comprehensive review summarizing effect of electrospinning parameters and.pdf

7/26/2019 Comprehensive review summarizing effect of electrospinning parameters and.pdf

1/65

Accepted Manuscript

Comprehensive review summarizing effect of electrospinning parameters and

potential applications of nanofibers in biomedical and biotechnology

Adnan Haider, Sajjad Haider, Inn-Kyu Kang

PII: S1878-5352(15)00327-5

DOI: http://dx.doi.org/10.1016/j.arabjc.2015.11.015

Reference: ARABJC 1808

To appear in: Arabian Journal of Chemistry

Received Date: 20 August 2015

Accepted Date: 23 November 2015

Please cite this article as: A. Haider, S. Haider, I-K. Kang, Comprehensive review summarizing effect of

electrospinning parameters and potential applications of nanofibers in biomedical and biotechnology, Arabian

Journal of Chemistry(2015), doi: http://dx.doi.org/10.1016/j.arabjc.2015.11.015

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers

we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and

review of the resulting proof before it is published in its final form. Please note that during the production process

errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
http://dx.doi.org/10.1016/j.arabjc.2015.11.015http://dx.doi.org/http://dx.doi.org/10.1016/j.arabjc.2015.11.015http://dx.doi.org/http://dx.doi.org/10.1016/j.arabjc.2015.11.015http://dx.doi.org/10.1016/j.arabjc.2015.11.015


2/65

Full title

Comprehensive review summarizing effect of electrospinning parameters and potential

applications of nanofibers specifically in biomedical field and biotechnology

Short title

A comprehensive review on electrospining


3/65

Comprehensive review summarizing effect of electrospinning parameters and potential

applications of nanofibers in biomedical and biotechnology

Adnan Haidera, Sajjad Haider

b, Inn-Kyu Kang*

a

a

Department of Polymer Science, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu, 702-701, South Korea

bDepartment of Chemical Engineering, College of Engineering, King Saud University, P.O.

Box 800, Riyadh, 11421, Saudi Arabia

Abstract

Nanotechnology is a budding technology that has been identified as a vital scientific and

commercial venture with global economic benefits. With the increasing knowledge of

nanomaterial manufacturing techniques, research groups around the globe are focusing more

on the preparation of nanomaterials for various applications. Among the various techniques

reported in the literature, electrospinning has gathered significant interest because of its

ability to fabricate nanostructures with unique properties such as a high surface area and

inter/intra fibrous porosity. Electrospinning has been the most widely used technique in the

late 20th (1990) and early 21st (2000) centuries (Reneker and Yarin 2008). Since its first use in

the early 20th

(1900) (Cooley 1900) century, significant improvements have been made in the

instrument design, material used, and nanomaterials produced. The production of

nanomaterials (nanofibers) via electrospinning is affected by many operating parameters.

This review paper will provide an overview of the electrospinning (applied electric field,

distance between the needle and collector and flow rate, needle diameter), solution (polymer

concentration, viscosity, solvent and solution conductivity) and environmental (relativity

humidity and temperature) parameters that affect the nanofibers fabrication and the

application of nanofibers in tissue engineering, drug delivery systems, wound dressings,

antibacterial study, filtration, desalination, protective clothing fabrication, and biosensors.

Keywords: Electrospinning, Parameters Effect, Biomedical Applications


4/65

1. Introduction

Since the late 20thcentury, electrospinning has been garnering increasing attention in

the scientific community, as well as in industry, and is considered to be a vital scientific and

commercial venture with global economic benefits. In the literature, various techniques are

reported for the fabrication of nanomaterials. These include drawing-processing, template-

assisted synthesis, self assembly, solvent casting, phase separation, and electrospinning

techniques (Lim et al. 2006; Peng et al. 2013; Yongquan et al. 2012). With the increasing

knowledge about nanotechnology, especially related to nanoparticles, nanostructures, and,

more specifically, the preparation of scaffolds, electrospinning has become the most

frequently used technique. This technique is preferred over solvent casting and phase-

separation because the nanofibers produced with electrospinning possess a high surface area

to volume ratio and large number of inter-/intra fibrous pores (Reneker and Fong 2006). In

addition to the previously mentioned properties, it has the advantages of being easy to use.

The growing literature on electrospinning has helped this technique to pave the way for

advancements in areas like bioengineering, environmental protection, sensors, catalysis and

electronics (Chen et al. 2007a; Katepalli et al. 2011; Kijeska et al. 2012) . With the ability to

fabricate nanostructures from various types of raw materials, ranging from natural and

synthetic polymers to composites (consisting of organic and inorganic components), an

increasing number of scientists are attracted to this highly effective technique for the

preparation of various nanostructures, which can find applications in almost every field. For

examples electrospun nanofiber has also played a pivotal role in the area of biomaterials. The

importance of electrospun nanofiber in the biomedical field can be determined from the fact

that numerous articles are being published on a regular basis highlighting its importance in

the area of biomedical engineering using biocompatible and biodegradable (natural or

synthetic) polymers. Electrospun nanofibers scaffolds can be tailored in accordance with the


5/65

purpose of their use. Such hybrid nanofibers scaffolds play an important role in providing a

familiar environment to the cells, which ultimately results in their better attachment,

proliferation, and differentiation (Haider et al. 2014b; Ostrowska et al. 2014) For example,

collagen fibril has been known to enhance the interaction between cells and scaffolds.

Similarly, electrospun nanofibers scaffolds are also used as a drug delivery carrier for

carrying drugs to their target sites (Wei and Ma 2008). Beside, biomedical application,

electrospun nanofiber has also found application in the protection of environment (both air

and water) as affinity membrane (Subramanian and Seeram 2013) (Feng et al. 2013) .

Electrospun nanofiber could also be used for producing high-surface-area chemical and

biological nanosensors (Huang et al. 2003b). Researchers have further emphasized that

sensors utilizing electrospun nanofibers could show enhanced sensing abilities for various

chemicals such as a nitro compound (2,4-dinitrotulene DNT), mercury, and ferric ions,

compared to a conventional thin film (Schulte 2005). In addition to chemical and biological

sensors, highly sensitive polymeric nanofibers optical sensors have also been fabricated from

fluorescent polymers (Lee et al. 2002; Wang et al. 2002a). Furthermore, ultrafine electrospun

nanofiber scaffolds had also been used for the preparation of nanotubes, which are of prime

importance in various industries (Hohman et al. 2001). Nanotubes are prepared by coating the

electrospun nanofibers with the raw material of the nanotubes followed by evaporation of

solvent or thermal degradation of polymers. Physical and chemical vapor deposition

technique has also been adopted using poly(L-lactide) (PLA) and poly(tetramethylene

adipamide) (PA) as templates (Bognitzki et al. 2001); (Hou et al. 2002a; Huang et al. 2003b).

Until now, electrospun nanofibers have been prepared from approximately 100

different polymers with both synthetic and natural origins. All of these nanofibers have been

prepared using either solvent or melt spinning. However, even with the widespread use of the


6/65

electrospinning technique, the understanding of this method is still very limited. This review

paper will help to provide an overview of the electrospinning (applied electric field, distance

between the needle and collector and flow rate,), solution (solvent, polymer concentration,

viscosity and solution conductivity) and environmental (humidity and temperature)

parameters that affect the nanofibers fabrication. It will also help in understanding the

application of nanofibers in biomedical field, in addition to their use in filtration, protective

clothing fabrication and biosensors. The review paper will save the reader time and energy by

making available all the aforementioned information in one document.

2. Electrospinning and its mechanism

Extensive research has been done on the electrospinning technique (Pillay et al. 2013). Based

on the reported research, the basic electrospinning setup (Figure 1a)mainly comprised of

four main parts: a glass syringe containing a polymer solution, metallic needle, power supply,

and metallic collector (with a variable morphology). The electrospinning process begins

when electric charges move into the polymer solution via the metallic needle. This causes

instability within the polymer solution as a result of the induction of charges on the polymer

droplet. At the same time, the reciprocal repulsion of charges produces a force that opposes

the surface tension, and ultimately the polymer solution flows in the direction of the electric

field (Figure 1b). A further increase in the electric field causes the spherical droplet to

deform and assume a conical shape. At this stage, ultrafine nanofibers emerge from the

conical polymer droplet (Taylor cone), which are collected on the metallic collector kept at

an optimized distance. A stable charge jet can be formed only when the polymer solution has

sufficient cohesive force. During the process, the internal and external charge forces cause

the whipping of the liquid jet in the direction of the collector. This whipping motion allows

the polymer chains within the solution to stretch and slide past each other, which results in


7/65

the creation of fibers with diameters small enough to be called nanofibers (Bae et al. 2013;

Haider et al. 2013).

Figure 1

3. Effects of parameters on electrospinning

There are several factors that affect the electrospinning process. These factors are classified

as electrospinning parameters, solution and environmental parameters. The electrospinning

parameters include the applied electric field, distance between the needle and collector, flow

rate, and needle diameter. The solution parameters include the solvent, polymer concentration,

viscosity and solution conductivity. The environmental parameters include relativity humidity

and temperature. All of these parameters directly affect the generation of smooth and bead-

free electrospun fibers. Therefore, to gain a better understanding of the electrospinning

technique and fabrication of polymeric nanofibers, it is essential to thoroughly understand the

effects of all of these governing parameters.

3.1 Effect of applied voltage

Generally, it is a known fact that the flow of current from a high-voltage power supply into a

solution viaa metallic needle will cause a spherical droplet to deform into a Taylor cone and

form ultrafine nanofibers at a critical voltage (Figure 2a, b, and c) (Laudenslager and

sigmund. 2012). This critical value of applied voltage varies from polymer to polymer. The

formation of smaller-diameter nanofibers with an increase in the applied voltage is attributed

to the stretching of the polymer solution in correlation to the charge repulsion within the

polymer jet (Sill and von Recum 2008). An increase in the applied voltage beyond the critical

value will result in the formation of beads or beaded nanofibers. The increases in the diameter

and formation of beads or beaded nanofibers with an increase in the applied voltage are

attributed to the decrease in the size of the Taylor cone and increase in the jet velocity for the


8/65

same flow rate. Deitzel et al. reported bead formation with an increase in the applied voltage

using poly(ethylene oxide) (PEO)/water. Similar results were also reported by Meechaisue et

al. andZong et al (Deitzel et al. 2001). Furthermore, the diameter of the nanofibers was also

reported to increase with an increase in the applied voltage. This increase in the diameter was

attributed to an increase in the jet length with the applied voltage (Figure 2) (Baumgarten

1971).

Figure 2

3.2 Effect of solution flow rate

The flow of the polymeric solution through the metallic needle tip determines the

morphology of the electrospun nanofibers. Uniform beadless electrospun nanofibers could be

prepared viaa critical flow rate for a polymeric solution. This critical value varies with the

polymer system. Increasing the flow rate above the critical value could lead to the formation

of beads. For example, in case of polystyrene, when the flow rate was increased to 0.10

mL/min, bead formation was observed. However, when the flow rate was reduced to 0.07

mL/min, bead-free nanofibers were formed. Increasing the flow rate beyond a critical value

not only leads to increase in the pore size, fiber diameter but also to bead formation (due to

incomplete drying of the nanofiber jet during the flight between the needle tip and metallic

collector) (Megelski et al. 2002a). Because increases and decreases in the flow rate affect the

nanofiber formation and diameter, a minimum flow rate is preferred to maintain a balance

between the leaving polymeric solution and replacement of that solution with a new one

during jet formation (Megelski et al. 2002a; Zeleny 1935). This will also allow the formation

of a stable jet cone and sometimes a receded jet (a jet that emerges directly from the inside of

the needle with no apparent droplet or cone). Receded jets are not stable jets, and during the

electrospinning process, these jets are continuously replaced by cone jets. As a result of this

phenomenon, nanofibers with a wide range diameter are formed (Figure 3f) (Shamim et al.


9/65

2012). In addition to bead formation, in some cases, at an elevated flow rate, ribbon-like

defects (Megelski et al. 2002b) and unspun droplets (Figure 3g)have also been reported in

the literature (Shamim et al. 2012). The formation of beads and ribbon-like structures with an

increased flow rate was mainly attributed to the non-evaporation of the solvent and low

stretching of the solution in the flight between the needle and metallic collector. The same

effect could also be attributed to an increase in diameter of the nanofibers with an increase in

the flow rate (Li and Wang 2013). The presence of the unspun droplets is attributed to the

influence of the gravitational force (Shamim et al. 2012).Another important factor that may

cause defects in the nanofiber structure is the surface charge density. Any change in the

surface charge density may also affect the morphology of the nanofiber. For instance, Theron

et al. revealed that the flow rate and electric current are directly related to each other. They

studied the effects of the flow rate and surface charge density using various polymers,

including polyethylene oxide (PEO), polyacrylic acid (PAA), polyvinyl alcohol (PVA),

polyurethane (PU), and polycaprolactone (PCL). In the case of PEO, they observed that an

increase in the flow rate simultaneously increased the electric current and decreased the

surface charge density. A reduction in the surface charge density will allow the merging of

electrospun nanofibers during their flight toward the collector. This merging of nanofibers

facilitates the formation of garlands (Reneker et al. 2002; Theron et al. 2004).

Figure 3

3.3 Effect of needle to collector distance and needle diameter

The distance between the metallic needle tip and collector plays an essential role in

determining the morphology of an electrospun nanofiber. Similar to the applied electric field,

viscosity, and flow rate, the distance between the metallic needle tip and collector also varies


10/65

with the polymer system. The nanofiber morphology could be easily affected by the distance

because it depends on the deposition time, evaporation rate, and whipping or instability

interval (Matabola and Moutloali 2013). Hence, a critical distance needs to be maintained to

prepare smooth and uniform electrospun nanofibers, and any changes on either side of the

critical distance will affect the morphology of the nanofibers (Bhardwaj and Kundu 2010).

Numerous research groups have studied the effect of the distance between the needle tip and

collector and concluded that defective and large-diameter nanofibers are formed when this

distance is kept small, whereas the diameter of the nanofiber decreased as the distance was

increased (Baumgarten 1971; Matabola and Moutloali 2013; Wang and Kumar 2006).

However, there are cases where no effect on the morphology of the nanofiber was observed

with a change in the distance between the metallic needle and collector (Zhang et al. 2005a).

3.4 Effects of polymer concentration and solution viscosity

The electrospinning process relies on the phenomenon of the uniaxial stretching of a charged

jet. The stretching of the charged jet is significantly affected by changing the concentration of

the polymeric solution. For example when the concentration of the polymeric solution is low,

the applied electric field and surface tension cause the entangled polymer chains to break into

fragments before reaching the collector (Haider et al. 2013; Pillay et al. 2013). These

fragments cause the formation of beads or beaded nanofibers. Increasing the concentration of

the polymeric solution will lead to an increase in the viscosity, which then increases the chain

entanglement among the polymer chains. These chain entanglements overcome the surface

tension and ultimately result in uniform beadless electrospun nanofibers. Furthermore,

increasing the concentration beyond a critical value (the concentration at which beadless

uniform nanofibers are formed) hampers the flow of the solution through the needle tip (the

polymer solution dries at the tip of the metallic needle and blocks it), which ultimately results


11/65

in defective or beaded nanofibers (Haider et al. 2013). The morphologies of the beads depict

an interesting shape change from a round droplet-like shape (with low-viscosity solutions) to

a stretched droplet or ellipse to smooth fibers (with sufficient viscosity) as the solution

viscosity changes (Figure 4a-d) (Shamim et al. 2012). Similar phenomenon was found by

Fong et al when they electrospun PEO by varying it viscosity (Figure 4e-h) (Fong et al.

1999). Zong et al. while studying poly(d,l-lactic acid) (PDLA) and poly(l-lactic acid) (PLLA)

also observed that the shape of the beads changes with viscosity (Zong et al. 2002). The

effect of the concentration/viscosity on the morphology of the nanofibers was also reported

by Doshi et al. Working with PEO, they concluded that the optimum viscosity for the

generation of electrospun nanofibers is 800-4000 cp (Doshi and Reneker 1995). In addition to

the work of Doshi et al., an experiment on a polyacrylonitrile (PAN) polymer solution

showed that smooth electrospun nanofibers could be prepared when the viscosity of the

solution was kept at 1.7-215 cp. Hence, it can be concluded that in addition to the

electrospinning parameters, the determination of the critical value of the

concentration/viscosity is also essential to obtain beadless nanofibers (Baumgarten 1971).

Figure 4

3.5. Effect of Solution conductivity

Solution conductivity not only affects the Taylor cone formation but also helps in controlling

the diameter of the nanofibers. At very low solution conductivity, the surface of the droplet

will have no charge to form a Taylor cone as result no electrospinning will take place.

Increasing the conductivity of the solution to a critical value will not only increase the charge

on the surface of the droplet to form Taylor cone but will also cause decrease in the fiber

diameter (Sun et al. 2014). Increasing the conductivity beyond a critical value will again

hinder the Taylor cone formation and electrospinning. This phenomenon could be explained


12/65

by taking into consideration the entire electrospinning process. Electrospinning process is

dependent on the Coulomb force between the charges on the surface of the fluid and the force

due to the external electric field. However, the formation of the Taylor cone is governed

largely by the electrostatic force of the surface charges created by the applied external electric

field (the component of the field that is tangential to the surface of the fluid induces this

electrostatic force). An ideal dielectric polymer solution will not have enough charges in the

solution to move onto the surface of the fluid; hence, the electrostatic force generated by the

applied electric field will not be insufficient to form a Taylor cone and initiate

electrospinning process. In contrast, a conductive polymer solution will have sufficient free

charges to move onto the surface of the fluid and form a Taylor cone and initiate the

electrospinning process. The conductivity of a polymer solution could be controlled by the

addition of an appropriate salt to the solution. The addition of salt affect the electrospinning

process in two ways; (i) it increases the number of ions in the polymer solution, which results

in the increase of surface charge density of the fluid and the electrostatic force generated by

the applied electric field and (ii) it increases the conductivity of the polymer solution, which

results in the decrease in tangential electric field along the surface of the fluid. However

when this tangential electric field is extensively decreased with the increase in conductivity

of the solution, the electrostatic force along the surface of the fluid diminishes, which

negatively affect the formation of the Taylor cone. Coulomb and electrostatic forces together

influences the elongating and thinning of the straight jet portion. The length of the straight jet

portion and the behavior of the whipping jet region have a significant influence on the

diameter of the nanofibers. The stretching in the whipping region due to the surface charges

draws the fluid jet into the nanoscale (Angammana and Jayaram 2011). A number of research

groups have studied the effect of the salt on the diameter of the nanofibers, for example Zong

et al investigated the effect of different salts (KH2PO4, NaH2PO4, and NaCl in 1%W/V)on


13/65

the diameter of poly(D,L-lactic acid) (PDLLA). They observed that after adding the salt to

the polymer solution separately, the nanofibers were not only smooth, beadles but were also

of small diameter compared to pristine nanofibers (Zong et al. 2002). A similar observation

was also reported by Choi et al, when they add a small amount of benzyl trialkylammonium

chlorides to poly(3-hydroxybutyrate-co-3-hydroxyvalerate) solution, the average diameter

decreased to 1.0m(Choi et al. 2004).

3.6. Role of solvent in electrospinning

The selection of the solvent is one of the key factors for the formation of smooth and beadless

electrospun nanofiber. Usually two things need to be kept in mind before selecting the

solvent. First, the preferred solvents for electrospinning process have polymers that are

completely soluble. Second, the solvent should have a moderate boiling point. Its boiling

point gives an idea about the volatility of a solvent. Generally volatile solvents are fancied as

their high evaporation rates encourage the easy evaporation of the solvent from the

nanofibers during their flight from the needle tip to collector. However, highly volatile

solvents are mostly avoided because their low boiling points and high evaporation rates cause

the drying of the jet at the needle tip. This drying will block the needle tip, and hence will

hinder the electrospinning process. Similarly, less volatile solvents are also avoided because

their high boiling points prevent their drying during the nanofiber jet flight. The deposition of

solvent-containing nanofibers on the collector will cause the formation of beaded nanofibers

(Lannutti et al. 2007; Sill and von Recum 2008). Numerous research groups have studied the

effects of the solvent and solvent system on the morphology of nanofibers (Figure 5)

( Kanani and Bahrami 2011) and concluded that similar to the applied voltage, the solvent

also affects the polymer system (Fong et al. 1999). Furthermore, the solvent also plays a vital

role in the fabrication of highly porous nanofibers. This may occur when a polymer is


14/65

dissolved in two solvents: one of the solvents will act as a non-solvent. The different

evaporation rates of the solvent and non-solvent will lead to phase separation and hence will

results in the fabrication of highly porous electrospun nanofibers (Figure 5f) (Sill and von

Recum 2008). Similar results were also reported by Zhang et al. (Zhang et al. 2006b).

Megelski et al. prepared porous nanofibers by varying the ratio of tetrahydrofuran (THF) and

dimethylformamide (DMF) (Megelski et al. 2002a). In addition to the volatile nature of the

solvent, its conductivity and dipole-moment are also very important. To investigate the

effects of the conductivity and dipole-moment, Jarusuwannapoom et al. tested 18 solvents

and came to the conclusion that out of the 18 solvents used, only five solvents (ethyl acetate,

DMF, THF, methyl ethyl ketone, and 1,2- dichloroethane) could feasibly be used for the

electrospinning of polystyrene polymeric solution, because these solvents exhibited

comparatively better conductivity and dipole-moment values (Jarusuwannapoom et al. 2005).

Figure 5

3.7. Effect of humidity and temperature

Beside the electrospinning and solution parameters, recently it has been reported that

environmental (ambient) factors such as relative humidity and temperature also affect the

diameter and morphology of the nanofibers (Huan et al. 2015; Pelipenko et al. 2013).

Humidity cause changes in the nanofibers diameter by controlling the solidification process

of the charged jet. This phenomenon is, however, dependent on the chemical nature of the

polymer. Pelipenko et al., studied the change in nanofibers diameter with change in

humidity using PVA, PEO and their blend solution PVA/hyaluronic acid (HA),

PEO/(chitosan(CS)). They observed that the diameter of the nanofibers decreased from

667nm to 161nm (PVA) and 252 nm to 75nm (PEO) with increase in humidity from 4 to

60 %. For the blend the decrease was even more for example decrease in humidity from 4 %


15/65

to 50 %, the diameter of the nanofibers for PVA/HA decreased from 231nm to 46 nm and

for PEO/CS from 231 nm to 46 nm. Further increase in humidity led to bead fiber for

individual polymers and almost no electrospinning for the blends (Pelipenko et al. 2013). A

similar decrease in the nanofibers diameter of PEO with increase in humidity is also reported

by Park et al (Park and Lee 2010). Humidity also plays an important role in the creation of

porous nanofibers when binary solvent system is used.. Bae et al., used polymethyl

methacrylate (PMMA) and a binary solvent system (dichloromethane (DCM):

Dimethylformamide (DMF) in 8:2 ratio to produce highly porous nanofibers. The creation

of the pores was attributed to the different evaporation rates of the two solvent. The more

volatile solvent (DCM) starts to evaporate faster than the less volatile solvent (DMF). (while

the fibers are flying towards the collector; Figure 6). This difference in rates of evaporation

of the two solvents causes a cooling effect, a phenomenon similar to perspiration. This

cooling effect result in the condensation of water vapor into water droplets (as also observed

during cloudy conditions or in fog). The water droplets settle on the fibers. As water is

miscible with DMF, hence the two mix well with each other on the inner and outer surfaces

of the fibers. The complete evaporation of the solvents and the water droplets from the

fibers results in the formation of porous PMMA electrospun fibers (Figure 6) (Bae et al.

2013). Temperature causes two opposing effects to change the average diameter of the

nanofibers (i) it increases the rate of evaporation of solvent and (ii) it decreases the viscosity

of the solution. The increase in the evaporation of the solvent and the decrease in the

viscosity of the solution work by two apposite mechanisms, however, both lead to decrease

in the mean fiber diameter. A similar observation was reported by Vrieze et al using

cellulose acetate (CA) and poly(vinylpyrrolidone) (PVP) (De Vrieze et al. 2009).

Figure 6.


16/65

4. Application of electrospun nanofibers

Nanomaterials in the forms of tubes, wires, rods, spheres, and fibers have been assembled

into macrostructures with different designs for a variety of high technology applications

(Laurencin et al. 2008). Electrospinning technology has been used for the fabrication and

assembly of nanofibers into membranes, which have extended the range of potential

applications in the biomedical, environmental protection (Table 1), nanosensor,

electronic/optical, and protective clothing fields. In the biomedical field, it is now an

established fact that almost all tissues and organs such as skin, collagen, dentin, cartilage, and

bone, in one way or another, have some sort of resemblance to highly organized, hierarchical,

nanosize fibrous structures. Therefore, research on biomedical applications has focused on (i)

the generation of fibrous scaffolds for tissue engineering, (ii) wound dressing, (iii) drug

delivery mechanisms, and (iv) enzyme immobilization to achieve faster reaction rates in

biological reactions (Metreveli et al. 2014). There are many articles in the literature that

highlight the importance of the biomedical applications of electrospinning (Haider et al.

2014a; Haider et al.). Because of their unique properties such as their morphology

(dimensions), high surface-area-to-volume ratio, and inter/intra fibrous porosity, electrospun

nanofibers are regarded as promising scaffold materials. They have shown the ability to

initiate/evoke (stimulate) special biological responses in cells when cells are cultured on them.

Furthermore, nanofibrous scaffolds have shown enhanced cell adhesion, stimulated cell

growth, protein adsorption, and assisted in cell differentiation (Mattiace et al. 2008; Woo et al.

2003). In addition to biomedical applications, nanofibers have been widely studied as a

potential filter material in the environmental protection field. Based on the design and

construction of the membrane, and the size of the contaminants, filters are of two main types:

nanofilters and microfilters. To achieve the easy removal of a targeted contaminant, the filter

membrane should have pores or passage channels. These channels allow liquid and particles


17/65

with the appropriate dimension to pass, while arresting the particles or contaminants with a

larger particle size. For instance, one of the most commonly used filters in daily life is a

paper coffee filter, which has the ability to prevent the movement of large and undissolved

particles through its pores, while allowing dissolved particles with smaller diameters to pass

(Gupta et al. 2014; Haider et al. 2015; Haider et al. 2014c). In addition to normal fibrous

filter membranes, researchers have developed a new type of fibrous membrane known as an

affinity membrane. These membranes have selective sites that assist in the selective

immobilization of targets and removal of the target contaminant. This membrane has shown

an extensive range of applications in both the environmental and biomedical engineering

fields (Haider et al. 2014a; Haider et al. 2014b; Haider et al. 2013; Haider et al. 2015).

Table 1: Natural biopolymer electrospun nanofibers (Haider et al. 2015).

MaterialsSolvent

systemRef Materials

Solvent

systemRef

Cellulose

acetate

Acetone/DM

Ac

(Deng et

al. 2013)

Chitosan TFA(Schiffman and

Schauer 2007)

Chitin HFIP/PBS

(Holzwar

th and

Ma

2011)

Hyluronic

acidDMF/Water (Ji et al. 2006)

Silk

FibroinFormic acid

(Hang et

al. 2012)Fibrinogen -

(Wnek et al.

2003)

Gelatin TFE/ HFIP

(Huang

et al.

2004)

Elastine Water (Huang et al.2000)

Collagen HFIP(Rho et

al. 2006)Soy protein HFIP

(Har-el et al.

2014)

Wheat

proteinHFIP

(Woerde

man et

al. 2005)

Whey

protein

Acidic

aqueous

solution

(Sullivan et al.

2014)


18/65

4.1. Tissue engineering

A range of methods has been reported in the literature for the fabrication of tissue

engineering scaffolds. However, in the past decade, nanofibers systems have been targeted

for the preparation of scaffolds for tissue engineering (Vasita and Katti 2006). For the

regeneration of tissue, biocompatible and biodegradable fibrous scaffolds are generally

preferred over conventional scaffolds because of their unique nature and ability to provide the

target cells/tissues with a native environment by mimicking the extracellular matrix.

Therefore, the use of electrospun nanofibers in tissue engineering is increasing with the every

passing day (Sun et al. 2014). The literature published on tissue engineering utilizing

electrospun nanofibers has so far surpassed the literature published on the conventional

materials. Fibrous scaffolds not only have shown an impact on the cell-to-cell interaction but

have also increased the interaction between the cells and matrix (Li et al. 2002). Because of

the aforementioned properties and similarities between the hierarchical structure of

electrospun nanofiber scaffolds and the natural extracellular matrix, electrospun nanofiber

scaffolds have exhibited an excellent cell growing capability (Friess 1998). Furthermore,

until recently researchers have mainly focused on bio/natural polymers (hyaluronic acid,

alginate, collagen, silk protein, fibrinogen, chitosan, starch, and poly(3-hydroxybutyrate-co-

3-hydroxyvalerate (PHBV)) for tissue engineering, because these polymers showed excellent

biocompatibility and biodegradability (Almany and Seliktar 2005; Pavlov et al. 2004;

Prabhakaran et al. 2013; Yoo et al. 2005). However, more recently, attempts have been made

to utilize a wide range of natural and synthetic polymers for the regeneration of new tissues,

specifically cartilage tissue (Rho et al. 2006), dermal tissue (Bhardwaj and Kundu 2010), and

bones (Chen et al. 2006). Among the synthetic polymers, poly(lactic acid-co-glycolic acid)

(PLGA) is considered to be the ideal material for tissue regeneration because of its tunable

and biodegradable nature, easy spinnability, and the presence of multiple focal adhesion


19/65

points. Silk fibroin is another polymer fiber that in a blended form with bone morphogenetic

protein 2 (BMP-2) and hydroxyapatite nanoparticles (nHAP) has exhibited excellent bone

tissue regeneration (Li et al. 2006). Researchers have also explored the potential application

of poly caprolactone (PCL) in bone tissue regeneration. The results obtained revealed that

PCL electrospun nanofiber scaffolds enhanced the MC3T3-E1 pre-osteoblasts cell adhesion

and proliferation as well as assisted in the differentiation of the cell (Wong et al. 2014). A

huge amount of literature is available on the tissue engineering applications of electrospun

nanofibers (Yoshimoto et al. 2003), However, there are some limitations in the use of

electrospun nanofiber scaffolds in tissue engineering. One such hurdle is the infiltration of the

cells inside the scaffolds because of the smaller intra-fiber pore size. In order to overcome

this hurdle, various attempts have been made to fabricate scaffolds with a larger intra-fiber

pore size to allow the scaffolds to present a 3D environment instead of a 2D environment. As

compare to conventional 2D electrospun scaffold, 3D scaffolds have more exposed inner

surface area and pore size, and therefore show enhanced infiltration of cell. Literature shows

that cells migrated approximately up to 4 mm and exhibited a spatial cell

distribution. Therefore excellent biocompatibility, physical and spatial geometries of 3D

electrospun scaffolds are important in tissue engineering applications such as nerve

regeneration, vascular grafts, and bone regeneration, etc (Sun et al. 2014). Researchers are

therefore trying various options to fabricate 3D scaffolds. One method to fabricate 3D

scaffolds could be the fabrication of nanofibers scaffolds by combining multiple polymers.

Because of the different solubilities and stretching characteristics of the polymers in the flight

between the needle and collector, fibers with different diameters will be created which will

result in a controlled intra-fiber pore size. The controlled large intra-fiber pore size will result

in the infiltration of cells into the electrospun blended nanofibers scaffold. Besides the pores,


20/65

using the wettability of the polymers blends a promising construct could be prepare, which

could promote cells infiltration, adhesion (Moffa et al. 2013).

4. 2. Drug delivery

Delivering drugs in the most feasible physiological manner is of prime importance in the

medical field. Providing a drug with a smaller size and suitable coating material enhances its

ability to be digested or absorbed by the targeted site. Targeted drug delivery using

electrospun nanofibers banks on the idea that the drug dissolution rate increases with an

increase in the surface area of the carrier and the drug itself. Numerous reports have been

published highlighting the benefits of using electrospun nanofibers as a drug delivery carrier

(Table 2) (Kenawy et al. 2002b). Until now, many kinds of drugs, including anticancer

agents, proteins, antibiotics, ribonucleic acid (RNA), and deoxyribonucleic acid (DNA), have

been loaded on electrospun nanofibers (Hu et al. 2014). One such example is the loading of

bovine serum albumin (BSA protein), whose loading mechanism is depicted in Figure 7(Ma

et al. 2006; Nasreen et al. 2013b). The diversity offered by the electrospinning technique,

simply by tuning its parameters according to the target study, has made the use of

electrospinning in drug delivery and tissue engineering highly attractive. Various methods

such as incorporating the drug into the electrospun nanofibers and coating the drug on the

surface of the electrospun nanofibers have been employed for the preparation of electrospun

nanofiber scaffolds, which can act as a nano-cargo carrier. All of these methods can be

helpful in providing a controlled and sustained release of a drug at the target site by simply

tailoring the drug-release kinetics (Sill and von Recum 2008). In addition to the controlled

and sustained release of a drug, the electrospinning technique has also shown enhanced

therapeutic efficacy and reduced toxicity. Multiple drugs can be loaded into the electrospun


21/65

nanofiber. Keeping in mind the versatility shown by nanofiber as a carrier, many research

groups around the world have extensively studied the role of electrospun nanofiber as a drug

delivery system (DDS) (Chung and Park 2007). They have evaluated the effects of

biodegradable and non-biodegradable polymers on sustained drug delivery using electrospun

nanofibers (Papkov et al. 2007). For instance, Kenway et al. used the biodegradable polymer

PCL, non-biodegradable polymer polyurethane (PU), and their blend as drug delivery carriers

(Kenawy et al. 2009). However, they could not find any difference between the drug-release

rates of PCL, PU, and their blend. The only difference was the improved mechanical

properties of the blend. Further, to investigate the effect of the carrier on the release of the

drug, the drug-release profiles of nanofibers made of poly(lactic acid) (PLA), poly(ethylene-

co-vinyl acetate) (PEVA), and their blend (50:50 ratio) were compared using a commercially

available DDS and cast films. The results revealed that the electrospun nanofiber carriers

exhibited better drug-release profiles than the drug carrier made by the conventional casting

technique (Kenawy et al. 2002a). More recently, many naturally available polymers have also

been tested as DDSs. For instance, Yang et al. prepared PVA/gelatin composite nanofiber

scaffolds and investigated the various factors that govern the drug-release profile of raspberry

ketone (RK). The electrospun nanofiber carrier initially showed a burst release of the drug,

but over time, this burst release was changed to a sustained release. Three parameters were

found to govern the release profile of RK: (i) the PVA/gelatin ratio, (ii) crosslinking time of

the glutaraldehyde vapor, and (iii) amount of loaded drug (Kanani and Bahrami 2010; Yang

et al. 2007b). Haider et al. published articles on the use of PLGA electrospun nanofiber as a

drug carrier for calcium apatites, as well as growth factors. They concluded that PLGA

electrospun nanofibers could be effectively used for targeted drug delivery at a target site

(Haider et al. 2014a; Haider et al. 2014b). In addition to the conventional loading of a drug

into nanofibers, the effect of modulating the polymer dissolution rate, which can be used to


22/65

govern the release rate of the drug (Kim and Fassihi 1997). The blending of a drug and its

carrier via the electrospinning technique can be helpful in the fabrication of different

hierarchical structures. This could be achieved using various strategies, including adding the

drug to the nanofiber as nanoparticles, fabricating a fibrous blend of the drug and carrier,

blending both the drug and carrier with fibrous materials, and encapsulating the drug into

electrospun nanofibers. However, because the fabrication of a drugdelivery carrier (DDC) in

the form of nanofibers is still in the early stage, a convenient drug delivery carrier is yet to be

established (Huang et al. 2003b). Some of the examples of the aforementioned loading of

drugs into the electrospun nanofiber have been depicted in Figure 9.

Figure 7

4. 3. Immobilization of enzymes

Enzymes are usually immobilized on inert non soluble material for improving the durability

and maintaining the properties of the enzymes such as bioprocessing and controlling reaction

for longer duration (Jia et al. 2002; Xie and Hsieh 2003). Similar to a drug carrier, the

enzymatic activity greatly depends on the properties of the carrier material, such as its

biocompatibility and durability, as well as its hydrophobic or hydrophilic nature (water

contact angle) (Ye et al. 2005). Various methods have been implemented for the preparation

of an enzyme carrier, such as gel matrices, porous particles, and porous membranes

(Bhardwaj and Kundu 2010; Martinek et al. 1977). Among these, the unique properties of

electrospun nanofibers can efficiently relieve the hurdles that usually hinder the catalyzing

ability of an enzyme immobilized on a carrier material. Jia et al. (2002) fabricated a

polystyrene electrospun nanofiber carrier to carry -chymotrypsin. From an analysis of the

obtained results, they concluded that the hydrolytic activity of the enzyme was increased by

65% compared to the free enzyme (Jia et al. 2002). Similarly, a silk fibroin (SF) electrospun


23/65

nanofiber carrier exhibited a 90% -chymotrypsin retainment activity for an electrospun

nanofiber over 24 hr (Lee et al. 2005). Ye et al. fabricated a poly (acrylonitrile-co-maleic acid)

electrospun nanofiber for the immobilization of lipase. The activity retention of the poly

(acrylonitrile-co-maleic acid) carrier nanofibers for lipase was several times higher than those

calculated for a hollow fiber membrane (Ye et al. 2005). Similarly, Li et al. opted for an

amidination reaction, using pristine polyacrylonitrile nanofibers as the carrier for lipase. They

revealed that the conjugation of enzymes on the carrier electrospun nanofibers exhibited a

higher enzyme loading ability compared to other immobilization techniques (Li et al. 2007).

Moreover, Kim et al. highlighted the use of an advanced hierarchical structure that can

support, as well as enhance, the enzyme loading ability of the electrospun nanofibers.

Therefore, they fabricated a poly (-caprolactone) and poly (D, L-lactic-co-glycolic acid)-b

poly (ethylene glycol)-NH2 (PLGA-b-PEG-NH2) block copolymer with biocompatibility, as

well as a high surface area, for the covalent immobilization of enzymes (Kim and Park

2006b). The dual electrospinning technique has been implemented by researchers for the

fabrication of electrospun nanofibers to increase the immobilization of enzymes. Huang et al.

used phospholipid side moieties for the fabrication of electrospun nanofiber scaffolds with

mean diameters smaller than 90 nm. The nanofiber exhibited an excellent enzyme

immobilization capacity and enhanced biocompatibility (Figure 8) (Huang et al. 2006a).

Furthermore, they also suggested various possible ways of carrying enzymes to their target

site via electrospun nanofiber. However, there are certain limitations that have hindered the

wide-scale use of such approaches. These include (i) the encapsulation of enzymes and (ii) a

limitation on the immobilization of those enzymes on the surface of the fibers, which need to

interact directly with the nuclei of the cells. Therefore, in order to avoid such hurdles, the

sustained release of target molecules can be tailored using materials responsive to local

external cues (Kim and Yoo 2010). Furthermore, apart from the immobilization of enzymes


24/65

or other biomolecules, the surface of the nanofiber can be modified with various chemicals in

order to regulate the release of biomolecules from the nanofiber surface (Im et al. 2010;

Theron et al. 2005).

Figure 8

4. 4. Wound dressing

Wound healing is a dynamic process that follows an intricate sequence of events, including

homeostasis, inflammation, proliferation, and remodeling (Martin 1997). This sequence is

controlled by various factors, signaling molecules, and cells. The roles of these factors are

still not completely known. Much work is needed to identify and understand the roles of these

factors. Therefore, wound dressing plays a pivotal role in the protection of a wound site,

elimination of exudates, appearance, and inhibition of microorganisms. Work on the

preparation of conventional wound dressings began in the early stages of human civilization

when they came to understand that a blister healed quicker if not broken. As previously

mentioned, wounds provide a favorable environment for microbial growth. Therefore, a

wound dressing agent has to play a multifunctional role. First, an ideal wound dressing

should provide a nice moist environment for the wound site to enhance wound healing.

Second, it should have the ability to cope with microbes, specifically antibiotic resistant

bacteria (Behm et al. 2005; Jones et al. 2004). Therefore, wound dressings prepared using an

electrospinning technique provide numerous advantages over wound dressing agents

prepared using conventional methods (Gao et al. 2014). The unique properties of electrospun

nanofiber scaffolds, such as their inter- and intra-fiber pores and high surface area, stimulate

the response of fibroblastic cells by quickly activating cell signaling pathways. Furthermore,

an electrospinning technique can be used because of its potential application in the

fabrication of cosmetic masks, which are used for skin cleansing and skin healing (Smith et al.

2001). The high surface area of an electrospun skin mask facilitates the flow of additives


25/65

from and to the skin. Apart from the transfer of additives through a skin mask, an electrospun

skin mask can easily be applied and removed from the skin without inducing pain (Huang et

al. 2003a). Moreover, various factors that are essential for the nourishment/treatment of the

skin can be incorporated into the electrospun nanofiber matrix, which can assist in the

treatment of the skin (Si et al. 2014). Because of the aforementioned properties, electrospun

nanofiber has the potential to be used in the fabrication of a skin mask. Some strategies used

to prepare a suitable wound dressing with antibacterial properties are shown in Figure 9.

Figure 9

Furthermore, these scaffolds also attract cells to the dermal layer, which has the ability to

excrete vital extracellular materials that assist in the repair of damaged tissues, including

cytokines, collagen, and growth factors (Chen et al. 2008a). Non-woven nanofibers are very

suitable as wound dressing agents (Deitzel et al. 2001; Van et al. 2006; Zhang et al. 2005b).

Therefore, the electrospinning technique has been used to prepare various nanofiber scaffolds

from raw materials like collagen (Behm et al. 2005), PEO, hydrophilic polymers such as

PVA, gelatin, chitosan, chitin, polyurethane, and polyesters, which have played pivotal roles

as wound dressing agents. (Khil et al. 2003b). Powell and coworkers conducted a

comparative study on collagen nanofiber scaffolds fabricated from bovine collagen using

freeze-drying and an electrospinning technique (Powell et al. 2008). From the analysis of the

obtained data, they concluded that electrospun nanofiber scaffolds can be a better substitute

than scaffolds prepared by the freeze drying method. The better performance of the

electrospun nanofiber scaffolds was attributed to the better cellular organization on the

nanofibers compared to the conventional freeze-dried scaffolds. Furthermore, another

research group revealed that electrospun collagen nanofiber scaffolds treated with type 1


26/65

collagen and laminin exhibited better cytocompatibility as compared to the untreated collagen

nanofiber scaffolds (Rho et al. 2006). Similar attempts have been made to fabricate blended

electrospun nanofiber scaffolds using various biocompatible polymers like chitosan and PEO

(Chen et al. 2008a; Chen et al. 2008c; Chen et al. 2007b; Huang et al. 2001). Chen and

coworkers prepared composite electrospun nanofiber scaffolds composed of polyethylene

oxide, type I collagen, and chitosan, with the ability to be crosslinked via glutaraldehyde

vapors. This electrospun composite nanofiber scaffold was subjected to various cyto-

compatibility experiments. The results obtained from those cyto-compatibility experiments

suggested that the electrospun composite nanofiber scaffolds were non-cytotoxic. Because

chitin and chitosan have structural similarities with glycosaminoglycans (GAGs, the main

component of proteoglycans), their antibacterial activity makes this electrospun nanofiber

scaffold one of the candidates to be used in the regeneration of skin tissues. Scientists also

reported the effectiveness of using chitin, either in a pure or blended form with other

polymeric materials (Noh et al. 2006; Yoo et al. 2008). Bhattarai et al. prepared chitosan-

based nanofibers scaffolds containing PEO, chitosan, and Triton X-100 (Bhattarai et al. 2005).

Based on the biocompatibility data analysis, they came to the conclusion that the electrospun

composite nanofiber scaffolds facilitated the adhesion of human osteoblastic cells. Xu et al.

fabricated chitosan/PLA blend micro/nanofibers by using electrospinning technique (Xu et al.

2009). The blended micro/nanofibers scaffolds were assumed to mimic the extracellular

matrix, and thus ultimately will provide a native environment to the cells. Therefore, they

suggested the use of this kind of blended micro/nanofiber electrospun scaffold in tissue

engineering. Gholipour et al. prepared electrospun blended nanofiber scaffolds comprised of

chitosan-PVA. Based on their results, they concluded that a 25/75 chitosan/PVA ratio was the

most suitable for the preparation of blended nanofiber scaffolds. Furthermore, they subjected

the samples to in vitro studies. The results of these in vitro studies suggested that the


27/65

chitosan/PVA electrospun nanofiber scaffolds exhibited excellent antibacterial activity

against gram-negative bacteria. Thus, the scaffolds could be used as a wound dressing

(Gholipour et al. 2009). Besides chitosan, the role of gelatin in the biomedical field cannot be

denied. Rujitanaroj et al. fabricated ultrafine gelatin nanofiber scaffolds that exhibited

excellent antibacterial properties against some common bacterial strains usually found in

burn wounds (Rujitanaroj et al. 2008). PU is another polymer that has frequently been used in

wound dressings because of its unique carrier non-cyto-toxic properties and good oxygen

permeability (Khil et al. 2003a). As PU nanofibers scaffolds are non-toxic, they provide

cultured cells with a native environment, which allows the cells to efficiently proliferate on

the PU nanofiber scaffolds. Verreck et al. fabricated PU nanofibers scaffolds loaded with the

drugs itraconazole and ketanserin, with potential use in wound healing applications (Verreck

et al. 2003). Kumbar et al. fabricated electrospun PLGA nanofiber scaffolds with fiber

diameter ranges of 150-225, 200-300, 250-467, 500-900, 600-1200, 2500-3000, and 3250-

6000 nm for their possible application in skin tissue regeneration (Kumbar et al. 2008). Based

on the morphology of the fibroblastic cells cultured on the surface of the PLGA nanofiber

scaffolds, it was concluded that all of the scaffolds exhibited a biocompatible nature, but the

most favorable environment was provided by the nanofiber scaffolds with diameters in the

range of 3501100 nm. Therefore, based on the purpose and need, skin substitutes can

successfully be fabricated using the electrospinning technique (Kanani and Bahrami 2010).In

order to construct suitable skin substitutes and achieve our future goals, material-wound

interactions should first be understood. Later, various strategies for optimizing such

properties can be formulated. Table 2 gives a summarized list of polymers and their

biomedical applications.

Table 2: Natural polymers and some of their targeted applications (Khan 2012).


28/65

Polymer Solvent Fiber

diameter

Ref

Drug Delivery System

Drug Delivery System:(a) Poly(-caprolactone) (shell)

+ Poly(ethylene glycol) (core)

222,trifluoroethanol

(b) Water200350 nm

(Zhang et al.

2006c)

(a) Poly(-caprolactone) and

Poly(ethylene glycol) (shell)Dextran (core)

Chloroform

and DMF, Water

15 m (Jiang et al.

2006)

Poly(-caprolactone) (shell)Poly(ethylene glycol) (core)

Chloroformand DMF, Water

500700 nm (Jiang et al.2005)

Poly(-caprolactone-co-ethylethylene phosphate)

DCM and PBS ~4 m (Chew et al.2005)

Poly(D-L-lactic-co-glycolicacid), PEG-b-PLA and PLA

DMF 260250 nm (Xu et al.2006a)

Poly(D-L-lactic-co-glycolic acid) DCM 110 m (Liang et al.2005)

Poly(D-L-lactic-co-glycolic acid) THF:DMF 400600 nm (Haider et al.2014a;

Haider et al.2014b;

Haider et al.)

Poly(L-lactide-co-glycolide)

and PEG-PLLA

Chloroform 6901350

nm

(Xie and

Wang 2006)

General Tissue Engineering

Poly(-caprolactone) Chloroform and

methanol

210 nm (Pham et al.

2006)Poly(-caprolactone) (core) + Zein

(shell)

Chloroform and

DMF

500900 nm (Jiang et al.

2007)

Poly(-caprolactone) (core) +

Collagen (shell)

2,2,2-

trifluoroethanol

500 nm (Zhang et al.

2005c)

Poly(D-L-lactic-co-glycolic acid)

and PLGA-b-PEG-NH2

DMF and THF 4001000

nm

(Kim and

Park 2006a)

Poly(D-L-lactide-co-glycolide) DMF and THF 500800 nm (Eichhorn

and Sampson2005)

Poly(ethylene glycol-co-lactide) DMF and acetone 14 mm (Yang et al.

2007a)

Poly(ethylene-co-vinyl alcohol) 2-propanol and

water

0.28.0 mm (Chuangchote

and Supaphol

2006)

Collagen HFP 180250 nm (Kenawy et

al. 2003)

Gelatin 2,2,2-

trifluoroethanol

0.299.10

mm

(Song et al.

2008)

Fibrinogen HFP 120610 m (Ayres et al.

2006)

Poly(glycolic acid) and chitin HFP 130380 nm (Lopes-da-Silva et al.


29/65

2009)

Vascular Tissue EngineeringPoly(-caprolactone) Chloroform and

DMF

0.21 nm (Zuwei Ma et

al. 2005)

Poly(L-lactide-co--caprolactone) Acetone 200800 nm (Inoguchi et

al. 2006; Xuet al. 2004)

Poly(propylene carbonate) Chloroform 5 m (Zhang et al.2006a)

Poly(L-lactic acid)andhydroxyapatite

DCM and 1,4-dioxane

300 nm (Ji et al.2006)

Chitin HFP 0.168.77

nm

(McManus et

al. 2007)

4.5. Anti-bacterial studies

Because of the aforementioned properties of electrospun nanofibers, numerous types of

electrospun hybrid nanofiber scaffolds with antimicrobial effects have been fabricated by

various research groups. For instance, researchers fabricated PAN/Ag composite nanofiber

scaffolds for their possible antimicrobial effect. The results showed the capability of PAN/Ag

nanofiber scaffolds to inhibit both gram-positive (Basillus cereus) and gram-negative

(Escherichia coli)bacterial growth. The antimicrobial effect of PAN nanofiber scaffolds was

further assessed by immobilizing amidoxime (having antimicrobial effect) onto PAN

nanofiber scaffolds. The immobilization of the amidoxime gave a significant antimicrobial

property to the PAN nanofiber scaffolds. This was evident from the fact that it completely

killed theE. coliand S. aurieusbacterial strains. The possible mechanism behind the killing

of those bacterial strains was the binding ability of amidoxime to magnesium (Mg 2+) and

calcium (Ca2+) ions, which are very essential for bacterial survival. The binding of these

metals to the membrane with amidoxime rather than bacterial cells disturbed the balance,

which therefore hindered the normal functions of the bacteria which ultimately resulted in

their death (Zhang et al. 2011). The same phenomenon was also revealed for a PAN

nanofiber scaffold dipped in an AgNO3solution and PAN/Ag nanofiber scaffolds (Zhang et


30/65

al. 2011). Table 3) provides a summarized list of the polymers and antibacterial agents used

in anti-bacterial nanofiber scaffolds.


31/65

Table 3. Polymers electrospun and used in antibacterial study (Gao et al. 2014).

Electrospun polymer Antibacterial agents Method of incorporation Ref:

Antibiotics

PLA, PEVA, PLA/PCL,

PEUU/PLGA

Tetracycline Mixing (Hong et al.

2008;

Kenawy et al.

2002a;

Zahedi et al.2012)

PLGA Cefoxitin Mixing (Kim et al.

2004)

PLA Mupirocin Mixing (Thakur et al.

2008)

coPLA, coPLA/PEG,

PU

Ciprofloxacin Mixing (Toncheva et

al. 2012;Unnithan et

al. 2012)

PLAGA Cefazolin Mixing (Katti et al.

2004)

PLGA Amoxillin Mixing (Chen et al.

2013)

PLA, PLA/Collagen,

PCL

Gentamycin Core/sheath (Huang et al.

2006b;Torres-Giner

et al. 2012)

PLLACL Tetracycline Core/sheath (Su et al.2009)

PMMA/nylon Ampicillin Core/sheath (Sohrabi et al.2013)

PLGA Amoxicllin Adsorption/encapsulation

on nanostructure

(Wang et al.

2012c; Zheng

et al. 2013)

Nanobiotics

PCL/PLA Triclosan Mixing (del Valle etal. 2011)

PLA Triclosan Complexing with g-CD (Kayaci et al.2013)

CA Chlorhexidine Mixing (Chen et al.

2008b)

PAN, PLA, PLA/PEG QACs Mixing (Gliciska et

al. 2013;

Toncheva et

al. 2011)

CA/PEU PHMB Mixing (Liu et al.

2012)

PAN PHMB Covalent immobilization (Mei et al.

2012)PAN N-Halamine Mixing (Ren et al.


32/65

2013)

PEO/chitosan K5N8Q Mixing (Spasova et

al. 2004)

Metal and metal oxides

PDLLA, PEO Antibacterial peptides

AgNps

Mixing (Heunis et al.

2011)PVDF, PVA/PU, nylon6, PLGA, PBS

AgNP NP dispersion (Lin et al.2014; Park et

al. 2009; Tianet al. 2013;

Xing et al.2011; Yuan et

al. 2010)

Nylon 6, PAN, PLLCL,

PCL, PVA

AgNP Synthesis in polymer

solution

(Chae et al.

2011;Mahapatra et

al. 2012;Montazer and

Malekzadeh2012; Paneva

et al. 2011;

Pant et al.

2011;

Rujitanaroj et

al. 2010; Shi

et al. 2011a;

Shi et al.2011b; Sichani

et al. 2010;

Wang et al.

2012b)

PLA, PCL, PAN, PVA,

PEO

AgNP In situsynthesis (An et al.

2009; Au et al.

2012; Gao et

al. 2014;

Gilchrist et al.

2013; Hang et

al. 2010;Mahapatra etal. 2012; Xu et

al. 2006b)

PLA/chitosan AgNP In situsynthesis (Au et al.

2012; Zhao et

al. 2012)

PEO/chitosan AgNP In situsynthesis (An et al.

2009)

PVA/chitosan AgNP In situsynthesis (Hang et al.

2010)

PVA/chitosan AgNP NP dispersion (Abdelgawad

et al. 2014)


33/65

PEO/chitosan AgNP NP dispersion (Fouda et al.

2013)

PU, PVA, silk fibroin ZnO, TiO2 Dispersion (Lee and Lee

2012; Lee2009; Pant et

al. 2011)PU TiO2 In situsynthesis (Yan et al.

2011)

Nylon 6 ZnO Electrospray on surface

PMMA ZnO/TiO2 Synthesis in solution (Hwang andJeong 2011)

Chitosan

PLA, PVA Chitosan derivatives Blending (Alipour et al.

2009; Ignatova

et al. 2009;

Ignatova et al.2006)

PET, PCL, PEO Chitosan Blending (Cooper et al.

2013; Jung et

al. 2007;

Kriegel et al.

2009; Sadri et

al. 2012)

PLA Chitosan Core/shell (Nguyen et al.

2011)

5. Other applications

Besides the various potential applications of electrospun nanofibers in the biomedical area,

they have also found applications in filtration, desalination, protective clothing, and in

sensors. Numerous polymers have been used in the aforementioned applications of

electrospun nanofiber scaffolds. For example cellulose acetate (CA) and PVA have been used

in the removal of toxic chromium ions. PVDF and PAN-based carbon nanofiber (CNF) have

applications in desalination, etc. Polymers with a piezoelectric effect such as PVDF can be

used for the preparation of nanofibrous piezoelectric devices (Huang et al. 2003b). Detailed

descriptions of these applications are given in the following sections.


34/65

5.1. Filtration

Various heavy metals are used in the manufacturing processes of various industries. The ions

released in effluent, can cause severe damage to human health and the environment. Heavy

metal ions can easily be mixed with into the water reservoir (that acts as a carrier), which

distributes metal ions to the surroundings (Nasreen et al. 2013c). The separation of metal ions

from reservoir water is a serious problem. Therefore, researchers have focused on addressing

this issue. Among the various metal ions, chromium (Cr) is considered to be the most toxic

heavy metal due to its carcinogenic effect in humans. Several researchers have reported that

electrospun nanofibers are of prime importance in filtering heavy metal ions from

contaminated water because of their unique surface-to-length ratio and interconnected

porosity contrary to conventional materials. Pristine polymers, functionalized polymers, and

polymer composites offer excellent capability for the removal of Cr (IV). Taha et al.removed

19.45 mg/g of Cr (IV) using amine-functionalized cellulose acetate/silica nanofibers. The

mechanism of the metal ion and polymer interaction was electrostatic (Taha et al. 2012). A

much higher amount of Cr (IV) (97 mg/g) was removed when cellulose acetate (CA) was

replaced with PVA. Besides using the previously mentioned polymers, PAN/FeCl3has shown

two advantages. (i) It increased the removal of chromium (IV) (110 mg/g) and (ii) helped to

convert Cr (IV) to Cr (III), which is assumed to be less harmful (Nasreen et al. 2013a). A

similar conversion of Cr (IV) to Cr (III), with an increased removal of Cr (IV) (150 mg/g),

was reported by Li et al.using polyamide 6 and FexOy. According to them, the synthesis of

iron nanoparticles and their protonated form helped in the conversion of Cr (VI) to Cr (III)

(Li et al. 2013).

HCrO4+ 3Fe2+ + 7H+ Cr3+ + 3Fe3+ + 4H2

The removal of Cr (VI), copper (II), and lead (II) using chitosan nanofiber scaffolds has also

been reported in the literature. Using chitosan, very high amounts of lead (Pb), i.e., 263.15


35/65

mg/g, and copper (Cu), i.e., 485.44 mg/g, were removed. These results with chitosan

nanofiber were much higher than those with conventional materials. They assumed that the

removal was based on electrostatic interaction (Li et al. 2013). Aliabadi et al. also studied the

removal of Cu (II), Pb (II), nickel (Ni (II)), and cadmium (Cd (II)) using chitosan/PEO

composite nanofiber scaffolds and concluded that the chitosan/PEO composite nanofibers

removed 229.2 mg/g of Cu, 249.9 mg/g of Ni, 195.1 mg/g of Pb, and 196.6 mg/g of Cd from

an aqueous solution (Aliabadi et al. 2013). Qin et al. fabricated cross-linked PVA electrospun

nanofibers using malefic acid as the cross linker and vitriolic acid as the catalyst to make

them stable in water. The filtration capability was found to be far better when the electrospun

nanofiber membrane was crosslinked at the sub-layers, which were prepared using spun-

bound and melt blown techniques (Qin and Wang 2008). Homaeigohar et al. studied the

benefits of using polyethersulphone (PES) electrospun nanofiber membranes supported with

polyethylene terephthalate (PET) sub-layers for the filtration of water. They concluded that

PES electrospun nanofiber membranes exhibited a high permeability for pure water, whereas

the permeability slowly decreased with an increase in the feed pressure. It was reported that

particles with a size of >1 m were removed within an hour by maintaining a low pressure

and very high flux. However, high rejection was achieved when particles with a size of


36/65

5.2. Desalination

To meet the increasing demands for pure drinking water, various techniques have been

implemented for the purification of water with a high salt content. These techniques include

membrane distillation (MD), electro dialysis (ED), freeze desalination (FD), and reverse

osmosis (RO). Because of their flux and cost effectiveness, electrospun membranes are

considered to be the most effective method for purifying saline water. Such electrospun

nanofiber scaffold/membranes are used as self-supporting membranes for the purpose of

desalination. The application of electrospun nanofiber scaffold/membranes in the purification

of water was also explored by the Ramakrishna group (Feng et al. 2008; Kaur et al. 2012).

They highlighted that electrospun nanofiber scaffold/membranes can remain stable for up to 4

weeks. Therefore, these electrospun nanofiber scaffold/membranes can be used as an

alternative to conventional distillation membranes. The blending of clay nanoparticles with

PVDF followed by electrospinning was carried out for a direct contact membrane distillation

(DCMD) process and up to 99.95% salt rejection was achieved by (Prince et al. 2012).

Moreover, the potential application of PAN-based CNFs was also explored for capacitive

deionization by Wang et al. (Wang et al. 2012a). They observed a higher electro-sorption

capacity (4.64 mg/g) for CNFs than for other materials such as activated carbon (3.68),

woven carbon fibers (1.87), carbon aerogel (3.33), carbon nanotubes (CNTs)-CNFs (3.32),

mesoporous carbon (0.69), and graphene (1.85 mg/g), which showed that electrospun

nanofiber scaffold/membranes could potentially be applied in the electrochemical capacitive

deionization of highly salinated seawater (Nasreen et al. 2013c; Tijing et al. 2014). A

summarized list of the various electrospun polymer scaffolds used in desalination is given in

Table 4.


37/65

Table 4: Electrospun nanofibers in desalination applications ( Haider et al. 2015).

Middle layer

(electrospun

nanofiber)

Third layer Solute Method Flux

(L/m2/h)

Rejection (%)

PVA/MWNT orPebax/MWNTover PETsubstrate

- Oil/water TFNC bycoating 330 or 160 N/A

PVA or Pebaxover PET

substrate

- Oil/water TFNC bycoating

130 or 58 PVA coated>99.5

10&4 wt% ofPAN over PET

substrate, rotatingcollector

- Oil/water TFNC bycoating

TFNC an orderof magnitude >

com

99.5% betterthan com. NF

PAN Polyamides MgSO4 TFNC by

interfacial

TFNC 38% >

com. NF 270

TFNC and

com. arecomparable

PVDF Polyamides MgSO4NaCl

TFNC byinterfacial

0.660.66

75.770.2

PAN Polyamides MgSO4 InterfacialTFNC1TFNC2

--81

-8884.2

First layer 8 or10wt% PANSecond layer 4 or

6 or 8 wt% PAN

Polyamides MgSO4NaCl

Interfacial 220200

8989

PVDF - 6 wt%NaCl

AGMD 1112 kg (ms-h) N/A

PVDFPVDF-claynanocomposites

- NaCl DCMD N/A 98.2799.95

PET/PS Polyamide NaCl Interfacial 1.13 L/m-hr-bar -

5.3. Protective clothing

In military, protective clothing is primarily expected to help maximize the survivability,

sustainability, and combat effectiveness of the individual soldier against extreme weather

conditions, ballistics, and nuclear, biological, and chemical (NBC) warfare (Nurwaha et al.

2013). During war, protective clothing with particular functions against chemical warfare

agents such as sarin and soman, and breathing apparatus, which help prevent the inhalation

and absorption through the skin of mustard gas, gain special importance for combatants in

conflicts and civilian populations during terrorist attacks. The current protective clothing


38/65

containing charcoal absorbents has its limitations in terms of water and air permeability, the

extra weight imposed, and flammability. Therefore, a lightweight and breathable fabric that is

permeable to both air and water vapor, but insoluble in all solvents and highly reactive with

nerve gases and other deadly chemical agents, is desirable. Because of their great surface area,

nanofiber fabrics could help in neutralizing chemical agents as well as the impedance of the

air and water vapor permeability of clothing (Huang et al. 2003b). Electrospinning results in

nanofibers that are laid down in layers that have high porosity but a very small pore size,

providing good resistance to the penetration of toxic chemical agents from an aerosol (Gibson

et al. 1999). Moreover, various methods have been adapted for the surface modification of

electrospun nanofibers to further enhance their protection capability against toxic materials.

One of the methods used to improve their protection ability is modifying the surface of the

electrospun nanofiber with reactive groups such as chloramines, cyclodextrins, and oximes,

which have the capability to bind and neutralize the threat of toxic materials (Bhardwaj and

Kundu 2010; Gopal et al. 2006). Nattanmai et al. conducted an experiment using MgO

nanoparticles embedded in nylon 6 nanofibers. Based on the results, it was concluded that

nylon 6 composite nanofibers containing MgO nanoparticles have good anti-flammable

properties. Hence, it was proposed that nylon 6/MgO composite nanofiber scaffolds could be

a good addition to the antiflammable clothing used in wars (Nattanmai Raman et al. 2014) .

Preliminary investigations on electrospun nanofibers have also revealed that compared to

conventional textiles, electrospun nanofiber clothes have minimal impedance to moisture

vapor diffusion, are extremely efficient in trapping aerosol particles, and promise protection

(Gibson et al. 2001; Gibson et al. 2002).


39/65

5.4. Sensor applications

Electrospun nanofibers have potential in the fabrication of sensing devices because they have

a high surface area, which ultimately enhances their sensitivity as a sensor. Researchers have

used PLGA electrospun nanofiber scaffolds as chemical sensors to prepare new sensing

devices (Huang et al. 2003b). Fluorescent electrospun nanofiber could be another material

with potential in sensors (Lee et al. 2002; Wang et al. 2002a; Wang et al. 2002b). Initial

reports suggests that the sensing abilities of the electrospun nanofiber scaffolds used for the

detection of a nitro compound (2,4-dinitrotulene, DNT), mercury ions, and ferric ions are

many times higher in magnitude than the traditional thin films. Nanorods/nanotubes prepared

from different materials such as ceramics, metals, carbon, and polymers are given priority

because of their potential applications in various industries. An electrospinning technique can

be used to prepare ultrafine nanofibers, which can be used as a template for the preparation of

various nanorods/nanotubes (Bognitzki et al. 2000; Hou et al. 2002b). For instance, nanotube

materials can be coated on an electrospun nanofiber template, and then nanorods/nanotubes

can be obtained by the solvent extraction or thermal degradation of the template. For the

preparation of nanorods/nanotubes, the electrospun template used must be stabile during

coating and must be degradable without disturbing the nanorods/nanotubes. Bognitzki et al.

used a PLA nanofiber template and obtained various composites of poly(p-xylene) (PPX) and

metal (aluminum) nanotubes (with wall thickness in the range of 0.11 mm) (Bognitzki et al.

2000). Hou et al. employed a similar technique but used a fine diameter electrospun nanofiber

template, which resulted in the fabrication of very thin nanotubes (Hou et al. 2002a; Huang

2001). Furthermore, Baniasadi et al. fabricated stretchable PVDF-TrFE lectrospun nanofiber

with piezoelectric structures. They observed an increase in the overall tensile strength by

twisting the electrospun ribbons(Baniasadi et al. 2015). This kind of scaffold could be used in


40/65

the fabrication of piezoelectric and energy harvesting devices (Figure 11) (Fang et al. 2013;

Persano et al. 2015)

Figure 11

5.5. Future direction

With the increasing knowledge in the field of nanotechnology, many techniques are being

employed for the synthesis of materials at the nanometer level. Electrospinning is considered

to be one of the most efficient techniques used for the synthesis of nanomaterials. Although

this technique was discovered way back in the 19thcentury, the bulk of the work has been

done in the late 1990s and early part of the 21st century. The work in the field of

electrospinning has intensified more recently. Many polymer and high-molecular-weight

compounds with sufficient viscosity have been electrospun. At present, not only can the

morphology and inter- and intra-porosities be controlled, but the dimension and direction of

the nanofiber deposition can also be controlled. All of these factors have led to the extensive

utilization of nanofibers in almost every field, including filtration, enzyme immobilization, as

sensing membranes, cosmetics, protective clothing, affinity membranes, tissue engineering

scaffolds, drug delivery, and wound healing applications. In biomedical applications and

particularly in tissue engineering, it is very important for artificial scaffolds to mimic the

original biological structure and exhibit similar biological properties. Therefore, more work is

needed to provide a natural environment for cells and avoid toxicity, which would lead to

greater cell proliferation. This could be done by the immobilization of spacers (functional

groups) onto scaffolds. Such immobilizing species should also be biocompatible. A similar

approach is also finding much interest in environmental and sensor applications. However,

environmental applications with surface functionalized nanofibers are facing few challenges

that need to be tackled. These include a capacity reduction and kinetic slowness after surface


41/65

modifications. Similarly, the adsorption and removal capacities of nanofibers are dramatically

reduced after regeneration. The first effect is related to the porosity of the membrane, which

changes after surface modification, whereas the latter is related to the occupation of the

adsorption sites by water molecules. Hence, it is suggested that surface functionalization

strategies should be designed that not only avoid pore changes but also prevent a decrease in

the adsorption capacities after desorption. In sensor applications, the surface group should

have an affinity for the material that needs to be determined.

6.

Conclusion

Electrospinning is a simple, unique, versatile, and cost-effective technique that is widely used

for the fabrication of non-woven fibers with a high and tunable porosity and high surface area.

The morphology of the electrospun nanofibers is significantly affected by various parameters

such as the polymer concentration, viscosity, molecular weight, applied voltage, tip-to-

collector distance, and solvent. By controlling these parameters, it is possible to easily

fabricate electrospun nanofiber scaffolds for the desired function. Electrospun nanofiber

scaffolds/membranes have found numerous potential applications in almost every field,

including filtration, enzyme immobilization, sensing membranes, cosmetics, protective

clothing, affinity membranes, tissue engineering scaffolds, drug delivery, and wound healing

applications. Because of the aforementioned properties of electrospun nanofibers, the

electrospinning technique is considered to play a vital role in different biomedical fields and,

more specifically, in the area of tissue engineering. Despite having unique properties,

electrospun nanofibers have a few limitations. One hurdle is the poor infiltration of cells into

electrospun nanofiber scaffolds. However, progress is being made to fabricate electrospun

nanofiber scaffolds with enhanced cell infiltration ability, which will actually allow them to


42/65

act as 3D scaffolds. In general, the electrospinning technique has exhibited excellent potential

to be used in various fields, specifically in the field of tissue engineering.

Acknowledgement

This work was supported by the Basic Research Laboratory Program (No. 2011-0020264)

and General Research Program (2013 RIAIA 2005148) from the Ministry of Education,

Science and Technology of Korea.

References

A.M. Abdelgawad, S.M. Hudson, O.J. Rojas Antimicrobial wound dressing nanofiber mats from

multicomponent (chitosan/silver-NPs/polyvinyl alcohol) systems Carbohydrate Polymers, 100 (0)

(2014), pp. 166-178.M. Aliabadi, M. Irani, J. Ismaeili, H. Piri, M.J. Parnian Electrospun nanofiber membrane of

PEO/Chitosan for the adsorption of nickel, cadmium, lead and copper ions from aqueous solution

Chemical Engineering Journal, 220 (0) (2013), pp. 237-243.

S.M. Alipour, M. Nouri, J. Mokhtari, S.H. Bahrami Electrospinning of poly(vinyl alcohol)water-

soluble quaternized chitosan derivative blend Carbohydrate Research, 344 (18) (2009), pp. 2496-

2501.

L. Almany, D. Seliktar Biosynthetic hydrogel scaffolds made from fibrinogen and polyethylene

glycol for 3D cell cultures Biomaterials, 26 (15) (2005), pp. 2467-2477.

J. An, H. Zhang, J. Zhang, Y. Zhao, X. Yuan Preparation and antibacterial activity of electrospun

chitosan/poly(ethylene oxide) membranes containing silver nanoparticles Colloid Polym Sci, 287

(12) (2009), pp. 1425-1434.C.J. Angammana, S.H. Jayaram Analysis of the Effects of Solution Conductivity on Electrospinning

Process and Fiber Morphology Industry Applications, IEEE Transactions on, 47 (3) (2011), pp. 1109-

1117.

H. Au, L. Pham, T. Vu, J. Park Fabrication of an antibacterial non-woven mat of a poly(lactic

acid)/chitosan blend by electrospinning Macromolecular Research, 20 (1) (2012), pp. 51-58.

C. Ayres, G.L. Bowlin, S.C. Henderson, L. Taylor, J. Shultz, J. Alexander, T.A. Telemeco, D.G. Simpson

Modulation of anisotropy in electrospun tissue-engineering scaffolds: Analysis of fiber alignment

by the fast Fourier transform Biomaterials, 27 (32) (2006), pp. 5524-5534.

H.-S. Bae, A. Haider, K.M.K. Selim, D.-Y. Kang, E.-J. Kim, I.-K. Kang Fabricatio

comprehensive review summarizing effect of electrospinning parameters and.pdf

Documents