Biomedical Engineering Reference
In-Depth Information
cardiovascular implants. Polymers represent the largest
class of biomaterials. In this section, we will consider the
main types of polymers, their characterization, and
common medical applications. Polymers may be derived
from
Molecular weight
In polymer synthesis, a polymer is usually produced with
a distribution of molecular weights. To compare the
molecular weights of two different batches of polymer, it
is useful to define an average molecular weight. Two
statistically useful definitions of molecular weight are the
number average and weight average molecular weights.
The number average molecular weight (
M
n
) is the first
moment of the molecular weight distribution and is
an average over the number of molecules. The weight
average molecular weight (
M
w
) is the second moment of
the molecular weight distribution and is an average over
the weight of each polymer chain. Equations 3.2.2.1 and
3.2.2.2 define the two averages:
M
n
¼
P
N
i
M
i
natural
sources,
or
from
synthetic
organic
processes.
The wide variety of natural polymers relevant to the
field of biomaterials includes plant materials such as
cellulose, sodium alginate, and natural rubber, animal
materials such as tissue-based heart valves and sutures,
collagen, glycosaminoglycans (GAGs), heparin, and
hyaluronic acid, and other natural materials such as
deoxyribonucleic acid (DNA), the genetic material of all
living creatures. Although these polymers are un-
doubtedly important and have seen widespread use in
numerous applications, they are sometimes eclipsed
by the seemingly endless variety of synthetic poly-
mers that are available today. Synthetic polymeric bio-
materials range from hydrophobic, non-water-absorbing
materials such as SR, polyethylene (PE), polypropylene
(PP), poly(ethylene terephthalate) (PET), polytetra-
fluoroethylene (PTFE), and poly(methyl methacrylate)
(PMMA) to somewhat more polar materials such as
poly(vinyl chloride) (PVC), copoly(lactic-glycolic acid)
(PLGA), and nylons, to water-swelling materials such
as poly(hydroxyethyl methacrylate) (PHEMA) and
beyond, to water-soluble materials such as poly(ethy-
lene glycol) (PEG or PEO). Some are hydrolytically
unstable and degrade in the body while others may
remain essentially unchanged for the lifetime of the
patient.
Both natural and synthetic polymers are long-chain
molecules that consist of a large number of small re-
peating units. In synthetic polymers, the chemistry of the
repeat units differs from the small molecules (mono-
mers) that were used in the original synthesis pro-
cedures, resulting from either a loss of unsaturation or
the elimination of a small molecule such as water or HCl
during polymerization. The exact difference between the
monomer and the repeat unit depends on the mode of
polymerization, as discussed later.
The task of the biomedical engineer is to select a bio-
material with properties that most closely match those
required for a particular application. Because polymers
are long-chain molecules, their properties tend to be
more complex than those of their short-chain precursors.
Thus, in order to choose a polymer type for a particular
application, the unusual properties of polymers must be
understood.
This section introduces the concepts of polymer syn-
thesis, characterization, and property testing as they are
relevant to the eventual application of a polymer as
a biomaterial. Following this, examples of polymeric
biomaterials currently used by the medical community
are cited and their properties and uses are discussed.
P
N
i
(3.2.2.1)
M
w
¼
P
N
i
M
i
P
N
i
M
i
(3.2.2.2)
where
N
i
is the number of moles of species
i
, and
M
i
is
the molecular weight of species
i
.
The ratio of
M
w
to
M
n
is known as the polydispersity
index (PI) and is used as a measure of the breadth of the
molecular weight distribution. Typical commercial poly-
mers have polydispersity indices of 1.5-50, although
polymers with polydispersity indices of less than 1.1 can
be synthesized with special techniques. A molecular
weight distribution for a typical polymer is shown in
Fig. 3.2.2-1.
Linear polymers used for biomedical applications
generally have
M
n
in the range of 25,000 to 100,000 and
M
w
from 50,000 to 300,000, and in exceptional cases,
such as the PE used in the hip joint, the
M
w
may range up
to a million. Higher or lower molecular weights may be
necessary, depending on the ability of the polymer chains
Fig. 3.2.2-1 Typical molecular weight distribution of a polymer.
