Biomedical Engineering Reference
In-Depth Information
semicrystalline
L
-PLA is preferred in applications where
high mechanical strength and toughness are required,
such as sutures and orthopedic devices.
PLA and PGA and their copolymers have been in-
vestigated for more applications than any other degradable
polymer. The high interest in these materials is based, not
on their superior materials properties, but mostly on the
fact that these polymers have already been used suc-
cessfully in a number of approved medical implants and
are considered safe, nontoxic, and bio-compatible by
regulatory agencies in virtually all developed countries.
Therefore, implantable devices prepared from PLA,
PGA, or their copolymers can be brought to market in
less time and for a lower cost than similar devices pre-
pared from novel polymers whose biocompatibility is still
unproven.
Currently available and approved products include
sutures, GTR membranes for dentistry, bone pins, and
implantable drug delivery systems. The polymers are also
being widely investigated in the design of vascular and
urological stents and skin substitutes, and as scaffolds for
tissue engineering and tissue reconstruction. In many of
these applications, PLA, PGA, and their copolymers have
performed with moderate to high degrees of success.
However, there are still unresolved issues: First, in tissue
culture experiments, most cells do not attach to PLA or
PGA surfaces and do not grow as vigorously as on the
surface of other materials, indicating that these polymers
are actually poor substrates for cell growth
in vitro.
The
significance of this finding for the use of PLA and
PGA as tissue engineering scaffolds
in vivo
is currently
a topic of debate. Second, the degradation products of
PLA and PGA are relatively strong acids (lactic acid
and glycolic acid). When these degradation products
accumulate at the implant site, a delayed inflammatory
response is often observed months to years after im-
plantation (Bergsma
et al.,
1995; Athanasiou
et al.,
1998;
T¨rm¨l¨
et al
., 1998).
Table 3.2.7-2 Some ''short-term'' medical applications of
degradable polymeric biomaterials
Application
Comment
Sutures
The earliest, successful
application of synthetic
degradable polymers in human
medicine.
Drug delivery devices
One of the most widely
investigated medical
applications for degradable
polymers.
Orthopedic fixation devices
Requires polymers of
exceptionally high mechanical
strength and stiffness.
Adhesion prevention
Requires polymers that can form
soft membranes or films.
Temporary vascular grafts and
stents made of degradable
polymers
Only investigational devices are
presently available. Blood
compatibility is a major concern.
Tissue engineering or guided
tissue regeneration scaffold
Attempts to recreate or improve
native tissue function using
degradable scaffolds
A
temporary support device
is used in those circum-
stances in which the natural tissue bed has been weak-
ened by disease, injury, or surgery and requires some
artificial support. A healing wound, a broken bone, or
a damaged blood vessel are examples of such situations.
Sutures, bone fixation devices (e.g., bone nails, screws, or
plates), and vascular grafts would be examples of the
corresponding support devices. In all of these instances,
the degradable implant would provide temporary, me-
chanical support until the natural tissue heals and regains
its strength. In order for a temporary support device to
work properly, a gradual stress transfer should occur: As
the natural tissue heals, the degradable implant should
gradually weaken. The need to adjust the degradation
rate of the temporary support device to the healing of the
surrounding tissue represents one of the major challenges
in the design of such devices.
Currently, sutures represent the most successful ex-
ample of a temporary support device. The first synthetic,
degradable sutures were made of PGA and became
available under the trade name ''Dexon'' in 1970. This
represented the first routine use of a degradable polymer
in a major clinical application (
Frazza and Schmitt,
1971
). Later copolymers of PGA and PLA were de-
veloped. The widely used ''Vicryl'' suture, for example, is
a 90:10 copolymer of PGA/PLA, introduced into the
market in 1974. Sutures made of PDS became available
in the United States in 1981. In spite of extensive
Applications of synthetic,
degradable polymers
as biomaterials
Classification of degradable medical
implants
Some typical short-term applications of biodegradable
polymers are listed in
Table 3.2.7-2
. From a practical
perspective, it is convenient to distinguish between five
main types of degradable implants: the temporary sup-
port device, the temporary barrier, the drug delivery
device, the tissue engineering scaffold, and the multi-
functional implant.
