Biomedical Engineering Reference
In-Depth Information
and fibers, as coatings on another substrate, and into
more complex geometries and foams.
It is important to realize that the presence of
processing and functional aids can affect other properties
of a polymer. For example, plasticisers are added to rigid
PVC to produce a softer material, e.g., for use as dialysis
tubing and blood storage bags. But additives such as
plasticizers and mold release agents may alter the surface
properties of the material, where the tissues come into
contact with the polymer, and may also be extracted into
body fluids.
Prior to use, materials must also be sterilized. Agents
used to reduce the chances of clinical infection include,
steam, dry heat, chemicals, and irradiation. Exposing
polymers to heat or ionizing radiation may affect the
properties of the polymer, by chain scission or creating
cross-links. Chemical agents such as ethylene oxide may
also be absorbed by a material and later could be released
into the body. Therefore, devices sterilized with ethylene
oxide require a period of time following sterilization for
any residues to be released before use.
T
g
T
c
T
m
Temperature
Fig. 3.2.2-15 DSC thermogram of a semicrystalline polymer,
showing the glass transition temperature (T
g
), the crystallization
temperature (T
c
), and the melting temperature (T
m
) of the polymer
sample.
Fig. 3.2.2-15.
The areas under the peaks can be quanti-
tatively related to enthalpic changes.
Surface characterization
Polymeric biomaterials
Surface characteristics of polymers for biomedical ap-
plications are critically important. The surface composi-
tion is inevitably different from the bulk, and the surface
of the material is generally all that is contacted by the
body. The main surface characterization techniques for
polymers are X-ray photoelectron spectroscopy (XPS),
contact angle measurements, attenuated total reflectance
Fourier transform infrared (ATR-FTIR) spectroscopy,
and scanning electron microscopy (SEM). The tech-
niques are discussed in detail in Section 3.1.4.
PMMA is a hydrophobic, linear chain polymer that is
transparent, amorphous, and glassy at room temperature
and may be more easily recognized by such trade names
as Lucite or Plexiglas. It is a major ingredient in bone
cement for orthopedic implants. In addition to toughness
and stability, it has excellent light transmittance, making
it a good material for intraocular lenses (IOLs) and hard
contact lenses. The monomers are polymerized in the
shape of a rod from which buttons are cut. The button or
disk is then mounted on a lathe, and the posterior and
anterior surfaces machined to produce a lens with de-
fined optical power. Lenses can also be fabricated by melt
processing, compression molding, or casting, but lathe
machining methods are most commonly used.
Soft contact lenses are made from the same methacry-
late family of polymers. The substitution of the methyl
ester group in methylmethacrylate with a hydroxyethyl
group (2-hydroxyethyl methacrylate or HEMA) pro-
duces a very hydrophilic polymer. For soft contact lenses,
the poly(HEMA) is slightly cross-linked with ethylene
glycol dimethyacrylate (EGDMA) to retain dimensional
stability for its use as a lens. Fully hydrated, it is a swollen
hydrogel. PHEMA is glassy when dried, and therefore,
soft lenses are manufactured in the same way as hard
lenses; however, for the soft lens a swelling factor must
be included when defining the optical specifications. This
class of hydrogel polymers is discussed in more detail in
Section 3.2.5.
Polyacrylamide is another hydrogel polymer that is
used in biomedical separations (e.g., polyacrylamide gel
Fabrication and processing
Before a polymer can be employed usefully in a medical
device, the material must be manipulated physically,
thermally, or mechanically into the desired shape. This
can be achieved using the high-molecular-weight poly-
mer at the start of the process and may require additives
in the material to aid processing, or the end use. Such
additives can include antioxidants, UV stabilizers, rein-
forcing
fillers,
lubricants,
mold
release
agents,
and
plasticizers.
Alternatively, polymer products can be fabricated into
end-use shapes starting from the monomers or low-
molecular-weight prepolymers. In such processes, the
final polymerization step is carried out once the pre-
cursors are in a casting or molding device, yielding a solid,
shaped end product. A typical example is PMMA dental
or bone cement, which is cured
in situ
in the body.
Polymers can be fabricated into sheets, films, rods, tubes,
