- Email: [email protected]
Self crimped and aligned fibers
Self crimped and aligned fibers Electrospinning is one of the most successful means of producing nanofibers with enormous application potential. New methods of producing nanofibers without high voltages are presently being explored by researchers around the globe. In the present work, a facile method of producing aligned and crimped fibers with polycaprolactone by rotating the needle assembly is described. Thinakaran Senthilrama, Loordhuswamy Amalorpava Marya, Jayarama Reddy Venugopalb, Lakshmanan Nagarajana, Seeram Ramakrishnab, and Venkateshwarapuram Rengaswami Giri Deva* aDepartment of Textile Technology, Anna University, Chennai, India bNanoscience and Nanotechnology Initiative, Faculty of Engineering, National University of Singapore, Singapore *E-mail: [email protected]
Nanofibers have been extensively used in various applications,
engineering, as well as fuel cells6 and gene delivery7. Aligned nanofibers
such as tissue engineering, filtration, drug delivery, solar cells,
are known to aid in the regeneration of highly organized structures,
sensors, and batteries. The use of fibers has garnered interest
like tendons, nerve cells, and ligaments and can provide topographic
among the global research community thanks to its properties,
guidance to cells thereby facilitating cell adhesion, proliferation, and
which may be tailor-made. The large surface area offered by these
migration.
nanofibers has paved the way for these techniques to be utilized in various innovative applications. The nanofibers can be arranged
Numerous attempts have been made by researchers to produce aligned nanofibers using electrospinning, using such approaches
in a random fashion as a mat, resulting in pores ranging in size from 0.5 to 1.5 μm. Pores present in the nanofiber mat, coupled with the high surface area, makes these the ideal candidate for scaffolds. Moreover, it has been reported that the nanofiber mats mimic the hierarchal structure of the extracellular matrix (ECM) which is critical for cell adhesion and proliferation1,2. Electrospinning has to date been the most successful and versatile method of producing nanofibers with random and aligned configurations, even though conventional fiber production routes such as wet spinning, melt spinning, and gel spinning have been able to produce fibers and filaments with sub-micron diameters, which can be made into nonwoven forms. Aligned nanofibers have potential applications in bone3, nerve4, skeletal muscle5, and vascular tissue
226
MAY 2011 | VOLUME 14 | NUMBER 5
Fig. 1 Schematic diagram of the rotating needle assembly.
METHODS & MATERIALS
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Fig. 2 SEM and optical micrographs of aligned and crimped fibers: (a) aligned fibers, (b) aligned bunch of fibers, (c) intertwined structure, (d) crimped configuration of fibers, (e) crimped path within the fiber, (f) interconnected un-stretched fibers, (g) interconnected stretched fibers, and (h) crimped single fiber.
as a wire drum collector8, high speed rotating drum9, patterned
Attempts are now being made to generate nanofibers without an
electrodes10, collection on a liquid medium followed by drawing11,
applied voltage by using centrifugal force13,14. This facile method of
and two pole air gap
electrospinning12.
The formation of aligned fibers
preparing nanofibers is likely to overtake the electrospinning process.
using electrospinning technology also has certain limitations; with
The set-up used in the above process is based on the principle of a
increasing thickness of the mat, the alignment of fibers is lost due to
cotton candy machine and in the present work, we propose a simple,
the presence of residual charge present on the fibers which hinders
modified arrangement for producing strongly aligned fibers. The
further deposition.
schematic diagram of this novel set-up is presented in Fig. 1. In the
However, the major drawback of electrospinning is that the
present set-up a syringe with a 24 G needle was loaded onto the motor
production rate is low and the generation of fibers is dependent upon
shaft by means of a chain loop. The speed of the motor can be varied
a number of factors such as concentration, voltage applied, and the
from 1000 to 10 000 rpm by a variable controller. Polycaprolactone
distance between the needle and the
collector13.
The most important
(PCL) was taken as prototype polymer for fabricating aligned
limiting factor in electrospinning certain polymers is that ambient
nanofibers. A polymer solution with the concentrations in the range
conditions also play a significant role in the production of nanofibers.
of 3 – 15 % w/v was prepared by using chloroform as the solvent. The
MAY 2011 | VOLUME 14 | NUMBER 5
227
METHODS & MATERIALS
(a)
(b)
(c)
(d)
Fig. 3 SEM micrographs of fibers (a), (b) at a 15 % concentration and 2000 rpm; (c) at 6 % concentration and 4000 rpm; (d) at 6 % concentration and 7000 rpm.
major parameters that influence the production of aligned 3D pattern
with fibers, as shown in Fig. 2e. This leads to the overall crimping of
are the concentration, speed of rotation, and, most importantly, the
the fibers in the mat at higher speeds. When tension is applied to the
distance between the needle and the collector.
mat, the fiber becomes aligned; the SEM micrographs with and without
The speed of the rotation of the assembly was varied from 1000 rpm in steps of 2000 rpm and it was found that fibers were formed for all
tension are shown in Figs. 2f and g. The second parameter that was altered during production of fibers
polymer concentrations (3 – 15 %). The scanning electron microscope
was the concentration of the polymer. It was observed that fibers
(SEM) micrographs of the fibers are given in Figs. 2a and b. The
could even be formed with a low concentration of 3 %, thereby
morphology (450 – 750 nm) of the fibers was not significantly altered in
showing tremendous commercial viability compared to that of
the speed range studied; however, the fibers are likely to be intertwined
electrospinning which requires a general concentration of 8 – 15 %.
(Fig. 2c) and crimped (Figs. 2d, f, and h) at speeds above 7000 rpm,
During experiments using higher concentrations (15 %) and low
as seen in the SEM and optical microscope images. Such behavior
speeds (2000 rpm) it was observed that a uniform formation of beads
may be explained as resulting from the high shear force acting on the
and fibers was produced as shown in Figs. 3a and b. This may be
polymer as it is ejected out of the needle tip, thereby creating high
due to the high viscosity of the polymer and the low shearing force
internal tension within the fibers. As the next polymer jet is ejected,
acting on the polymer solution as it is ejected out of the needle tip.
the former releases the internal tension and crimps, which may also be
The degree of crimping of the fibers in the mat was also influenced
attributed to the viscoelastic nature of the polymers. As further polymer
by both the concentration and the speed. Crimping was observed
jets come out successively, the internal crimped structure gets wrapped
for concentrations above 6 % and at speeds higher than 7000 rpm (Figs. 3c and d).
Instrument citation
The formation of the fibers was also highly influenced by the third parameter: the distance between the needle tip and the collector,
Hitachi S – 3400, scanning electron microscope
which was adjusted by altering the loop length of the chain attached
Paramount digital projectino, optical microscope
to the motor shaft. The polymer concentration and the speed of the motor rotation was kept constant to study the influence of the air
228
MAY 2011 | VOLUME 14 | NUMBER 5
METHODS & MATERIALS
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 4 Schematic diagram and photographs of (a), (b) uniform coatings of polymer; (c), (d) radial alignment of fibers; and (e), (f) 3D aligned, bundled fibers.
gap on the nature of the fibers produced. At shorter length of 8 cm, it
In summary, a new, modified, simple set-up for the production
was observed that the polymer was uniformly coated on the collector,
of nanofibers has been developed using a rotating needle assembly.
indicating that the time for solvent evaporation was less and that
This set-up is in the trial stage for the production of fibers such as
fibrillation of the polymer jet did not occur (Figs. 4a and b). With
acrylic, nylon, and polyurethane. The studies on polymers mentioned
further increase in length (12 cm), it was observed that the time for
above have shown promising results, and further studies on various
solvent evaporation was greater, and a thin jet of polymer was adhered
application are to be carried out in the near future. The proposed setup
to the wall of the collector. This acted as a nucleus, and with further
can be utilized to develop fibers with various geometries and can be
rotation of the needle assembly the polymer was stretched, leading
used in applications such as regenerative medicine and energy.
to fiber formation in a radial fashion, as shown in Figs. 4c and d. At lengths greater than 16 cm, the polymer jet did not have chance to
Acknowledgement
adhere to the wall of the collector; instead, fibers were formed in
T. Senthil Ram acknowledges DST-PURSE (Proceeding No. 110001/
the air between the collector and the needle tip, in a circular fashion,
PD2/2008) Department of Science and Technology (DST), Government
forming an aligned fiber bundle (Figs. 4e and f).
of India.
REFERENCES 1. Ramakrishna, S., et al., Mater Today (2006) 9(3), 40.
8. Katta, P., et al., Nano Lett (2004) 4(11), 2215.
2. Huang, Z-M., et al., Compos Sci Technol (2003) 63, 2223.
9. Bian, W., and Bursac, N., Biomaterials (2009) 30, 1401.
3. Meng, Z. X., et al., Mater Sci Eng C (2010) 30, 1204.
10. Li, D., et al., Nano Lett (2005) 5(5), 913.
4. BingWang, H., et al., J Neural Eng (2009) 6, 016001.
11. Teo, W-E., et al., Polymer (2007) 48, 3400.
5. Aviss, K. J., et al., Eur Cells Mater (2010) 19, 193.
12. Balendu, S., et al., Acta Biomater (2011) 7, 20.
6. Tamura, T., et al., Nano Lett (2010) 10, 1324.
13. Badrossamay, M. R., et al., Nano Lett (2010) 10, 2257.
7. Melechko, A. V., et al., J Appl Phys D (2009) 42, 193001.
14. Sarkar, K., et al., Mater Today (2010) 13, 11.
MAY 2011 | VOLUME 14 | NUMBER 5
229
Nanofibers have been extensively used in various applications,
engineering, as well as fuel cells6 and gene delivery7. Aligned nanofibers
such as tissue engineering, filtration, drug delivery, solar cells,
are known to aid in the regeneration of highly organized structures,
sensors, and batteries. The use of fibers has garnered interest
like tendons, nerve cells, and ligaments and can provide topographic
among the global research community thanks to its properties,
guidance to cells thereby facilitating cell adhesion, proliferation, and
which may be tailor-made. The large surface area offered by these
migration.
nanofibers has paved the way for these techniques to be utilized in various innovative applications. The nanofibers can be arranged
Numerous attempts have been made by researchers to produce aligned nanofibers using electrospinning, using such approaches
in a random fashion as a mat, resulting in pores ranging in size from 0.5 to 1.5 μm. Pores present in the nanofiber mat, coupled with the high surface area, makes these the ideal candidate for scaffolds. Moreover, it has been reported that the nanofiber mats mimic the hierarchal structure of the extracellular matrix (ECM) which is critical for cell adhesion and proliferation1,2. Electrospinning has to date been the most successful and versatile method of producing nanofibers with random and aligned configurations, even though conventional fiber production routes such as wet spinning, melt spinning, and gel spinning have been able to produce fibers and filaments with sub-micron diameters, which can be made into nonwoven forms. Aligned nanofibers have potential applications in bone3, nerve4, skeletal muscle5, and vascular tissue
226
MAY 2011 | VOLUME 14 | NUMBER 5
Fig. 1 Schematic diagram of the rotating needle assembly.
METHODS & MATERIALS
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Fig. 2 SEM and optical micrographs of aligned and crimped fibers: (a) aligned fibers, (b) aligned bunch of fibers, (c) intertwined structure, (d) crimped configuration of fibers, (e) crimped path within the fiber, (f) interconnected un-stretched fibers, (g) interconnected stretched fibers, and (h) crimped single fiber.
as a wire drum collector8, high speed rotating drum9, patterned
Attempts are now being made to generate nanofibers without an
electrodes10, collection on a liquid medium followed by drawing11,
applied voltage by using centrifugal force13,14. This facile method of
and two pole air gap
electrospinning12.
The formation of aligned fibers
preparing nanofibers is likely to overtake the electrospinning process.
using electrospinning technology also has certain limitations; with
The set-up used in the above process is based on the principle of a
increasing thickness of the mat, the alignment of fibers is lost due to
cotton candy machine and in the present work, we propose a simple,
the presence of residual charge present on the fibers which hinders
modified arrangement for producing strongly aligned fibers. The
further deposition.
schematic diagram of this novel set-up is presented in Fig. 1. In the
However, the major drawback of electrospinning is that the
present set-up a syringe with a 24 G needle was loaded onto the motor
production rate is low and the generation of fibers is dependent upon
shaft by means of a chain loop. The speed of the motor can be varied
a number of factors such as concentration, voltage applied, and the
from 1000 to 10 000 rpm by a variable controller. Polycaprolactone
distance between the needle and the
collector13.
The most important
(PCL) was taken as prototype polymer for fabricating aligned
limiting factor in electrospinning certain polymers is that ambient
nanofibers. A polymer solution with the concentrations in the range
conditions also play a significant role in the production of nanofibers.
of 3 – 15 % w/v was prepared by using chloroform as the solvent. The
MAY 2011 | VOLUME 14 | NUMBER 5
227
METHODS & MATERIALS
(a)
(b)
(c)
(d)
Fig. 3 SEM micrographs of fibers (a), (b) at a 15 % concentration and 2000 rpm; (c) at 6 % concentration and 4000 rpm; (d) at 6 % concentration and 7000 rpm.
major parameters that influence the production of aligned 3D pattern
with fibers, as shown in Fig. 2e. This leads to the overall crimping of
are the concentration, speed of rotation, and, most importantly, the
the fibers in the mat at higher speeds. When tension is applied to the
distance between the needle and the collector.
mat, the fiber becomes aligned; the SEM micrographs with and without
The speed of the rotation of the assembly was varied from 1000 rpm in steps of 2000 rpm and it was found that fibers were formed for all
tension are shown in Figs. 2f and g. The second parameter that was altered during production of fibers
polymer concentrations (3 – 15 %). The scanning electron microscope
was the concentration of the polymer. It was observed that fibers
(SEM) micrographs of the fibers are given in Figs. 2a and b. The
could even be formed with a low concentration of 3 %, thereby
morphology (450 – 750 nm) of the fibers was not significantly altered in
showing tremendous commercial viability compared to that of
the speed range studied; however, the fibers are likely to be intertwined
electrospinning which requires a general concentration of 8 – 15 %.
(Fig. 2c) and crimped (Figs. 2d, f, and h) at speeds above 7000 rpm,
During experiments using higher concentrations (15 %) and low
as seen in the SEM and optical microscope images. Such behavior
speeds (2000 rpm) it was observed that a uniform formation of beads
may be explained as resulting from the high shear force acting on the
and fibers was produced as shown in Figs. 3a and b. This may be
polymer as it is ejected out of the needle tip, thereby creating high
due to the high viscosity of the polymer and the low shearing force
internal tension within the fibers. As the next polymer jet is ejected,
acting on the polymer solution as it is ejected out of the needle tip.
the former releases the internal tension and crimps, which may also be
The degree of crimping of the fibers in the mat was also influenced
attributed to the viscoelastic nature of the polymers. As further polymer
by both the concentration and the speed. Crimping was observed
jets come out successively, the internal crimped structure gets wrapped
for concentrations above 6 % and at speeds higher than 7000 rpm (Figs. 3c and d).
Instrument citation
The formation of the fibers was also highly influenced by the third parameter: the distance between the needle tip and the collector,
Hitachi S – 3400, scanning electron microscope
which was adjusted by altering the loop length of the chain attached
Paramount digital projectino, optical microscope
to the motor shaft. The polymer concentration and the speed of the motor rotation was kept constant to study the influence of the air
228
MAY 2011 | VOLUME 14 | NUMBER 5
METHODS & MATERIALS
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 4 Schematic diagram and photographs of (a), (b) uniform coatings of polymer; (c), (d) radial alignment of fibers; and (e), (f) 3D aligned, bundled fibers.
gap on the nature of the fibers produced. At shorter length of 8 cm, it
In summary, a new, modified, simple set-up for the production
was observed that the polymer was uniformly coated on the collector,
of nanofibers has been developed using a rotating needle assembly.
indicating that the time for solvent evaporation was less and that
This set-up is in the trial stage for the production of fibers such as
fibrillation of the polymer jet did not occur (Figs. 4a and b). With
acrylic, nylon, and polyurethane. The studies on polymers mentioned
further increase in length (12 cm), it was observed that the time for
above have shown promising results, and further studies on various
solvent evaporation was greater, and a thin jet of polymer was adhered
application are to be carried out in the near future. The proposed setup
to the wall of the collector. This acted as a nucleus, and with further
can be utilized to develop fibers with various geometries and can be
rotation of the needle assembly the polymer was stretched, leading
used in applications such as regenerative medicine and energy.
to fiber formation in a radial fashion, as shown in Figs. 4c and d. At lengths greater than 16 cm, the polymer jet did not have chance to
Acknowledgement
adhere to the wall of the collector; instead, fibers were formed in
T. Senthil Ram acknowledges DST-PURSE (Proceeding No. 110001/
the air between the collector and the needle tip, in a circular fashion,
PD2/2008) Department of Science and Technology (DST), Government
forming an aligned fiber bundle (Figs. 4e and f).
of India.
REFERENCES 1. Ramakrishna, S., et al., Mater Today (2006) 9(3), 40.
8. Katta, P., et al., Nano Lett (2004) 4(11), 2215.
2. Huang, Z-M., et al., Compos Sci Technol (2003) 63, 2223.
9. Bian, W., and Bursac, N., Biomaterials (2009) 30, 1401.
3. Meng, Z. X., et al., Mater Sci Eng C (2010) 30, 1204.
10. Li, D., et al., Nano Lett (2005) 5(5), 913.
4. BingWang, H., et al., J Neural Eng (2009) 6, 016001.
11. Teo, W-E., et al., Polymer (2007) 48, 3400.
5. Aviss, K. J., et al., Eur Cells Mater (2010) 19, 193.
12. Balendu, S., et al., Acta Biomater (2011) 7, 20.
6. Tamura, T., et al., Nano Lett (2010) 10, 1324.
13. Badrossamay, M. R., et al., Nano Lett (2010) 10, 2257.
7. Melechko, A. V., et al., J Appl Phys D (2009) 42, 193001.
14. Sarkar, K., et al., Mater Today (2010) 13, 11.
MAY 2011 | VOLUME 14 | NUMBER 5
229