Biomedical Engineering Reference
In-Depth Information
with a wide range of versatility in terms of tailoring their physical properties, blood and tissue com-
patibility, and more recently, their biodegradation character. Softer and more fl exible polyurethanes
result when linear difunctional polyethylene glycol segments, commonly called polyether polyols,
are used to create the urethane links. This strategy is used to make elastomeric fi bers and soft
rubber parts, as well as foam rubber. More rigid products result if polyfunctional polyols are used,
as these create a three-dimensional cross-linked structure, which again, can be in the form of low-
density foams. The mechanical properties, density, hardness and other physical properties, can be
modifi ed by selection of appropriate monomers.
Poyurethanes have had a signifi cant role in the development of medical devices ranging from
catheters to artifi cial heart components. Biostable medical devices have been fabricated from poly-
urethanes due to their durability, elasticity, good fatigue resistance, and biotolerance. Furthermore,
the propensity for bulk and surface modifi cations allow attachment of biologically active species
such as anticoagulants and other biomolecules, which aid in the healing process. Segmented poly-
urethanes also show low bacterial adhesion and hemocompatibility in addition to good mechanical
properties.
67,68
Polyester urethanes are hydrolytically unstable and hence are being increasingly mod-
ifi ed with silicone containing moeities to restrict the chronic
in vivo
instability. Thus, devices such as
breast implants and cardiovascular devices that require long-term biostability have caused concern.
Polyurethanes have been the subject of extensive investigation in terms of biodegradation studies
69
and as a consequence, not only has there has been a move to develop more biostable implants but also
to develop a new class of bioresorbable materials with all the versatility of PUs in terms of physical
properties and biocompatibility. The designs of biodegradable PUs have incorporated comonomers
such as lactide and glycolide, polycaprolactone units, and polyethylene oxide.
Polyurethane foam breast implants were fi rst used in 1970s as a biostable implant.
70,71
In the late
1980s, it was repor ted it hat
in vitro
degradation of polyurethane could lead to formation of substances
known to be carcinogenic in animals.
72
This information raised concerns on the potential carcino-
genic effect of polyurethane breakdown products in humans; however, with the very low risk associ-
ated, this product was not banned by the The United States Food and Drug Administration (FDA).
Subsequently, these implants were voluntarily withdrawn from the market in 1991. Polyurethane
coatings have been used to decrease bacterial adhesion on implant surfaces
73,74
and enhanced blood
compatibility has been achieved through different methods. There is a vast amount of literature on
this subject and a detailed discussion is outside the scope of this chapter.
15.3 NATURAL DEGRADABLE POLYMERS
Polysaccharides form a group of polymers that have found extensive use in biomedical application.
The nontoxicity of the polymers, their monomeric residues, facile chemical modifi cations to alter
fl uid uptake, their swellability, and a wide variety of chemical structures with excellent properties
make them a versatile group of polymers for medical application.
15.3.1 A
LGINATES
Alginic acid is an insoluble polysaccharide and the sodium, potassium, or ammonium salts are the
alginates. Alginate is a water-soluble linear polysaccharide extracted from brown seaweed and is
composed of alternating blocks of 1,4-linked α
-
l-guluronic (G) and β-d-mannuronic acid (M) resi-
dues. The structure is shown in Figure 15.9 and the binding between the two residues in the alginate
molecule. Alginates are not random copolymers and the pattern of the two residues, D and M, differ
according to the source of the algae, each of which have different conformational preferences and
behavior, for example, the M/G ratio of alginate from
Macrocystis pyrifera
is about 1.6 whereas that
from
Laminaria hyperborea
is about 0.45.
The β-d-mannuronic acid blocks and α-l-guluronic acid blocks can be arranged in different
proportions and sequences along the polymer chain and the composition; sequences of the residues
