Plasma-assisted electrospinning: the many facets of a process
M. Alessandri1, L. Calzà1, V. Colombo2,3, L. S. Dolci1, A. Fiorani5, D. Fabiani3,4,
M.L. Focarete1,5, E. Ghedini2,3, M. Gherardi2, C. Gualandi3, R. Laurita2,
A. Liguori2, S. D. Quiroga1,5, P. Sanibondi2, M. Zaccaria4
Alma Mater Studiorum – Università di Bologna
1
Health Sciences and Technologies - Interdepartmental Center for Industrial Research (HST-CIRI), Via Tolara di Sopra
50, 40064 - Ozzano dell'Emilia, Italy
2
Department of industrial engineering (DIN), Via Saragozza 8, 40126 Bologna, Italy
3
Industrial Research Centre for Advanced Mechanics and Materials (CIRI-MAM)
4
Electrical, Electronic and Information Engineering Department (DEI),Viale Risorgimento 2, 40136 Bologna, Italy
5
Department of Chemistry ‘G. Ciamician’, Via Selmi 2, 40126 Bologna, Italy
Abstract: Atmospheric pressure non equilibrium plasma treatments have been implemented
to produce solvent-free Poly-L-lactic acid (PLLA) scaffolds, to improve biocompatibility of
PLLA mats and to increase electrolyte uptake of Poly(vinylidene fluoride) (PVDF) separators
for lithium batteries. It has been observed that plasma treatments are a suitable way to
improve polymeric solution electrospinnability, to functionalize PLLA scaffold and to
produce enhanced PVDF separators for energy storage devices.
Keywords: Not-thermal plasma, dielectric barrier discharge, plasma jet, solvent-free scaffold,
increased biocompatibility, enhanced electrospun separators for lithium batteries.
1. Introduction
Electrospinning (ES) is an innovative method to produce
polymeric fibers, with diameters ranging from tens of
nanometers to a few microns, through a jet of an
electrostatically charged molten polymer or polymeric
solution stretched from an high voltage needle to a
grounded plate, where a solid non-woven mat can be
collected after the evaporation of the solvent during the
flight of the jet [1].
Atmospheric pressure non-equilibrium plasma is a
cocktail of charged particles, reactive species, UV rays,
heat flux and electric field, which can effectively support
various phases of the electrospinning process, such as the
preparation of the polymeric solution, the evaporation of
solvents during fiber formation and the surface
modification of the produced mats. Indeed, atmospheric
pressure plasma treatment of an aqueous polymeric
solution has been demonstrated to induce an improvement
of electrospinnability [2] through the modification of the
polymeric solution properties that mainly influence the
electrospinning process, such as conductivity, viscosity
and surface tension, leading to the production of
nanofibers with a lower amount of beads with respect to
fibers produced from an untreated solution. Plasma
treatment during the evaporation phase, i.e. in flight
treatment, can improve the adhesion of the fibers on a
substrate [3]. Plasma treatment of electrospun scaffolds is
able to improve material biointegration by promoting cell
adhesion, proliferation and differentiation [4,5], thanks to
the introduction of specific functional groups [6,7].
In this work, results concerning the treatment of the
polymer solution and the surface modification of
electrospun mats will be shown. Plasma treatment of a
Poly-L-lactic acid (PLLA) in 100% dichloromethane has
been carried out to produce a solvent free nanofibrous mat
for biomedical application. Post treatment of PLLA
scaffold has been implemented to functionalize mat and
increase biocompatibility, while the treatment of
Poly(vinylidene fluoride) (PVDF) electrospun mats has
induced an increase of electrolyte uptake for innovative
energy storage devices.
2. Materials
Plasma apparatus
Different atmospheric pressure plasma sources have
been used for each application. Polymeric solution
treatment has been carried out by means of a multi-gas
plasma jet developed by the authors [8]; this source is
provided of two separate gas inlets to control the
production of reactive species generated by the plasma jet.
The primary gas is usually Ar, while O2, N2 or Air can be
used as secondary gas. In this work Ar is used as primary
gas, while no secondary gas is used. In Figure 1 is
reported plasma jet source.
Figure 1. Plasma jet operated in Ar at flow rate 2 slpm, driven
by the nanopulsed power generator, PRF 1 kHz and PV 17 kV.
Plasma treatment of PLLA electrospun scaffolds has
been carried out by means of Linear Corona (LC) plasma
source, shown in Fig.2, composed of a housing made of
dielectric material and a sharp stainless steel blade, 36
mm wide and 0.1 mm thick, as high voltage electrode; the
housing is provided of a gas inlet, a compensation
chamber and two parallel gas channels.
were then incubated with 1,4-diaminobutane, conjugated
with FITC 10-3M in carbonate buffer for 2 hours a RT
and finally shaken and rinsed four times for 5 minutes
each. Untreated (PLLA) and plasma-treated PLLA
(PLLA-LC) FITC conjugate were embedded with
glycerol 0.1% 1,4-phenylendiamine and stored at -20°C
until use. Control samples to evaluate the unspecific
binding of FITC were prepared removing EDC and sNHS
from the conjugation process. FITC conjugated PLLA
and PLLA-LC scaffolds were observed by NIKON
eclipse E600 (Nikon, Italy) equipped with digital CCD
camera Q imaging Retiga 20002V (Q Imaging, Surry, BC,
Canada). The images were analyzed using Imaging NIS
Elements software.
Figure 2. 3D drawings of the LC plasma source: rendering (left)
and section (right)
Electrolyte solution for lithium batteries
Electrolyte solution was composed by Ethylene
Carbonate (EC):Dimethyl Carbonate (DMC) (EC:DMC
(50:50 w/w)). EC and DMC were purchased from
Sigma-Aldrich and used without any further purification.
Post treatment of PVDF separator has been performed
with a dielectric barrier discharge (DBD), with an high
voltage aluminium electrode (60 mm x 60 mm surface), a
Pom-C dielectric (1 mm thickness) and a gap between
dielectric and the grounded plate of 1mm.
Plasma sources have been alimented by a pulse generator
producing high voltage pulses with a rise time of few
kV/ns, a peak voltage (PV) of 7-20 kV into a 100-200 Ω
load impedance and a maximum pulse repetition
frequency (PRF) of 1000 Hz.
Electrospinning apparatus
The electrospinning (ES) apparatus, made in house,
was composed of a high voltage DC power supply
(Spellman, SL 50 P 10/CE/230), a syringe pump
(KDScientific 200 series), a glass syringe, a stainless steel
blunt-ended needle (inner diameter: 0.84mm) connected
with the power supply electrode and a grounded
aluminium plate-type collector (7 × 7 cm2). The polymer
solution was dispensed through a Teflon tube to the
needle that was vertically placed on the collecting plate
Polymeric solutions for membranes production
Poly(L-lactic acid) (PLLA) (Lacea H.100-E) (Mw = 8.4
x 104 g/mol, PDI = 1.7) was supplied by Mitsui Fine
Chemicals. PLLA was dissolved at a concentration of
13% w/v in dichloromethane (DCM):dimethylformamide
(DMF) (65:35 v/v) and at a concentration of 10% w/v in
pure DCM. Poly(vinylidene fluoride) (PVDF) was
dissolved at a concentration of 15% w/v in Acetone
(Ac):Dimethyl sulfoxide (DMSO) (70:30 v/v). DCM,
DMF, Ac and DMSO were purchased from
Sigma-Aldrich and used without any further purification.
PLLA Scaffold functionalization
PLLA scaffolds
first
was
activated
with
1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)
and N-Hydroxysuccinimide (sNHS); activated scaffolds
PLLA and PVDF morphological characterization
Scanning Electron Microscope (SEM) observations
were carried out using a Philips 515 SEM by applying an
accelerating voltage of 15 kV on samples sputter coated
with gold. The distribution of fiber diameters was
determined through the measurement of about 250 fibers,
by means of an image acquisition and analysis software
(EDAX Genesis), and results were given as the average
diameter±standard deviation.
Weight measurements
Dry and soaked in electrolyte membranes were
weighted by means of an electronic balance (Kern, mod.
ABT 120-4m, p=0,1 mg).
3. Results
In this section, results concerning pretreatment of
PLLA electrospinning polymeric solution, treatment of
PLLA mats for biomedical applications and treatment of
PVDF separators for energy storage will be shown.
Pretreatment of PLLA electrospinning polymeric solution
For biomedical applications electrospun nanofibrous
mats represent an interesting resource, however, the risk
of residual toxic solvent traces dramatically reduce
scaffold biocompatibility. Usually, bead-free fibres
electrospun PLLA fibres are fabricated from a mixed
solution of DCM:DMF. The removal of DMF, a high
boiling point solvent, from the solution could be suitable
for the production of solvent-free scaffolds.
Figure 3 shows PLLA fibres obtained from
electrospinning a PLLA 13% w/v solution in DCM:DMF
(65:35 v/v). Fiber diameter range is typically
sub-micrometric, in the range of hundreds nanometers.
However, PLLA in pure DCM solution could be
electrospun at higher polymer concentration (e.g. 20%
w/v, SEM image in Figure 4 A), but fibers with
micrometric diameter are typically produced; on the other
hand at lower concentrations there was no fiber formation,
as reported in Figure 4 B.
Figure 6. SEM image of fibers electrospun from a plasma
exposed PLLA (10% w/v) in 100% DCM solution, after 180 (A)
and 210 (B) minutes from the treatment. Plasma parameters: 2
slpm Ar flow rate, PV = 27 kV, PRF = 1000 Hz and treatment
time = 120 seconds.
Figure 3. SEM image of electrospun PLLA mat (polymer
dissolved at a concentration of 13% w/v in DCM:DMF (65:35
v/v).
Figure 7. SEM images of fibers electrospun from an exposed
PLLA solution (6 ml at 10% w/v) in 100% DCM, using 2 slpm
Ar flow rate, treatment time = 120 s and PV=17 kV and PRF =
1000 Hz for image A, PV = 27 kV and PRF= 500 Hz for image
B.
Figure 4. SEM images of electrospun PLLA dissolved at a
concentration of 20% (A) and 10% (B) w/v in 100% DCM.
Solutions of PLLA in pure DCM have been exposed to
plasma in order to evaluate the possibility to improve
their electrospinnability. The electrospinnability was
evaluated through the comparison of the electrospun fiber
morphology, by means of SEM observations. PLLA
solutions were electrospun by using the following
conditions: voltage = 15 kV, needle-to-collector distance
= 15 cm, flow rate = 0.015 ml/min. The plasma source
was driven by the nanopulsed power supply: good quality
nanofibers (average diameter = 0,39 ± 14 µm) without
bead-like defects were electrospun (Figure 5).
Figure 5. SEM image of fibers electrospun from a plasma
exposed PLLA (12 ml at 10% w/v) in 100% DCM solution.
Plasma parameters: 2 slpm Ar flow rate, PV = 27 kV, PRF =
1000 Hz and treatment time = 120 seconds.
Treatment effectiveness persisted for 180 minutes after
plasma treatment (Figure 6 A); after 210 minutes the
presence of beads along the fiber axis has been observed
(Figure 6 B).
The effect of PV and PRF on the electrospinnability was
also investigated (Figure 7) highlighting the decay in fiber
quality when these parameters are decreased.
Treatment of PLLA mats for biomedical applications
PLLA electrospun nanofibrous scaffolds have been
widely used as artificial scaffolds to mimic the fibrillar
arrangement of the extracellular matrix (ECM) both from
a morphological and a mechanical point of view. PLLA,
and polyesters in general, are hydrophobic materials. In
order to increase hydrophilicity and improve
biointegration of PLLA scaffolds, electrospun mats have
been treated by mean of LC plasma at atmospheric
pressure.
Cell viability and proliferation on a material is strongly
linked to the introduction of specific functional groups on
the surface. In order to characterize the effect of LC
plasma treatment on PLLA scaffolds, chemical
conjugation reactions have been carried out under
introduction of FITC molecules on its surface and the
introduction of carboxyl groups, as coupling sites, has
been studied. FTIC fluorophore has been used to
characterize the surface “activation”. The introduction of
carboxyl groups on PLLA scaffolds after LC plasma
treatment was characterized by fluorescence analysis,
after the chemical conjugation with FITC molecules. The
signal to background ratio of plasma treated PLLA
scaffolds increased of more than five times compared to
untreated scaffolds, while no relevant value of
fluorescence intensity was registered for control scaffolds
(unspecific binding of FITC), as highlighted in Figure 8A.
In order to determine the aging of plasma modification,
FITC conjugation and fluorescence analysis were
performed at different times after plasma treatment;
Figure 8B shows that the intensity of fluorescence
remains constant for at least 6 days after plasma
treatment.
A
B
Figure 9. Separator uptake values.
Figure 8. Mean intensity of COOH-FITC binding (A), Aging of
mean intensity of COOH-FITC binding (B).
Treatment of PVDF separators for energy storage
Electrospun nanofibrous mats could be also used as
separators in lithium batteries, due to their high surface
area and porosity that can ensure a good electrolyte
uptake. Any presence of beads in the fibers decreases
mechanical properties of the mat: plasma jet exposure of
the PVDF polymeric solution was performed to avoid
beads formation. Ionic conductivity and lithium cell
capacity are proportional to the electrolyte solution
quantity [9]: any increase of the electrolyte content causes
an increase of batteries performances. In order to increase
electrolyte uptake, PVDF electrospun separators have
been treated by atmospheric pressure plasma to induce a
surface chemical modification of the mat. PVDF solutions
were electrospun by using the following conditions:
voltage = 15 kV, needle-to-collector distance = 15 cm,
flow rate = 0,01 ml/min. Electrolyte uptake was realized
by soaking the separators in a solution composed by
Ethylene Carbonate: Dimethyl Carbonate (50:50 w/w)
and calculated according to: Uptake [%] = [(masswet massdry)/massdry]*100.
In Figure 9 uptake results are presented for a) pristine
mat (Control), b) mat electrospun from a solution exposed
to plasma jet (Pre-Treated), c) mat electrospun form an
unexposed solution and treated by DBD (Post-Treated), d)
mat electrospun from a solution exposed to plasma jet and
treated by DBD (Pre and Post Treated). Plasma treatment
enhanced electrolyte uptake for soaking time up to 20
minutes and the best result has been achieved when both
Pre and Post Treatment have been performed. The highest
uptake (about 700%) was achieved for Pre and Post
Treated mats, while a significantly lower uptake of 500%
was achieved for pristine mats.
4. Conclusion
Electrospinning can be applied in several fields, such as
biomedical applications, energy storage, composite
materials and filters. Limits of such materials lead to
research novel methods to improve their properties.
Plasma assisted improvement of electrospinnability
has been implemented in order to produce solvent-free
PLLA nanofibrous scaffolds, suitable in tissue
engineering. Moreover non-thermal atmospheric plasma
treatment is able to promote functionalization and
biocompatibilization of PLLA electrospun scaffold. A
drastic increase of –COOH concentrations can be
registered on the LC modified substrate compared to
pristine scaffold.
Furthermore a pre and post treatment of PVDF
electrospun separator can enhance the electrolyte uptake,
leading to the production of innovative Li-ion batteries
with a higher capacity than that of conventional ones.
Future studies will be dedicated to the in-flight
treatment of polymer jet and to the investigation of
plasma interaction with polymer solutions.
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