Application of Electrospun Hydrophobic Layer

Hydrophobic materials when electrospun to form a network of fibers have been shown to significantly increase its surface water contact angle compared to its film form. Electrospun fibers and its increased hydrophobic characteristics of the network may be used in the form of a coating on a surface or a standalone membrane. Diaa and Jaafar (2017) have shown that a layer of electrospun coating was able to render the surface of metal, ceramic and glass surface superhydrophobic. Water contact angle prior to coating with electrospun fibers have shown that all three materials are hydrophilic with water contact angle of metal, glass and ceramic as 60 °, 48 ° and 0 ° respectively. The ceramic test material is porous and the water droplet gets absorbed into it upon contact. With electrospun beaded polystyrene (PS) fibers on the surface of metal, a water contact angle of 161 ° was recorded. For ceramic and glass test subjects, electrospun beaded PS/TiO₂ nanoparticles coating on them gave water contact angle of 154 ° for both. With static contact angle greater than 150 °, this satisfy one of two conditions to consider the surface superhydrophobic. Several applications have been developed using this electrospun fiber characteristic.

[Karim et al. Journal of Nanomaterials, vol. 2011, Article ID 979458, 7 pages, 2011. doi:10.1155/2011/979458. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Self-cleaning surface

A super hydrophobic surface is also known to be self-cleaning as any water that splashes into it would roll off the surface, picking up any dirt along the way. However, if the contaminant is hydrophobic, then it may adhere to the surface instead of being washed off. An alternative mechanism employed. Naef and Seeger (2023) constructed an electrospun self-cleaning membrane with super hydrophobic and photocatalytic properties. The materials used to form the fibers were polystyrene (PS), polydimethylsiloxane (PDMS) and graphitic nitride. PS was used as a carrier material and its tendency to form porous or rough fibers from electrospinning would increase the hydrophobicity of the membrane. PDMS is a hydrophobic polymer and this will also enhance the hydrophobicity of the resultant membrane. Graphitic nitride is a photocatalytic material which would give the membrane photocatalytic properties. At optimum composition of PS/PDMS/graphitic nitride, a water contact angle of more than 150° and a sliding contact angle of less than 20° was obtained. The membrane was tested with hydrophilic and hydrophobic pollutants simulated using methylene blue in aqueous solution and ethanol. With aqueous methylene blue, the hydrophilic pollutant just rolled off the super hydrophobic membrane surface hence demonstrating the self-cleaning effect. With hydrophobic pollutants, the pollutant may be washed off but with more hydrophobic pollutant that adheres to the surface, the surface of the membrane can be cleaned by photocatalysis. 15 min of UV radiation was able to remove the blue coloring. However, when the water evaporated before dye had fully oxidized, about 3 h was required. This showed the effectiveness of using a super hydrophobic membrane with photocatalytic property as a self-cleaning substrate or coating.

Anti-rain on glass windows/car windscreen

On a standing glass window or a tilted car windscreen, the ability to drain off rain water fallen onto its surface is important to maintain a clear sight through the glass. A more hydrophobic layer will encourage water on the surface to roll away under its weight. Availability of portable electrospinning device has made it possible for the use of electrospun fibers as a coating on glass window for this purpose. A thin layer of electrospun fibers is sufficient to create an anti-rain surface while maintaining good transparency.

For outdoor anti-rain applications, it is important to determine the weathering effect on the electrospun hydrophobic coating. Jaafar and Aldabbagh (2019) conducted an accelerated weathering study on a glass surface coated with electrospun polystyrene (PS), polymethylmethacrylate (PMMA), silicon rubber (Si) and with TiO₂ added to the polymer. The specimens were being subjected to ultraviolet lights, high temperature up to 50°, humidity and rain for 6 months. The contact angle was taken before and after the accelerated weathering test. Electrospun PS and Si coating with and without TiO₂ showed significant drop in contact angle from values above 100° to about 90°. In particular, electrospun PS and PS/TiO₂ coating has an initial contact angle of 160° and 154° respectively and their reduction is most significant. Electrospun PMMA and PMMA/TiO₂ has initial contact angle of 152° and 125° respectively and after accelerated weathering, has contact angle of 134° and 124° respectively. Of these materials, PMMA/TiO₂ is the most promising coating with a minor drop in hydrophobicity.

Easy maintenance sensor

A superhydrophobic surface is good for removal of aqueous droplets for re-generation of the sensor surface. Liu et al (2016) constructed an upconversion-luminescence-membrane using electrospinning for single droplet ultrasensitive fluorescence sensing. The material, Ln³⁺-doped (Yb³⁺,Tm³⁺ or Yb³⁺, Er³⁺ co-doped) NaYF₄ nanoparticle (NP)/polystyrene (UCLNPs/PS) nanofiber membrane has a water contact angle of 153°. Due to its superhydrophobic characteristic, the target droplet for fluorescence detection can be easily removed without residues and freeing the surface for the next target droplet.

Electrospun membrane for distillation

For an electrospun membrane to be used in membrane distillation, it must be able to maintain a separation between the feed and permeate. Thus, a superhydrophobic membrane is preferred as it is better able to maintain water separation. Having superhydrophobic characteristics due to the physical arrangement of the fibers may not be adequate to maintain separation between the feed and permeate. Electrospun membrane that is initially hydrophobic may be wetted after a few hours of operation. It may be necessary for electrospun fibers to undergo further treatment to increase the hydrophobicity of the material. Zhou et al (2014) used a commonly known superhydrophobic material polytetrafluoroethylene (PTFE), for fabrication into nanofiber membrane. Since PTFE is resistance to most solvents, they used a suspension of polytetrafluoroethylene (PTFE) fine particles in water for blending with water soluble polyvinyl alcohol (PVA). The PVA with PTFE particles were electrospun and the PVA component removed through sintering up to 380°C for 30 minutes. At this temperature the PTFE particles melted and fused together to form an interconnected nanofibrous network of PTFE. The resultant membrane showed a water contact angle of 156.7°C. When tested for vacuum membrane distillation, a pure water flux of 15.8kg/m²h and a stable salt rejection of more than 98% for ten hours were recorded.

Reducing Bacterial Adhesion

A superhydrophobic surface is known to reduce bacterial adhesion due to trapped air which reduces the contact surface available for the bacteria to gain a foothold. Electrospinning fibers to give a superhydrophobic surface that prevent biofilm formation or easy removal of biofilm may reduce the dependence of using biocides or antibiotics to remove bacteria. Yuan et al (2017) demonstrated potential of electrospun polystyrene fiber with fluorinated surface in preventing bacterial adhesion. The fluorinated fibers exhibited a water contact angle of 168°. Escherichia coli cultured on the fibers showed preferential attachment on the crest of the fibers. With fluorinated electrospun fibers membrane, the amount of bacteria adherence was significantly less than un-fluorinated membrane. Further, subsequent washing of the fluorinated electrospun membrane with bacteria was able to effectively removed the attached bacteria thereby demonstrating self-cleaning property.

Overview of the effect of hydrophilic and hydrophoic electrospun fibrous layer on bacterial adhesion. [Yuan et al. RSC Adv. 2017;7: 14254. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Food Packaging

A transparent and superhydrophobic film is useful as food packaging material as it prevents accumulation of moisture on the surface of the film. Pardo-Figuerez et al (2018) used a combination of electrospinning and electrospraying to create a superhydrophobic coating on polyethylene terephthalate (PET) film as transparent food packaging. Electrospinning of polylactide (PLA) solution was first carried to deposit a layer of fibers on the PET film. This increases the hydrophobicity of the film from 82° to 96° using water contact angle test. Unfortunately, annealing of the bilayer film at temperatures ranging from 90°C to 170°C reduces the water contact angle to 73 °C at 170°C annealing temperature. This may be attributed to the partial loss of fiber structure especially at temperatures above 120°C. To increase hydrophobicity of the film, SiO₂ microparticles were electrosprayed on top of electrospun PLA fibers. An optimum annealing temperature of 160°C was found to create good adhesion between the fibers, SiO₂ microparticles and the film. A water contact angle of 171° and sliding angle of 6° were obtained thus making the film superhydrophobic. The film also exhibited good optical transparency following heat treatment.

Controlled Drug Release

The hydrophobicity of a membrane will have an influence on the rate of water penetration into its depth. In the Cassie-Baxter model, hydrophobicity of a material is due to air pockets between the pores of a material. In electrospun membrane, the pores formed by the interconnecting fibers were able to trap air and this makes membrane made of non-water absorbent material hydrophobic. As diffusion of embedded substances requires contact with water or fluid, increasing the time taken for water penetration into the membrane will certainly reduces the rate of drug release. Yohe et al (2012) electrospun poly(ε-caprolactone) (PCL)/poly(glycerol monostearate-co-ε-caprolactone) ( PGC-C18) to investigate the effect of hydrophobicity in the release of a model bioactive agent (SN-38). Comparing the release rate of SN-39 for 10% doped PGC-C18 electrospun meshes, melted 10% doped PGC-C18 electrospun meshes and degassed 10% doped PGC-C18 electrospun meshes, the time taken for the release of 50% of SN-38 was about 40 days, 12 days and 1 day respectively. This shows that the pores within the electrospun mesh played an important role to spread the drug release over a longer duration. Habibi et al (2018) constructed a three layered drug-loaded biodegradable nanofibrous scaffolds comprising of polylactic acid (PLA) (top)/ drug- loaded polyvinyl alcohol (PVA) (second) and PLA-PVA (bottom) nanofibers for sustained release of naltrexone. Since PLA is hydrophobic, a thicker layer of electrospun PLA or a greater concentration of PLA in the mixture result in slower naltrexone release rate. Conversely, adding more PVA which is water soluble to the mixture increase the drug release rate due to greater water penetration. At optimal parameters, sustained release of the drug lasts more than 30 days.

Proposed mechanism of a drug-eluting 3D superhydrophobic material in a metastable Cassie state. Over time, water slowly displaces air content from the material with the transition from the metastable Cassie state to the stable Wenzel state. If treated as iterative surfaces, water will slowly penetrate each individual surface over time enabling prolonged drug release [Yohe et al 2012].

Electrospun membranes may also be treated to render it more hydrophobic to prolong its drug release. Han et al (2019) used chemical cross-linking to increase hydrophobicity and reduce degradation rate of electrospun lutein-loaded polyvinyl alcohol/sodium alginate (PVA/SA) nanofibers which in turn reduces its drug release rate. With longer cross-linking duration, the longer the sustained drug release. The drug release mechanism may also be altered by the cross-linking duration. The drug release rate from a cross-linked duration of 1h and 5h was 12.5% per hour and 0.85% per hour respectively. For a cross-linking duration of an hour, the PVA/SA nanofibers remain hydrophilic and drug release is determined by diffusion and dissolution of the polymer in water. When the cross-linking duration is 5h, the nanofibrous membrane becomes hydrophobic and the main release mechanism is from diffusion.

Corrosion Resistance

To improve corrosion resistance of a metal substrate, a commonly used method is to use a protective barrier. Electrospun membrane from hydrophobic polymer is known to exhibit greater hydrophobicity than the same material in the form of film. Since the presence of water generally increases the rate of corrosion of metal, an electrospun hydrophobic layer on the metal surface will help to protect the underlying metal substrate from corrosion. Iribarren et al (2019) demonstrated the protective benefit of electrospun polyvinyl chloride (PVC) with nanoparticles of a corrosion inhibitor like ZnO on aluminum alloy 6061T6. Aluminum with electrospun PVC coating with and without ZnO nanoparticles showed reduction in corrosion current density by two orders of magnitude. Heat treatment was carried out to improve adhesion of electrospun coating to the metal substrate. Electrospun coatings with and without heat treatment showed a minimum water contact angle of 120° (at heat treatment temperature of 120°C) with higher heat treatment temperature lead to a corresponding reduction in its water contact angle. The reduction in water contact angle with increasing heat treatment temperature may be due to increased fusion between fibers and lowers the surface roughness. Interestingly, electrospun pure PVC fibers heated to 80°C (Tg) showed protection efficiency of 99.01%. However, the same temperature treatment on PVC/ZnO has a protection efficiency of 95.12%. While increasing the heat treatment temperature of electrospun pure PVC fibers to 100°C reduces the protection to 97.38%, the same heat treatment on PVC/ZnO increases the protection to 99.19%. The reduction of protection in the heat treated pure PVC fibers may be attributed to reduction in its water contact angle. In the presence of ZnO nanoparticles, the same molecular relaxation may improve the distribution of ZnO nanoparticles within the PVC matrix, hence increasing the level of protection.

Sensor sensitivity

One of the most basic forms of sensors is the lateral-flow analysis (LFA) strips. These strips are usually made of nitrocellulose with the sensing material attached to its surface. However, a limitation is its loading capacity and sensitivity. Wang et al (2021) used electrospinning to coat a nitrocellulose nanofibrous layer on commercial nitrocellulose membrane to increase the surface porosity and surface area which increases protein adsorption. Increasing thickness of the electrospun nitrocellulose nanofibrous layer also increases the hydrophobicity of the membrane and this reduces the flow rate which increases the opportunity of antigen-antibody reaction. Electrospun nitrocellulose fiber coated membrane (ENC) at optimum coating thickness has a protein adsorption rate close to 12% while corresponding commercial nitrocellulose membrane (CNC) has an adsorption rate of only 5.3%. Compared to the lower detection limit of CNC, the sensitivity was increased by 50 times in ENC. Beyond the optimum electrospun fiber thickness, the sensitivity starts to drop. This has been attributed to increasing hydrophobicity as the thickness of electrospun nitrocellulose fiber layer increases which reduces protein adsorption.

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