Biomedical Engineering Reference
In-Depth Information
2. Titanium biomaterial characteristics
Titanium and some of its alloys Ti6Al4V, TI6AL7Nb, Nitinol, are classified as biologically
inert biomaterials and are commercially used in orthopaedic and dental application. This is
a result of their outstanding corrosion resistance and inertness. The inertness of those
materials is due to titanium oxide (TiO
2
) which spontaneously covers them after exposure to
ambient air or water according to the following reactions:
Ti + O
2
→ TiO
2
Ti + 2H
2
O → TiO
2
+ 4H
+
+ 4e
-
The stability of TiO
2
can be compromised only by complexing species such as HF or H
2
O
2
and can lead to its dissolution. Without those species TiO
2
is thermodynamically stable in
the wide range of pH = 2-12 (Schenk, 2001) and by this CP-Ti is totally corrosion resistant in
the presence of neutral physiological solutions. In the case of Al-containing alloys (Ti6Al4V,
Ti6Al7Nb) acidification of solutions make them more prone to dissolution than CP-Ti
(Ruzickova et al., 2005).
In present work our research concentrates on CP Titanium as on precursor of another
titanium alloys. Along excellent corrosion resistance, inertness to human body and excellent
osseointegration titanium offers more suitable properties for implantable biomaterials. Some
of those properties include: fatigue resistance, strength, density, elastic module, etc. The
density of around 4.5 g.cm
-3
makes it almost two times lighter than cobalt-chromium or
316L stainless steel alloys. As pure element CP titanium also excludes possibility of leaching
harmful elements which could be detrimental to surrounding tissues as can be in case of
316L stainless steel and Nitinol where possibility of leaching nickel is reality. The biggest
disadvantage of CP-Ti is its relatively low wear resistance and by this it should not be used
in devices where contact wear is unavoidable as for example modular interface corrosion
between Co-Cr heads and Ti stem in total hip replacement prosthesis (Singh &, Dahotre,
2007; Salvati et al., 1995).
2.1 Mechanism of passive film formation and growth
Even that the passive film on titanium is only in nanometers range it creates very protective
barrier against biological environment of human body. Its protectiveness depends on
several features: morphology, homogeneousness, thickness, kind and quantity of foreign
chemicals species incorporated in it. All of those properties of passive film determine the
tunneling rate end speed of ions moving through the film as well as its dissolution by
surrounding fluids.
According to (Cabrera & Mott, 1948) high field mechanism for oxide film formation and
growth theory the main prerequisite is adsorption of oxygen on bare titanium surface which
creates oxide monolayer (Fig. 1). The next step is electron tunneling from titanium to
monolayer of adsorb oxygen which by adding electrons became electron traps on the outer
surface of the oxide. As the number of electron traps increases the potential drop across the
films grows. The drop of potential creates the electric field across the passive film which
lowers the activation energy necessary for further ions transport though passive film. The
oxide on titanium is classified as N-type semiconductor which means that anion transport
through film is dominant way of film grow and is due to oxygen ions movement toward
titanium metal. The thickening of oxide film increases the activation energy necessary for
