LASER PROCESSING OF ORTHOPEDIC BIOMATERIALS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 292

materials show a clear active to passive transition at E Corr values close to 0 V. There
is no signifi cant difference in the free potential values, E corr for these materials. In
contrast to testing in 0.1 N HCl, in Ringers Solution none of the three materials,
Ti-6Al-4V, TNZT, or TNZT + 2B, exhibited any breakdown of the protective
oxide fi lm formed on the surface even at potentials as high as 5V. The critical
current density value for TNZT alloy ( ∼ 17 uA/cm 2 ) is marginally higher than

that observed for Ti - 6Al - 4V ( 15.36 uA/

cm 2 ) exhibited similar critical current densities. The anodic polarization curve
shows no breakdown suggesting the formation of a very stable passive oxide
fi lm on the surface. Thus, it can be inferred that all the three materials are highly
corrosion resistant in Ringers solution. The typical values of E tp or E br , (break-
down potential) reported under similar conditions of testing in Ringer's solution,
for Stainless Steels and Co-Cr-Mo alloys are

∼ 14 uA/cm 2 ). Ti - 6Al - 4V and TNZT + 2B ( ∼

300 - 500 mV, which can easily
be reached in the body in the case of an infl ammation [35] . The substantially
higher values (

∼

5 V) of E tp for Ti - 6Al - 4V, TNZT and TNZT + 2B indicate
that these materials have superior corrosion resistance in simulated body fl uid
conditions.

>

9.3.3.5 XPS Studies of the Passive Oxide Film on LENS™ Deposited
TNZT. The superior biocompatibility and corrosion resistance of titanium alloys
are attributed to the stable and dense passive oxide fi lm that forms on the surface
in the presence of an oxidizing media. The bio-adhesion of titanium is achieved
by free OH - groups that are available in the pH region from 2.9 to 12.7 on the
surface of the oxide layer. These groups react with the bio molecules and are
infl uenced by the chemical and biological characteristics of the surface fi lm. The
interaction of the surface oxide fi lm with the bio molecules from the body fl uids
generates a fi lm called the bio fi lm. The host body initiates a series of reactions
in response to this bio fi lm thereby enabling the development of the interface
between the implant and the surrounding tissue.
Typical hostile bio-responses after implant surgery include the buildup of
monocytes and macrophages (after one day), formation of granulation tissue con-
taining fi broblasts and type III collagens (after fi ve days) and fi nally attack of
foreign body giant cells. Thus, from the perspective of biocompatibility, it is
important to develop a better understanding of the properties of the oxide fi lm
forming on the surface of the implant alloy. These properties include composition,
thickness and the structure of the oxide layer. Surface analytical techniques, such
as x-ray photoelectron spectroscopy (XPS), can be effectively used to probe the
thin surface oxide layers, created by anodic polarization tests on these LENS™
deposited TNZT alloys [36]. There have been several studies related to the oxide
layer forming on electrochemically tested (or treated) Ti-6Al-4V alloys. In most
cases, the results indicate that the surface oxide layer on Ti-6Al-4V comprises
mainly of TiO 2 with small amounts of sub oxides TiO and Ti 2 O 3 . Hence, in the
present case, a detailed XPS analysis of the surface oxide layer formed on LENS™
deposited TNZT has been carried out.