Solution Blending

TEM images of MWNTs/PU nanofibers without thermal treatment [Liu et al 2012]

Solution blending is probably the most commonly used method for introducing alternative property or function to the core material. This method simply involves mixing of two parts in a solution followed by electrospinning of which only one part solution needs to be electrospinnable. In drug release application, electrospinning is generally more tolerant of changes in the solution properties due to additives compared to other nanofiber production methods. Scaffaro et al (2023) did a comparison of nanofibers blended with Carvacrol or Chlorhexidine produced by electrospinning (ES) or solution blow spinning (SBS). The addition of drugs to the solution did not have a significant impact on the electrospun nanofiber output. However, with SBS, the changes in solution properties resulted in beaded fibers and bundling of the fibers. This demonstrated the greater tolerance of electrospinning in the production of functionalized nanofibers through blending.

SEM micrographs of PLA nanofibers membranes obtained by electrospinning (ES) or solution blow spinning (SBS) of ES-PLA, SBS-PLA, ES-PLA/CHX, SBS-PLA/CHX, ES-PLA/CHX/GNP, SBS-PLA/CHX/GNP (a-c,a'-c'), and relative averages diameters (d) [Scaffaro et al 2023].

In many cases blending is used where the desired material is unable to form fibers thus a companion, electrospinnable polymer is used. Colloids where one part is non-soluble such as carbon nanotubes, silica particles [Lim et al 2006], hydroxyapatite, have been suspended in solution and electrospun to form fibers. Other more commonly introduced materials are polymers and soluble salts. Three possible solution blending scenarios are commonly encountered, first scenario is the materials for mixing are soluble in a common solvent, second is that there is no common solvent to dissolve both materials and the third is one material is insoluble as mentioned earlier. Given the versatility of blending, combinations of the three scenarios may be used together in fiber spinning [Nie et al 2009].

Despite the ease of using blending to introduce a separate property to the material, a major drawback is the potential leaching of the added material since there is no chemical bonding between the base and added material. Leaching of blended material is dependent on the material mixture as there are also studies that showed no leaching of the blended compound. Guo et al (2021) constructed an electrospun metal-organic frameworks (MOF) hybrid nanofiber membrane for the removal of As(III) and As(V) from water. Synthesized Zr-based MOF (UiO-66) was blended into a polyacrylonitrile (PAN) solution for electrospinning into nanofiber membranes. The UiO-66/PAN nanofibers membrane was found to be stable with no detectable MOF leakage after five cycles of recycling. With the MOF firmly embedded within the polymer matrix, the maximum adsorption efficiency for arsenic was found to be 93% at 10 wt% loading which was close to that of pure MOFs. The UiO-66/PAN nanofiber has a diameter of 324?nm, and at this small diameter, the MOF within the PAN matrix and the As ions may be sufficiently close for efficient adsorption. In cases where the material lost is very slow or insignificant, the benefit of this method may outweigh this weakness.

Materials with common solvent

Finding a common solvent is the best approach to preparing a solution for electrospinning. This is especially essential where the ratio of the mixture goes from 0 to 1. For synthetic polymers, solvents like dichloromethane, chloroform and dimethylformamide are a good start. Where proteins (eg. collagen) are involved, 1,1,1,3,3,3-hexafluoro-2-propanol may be suitable. However, in cases where one polymer is non-polar and the other material is polar, it may not be possible to find a common solvent.

Table 1. Selected examples of materials dissolved in a common solvent for electrospinning.

Material Mixture	Common Solvent	Reference
Polypyrrole and polyethylene oxide	Chloroform	Chong et al 2013
Polydioxanone, elastin, and collagen	1,1,1,3,3,3 hexafluoro-2-propanol	McClure et al 2009
Polyacrylonitrile and AgNO3	N, N -Dimethyl formamide	Lala et al 2007
Silk fibroin, nerve growth factor and polyethylene oxide	Water	Madduri et al 2010

Materials with dissimilar solvent

Materials at the opposite ends of the water-solubility scale are unlikely to have a common solvent. In cases where there is no common solvent, it may be necessary to dissolve the material in their individual solvent before mixing them together. However, bearing in mind that the material does not dissolve in the other solvent, there is a limit to the ratio which both solutions would mix without precipitating out the other material. Generally, the non-electrospinnable material accounts for less than 10% of the mixture's mass.

Table 2. Selected examples of materials dissolved in its own solvent before mixing for electrospinning.

Solution 1	Solution 2	Reference
Chondroitin sulfate in water	Collagen Type I in trifluoroethanol	Zong et al 2005

In cases where mixed solvents are used, adding all components and solvents together in a single step may result in poor particle dispersion. Optimizing the sequence of component additions may enhance the uniformity of particle dispersion. Going et al (2015) demonstrated this importance with cellulose nanocrystals (CNCs) dispersed in a co-solvent solution of polycvinylpyrrolidone (PVP). By predispersing CNCs in deionized water prior to addition into methanol/PVP solution, the homogeneity of the mixture is vastly improved. Without predispersion, the homogeneity of 5 wt% CNCs in PVP solution lower than 10 wt% CNCs predispersed and adding into PVP solution. Their results showed that it is possible to achieve good dispersion without the use of surfactants.

One part insoluble

Particles may be added to a solution as long as they do not agglomerate during the electrospinning process. Carbon nanotubes are commonly added to electrospinning solution to create a composite of higher strength. Surfactants are sometimes added to ensure uniform dispersion of the particles or increase the particles loading in the solution [Diouri et al 2013]. Sonication may also be used to mechanically disperse the particles prior to electrospinning [Li et al 2013].

Table 3. Selected examples of particles added into a solution for electrospinning.

Particle	Solution	Reference
Multi-Walled Carbon Nanotube	Polyvinyl alcohol, 12 wt% in water	Diouri et al 2013
Single Walled Carbon Nanotube	Poly(methyl methacrylate), 15 wt% in chloroform	Sundaray et al 2008
Organically modified montmorillonite	Poly(vinylidene difluoride) with organically modified montmorillonite, 20 w/v% in N,N-Dimethyl formamide	Liu et al 2010
Fe3O4	poly(lactic-co-glycolic acid), 24 w/v% in Tetrahydrofuran / N,N-Dimethyl formamide (V/V, 3/1)	Li et al 2013
Hydroxyapatite	Collagen 7 w/v% in 1,1,1,3,3,3-hexafluoro-2-propanol	Teng et al 2008

To maintain dispersion of particles at higher concentration, surface treatment of the particles are often used. Hou et al (2019) showed the advantage of using cetyltrimethylammonium chloride (CTAC) as a surfactant to modify the surface of Graphene Oxide (GO) for blending into polyacrylonitrile (PAN) solution. Treated GO sheet is more hydrophilic than bare GO sheet. Without treatment, GO loading into PAN solution for electrospinning is only up to 4 wt%. However, treated GO may be loaded up to 30 wt%. The presence of CTAC surfactant on GO improves the dispersity and compatibility of GO in polymer precursor solution. For modified GO loading up to 30 wt%, smooth fibers may be obtained from electrospinning. However, when the loading increases to 40 wt%, fibers could not be form.

In most studies, nanoparticles are reported to be well dispersed in electrospun fibers when it has been properly dispersed in the solution prior to electrospinning. In the absence of surfactant, nanoparticles may require additional assistance to ensure uniform dispersion within the fiber matrix. Zhu et al (2022) used ultrasonic-assisted electrospinning for uniform dispersion of starch-capped Ag nanoparticles in polyvinylpyrrolidone (PVP) fibers. In this setup, the mixture of AgNPs and PVP solution was passed through a tube that coiled round an ultrasonic generator before ending with a metal nozzle. As the solution dispersion flow pass the ultrasonic generator, the ultrasonic vibration breaks up any AgNPs aggregates. The solution dispersion then travels a short distance to the nozzle where a high voltage is applied for electrospinning. Comparing the dispersion of the nanoparticles between electrospinning with and without ultrasonic-assistance, it is apparent in 0.6 wt% concentration of AgNPs formed long strips of aggregated nanoparticles within the fibers without ultrasonic-assistance. With ultrasonic assistance, the AgNPs are evenly dispersed in the electrospun fiber. Hence this setup presents a relatively simple method of dispersing nanoparticles in the solution just before electrospinning.

Apparatus scheme of fabricating AgNPs-PVP nanofiber by ultrasonic-assisted electrospinning [Zhu et al 2022].

TEM images of 0.6 wt.% AgNPs loading on 10 wt.% PVP electrospun fibers without Ultrasonic-Assisted (left) and with Ultrasonic-Assisted (right) [Zhu et al 2022].

Multi-components blending

Solution blending is such a straightforward method of incorporating functional additives to electrospun fibers that it is easy to create multi-functional nanofibers. Gao et al (2016) constructed electrospun fiber made of gelatin, chitosan, hydroxyapatite (HA) and graphene oxide (GO) through solution blending. Both chitosan and gelatin were dissolved in the same solvent while HA and GO were added as nanoparticles in a solution suspension. With GO, the resultant membrane was found to exhibit anti-bacterial properties. Adsorption of bovine serum albumin (BSA) was also found to be good on the composite fibers. This improves its potential use as a bone tissue scaffold.

Various nanoparticles may also be brought together by blending and electrospinning. Chang et al (2023) electrospun a tungsten oxide/tin oxide/silver/poly (methyl methacrylate) (WO₃/SnO₂/Ag/PMMA) sensing material to detect breath acetone. The WO₃/SnO₂/Ag/PMMA nanofibers were prepared by mixing nanoparticles (NPs) of WO₃, SnO₂ and Ag into PMMA solution followed by electrospinning. Addition of Ag NPs has been shown to enhance the detection limit of sensing material from surface plasmon resonances but with Ag/PMMA, the detection of acetone vapor was only at 100 ppm. Introduction of SnO₂ would increase the selectivity while WO₃ addition gave the mixture a gasochromic property. With PMMA as the binding material, the combination of WO₃, SnO₂ and Ag NPs demonstrated a synergistic effect which gave the material a higher extinction change and reduced response time.

It is often difficult to blend hydrophobic substances into polar solution. To overcome this constraint, a carrier molecule such as cyclodextrin may be used. Cyclodextrin has a hydrophobic inner ring with hydrophilic outer shell which allows it to trap small hydrophobic molecules inside and the combined complex molecule is rendered more soluble in polar solution. Rezaei et al (2019) used βCD for complexing with curcumin to improve the compound solubility in polyvinyl alcohol (PVA)/gum solution and improve loading efficiency. With pure curcumin, the maximum amount of curcumin that can be added to the PVA/gum solution for electrospinning without beads on the fibers is just 2%(w/w). However, at this concentration, the loading efficiency of curcumin in gum/PVA/curcumin nanofibers is just 65%. With complexing of curcumin in βCD before blending in gum/PVA solution, up to 4%(w/w) can be loaded without beads on the electrospun nanofibers. Further, loading efficiency was around 92-95%.

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