Biomedical Engineering Reference
In-Depth Information
double bond and molecular weight are key for control of
the final
mechanical properties and degradation time.
However, PPF and other biodegradable polymers lack the mechanical
strength required for tissue engineering of load-bearing bones (Mistry and
Mikos, 2005). The development of composite and nanocomposite materials
combining inorganic particles, e.g. apatite component (i.e. the main
constituent of the inorganic phase of bone (Ma, 2004)), bioactive glasses,
carbon nanostructures (e.g. nanotubes, nanofibers and graphene), and metal
nanoparticles has been investigated.
17.3.1 Hydroxyapatite (HA)-based nanocomposites
HA promotes bone ingrowth and is biocompatible because around 65 wt%
of bone is made of HA, Ca
10
(PO
4
)
6
(OH)
2
. Natural or synthetic HA has been
intensively investigated as a major component of scaffold materials for bone
tissue engineering (Knowles, 2003). The Ca/P ratio of 1.50-1.67 is the key to
promoting bone regeneration. Recently, much better osteoconductive
properties in HA by changing composition, size and morphology have
been reported (Gay et al., 2009). Nano-sized HA (nHA) may have other
special properties due to its small size and huge specific surface area. A
significant increase in protein adsorption and osteoblast adhesion on nano-
sized ceramic materials was reported by Webster et al. (2000).
Figure 17.1 shows the rod-shaped morphology of nHA with particle
width ranging from 37 to 65 nm and length from 100 to 400 nm (Nejati et al.,
2008). The compressive strength of bioceramics increases when their grain
size is reduced to the nanolevel (El-Ghannam et al., 2004).
Nanocomposites based on HA particles and biopolymers have attracted
much attention for their good osteoconductivity, osteoinductivity, biode-
gradability and high mechanical strength. Wei and Ma (2004) mimicked the
size scale of HA in natural bone and showed that the incorporation of nHA
improved the mechanical properties and protein adsorption of the
composite scaffolds whilst maintaining high porosity and suitable micro-
architecture.
Nejati et al. (2008) reported on the effect of the synthesis nHA on the
scaffold's morphology and mechanical properties in poly(L-lactic acid)
(PLLA)-based nanocomposites. The morphology and microstructure of the
scaffolds were examined using a scanning electron microscope (SEM) (Fig.
17.2). The nanocomposite scaffold (Fig. 17.2(c) and (d)) maintained a
regular internal ladder-like pore structure similar to a neat PLLA scaffold
(Fig. 17.2(a) and (b)) with a typical morphology processed by thermally-
induced phase separation (Nam and Park, 1999). Rod-like nHA particles
are distributed within the pore walls and no aggregation appears in the
pores (Fig. 17.2(e) and (f)). However, the nanocomposite exhibits little effect
