Biomedical Engineering Reference
In-Depth Information
Thus, the design and synthesis of new, degradable bio-
materials is currently an important research challenge, in
particular within the context of tissue engineering where
the development of new biomaterials that can provide
predetermined and controlled cellular responses is
a critically needed component of most practical appli-
cations of tissue engineering (
James and Kohn, 1996
).
Degradable materials must fulfill more stringent re-
quirements in terms of their biocompatibility than non-
degradable materials. In addition to the potential
problem of toxic contaminants leaching from the implant
(residual monomers, stabilizers, polymerization initia-
tors, emulsifiers, sterilization by-products), one must
also consider the potential toxicity of the degradation
products and subsequent metabolites. The practical
consequence of this consideration is that only a limited
number of nontoxic, monomeric starting materials have
been successfully applied to the preparation of degrad-
able biomaterials.
Over the past decade dozens of hydrolytically un-
stable polymers have been suggested as degradable bio-
materials; however, in most cases no attempts have been
made to develop these new materials for specific medical
applications. Thus, detailed toxicological studies
in vivo,
investigations of degradation rate and mechanism, and
careful evaluations of the physicomechanical properties
have so far been published for only a very small fraction
of those polymers. An even smaller number of synthetic,
degradable polymers have so far been used in medical
implants and devices that gained approval by the Food
and Drug Administration (FDA) for use in patients. Note
that the FDA does not approve polymers or materials per
se, but only specific devices or implants. As of 1999, only
five distinct synthetic, degradable polymers have been
approved for use in a narrow range of clinical applica-
tions. These polymers are PLA, PGA, polydioxanone
(PDS), polycaprolactone (PCL), and a poly(PCPP-SA
anhydride) (see later discussion). A variety of other
synthetic, degradable biomaterials currently in clinical
use are blends or copolymers of these base materials such
as a wide range of copolymers of lactic and glycolic acid.
Note that this listing does not include polymers derived
from animal sources such as collagen, gelatin, or hyaloronic
acid.
Recent research has led to a number of well-established
investigational polymers that may find practical applica-
tions as degradable implants within the next decade. It is
beyond the scope of this section to fully introduce all of
the polymers and their applications under investigation,
thus only representative examples of these polymers are
described here. This section will concern itself mostly
with
synthetic
degradable polymers, as
natural
polymers
(e.g., polymers derived from animal or plant sources)
are described elsewhere.
Table 3.2.7-1
provides an
overview of some representative degradable polymers.
Table 3.2.7-1 Degradable polymers and representative applications
under investigation
Degradable polymer
Current major research
applications
Synthetic degradable polyesters
Poly(glycolic acid), poly(lactic
acid), and copolymers
Barrier membranes, drug
delivery, guided tissue
regeneration (in dental
applications), orthopedic
applications, stents, staples,
sutures, tissue engineering
Polyhydroxybutyrate (PHB),
polyhydroxyvalerate (PHV), and
copolymers thereof
Long-term drug delivery,
orthopedic applications, stents,
sutures
Polycaprolactone
Long-term drug delivery,
orthopedic applications, staples,
stents
Polydioxanone
Fracture fixation in non-load-
bearing bones, sutures,
wound clip
Other synthetic degradable polymers
Polyanhydrides
Drug delivery
Polycyanoacrylates
Adhesives, drug delivery
Poly(amino acids) and
''pseudo''-Poly(amino acids)
Drug delivery, tissue
engineering, orthopedic
applications
Poly(ortho ester)
Drug delivery, stents
Polyphosphazenes
Blood contacting devices, drug
delivery, skeletal reconstruction
Poly(propylene fumarate)
Orthopedic applications
Some natural resorbable polymers
Collagen
Artificial skin, coatings to
improve cellular adhesion,
drug delivery, guided tissue
regeneration in dental
applications, orthopedic
applications, soft tissue
augmentation, tissue
engineering, scaffold for
reconstruction of blood vessels,
wound closure
Fibrinogen and fibrin
Tissue sealant
Gelatin
Capsule coating for oral drug
delivery, hemorrhage arrester
Cellulose
Adhesion barrier, hemostat
Various polysaccharides such as
chitosan, alginate
Drug delivery, encapsulation of
cells, sutures, wound dressings
Starch and amylose
Drug delivery
