NANOMATERIALS FOR IMPROVED ORTHOPEDIC AND BONE TISSUE ENGINEERING APPLICATIONS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 224

Another highly studied ceramic material in composites is zirconium dioxide
or zirconia (ZrO 2 ). It enhances fracture toughness in polymers or other ceramics.
A novel orthopedic bioceramic is the zirconia (ZrO 2 ) - hydroxyapatite (HA)
composite which has good biocompatibility properties [115]. CaP and phosphate-
based glass composite coatings have been shown to strengthen ZrO 2 and to
improve biocompatibility. The proliferation and alkaline phosphatase activities of
osteoblasts on such composite coatings were improved by approximately 30-40%
when compared to ZrO 2 substrates alone, and were comparable to the pure HA
coating. These fi ndings suggest that the CaP and P-glass composites are poten-
tially useful for coating traditional biomaterials used as a hard tissue implant, due
to their morphological and mechanical integrity, enhanced bioactivity, and
favorable responses of osteoblast-like cells [116]. Kong et al. studied HA-added
zirconia - alumina (ZrO 2 - Al 2 O 3 ) nanocomposites in load-bearing orthopedic
applications. The HA-added zirconia-alumina nanocomposites contained bipha-
sic calcium phosphates (BCP) of HA/TCP and had higher fl exural strength than
conventionally mixed HA - added zirconia - alumina composites. In vitro tests
showed that the proliferation and differentiation of osteoblasts on this nanocom-
posite gradually increased as the amount of HA added increased [117].
Synthetic biodegradable polymers are widely utilized in the fabrication of
scaffolds for bone tissue engineering [118]. Because carbon nanotubes (CNTs)
have a very high strength to weight ratio for greater mechanical properties,
they can be used as a reinforcing material for polymer composites. For example,
Webster et al. developed a carbon nanofi ber reinforced polycarbonate urethane
(PU) composite in an attempt to determine the possibility of using either
carbon nanofi bers or strips of CNTs, as either neural or orthopedic prosthetic
devices. The results have shown that such composite biomaterials have the poten-
tial to promote osteoblast functions [119]. Also, Shi et al. used single-walled
carbon nanotubes (SWNTs) as reinforcing materials for poly(propylene fuma-
rate) (PPF). Such composites are used as an injectable highly porous scaffold
for load-bearing bone repair [120]. Moreover, it is believed that electrical stimula-
tion enhances osteoblast function on such CNT composites. Conductivity of poly-
lactic acid (PLA), which is an insulator, can be enhanced by adding CNTs. Studies
have shown that osteoblast proliferation increased signifi cantly on such com-
posites after exposing them to electrical stimulation [121]. Thus, great promise
exists for many nanocomposites for orthopedic applications.
7.3.5 Role of Chemistry
In bone tissue engineering, nanotechnology is also being used in conjunction with
changes in implant surface chemistries. For example, for titanium, the surface
oxide layer has many qualities regarded as important for optimal reactions with
bone. The excellent biocompatibility of Ti-based materials can be attributed
to the high corrosion resistance (low metal ion release) and, moreover, to the
good bone-binding ability of TiO 2 which covers the implant surface. An electro-
chemical method known as anodization or anodic oxidation (Figure 7.11) is a