Biomedical Engineering Reference
In-Depth Information
With regard to hardness and modulus enhancement, our nanoindentation results show that the
treated surfaces possess higher Young's modulus and hardness than the untreated control surface.
Hence, the surface mechanical properties of the treated samples are enhanced. The modifi ed
surfaces not only possess better corrosion resistance, but also are capable of resisting mechanical
shocks. The effi cacy of using PIII to strengthen the materials, surface mechanical properties such
as hardness and elastic modulus [141] depends on the amorphous matrix composition and the size
of precipitates [142]. Additionally, the corrosion resistance seems to be directly proportional to
the surface conditions of metals. For instance, smooth surfaces usually give rise to higher cor-
rosion resistance. A crack-free surface is always advantageous because of the reduced chance of
localization of corrosive agent. Chemically inert materials such as metal oxides, nitride, or car-
bide can effectively reduce the permeability of the corrosive agent [143]. The wetting properties
also govern the anticorrosion capability of a material.
Compared to the other treated and untreated NiTi alloys, the
in vitro
cell culture study indicates
that the NiTi alloy after nitrogen implantation exhibits good biocompatibility. The cell prolifera-
tion rate on nitrogen-treated surfaces appears to be as good if not better than untreated NiTi alloy
at the later time points [99,102,103,106,109]. This fi nding can be explained by Piscanec's study
[144]. They reported the growth of the calcium phosphate phase on TiN-coated titanium implants,
but no such activities were observed on the untreated titanium implants. The surface composition
analysis revealed that this layer consisted of mixed precipitates of TiO
x
N
y
oxynitride. This layer
promoted the deposition of Ca ions because of negative charges localized on the surface after the
surface treatment. Therefore, this coating was favorable to the formation of bone-like materials
under
in vivo
conditions. It was believed that the TiO
x
N
y
oxynitride layer also existed on the nitro-
gen-implanted NiTi alloy.
19.5
SURFACE MODIFICATION OF BLOOD-CONTACTING MATERIALS
19.5.1 DLC T
HIN
F
ILMS
Diamond like carbon (DLC) thin fi lms have been widely studied and used for many industrial
purposes in the past 20 years due to their superior chemical, optical, electrical, and tribological
properties. Their particularly favorable attributes include low friction coeffi cient, high hardness,
and high wear resistance. They have been commercially used as the coating materials in many
applications, for instance, shavers, cutting and drilling tools, protective coating for magnetic
media and optical lenses. However, there has also been a lot of interest in DLC as biomedical
coating materials since the materials not only possess excellent chemical and mechanical proper-
ties, but also are in general biocompatible [145-147]. Conventionally, metals or metal alloys are
the dominant materials in medical implants, but relatively poor wear and corrosion resistance and
inadequate biocompatibility are the drawbacks of some metallic materials. Therefore, DLC has
been suggested to be a replacement in some applications. However, entire substitution of most
medical devices with DLC is not practical. Hence, surface modifi cation approach such as coating
techniques is one of the potential solutions.
Initial studies of DLC in biomedical applications are mainly in the orthopedic fi eld such as
surface modifi cation of artifi cial total hip and knee joints [148-151]. Most studies show that the
DLC fi lms are biocompatible, and the most important fact is that the tribological properties can
be greatly improved by DLC fi lms. However, in the cardiovascular fi eld, there are fewer reports
in the literature. Referring to the commercial products of cardiovascular devices such as artifi cial
heart valves, rotary heart pumps, and stents, the crucial requirement of biocompatibility is the
bioinertness to blood. In other words, the implants must avoid the occurrence of thrombogenesis.
In practice, the hemocompatibility of the devices is not adequate and patients should continuously
take anticoagulation medication after receiving implants. Hence, the development of a material with
