Biomedical Engineering Reference
In-Depth Information
to have superior mechanical properties, such as high hardness, low stiction, low friction
coefficient, and low surface roughness. The material also has good chemical and biologi-
cal properties, such as resistance to chemical corrosion and biocompatibility, and thus has
attracted the attention of researchers around the world on studying and developing it for
biological and biomedical applications.
Before discussing the various biological/biomedical studies done over the years, it is
important to have an understanding on the material itself. In this section, the material,
amorphous carbon, and some useful ways of characterizing it will be covered. In addition,
an introduction on how to determine its surface characteristics will be briefly discussed.
Bonding in Amorphous Carbon
Amorphous carbon contains both sp
3
and sp
2
bonded carbon (refer to Figure 2.1). It is also
called “diamond-like carbon.” In the sp
3
configuration, as with diamond, a carbon atom's
four valence electrons are each assigned to a tetrahedrally directed sp
3
orbital with an
angle of 109.5° from each other, which makes a strong
σ
bond to an adjacent atom. In
the sp
2
configuration, as in graphite, three of the four valence electrons enter trigonally
directed sp
2
orbitals equally separated by an angle of 120°, which form
σ
bonds in a plane.
The fourth electron of the sp
2
atom lies in a
π
orbital, normal to the
σ
bonding plane. This
π
orbital forms a weaker
π
bond with a
π
orbital on one or more neighboring atoms.
Amorphous carbon bonding structures and electronic properties have been widely
studied using ab initio molecular dynamics of the first principles. Both sp
2
-rich a-C [2] and
sp
3
-rich a-C (sp
3
fraction > ~80% is popularly known as the tetrahedral amorphous carbon
or ta-C) [3] models were studied. Results have shown an increase in sp
2
hybridizations
when the temperature is increased. It is therefore apparent that in order to fabricate high-
quality amorphous carbon with properties that are near diamond (high sp
3
bondings), the
synthesis process should be carried out at room temperature or lower.
Amorphous Carbon Growth Mechanism
Lifshitz et al. [4, 5] used the Auger analysis of the depth profiles of medium energy carbon ions
incident on nickel substrates to show that the growth was subsurface and denoted the process
as “subplantation” (low-energy subsurface implantation). This process advances as follows:
(a) Penetration
Penetration of the impinging species into the subsurface layers of the substrate,
with the penetration depth and distribution depending on the mass and energy
π
sp
3
sp
2
FIGURE 2.1
sp
3
and sp
2
hybridized bonding of carbon.
