Biomedical Engineering Reference
In-Depth Information
perfect orientation along the axis perpendicular to the planes of graphite. The structure
of HOPG is well defined than PG, and its surface has a mirror-like finish. Several forms of
HOPG have been developed [19-22].
Glassy Carbon
The possibility of using glassy carbon (GC) as an electrode material was pioneered by
Zittel and Miller [23]. Unlike PG, which is anisotropic, GC has an isotropic structure.
GC is highly resistant to chemical attack and electrically conductive. In addition, GC is
impermeable to liquids and gases. GC can be prepared in several ways. Of all the meth-
ods reported, the simplest one is reported by McBride and Evans [24]. In this method, a
GC rod of 3-mm diameter is sealed inside a 5-mm-long glass tubing with epoxy cement.
A few millimeters of carbon are then exposed from the end of the glass rod and pol-
ished with emery paper, followed by alumina, until a mirror finish is obtained. In another
method, GC can be obtained by the heat treatment of polymers of phenol or formaldehyde
or by the heat treatment of poly(acrylonitrile). Upon heating the polymers at temperatures
above 3000°C under pressure, atoms such as hydrogen, oxygen, and nitrogen are released
from the polymeric chain, leaving behind the extended, conjugated, and interwoven
sp
2
carbon structure, and this accounts for the hardness of GC. However, the original
polymeric backbone remains unaltered, and it prevents the formation of extended gra-
phitic domains. GC electrodes are reactive, and the functional groups present provide an
opportunity for modifying the electrode surface [148] to detect targeted biological and
biomedical applications.
Carbon Fibers
CFs are known for their high tensile strength and light weight. The diameter of the CFs
varies between 0.1 and 100 µm. CFs can be obtained in many ways. However, most of the
CFs used in these ways is made either by the heat treatment procedures that are used in
the preparation of GC or by the catalytic chemical vapor deposition (CVD) technique. In
the usual heat treatment method, the polymers are heated up to 3000°C. The molten poly-
mer is then woven or extruded to obtain CFs. Methods involving heat treatment above
3000°C usually result in the formation of fibers that possess a high tensile strength [8]. At
such a high temperature, carbon graphitizes and aligns its axis alongside the axis of the
fiber, which accounts for a high Young's modulus [25-27]. The microstructure of CFs varies
according to the method. Catalytic dehydrogenation of hydrocarbons on metals such as
Fe and Ni forms the basis of catalytic CVD [8]. The fibers resulting in this manner possess
onion type of structures and are stronger but shorter in length than the fibers obtained
from polymers.
Carbon Black
Carbon black (CB) is not widely used in any applications. However, there are quite a few
methods to synthesize CB. In all the methods, the basic principle involves thermal decom-
position of hydrocarbons [8, 13]. The particles of the CB vary in its size from 300 to 5000 Å
[13]. CB is a collection of many smaller microcrystallites. The surface area of the CB is on
the order of 5000 m
2
g
-1
. Unlike graphite, CB has a disordered structure that accounts for
its lower conductivity and lower mechanical strength.
© 2011 by Taylor & Francis Group, LLC
