Biomedical Engineering Reference
In-Depth Information
of certain ions from and corrosion products of metallic implants. Of
course, neither metals nor alloys are biomimetic (the term biomimetic
can be defined as a processing technique that either mimics or
inspires the biological mechanism, in part or whole [195]) in terms
of chemical composition because there are no elemental metals in
the human body. In addition, even biocompatible metals are bioinert:
while not rejected by the human body, any metallic implants cannot
actively interact with the surrounding tissues. Nevertheless, in some
cases (especially when they are coated by calcium orthophosphates;
however, that is another story) the metallic implants can show a
reasonable biocompatibility [196]. Only permanent implants are
made of metals and alloys, in which degradation or corrosion is not
desirable. However, during recent years a number of magnesium
implants have been proposed which are aimed to degrade in the
body in order to make room for ingrowing bones [193, 197, 198].
6.3.3.2 Glasses and glass-ceramics
Special types of glasses and glass-ceramics are also suitable materials
for biomedical applications [199-201] and a special Na
O - CaO -
2
[13, 28, 30, 31, 202, 203] is the
most popular among them. They are produced via standard glass
production techniques and require pure raw materials. Bioglass
SiO
- P
O
glass named Bioglass
®
2
2
5
®
is
a biocompatible and osteoconductive biomaterial. It bonds to bone
without an intervening fibrous connective tissue interface and, due
to these properties, it has been widely used for filling bone defects
[204]. The primary shortcoming of Bioglass
®
is mechanical weakness
and low fracture toughness due to an amorphous two-dimensional
glass network. The bending strength of most Bioglass
®
compositions
is in the range of 40-60 MPa, which is not suitable for major load-
bearing applications. Making porosity in Bioglass
-based scaffolds
is beneficial for even better resorption and bioactivity [205].
By heat treatment, a suitable glass can be converted into glass-
crystal composites containing crystalline phase(s) of controlled
sizes and contents. The resultant glass-ceramics can have superior
mechanical properties to the parent glass as well as to sintered
crystalline ceramics. The bioactive A-W glass-ceramics is made from
the parent glass in the pseudoternary system 3CaO·P
®
O
- CaO·SiO
-
2
5
2
MgO·CaO·2SiO
, which is produced by a conventional melt-quenching
method. The bioactivity of A-W glass-ceramics is much higher than
that of sintered HA. It possesses excellent mechanical properties and
2
Search WWH ::
Custom Search