Biomedical Engineering Reference
In-Depth Information
Since diverse biomedical applications require different
configurations and shapes, the graded (or gradient) porous
bioceramics can be grouped according to both the overall shape
and the structural configuration [375]. The basic shapes include
rectangular blocks and cylinders (or disks). For the cylindrical
shape, there are configurations of dense core—porous layer, less
porous core—more porous layer, dense layer—porous core and
less porous layer—more porous core. For the rectangular shape, in
the gradient direction, i.e., the direction with varying porosity, pore
size or composition, there are configurations of porous top—dense
bottom (same as porous bottom—dense top), porous top—dense
center—porous bottom, dense top—porous center—dense bottom,
etc. Concerning biomedical applications, a dense core—porous layer
structure is suitable for implants of a high mechanical strength and
with bone ingrowth for stabilization, whereas a less porous layer—
more porous core configuration can be used for drug delivery
systems. Furthermore, a porous top—dense bottom structure can be
shaped into implants of articulate surfaces for wear resistance and
with porous ends for bone ingrowth fixation; while a dense top—
porous center—dense bottom arrangement mimics the structure
of head skull. Further details on bioceramics with graded porosity
might be found in literature [375].
4.6
Biological Properties and
in vivo
Behavior
The most important differences between bioactive bioceramics and
all other implanted materials are the following: inclusion in the
metabolic processes of the organism; adaptation of either surface
or the entire material to the biomedium; integration of a bioactive
implant with bone tissues at the molecular level or the complete
replacement of a resorbable bioceramics by healthy bone tissues. All
of the enumerated processes are related to the effect of an organism
on the implant. Nevertheless, another aspect of implantation is also
important—the effect of the implant on the organism. For example,
using of bone implants from corpses or animals, even after they
have been treated in various ways, provokes a substantially negative
immune reactions in the organism, which substantially limits the
application of such implants. In this connection, it is useful to dwell
on the biological properties of bioceramic implants, particularly
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