Biomedical Engineering Reference
In-Depth Information
morphologies. In particular, it can result in high purity of the substrate surface,
superior than the anodization process which has been reported to have excess
fluoride on the titanium surface if not carefully cleaned [51]. Unfortunately, to
date, there have been no known accurate comparisons of bone growth on these
three novel metal nanostructured surfaces, but
it
is clear that each shows
advantages over currently implanted titanium.
Nanostructured ceramics
Ceramics are non-metallic inorganic materials which usually possess excellent
biocompatibility properties and in some cases, have a high degree of biodegrad-
ability in the physiological environment. For example, HA (Ca
10
(PO
4
)
6
(OH)
2
) is
one of the most popular bone graft substitutes and filler materials. As mentioned
earlier in this chapter, HA is one of the main components in natural bone, so it
should not be surprising that others have reported that HA has good osteo-
conductive properties to form a tight bond with bone [52]. Although traditional
bioceramics have long served as bone substitutes, they have a variety of implant
failure modes due to insufficient prolonged osseointegration, poor mechanical
properties, osteolysis and detrimental ceramic implant wear debris. Therefore,
nanostructured ceramics (as opposed to conventional micron-structured
ceramics) which mimic the natural nanostructure of human bone have become
novel candidates for improving bone implant performance [53].
With decreased grain size and pore diameters (if porous), surface area,
roughness and surface energy of nanostructured ceramics dramatically increase
compared with micron-structured ceramics. For instance, nanostructured
alumina, HA and titania have improved wettability properties compared with
conventional ceramics [54]. As another example, a 23 nm grain size alumina
compact prepared by compressing nanoceramic particles had approximately
50% more surface area for cell adhesion than a 177 nm grain size alumina;
similarly, a 32 nm grain size titania had about 35% more surface area than that
of a 2.12m grain size titania [55]. Besides the large surface area of nano-
ceramics to increase cell adhesion, researchers have also found that the special
surface properties (namely, surface energy) of nanomaterials are closely related
to their observed excellent biocompatibility and osseointegration properties. An
example, nanophase HA with a 67 nm grain size significantly enhanced osteo-
blast adhesion and inhibited the adhesion of competitive cells that synthesize
soft fibrous tissue (fibroblasts) compared to conventional HA [54]. The reason
that nano-HA selectively enhanced osteoblast adhesion over fibroblasts could
be due to the adsorption of specific proteins necessary for bone cell functions
(such as fibronectin) [54, 55]. Some nanoceramics have also demonstrated an
antibacterial property even when the respective conventional ceramic is not
antibacterial. For instance, nanophase ZnO and TiO
2
inhibited Staphylococcus
epidermidis (a common bacterium contributing to undesirable biofilm
