Page 333 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

310 BIOMATERIALS

Polyurethane is used as a coating for pacemaker leads and for angioplasty balloons. Silicones are
used for a variety of catheters, soft contact lenses, and foldable intraocular lenses.
This chapter begins with an overview of polymer science topics, including synthesis, structure, and
mechanical properties. The remainder of the chapter will discuss individual polymers, including their
applications and properties. The polymers are presented in the following order: water-soluble poly-
mers, gelling polymers, hydrogels, elastomers, and finally rigid polymers. These five categories are
roughly ordered from low to high modulus (i.e., high to low compliance). Water-soluble polymers in
solution do not have an elastic modulus since they are fluids, so these are presented first. In fact, most
polymers do not have a true elastic modulus since they are viscoelastic and exhibit solid and viscous
mechanical behavior, depending on the polymer structure, strain rate, and temperature.
Natural tissues are continuously repaired and remodeled to adjust to changes in the physiologic
environment. No current synthetic biomaterial or biopolymer can mimic these properties effectively.
Consequently, the ideal biomaterial or biopolymer performs the desired function, then eventually
disappears and is replaced by natural tissue. Therefore, degradable polymers are of great interest to
the biomedical engineering community. Polylactides and their copolymers are currently used as bone
screw and sutures since they have good mechanical properties and degrade by hydrolysis so that they
can, under optimum conditions, be replaced by natural tissue.
In addition to classification as water-soluble polymers, gelling polymers, hydrogels, elastomers,
and rigid polymers, polymers can also be classified as bioinert, bioerodable, and biodegradable.
Bioinert polymers are nontoxic in vivo and do not degrade significantly even over many years.
Polymers can degrade by simple chemical means or under the action of enzymes. For the purposes
of this chapter, bioerodable polymers such as polylactide are those that degrade by simple chemical
means and biodegradable are those that degrade with the help of enzymes. Most natural polymers
(proteins, polysaccharides, and polynucleotides) are biodegradable, while most synthetic degradable
polymers are bioerodable. The most common degradation reactions for bioerodable polymers are
hydrolysis and oxidation.

13.2 POLYMER SCIENCE

13.2.1 Polymer Synthesis and Structure

Polymers are frequently classified by their synthesis mechanism as either step or chain polymers.
Step polymers are formed by stepwise reactions between functional groups. Linear polymers are
formed when each monomer has two functional groups (functionality = 2). The second type of poly-
merization is chain polymerization where monomers are added one at a time to the growing polymer
chain.
Most polymerization techniques yield polymers with a distribution of polymer molecular
weights. Polymer molecular weight is of great interest since it affects mechanical, solution, and melt
properties of the polymer. Figure 13.1 shows a schematic diagram for a polymer molecular weight
distribution. Number of average molecular weight M averages the molecular weight over the num-
n
ber of molecules, while weight of average molecular weight M averages over the weight of each
w
polymer chain. Equations (13.1) and (13.2) define M and M .
w n
∑ NM
M = i i (13.1)
n
∑ N i
∑ NM 2
M = i i (13.2)
w
∑ NM i
i
where N is the number of polymer chains with molecular weight M .
i i