Biomedical Engineering Reference
In-Depth Information
mineralised tissues. However, these products have low compression strength values - in the
interval 10-40 MPa - and are therefore questioned as load bearing implants.
Materials based on Ca-aluminate (CA) and Ca-silicate (CS) with chemistry similar to that of
Ca-phosphates (CP) contribute to some additional features of interest with regard to dental
and orthopaedic applications (Scriviner, 1988). The inherent difference in water uptake
between the CA/CS-systems and CP gives benefits as:
Higher mechanical strength.
Possibility to add fillers, e.g. for improved radio opacity.
Tuneable handling properties, e.g. rheology.
Materials based on Ca-aluminate and Ca-silicate thus contribute to some additional features
of interest with regard to dental and orthopaedic applications. These features are related to
the high amount of water involved in the curing process, the early and high mechanical
strength obtained, and the biocompatibility profile including in situ reactions with
phosphates ions of the body fluid. Compressive strength of cements based on Ca-silicate
and Ca-aluminate is in the range 50-250 MPa depending on the water to cement ratio. The
mechanical properties tested for the three Ca-based cement systems are compiled in Table 1
(Kraft, 2002, Loof et al 2003, Engqvist et al 2006).
Ca-phosphate
based material
Compressive strength (MPa) 100-200 100-150 < 100
Flexural strength (MPa) 30-60 30-40 20-30
Young´s modulus (GPa) Approx. 15 Approx. 11 Approx. 3
Table 1. Mechanical property profile of the Ca-aluminate, Ca-silicate and Ca-phosphate
systems
Property profile after 7 days
Ca-aluminate
based material
Ca-silicate based
material
The water content involved in the hydration of the different chemically bonded ceramics are
presented in Table 2.
System
Typical phase(s)
Oxide formula
Mol % H 2 O
Weight-% in
hydrated product
Ca-
phosphate
Apatite
10CaO 3P 2 O 5 H 2 0
7
Approx 5
Ca-aluminate Katoite + gibbsite
3CaO Al 2 O 3 6 H 2 0
+ Al 2 O 3 2H 2 O
> 60
Approx 25
Ca-silicate
Tobermorite +
amorphous phases
5CaO 6SiO 2 5H 2 O
+ Ca, Si)H 2 O
> 30
Approx 20
Table 2. The three chemically bonded ceramic systems most used for biomaterials
2.1 Materials and basic function
2.1.1 Main chemistry
The injectability and handling features of the chemically bonded ceramics is mainly caused
by the added water as the reacting phase with the powdered cements. This reaction is an
acid-base reaction where water acts as a weak acid and the cement powder as a base. Several
cement phases exist in the CaO - Al 2 O 3 (CA) and in the CaO-SiO 2 (CS) systems, see Figure 1,
but only a few are suitable as injectable materials. For one of the most attractive phases -
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