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CHAPTER 7
PREPARATION AND PROPERTIES OF PLLA/PLCL
FILAMENTS FOR POTENTIAL USE AS A
MONOFILAMENT SUTURE
7.1 INTRODUCTION
In recent years, biodegradable polymers have attracted increasing
attention as a candidate for use in biomedical applications (Furuhashi et al
2006). Among medical devices made out of biodegradable polymers, fibrous
materials are playing an increasing important role. This is attributed to the
development of sophisticated surgical procedures, as well as the
advancements made in synthetic fibres. The use of fibres as absorbable
sutures represents a major biomedical application of this form of polymer. If
fibres are to be used as absorbable sutures in medicine and surgery it is
necessary to meet a number of specific criteria pertinent to biological
properties, tensile properties, handling characteristics and surface properties
(Penning et al 1993).
Today the selection of a proper suture is not a simple task. In
earlier days it was much less difficult for the surgeon to select the sutures due
to the only availability of natural suture materials. With the introduction of
synthetic polymer materials, particularly synthetic degradable materials, a
revolution have occurred in this field. A wide variety of new suture materials
were developed in order to satisfy the requirements of modern surgery.
However, there is no single suture material which can fulfill all the crucial
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requirements of sutures. The surgeon should choose the right suture for the
type of surgery that he is performing because different tissues have differing
requirements for suture support. The present surgeon has several choices of
suture material available and he may choose them based on availability and
his familiarity (Pillai et al 2010).
Poly (L-lactic acid) (PLLA) represents a class of materials of
growing importance, especially in the field of biomedical engineering due to
its favorable chemical, biological, and mechanical characteristics. These
characteristics include structural simplicity, comparatively easy synthesis, and
compatibility with biological tissues, applicability of mechanical
characteristics, and a simple degradation mechanism with no complex
products (Jamshidi et al 1988).
PLLA can be transformed by spinning into filaments for
subsequent fabrication of desirable textile structures. Several researchers have
prepared PLLA fibres by melt spinning and solution spinning. Cicero et al
(2001) employed two step melt-spinning to produce PLLA fibres. Higher-
order structures and mechanical properties of stereocomplex-type poly (lactic
acid) melt spun fibres were studied by Furuhashi et al (2006). Eling et al
(1982) reported that the PLLA fibres prepared by dry spinning have higher
mechanical properties than melt spinning. Wet spinning of PLLA was carried
out by Nelson et al (2003). It was reported that solvent systems, polymer
blends, and winding rates alter mechanical and morphological properties of
the fibres. Gupta et al (2006) used a dry-jet-wet spinning and hot drawing
process to obtain PLLA fibres.
In a recent study PLLA hompolymer fibres has been prepared using
dry-jet-wet spinning technique and observed that the glass transition
temperature (Tg) and crystallinity are 76°C and 48% respectively, which is
higher for suture applications (Gupta et al 2006). Furthermore,
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Hiljanen-Vainio (1996) also reported that for monofilament suture purposes
PLLA is too stiff and inflexible due to its greater glass transition temperature
and crystallinity, therefore it is difficult to make knots at room temperature.
The knottability of the fibres may be improved by modifying the PLLA fibre
properties through reducing the glass transition temperature and crystallinity.
Blending of PLLA with other polymers has been recognized as a valuable
tool for modifying the properties of PLLA (Sodergard and Stolt 2002).
However, manifestation of superior properties depends upon the miscibility of
polymers at the molecular level (Hiljanen-Vainio 1996).
Thus, several polymer blend systems have been investigated, such
as PLLA / Poly (ethylene glycol) (PEG) (Sheth et al 1997), PLLA/Poly (D, L-
lactic acid) (PDLLA) (Tsuji et al 1991), PLLA/PCL (Yang et al 1997),
PDLLA/PCL (Tsuji and Ikada 1996). However, most of the blends are found
to be immiscible or partly miscible depending on their composition. Hiljanen-
Vainio (1996) produced different combinations of PLLA/PLCL using an
injection molding apparatus and stated that blending of PLLA with
Poly (L-Lactide-co- -caprolactone) (PLCL) provides a unique means to
achieve intimately compatible blends.
Previous studies in the literature showed that, no extensive study is
carried out for PLLA/PLCL fibres using dry–jet–wet spinning technique
(Gupta et al 2006). In this study attempts have been made to prepare
PLLA/PLCL fibres by dry-jet-wet spinning. The main advantages of
producing the fibres by this technique are that the minimum polymer
degradation occurs during the spinning process and leads to better molecular
chain orientation, which ultimately results in the development of the fibres
with superior properties (Gupta et al 2006). The effect of draw ratio on the
PLLA/PLCL fibres is also studied.
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7.2 MATERIALS AND METHODS
7.2.1 Materials
Two kinds of polymers, poly L-lactic acid (PLLA) and poly
(lactide-co- -caprolactone) (PLCL) (both obtained from Polymer laboratory,
University of Uppsala, Sweden) were used in this study. Both the polymers
were stored under vacuum at ambient conditions. Chloroform and methanol
were supplied by Merck India. Chloroform was dried over P2O5 prior to the
polymer dissolution. The intrinsic viscosity of the dried PLLA and PLCL
were 3.45 dL/g and 3.27 dL/g respectively.
7.2.2 Methods
The process of manufacture of suture is presented in the Figure 7.1.
The following section discusses the each step in detail.
7.2.2.1 Spinning
PLLA/PLCL (90%/10% w/w) fibres were prepared by dry-jet-wet
spinning of the polymer from chloroform solution and with methanol as the
precipitating medium. Polymer was dried at 100°C under vacuum for 24
hours prior to the spinning. The polymer was dissolved in chloroform using a
moisture-free glass assembly, under constant stirring at ambient conditions
for 24 hours. The polymer solution was subsequently spun on a dry–jet–wet
spinning machine as shown in Figure 7.2.
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Figure 7.1 Process of Manufacture of PLLA/PLCL Monofilament Suture
Sourcing of PLLA/ PLCLpolymers and other chemicals
Spinning by Dry-jet-wet spinningmachine
Drawing and heat settingof as-spun fibres
Characterization of PLLA/PLCLmono filaments
Tenacity measurement
Knot strength
measurement
Glass transition
temperature
Crystallinity
Surface structure analysis
Comparison of properties ofPLLA homopolymer with
PLLA/PLCL fibres
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Spinneret Coagulation bath
Spinningzone
PLLA/PLCLMonofilamentfibres
Take up rollerRam
Cylinder
Figure 7.2 Schematic Diagram of Dry-Jet-Wet Spinning
The polymer solution was extruded through the spinneret of 0.5
mm into the coagulation bath containing methanol. Both the coagulation bath
had methanol as the coagulant and fibre was collected using two sets of take
up rollers, both operating at the speed of 6m/min. The air gap between the
spinneret and coagulation bath was kept as 25 mm. The fibre was collected on
bobbins and was subjected to drawing process in the second step.
7.2.2.2 Drawing
The as spun fibre produced at a take-up speed 6 m/min was
subjected to drawing and heat setting operations. The fibre was subjected to
two stages drawing with varying draw ratios in the range of 3–15 at the
drawing temperature of 75°C, followed by the heat setting at 100°C under
taut conditions.
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7.2.2.3 Tenacity and knot strength tests
Instron tensile tester was used to measure the tenacity and knot
strength of PLLA/PLCL sutures. The PLLA/PLCL sutures were tested for
tenacity and knot strength at a gauge length of 150 mm and extension rate of
90 mm/min. In the knot strength measurement, knot was formed with square
knot method as described previously. The details of the knot tying procedure
have been explained in chapter 3, section 3.3.4. For each test method at least
20 readings are taken.
7.2.2.4 Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry (DSC) was performed using a
Perkin Elmer DSC-7 system. Vacuum-dried samples were loaded and the
thermograms were run in the temperature range of 40–200°C under nitrogen
atmosphere at a heating rate of 10°C/min. The heat of fusion ( Hf) values
were obtained from the area under the melting thermograms. The crystallinity
was obtained by the following expression:
Crystallinity (%) = Hf / Hf (cry) X 100
Where, Hf is the heat of fusion of the sample and Hf (crys) is the heat of
fusion of 100% crystalline PLLA and is taken as 93.7 J/g.
7.2.2.5 X-Ray Diffraction
PLLA/PLCL monofilaments were analyzed using a Phillips wide-
angle X-ray diffraction apparatus equipped with a scintillation counter. X-ray
scans were made over a 2 range of 10°-35.
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7.2.2.6 Scanning Electron Microscope
The surface characteristics of fibres were studied using a
STEREOSCAN 360 (Cambridge Scientific Industries Ltd.), scanning electron
microscope after coating them with silver.
7.2.2.7 Statistics
The datas are expressed as mean ± standard deviation. Statistical
analysis was carried out using the unpaired student’s t test. A value of P <0.05
has been considered to be statistically significant.
7.3 RESULTS AND DISCUSSIONS
7.3.1 Draw Ratio and Its Effect on Tenacity and Knot Strength
The PLLA/PLCL fibres are prepared by dry-jet-wet spinning
method. The as spun fibres obtained using the dry-jet-wet spinning is
subjected to drawing and heat-setting operations. The drawing of the fibre is
carried out at a temperature range of 75°C, followed by the heat setting at
100°C. It is important to mention that in the spinning conditions, a maximum
draw ratio of 15 could be achieved. This draw ratio is comparatively higher
than that reported so for in melt spinning of PLLA homopolymer fibres. The
achieved higher draw ratio may be due to the fact that the chain entanglement
is largely suppressed in the solution state as compared with the polymer melt.
Since the entanglements are the key factors in limiting the maximum
drawability of a polymer, the low entanglements are effectively transferred to
the fibre structure (Fu et al 2002).
Further, it is observed that the draw ratio obtained for PLLA/PLCL
fibre is also higher than that of PLLA homopolymer fibres obtained by dry-
jet-wet spinning. The higher draw ratio in PLLA/PLCL fibres may be
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interpreted in terms of a reduction of molecular interactions associated with
the introduction of the flexible polymer PLCL. Smook et al (1990)
demonstrated that the maximum draw ratio of flexible polymers is governed
by molecular attraction forces expressed as the cohesive energy for a wide
range of polymers. Based on this relation one expects a higher drawability for
PLLA/PLCL fibres.
The variation of tenacity of the PLLA/PLCL fibre with the draw
ratio is presented in Figure 7.3. It is clear from the figure that tenacity
increases with increase in draw ratio and reaches a maximum value of
30cN/tex for the PLLA/PLCL fibre at draw ratio of 9 and then decreases with
further increase in draw ratio. The increase in tenacity with increase in draw
ratio is due to the high orientation of molecular chains and crystallinity.
Beyond draw ratio 9, tenacity decreases, the decrease in strength can be
associated with void generation due to overdrawing.
Figure 7.3 Tenacity as a Function of Draw Ratio for PLLA/PLCL Fibre
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An effect of draw ratio on knot strength is similar to that of the
tenacity as shown in Figure 7.4. The maximum knot strength of 22cN/tex for
blended fibre is achieved for a draw ratio of 9. Further it is observed that the
maximum tenacity and knot strength value achieved for the PLLA/PLCL
fibre is substantially lower than that reported for PLLA fibre. Further it is
observed that the maximum tenacity of 30 cN/tex and knot strength value of
22cN/tex achieved for the PLLA/PLCL fibre is substantially lower than the
tenacity value of 36 cN/tex and knot strength value 27 cN/tex reported for
PLLA fibres by Gupta et al (2006). The decrease of crystallinity of
PLLA/PLCL fibres is due to the presence of PLCL, which interrupt the chain
regularity and inhibit the formation of highly ordered structures.
Figure 7.4 Knot Strength as a Function of Draw Ratio for PLLA/PLCL
Fibres
7.3.2 Differential Scanning Calorimetry Analysis
The results of DSC heating scans of the PLLA/PLCL as spun and
the fibres drawn fibres at draw ratio (DR) 9 are shown in Figure 7.5. The
glass transition temperature of the asspun and the drawn fibres are 50°C and
58°C respectively. As the glass transition is associated with the mobility of
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the fibre in the amorphous region, orientation is introduced in these regions
during the drawing process. Further, the crystalline regions in a semi
crystalline polymer are known to act as the anchors between the amorphous
regions and therefore tend to restrict the molecular mobility (Jamshidi et al
1988).
The observed behavior of the Tg enhancement with drawing and
heat setting may, therefore, be attributed to the cumulative effect of the
enhanced orientation in the amorphous region and the high crystallinity.
Similar results are observed by Cicero et al (2002). Furthermore, it is
important to mention that the maximum Tg value of 58°C for the PLLA/PLCL
fibres is considerably lower than the Tg value 76°C stated for PLLA
hompolymer fibres by Gupta et al (2006). This reduction in Tg value for the
PLLA/PLCL fibre may be due to the increase in the chain flexibility and
mobility associated with the introduction of PLCL.
Endo
Temperature (0C)
160 200 220180140120605040 80
Asspun
DR 9
Exo
Figure 7.5 DSC Thermograms of the PLLA /PLCL Fibres
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7.3.3 X-Ray Diffraction Analysis
The X-ray diffraction pattern of the PLLA/PLCL asspun fibre and
the fibres drawn fibres at draw ratio (DR) 9 are presented in the Figure 7.6.
Sharp peaks with higher intensity are observed in case of drawn fibre as
compared to asspun fibre. This shows that X-ray crystallinity is higher in
drawn fibre as compared to asspun fibres. This could be due to orientation-
induced crystallization. All diffractions lie at identical angles, and no
additional diffraction peaks are visible in the drawn fibres, indicating that the
crystalline phase does not change during drawing process. More over it is
observed that the maximum value of crystallinity of 40 % for the
PLLA/PLCL fibre is also lower than the crystallinity 48% stated for PLLA
homopolymer fibres by Gupta et al (2006). The decrease of crystallinity of
PLLA/PLCL fibres is due to the presence of PLCL, which interrupt the chain
regularity and inhibit the formation of highly ordered structures.
Intensity
10 20 30
DR= 9
Asspun
Angle 2
Figure 7.6 X-Ray Diffraction Patterns of PLLA/PLCL Fibres
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7.3.4 Surface Topography from SEM
The Scanning electron micrograph of the asspun and drawn
PLLA/PLCL fibres is presented in Figure 7.7. The asspun fibre shows a
porous structure throughout the surface, whereas considerable change in the
surface morphology is observed in the drawn fibres, this may be due to the
collapse of the porous structure.
(a) (b)
(c)
Figure 7.7 SEM Micrographs of PLLA /PLCL Filaments (a) Asspun
(b) Drawn at Draw Ratio 3 (c) Drawn at Draw Ratio 9
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7.3.5 Comparison of Properties of PLLA Homopolymer Fibres with
PLLA/PLCL Fibres
The properties of PLLA homopolymer fibres and PLLA/PLCL
fibres are presented in the Table 7.1. From the Table 7.1, it is noted that the
maximum tenacity of 30 cN/tex and knot strength value of 22cN/tex achieved
for the PLLA/PLCL fibre is substantially lower than the tenacity value of 36
cN/tex and knot strength value 27 cN/tex reported for PLLA fibres by Gupta
et al (2006). The decrease of crystallinity of PLLA/PLCL fibres is due to the
presence of PLCL, which interrupt the chain regularity and inhibit the
formation of highly ordered structures. Furthermore, it is noted that the
maximum Tg value for the PLLA/PLCL fibres is 58°C that is considerably
lower than the Tg value 76°C stated for PLLA hompolymer fibres by Gupta et
al (2006). This reduction in Tg value for the PLLA/PLCL fibre may be due to
the increase in the chain flexibility and mobility associated with the
introduction of PLCL. More over it is observed that the maximum value of
crystallinity of 40 % for the PLLA/PLCL fibre is also lower than the
crystallinity 48% stated for PLLA homopolymer fibres by Gupta et al (2006).
The decrease of crystallinity of PLLA/PLCL fibres is due to the presence of
PLCL, which interrupt the chain regularity and inhibit the formation of highly
ordered structures.
Table 7.1 Comparison of Properties of PLLA homopolymer fibres with
PLLA/PLCL fibres
Property PLLA homopolymer fibres PLLA/PLCL fibres
Tenacity (cN/tex) 36 30
Knot strength (cN/tex) 27 22
Tg (°C) 76 58
Crystallinity (%) 48 40
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7.4 CONCLUSIONS
The dry–jet–wet spinning process is employed to spin poly
(lactic acid) (PLLA)/ poly (lactide-co- -caprolactone) (PLCL)
(90/10 %) fibres. The as spun fibres are subsequently subjected
to two-step process of drawing and subsequent heat setting. It is
important to mention that a maximum draw ratio of 15 could be
achieved for PLLA/PLCL fibres.
The effect of draw ratio on tenacity and knot strength are
studied. The tenacity value increases with increase in draw ratio
and a maximum value of 30cN/tex is achieved at draw ratio 9.
Beyond draw ratio 9 tenacity decreases. The effect of draw ratio
on knot strength is similar to that of the tenacity. The maximum
knot strength of 22cN/tex is achieved at draw ratio 9 for
PLLA/PLCL fibres.
The addition of PLCL in PLLA, in order to reduce the glass
transition temperature and crystallinity has shown a strong
effect on the fibre properties.
The fibre with the crystallinity of 40% and glass transition
temperature of 58°C are achieved for a draw ratio 9, which is
considerably lower than the PLLA homo polymer fibres.
The asspun fibre shows a porous structure throughout the
surface, whereas considerable change in the surface
morphology is observed in the drawn fibres.
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In conclusion, manufacture of PLLA/PLCL fibres using dry-jet-
wet spinning technique may be an effective way to modify the
property of PLLA and more suitable to use in suture
applications.