Biomedical Engineering Reference
In-Depth Information
electrophoresis, or PAGE). The mechanical properties
and the degree of swelling can be controlled by cross-
linking with methylene-bis-acrylamide (MBA). Poly(N-
alkylacrylamides) are environmentally sensitive, and the
degree of swelling can be altered by changes in tempera-
ture and acidity. These polymers are discussed in more
detail in Section 3.2.6 and; see also
Hoffman (1997)
.
Polyacrylic acids also have applications in medicine.
They are used as dental cements, e.g., as glass ionomers.
In this use, they are usually mixed with inorganic salts,
where the cation interacts with the carboxyl groups of
the acid to form physical cross-links. Polyacrylic acid is
also used in a covalently cross-linked form as a mucoad-
hesive additive to mucosal drug delivery formulations.
Polymethacrylic acid may also be incorporated in small
quantities into contact lens polymer formulations to
improve wettability.
PE is used in its high-density form in biomedical ap-
plications because low-density material cannot withstand
sterilization temperatures. It is used as tubing for drains
and catheters, and in ultrahigh-MW form as the acetab-
ular component in artificial hips and other prosthetic
joints. The material has good toughness and wear re-
sistance and is also resistant to lipid absorption. Radiation
sterilization in an inert atmosphere may also provide
some covalent cross-linking that strengthens the PE.
PP is an isotactic crystalline polymer with high rigidity,
good chemical resistance, and good tensile strength. Its
stress cracking resistance is excellent, and it is used for
sutures and hernia repair.
PTFE, also known as PTFE Teflon, has the same
structure as PE, except that the four hydrogens in the
repeat unit of PE are replaced by fluorines. PTFE is a very
high melting polymer (
T
m
¼
327
C) and as a result it is
very difficult to process. It is very hydrophobic, has ex-
cellent lubricity, and is used to make catheters. In mi-
croporous form, known generically as e-PTFE or most
commonly as the commercial product Gore-Tex, it is
used in vascular grafts. Because of its low friction, it was
the original choice by Dr. John Charnley for the acetab-
ular component of the first hip joint prosthesis, but it
failed because of its low wear resistance and the resultant
inflammation caused by the PTFE wear particles.
PVC is used mainly as tubing and blood storage bags in
biomedical applications. Typical tubing uses include blood
transfusion, feeding, and dialysis. Pure PVC is a hard,
brittle material, but with the addition of plasticizers, it can
be made flexible and soft. PVC can pose problems for
long-term applications because the plasticizers can be
extracted by the body. While these plasticizers have low
toxicities, their loss also makes the PVC less flexible.
Poly(dimethyl siloxane) (PDMS) or SR is an extremely
versatile polymer, although its use is often limited by
its relatively poor mechanical strength. It is unique in that
it has a silicon-oxygen backbone instead of a carbon
backbone. Its properties are less temperature sensitive
than other rubbers because of its very low
T
g
. In order to
improve mechanical properties, SR is usually formulated
with reinforcing silica filler, and sometimes the poly-
siloxane backbone is also modified with aromatic rings
that can toughen it. Because of its excellent flexibility and
stability, SR is used in a variety of prostheses such as finger
joints, heart valves, and breast implants, and in ear, chin,
and nose reconstruction. It is also used as catheter and
drainage tubing and in insulation for pacemaker leads. It
has also been used in membrane oxygenators because of its
high oxygen permeability, although porous PP or poly-
sulfone polymers have recently become more used as
oxygenator membranes. Silicones are so important in
medicine that details on their chemistry are provided in
Section 3.2.3.
PET is one of the highest volume polymeric biomateri-
als. It is a polyester, containing rigid aromatic rings in
a ''regular'' polymer backbone, which produces a high-
melting (
T
m
¼
267
C) crystalline polymer with very high
tensile strength. It may be fabricated in the forms of knit,
velour, or woven fabrics and fabric tubes, and also as non-
woven felts. Dacron is a common commercial form of PET
used in large-diameter knit, velour, or woven arterial grafts.
Other uses of PET fabrics are for the fixation of implants
and hernia repair. PET can also be used in ligament re-
construction and as a reinforcing fabric for tissue re-
construction with soft polymers such as SR. It is used in
a nonwoven felt coating on the peritoneal dialysis shunt
(where it enters the body and traverses the skin) to enhance
ingrowth and thereby reduce the possibility of infection.
PEG is used in drug delivery as conjugates with low
solubility drugs and with immunogenic or fairly unstable
protein drugs, to enhance the circulation times and sta-
bilities of the drugs. It is also used as PEG-phospholipid
conjugates to enhance the stability and circulation time
of drug-containing liposomes. In both cases it serves to
''hide'' the circulating drug system from immune recog-
nition, especially in the liver. PEG has also been immo-
bilized on polymeric biomaterial surfaces to make them
''nonfouling.'' PEGs usually exist in a highly hydrated
state on the polymer surfaces, where they can exhibit
steric repulsion based on an osmotic or entropic mech-
anism. This phenomenon contributes to the protein- and
cell-resistant properties of surfaces containing PEGs
(See Section 3.2.13).
Regenerated cellulose, for many years, was the most
widely used dialysis membrane. Derivatives of cellulose,
such as CA, are also used, since CA can be melt
processed as hollow fibers for the hollow fiber kidney
dialyser. CA is also used in osmotic drug delivery devices.
Polymerization of bisphenol A and phosgene produces
polycarbonate, a clear, tough material. Its high impact
strength dictates its use as lenses for eyeglasses and safety
glasses, and housings for oxygenators and heart-lung
