteristics of the material (
[25]
); however, still alumina and silica, which form the skeletal backbone
of the glass, are the main structural components
[27]
. These components are melted at high tempera-
ture (fusion of oxides at temperatures between 1100 and 1500
C). The use of high temperatures,
particularly on an industrial scale, consumes a huge quantity of energy since the oxide links need to
be ruptured and then formed again for the synthesis of glass. This makes production cost higher.
Alternatively, soft chemistry has been used for synthesis of glasses because this route yields more
homogeneous materials using lower processing temperatures than the conventional fusion method.
The other methods are flame spraying and inductively coupled radiofrequency plasma spraying
techniques
[28,29]
, spray drying method
[30]
, and sol
gel technique
[31]
.
The three-dimensional structure of the glass particle is based on an aluminosilicate network.
The Si
4
1
ions reside at the interstices formed by four oxygen anions, where Al
3
1
plays a dual role
in the glass matrix. It can be substituted for a Si
4
1
, therefore, consider as a network-forming ion.
The negative charge is offset by other network-dwelling ions such as sodium (Na
1
) or calcium
(Ca
2
1
). Network-dwelling ions do not take part in the three-dimensional network but reside within
the glass. If sufficient sodium or calcium ions are not present to maintain charge neutrality, then
the aluminum ions will be network dwelling, presumably as an oxide, fluoride, or phosphate also
considered network dwelling
[32,33]
. The proper glass network should be formed before the glass
is reactive to an acidic polymer. This occurs when counter ions are present and the Al/Si ratio is
close. The glass has loosely bound negative charges that attack by the carboxylic acid from poly-
mers and disrupt the three-dimensional matrix. Al
3
1
ions, along with other ions, are released and
form ionic bonds with polymers. The Al/Si ratio is the main factor controlling the rate of setting
reaction in GIC. The ratio of Al
2
O
3
to SiO
2
is critical for accurate reactivity and must be 1:2 or
more by mass for cement formation. Furthermore, the hydrolytic stability of GIC also depends on
Al/Si ratio; higher ratios of Al/Si increase the stability of the cement
[25,34]
. The entrance of Al
3
1
ions in the network-forming sites increases the susceptibility of the glass structure to acid attack
because the negative charge on the network is increased. Other ion concentrations also play a role
in the setting and properties of GIC. The presence of Na
1
ions in the glasses has adverse effects on
the hydrolytic stability. Currently, in GIC the amount of CaF
2
has been decreased, and the Al
2
O
3
/
SiO
2
ratio has been changed to enhance the esthetics and the degree of transparency. CaF
2
has
been added as a flux to decrease the melting point (the structure can be melted at a more economi-
cal temperature below 1350
C). The addition of ions of lanthanum (La), strontium (Sr), barium
(Ba), or zinc (Zn) provides radiopacity for the cement.
