NANOMATERIALS FOR IMPROVED ORTHOPEDIC AND BONE TISSUE ENGINEERING APPLICATIONS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 213

forms a thick extracellular matrix and eventually a biofi lm around bone
implants [58], which leads to implant failure. Thus, decreased Staphylococcus
epidermidis functions on implants are desirable. Nanophase zinc oxide and
titania have indeed been observed to signifi cantly inhibit Staphylococcus epider-
midis functions [57] .
It was elucidated that the special surface topography as well as wettability of
nanophase ceramics may be the main reason for the promoted bone cell and in-
hibited bacteria functions on nanoceramics. At the tissue-implant interface, the
fi rst and necessary step for success is the adsorption of specifi c proteins. Many
researchers have demonstrated that the protein adsorption process depends on
surface properties (such as hydrophilicity, charge density, roughness, and surface
energy) [60,61]. For instance, on hydrophilic and high nanorough surfaces, studies
have shown maximum vitronectin, fi bronectin and laminin adsorption [16,60,61].
Specifi cally, the highest concentration of vitronectin (a protein known to mediate
anchorage-dependent cell adhesion) adsorption on nanophase ceramics may
explain the subsequent enhanced osteoblast adhesion on nanophase ceramics
[16]. Secondly, proteins will interact with specifi c cell membrane receptors (inte-
grins) to aid in cell adhesion on the substrate and at the same time inhibit other
nonspecifi c cell adhesion. It is apparent that surface properties are closely related
to anchorage-dependent cell (such as osteoblasts and osteoclasts) adhesion to a
biomaterial surface. After that, cell proliferation, differentiation, and extra-
cellular matrix deposition can happen and will lead to a successful integration
with surrounding tissue. Many current attempts, therefore, have focused on nano-
surface modifi cations of conventional orthopedic materials in order to promote
anchorage-dependent osteoblast adhesion on implant surfaces. Importantly,
promising results have been observed on nano ceramics without any modifi ca-
tion; it is their raw surface that works.

7.3.1.2 Enhanced Mechanical Properties of Nanophase Ceramics.
Mechanical properties (such as hardness, ductility, stiffness, bending, compressive
and tensile strengths) of biomaterials play a crucial role in the long-term suc-
cess of orthopedic implants. Many attempts and efforts have ensued to improve
mechanical properties of current metals, ceramics and polymers in order to bio-
mimic those of physiological bone. Unlike metals, ceramics (such as alumina and
HA) have little ductility and have low fracture toughness (0.8- 1.2 MPa · m − 1/2 for
HA). They cannot tolerate damage and are vulnerable to crack initiation and
propagation. They, therefore, cannot be applied in some loading-bearing appli-
cations. Table 7.2 shows the mechanical properties of traditional ceramics com-
pared to natural bone as well as to other implant materials.
To date, it is well known that decreasing grain sizes into the nanoscale (dia-
meter 12-20 nm) increases strength, hardness and plasticity for both metals

and ceramics. Theoretically, the well-known empirical Hall-Petch equation which
relates yield stress (

>

y ) to average grain size ( d ) can exhibit remarkably increased
strength when reducing grain size from the micronmeter to the nanometer
regime [63] :

σ