Journal of Membrane Science 427 (2013) 207–217

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Journal of Membrane Science

journal homepage: www.elsevier.com/locate/memsci

Tailored fibro-porous structure of electrospun polyurethane membranes,

their size-dependent properties and trans-membrane glucose diffusion
Ning Wang a, Krishna Burugapalli a, Wenhui Song b,n, Justin Halls a, Francis Moussy a,
Yudong Zheng c, Yanxuan Ma c, Zhentao Wu d, Kang Li d
a
Brunel Institute for Bioengineering, Brunel University, Uxbridge, London, UK
b
Wolfson Centre for Materials Processing, Brunel University, Uxbridge, London, UK
c
School of Materials Science & Engineering, University of Science and Technology, Beijing, China
d
Department of Chemical Engineering, Imperial College, London, UK

a r t i c l e i n f o abstract

Article history: The aim of this study was to develop polyurethane (PU) based fibro-porous membranes and to
Received 25 June 2012 investigate the size-effect of hierarchical porous structure on permeability and surface properties of the
TM
Received in revised form developed electrospun membranes. Non-woven Selectophore PU membranes having tailored fibre
8 August 2012
diameters, pore sizes, and thickness were spun using electrospinning, and their chemical, physical and
Accepted 16 September 2012
Available online 7 October 2012
glucose permeability properties were characterised. Solvents, solution concentration, applied voltage,
flow rate and distance to collector, each were systematically investigated, and electrospinning conditions
Keywords: for tailoring fibre diameters were identified. Membranes having average fibre diameters—347, 738 and
TM
Selectophore polyurethane 1102 nm were characterized, revealing average pore sizes of 800, 870 and 1060 nm and pore volumes of
Electrospinning process
44%, 63% and 68% respectively. Hydrophobicity increased with increasing fibre diameter and porosity.
Fibro-porous membranes
Effective diffusion coefficients for glucose transport across the electrospun membranes varied as a
Glucose diffusion
function of thickness and porosity, indicating high flux rates for mass transport. Electrospun PU
membranes having significantly high pore volumes, extensively interconnected porosity and tailorable
properties compared to conventional solvent cast membranes can find applications as coatings for sensors
requiring analyte exchange.
& 2012 Elsevier B.V. All rights reserved.

1. Introduction be varied to obtain different fibre sizes, orientation and the

resulting electrospun 3D structures [2].
Electrospinning is a fibre spinning technology wherein fibres Electrospun membranes, having unique non-woven fibro-porous
having diameters from 2 nm to 10s of mm are spun under the structure, offer small pores sizes, large pore volumes, and good
influence of electric field [1,2]. The advantage of electrospinning is mechanical stability per unit weight, leading to their wide-spread
that submicron fibre diameters can be achieved, which is not use as nano-filtration membranes for air and water purification
feasible with conventional dry or wet spinning methods. Under the [5,6]. As filtration membranes, they were shown to support high flux
influence of electric field, a flowing dielectric liquid stretches as it rates for air, moisture and fluid permeability, which is essential for
emerges from a needle or a hole in a flat end (spinneret) to form a light-weight garments, wound dressings as well as high-pressure air
cone, commonly referred to as Taylor cone, from the tip of which and water filtration systems [6]. More recently, electrospun bioma-
tiny droplets lift off (electrospraying) and if the applied voltage is terials have been of particular interest for biomaterials and tissue
sufficiently large, the droplets lifting off coalesce to form (usually engineering due to their remarkable similarity to nano-fibrous
one) thin jet of liquid (electrospinning) [2–4]. The formation of the natural extracellular matrix (ECM) [7–9]. They have been tested as
Taylor cone and the thin jet of liquid are also dependent on the scaffolds for tissue engineering of blood vessel, skin, bone, cartilage,
viscosity and the electrical resistivity of the polymer solution [2–4]. skeletal muscle, neural and many other soft tissues [7–9]. Their
Provided the solvent containing the polymer evaporates fast larger surface area per unit volume also prompted development as
enough, the thin jet forms the fibre. The continuous electrospun electrodes for sensing applications and carriers for drug delivery
fibre is collected on a grounded collector, whose configuration can [7,10].
However, to our knowledge, they have not been reported as
mass-transport limiting or anti-fouling membranes for implantable
n
Corresponding author Tel.: þ44 1895266123; fax: þ44 1895269737. biosensors that require analyte exchange. Biocompatible membrane
E-mail address: wenhui.song@brunel.ac.uk (W. Song). technologies are of major importance in medical applications.

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.memsci.2012.09.052
208 N. Wang et al. / Journal of Membrane Science 427 (2013) 207–217

Coatings for biosensor and other medical devices are among a 12 wt%), flow rate (0.1 to 1.2 ml/h), applied electric potential
number of applications of life-saving diagnosis, monitoring and (11–25 kV) and distance between spinneret and collector (18–24 cm),
therapeutic devices. The coatings are often required to operate in were varied to optimize the parameters for electrospinning PU at
complex biological fluids such as blood or plasma. Selective trans- ambient conditions (2373.32 1C room temperature and 35–47%
port of targeted analyte molecules, sieving of large biomolecules, relative humidity). Electrospinning time was varied from 1 to
biocompatibility—preventing biofouling and fibrous encapsulation 120 min to obtain membranes of different thicknesses. The mem-
and robust mechanical properties are essential for making inter- branes were dried at room temperature and stored in a power
ference free and long-term sensing especially when implanted in assisted vacuum desiccator until further use.
the body.
Solvent cast polyurethanes have been widely used as mass- 2.2. Infrared spectroscopy
transport limiting membranes for implantable biosensors due to
their excellent hydrolytic stability and good biocompatibility An ATR-FTIR spectrophotometer (PerkinElmer Inc.) was used
[6,11]. However, their permeability to solute (e.g. glucose) trans- to investigate any qualitative changes to the polymer structure
port is susceptible to huge variations (within and between the due to electrospinning, and also to qualitatively monitor any
manufacture batches) due to their unpredictable pore geometries residual solvents in the electrospun membranes. Each spectrum,
and pore size distributions [12,13]. The hypothesis for our overall acquired in transmittance mode, was an average of 128 scans at a
research was that the highly interconnected porous structure resolution of 4 cm  1.
of PU electrospun coatings would provide highly controllable
permeability for solute transport, as well as bio-mimicking 3-D 2.3. Morphology
fibro-porous structure of the natural extracellular matrix for
implantable biosensors. The quality of electrospun membranes being prepared was first
However, before the membranes can be applied on sensors, it screened visually under an optical microscope (LEICA S60) to ascer-
was first essential to determine the electrospinning parameters for tain the uniformity of the fibre being formed. Thereafter, morphology
obtaining the fibres of desired diameters and membrane structure. of small samples of the different electrospun membranes were
Hence, in this study, we systematically evaluated the different sputter coated for 30 s with gold using an AGAR high-resolution
parameters namely, solvents, solution concentration, applied sputter-coater and observed under SEM (Zeiss Supra 35VP field
voltage, flow rate and distance to collector for electrospinning emission SEM (FESEM) in SE mode).
TM
the commercial Selectophore PU. Different combinations of para-
meters were identified to electrospin different PU fibre membranes 2.4. Fibre diameter and membrane thickness
(on a flat plate collector) having average fibre diameters specifi-
cally in the range between 100 and 1500 nm. Following optimiza- The fibre diameters were measured on SEM images using a
tion, a set of PU membranes having average fibre diameters of 347, software programme for length measurements developed in-house
738 and 1102 nm were chosen for extensive characterization of using Matlab. For each measurement, the software first requires a
membrane properties. These sets of membranes were primarily line to be drawn at one edge along the length of the fibre followed
intended for application as mass-transport limiting membranes for by a second line perpendicular to the first line drawn across to the
implantable glucose biosensors. Hence, in this paper, we further other edge of the fibre to obtain the actual and the accurate
report the characterization of their properties using FTIR (qualita- measurement for fibre diameter. The accuracy of these measure-
tive chemical stability), SEM (morphology, fibre diameter, and ments was cross-confirmed with Image J image analysis software.
membrane thickness), bubble point and gravimetry (porosity), For the measurement of fibre diameter, a total of 160 measure-
contact angle (hydrophilicity) and glucose diffusion across the ments were made on 8 different SEM images, each representing
membranes’ cross-section (permeability to analyte/solute). a non-overlapping random field of view for each electrospun
membrane configuration. To measure thickness, the membranes
were snap-frozen using liquid nitrogen, followed by cutting using
2. Materials and methods
a scalpel and images of their cross-sections obtained using SEM.
TM The above-mentioned software was used for measuring membrane
Selectophore PU [a medical-grade aliphatic poly(ether-
thickness. Membrane thicknesses were also measured using a
urethane)], Tetrahydrofunan (THF), N,N-Dimethylformamide
digital micrometer.
(DMF), bovine serum albumin (BSA), glutaraldehyde grade I
(50%) (GTA), glucose oxidase (GOD) (EC 1.1.3.4, Type X-S, Aspergil-
2.5. Pore size and porosity
lus niger, 157,500U/g, Sigma), ATACS 5104/4013 epoxy adhesive,
Brij 30, D-(þ)-glucose and 0.01 M phosphate buffered saline (PBS)
The pore size for the different membranes was measured using
tablets were purchased from Sigma-Aldrich–Fluka. Teflon-coated
extrusion porosimetry (also called bubble point measurement) as
platinum–iridium (9:1 in weight, | 0.125 mm) and silver wires
reported earlier [14]. Membranes electrospun for 2 h were used
(| 0.125 mm) were obtained from World Precision Instruments,
for bubble point measurements. The pressure needed to blow air
Inc. (Sarasota, FL).
through a liquid filled membrane was used to determine the
TM bubble point. The range of pore sizes (radius a) was calculated
2.1. Electrospinning Selectophore PU using the Young–Laplace equation [14] (Eq. (1)):

A typical vertical setup consisting of a high voltage power 2gst cos y

a¼ ð1Þ
supply (EL30R1.5, Glassman High Voltage Inc., Hampshire, UK), a DP
syringe pump (Fusion 100), a PTFE tubing (| 1/160 ) connecting a where DP is the differential pressure, gst the surface tension of the
10 ml plastic syringe (BD) to a 22G stainless steel needle (BD, flat- wetting liquid and y the wetting angle. For a completely wetted
tip, FISHMAN, UK) spinneret, and a 16  16 cm2 grounded steel membrane with all pores filled with the wetting liquid, cos y is 1
plate collector was used for electrospinning PU (Supplementary [14]. This is valid if the walls of the pores are assumed to be
Fig. 1). Solvents (THF and DMF mixed in weight ratios of 100:0, straight with a sharp edge, as there would be a film of the wetting
70:30, 50:50, 30:70 and 0:100), solution concentration (8, 10 and liquid presenting on the pore wall. In any cases, for instance in
 N. Wang et al. / Journal of Membrane Science 427 (2013) 207–217 209

our study, a contact angle (y) of 201 was used giving a value of Fick’s law and mass balance conditions between the two cham-
0.94 (cos 201). bers A and B given Eq. (4): [18,19].
The porosity of the membranes was determined using gravimetry. C B C A t
The dimensions of the membranes were measured using micrometer ¼ et ð4Þ
C B0 C A0
(No. 293-832 Mitutoyo, Japan) and ruler to obtain thickness and
surface area. The weight of the sample was measured using a high with
sensitivity balance (Fisher Scientific, resolution 0.01 mg). The result- d V AV B
ing data was fitted in Eqs. (2) and (3) to obtain the apparent density t¼
Def f S ðV A þ V B Þ
(rapp) and porosity (e) of the membranes.
where CA, CB and VA and VB are the concentration and the volume
m
rapp ¼ ð2Þ of chambers A and B, l is the membrane thickness, S is the
dA
membrane area, and Deff is the effective diffusion constant in the
rapp membrane. The mean relaxation time t for each membranes with
e ¼ 1 ð3Þ different thickness was calculated by linear regression of
rb
ln(CB  CA)/CB0 CA0) versus t from experimentally measured
where m¼the mass of the membrane (g), d¼the thickness of the values of tracer concentration of chamber B at different times,
membrane (cm), A¼the area of nanofibrous mat (cm2), rb ¼the bulk and then Deff could be calculated from the mean relaxation time t.
TM
density of materials (g/cm3). The bulk density for Selectophore PU
3
as reported by the manufacturer was 1.04 g/cm [15].
2.8. Glucose biosensor

2.6. Contact angle measurements A miniature coil-type implantable glucose biosensor, routinely
used in our laboratory, [12,20] was used to continuously monitor
The contact angles for a drop of distilled water on electrospun the diffusion of glucose across the electrospun PU membranes
membranes were measured using a contact angle instrument (Section 2.7). The amperometric sensor is a two electrode system
(OCA15þ, Data-physics, Germany) at room temperature. A single based on Pt–Ir working and silver/silver chloride (Ag/AgCl)
drop of 1 mL DI water was dropped on the surface of a flat reference electrodes. The working electrode consisted of a Pt–Ir
10  10 mm membrane using a syringe perpendicular and image coil reinforced with cotton, which was coated with an inner
captured in o1 s after the water droplet became stable on the glucose oxidase enzyme layer and an outer epoxy-polyurethane
surface. This process was repeated four times on each membrane. mass-transport limiting membrane. The sensor response currents
The contact angles were then measured using the instrument’s were logged using Apollo 4000 Amperometric Analyzer (World
SCA20 software. Precision Instruments Inc., Sarasota, FL) at 0.7 V versus Ag/AgCl
reference electrode and correlated with changes in glucose con-
2.7. Diffusion test centrations as reported earlier [12,20].

The effects of fibre diameter, thickness and porosity of electro- 2.9. Statistical analysis
spun PU membranes on their permeability to glucose were tested
using a biodialyser (singled-sided biodialyser system with mag- Statistical analyses were done using statistical software (SPSS
net, 1 ml, Sigma-Aldrich) in a beaker as a two-component diffu- v.15). One-way analysis of variance (ANOVA), using Tukey’s test
sion chamber. The biodialyser has an integrated magnet, 1 ml for post hoc evaluation was used to identify statistical differences
sample well and a threaded cap ring to mount a membrane on top of Po0.05.
of the sample well exposing a 113.14 mm2 membrane area for
diffusion. Membranes for diffusion test were prepared using a
ring-shaped aluminium template having an inner circle of 12 mm 3. Results and discussion
diameter cut out and an outer diameter of 22 cm. Following
electrospinning, the membranes on the template ring were 3.1. Optimization of electrospinning parameters
harvested and soaked in PBS (pH 7.4) at 37 1C for seven days to
ensure equilibrium swelling of the membranes. For diffusion test, Each of the parameters: solvents, flow rate, applied potential,
the donor solution chamber (A) of the biodialyser was filled with distance from spinneret to collector and solution concentration
1 ml of glucose solution in PBS and the wet membrane was was individually varied while keeping all the other parameters
mounted and secured with the treaded cap ring. The assembly constant to study each of their influence on fibre structure and
was immersed in 49 ml of receiving PBS without glucose (cham- diameter.
ber B) and rotated slowly (  250 rpm) at a fixed rate and
temperature in order to minimise boundary layer effects (diffu- 3.1.1. Solvents
sion resistance at the interface between the liquid and mem- For assessing the solvent effects on electrospun PU fibre
brane) [16,17]. Pre-calibrated amperometric glucose sensors structure, 20 kV applied voltage, 1 ml/h flow rate, 22 cm distance
made in-house were immersed in the receiving PBS to continu- to collector and 10 wt% solution concentration were used, while
ously log the changes in glucose concentration (as described in varying the solvent composition, having THF and DMF at ratios of
Section 2.8). The donor chamber solution concentration was 100:0, 70:30, 50:50, 30:70 and 0:100 (w/w). The morphology and
chosen such that the eventual equilibrium glucose concentration fibre diameter distribution histograms for the electrospun mem-
of receiver solution reaches 30 mM. The experiment was run branes as a function of solvent composition are presented in
overnight to ensure equilibrium was reached. The sensor Fig. 1. When PU was electrospun with its solution in 100% THF,
response current was converted to concentration versus time hollow elongated spheroid shaped beads of 2–10 mm diameter
(flux) curve. The effective diffusion coefficient was determined formed (Fig. 1A) and the spinneret needle tip often got blocked
from the time dependent glucose concentration assuming that a during the electrospinning process. This was mainly due to the
quasi-steady-state concentration condition within the membrane. rapid evaporation of THF, which has a low boiling point, viscosity
Based on the assumptions, combining the diffusion equation with and surface tension (Table 1). Although much less prominent, the
210 N. Wang et al. / Journal of Membrane Science 427 (2013) 207–217

Table 1
Solvents used in this study and their properties [49].

Solvent Boiling Surface tension Dielectric Dipole Viscosity

point (1C) (mN/m) constant moment (Pa s)

THF 66 26.4 7.5 1.63 5.5  10  4

DMF 153 37.1 38.3 3.82 9.2  10  4

Table 2
Systematic evaluation of effects of the electrospinning parameters namely,
solvents, flow rate, applied voltage, distance to collector and solution concentra-
TM
tion on the fibre diameter of Selectophore PU (Mw  100,000).

Solvents (%) Flow Applied Distance Solution Fibre

wt/wt rate voltage from concentration diameter
(ml/h) (kV) spinneret to (%, wt/wt) (nm)
THF DMF collector
(cm)

100 0 1 20 22 10 486 7231a

70 30 1 20 22 10 735 7362a
50 50 1 20 22 10 738 7131
30 70 1 20 22 10 538 7208
0 100 1 20 22 10 517 7340a
50 50 0.1 20 22 10 445 7109
50 50 0.2 20 22 10 624 7117
50 50 0.6 20 22 10 670 7122
50 50 1.0 20 22 10 738 7131
50 50 1.2 20 22 10 695 7129
50 50 1.0 15 22 10 1120 7247
50 50 1.0 20 22 10 738 7131
50 50 1.0 25 22 10 551 7111
50 50 1.0 20 12 10 1011 7161
50 50 1.0 20 17 10 845 7115
50 50 1.0 20 22 10 712 790
50 50 1.0 20 27 10 737 7111
50 50 1.0 20 32 10 885 7128
50 50 1.0 20 22 8 423 792a
50 50 1.0 20 22 10 738 7131
50 50 1.0 20 22 12 936 7197

a
Indicates bead formation.

synergetic effects of the high dielectric constant, dipole moment,

boiling point and surface tension of DMF (Table 1) are attributed
to increase the surface tension, decrease the volatility of the
mixed solvent, increase conductivity of the blend-solvent solu-
tions, and thus improve the mass throughput of the solutions
from the spinneret, preventing bead formation and clogging of
spinneret, in agreement with other reports in the literature [21].
Beyond the optimal conductivity the higher charge density in the
electrospinning jet causes bending instability and increases jet
path and stretching, resulting in a reduction in fibre diameter.
Fig. 1. Morphology and fibre diameter distribution histograms for the electrospun However, when PU was electrospun in pure DMF, although not
membranes as a function of solvent composition—ratios of THF: DMF at 100:0 phase-separated and hollow, spindle shaped beads formed
(A, F), 70:30 (B, G), 50:50 (C, H), 30:70 (D, I), 0:100 (E, J). PU solution concentration
(Fig. 1E). Such morphology was also reported in other studies
of 10% (w/w), 20 kV applied voltage, flow rate of 1 ml/h and 22 cm distance to
collector, were kept constant. [22–24], attributed to the higher surface tension but relatively
lower viscosity of PU solution in 100% DMF. As a result, for
optimization of rest of the parameters THF:DMF solvent ratio was
hollow spheroid shaped beads still formed when a 70:30 ratio fixed at 1:1.
THF:DMF solvent mixture was used (Fig. 1B). With increasing
DMF in the mixed solvent to 50:50 (1:1) and 30:70 THF to DMF
solvent ratios, the bead formation was eliminated (Fig. 1C and D). 3.1.2. Solution flow rate
However when PU was electrospun in 100% DMF spindle shaped Flow rate was varied from 0.1 to 2 ml/h, while maintaining
beads formed (Fig. 1E). Overall, the fibre diameter measurements solution concentration at 10%, voltage at 20 kV and distance to
(Fig. 1F–J, Table 2) in conjunction with morphology (Fig. 1A–E) collector at 22 cm. For flow rates from 0.1 to 1.2 ml/h, no bead
revealed an increase in fibre diameter with a corresponding formation was observed. The size of the Taylor cone increased
narrower fibre diameter distribution until 50:50 THF:DMF and with increasing flow rate. But, when the flow rate reached 2 ml/h,
vice versa, clearly indicate the effect of the polarity and evapora- the size of droplet was too big to suspend at the tip of the needle
tion rate of solvents on the electrospinning of PU. It is clear that resulting in the free fall of droplets on the collector. As listed in
 N. Wang et al. / Journal of Membrane Science 427 (2013) 207–217 211

Table 2, the average fibre diameter increased up to a flow rate of 3.1.5. Solution concentration
1 ml/h and thereafter it appeared to decrease. However, for flow For a given molecular weight of a polymer and solvent chosen,
rates o1 ml/h, at the set applied voltage of 20 kV, the electro- its solution concentration is the primary factor affecting its
spinning jet emerging from the Taylor cone was not stable and viscosity and therefore its electrospinnability. For electrospinning
continuous across the electrospinning duration, indicating 1 ml/h PU, 3 concentrations, namely 8%, 10% and 12% were tested, while
as the optimum flow rate for the fixed parameters. The increase in having 1:1 for THF:DMF solvent ratio, 20 kV applied voltage, 1 ml/h
fibre diameters with increasing flow rate up to 1 ml/h can be flow rate and 22 cm distance to collector constant (Table 2).
attributed to the increasing volume of polymer solution in the The morphology and fibre diameter distribution histograms are
Taylor cone. However, the increase peaked and started to presented in Fig. 1C and H and Fig. 2. Bead formation was
decrease with further increase in flow rate. The decrease is observed for an 8% PU solution in 1:1 THF:DMF (Fig. 2A), indicat-
apparent from the fact that for the increasing volume of polymer ing a less viscous solution. At 10% and 12% PU solution concen-
solution in the Taylor cone in conjunction with increasing the trations, the viscosity was sufficient to prevent bead formation
flow rate, a corresponding increase in surface charge (induced by (Figs. 1C and 2C). A trend of an increase in the average fibre
increasing the applied voltage) is essential to stretch the Taylor diameter having wider distribution with increasing concentration
cone, stabilize the jet and reduce the fibre diameter [25]. was observed as illustrated in Fig. 2. It has been proven that higher
viscosity allows ionic charge distribution among the polymer chains
and solvent molecules such that the polymer solution is stretched
to form uniform fibres [34]. The increasing viscosity with increasing
3.1.3. Applied voltage between spinneret and flat-plate collector
concentration is said to prevent the charged jet from splaying and
In contrast to the flow rate, an increase in applied voltage
splitting, thus reducing the jet’s path and bending instability [35]
resulted in a significant decrease in average fibre diameters
resulting in increasing fibre diameter. The lower splaying, in turn,
(Table 2). The membranes were electrospun with 10% PU solution,
also causes the fibres to be deposited in a smaller area on the
1 ml/h flow rate and 22 cm distance to collector. At voltages
collector. Srivastava et al. explain the increasing fibre diameter with
below 11 kV, no jet emerged from Taylor Cone and the PU
increasing polymer solution concentration to be induced by the
solution dripped. Further, with increasing applied voltage the
faster evaporation of low solvent content and the greater viscoe-
Taylor Cone receded closer to spinneret tip. Also at 25 kV,
lastic forces acting against the columbic forces of the charges results
occasionally, secondary jets formed during electrospinning. The
in the less stretching of fibres and thus the formation of thicker
different jets repelled each other causing thinner fibres depositing
fibres [24]. The results achieved in this work further reiterate that
on a larger area on the collector. It is worthwhile to note that
the polymer solution concentration and the choice of solvents play
higher voltages resulted in a more uniform and narrow fibre
a critical role on the resultant structure and diameter of the
diameter distributions (Table 2). The effect of applied voltage on
resultant electrospun fibres.
the diameter and structure of fibres has been a debatable issue.
This study as well as other reports in the literature suggested a
decrease in fibre diameter with an increase in applied voltage
3.1.6. Optimized parameters
[26,27]. This could be due to the higher surface charge and the
To generate finer fibres, 8 wt% PU solutions in three different
lower volume of polymer solution available in the receding Taylor
solvent mixture of THF to DMF ratios of 3:1, 1:1 and 2:3 (w/w) were
cone with increasing applied voltage. In contrast, some studies
tested. For the 8% PU solution in 3:1 THF:DMF solvent mixture, the
report increasing applied voltage to cause no change or even an
external electrostatic strength was varied by applied voltages
increase in fibre diameter [28–32]. The divergent observations on
between 9 and 27 kV. The flow rate was also varied to suit the
the effects of applied voltage on fibre characteristics are thought
applied voltage such that the electrospinning jet forms, while
to be influenced by the polymer properties and choice of solvents,
maintaining the distance to collector constant at 22 cm. Irrespective
further supporting the need for polymer specific process optimi-
of the changes in the applied voltage and flow rates, 8% PU solution
zation for electrospinning.
in 3:1 THF:DMF solvent mixture always resulted in bead formation,
and no jet formed below 10 kV. Similarly, for 8% PU in 1:1 THF:DMF
solvent mixture, irrespective of the applied voltage and flow rate,
3.1.4. Distance between the spinneret tip and collector beads formed and their density increased with increasing applied
Distance between the spinneret tip and collector influences voltage, with the exception of 27 kV, where beads began to join to
the flight time for the electrospinning jet and the electric field form thick fibres. Finally, an 8% PU solution in 2:3 (40:60) THF to
strength. To obtain independent fibres, the electrospinning jet DMF produced beadless and uniform fibres.
must be allowed enough time for most of the solvent to evapo- Based on the extensive electrospinning process optimization
rate. Short distances between the spinneret and collector cause experiments presented above, it is evident that electrospinning
stronger electric fields, which in turn accelerate the jet. As a can be used to generate membranes with desired fibre diameters.
result, there may not be sufficient time for solvent to evaporate The typical influence of the different solution and processing
before the fibre hits the collector [33]. Provided there is sufficient parameters on the structure of the electrospun fibres and their
electrostatic field strength, increasing the distance between membranes was further reiterated by our study results and more
the spinneret and needle causes stretching of fibres, reducing importantly, the solvent compositions, solution concentrations,
the fibre diameter. However, beyond an optimum distance, the flow rates, applied voltages, and distances to collector were
strength of the electrostatic field decreases, thus lowering the identified for preparing electrospun PU membranes having
stretching effect and increase in fibre diameter [31]. As sum- desired fibre diameters. Towards the goals of achieving mem-
marised in Table 2, increasing the distance from 17 up to 22 cm branes with different fibre diameters and interconnected porous
leads to a decrease in the average fibre diameter and thereafter structures, three membrane configurations (Table 3 and Fig. 3)
the reverse trend was observed. Thus for the set parameters of having average fibre diameters of 347.4 nm, 738.4 nm and
10% solution, 21 kV applied voltage and 1 ml/h flow rate, an 1102.3 nm, designated as 8PU, 10PU and 12PU membranes
optimum distance of 22 cm between spinneret and collector was respectively, were shortlisted to study the effect of the different
identified. At this condition, an evenly distributed nano-web with diameters on morphology, porosity, mechanical, surface hydro-
the lowest average fibre diameter was achieved. philicity and solute diffusion properties.
212 N. Wang et al. / Journal of Membrane Science 427 (2013) 207–217

Fig. 2. SEM images of the PU electrospun fibrous webs and their fibre diameter distribution with different concentration at 8 wt% (A, D), 10 wt% (B, E), 12 wt% (C, F),
constant THF:DMF solvent ratio of 50:50, voltage of 20 kV, DTC of 22 cm, and flow rate of 1.0 ml/h.

Table 3
Electrospinning parameters for the three shortlisted membrane configurations.

Designation PU solution concentration (wt%) Solvent THF:DMF Voltage Feed rate Distance to Fibre diameter (nm)
(%) (wt:wt) (kV) (ml/h) collector (cm)

8PU 8 40:60 21 0.6 22 347 7 87

10PU 10 50:50 20 1.0 22 738 7 131
12PU 12 50:50 20 1.2 22 1102 7 210

3.2. Characterization of electrospun PU membranes stretching vibration, which shifted slightly to a higher wave
number after electrospinning, suggestive of formation of local
3.2.1. Chemical structure and residual solvent evaluation using FTIR packing or orientation of related soft chain segments in the
spectroscopy confinement of micro-/nano-fibrous structure during electrospin-
Qualitative investigation of changes to chemical structure of PU ning. Otherwise, the high applied voltages during electrospinning
and any residual solvents in the electrospun membranes was done did not affect chemical structure of PU. Furthermore, the strong
using ATR-FTIR spectroscopy. Solvents, a solvent cast PU film and absorption peaks characteristic for DMF, CQN at 1657 cm  1, and
as-supplied PU were tested as controls. The FTIR spectra (Fig. 4) of THF, C–F at 1065 cm  1, were not observed in the spectra for
electrospun PU membranes had the characteristic absorption peaks solvent cast films or electrospun PU membranes, indicating no
at 3324, 2855, 1715, 1530, 1232, 1111 and 779 cm  1 which are residual solvents following the drying cycles in vacuum desiccator.
assigned to n (N–H), n (C–H), n (CQO), n (CQC), n (C–C), n (C–O)
and n (C–H) respectively on substituted hexane ring [36,37]. The
spectra of solvent cast PU, as-supplied PU pellet and electrospun 3.2.2. Thickness versus electrospinning time
PU membranes all had similar absorption peaks with the exception Thickness is an essential parameter that influences the perme-
of the absorption peak at about 1111 cm  1 concerning the C–O–C ability and transport properties of a membrane. For the chosen
 N. Wang et al. / Journal of Membrane Science 427 (2013) 207–217 213

Fig. 3. Morphology of the membranes (A) 8PU, (B) 10PU, and (C) 12PU and (D) their respective fibre diameters. The average fibre diameter for each membrane
configuration is statistically different from the other two membranes.

Fig. 4. ATR-FTIR spectra for DMF, THF, as-supplied PU pellets, solvent cast PU film Fig. 5. Thickness of the electrospun membranes: (A) 8PU, (B) 10PU, and (C) 12PU
and electrospun PU membrane. as a function of electrospinning time.

three membrane configurations, namely 8PU, 10PU and 12PU,

membrane thicknesses increased linearly with increasing electro-
spinning time for 12PU (R2 of 0.9931) and 10PU (R2 of 0.9893)
respectively, while that for 8PU was not as linear (R2 of 0.9166)
(Fig. 5). The increase in membrane thickness as a function of
increasing PU solution concentration was as expected. However,
the relatively smaller slope for increase in membrane thickness
for 8PU as a function of increasing electrospinning time (Fig. 5)
can be attributed to fusion fibres with neighbouring stacking
layers (Fig. 3A) potentially due to incomplete evaporation of
solvents during electrospinning.

3.2.3. Pore size and porosity

The membranes 8PU, 10PU and 12PU were prepared using a
2 h electrospinning time to obtain membranes thick enough to
meet the safety requirement for the bubble point measuring Fig. 6. Pore size distributions for the electrospun membranes 8PU, 10PU and
apparatus. Fig. 6 shows the pore size distribution of the three 12PU, as determined by bubble extrusion porosimetry.
214 N. Wang et al. / Journal of Membrane Science 427 (2013) 207–217

electrospun membrane configurations measured using bubble diameters ( 347 nm), pore size ( 800 nm) and pore size distribu-
point measurements. 8PU, 10PU and 12PU having fibre diameters tion (68% between 700 and 900 mm), and about 44% pore volume
of about 347, 738 and 1102 nm respectively were determined to (Fig. 6 and Table 4), 8PU is anticipated to allow high flux/flow of
have radii for majority of their pores and corresponding distribu- solvents or solutes across, while acting as a mass-transport limiting
tion (%) at about 800 nm (68%), 870 nm (36%) and 1060 nm (29%) membrane.
respectively from Fig. 6. The results revealed an increase of the
average pore size with increasing fibre diameter. The pore size
distribution was very narrow for 8PU compared to wide distribu- 3.2.4. Surface contact angle measurement
tions for 10PU and 12PU membranes. The reducing percentage of The hydrophobic and hydrophilic natures of membranes have
the majority of pores having uniform size is indicative of wider an important role in biosensor applications through their effects
pore size distributions with increasing fibre diameters. on the diffusion of analytes, protein adsorption and foreign body
The results for fibre packing density and pore volume determined reactions. The surface wetting analysis using contact angle
by gravimetry shown in Table 4 also reiterate the increasing porosity measurements demonstrated an increase in hydrophobicity as a
as a function of fibre diameter. Gravimetry method revealed a function of increasing fibre diameters and porosity for electro-
decreasing fibre packing density and an increasing pore volume with spun PU membranes. Fig. 7 illustrates the typical morphology of a
increasing fibre diameter. The fibre packing density (n) is significantly 1 ml droplet of DI water on the different PU membranes and their
higher and pore volume (#) significantly lower for 8PU when corresponding contact angles. The solvent cast non-porous PU
compared to 10PU and 12PU membranes. The pore volumes were film showed the smallest contact angle of about 861, while the
about 44%, 63% and 68% respectively for 8PU, 10PU and 12PU, which electrospun PU membranes, 8PU, 10PU and 12PU had 104.31,
is much higher than conventional porous membranes that usually 116.11 and 122.51 respectively. The static contact angle measured
have up to 34% pore volumes having lesser pore interconnectivity is determined by the surface chemistry and the surface roughness
[13]. In addition, if the pores size distribution is narrow and porosity of the membrane. Cassie–Baxter model (Cassie’s law) for two
in sub-micron dimension, they could function as size or mole- component composite smooth surface [38–40] describes the
cular weight cut-off membranes. Having significantly smaller fibre contact angle y on a solid surface changes to yn if the liquid is

Table 4
Fibre packing density and pore volume estimations for electrospun PU membranes based on gravimetry, calculated using Eqs. 2 and 3.
The electrospinning time for the membranes was 2 h, and n¼3 per test membrane.

Sample designation Thickness (lm) Fibre diameter (lm) Fibre packing density (g/cm3) Pore volume (%)

8PU 28.4 75.4 0.347 7 0.087 0.580 70.028* 44.19 7 2.54#

10PU 86.0 712.9 0.738 7 0.131 0.408 70.013 62.87 7 1.18
12PU 180.6 742.2 1.102 7 0.210 0.381 70.020 65.40 7 1.85

n
The fibre packing density is significantly higher for 8PU when compared to 10PU and 12PU membranes.
#
Pore volume significantly lower for 8PU when compared to 10PU and 12PU membranes.

PU film ~86° 8PU membrane 10PU membrane 12PU membrane

~104° ~116° ~123°

12PU
10PU

8PU

Fig. 7. Contact angle measurements: (A) the morphology of water droplet (1 mL) on solvent cast PU film and the three electrospun PU membrane variables; and (B) contact
angles as a function of fibre diameter. Contact angles (y) less than 901 are indicative of hydrophilic surface and greater than 901 indicative of a relatively hydrophobic
surface. A zero contact angle represents rapid and complete wetting.
 N. Wang et al. / Journal of Membrane Science 427 (2013) 207–217 215

suspended on the tops of microstructures of porous surface:

n
cosy ¼ jðcos y þ 1Þ1 ð5Þ
where: j is the area fraction of the solid that touches the liquid.
Based on Cassie–Baxter model, the areas fraction of the actual
contact surface for the membrane, 8PU, 10PU and 12PU, was
estimated to be 30.9%, 1.5% and 0.04% respectively from the mea-
surement results of apparent contact angle above using Eq. (5).
Clearly the actual contact area reduced substantially with increas-
ing fibre diameter, having only 0.04% area touching the liquid in
12PU. Increased pore size and porosity of the 12PU membrane
with larger fibre diameter are envisaged to almost full of air
pocket phase (the contact angle of water in air ¼1801) at the
surface of the membrane, enhancing the hydrophobic nature of
PU. Similar observations were also reported earlier [39,41]. The
bigger pores on the electrospun membrane surface are said to
entrap more air causing the surface to be more hydrophobic
[42–44]. Liu et al. also suggest that the smaller the pore in the
material, the higher their capillary effect contributing to faster
water absorption, thus decreasing the contact angle with decreas-
ing pore sizes [45]. This reiterates that the actual contact surface
area is the controllable parameter for changing the surface
wetting nature. The results indicate that the hydrophilicity–
hydrophobicity balance for a particular polymer can be adjusted
to some extent using straightforward electrospinning technology
towards generating biocompatible surfaces.

3.2.5. Glucose diffusion test

Considering the intended application as coatings for glucose
biosensors (model sensors), it was important that the designed
coatings cause minimal decrease in sensitivity, which we hypo-
thesized can be achieved by increasing the flux of trans-membrane
transport of glucose. Hence, we first screened the different electro-
spun membrane configurations, optimized in this study, for their
permeability to glucose using rotating biodialysers as a function of
membrane thickness and porosity. Three different electrospinning
times namely 2.5, 5 and 10 min were used to obtain 8PU, 10PU and
12PU membranes having fine differences in thickness (Table 4). The
original glucose diffusion profiles had 10 data points recorded
per second. To minimize the enormity of the data for statistical
analysis, we extracted readings for the time points 2, 5, 10, 20, and
6.0E-04 2.5 5 10 40 min for each of the membranes. The resulting glucose diffusion
# profiles for the different membranes as a function of time are
5.0E-04 shown in Fig. 8A–C.
Based on the two-chamber diffusion model described in Section
2.7, the mean relaxation times, t, for each type of membranes were
4.0E-04 determined by best fitting using Eq. (4), and their corresponding
Deff(mm2/s)

effective diffusion coefficients were calculated by inputting thick-

* ness and area of the membranes. The results are listed in Table 5,
3.0E-04 ##
c which are mean values from 5 membranes tested for each type
a b
of the membranes. The values of the effective diffusion coefficient
2.0E-04 for the fibrous membranes fall within a wide range from
## ##
(0.42970.45)  10  5 to 4.8870.44)  10  4 mm2/s as a function
** aa
1.0E-04 cc of membrane thickness and membrane structure. The highest Deff
##
bb (4.8870.44)  10  4 mm2/s observed for 10PU2.50 is close to the
glucose diffusion coefficient in water (D¼6.73  10  4 mm2/s) [46]
0.0E+00
indicating a high flux rate mass transport through the membranes.
8PU 10PU 12PU
Such behaviour can be attributed to the highly interconnected
Fig. 8. Glucose diffusion across (A) 8PU, (B) 10PU and (C) 12PU membranes, and porous network structure having about 44% to 69% pore volume
(D) the trends of the average relaxation time and effective diffusion coefficient of the fibro-porous electrospun membranes (Table 4), which is
calculated from the diffusion measurement as a function of time, and thickness significantly different to conventional porous membranes that
(electrospinning times of 2.5, 5 and 10 min). Data is represented as Mean7 SE
have pore volume up to 34%, without compromising on mechanical
(standard error) of mean, n¼ 5.
properties [13]. Moreover, the effective diffusion coefficient of
the membrane can be tailored by producing finer nano-fibrous
nanoporous structure similar to various types of nano-membranes
216 N. Wang et al. / Journal of Membrane Science 427 (2013) 207–217

Table 5 and systematically investigated and sets of parameters for spin-

The thickness of electrospun membranes used for diffusion tests and their ning PU membranes having desired fibre diameters were identi-
corresponding average relaxation time and effective diffusion coefficient as a
fied. Three membrane configurations designated as 8PU, 10PU
function of electrospinning time. n¼ 5.
and 12PU having average fibre diameters 347, 738 and 1102 nm
Sample- Mean of Average Effective diffusion respectively were chosen and tested for membrane properties.
electrospinning thickness relaxation time, coefficient, Electrospinning process did not affect the chemical structure of
time (min) (lm) s (min) Deff (mm2/s) PU except for a slight shift in the peak for the –C–O–C– in the PU
Watera – – (6.73)E-04
backbone indicating a change in conformation of the PU in the
8PU-2.5 15.3 12.31 72.84 (2.067 0.61)E-04 electrospun membrane. Pore sizes and volume increased with
8PU-5 15.6 14.16 70.78 (1.797 0.10)E-04 increasing fibre diameters. Thickness of the 12PU and 10PU
8PU-10 16.0 26.38 74.38 (1.007 0.17)E-04 membranes spun on a flat-plate collector increased linearly with
10PU-2.5 17.5 5.887 0.55 (4.88þ 0.44)E-04
increasing electrospinning time. But that of 8PU was not as linear,
10PU-5 19.0 20.797 1.71 (1.497 0.13)E-04
10PU-10 21.9 86.19 728.97 (4.567 1.90)E-05 potentially due to the fusing of neighbouring fibres resulting for
12PU-2.5 18.9 197 0.56 (1.637 0.05)E-04 slower drying of the relatively higher DMF content in the feed PU
12PU-5 21.5 17.95 74.02 (2.017 0.41)E-04 solution. Increasing fibre diameters and their corresponding
12PU-10 26.4 100.927 10.52 (4.297 0.45)E-05 larger pore sizes resulted in increased surface hydrophobicity of
a
[46].
electrospun PU membranes. The large pore volumes in the
electrospun PU membranes resulted in higher glucose flux rates,
reported such as the nanoporous polyethylene membrane (0.18  which can be further tuned by varying the fibre diameters and
10  4 mm2/s) and a 20 nm asymmetric alumina membrane from membrane thicknesses. The thinner the membranes, the closer
Whatman (1.39  10  4 mm2/s) [47]. were their trans-membrane glucose diffusion coefficients to that
As a function of membrane thickness (electrospinning time), a of glucose in water without any membranes. The fibre-size-
increasing trend of the average relaxation time (t) and reduction dependent properties of electrospun PU membranes, especially
of effective diffusion coefficients (Deff) confirms a decrease in the their high and tuneable flux rates for trans-membrane glucose
rate of diffusion of glucose across each of the three membrane diffusion is anticipated to find them applications as coatings for
configurations 8PU, 10PU and 12PU with increasing electrospin- sensors and other biomedical devices requiring analyte exchange.
ning time from 2.5, 5 to 10 min (Fig. 8A–D and Table 5). Further studies elucidating the methods for coating miniature
Particularly in the cases of 10PU and 12PU, the increase of t glucose biosensors with electrospun PU membrane configurations
and reduction of Deff tends to be more drastic compared with 8PU, reported in this paper, the coatings’ efficacy as mass-transport
which is consistent with the steeper increase in thickness of the limiting and biocompatible membranes, and their pre-clinical
membrane causing increased diffusion distance throughout the biocompatibility and functional efficacy will feature in future
thicker membrane produced (Fig. 5). Membrane configurations in papers.
terms of fibre diameter, porosity, pore size and pore size dis-
tribution also influence the diffusion kinetics of the membranes.
The trend of effective diffusion coefficient in correlation to the Acknowledgements
membrane configuration is not that straightforward (Fig. 8D).
Maxwell model predicts an increase of effective diffusion coeffi- This research is supported by Brunel University, the Royal
cient with increasing porosity of the membrane based on a Society research Grant (RG100129), the Royal Society-NSFC inter-
simplified periodically spaced porous structure [48]. Given the national joint project Grant (JP101064) and the National Institute
relatively close thickness of three membranes electrospun for of Health (NIH/NIBIB, Grant R01EB001640).
2.5 min (Table 5), Deff ¼(4.8870.44)  10  4 mm2/s for 10PU2.5
with higher porosity and pore size more than doubles compared
with Deff ¼(2.06 70.61)  10  4 mm2/s for 8PU2.5, but Deff ¼ Appendix A. Supporting information
(1.6370.06)  10  4 mm2/s for 12PU2.5 with the largest porosity
and pore size is unexpectedly lower. Such variations in magni- Supplementary data associated with this article can be found
tudes at each time point between the groups, 8PU, 10PU and in the online version at http://dx.doi.org/10.1016/j.memsci.2012.
12PU, can be attributed to subtle variations in their thicknesses 09.052.
and fibro-porous structure resulting from the differences in
electrospinning parameters and ambient conditions during elec-
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