|
Abstract
Biodegradable polymers are degraded in the biologically
assisted degradation process. They offer many advantages
over other materials because of their bio absorbable and
resorbable nature in biological systems. In spite of their
such advantages they are drawing lot of criticism since
their degradation products are not always free from
effects which are responsible for many biological
disorder. Just for example, some types of polymers have
been extensively used in surgical application for last few
decades as suture materials. But they have been found to
cause problems since they were typically made from a
single polymer system. They either degrade too quickly and
reacts with tissues or take long time to degrade and thus
offer no advantages over other conventional material like
metal. Therefore, in order to avoid such problem necessity
of making tailored polymer structure is recently felt.
INTRODUCTION
The most common bio-degradable polymers previously used in
orthopedics were created from L lactide, D lactide and
glycolide, L lactide, D lactide Glycolide and Tri-methylene
carbonate (TMC) are based on single monomer units (Fig.1)
Fig. 1 Monomer Units
They found to cause problem when used as implant
materials. The degradation of PLA and PGA generally
involves random hydrolysis of their ester bonds. PLA
degrades to form lactic acid which is normally present in
the body. This acid then enter tri carboxylic acid and
excreted as water and carbon dioxide. No significant
amount of accumulation or degradation product on PLA have
been reported in any of the vital organs, but the
degradation rate depends on manufacturers configurational
structure, co polymer ratio, crystallinity, molecular
weight, morphology, stresses, amount or residual monomer,
porosity and site of implantation.
Carbon (C13) labelled PLA has demonstrated little
radioactivity in feces or urine indicating that most of
the degradation products are released through respiration.
Concerns about the biocompatibility of these materials
have been raised when PLA produced toxic solutions
probably as a result of acidic degradation. Another
concern is the release of small particles during
degradation which can trigger an inflammatory response.
Poly (glycolic acid) (PGA) is a rigid thermoplastic
materials with high crystallinity (46-50%) The glass
transition temperatures of PGA are 36 and 2250C. Because
of high crystallinity PGA is not soluble in most organic
solvent, the exceptions are highly fluorinated organic
solvents such as hexefluoro isopropanol.
Although common processing techniques such as extrusion,
injection and compression moulding can be used to
fabricate PGA into various forms, its high sensitivity to
hydrolytic degradation requires careful control of
processing conditions. Porous scaffolds and foams can be
also fabricated from PGA, but the properties and
degradation characteristics are affected by the type of
processing technique. Solvent casting, particular
bleaching method and compression moulding are also used to
fabricate PGA based implants. |
The preferred method for
preparing high molecular weight PGA is ring opening
polymerization of glycolide, the cyclic dimmer of glycolic
acid and both solution and melt polymerization can be
used. Although it is possible to synthesis these polymers
by acid catalysed polycondonsation of respective acids,
the resulting polymers generally have a low molecular
weight and often poor mechanical properties. The
attractiveness of PGA as a biodegradable polymer in
medical application is that its degradation product
glycolic acid is a natural metabolite. A major application
of PGA is in resorbable sutures.
Numerous studies have
established a simple, degradation mechanism via
homogeneous erosion. The degradation process occurs in two
stages, the first involve the diffusion or water into the
amorphous regions of the matrix and simple hydrolytic
chain session of ester groups. The second stage of
degradation involves largely the crystalline area of the
polymer, which becomes predominant when the majority of
the amorphous regions have been eroded.
In a study of Dexon sutures in vitro the first stage
degradation predominates during the first 21 days and a
further 28 days for the degradation of the crystalline
regions, after 49 days, the reported weight loss is around
42% with complete loss of mechanical properties. Although
the degradation product glycolic acid is resorbable at
high concentration, they can cause an increase of
localized acid concentration resulting in tissue damage.
The ultimate fate of glycolic acid in vivo is
considered to be conversion to carbon dioxide and water,
with removal from the body via the respiratory system.
Poly(lactic acid) is present in three isomeric forums
D(-) L (+) and racimic (D, L) and the polymers are usually
abbreviated to indicate the chirality. Poly (L) LA and
Poly(D) LA are semi crystalline solids with similar rates
of hydrolytic degradation as PGA.
PLA is more hydropholic than PGA. For most application
the (L) isomer of lactic acid (LA) is chosen because it is
preferentially metabolized in the body.
Effect of homopolymer and copolymer structures
It has been found that some of the biodegradable
implant cause problem in the body since they take lot of
time degradation because of the homopolymer structure. L-lactide,
D-lactide, glycolide and tri-methylene carbonate (TMC) are
single units known as monomers (Fig.1). These monomers can
be formed in to chains called polymers Fig.2. For example,
L– lactide monomers can join to form the Poly Llactide (PLLA).
Fig. 2 Polymers from monomers.
A polymer can consist or a single type of monomer
creating a homopolymer (Fig.3) or a polymer can consist of
two or more types of monomer creating copolymer (Fig.4)
Fig. 3 Representation of homo polymer. |
