Biomedical Engineering Reference
In-Depth Information
bypass machine. Polycarbonate macrodiols have also
been used to prepare copolymers such as polyurethanes.
Polycarbonate segments may confer enhanced biological
stability to a material.
Nylon is the name originally given by Du Pont to
a family of polyamides; the name is now generic, and
many other companies make nylons. Nylons are formed
by the reaction of diamines with dibasic acids or by the
ring opening polymerization of lactams. Nylons are used
as surgical sutures (see also Section 3.2.4).
talline reinforcing blocks, are composed of a diisocyanate
and a chain extender. The diisocyanates most commonly
used are 2,4-toluene diisocyanate (TDI) and methylene
di(4-phenyl isocyanate) (MDI), with MDI being used in
most biomaterials. The chain extenders are usually
shorter aliphatic glycol or diamine materials with two to
six carbon atoms. The ''soft'' blocks in polyurethanes are
typically polyether or polyester polyols whose
T
g
values
are much lower than room temperature, allowing them
to give a rubbery character to the materials. Polyether
polyols are more commonly used for implantable devices
because they are stable to hydrolysis. The polyol mo-
lecular weights tend to be on the order of 1000 to 2000.
Polyurethanes are tough elastomers with good fatigue
and blood-containing properties. They are used in pace-
maker lead insulation, catheters, vascular grafts, heart
assist balloon pumps, artificial heart bladders, and wound
dressings.
Biodegradable polymers
PLGA is a random copolymer used in resorbable surgical
sutures, drug delivery systems, and orthopedic appliances
such as fixation devices. The degradation products are
endogenous compounds (lactic and glycolic acids) and as
such are nontoxic. PLGA polymerization occurs via
a ring-opening reaction of a glycolide and a lactide, as
illustrated in
Fig. 3.2.2-6.
The presence of ester linkages
in the polymer backbone allows gradual hydrolytic deg-
radation (resorption). The rate of degradation can be
controlled by the ratio of polyglycolic acid (PGA) to PLA.
Final remarks
The chemistry, physics, and mechanics of polymeric ma-
terials are highly relevant to the performance of many
devices employed in the clinic today. Polymers represent
a broad, diverse family of materials, with mechanical
properties that make them useful in applications relating
to both soft and hard tissues. The presence of functional
groups on the backbone or side chains of a polymer
also means that they are readily modified chemically
or biochemically, especially at their surfaces. Many
researchers have successfully altered the chemical and
biological properties of polymers, by immobilizing anti-
coagulants such as heparin, proteins such as albumin for
passivation and fibronectin for cell adhesion, and cell-
receptor peptide ligands to enhance cell adhesion, greatly
expanding their range of applications (see Section 3.2.16).
Copolymers
Copolymers are another important class of biomedical
materials. A copolymer of tetrafluoroethylene with
a small amount of hexafluoropropylene (FEP Teflon) is
used as a tubing connector and catheter. FEP has a crys-
talline melting point near 265
C compared with 327
C
for PTFE. This enhances the processability of FEP com-
pared with PTFE while maintaining the excellent chem-
ical inertness and low friction characteristic of PTFE.
Polyurethanes are block copolymers containing ''hard''
and ''soft'' blocks. The ''hard'' blocks, having
T
g
values
above room temperature and acting as glassy or semicrys-
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