Biomedical Engineering Reference
In-Depth Information
accompanies a new medical device with high
value in end use that can substantiate the high
cost of research and development, testing, and
building of new manufacturing lines.
An example is furnished by polyurethanes
widely used in catheter manufacturing. It was
discovered that certain polyurethane formula-
tions just happen to work well as catheters. This
is not to say that all polyurethanes are biocompat-
ible or that existing polyurethane catheters are
perfect [36] . Certainly, academic and some indus-
trial researchers have invested considerable effort
in improving polyurethane biocompatibility and
to understand why these materials happen to
exhibit these excellent properties [37] . But because
the original aromatic polyurethane formulations
Pellethane , Biomer , and Tecoflex, introduced more
than 20 years ago by the Upjohn, Ethicon, and
Thermetics corporations, respectively [38] ,
worked fairly well, these are the formulations
still in widespread use by catheter manufacturers
today. Companies and technologies have been
bought and sold, but aromatic polyurethane for-
mulations remain basically the same. Effectively,
what has happened is that trial and error led to
polyurethane formulations that meet basic medi-
cal and manufacturing requirements. The success
of these imperfect materials effectively blocks
continuous improvement, and catheter develop-
ment is frozen in time.
modification to coatings, sometimes used in
combination. Surface modification is also used
as a tool to study the biological response to mate-
rials. A range of surface chemistries or energies
incrementally sampling the available range can
be prepared, allowing researchers to explore
how the biological response to materials changes
in response to these different surface properties
[19-22] . Surface modification is a very important
tool in the medical device designer's kit.
As mentioned in Section 8.1.3 , there are sev-
eral topics [4-8] and review articles [9, 10] that
supplement a substantial literature detailing
specific biomaterial surface modification tech-
niques and methods. The following subsections
neither repeat these summaries nor review
recent literature in detail. Rather, the scope of
this section is to discuss broad classes of bioma-
terial surface modification methods in the con-
text of the mechanism of biocompatibility
diagrammed in Figures 8.3 and 8.4 .
8.3.1 Wettability
One of the early discoveries in the development
of biomaterials was that water wettability (sur-
face energy) had a great effect on the biological
response. It is unclear just when and for what
purpose the first discovery of the effect of water
wettability was made, but certainly changing
the surface chemistry of materials used in cul-
turing animal cells in the laboratory, called tis-
sue culture, discussed further in Section 8.3.1.1 ,
must have been among the earliest [26] .
Altering water wettability of biomaterial
surfaces influences adsorption and adhesion—
the two most important manifestations of surface
chemistry/energy ( Box 8.3 ). Interpreted in terms
of Figure 8.4 , changing water wettability affects
the chemistry of vicinal water that guides
formation of the dynamic interphase upon
contact with a biological milieu. Exactly how
this occurs is not known. Understanding the
various molecular interactions involved and
how these molecular interactions control the
8.3 SURFACE MODIFICATION OF
BIOMATERIALS
The primary motivation behind surface modifi-
cation is that materials with desirable physical
properties, such as strength or flexibility, are fre-
quently not biocompatible due to adverse sur-
face-mediated reactions with the biological
milieu. Modifying the surface allows medical-
device designers to retain desirable physical
properties while improving biocompatibility to
a useful level. There are many methods of sur-
face modification ranging from chemical
 
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