Biomedical Engineering Reference
In-Depth Information
Examples of polymers used in medical devices and their mechanical properties are listed in
Tables 5.1 and 5.2.
The large macromolecules of commercially useful polymers are synthesized by combin-
ing many (poly) smaller molecules (mers) in a process termed
. Polymeriza-
tion may proceed by addition (or chain reaction) polymerization, in which monomer
units are attached one at a time and then terminated, or by condensation (or step reaction)
polymerization, in which several monomer chains are combined and a by-product of the
reaction, such as water, is generated. Additives such as fillers, plasticizers, stabilizers, and
colorants typically are used in polymer synthesis to enhance the mechanical, chemical,
and physical properties. It is now known that some of the additives used in manufacturing
plastic water bottles have hormone-disrupting activity or are carcinogens, and this is an
example why polymer additives used for biomedical applications must be carefully screened
prior to being implanted in a human.
Polymers can be classified as thermoplastic or thermosetting. A thermoplastic polymer
has a linear or branched structure. As a solid it is like a bowl of spaghetti in that the chains
can slide over one other. With heating, the chains can slide more easily, and the polymer
melts or flows. Thus, thermoplastic polymers can be heated, melted, molded, and recycled.
Differences in properties can be achieved with the addition of different ligands. PVC is
more rigid than PE because the chlorine atoms are larger and tend to prevent the sliding
of one molecule over another. Polymethylmethacrylate (PMMA), as shown in Table 5.2,is
stronger, stiffer, and much more brittle than UHMWPE. In this case, two of the four hydro-
gen atoms are replaced, one with a methyl group (CH
3
) and the other with an acrylic group
(COOCH
3
). These large side-groups make sliding much more difficult, hence the increase in
strength and modulus. They also make it difficult for the molecules to orient in an orderly,
crystalline pattern. As a result of this amorphous structure, PMMA (Plexiglas
W
or Lucite
W
)
is optically transparent.
Thermoplastic polymers can also degrade in the body by undergoing a hydrolysis
reaction in which water causes a breakdown of the polymer chains. Poly(D,L-lactide-co-
glycolide) or PLGA is an example of a thermoplastic polyester that is incorporated in a
variety of Food and Drug Administration (FDA) approved biomedical devices such as
surgical sutures and orthopedic fixation screws and plates. Vicryl sutures are made of a
PLGA copolymer that is 90:10 glycolic and lactic acid, respectively. Polyglycolic acid
resorbs faster than polylactic acid, and by varying the ratio within the copolymer, the
biomaterials scientist can adjust the composition to achieve the desired degradation rate.
As PLGA degrades, chemical by-products that are naturally present in the body are released,
but they do cause a decrease in local pH that can be harmful to new tissue formation.
In contrast, a thermosetting polymer is composed of chains that are cross-linked. They do
not melt with heating but degrade. The term
polymerization
implies that there is a chemical reac-
tion, often involving heat, that results in setting in a three-dimensional cross-linked structure.
A common example is “5-minute epoxy.” When the two parts are mixed, the catalyst causes
setting and cross-linking of the epoxy. Once set, it cannot be heated and reused. The amount
of cross-linking affects the mechanical properties. A few cross-links are used to make rubber
gloves. Adding more sulfur and cross-linking produces a car tire. Even more cross-links are
added to make the hard casing of a car battery.
thermoset
