Surface Adhesion

Figure 1. Depositing particles on the electrospinning jet	Figure 2. Electrospinning and electrospraying on opposite sides of a rotating drum.

To reduce deterioration in the property of nanofibers from radiation or chemical treatment, surface adhesion may be used to incorporate functional material. Surface adhesion may come from secondary forces such as hydrogen bonding, van der Waals forces or through partial melting of deposited particles on the nanofiber surface.

The simplest form of surface adhesion involves dipping the electrospun fibers into a solution or suspension of conductive material such that the conductive material adheres to the surface of the fibers. Kang et al [2007] dipped electrospun silk membrane into a suspension of multi-walled carbon nanotube (MWCNT) in water containing Triton X-100. The MWCNT was found to adhere to the surface of the silk nanofiber and remained adhered after washing with de-ionized water. Francavilla et al (2021) dipped electrospun polycaprolactone (PCL) into a suspension of graphene nanoplatelets (GNPs) in ethanol to create a piezoresistive sensor. Electrospun PCL fibers may retain some positive charges on its surface and this would facilitate adhesion of GNPs on its surface. A maximum electrical conductivity value of 3.17 S/m was obtained for 2%GNPs-PCL sample but this drops to 0.08 S/m after 5 washing cycles. All GNPs-PCL samples showed high sensitivity to external pressures and excellent durability to repetitive pressing. Unfortunately, the membrane is only subjected to 5 washing cycles and it is unknown whether there will be a minimum threshold of GNPs retention after a certain number of washing.

FESEM images of (a) and (b) PCL microfibers; (c) and (d) 0.2%GNPs-PCL membranes; (e) and (f) 2%GNPs-PCL membranes. The magnifications used for these images was of 10,000x (10 µm) and 50,000x (2 µm), from the left to the right [Francavilla et al 2021].

Instead of having a two steps process for adhering nano-particles onto the surface of nanofibers, Zheng et al (2014) electrospun cellulose directly into an an aqueous suspension of magnesium hydroxide nanoparticles. As the cellulose fibers are formed in the aqueous suspension, the magnesium hydroxide nanoparticles adhered on the surface of the fibers. An advantage of this method is that the distribution of the nanoparticles across the bulk nanofibrous structure is likely to be more uniform than having the coating as a separate process. Adhesion of particles to the electrospun fiber depends very much on the material pair. In some cases presence of residual solvents on the electrospun fibers may cause aggregation of nanoparticles after dipping into the nanoparticles suspension. Phan et al (2020) showed that this is the situation with electrospun PEICT (terpolyester of a renewable isosorbide (ISB) monomer, ethylene glycol, 1,4-cyclohexane dimethanol, and terephthalic acid) membrane and adhesion of ZnO nanoparticles. They found that post treatment of electrospun PEICT membrane was needed to prevent aggregation of ZnO nanoparticles coating. Residual solvent, trifluoroacetic acid, from the electrospun nanofibers may have leached out and created a local acidic condition which favors metal ions aggregation. PEICT membrane that was dipped in ZnO nanoparticles suspension exhibited significant agglomeration of nanofibers, nanofiber mat shrinkage and particle aggregation. However, if the PEICT membrane was subjected to washing with ethanol prior to dipping into ZnO nanoparticles suspension,composite nanofiber membrane remains well-defined with a homogeneous distribution of nanoparticles on the nanofiber surface.

TEM images showing the surface morphologies of (a) an as-spun PEICT nanofiber, (b) a PEICT/ZnO composite nanofiber without post-electrospinning treatment with ethanol, and (c) PEICT/ZnO composite nanofibers treated with ethanol before ZnO nanoparticle loading [Phan et al 2020]

Kang et al [2012] demonstrated the use of hydrogen bonding to coat nanofiber with egg-shell membrane. Firstly, electrospinning was carried out to form nanofibers from caprolactone and catechin blend. When submerged in egg-shell membrane solution, catechin diffuses to the surface of the fiber and activates the precipitation of egg-shell membrane due to the formation of hydrogen bonding between them.

Dry powder impregnation technology may also be used to spread active ingredients throughout the electrospun membrane without wetting it. Teno et al (2020) used this process to load electrospun pullulan membrane with chitin nanofibrils-nanolignin-glycyrrethinic acid (CLA) complexes. Pullulan was electrospun to form a membrane before CLA particles were coated on one side. An alternating electric field was then applied from the other side such that the particles were dispersed across the porosities of the membrane. The size of the CLA particles was less than 10 µm with most of the particles with a diameter of 3 µm. The electrospun pullulan fiber has an average diameter of 1.13 µm. The CLA particles were homogeneously distributed and adhering to the surface of the pullulan fibers.

(a) Pullulan electrospun mesh being inserted into the pilot equipment for dry impregnation. (b) Sketch of the dry impregnation technology (S-Preg). (c) Pullulan electrospun mesh impregnated with CLA complexes [Teno et al 2020].

Representative SEM micrograph (2500×) of electrospun pullulan mesh impregnated with CLA, visible as microparticles attached to the fibers. Scale bar is 40 microns [Teno et al 2020].

Simultaneous electrospinning and electrospraying may be used to incorporate functional nanoparticles on the surface of the nanofibers within the membrane. Alternatively, an ultrasonic atomizer [Dong et al 2013], airbrush or other spraying device may be used in place of electrospraying. Spraying may be directed at the electrospinning jet such that the particles are attached to the fiber surface prior to deposition on the mat [Xuyen et al 2009] or at the opposite side of a rotating collector [Jaworek et al 2009]. Almost any kind of nanoparticles can be incorporated on the surface the nanofibers using this method. TiO₂, MgO and Al₂O₃ nanoparticles have been coated uniformly across the thickness of the membrane using simultaneous electrospinning and electrospraying [Jaworek et al 2009].

Chemically active nanoparticles on the surface of the nanofibers will increase the activity of the composite. However there is a risk that the nanoparticles may get dislodged from the nanofiber and causes health and environmental risks. Zhang et al [2013] tested the stability of the TiO₂ particles on nylon-6 membrane by sonication in ultrasonic bath for a few minutes. The internal microstructure of the membrane remains unchanged and this demonstrated the adhesion stability of the particles to the membrane. Post spinning process such as heat treatment may be required to ensure better bonding or fusion between the particles and the fibers. Trejo and Frey (2015) conducted a study of carboxylic acid coated iron oxide nanoparticles (CA-Fe3O4 NPs) applied to Nylon 6 nanomembranes using simultaneous electrospinning and electrospraying. Their study showed that over 97% of the nanoparticles remain adhered to the fiber surface after 60 minutes of washing. The retention ability of the nanoparticles on the nanofibers is dependent on the pH of the washing solution.

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