Bioactive Glass-Based Scaffolds for Bone Tissue Engineering - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 206

Fig. 5 SEM micrographs showing the structure of bone. Specimens are taken from human
mandible showing high-density structure (left), and a low-density, sponge-like structure (right).
(Figures courtesy of R. Detsch, University of Erlangen-Nuremberg, Germany)
remain healthy. In the literature, depending on the measurement technique and
parameters used, the source of bone and the structural variation in bone from a
given source, a wide range of values has been reported for the compressive
modulus of trabecular (0.12-1.1 GPa) and cortical bone (11.5-17 GPa) (Table 2 ).
Fu et al. [ 93 ] reported a compressive strength of 11 ± 1 MPa and compressive
modulus of 3.0 ± 0.5 GPa for magnesium- and potassium-substituted bioactive
glass-ceramic scaffolds (porosity = 85 ± 2%, pore size = 100-500 lm), which
match the highest values reported for human trabecular bone (Table 2 ). Interest-
ingly, these values are more than ten times higher than compressive strengths
reported for 45S5 Bioglass -based scaffolds [ 72 ] of similar porosity and prepared
by the same foam replication method. This finding confirms that glass composition
and sintering parameters also affect the mechanical properties of glass-ceramic
scaffolds. Upon immersion in SBF, Fu et al. [ 93 ] observed a nanostructured
HA layer formed on the surface of the porous scaffolds within 7 days, indicating
the in-vitro bioactivity of the scaffolds. Such HA nanocrystals are found in human
bone and are believed to be beneficial for increased cell adhesion, proliferation and
greater tissue growth into the scaffold [ 122 - 124 ]. Cell culture results and scanning
electron microscopy (SEM) observations presented in Ref. [ 93 ] confirmed an
excellent attachment and subsequent proliferation of osteoblastic cells.
Engineering constructs with graded porosity represent an interesting approach
to the development of bone TE scaffolds. Vitale-Brovarone et al. [ 102 ] and
Bretcanu et al. [ 98 ] manufactured highly porous bioactive glass-ceramic scaffolds
with tailored porosity gradients in order to mimic the morphology and lightweight
structure of human bone, formed by cortical (compact bone with dense structure)
and cancellous bone (trabecular bone with highly porous structure) (Fig. 5 ).
Trabecular bone represents only about 20 wt% of the skeletal mass, but has a
nearly ten times greater surface-to-volume ratio (100-300 cm 2 /cm 3 ) than compact