Biomedical Engineering Reference
In-Depth Information
pH 7.4
Table 7.2-3 Chemical structure of suture materials
100
Suture
Chemical composition
(wt ratio)
75
MONOCRYL
Glycolide: 3 -caprolactone ¼ 75:25
MAXON
Glycolide:trimethylene carbonate ¼
67.5:22.5
50
BIOSYN
Glycolide:dioxanone:trimethylene
carbonate ¼ 60:14:26
25
PDS II
p-Dioxanone ¼100
0
P(LA/CL)
L -Lactide: 3 -caprolactone ¼ 80:20
0
5
10
15
Week
ETHILON
Caprolactam ¼ 100
Fig. 7.2-8 Tensile strength change of monofilament sutures upon
immersion in buffer solution of pH 7.4 at 37 C. Open circles,
MONOCRYL; open squares, PDS II; open triangles, MAXON; solid
circles, BIOSYN; crossed squares, P(LA/CL); solid squares,
ETHILON; and solid triangles, PROLENE.
PROLENE
Propylene ¼100
months at in vitro degradation, and are absorbed in less
than a year. In contrast, copolymers of TMC and CL
degrade more slowly than TMC-DLLA copolymers. The
TMC-CL copolymers with high contents of CL are semi-
crystalline, very flexible, and tough, so that they can main-
tain mechanical properties for more than 1 year when
incubated in buffer solution at pH 7.4 and 37 C.
In addition to A-B type copolymers, A-B-A type
triblock copolymers have been actively synthesized. For
instance, PEG-PLA-PEG triblock copolymers can be
synthesized as follows. In the first step, diblock co-
polymers of methoxy(Me).PEG-PLA are prepared by
ring-opening polymerization of LA in the presence of
Me.PEG-OH. Then, the resultant diblock copolymer is
reacted with hexamethylene diisocyanate (HMDI) at a
high temperature to connect the two diblock copolymer
chains. The reasoning for block copolymerization of
LA with PEG is to reduce the surface hydrophobicity of
PLA scaffolds or make suspension in aqueous media.
Many bioabsorbable polymers like PLA and PGA
lack functional groups which facilitate further function-
alization. Therefore, poly(LA- co -lysine) was synthesized
change the degradation kinetics of each homopolymer,
but copolymerization of monomer B with monomer A
converts homopolymer B to a bioabsorbable polymer
component. The scheme is illustrated in Fig. 7.2-9 for
the equimolar copolymerization of monomers A and B.
The average continuous sequence of each monomer
in the copolymer chain is governed by polymerization
conditions such as initiator and temperature. If the
continuous sequence of monomer B in the copolymer
chain is shorter than a critical length below which olig-
omers B are soluble or dispersible in aqueous media, the
copolymer A-B becomes bioabsorbable owing to the
degradation of monomer A unit. A typical example is
LA-CL copolymers that are absorbed in the body at rates
higher than LA homopolymer and CL homopolymer that
is virtually non-absorbable. Furthermore, this copoly-
merization converts the brittle LA homopolymer into
much more rubber-like, tough polymer. Figure 7.2-10
shows how copolymerization of LLA with CL yields
polymers with low Young's moduli [4] and high re-
sorption rates. The DLLA copolymerization with CL will
produce copolymers with properties different from
those of LLA-CL copolymers, since long sequences
dominating in the LLA chains will result in small crys-
tallite formation by associating together, in marked con-
trast with DLLA sequences that do not have any
potential to crystallize.
Copolymerization of DLLA or CL with TMC has also
been attempted. The TMC homopolymer of high MW is
an amorphous elastomer that shows good mechanical
performance, combining high flexibility with high tensile
strength, but degrades very slowly at pH 7.4 and 37 C.
High MW copolymers of TMC and DLLA with 20-
50 mol% of TMC are amorphous, relatively strong elas-
tomers, can maintain mechanical properties up to 3
Copolymerization of monomers A and B
A + B
AABABBAABBAB
Comonomers
Copolymer A-B
A
A
B
B
A
Mixing of polymers A and B
B
B
A
B
A
A
A
B
B
B
A
B
A
A
A
A
A
A
A
A
A
B
A
A
A
B
B
B
Polymer A
A
A
A
A
B
A
A
B
BBBBBBB
Polymer B
Blend of polymer A and B
Fig. 7.2-9 Difference between copolymer A-B and blend of
polymer A and polymer B.
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