Bioactive Glass-Based Scaffolds for Bone Tissue Engineering - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 211

bioactive properties, degradation characteristics in vitro and in vivo behavior, as
well as mechanical properties (Table 3 ).
The mechanical strength of most of today's available porous polymer/BG
composite scaffolds is inadequate for bone substitution because they are at least
one order of magnitude weaker than natural cancellous bone and orders of mag-
nitude weaker than cortical bone. Moreover there is still limited understanding of
how microstructure features (e.g., geometry of struts, pore size distribution, pore
orientation, interconnectivity, morphology and distribution of the BG filler)
affect the scaffold's mechanical response and its functional performance [ 141 ].
In addition, insufficient particle-matrix bonding is considered a possible reason for
the low mechanical properties of these composites. With regard to the latter, two
key issues have to be solved to effectively improve the material properties of
scaffolds by adding bioactive glass particles as filler: (1) interfacial bonding and
(2) the proper, homogeneous dispersion of the individual particles in the matrix
(e.g., by particle surface functionalization). According to the concepts of the
composites theory [ 142 ], load transfer at the filler/matrix interface is key to
achieving strengthening and stiffening, which depends on the quality of interfacial
bonding between the two phases (filler and matrix). Strong interfacial bonding is
therefore a significant requirement for improving the mechanical properties of
biodegradable polymer composite scaffolds.
Blaker et al. [ 144 ] have developed highly porous (porosity & 94%) poly
(d,l-lactide) (PDLLA)/Bioglass foams using thermally induced phase separation
(TIPS). The scaffolds exhibited a bimodal and anisotropic pore structure, with
tubular micropores of &100 lm in diameter, and with interconnected micro-pores
of &50-10 lm, along with anisotropic mechanical properties. With respect to the
direction of the tubular pores, similar axial yield strengths of about 0.08 MPa were
found for all composites (0, 4.8, 28.6 wt% Bioglass ), whereas a higher axial
compressive modulus (1.2 MPa) was obtained for 28.6 wt% Bioglass containing
scaffolds compared to the pure PDLLA constructs (0.89 MPa). The yield strength
values reported in Ref. [ 144 ] are considerably lower than those for cancellous bone
[ 117 ], so a further improvement is necessary to increase the mechanical perfor-
mance up to the levels required for bone TE applications. The compressive moduli
are in the range of those determined for trabecular bone, but lower than those for
cortical bone (see Table 2 ).
Other authors have found, however, considerably higher mechanical strength
for their composite scaffolds [ 47 , 151 ]. Maquet et al. [ 151 ], for example, have
reported highly porous (porosity [ 90%) PDLLA and PLGA scaffolds containing
50 wt% Bioglass , exhibiting compressive moduli of about 21 and 26 MPa,
respectively: a factor 1.5-2.5 higher than the values of the pure polymer scaffolds.
Lu et al. [ 47 ] determined for PLGA scaffolds incorporated with 25 wt% Bioglass
(porosity = 43%, pore diameter = 89 lm) a compressive modulus of about 51 MPa,
and compressive strength of about 0.42 MPa, which are in the range of values
reported for trabecular bone (Table 2 ), but at the cost of porosity (43%).
Interestingly, numerical analyses presented in Ref. [ 144 ] showed that the com-
pressive modulus of the composite foams can be well predicted by micromechanic