Biomedical Engineering Reference
In-Depth Information
Table 2 Mechanical properties of human cancellous and cortical bone in comparison to dense
bioactive glass (45S5 Bioglass
)
Material property
Bioglass
45S5
Trabecular bone
Cortical bone
Compressive strength (MPa)
0.1-16 [
114
,
115
]
130-200 [
34
,
114
]
500 [
34
]
Tensile strength (MPa)
n.a.
50-151 [
34
]
42 [
62
]
Compressive modulus (GPa)
0.12-1.1 [
116
,
117
]
11.5-17 [
67
]
n.a.
Young's modulus (GPa)
0.05-0.5 [
34
,
118
]
7-30 [
6
,
34
,
118
]
35 [
62
]
Fracture toughness (MPa m
1/2
)
n.a.
2-12 [
34
,
62
]
0.9 [
119
,
120
]
n.a. not available
of 25 ± 3 MPa, compressive modulus of 1.2 GPa, and pore width of 90-110 lm,
compared to values of 10 ± 2 MPa, 0.4 GPa, and 20-30 lm, respectively, for the
lamellar scaffolds. The compressive strength of these columnar bioactive glass
scaffolds is [1.5 times higher than the highest strength reported for trabecular
bone (0.1-16 MPa, see Table
2
). In addition, the cellular response of murine post-
osteoblasts/pre-osteocytes to columnar scaffolds indicated that these structures
were the most favorable for cell proliferation, migration and mineralization (e.g.,
bone nodule formation, alkaline phosphatase activity). From the results of their
study [
94
], the authors claimed that 13-93 bioactive glass scaffolds with columnar
microstructure are promising candidate materials for the repair and regeneration of
load-bearing bones in vivo. It is interesting to note in this regard that highly porous
lamellar HA scaffolds (porosity & 50-70%) fabricated by freeze casting exhibited
2.5-4 times higher compressive strength (&20-140 MPa) than conventional
porous HA [
111
].
Multi-directional, anisotropic mechanical properties of scaffolds have been also
reported by Baino et al. [
91
]. They prepared glass-ceramic scaffolds containing
fluoroapatite and investigated their mechanical, structural and bioactive properties
upon soaking in simulated body fluid (SBF). The scaffolds had interconnected
macropores (porosity = 23.5-50%) and orthotropic mechanical properties, with
compressive strength values in the range 20-150 MPa. Thick hydroxyapatite
layers were formed on the surface of the scaffolds after 7 days of immersion in
SBF, demonstrating the scaffold's excellent bioactivity. Compressive strength
values reported in Ref. [
91
] are considerably higher than those found for bioactive
glass-ceramic scaffolds with similar porosities (porosity = 54-73%) prepared by
the foam replication technique [
113
]. The latter scaffolds formed from SiO
2
-P
2
O
5
-
CaO-MgO-Na
2
O-K
2
O bioactive glass had a compressive strength of 1.3-5.4
MPa [
113
].
Ideally, the elastic modulus of the scaffold should be comparable to that of the
tissue to be replaced in order to promote load transfer and minimize stress
shielding, reducing the problems of bone resorption [
121
]. Stress shielding
describes the mismatch in elastic moduli between biomaterial and the adjacent/
surrounding bone. In cases of large elastic mismatch, bone becomes ''stress
shielded'', which is undesirable since living bone must be under some stress to
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