Inorganic and Composite Bioactive Scaffolds for Bone Tissue Engineering - Biomaterials Fabrication and Processing - page 10

TABLE 1.4
Mechanical Properties of Hydroxyapatite, 45S5 Bioglass, Glass-Ceramics, and Human
Cortical Bone

Fracture
Toughness
(MPa √ __

Compression
Strength (MPa)

Tensile Strength
(MPa)

Elastic Modulus
(GPa) Ceramics m ) References

45S5 Bioglass 500 42 35 0.5-1 42,73

A-W 1080 215 (bend) 118 2.0 26

Parent glass of A-W NA 72 (bend) NA 0.8 26

Bioverit I 500 140-180 (bend) 70-90 1.2-2.1 74

Cortical bone 130-180 50-151 12-18 6-8 28,43-46

and even turn a bioactive glass into an inert material [71]. This is one of the disadvantages that
limit the application of bioactive glasses as scaffold materials, as full crystallization occurs prior to
signifi cant densifi cation upon heat treatment (i.e., sintering) [72]. Extensive sintering is necessary
to densify the struts of a scaffold, which would otherwise be made up of loosely packed particles
and thus the structure would be too fragile to handle. Most recently, Boccaccini's group at Imperial
College London [17] reported on a phase transformation from a mechanically competent crystalline
phase to a biodegradable amorphous calcium phosphate in 45S5 Bioglass-derived scaffolds. This
phase transition, which takes place in a biological environment at body temperature, couples the
two required properties (mechanical strength and biodegradability) in a single scaffold. A detailed
characterization of this material is given in Section 1.3.3.4.
In summary, like hydroxyapatite and related calcium phosphates, bioactive glasses exhibit good
biocompatibility and osteoconductivity. At the same time, all these materials, except 45S5 Bioglass-
derived glass-ceramics, encounter a similar disadvantage, that is, a mechanically strong scaffold
has to be achieved through crystallization, which unfortunately hampers the biodegradability of
these materials.

1.3.3 B IOCERAMICS : G LASS -C ERAMICS
Glasses can be strengthened by the formation of crystalline particles in the glass matrix upon heat
treatment in the relevant glass-crystal region of its phase diagram. The resultant glass-ceramics usually
exhibit better mechanical properties than both the parent glass and sintered crystalline ceramics (e.g.,
sintered hydroxyapatite) (Table 1.4). There are many biomedical glass-ceramics available for the repair
of damaged bones. Among them, apatite-wollastonite (A-W), Ceravital, and Bioverit glass-ceramics
have been intensively investigated [16,18]. Recently, a 45S5 Bioglass-derived glass-ceramic showed a
great potential as a tissue-engineering scaffold material, as mentioned above (Section 1.3.2.3).

1.3.3.1 A-W Glass-Ceramics

In A-W glass-ceramic, the glass matrix is reinforced by β-wollastonite (CaSiO 3 ) crystals and a
small amount of apatite phase, which precipitate successively at 870°C and 900°C, respectively
[75]. Some mechanical properties of this glass-ceramic have been listed in Table 1.4. The high
bending strength (215 MPa) of A-W glass-ceramic is due to the precipitation of wollastonite as well
as apatite. These two precipitates also give the glass-ceramic a higher fracture toughness than that
of both the glass and ceramic phases. It is believed that wollastonite effectively prevents straight
propagation of cracks, causing them to defl ect or branch out [26,75-77].
A-W glass-ceramic is capable of binding tightly to a living bone in a few weeks after implanta-
tion, and the implants do not deteriorate in vivo [78]. The excellent bone-bonding ability of A-W