Biomedical Engineering Reference
In-Depth Information
glass-ceramic is attributed to the glass matrix and apatite precipitates, whereas the
in vivo
stability
as a whole is due to the inertness of β-wollastonite. Although the long-term integrity
in vivo
is desir-
able in the application of nonresorbable prosthesis, the material does not match the goal of tissue
engineering, which demands biodegradable scaffolds.
1.3.3.2
Ceravital Glass-Ceramics [79]
“Ceravital” was coined to mean a number of different compositions of glasses and glass-ceramics
and not only one product. Their basic network components include SiO
2
, Ca(PO
2
)
2
, CaO, Na
2
O,
MgO, and K
2
TiO2, with ceramic additions being Al
2
O
3
, Ta
2
O
5
, TiO
2
, B
2
O
3
, Al(PO
3
)
3
, SrO, La
2
O
3
, or
Gd
2
O
3
. This material system was developed as solid fi llers in the load-bearing conditions for the
replacement of bone and teeth. It turned out, however, that their mechanical properties do not serve
the purpose, and there has been virtually no research on the application of this material in tissue-
engineering scaffolds.
1.3.3.3 Bioverit Glass-Ceramics
[74]
Bioverit products are mica-apatite glass-ceramics. Mica crystals (aluminum silicate minerals) give
the materials good machinability, and apatite crystals ensure the bioactivity of the implants. The
mechanical properties of Bioverit materials (Table 1.4) allow them to be used as fi llers in dental
application. As regards bioreactivity, Bioverit implants show a hydrolytic stability
in vivo
. As for
Ceravital glass-ceramics, no signifi cant research has been carried out regarding the use of this
glass-ceramic in tissue engineering.
1.3.3.4
45S5 Bioglass-Derived Glass-Ceramics
In 2005, Chen et al. [80] fabricated a 3-D, highly porous, mechanically competent, bioactive and
biodegradable scaffold for the fi rst time by the replication technique using 45S5 Bioglass pow-
der. Under an optimum sintering condition (1000°C/h), nearly full densifi cation of the foam struts
occurred and fi ne crystals of Na
2
Ca
2
Si
3
O
9
are formed, which conferred the scaffolds the highest
possible compressive and fl exural strength for this foam structure. Important fi ndings in this work
are that the mechanically strong crystalline phase Na
2
Ca
2
Si
3
O
9
can transform into an amorphous
calcium phosphate phase after immersion in simulated body fl uid (SBF) for 28 days and that the
transformation kinetics can be tailored by controlling the crystallinity of the sintered 45S5 Bio-
glass. As such, it was demonstrated that the goal of an ideal scaffold that provides good mechanical
support temporarily while maintaining bioactivity and that can biodegrade at later stages at a tailor-
able rate can be achieved with these Bioglass-based scaffolds [17].
1.3.4 N
ATUR ALLY
O
CCURRING
B
IOPOLYMERS
Much research effort has been focused on naturally occurring polymers such as demineralized bone
ECM [81], purifi ed collagen [82,83], and chitosan [84] for tissue engineering applications. Theoreti-
cally, naturally occurring polymers should not cause response of foreign materials when implanted.
They provide a natural substrate for cellular attachment, proliferation, and differentiation in their
native state. For these reasons, naturally occurring polymers could be a favorite substrate for tis-
sue engineering [28]. Table 1.5 provides a list of some of the naturally occurring polymers, their
sources, and applications. Among them, collagen and chitosan are most widely investigated for
bone engineering and are briefl y discussed here.
1.3.4.1
Collagen and ECM-Based Materials
The most commonly used naturally occurring polymer is the structural protein collagen. Bioma-
terials derived from ECM include collagen and other naturally occurring structural and functional
