Electrospun membrane hydrophilicity

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]

Electrospun membrane is well known for its hydrophobicity due to its surface roughness. However, not all applications favour hydrophobicity and in some cases, it is preferred that the electrospun membrane is hydrophilic. For example, in aqueous filtration applications, a hydrophilic surface may be preferred to reduce protein adhesion, in micro- and ultra-filtration and forward osmosis applications. The advantage of nanofibers is its high surface area to volume ratio but lower fiber diameter generally increases hydrophobicity. Therefore, it is necessary to find out ways to reduce the membrane hydrophobicity while retaining the advantage of low fiber diameter.

Surface Treatment

A hydrophobic polymer may be made more hydrophilic with high energy surface treatment such as plasma treatment, ultra-violet (UV) treatment or radiation. Zhu et al (2015) used cold atmospheric plasma treatment to improve the wettability of hydrophobic poly(ε-caprolactone) (PCL) fibers membrane. This method uses ionized gas at temperature close to room temperature to generate free radicals on the polymer. The free radicals will react with the gas present in the chamber to generate functional groups. By exposing the PCL nanofibers to 5 minutes of plasma treatment, the water contact angle of the membrane was reduced from 117° to 14°. Esmail et al (2021) conducted oxygen plasma treatment on electrospun poly(3-hydroxybutyrate) P(3HB), copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) P(3HB-co-3HV) and medium chain length poly(hydroxyalkanoates) (mcl-PHAs) blended with P(3HB-co-3HV). Following the plasma treatment, all three electrospun membranes showed complete wetting (θ=0). However, it is important to note that plasma treatment may result in degradation in the membrane. There were visible fractures on the fibers, in particular the copolymer P(3HB-co-3HV) fibers, and they were also found to exhibit a slight reduction in its average fiber diameter. Such physical deterioration of the fiber is due to physical etching phenomena, where the ions in the plasma hitting the fiber surface cause erosion of the material. When medium chain length poly(hydroxyalkanoates) (mcl-PHAs) was blended with P(3HB-co-3HV) and electrospun into fibers, the resultant P(3HB-co-3HV)/mcl-PHAs fibers showed a significant reduction in fiber diameter after oxygen plasma treatment but very little fractures. Electrospun mcl-PHAs alone is unable to maintain its fiber shape due to its stickiness and tackiness at room temperature. However, when blended with P(3HB-co-3HV), this behavior of mcl-PHAs may allow the fiber to withstand fracturing during plasma treatment.

Electrospun P(3HB-co-3HV) copolymers after 12 min of oxygen plasma exposure showing fractures on the fibers [Esmail et al 2021].

The main purpose of UV treatment and radiation of a material is for sterilization. However, this often has the unintended consequence of reducing its hydrophobicity. Duzyer et al (2013) showed that UV treatment of electrospun polyethylene terephthalate (PET) fibers results in a reduction in its surface contact angle from 133° to 97°. Similarly, for polycaprolactone, there is a reduction in its surface contact angle by Gamma irradiation depending on the dosage. At 65 kGy, Agustine et al (2015) showed that the contact angle of electrospun polycaprolactone membrane was reduced from 106° to 79°.

Blending

Blending is the most common method of introducing added functionality or properties to the base electrospun nanofiber material. Mixing a hydrophilic polymer into a solution of hydrophobic polymer certainly increase the hydrophilicity of the resultant electrospun fibrous membrane. Although increasing the content of hydrophilic component will increase the water absorbance capacity of the composite material, a balance is needed to maintain desirable fiber morphology. Hendrick and Frey (2014) increased the hydrophilicity of poly(lactic acid) (PLA) by introducing polyethylene glycol (PEG) group in the electrospinning solution. Comparing the wettability of the composite membrane where PEG homopolymer and PLA-b-PEG copolymer was added, the composite membrane containing the copolymer demonstrated better wettability. Asmatulu et al (2013) improves the hydrophilicity of polyvinyl chloride (PVC) by incorporating water soluble polyvinyl pyyrolidone (PVP). As expected, the water contact angle of the resultant material reduces with increasing PVP concentration. However, adding large amount of additives to reduce the surface contact angle of the electrospun membrane may result in a deterioration of other properties. To minimize the amount of additives, Vasita et al (2010) blended poly(lactide-co-glycolide) (PLGA) with small quantities of non-ionic surfactant Pluronic F-108. With just 2% Pluronic F-108 blended with the PLGA for electrospinning, the surface contact angle of the membrane was reduced from 120° to about 100° immediately and a slow reduction to 10° in less than a minute.

Surface coating

Surface coating is a useful method of introducing a hydrophilic surface to the nanofiber without introducing a secondary component to the matrix material which may compromise its integrity or mechanical properties. Huang et al (2014) coated electrospun polyacrylonitrile (PAN) and polysulfone (PSU) with hydrophilic polydopamine (PDA) by polymerization of dopamine. Comparing the contact angle before and after PDA coating, there is no signification change in PAN nanofiber membrane as PAN is already hydrophilic on its own. However, electrospun PSU with an initial contact angle of 146° drops to zero with the PDA coating. For cast PUS film, the reduction in contact angle is less significant from 79° to 71°.

Solvent

The type of solvent used in dissolving the polymer for electrospinning may influence the hydrophilicity of the resultant electrospun fibers. Deep eutectic solvent (DES) is a relatively young class of ionic liquids which has properties that make it safer and greener than conventional volatile organic solvents. It is non-volatile and has the potential to replace organic solvents for dissolution of polymers and extraction of certain ions and molecules. Khatri et al (2020) demonstrated the use of DES based on Quaternary salt (Choline chloride) as a hydrogen bond acceptor (HBA) and (Furfuryl alcohol) as a hydrogen bond donor (HBD) for the dissolution and electrospinning of Zein. By dissolving Zein in Choline chloride (HBA) and Furfuryl alcohol (HBD) with a ratio of 1:2 and a concentration of 45%, Khatri et al (2020) was able to produce beads-free Zein fibers using electrospinning (Zein-DES). Ethanol is the conventional solvent used for preparing Zein for electrospinning. Khatri et al (2020) compared the hydrophilicity of electrospun fibers prepared by dissolving Zein in DES (Zein-DES) and ethanol (Zein-C). Using water contact angle tests, electrospun Zein-DES was also found to be more hydrophilic than Zein-C. Comparing the wicking height of both nanofibers also showed Zein-DES to be much more hydrophilic than Zein-C. Greater hydrophilicity of Zein-DES may be attributed to greater -NH and -OH on the surface of the nanofibers resulting from the protonation by DES.

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