Application for hydrophilic electrospun fibers

Materials made of polymers which are generally hydrophobic tends to become more hydrophobic (exhibits higher surface water contact angle) after electrospinning compared to its cast film form. While this is beneficial if the aim is to get a hydrophobic surface, there are many applications which a hydrophilic surface is preferred.

In biomedical applications, a more hydrophilic surface generally encourages cell adhesion [Kim et al 2006]. A common method of rendering a hydrophobic material more hydrophilic is to use plasma treatment. Abbasi et al (2014) showed that mouse embryonic stem cells (mESCs) proliferation on plasma treated polycaprolactone (PCL) electrospun scaffold with O₂ to improve its hydrophilicity was significantly better than non-treated PCL electrospun scaffold. Liu et al (2014) also showed better adhesion of porcine mesenchymal stem cells (pMSCs) on plasma treated electrospun poly-l-lactic acid (PLLA) nanofibers. A hydrophobic surface lacks binding sites for cell receptors to bind onto. With plasma treatment and the introduction of oxygen-containing group, the surface becomes hydrophilic and offers suitable binding sites for the cells. However, it is important that hydrophilicity alone may not improve cell response [Schaub et al 2015]. The surface adhesion strength may differ for different cell types and material surface chemistry [Blackstone et al 2012].

Contact angle comparison of scaffolds with and without plasma treatment. Data are mean ± standard error of the mean, n = 3; *p<0.05. [Zhu et al. PLoS ONE 2015; 10(7): e0134729. doi:10.1371/journal.pone.0134729. cc by 4.0]

Hydrophilicity of the material, in particular, its absorption and swelling with water uptake may be used to influence drug release. Ko et al (2019) electrospun blend of polyurethane (PU) and cellulose acetate (CA) with paclitaxel (PTX) as the model drug. PU has good mechanical properties while CA is hydrophilic with high water uptake. With higher content of CA in the electrospun PU/CA composite, swelling of the electrospun membrane increases. Increased swelling also led to a corresponding increase in drug release. Comparing the drug release profile of electrospun PU, PU/CA (7:3) and PU/CA (3:7) membrane, they shared similar initial burst release rate before diverging. The similarity in the drug release rate at the initial phase can be attributed to drugs that are at the surface of the nanofibers. Once these exposed drugs were released, subsequent release is dependent on the diffusion of the drugs across the matrix.

Electrospun membrane has been investigated widely for use as a water filtration membrane. By making the filtration membrane hydrophilic, the filtration efficiency of the membrane may be improved due to reduced capillary pressure required for liquid penetration. This also potentially improves the flow rate of the water. Asmatulu et al (2013) tested the effectiveness of reducing the hydrophobicity of electrospun polyvinyl chloride (PVC) by blending with polyvinylpyrrolidone (PVP) to improve filtration performance. Flow rate of water through the more hydrophilic PVC/PVP was more than double that of PVC electrospun membrane alone. Further, the combination of the electrospun membrane and coagulation agent significantly improves particles removal from the suspensions.

Other applications for hydrophilic surface involve wicking of water away from an object or for absorbing water. Textile for clothing would benefit from this as it makes for greater wear comfort. Yadav et al (2016) investigated the use of cellulose acetate electrospun nanofibers for sanitary napkins and compared its performance against commercial sanitary napkins. Their results showed that cellulose acetate (CA) electrospun nanofibers membrane performed better than commercial sanitary napkins when tested using synthetic urine and saline solution. Zhang et al (2020) created a bi-layer electrospun fibrous membrane with a thin inner layer of hydrophobic thermoplastic polyurethane (TPU) and a thick outer layer of super hydrophilic polyacrylonitrile (PAN).The polyacrylonitrile (PAN) layer was coated dopamine (PDA) to enhance its wettability. Electrospinning of the thicker PAN layer was first carried out followed by functionalizing with PDA. The thin TPU was subsequently electrospun directly on the functionalized PAN layer to create the bi-layered membrane. The thin TPU layer is meant for moisture wicking to draw water away from the skin surface while the thicker PAN/PDA hydrophilic layer transports the moisture away. It was demonstrated that the hydrophobic TPU layer needs to be thin so that water can pass through the membrane. There is also an optimal thickness for the super hydrophilic PAN/PDA layer. When the PAN/PDA layer is too thin, water can easily pass through the bi-layer membrane. At optimal PAN/PDA membrane thickness, water capillary motion at the TPU layer pushes the water into the pores and the hydrophilic PAN/PDA layer draws in the water. However, beyond optimal PAN/PDA layer thickness, the increase in the bi-layer membrane thickness reduces its moisture wicking performance. With optimized bi-layer thickness, a moisture permeability of 9065 g m^-2 d^-1 and distinct breathability of 100 mm s^-1 (5.0 times higher than a commercial membrane) were recorded which makes it a promising material for applications such as moisture-wicking clothing.

A membrane with hydrophilic properties may be used for water collection from fog. The hydrophilic property of the membrane facilitates capture and condensation of water vapors although it should be sufficiently hydrophobic for the collected water to be transported to a collection reservoir. Aijaz et al (2023) constructed an electrospun poly(Lactic acid) (PLA)/poly (ethylene glycol)-poly(propyl glycol)-poly(ethylene glycol) (PEG-PPG-PEG) blended nanofiber membrane for the purpose of water collection from fog. WIth low concentration of PEG-PPG-PEG, there was hardly any water transport. At 7 and 10 %w/v PEG-PPG-PEG to PLA, the water transport rate was 3 and 8.1 mg.mm^-2h^-1 respectively. At higher concentration of PEG-PPG-PEG (30% w/v, the membrane was damaged. The electrospun membrane with 10 %w/v PEG-PPG-PEG to PLA showed the best water collection from fog with rate of water collection at 1.4 g.cm^-2h^-1 at saturation, an increase of 40% over pure PLA electrospun membrane.

Super hydrophilic surface is known to restrict bacterial adhesion and biofilm formation. This is an added advantage for water filtration membrane or as a surface coating to prevent bacterial proliferation. Goetz et al (2016) electrospun a superhydrophilic membrane using a blend of cellulose acetate and chitin nanocrystals. The resultant membrane with zero degrees surface water contact angle demonstrated a reduction in biofouling and biofilm formation. The membrane is also resistant to fouling in bovine serum albumin and humic acid fouling solutions. While the presence of chitin may contribute to the reduction in biofilm formation, a separate study by Yuan et al (2017) using O₂ plasma treated electrospun polystyrene membrane also showed reduced adhesion of bacteria, Escherichia coli (E. coli), on the membrane. Poor adhesion of E. coli and other Gram-negative bacteria has been attributed to hydrophobic lipopolysaccharide (LPS) surface which favours adhesion to more hydrophobic surface instead of hydrophilic surface.

Hydrophilicity of a membrane is more commonly investigated for organic materials. However, advantages of a hydrophilic surface may also be found in inorganic materials. Kim et al (2021) showed the effect of having a small amount of Fe- and N-doped graphene nanoplates (FeN@GnP) added to carbon nanofibers prepared by the carbonization of electrospun polyacrylonitrile (PAN) fibers. FeN@GnP was sonicated to form a suspension before PAN is added to form PAN/FeN@GnP solution for electrospinning. Electrospun carbon nanofiber (CNF) membrane was found to be hydrophobic with a surface contact angle of 143°. With the CNF/FeN@GnP membrane, the surface has a hydrophilic contact angle of 42°. Loading of the CNF/FeN@GnP membrane with Co₃O₄ nanoparticles using hydrothermal synthesis method showed uniform dispersion of Co₃O₄ nanoparticles over the surface of the membrane. In contrast, the same loading process on the hydrophobic CNF membrane saw the presence of Co₃O₄ aggregates. For application as high-performance supercapacitors, the Co₃O₄/CNF/FeN@GnP electrode showed 87.5% capacitance retention after 700 cycles. For Co₃O₄/CNF, the capacitance retention was 75.6%. Therefore well dispersed nanoparticles on the membrane were better able to support capacitance retention and this was made possible with a hydrophilic surface.

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