Biomedical Engineering Reference
In-Depth Information
and form such as braided fi bers, spun fi bers, single and multifi lament fi bers, plates, fi lms rods being
different, they give rise to different mechanical properties. PGA fi bers exhibit high strength and
modulus, and are braided to form sutures due to its high stiffness. PGA sutures can lose 50% of their
strength after 2 weeks and are generally totally absorbed within 4-6 months.
5
The biodegradation
of the microspheres of PLA and PGA has been discussed in detailed by Anderson et al.
6
and they
concluded that the rate of degradation is also dependent on the size of the microspheres in addition
to molecular weight, sterilization, and processing parameters of the polymers.
6
Biodegradable poly-
mers may be processed in a way similar to any engineering thermoplastic in that they can be melted
and formed into fi bers, rods, and molded parts. Final parts can be extruded, injection molded,
compression molded, or solvent spun, or cast.
7
The molecular weight correlates with fi nal properties
of the implant such as the mechanical properties, stability, and absorption time. Most processing
methods invariably infl uence the molecular weight, and factors such as residence time, temperature,
shear, moisture, oligomeric units, and impurities, contribute to the given effect.
8
For example, the
polymer poly(l-lactic acid) (PLLA) suffers chemical degradation at temperatures of 220
°
C and pre-
drying of polymers before processing leads to minimal changes in molecular weight.
The degradation of aliphatic polyesters occurs mainly through the bulk degradation through
hydrolysis. Hydrolytic degradation occurs as polymers (Figure 15.3) are in contact with tissue fl uids
or moisture and preferentially onsets at the amorphous regions of the polymeric network. Hydro-
lytic attack causes cleavage at the ester bonds, resulting in chain scission yielding low molecular
weight species in the initial stages. Chain scission may also occur in presence of nonspecifi c ester-
ases and carboxy peptidases, which can break down PGA into glycolic acid units. Both PLA and
poly(ε-caprolactone) (PCL) can also be degraded enzymatically. As the bulk of the polymer sub-
strate erodes, autocatalysis occurs within the core of the material due to the presence of the acidic
degradation products that are contained within the matrix, as they are unable to diffuse out. The
general trend observed in these semicrystalline polymers during the degradation is an increase in
crystallinity as most of the amorphous regions degrade preferentially, followed by the loss of the
crystalline phase. As this process continues, the polymer loses its mechanical strength and frag-
mentation occurs. Further hydrolysis yields smaller fragments that can be assimilated by phago-
cytes, while the soluble monomeric anions such as glycolate and lactate dissolve in the intercellular
fl uid. The glycolic acid is converted to glycine, then serine, and subsequently to pyruvic acid that
enters the TCA cycle and is eliminated as carbon dioxide and water. PLA is broken down into lactic
acid, which is converted to pyruvic acid and eliminated as carbon dioxide and water. PCL degrades
to hydrohexanoic acid, a metabolizable metabolite.
9
The degradation kinetics of poly(α-hydroxy acids) (PHA) is signifi cantly affected by the
molecular weight, morphology, and crystallinity. Properties such as crystallinity and morphology
are strongly dependent on the thermal processing and sterilization techniques, which have a bear-
ing on the rate of degradation, an important parameter in the designing of bioabsorbable devices.
In general, higher molecular weight polymers possess superior mechanical properties and slower
degradation rates in comparison with their lower counterparts. As the mechanism of degradation
is autocatalytic in nature, and hydrolysis of PLA/PGA yields acids, higher molecular weight poly-
mers have less acid-catalyzing groups present, and the longer chain length requires chain scission
to occur more drastically until the oligomeric units are small enough to diffuse through the matrix,
thus slowing the rate of degradation.
10
Morphology and crystallinity also play a major role in the
degradation of the semicrystalline polymers, the amorphous regions being more prone to degrada-
tion than the cr ystalline domains. The processing parameters also strongly infl uence the mechanical
properties, and increasing crystallinity through processing, annealing, and sterilization is refl ected
in the mechanical properties such as tensile strength and Young's modulus.
11
The mechanism of
degradation in these polymers is via bulk and surface erosion, and the primary driving force is the
relative hydrophilicity of the polymer that governs the ingress of aqueous solutions from the surface
to within the bulk. As degradation begins, the accumulation of water-soluble degradation products
causes an auto acceleration to occur, which causes more fl uids to ingress within the bulk, further
