Biomedical Engineering Reference
In-Depth Information
been investigated more in urological applications than in coronary applications. PUs have been
extensively used for medical device applications because of their excellent biocompatibility. These
polymers have been used as a coating material on stents to improve the antithrombogenic proper-
ties of Ta, corrosion resistance of Ni-Ti and biocompatibility of SS (stainless steel). It was also
reported that PU coatings can improve endothelialization. Thus, while some studies advocated the
use of PU for stents, others show contradictory evidence. Hence it is diffi cult to categorize PU as
a good or poor stent material. Although PU has been used successfully in many cardiovascular
devices (pacemaker lead wires, vascular grafts, artifi cial heart pumps, and inner surface coatings of
artifi cial heart), it does not necessarily mean that the coating may be benefi cial for stents. Fibrin, a
natural biopolymer and an insoluble protein, is known to be biocompatible and biodegradable with
viscoelastic properties
.
Fibrin has also been tested as a stent coating material with encouraging
results in a porcine model.
112
15.4.3 D
RUG
D
ELIVERY
Biodegradable polymers have been the subject of intense interest in the fi eld of controlled drug
delivery. The need for site-specifi c, targeted, and controlled release of drugs has been recognized
because it can improve bioavailability, including longer circulation time, and slower clearance,
which can decrease the systemic drug level in the body with the effi cacy of the improved drug as
breakdown of the drug is limited and patient compliance is not crucial. Controlled drug delivery is
a challenging fi eld as applications require drugs to be released either at one time, in days, weeks,
or months, or sustained release in different tissues, such as tumor sites, diseased blood vessels, and
others. Extensive research has been focused on improving and creating advanced drug delivery
systems.
115
-
117
The fact that active proteins can now be synthetically prepared and drugs can be pro-
duced by molecular biology-based techniques has stimulated the research in the fi eld of controlled
drug delivery. Furthermore, improvements in the effi cacy of the drug through the changes in the
molecular structure of the active protein molecules and the possibility of sustained release have had
a marked effect on patient health care.
Hydrogels based on drug delivery vehicles
118
are of current major interest with several products
available commercially. Degradable hydrogel systems are of potential interest in drug delivery.
West and Hubbell synthesized PLA-
b
-PEG-
b
-PLA hydrogels composed of PLA and PEGblock
copolymers for protein release applications.
119
Metters et al. developed scaling laws to predict the
degradation rates of PLA-
b
-PEG-
b
-PLA hydrogels based on macroscopic properties such as com-
pressive modulus and volumetric swelling ratio.
120,121
Controlled delivery devices are generally dif-
fusion-based release systems applicable to release of drugs intended for the systemic circulation, or
for a localized site. The basic approach developed by Langer provided the foundation of a wide vari-
ety of implantable and injectable devices made primarily from degradable materials. Drug-eluting
stents are now providing medical technology to decrease restenosis and are also able to treat more
complex lesions in coronary heart disease. For example, TAXUS paclitaxel-eluting stent in a soft,
hydrophobic, elastomeric triblock copolymer, poly(styrene-
b
-isobutylene-
b
-styrene), is an example
of this new class of product.
122
Polyurethane has also been investigated in drug delivery systems.
However, some studies have shown that PU coating can be accompanied by extensive infl ammatory
reactions.
123
15.5 CONCLUSION
Polymers are versatile materials that allow tailoring of its fi nal properties. The polymers discussed
in this chapter are typically biocompatible and are able to perform adequately for a number of
biomedical applications. Degradable polymers such as PLA, PGA, PCL, and PDS are able to offer
excellent suture materials, however these and related polymers have been relatively less successful
in providing adequate hard tissue replacement alternatives. Materials for bone reconstruction must
