Biomedical Engineering Reference
In-Depth Information
(Uemura et al., 2003). The scaffolds should biodegrade with a controlled
degradation rate and eventually disappear when the new tissue is fully
regenerated. In that sense, the three-dimensional space occupied by the
porous scaffolds would be replaced by newly formed tissue (Langer and
Vacanti, 1993; Holy et al., 1999). The development of new materials to
meet all these specifications is being addressed worldwide.
In recent years, several bioactive ceramics, such as hydroxyapatite,
tricalcium phosphate, biphasic calcium phosphate, and bioactive glasses
have been used in bone tissue engineering applications (Vallet-Regi, 2001).
Hydroxyapatite (HAp) is one of the most widely used synthetic ceramics due
to its chemical similarities to the inorganic component of hard tissue (Akao
et al., 1981; Dewith et al., 1981). Bioactive ceramic scaffolds can either
induce the formation of bone from the surrounding tissue or can act as a
carrier or guide for enhanced bone regeneration by cell migration,
proliferation and differentiation. But, as has already been highlighted,
ceramics are brittle and are not suitable for load-bearing applications (Paul
and Sharma, 2006). Therefore, the use of polymer-ceramic composites has
been suggested. These composites mimic the inorganic-organic composition
of natural bone, with nanometre size inorganic components (mainly bone-
like apatite). To make the mechanical properties more similar to those of
natural bone, nanoscale calcium phosphates are added to the polymer
matrix (Rho et al., 1998). Furthermore, with nanometric components, the
scaffold osteoconductivity and bone bonding ability are enhanced, and
osteoblasts and osteoprogenitor cells can adhere, migrate inside, differ-
entiate and synthesize new bone matrix (Ma et al., 2001; Zhang and Ma,
1999).
Poly(
-hydroxyl acids) such as poly(lactic acid) (PLA), poly(glycolic acid)
(PGA), poly(lactic acid-co-glycolic acid) (PLGA) and poly(
α
-caprolactone)
(PCL) satisfy many of the scaffold's material requirements and they have
already been used as scaffolding material for a variety of tissue engineering
applications, including bone (Lo et al., 1995). However, the highly porous
polymeric scaffolds are chemically hydrophobic, biologically inert and
relatively weak, which limits their use for bone tissue regeneration,
especially in the in vivo implant site.
Wei and Ma (2004) produced a nano-hydroxyapatite (nHAp)/PLLA
composite scaffold for bone tissue engineering. It was proved that the
compressive modulus increased significantly when the nHAp proportion
reached 30% of the composite. Moreover, the addition of nHAp increased
the protein absorption, thus improving cell adhesion. Kothapalli et al.
(2005) also proposed a nano-sized HA/PLLA composite scaffold. The
starting nanopowders had an average size of approximately 25 nm in width
and 150 nm in length. The obtained nanocomposite scaffold showed an
increase in compression modulus from 4.72MPa to 9.87MPa when the
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