A comprehensive review summarizing the effect of electrospinning parameters and potential applications of nanofibers in biomedical and biotechnology

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 via a metallic needle will cause a spherical droplet to deform into a Taylor cone and form ultrafine nanofibers at a critical voltage (Fig. 2a–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 with 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 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. and Zong 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 (Fig. 2) (Baumgarten, 1971).

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Figure 2

(a–c) Digital images showing the three stage deformation of the polyvinylpyrrolidone droplet under the influence of increasing electric field. The cartoon (d–f) shows the mechanism of the effect of charges on the polymeric droplets. The application of high voltage to the polymer solution held by its surface tension creates a charge on the surface of the liquid. Reciprocated charge repulsion and the contraction of the surface charges to the counter electrode cause a force directly opposite to the surface tension. As the intensity of the electric field is increased, the hemispherical drop formed at tip of the needle tip gets converted into conical shape (Laudenslager and Sigmund, 2012).

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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 via a 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 and 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., 2002). 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., 2002; 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 (Fig. 3f) (Shamim et al., 2012). In addition to bead formation, in some cases, at an elevated flow rate, ribbon-like defects (Megelski et al., 2002) and unspun droplets (Fig. 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 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).

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Figure 3

Formation of various jets with increasing flow rate (Shamim et al., 2012) of nylon 6. The SEM image shows wide range diameter of nanofiber (f), and the digital images show solution drop (g) and electrospun fibers of chitosan deposited on aluminum foil (h).

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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 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 (C. Zhang et al., 2005).

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 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 (Fig. 4a–d) (Shamim et al., 2012). Similar phenomenon was found by Fong et al. when they electrospun PEO by varying its viscosity (Fig. 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).

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Figure 4

Variation in morphology of electrospun nanofibers of PEO with viscosity: (a–d) schematic and (e–h) SEM micrographs. Reproduce with permission from the publisher (Luzio et al., 2014; Zander, 2013; Fong et al., 1999).

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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. Solution with lower 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 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 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 affects 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 influence 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 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 1.0 μm (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 (Fig. 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 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 result in the fabrication of highly porous electrospun nanofibers (Fig. 5f) (Sill and von Recum, 2008). Similar results were also reported by Y. Zhang et al. (2006)). Megelski et al. prepared porous nanofibers by varying the ratio of tetrahydrofuran (THF) and dimethylformamide (DMF) (Megelski et al., 2002). 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).

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Figure 5

SEM images of 5% PCL solutions dissolved in different solvents: (a) glacial acetic acid, (b) 90% acetic acid, (c) methylene chloride/DMF = 4/1, (d) glacial formic acid, (e) and formic acid/acetone, along with (f) SEM images of PVB nanofibers prepared from 10 wt% THF/DMSO (9/1 v/v) (Kanani and Bahrami, 2011; Lubasova and Martinova, 2011).

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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 667 nm to 161 nm (PVA) and 252 nm to 75 nm (PEO) with increase in humidity from 4% to 60%. For the blend the decrease was even more; for example, humidity decreased from 4% to 50%, and the diameter of the nanofibers for PVA/HA decreased from 231 nm 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 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 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 toward the collector; Fig. 6). This difference in rates of evaporation of the two solvents causes a cooling effect, a phenomenon similar to perspiration. This cooling effect results 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 (Fig. 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).

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Figure 6

A schematic diagram for formation of pores in nanofibers during electrospinning and FE-SEM images of the electrospun polymethyl methacrylate (PMMA) fibers with different humidity: (a) 15–25%, (b) 26–40%, (c) 41–55%, (d) 56–70%, (e) nonporous fiber cross-section, and (f) porous fiber cross-section (Bae et al., 2013).

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4.1 Tissue engineering

A range of methods have 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 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 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 (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, using the wettability of the polymers blends a promising construct could be prepared, which could promote cells infiltration and 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., 2002). 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 Fig. 7 (Ma et al., 2006; Nasreen et al., 2013). 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 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 (Sokolsky-Papkov et al., 2007). For instance, Kenway et al. used the biodegradable polymer PCL, non-biodegradable polymer 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 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., 2002). 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; D. Yang et al., 2007). 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 (A. Haider et al., 2014a, 2014b). In addition to the conventional loading of a drug into nanofibers, the effect of modulating the polymer dissolution rate can be used to 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 drug delivery 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., 2003). Some of the examples of the aforementioned loading of drugs into the electrospun nanofiber have been depicted in Fig. 9.

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Figure 7

Polysulfone (PSU) nanofibers acting as drug delivery carrier for BSA protein. Where MAA is methacrylic acid, EDAC is carbodiimide hydrochloride and NHS is N-hydroxysuccinimide (Ma et al., 2006; Nasreen et al., 2013).

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4.3 Immobilization of enzymes

Enzymes are usually immobilized on inert nonsoluble 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 α-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 nanofiber carrier exhibited a 90% α-chymotrypsin retainment activity for an electrospun nanofiber over 24 h (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 that calculated for a hollow fiber membrane (Ye et al., 2005). Similarly, Li et al. opted for an amidination reaction, they used 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(ε-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 (Fig. 8) (X.-J. Huang et al., 2006). 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 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).

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Figure 8

Schematic representation of fabrication of phospholipid-modified nanofibers by electrospinning process for lipase immobilization (X.-J. Huang et al., 2006).

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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 (Gallant-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 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., 2003). 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 Fig. 9.

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Figure 9

Various strategies used to prepare suitable wound dressing (Gao et al., 2014).

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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 (J.-P. Chen et al., 2008). Non-woven nanofibers are very suitable as wound dressing agents (Deitzel et al., 2001; Luong-Van et al., 2006; Y. Zhang et al., 2005). Therefore, the electrospinning technique has been used to prepare various nanofiber scaffolds from raw materials such as collagen (Gallant-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., 2003). 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 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 such as chitosan and PEO (J.-P. Chen et al., 2008; Z. Chen et al., 2007, 2008; 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 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., 2003). 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 350–1100 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.

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/sliver (Ag) nanofiber scaffolds to inhibit both Gram-positive (Bacillus 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 the E. coli and Staphylococcus aureus bacterial strains. The possible mechanism behind the killing of those bacterial strains was the binding ability of amidoxime to magnesium (Mg²⁺) 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 silver nitrate (AgNO₃) solution and PAN/Ag nanofiber scaffolds (Zhang et al., 2011). Table 3 provides a summarized list of the polymers and antibacterial agents used in anti-bacterial nanofiber scaffolds.

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., 2013). 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/ferric chloride (FeCl₃) has 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., 2013). 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 Fe_xO_y. 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). $${\text{HCrO}}_4-+3{\text{Fe}}^{2+}+7{\text{H}}^{+}\to {\text{Cr}}^{3+}+3{\text{Fe}}^{3+}+4{\text{H}}_2$$

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 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 polyethersulfone (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 μm 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 <1 μm were used (Homaeigohar et al., 2010). More recently, electrospun nanofibers have been functionalized with various functional groups. For instance the nitrile group in PAN nanofibers was converted to amidoxime. PAN–amidoxime nanofibers showed good removal abilities for Cu(II) and Pb(II) from aqueous water. Both the metal ions were found to have chemically bonded to the PAN–amidoxime (Saeed et al., 2008). Similarly, PAN nanofibers were functionalized with ethylenediamine (EDA) and diethylenetriamine (DETA) for the removal of organic color dyes (Fig. 10) (S. Haider et al., 2014, 2015a, 2015b).

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Figure 10

Dye interactions with DETA-g-PAN nanofibers membrane (Haider et al., 2015a, 2015b).

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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 G. Wang et al. (2012). 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., 2013; Tijing et al., 2014). A summarized list of the various electrospun polymer scaffolds used in desalination is given in Table 4.

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 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., 2003). 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 Magnesium oxide (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; Schreuder-Gibson et al., 2002).

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., 2003). Fluorescent electrospun nanofiber could be another material with potential in sensors (Lee et al., 2002; Wang et al., 2002a, 2002b). Initial reports suggest that the sensing abilities of the electrospun nanofiber scaffolds used for the detection of a nitro compound DNT, mercury ions (Hg(II)), and ferric ions (Fe(III)) 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., 2002). 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 0.1–1 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., 2002; Huang, 2001). Furthermore, Baniasadi et al. fabricated stretchable PVDF-TrFE electrospun 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 the fabrication of piezoelectric and energy harvesting devices (Fig. 11) (Fang et al., 2013; Persano et al., 2015)

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Figure 11

High-performance coils and yarns of polymeric piezoelectric nanofiber paper (Baniasadi et al., 2015).

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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 19th century, 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 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.