Biomedical Engineering Reference
In-Depth Information
specifications for use in humans. A surface modification
that is too complex will be difficult and expensive to
commercialize. It is best to minimize the number of steps
in a surface modification process and to design each step
to be relatively insensitive to small changes in reaction
conditions.
etc) to the surfaces of relatively inert hydrophobic
polymers. Within this category, three types of reactions
can be distinguished: grafting using ionizing radiation
sources (most commonly, a cobalt-60 or cesium-137
gamma radiation source) (Dargaville
et al.,
2003),
grafting using UV radiation (photografting) (Srinivasan
and Lazare, 1985; Matsuda and Inoue, 1990; Dunkirk
et
al.,
1991; Swanson, 1996), and grafting using high-energy
electron beams (Singh and Silverman, 1992). In all cases,
similar processes occur. The radiation breaks chemical
bonds in the material to be grafted, forming free radicals,
peroxides, or other reactive species. These reactive sur-
face groups are then exposed to a monomer. The
monomer reacts with the free radicals at the surface and
propagates as a free radical chain reaction incorporating
other monomers into a surface grafted polymer. Electron
beams and gamma radiation sources are also used for
biomedical device sterilization.
These high-energy surface modification technologies
are strongly dependent on the source energy, the radia-
tion dose rate, and the amount of the dose absorbed.
Gamma sources have energies of roughly 1 MeV (1 eV
¼
23.06 kcal/mol). Typical energies for electron beam
processing are 5-10 MeV. UV radiation sources are of
much lower energy (
<
6 eV). Radiation dose rates are low
for UV and gamma and very high for electron beams. The
amount of energy absorbed is measured in units of grays
(Gy) where 1 kilogray (kGy)
¼
1000 joules/kilogram.
Units of megarads (MR) are often used for gamma
sources; 1 MR
¼
1
10
6
ergs/gram
¼
10 kGy.
Three distinct reaction modes can be described: (a) In
the mutual irradiation method, the substrate material is
immersed in an oxygen-free solution (monomer
sol-
vent) that is then exposed to the radiation source. (b) The
substrate materials can also be exposed to the radiation
under an inert atmosphere or at low temperatures (to
stabilize free radicals). In this case, the materials are later
contacted with a monomer solution to initiate the graft
process. (c) Finally, the exposure to the radiation can take
place in air or oxygen, leading to the formation of per-
oxide groups on the surface. Heating the material to be
grafted in the presence of monomer, or addition of
a redox reactant (e.g., Fe
2
þ
) that will decompose the
peroxide groups to form free radicals, can initiate the
graft polymerization (in O
2
-free conditions).
Graft layers formed by energetic irradiation of the
substrate are often thick (
>
1
m
m) and composed of
relatively high-molecular-weight polymer chains. How-
ever, they are typically well-bonded to the substrate
material. Since many polymerizable monomers are
available, a wide range of surface chemistries can be
created. Mixtures of monomers can form unique graft
copolymers (Ratner and Hoffman, 1980). For example,
the hydrophilic/hydrophobic ratio of surfaces can be
controlled by varying the ratio of a hydrophilic and
Methods for modifying the surfaces
of materials
General methods to modify the surfaces of materials are
illustrated in
Fig. 3.2.14-1
, with many examples listed in
Table 3.2.14-2
. A few of the more widely used of these
methods will be briefly described. Some of the concep-
tually simpler methods such as solution coating of
a polymer onto a substrate or metallization by sputtering
or thermal evaporation will not be elaborated upon here.
Chemical reaction
There are hundreds of chemical reactions that can be
used to modify the chemistry of a surface. Chemical
reactions, in the context of this article, are those
performed with reagents that react with atoms or mol-
ecules at the surface, but do not overcoat those atoms or
molecules with a new layer. Chemical reactions can be
classified as nonspecific and specific.
Nonspecific reactions leave a distribution of different
functional groups at the surface. An example of a non-
specific surface chemical modification is the chromic acid
oxidation of PE surfaces. Other examples include the
corona discharge modification of materials in air; radio-
frequency glow discharge (RFGD) treatment of mate-
rials in oxygen, argon, nitrogen, carbon dioxide, or water
vapor plasmas; and the oxidation of metal surfaces to
a mixture of suboxides.
Specific chemical surface reactions change only one
functional group into another with a high yield and few
side reactions. Examples of specific chemical surface
modifications for polymers are presented in
Fig. 3.2.14-2
.
Detailed chemistries of biomolecule immobilization are
described in Section 3.2.16.
Radiation grafting and photografting
Radiation grafting and related methods have been widely
applied for the surface modification of biomaterials
starting in the late 1960s (Hoffman
et al.,
1972), and
comprehensive review articles are available (Ratner,
1980; Hoffman, 1981; Hoffman
et al.
, 1983; Stannett,
1990; Safrany, 1997). The earliest applications, particu-
larly for biomedical applications, focused on attaching
chemically reactable groups (
OH,
COOH,
NH
2
,
