Biomedical Engineering Reference
In-Depth Information
21.5.1 Substrates
The authors have been working in the area of medical devices, especially cardio-
vascular devices, for more than two decades, which included products such as
artifi cial heart valves, oxygenators and other cardio pulmonary support devices
(Muraleedharan et al., 2006). As part of these development activities, many
ma-terials such as cobalt-based alloys, titanium and its alloys, and ultra-high
molecular-weight polyethylene (UHMWPE) were evaluated for their physico-
chemical and biological behavior. Based on this experience, commercially-pure
titanium (conforming to ISO 5832—Implants for surgery—metallic materials
Part 2: Unalloyed titanium) and UHMWPE were selected as substrates.
The titanium samples were machined from annealed rods and polished using
a multi-stage process to obtain an average surface fi nish better than 0.2 micron Ra
(Kalyani Nair et al., 2003). UHMWPE samples were machined from extruded
rods and then polished using a solid state compaction process to achieve similar
levels of surface fi nish. These samples were cleaned using neutral soap in an ultra-
sonic cleaner prior to the coating process.
21.5.2 DLC Coating Process
DLC deposition was done on Ti and UHMWPE substrates using plasma-
enhanced chemical vapor deposition (PECVD) technique (M/s. ICC, Le Mée Sur
Seine, France). Prior to the coating, the samples underwent an ionic etching
process using argon plasma in order to improve the adhesion of the coating on
the substrate (Hauert, 2003). This ionic etching process considerably increases the
surface energy and liberates the chemical liaison allowing an increase in the adher-
ence. A mixture of methane and hydrogen is used as precursor and the process
parameters are adjusted to get a coating of thickness between 1
μ
m to 5
μ
m .
21.6 TITANIUM NITRIDE
Titanium Nitride (also known as Tinite, TiN) is a hard ceramic material, often
used as a coating on metal alloys to improve the substrate's surface properties. It
has low coeffi cient of friction and corrosion rates and hence is used in applica-
tions where friction, wear, and erosion are important design considerations. Tita-
nium nitride is extensively used in medical applications, especially as orthopedic
and dental biomaterials (Wisbey et al., 1987).
Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), and
ion implantation are the most widely employed techniques for deposition of TiN
coatings. PVD is a vapor deposition process either carried out using vacuum
evaporation techniques or using sputtering. In sputtering, a source (cathode) is
bombarded in a high vacuum with high energy inert gas ions producing a glow
discharge or plasma. Atoms from these are ejected and accelerated using external
bias towards the substrate to form the coating.
Search WWH ::
Custom Search