Dispersion of active substances in electrospun matrix

One-dimensional spatial confinement of quantum dots in the nanofiber. (a) Upper: scanning electron microscopy (SEM) image of electrospun nanofibers enveloping isolated quantum dots (iQDs). Lower: schematic figure of iQDs in the nanofiber. (b) Nanofibers enveloping iQDs were collected on aluminum foil and exhibited orange colored photoluminescence similar to the CdSe solution (inset) under ultraviolet light exposure (363nm). (c) Bright field and luminescence images (left and middle, respectively) of nanofibers enveloping iQDs. Overlay of the bright field and luminescence images (right) [Choi et al 2015].

Uniform dispersion of substances blended into electrospun fibers have been reported in many cases. Such good dispersion characteristic may be attributed to rapid solidification of the electrospinning jet to form fibers which reduces the time available for agglomeration or crystallization. However, it is still important to select suitable solvent for preparing the solution mixture to reduce aggregation [Choi et al 2015]. Having well dispersed substances in electrospun fibers plays an important role in enhancing its targeted functionality such as mechanical strength, fluorescence, predictable drug release and others.

Nanoparticles are known to exhibit unique properties and their high surface area makes them highly attractive for various functional applications. However, a main limitation in using them is their tendency to agglomerate. This significantly reduces their surface area and performance. Various nanoparticles have been blended into solution for electrospinning and they have been found to be well dispersed in the electrospun composite nanofiber. Suryavanshi et al (2017) showed that magnesium oxide nanoparticles (MgO NP)-loaded in electrospun polycaprolactone (PCL) were uniformly dispersed and this contributed to a significant improvement in its mechanical properties compared to neat PCL. Quantum dots have also been shown to be uniformly dispersed in electrospun fibers and this made it very useful in applications such as fluorescence-based sensor [Mahmoudifard et al 2014]. Where carbon nanotubes were used as fillers in electrospun fibers, they are usually found to be well dispersed and aligned along the fibers axis [Xu et al 2014].

Another common method is to blend salt precursor into the solution and reduced to nanoparticles after electrospinning into fibers [Chen et al 2017]. This method is commonly used to fabricate electrospun fibers embedded with silver nanoparticles to give it anti-bacterial functionality. Similarly, Wang et al (2007) fabricated poly(vinylpyrrolidone) (PVP) nanofibers embedded with gold nanoparticles by dissolving HAuCl₄ in PVP ethanol solution.

An advantage of drug encapsulation in electrospun fibers is that the spinning process reduces drug crystallization. Drug crystallization has been said to make the drug delivery system unstable and causes a drop in the drug loading efficiency and unpredictable release behavior [Kwak et al 2017]. Using water soluble fish gelatin (FG), Kwak et al (2017) demonstrated the lack of crystallization and the rapid release of caffeine in electrospun FG. For solvent cast FG film, with only 0.8 w/v% of caffeine added, spherical crystalline nucleus can be seen. Electrospun fibers in contrast can take up to 2% loading in FG without any disruption to the smooth fiber morphology. However, electrospinning may not reduce crystallization for all drugs loaded into the solution for fabrication into fibers. Seif et al (2015) found that the solvent polarity and its interaction with the drug has a major impact on its crystallization while electrospinning parameters only have a minor influence.

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