Biomedical Engineering Reference
In-Depth Information
contrast, the term ''erosion'' refers often to physical
changes in size, shape, or mass of a device, which could
be the consequence of either degradation or simply dis-
solution. Thus, it is important to realize that erosion can
occur in the absence of degradation, and degradation can
occur in the absence of erosion. A sugar cube placed in
water erodes, but the sugar does not chemically degrade.
Likewise, the embrittlement of plastic when exposed to
UV light is due to the degradation of the chemical
structure of the polymer and takes place before any
physical erosion occurs.
In the context of this section, we follow the usage
suggested by the Consensus Conference of the European
Society for Biomaterials (
Williams, 1987
) and refer to
''biodegradation'' only when we wish to emphasize that
a biological agent (enzyme, cell, or microorganism) is
causing the chemical degradation of the implanted
device. After extensive discussion in the literature, it is
now widely believed that the chemical degradation of the
polymeric backbone of poly(lactic acid) (PLA) is pre-
dominantly controlled by simple hydrolysis and occurs
independently of any biological agent (Vert
et al.
, 1991).
Consequently, the degradation of PLA to lactic acid
should not be described as ''biodegradation.'' In agree-
ment with Heller's suggestion (
Heller, 1987
), we define
a ''bioerodible polymer'' as a water-insoluble polymer
that is converted under physiological conditions into
water-soluble material(s) without regard to the specific
mechanism involved in the erosion process. ''Bioerosion''
includes therefore both physical processes (such as dis-
solution) and chemical processes (such as backbone
cleavage). Here the prefix ''bio'' indicates that the erosion
occurs under physiological conditions, as opposed to
other erosion processes, caused for example by high
temperature, strong acids or bases, UV light, or weather
conditions. The terms ''bioresorption'' and ''bioabsorp-
tion'' are used interchangeably and often imply that the
polymer or its degradation products are removed by
cellular activity (e.g., phagocytosis) in a biological envi-
ronment. These terms are somewhat superfluous and
have not been clearly defined.
3.2.7 Bioresorbable
and bioerodible materials
Joachim Kohn, Sascha Abramson, and
Robert Langer
Introduction
Since a degradable implant does not have to be removed
surgically once it is no longer needed, degradable poly-
mers are of value in short-term applications that require
only the temporary presence of a device. An additional
advantage is that the use of degradable implants can
circumvent some of the problems related to the long-
term safety of permanently implanted devices. A po-
tential concern relating to the use of degradable implants
is the toxicity of the implant
0
s degradation products.
Since all of the implant
0
s degradation products are re-
leased into the body of the patient, the design of a de-
gradable implant requires careful attention to testing
for potential toxicity of the degradation products. This
section covers basic definitions relating to the process of
degradation and/or erosion, the most important types of
synthetic,
degradable polymers available today, a classifi-
cation of degradable medical implants, and a number of
considerations specific for the design and use of degrad-
able medical polymers (shelf life, sterilization, etc.).
Definitions relating to the process
of erosion and/or degradation
Currently four different terms (biodegradation, bioero-
sion, bioabsorption, and bioresorption) are being used to
indicate that a given material or device will eventually
disappear after having been introduced into a living or-
ganism. However, when reviewing the literature, no clear
distinctions in the meaning of these four terms are evi-
dent. Likewise, the meaning of the prefix ''bio'' is not well
established, leading to the often-interchangeable use of
the terms ''degradation'' and ''biodegradation,'' or ''ero-
sion'' and ''bioerosion.'' Although efforts have been made
to establish generally applicable and widely accepted
definitions for all aspects of biomaterials research
(
Williams, 1987
), there is still significant confusion even
among experienced researchers in the field as to the
correct terminology of various degradation processes.
Generally speaking, the term ''degradation'' refers to
a chemical process resulting in the cleavage of covalent
bonds. Hydrolysis is the most common chemical process
by which polymers degrade, but degradation can also
occur
Overview of currently available
degradable polymers
From the beginnings of the material sciences, the de-
velopment of highly stable materials has been a major
research challenge. Today, many polymers are available
that are virtually nondestructible in biological systems,
e.g., Teflon, Kevlar, or poly(ether ether ketone) (PEEK).
On the other hand, the development of degradable bio-
materials is a relatively new area of research. The variety
of available, degradable biomaterials is still too limited to
cover a wide enough range of diverse material properties.
via
oxidative
and
enzymatic
mechanisms.
In
