Development of antimicrobial Ion Jelly fibers

Abstract

We report a method to obtain electrospun fibers based on ionic liquids and gelatin, exhibiting
antimicrobial properties.

Full text

Development of antimicrobial Ion Jelly fibers†
Renato dos Santos,
a
ˆ
Angelo Rocha,
b
Ana Matias,
c
Catarina Duarte,
c
Isabel S´
a-
Nogueira,
d
Nuno Lourenço,
b
Jo~
ao Paulo Borges*
e
and Pedro Vidinha*
a
We report a method to obtain electrospun fibers based on ionic liquids and gelatin, exhibiting
antimicrobial properties.
Introduction
The improvement of biodegradable biomaterials has greatly
impacted the development of modern biology and medicine.
1–3
The great advantage of these materials results from the fact that
they can be broken down and removed aer they have served
their purpose. The applications of this type of polymers goes
from surgical sutures and implants to more advanced
approaches, such as tissue engineering and drug delivery. To
meet this functional demand, materials should be designed so
as to exhibit certain physical, chemical, biological, biome-
chanical, and degradation properties. In this respect a wide
range of natural (e.g. collagen,
4,5
chitosan
6,7
gelatin
37
and
synthetic (e.g. PLGA
8
)) biodegradable polymers have been
researched.
Gelatin is one of the most versatile biomaterials to fulll the
above challenges. It is a widely available natural polymer that is
prepared through thermal denaturation of collagen, aer acid
or alkaline pre-treatment.
9
Gelatin is commonly used for
different pharmaceutical and medical applications due to its
biodegradability and biocompatibility in physiological envi-
ronments.
10
For instance, it is commonly used as a plasma
expander,
11
as an ingredient in drug formulations,
12
and as a
sealant for vascular prostheses.
13
Over the last years, several
modications have been introduced in the gelatin structure
with the aim to achieve new physical and chemical properties.
14
In this respect, we have recently reported a new biomaterial
obtained by cross-linking gelatin and an ionic liquid (IL),
designated Ion Jelly.
15
Ion Jelly can be described as a polymeric
material that combines the chemical versatility of an IL with the
morphological and mechanical versatility of gelatin. The
interest in the use of ILs has grown exponentially in the last
decade, mostly motivated by their tailor-made properties and
wide range of applications.
16
The combination of ILs with
gelatin has been shown to be an interesting and promising
strategy not only to prepare transparent, exible and conduct-
ing lms for different electrochemical devices,
15
but also as an
enzyme immobilization matrix
17
and hydrogels.
36
Recently, we reported the preparation of Ion Jelly bers
trough electrospinning in order to obtain high surface area
conductive materials.
18
Electrospinning is a technique that uses
electrostatic charges to produce bers from polymer melts or
polymer solutions.
19
Solid bers are obtained from electried
jets that are continuously elongated due to the electrostatic
repulsions between the surface charges and the evaporation of
solvent.
20
Some applications, such as drug delivery systems or
tissue engineering, usually rely on nanoscale or sub-micrometer
structures because of their unique properties, namely increased
surface area or the ability to mimic the structural dimension of
extracellular matrices.
21,22
Some of these systems are composed
of polymer bers, and electrospinning has been shown to be a
straightforward technique to produce bers that can reach
nanometer sizes.
23
Nevertheless, the number of reports on the
preparation of bers with intrinsic antimicrobial properties is
very limited.
24
Chitosan has a polycationic nature that imparts
non-toxic, biocompatible, biodegradable and bioactive proper-
ties
25,26
to chitosan based materials, but most other polymeric
bers have a propensity not to be biodegradable.
24
The development of a simple and friendly methodology that
could produce biodegradable antimicrobial bers in an easy
manner would be an important step for the development and
a
REQUIMTE, Departamento de Qu´
ımica, Faculdade de Ciˆ
encias e Tecnologia,
Universidade Nova de Lisboa, 2829-516 Caparica, Portugal. E-mail: pm.gomes@fct.
unl.pt
b
IBB-Institute for Biotechnology and Bioengineering, Centre for Biological and
Chemical Engineering, Instituto Superior Tecnico, Av. Rovisco Pais, 1049-001
Lisboa, Portugal. E-mail: nmtl@ist.utl.pt
c
IBET/ITQB-UNL –Instituto de Biologia Experimental e Tecnol´
ogica e Instituto de
Tecnologia Qu´
ımica e Biol´
ogica, Aptd. 12, 2780 Oeiras, Portugal
d
CREM –Centro de Recursos Microbiol´
ogicos, Faculdade de Ciˆ
encias e Tecnologia,
Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal.
E-mail: isn@fct.unl.pt
e
CENIMAT/I3N, Departamento de Ciˆ
encia dos Materiais, Faculdade de Ciˆ
encias e
Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal. E-mail:
jpb@fct.unl.pt
†Electronic supplementary information (ESI) available: See DOI:
10.1039/c3ra44258f
Cite this: RSC Adv., 2013, 3, 24400
Received 8th August 2013
Accepted 10th October 2013
DOI: 10.1039/c3ra44258f
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application of these materials. Our goal was the development of
Ion Jelly bers displaying antimicrobial properties, based on
the combination of an antimicrobial IL with gelatin. For that
purpose, choline based ILs were synthesized, combining
choline chloride and an antimicrobial acid–mandelic acid.
27
Concerning the use of ILs on the preparation of biomate-
rials, namely bers, there are some aspects that need to be
reected upon. One of the most important aspects is
biocompability. It is well known that the selection of both
cation and anion is crucial to obtain biocompatible ILs.
28
Choline ILs meet this criterion, and have been used in different
applications, including as cross-linking agents for collagen
based biomaterials.
29
In addition, choline based ILs have been
reported to be active antimicrobial agents against cocci, rods,
and fungi.
30
Herein we report the preparation of biocompatible Ion Jelly
bers through with intrinsic antimicrobial properties, using
electrospinning.
Experimental
Chemicals
All reagents and solvents were obtained commercially unless
otherwise noted, and appropriately puried if necessary.
Gelatin was purchased from Panreac (Cultimed). Choline
chloride was purchased from Alfa Aesar (98 + %). (R)-Mandelic
acid (98%) was purchased from Fluka.
Synthesis of ILs
The ILs [Ch][Ac] (choline acetate), [Ch][Ib] (choline ibuprofen-
ate), [Ch][Ma] (choline mandelate) and [Ch][Ti] (choline tiglate)
were synthesized in our laboratory and details are provided in
the ESI.†
Ion Jelly preparation
Aqueous gelatin solutions (blanks) –120 mg of gelatin were
dissolved in 280 mL of distilled water to obtain a concentration
of 30% (w/w). To produce Ion Jelly bers, gelatin was main-
tained at 30% (w/w) while water was replaced respectively by
40 mL, 80 mL and 120 mL in order to yield solutions with IL
concentrations of 10, 20 and 30% (w/w). All solutions were
prepared under magnetic stirring at 40 C. The water contents
of both IL and Ion Jelly materials were determined by Karl–
Fischer titration.
Electrospinning
The Ion Jelly solution was poured into a 1 mL syringe tted with
a 23-gauge needle, which was then placed on the infusion
syringe pump (KDS100) to control the solution feed rate. A
conducting ring, 15 cm diameter, was held coaxially with the
needle tip at its center, and electrically connected to it. The
needle and ring were directly connected to the positive output of
a high-voltage supply (Glassman EL 30 kV). Aer applying the
electric potential between the metallic syringe-tip and the
collector, the solution was continuously fed to the syringe-tip at
a constant ow rate, and accelerated towards the grounded
collector target by the ensuing electric eld (a standard elec-
trospinning setup). Stainless steel grids were used as collector.
All this equipment was located inside an acrylic box in order to
control temperature and humidity, which were kept constant in
all experiments.
Differential scanning calorimetry (DSC)
DSC results were obtained at Laborat´
orio de An´
alises
(REQUIMTE) using a Setaram DSC131 at a scan rate of 20 C
min
1
. Two cycles were performed and samples were kept at
90 C for 5 minutes between them. The results shown are those
for the second heating cycle. Thermograms are available in
the ESI.†
Quantication of IL in Ion Jelly electrospun bers
Quantication of IL –[Ch][Ma] was performed in a Beckman
Coulter DU –800 spectrophotometer. The calibration curve for
the IL quantication was obtained with the absorbance
readings of four IL –[Ch][Ma] standards. The blank used was a
gelatin aqueous solution. Details in the ESI.†
Fiber characterization
Optical microscopy (OM) and scanning electron microscopy
(SEM) were used to evaluate ber morphology. The optical
microscope used was an Olympus BH-2. For SEM analysis, the
bers were coated with a gold/palladium alloy with a Polaron
SC502 sputter coater, and the microscope used was a Zeiss DSM
962. Samples were suspended over two strips of carbon
conductive tape.
Antimicrobial activity assessment
The agar diffusion test –Kirby–Bauer disk diffusion method –
was used to evaluate the antimicrobial properties of IL –[Ch]
[Ma]. Paper disks (1 cm
2
) and Ion Jelly bers were put in agar
plates inoculated with 100 mL of the previously diluted pre-
inoculum and were incubated during the rst 3 hours at room
temperature followed by an overnight incubation at 37 C. In
the case of Ion Jelly bers, a 1 cm
2
stainless/plastic collector was
used. The bacteria used for the agar diffusion tests were Bacillus
subtilis 168 (B. subtilis) and Escherichia coli K-12 (E. coli). Both
were pre-inoculated in 5 mL of LB medium and incubated for 16
hours in a Sanyo orbital incubator Orbi-Safe Ts. For all anti-
microbial tests, the pre-inoculums were rst diluted in LB
medium in a 1 : 50 proportion prior to the inoculation of agar
plates (containing LB medium (LAB M) with 1.6% w/v of agar
concentration), except where mentioned otherwise.
Biocompatibility assessment
Choline salts were investigated for biocompatibility using the
3-[4,5-dimethyltriazol-2-y1]-2,5-diphenyltetrazolium (MTT) cell
viability assay with CaCo-2 cell lines. All the ILs were tested in a
concentration range up to 16 000 mM. CaCo-2 cells were
cultured in 0.5% fetal bovine serum (FBS) for 24 h and then
incubated for 4 h with the ILs in a 5% FBS medium. Aer this
period, 100 mL of the colorimetric reagent MTT (0.5 g L21) was
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added to each well and lefor 4 h. MTT is reduced to a purple
formazan product by mitochondrial reductase enzymes active
in viable cells and therefore the amount of formazan product is
proportional to the number of viable cells. Reaction was
stopped with 150 mL DMSO in each well and formazan was
quantied by measurement of the absorbance at 540 nm in a
plate reader. Each sample was incubated in three different wells
and three different measurements were obtained. The ratio
between the absorbance of IL treated cells and the absorbance
of solvent treated cells (control) was used to determine the cell
viability. This parameter was represented as a function of the
base 10 logarithm of the IL concentration (mM).
Results
Fiber production optimization
Gelatin is not oenly used in the electrospinning process
because of its high degree of hydrogen bonds. Highly polar
solvents, such as ILs, acidic conditions or temperature increase
are successful strategies for reducing intermolecular interac-
tions.
22,31,32
Ion Jelly (IJ) ber production through electro-
spinning was reported by Pimenta et al.,
18
who used [C
2
OHmim]
[BF
4
]. Based on the experience acquired then, using different
ILs, some adjustments were needed in order to obtain bers
with choline based ILs. Parameters such as polymer and IL
concentration, applied voltage, distance between the needle tip
and the collector, ow rate, solution conductivity, temperature
and humidity were tuned in order to obtain choline-based IJ
electrospun bers without defects. Details are provided in
the ESI.†
The optimized conditions for the electrospinning of IJ using
choline-based ILs are presented in Table 1.
Choline based IJ bers obtained through electrospinning
have mean sizes around the lower end of the micrometer range
(Table 2). It is also evident that changing the anion of these ILs
has little or no effect on ber diameter.
IJ-ber morphology can be evaluated by SEM images. Fig. 1
reveals that the electrospinning of choline-based IJ solutions
under conditions described in Table 1 yielded sub-micrometer,
defectless and bead-free bers. In line with previous observa-
tions, images suggest that changing the IL does not have a
signicant effect on ber morphology (Fig. 2–4).
Thermal characterization
Both choline-based ILs and IJs were submitted to DSC analysis
to analyze their phase transitions. The thermograms (presented
in the ESI†) reveal that the IJs keep their structural properties in
a wide range of temperatures. Glass transition temperatures
(T
g
), displayed in Table 3, were observed for IL –[Ch][Ac], IL –
[Ch][Ti], but not for IL –[Ch][Ib] and IL –[Ch][Ma]. On the other
hand, all IJs exhibited a T
g
, with the exception of IJ –[Ch][Ib]
(despite having a slight curve inection on the thermogram
curve around 25 C). Even though IL –[Ch][Ma] did not exhibit
aT
g
, the resulting IJ –[Ch][Ma] has a T
g
¼62.3 C. Noteworthy is
the absence of endothermic peaks in the IJs curves related to
denaturation of the triple-helix crystalline structure of gelatin.
Bigi et al. veried that various gelatin samples (with and
without cross-linking) showed a denaturation temperature
around 41 C.
37
The absence of endothermic peaks in this
region suggests that ILs interact with gelatin in such a way as to
hamper the ability of polymer chains to renaturate upon
cooling.
Antibacterial activity
One of the aims of the present work was to evaluate if IL
encapsulation in IJ electrospun bers was an effective way to
improve the antibacterial efficacy of a given IL. To evaluate this
possibility, IL –[Ch][Ma] and IJ –[Ch][Ma] bers were selected
for comparison of their antimicrobial properties with those of
mandelic acid, using the agar diffusion test. Our results show
that there is no loss of antibacterial properties of the mandelate
anion in the IL –[Ch][Ma]. However, there is a striking differ-
ence between the mandelic acid solution and IL –[Ch][Ma]
regarding antibacterial efficacy (see Table 4). This outcome is
possibly related with the higher viscosity of the IL and with
reduced diffusion in the paper disk matrix, or reduced perme-
ability of the IL in cell membranes, compared with
mandelic acid.
Additionally, a stainless steel square (1 cm
2
area) was used to
collect the IJ –[Ch][Ma] bers, that were aerwards placed on an
agar plate previously inoculated with E. coli. Our results show
that IJ bers clearly enhance the antibacterial efficacy of the IL –
[Ch][Ma]. In fact there is an almost four-fold increase on the IL
antimicrobial efficiency aer encapsulation of the IL in elec-
trospun bers. The same amount of IL can be released in a more
efficient way because the higher surface area of the bers
improves IL diffusion towards the medium and consequently
towards bacterial membranes, reducing the problem associated
Table 1 Optimized conditions for electrospinning of choline-based IJ solutions
IJ solution % IL v/v % Gelatin w/v Voltage (kV) Distance (cm) T(C) Humidity (%) Flow rate (mL h
1
)
IJ –[Ch][Ac] 15 30 15 15 40 20 [0.14]
IJ –[Ch][Ma] 23 30 15 15 40 20 [0.14]
IJ –[Ch][Ti] 15 30 15 15 40 20 [0.14]
IJ –[Ch][Ib] 23 30 15 15 40 20 [0.14]
Table 2 Mean diameters (mm) of IJ electrospun fibers
IJ-solution IJ –[Ch][Ac] IJ –[Ch][Ma] IJ –[Ch][Ti] IJ –[Ch][Ib]
Diameters
(mm)
1.08 0.25 1.06 0.23 1.23 0.33 0.96 0.23
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to high viscosity of this IL. This strategy enabled the IL to reach
nearly the same efficiency as mandelic acid (disk 2).
This antimicrobial assessment was also performed against
the Gram-positive bacteria B. subtilis. The same procedure for E.
coli was adopted. To avoid wire collector oxidation interference,
IJ bers were tested in a square-shaped plastic support instead.
For B. subtilis,IL–[Ch][Ma] encapsulation of IL in
IJ bers almost tripled its antimicrobial activity. Even though
Fig. 1 SEM images of IJ electrospun fibers at optimized conditions: upper left, IJ –[Ch][Ma]; upper right, IJ –[Ch][Ti]; bottom left, IJ –[Ch][Ib]; bottom right, IJ –[Ch][Ac].
Fig. 2 Agar diffusion tests for IJ –[Ch][Ma] (1), mandelic acid (2) and IL –[Ch][Ma] (3) against E. coli. (4) Stainless steel wire control. (5) Paper disk control.
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IJ –[Ch][Ma] ber membranes enhanced the IL properties, for B.
subtilis, mandelic aqueous solutions provided better results, in
opposition to the results obtained with E. coli cultures.
The results also suggest that B. subtilis is less sensitive to IL –
[Ch][Ma] and IJ –[Ch][Ma] than E. coli. As Gram-positive
bacteria, B. subtilis are more resistant to these antibacterials
agent due to presence of a higher amount of peptidoglycan on
the cell wall.
Biocompatibility assessment
Considering all the concerns about IL toxicity, the success and
practical applicability of any IL-Active Pharmaceutical Ingre-
dient (API) drug delivery system is dependent on toxicity
assessment. The toxicity of all choline based ILs used in this
work was evaluated using human colon adenocarcinoma cells,
Caco-2, which are a suitable and useful in vitro model for the
intestinal epithelial barrier studies.
33
The cytotoxicity assays
were performed as already described by Dias et al.
34
Briey,
Caco-2 cells were seeded at a density of 2 10
4
cells per well in
96-well plates and their media was replaced every 48 hours.
Aer cells reached conuence –72 hours aer seeding –the
medium was removed and cells were incubated for 4 h with
choline based ILs. The ILs were tested in a concentration range
of 500–15000 mM. The results obtained in this work show that
none of the choline-based ILs induce cytotoxicity in intestinal
epithelial cells aer 4 h of exposure in the concentration range
Fig. 3 Agar diffusion tests for IJ –[Ch][Ma] (6), mandelic acid (8) and IL –[Ch][Ma] (9) against B. subtilis. (7) Plastic wire control. (10) Paper disk control.
Fig. 4 Experimental cytotoxicity profiles of choline-based ILs in human colon
Caco-2 cells. Confluent cells were incubated with the different [Ch]-ILs for 4 h
before determination of viability.
Table 3 Glass transition temperature (T
g
)
IJ IL
[Ch][Ac] 68.6 84.5
[Ch][Ma] 62.3 —
[Ch][Ti] 78.6 70.7
[Ch][Ib] ——
Table 4 Antibacterial efficacy of mandelic acid, IL –[Ch][Ma] and IJ –[Ch][Ma] fibers against E. coli and B. subtilis
Disk/collector Agent Mandelate anion (mg) Inhibition area (cm
2
) Ratio 10
2
(cm
2
mg
1
)
E. coli 1IJ–[Ch][Ma] 8.05 0.71 1.55 (19.25 1.70)
2 Mandelic acid 8.15 0.03 1.47 (18.04 0.07)
3IL–[Ch][Ma] 8.37 0.06 0.39 (4.66 0.03)
B. subtilis 6IJ–[Ch][Ma] 8.29 0.74 1.06 (12.80 1.14)
8 Mandelic acid 9.06 0.03 3.46 (38.20 0.13)
9IL–[Ch][Ma] 8.88 0.06 0.39 (4.40 0.03)
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studied, except for choline ibuprofen. [Ch][Ib] drastically
decreased cellular viability to 50% aer 4 h, with an EC50 ¼
5827 mM, calculated from the dose–response curve using
GraphPad Prismasoware. However, this reduction was
predictable due to the inherent ibuprofen toxicity already
determined on Caco-2 cells (aer 24 h, EC50 ¼2200 mM).
35
Conclusions
Our results clearly show that the development of Ion Jelly bers
can be an interesting solution for the application of IL-APIs.
Moreover IL-APIs can also be considered non-toxic when
compared with other crystalline APIs in the market. With a wise
choice of ions, IL-APIs can be considered safe and their
encapsulation in IJ bers produced through electrospinning
can create a successful drug delivery system.
Acknowledgements
This work has been supported by Fundaç~
ao para a Ciˆ
encia e a
Tecnologia (FCT/MEC) through projects/grants PEst-C/EQB/
LA0006/2011, FCT-MEC through Strategic PEst-C/CTM/LA0025/
2013-2014, PEst-OE/EQB/LA0023/2011 PEst-OE/BIA/UI0457/
2011, PEst-OE/EQB/LA0004/2011 PTDC/EQU-EQU/104552/2008,
PTDC/EBB-EBI/099237/2008, PTDC/CTM/100244/2008, SFRH/
BPD/41546/2007 (P. Vidinha), SFRH/BPD/41175/2007 (N. Lour-
enço), and by FEDER. The NMR spectrometers are part of the
National NMR Network (RNRMN) and are funded by Fundaç~
ao
para a Ciˆ
encia e a Tecnologia.
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