Biomedical Engineering Reference
In-Depth Information
However, the final strength and in the case of dental applications the translucency (Engqvist
et al 2004), are reached after approximately a few days maturing.
It should be noted that for CBC materials, irrespective of chemistry (CA, CS or CPC), the
final porosity cannot be zero. When the hydrates precipitate there is a contraction of
approximately 10 %. The porosity is in the nanometer range and its exact amount is difficult
to determine, but < 10 % of the filled space (the original liquid-phase volume) is estimated to
be pores. However, this internal shrinkage related to hydration and precipitation yield no
total shrinkage of the bulk material. In contrast, a limited expansion close to zero, may be
induced (Kraft 2002). The porosity lowers the mechanical strength (although being in the
nanometer range) but also enables liquids to diffuse into, or even through, the hardened
CBC materials. This is an important feature when release of loaded drugs is a
complementary aspect of the injectable biomaterial.
3. Ca-aluminate - general description and property profile
Ca-aluminates comprise double oxides of CaO and Al
2
O
3
. Several intermediate phases exist
and these are - using the cement chemistry abbreviation system - C
3
A, C
12
A
7
, CA, CA
2
and
CA
6
, where C=CaO and A=Al
2
O
3
. See Fig. 1 above. Table 5 presents typical property data.
Property
Typical value
Interval*
Compression strength, MPa
150
60-270
Young´s modulus, GPa
15
10-20
Thermal conductivity, W/mK
0.8
0.7-0.9
Thermal expansion, ppm/K
9.5
9-10
Flexural strength, MPa
50
20-80
Fracture toughness, MPam
1/2
0.5
0.3-0.8
Corrosion resistance, water jet impinging, reduction in
mm
< 0.01
-
Radio-opacity, mm
1.5
1.4-2.5
Process temperature,
o
C
> 30
30-70
Working time, min
3
< 4
Setting time, min
5
4-7
Curing time, min
20
10-60
Porosity after final hydration, %
15
5-60
The interval is primarily related to the c/w ratio used, and the highest values are achieved with c/w
ratio close to that of complete hydration with no excess of water
Table 5. Mean property data of dental Ca-aluminate based materials (Kraft 2002, Lööf 2008,
Lööf et al 2004,2005, Hermansson et al 2008)
Due to reduced porosity based on the huge water uptake ability, the Ca-aluminate material
exhibit the highest strength among the chemically bonded ceramics. The inherent flexural
strength is above 100 MP based on measurement of the fracture toughness, which is about
0.7 - 0.8 MPam
1/2
. The actual flexural strength is controlled by external defects introduced
during handling and injection of the material. The thermal and electrical properties of Ca-
aluminate based materials are close to those of hard tissue, the reason being that Ca-
aluminate hydrates chemically belong to the same group as Ca-phosphates, the hard tissue
