MEDICAL DEVICES BASED ON BIOINSPIRED CERAMICS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 378

dependence with their shape. In hard woods (higher densities) the distribution of
pore size is bimodal and the packing of small pores around the big ones is random.
The bimodal distribution of the pore size is the way nature reaches a high poros-
ity, providing the necessary transfer of fl uids for a particular vegetal species, and,
at the same time keeping the wall thickness at adequate values. The strength of
the BioSiC ceramics fabricated from these woods is close to that predicted by a
random distribution of cylindrical pores (curve fi ve of Figure 11.17).
However, for higher porosities, the random distribution of pores loses inter-
connection as refl ected by the sudden decrease of strength in the curve fi ve of
Figure 11.17. At these lower densities the use of wall material available must be
optimized, and compact packed structures like the one from pine are found. In
pine wood, the number of neighbors is reduced to four and the cells have square
shape. The radial strength of the BioSiC ceramics fabricated from pine wood is
close to the expected values for an ideal cubic packing of tubular cells with square
shape (curve six of Figure 11.17). The high scatter on the strength of BioSiC fab-
ricated from pine wood, not found in other types of BioSiC ceramics (standard
deviations of around 60% versus a typical 20%), can be explained by the high
anisotropy of the strength between the axial and perpendicular direction, so small
misalignments can have important consequences on the measured strength. This
high anisotropy is also present within the plane perpendicular to the axial direc-
tion, where the strength is highly dependent on the orientation between compres-
sion direction and cell wall. The cells are stronger when compressed perpendicularly
to the cell wall than at an angle. The distribution of the square cells forming rings
in the tree, assures that any external force will be acting in the strongest orienta-
tion. Summarizing, as the density of the structures is reduced, the microstructure
of these biomimetic SiC-based materials change to maintain a good strength
value.
11.4 BIOINSPIRED CERAMICS COATED WITH
BIOACTIVE MATERIALS
When bioinert materials, such as titanium, Co-Cr alloys, stainless steel, alumina,
zirconia, and so on, are implanted into the body, a thin layer of fi brous tissue
forms around them, leading to the encapsulation of the implant [Anderson, 2004].
The interface between prosthesis and its host tissue is especially susceptible to
stress, and the mismatch in either biochemical or biomechanical factors can lead
to interfacial deterioration and eventual failure. As the isolation of the implant
from the body can have disastrous effects on some clinical applications, these
implants require a periodical revision and, eventually, a replacement due to,
among other factors, the lack of interaction with the receptor bone [Hench, 1993;
Ratner, 2004 ].
To solve these problems and produce cementless devices with enhaced
fi xation and osteointegration performance, a new approach leading to the for-
mation of a bond across the interface between the implant and the bone via