Functionalization of electrospun fiber

Functionalization allows the introduction of other properties to a single material nanofiber. A material with superior mechanical property or chemical stability may be incorporated with other desirable capability such as anti-bacterial, biocompatibility, catalytic or chemical sensing. The ability to introduce another functional feature to the nanofiber will significantly increase its performance and versatility to tailor to specific applications. Although it is possible to synthesize polymer with the desired functional capability prior to electrospinning, this article(s) shall focus on adding other functional capability during or after electrospinning.

As shown in Table 1, there are numerous methods of functionalization with varying degree of versatility. The functionalization methods may be used together to create multi-functional capability. Each method also has its own benefits and limitations and these have to be considered depending on the specific application. In some cases, a perceived limitation of the method may be suitable for another application. For example, leaching of material from a blended polymer may be undesirable if it is unintentional but this characteristic makes it suitable for drug release application. Xiang et al (2020) added permethrin to electrospun nylon 6 fibers using blending, dip coating and drip coating after plasma treatment. In terms of loading efficiency, having permethrin blended into a nylon solution before electrospinning is better than surface dip coating onto the spun fibers with or without surface plasma treatment. Plasma treated electrospun nylon fibers have a much higher permethrin loading compared to fibers without. Permethrin coated on plasma treated electrospun nylon fibers showed better permethrin retention compared to blended nylon fibers with permethrin loss of 30? and 78% respectively. There may be several factors that lead to the much lower retention of blended fibers. Firstly, blended fibers would already have higher loaded permethrin and there may be a limited capacity in the fiber matrix to hold them. Secondly, adhesion of permethrin molecules to plasma treated nylon surfaces may be much stronger than permethrin molecules just blended into the nylon fibers matrix.

Functionalization method	Location	Improve mechanical properties	*Reactive / Functional groups	Drug release	**Material Chemistry
Blending	Bulk	Yes	Yes	Yes	Yes
Plasma treatment	Surface	-	Yes	-	Yes
UV grafting	Surface	-	Yes	-	Yes
Electron Beam Radiation grafting	Surface	-	Yes	-	Yes
Wet chemical treatment	Surface	-	Yes	-	Yes
Adhesion	Surface	-	-	-	Yes
Direct covalent bonding	Surface	-	Yes	-	Yes
Layer-by-layer	Surface	-	Yes	Yes	Yes
Multi-components electrospinning (eg. core-shell)	Core/Surface	Yes	Yes	Yes	Yes
Sol-gel coating followed by sintering	Surface	-	-	-	Yes
Chemical vapor deposition	Surface	-	-	-	Yes
Inorganic catalytic deposition	Surface	-	-	-	Yes
Mineralization	Surface	Yes	-	-
Inorganic catalytic deposition	Surface	-	-	-	Yes
Sputtering	Surface	-	-	-	Yes

Zhan et al (2022) constructed multifunctional core-shell electrospun fibers for the purpose of diabetic wound healing. Four different materials were used in the fibers. Chitosan (CS) which exhibits antibacterial activity was used as the shell material so that the dressing would inhibit bacteria found on the wound. The core contains bioactive compounds, copper (Cu) salt and decellularized Wharton's jelly matrix (DWJM), and poly(L-Lactide-co-caprolactone) (PLCL). Cu is known to promote angiogenesis and endothelial migration and this will help wound healing at the intermediate stage. DWJM contains a natural extracellular matrix (ECM) and a source of endogenous growth factors to promote collagen deposition and wound healing. PLCL was added into the core matrix to provide stable electrospinning and mechanical strength for the composite fiber. In vivo test carried out using Sprague-Dawley (SD) rats diabetic model showed that the CTS/PLCL/DWJM/Cu core-shell nanofibers wound dressing exhibited the fastest healing compared to other wound dressings with one or more active substances absent. Nejaddehbashi (2023) used electrospinning to construct a double-layered nanofibrous mat with one layer containing silver sulfadiazine (SSD) for antibacterial properties and polycaprolactone (PCL) and the other layer made of collagen and PCL. The resultant mat was immersed in grape seed extract (GSE) solution (2% Wt) overnight to impart antioxidant properties to the electrospun mat. The mat was able to inhibit gram-positive and -negative bacteria. Using a rat diabetic wound model, the double-layered electrospun mat with GSE showed fully repaired skin on day 14 while the pure mat group was not completely healed. This demonstrated the positive impact of having both SSD and GSE in the electrospun wound dressing.

Coaxial nozzle has been used to electrospin core-shell fibers with two different functional materials. With multi-axial nozzles, fibers with multiple layers have been electrospun. This has the benefit of separating different functional materials to avoid any interference between them. Wang et al (2023) electrospun fibers with an inner core and 2 outer concentric layers. The core was a magnetic material, CoFe₂O₄/polymethyl methacrylate (PMMA). The intermediate layer was conductive polyaniline (PANI)/PMMA and a Tb(acac)₃bpy/PMMA insulated-fluorescent shell layer. This multi-layered fiber was electrospun through a tri-axial nozzle with the individual material extruded through their corresponding opening. The resultant electrospun fiber took the form of a microbelt with width of about 13 µm and the thickness of 4.45 µm. Of the materials properties, the fluorescent material was the most sensitive to the presence of other materials. When all three functional materials, CoFe₂O₄ nanoparticles, PANI and Tb(acac)₃bpy were mixed together with PMMA and electrospun as a single fiber, the fluorescence was the weakest due to the absorption of excitation and emission lights by CoFe₂O₄ nanoparticles and PANI. Similarly conductivity of the fiber was the weakest when PANI was blended with other functional materials due to the interference with charge transfer. Therefore, to ensure high functional output from respective materials, it is desirable to keep the functional materials separated on the same fiber.

In some instances, a combination of functionalizing steps are carried out to achieve the final product with the desired properties and functions. The ease of incorporating other substances into the polymer solution to be electrospun has enabled researchers to create novel composite materials. Huang et al (2019) was able to construct a novel poly(vinyl alcohol)/poly(acrylic acid)/Fe₃O₄/MXene@Ag nanoparticle composite nanofiber membrane with electrospinning for wastewater treatment. Electrospinning was first carried out to produce poly(vinyl alcohol)/poly(acrylic acid)/Fe₃O₄/MXene nanofibers. The composite nanofibers were subsequently stirred in AgNO₃ solution so that the Ag ions adhered and reduced to spherical Ag nanoparticles on the nanofibers. Liu et al (2022) constructed a multifunctional nanofibrous membrane using electrospun polylactic acid/hydroxyapatite (PLLA/HA) composite fiber as the base material. hydroxyapatite nanowires were blended into PLLA solution followed by electrospinning. This nanofibrous substrate material was then dipped in Sr²⁺, Cu²⁺ and Py in electrolyte solution for electrochemical deposition. The resultant fiber is a composite fiber made of PLLA/HA@SrHA/ Cu/PPy. Strontium was added to the fiber to improve osteoblast activity while copper ion is known to exhibit antibacterial properties and has been included in the composite fiber to reduce infections at the implant site. Polypyrrole was added as a regulator to encourage uniform distribution of Sr²⁺ and Cu²⁺ over the fiber surface. The resultant multifunctional PLLA/HA@SrHA/Cu/PPy composite fiber membrane showed close to 100% inhibition against Staphylococcus aureus and Escherichia coli bacteria. The PLLA/HA@SrHA/Cu/PPy composite fibers also showed good osteogenesis and angiogenesis properties through the culture of osteoblast and vascular endothelial cells (VEC) respectively.

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