Electrospun fibers as templates

Electrospun fibers are typically made out of polymers and they can be easily removed using suitable solvents or through sintering. Taking advantage of this, electrospun fibers have been used as template for the construction of micro-structures or creation of nano- and micro-features.

Hollow tubes

Electrospun nanofibers are often used in the construction of hollow tubes, in particular, inorganic tubes. These are often prepared by coating the fibers with the desired material followed by the removal of the fiber core. The challenge is to get a uniform coating around the fibers so that complete tubes can be obtained. Schneider and Naumann (2014) used dip coating to deposit tetra-ethoxysilane solution on polystyrene fibers followed by calcination. A second coating was carried out using spray coating to deposit a layer of cerium (III) chloride and zirconium oxychloride sol over the polystyrene/silica composite fibers. A second of calcination at 750 °C for 4 hours were carried out to form CeO2/ZrO2@SiO2 composite tubes.

Two-stage process to construct a hollow ceria/zirconia@silica by first electrospinning a polystyrene fiber template [Schneider and Naumann. Beilstein J. Nanotechnol. 2014; 5: 1152. This work is licensed under a Creative Commons Attribution 2.0 Generic.]

Another method of coating fibers is to use chemical vapor deposition which can be used to deposit organic polymers and inorganic materials. Liu (2004) used chemical vapor deposition to coat polylactide template fiber with poly(para-xylylene). Polylactide core was shown to be removable by boiling in chloroform for 12 hours or heat treatment at 280°C for 8 hours. Kim et al (2008) used atomic layer deposition, a type of chemical vapor deposition technique to deposit TiO₂ precursor (titanium tetraisopropyl oxide) on poly(vinyl pyrrolidone) (PVP) fibers. Calcination at 500°C was used to remove the inner PVP core.

To construct a metallic tube, metal material may be coated on template fibers by sputtering. Sintering is then carried out to remove the organic core. Pantojas et al (2008) coated polyethylene oxide fibers with palladium using sputtering. A sputtering duration of more than 250s is required to form complete tubes after sintering. However, the thickness of the wall section of the cylinder is not uniform due to line-of-sight deposition.

Similar to sputtering, thermal evaporation is another method of depositing inorganic materials and metal. A study by Wu et al (2013) showed that the choice of template polymer fiber has an influence on the quality of the tube formed and in their case, nano-trough since they are only interested in depositing on one side of the nanofibrous membrane. They have selected polyvinylpyrrolidone fiber for fabrication of nano-trough out of gold, platinum, silicon and ITO. Polyvinyl alcohol nanofibers were selected for copper, silver and aluminum. They first transfer the coated nanofibers on selected substrates such as plastic, paper, glass and textile without any surface treatment and using standard thin-film deposition technique. Removal of the core material is by using water or organic solvent to form nano-trough on the substrate surface. The adhesion between the nano-trough and the substrate was sufficient to resist peeling using scotch tape.

Nano or micro channels

Continuous fibers of electrospun membrane make it an ideal candidate to create either discrete or interconnected channels within a polymer block that are in the nanometer and submicrometer range [Gualandi et al 2013]. To create this structure, the polymer matrix needs to be a sufficiently fluid such that it can fill the spaces between the fibers. A vacuum pump may be used to remove any air pockets in the electrospun membrane. After the polymer matrix has cured, the electrospun fibers may be removed by dissolving in suitable solvent to form interconnected channels and tubes within the polymer block. Another way of removing the template or sacrificial material is through thermal depolymerisation. This method involves using thermal treatment for vaporization of the sacrificial template such that little or no residues are left. Gergely et al (2015) used electrospun poly(lactic acid) treated with tin catalyst as the sacrificial material to form hollow channels in epoxy, a thermosetting polymer. The presence of tin catalyst helps to lower the depolymerisation temperature. The hollow channels created by sacrificial electrospun fibers in the epoxy matrix resembles natural vascular network which may be used in various engineering applications. However, since electrospun fibers are typically in the low microns to the hundreds of nanometer diameter, the resultant ultra-small diameter channels may not be suitable for rapid cell migration. In this case, electrospinning may be used with other 3D printing methods to create favorable scaffold for cell migration and to maintain cell viability. The purpose of the channels formed by electrospun fibers is to provide inter-connecting channels and increasing its porosity. Sun et al (2016) demonstrated the usefulness of such a construct using gelatin as the main hydrogel scaffold, printed Pluronic F127 as sacrificial for macro-channels and electrospun PCL as sacrificial for inter-connected micro-channels. For the scaffold with the inter-connected micro-channels from electrospinning, seeded vascular endothelial cells (VEC) showed much better viability compared to scaffolds with only macro-channels.

Precision electrospinning methods such as near field electrospinning were able to control the deposition location of fibers down to the tens of microns. Such precision is able to deposit fibers between chips. Lee et al (2007) was able to construct chip-to-chip fluidic connectors using near field electrospinning with polyethylene oxide (PEO) as the sacrificial material. Parylene and SiO₂ was coated onto PEO fiber and the PEO fiber was subsequently removed using hot water. The resultant hollow channel of 150 nm to 2 µm with hydrophilic core has a capillary flow speed of 20 µm/s. Melt electrospinning may also be employed as a precision electrospinning technique when the electrospinning jet is stable. Also known as melt electrospinning writing (MEW), the long stable jet allows precise positioning and even stacking of the fibers. Haigh (2017) used melt electrospinning writing (MEW) to construct a stacked and elevated fibrous structure using polycaprolactone (PCL) for use as sacrificial template in a hydrogel. The hydrogel matrix comprising of copolymers of EtOx and 2-(3-butenyl)-2-oxazoline (ButenOx), with ratios of 380:20 or 190:10 were used. The fabricated fibrous structure was immersed in the hydrogel matrix prior to cross-linking under ultraviolet (UV) irradiation with a photoinitiator. PCL is later removed using a solvent. Due to the fibrous construct, channels were formed within the hydrogel which allows solution from outside the hydrogel matrix to flow inwards. As a tissue scaffold construct, the channels formed within the hydrogel will facilitate cell penetration and exchange of nutrients. In another use of melt electrospinning writing, Su et al (2021) combined techniques of melt electrospinning writing, micromolding, and skiving process to mass produce tadpole-like magnetic polycaprolactone/Fe₃O₄ (PCL/Fe₃O₄) microrobot. Melt electrospun PCL fibers were used as a sacrificial template to form PDMS channels. These PDMS channels were formed by layers of melt electrospun fibers stacked on top of one another. By varying the speed of the collector, melt electrospinning writing was able to produce fibers of different diameters. When the collector speed was 100 mm/min, the channel width after removing the fiber was 159 µm. When the collector speed was increased to 1500 mm/min, the channel width after removing the fiber was reduced to 25 µm. Injection molding was used to fill the PDMS channels with PCL/Fe₃O₄ solution. After solidification of the PCL/Fe₃O₄, the billets were removed and embedded in a frozen, water-soluble polymer for skiving into microslides. The tadpole-like microrobots were then released from the polymer by washing.

Characterization of the polydimethylsiloxane (PDMS) channels and magnetic polycaprolactone (PCL)/Fe3O4 assymetric billet. (A) Scanning electron microscopy (SEM) images of the cross-section of PDMS channels fabricated with different printing speeds using 0.9 mm needles. (B) The width and depth of the channels against the printing speeds. (C) SEM images of the cross-section of assymetric PDMS channels with designed shapes. (D) Optical microscope image of channel 2. (E) SEM image of magnetic PCL/Fe3O4 assymetric billet from channel 2. The values of speed were marked on each image and the unit was mm min-1 [Su et al 2021].

While electrospun fibers can be used to make nano or micro channels for other applications, electrospun mat may also be used to make replicas of its own surface topography. This creates a faster method of producing flat surfaces with the same surface topography as electrospun nanofiber mat. Ballester-Beltran et al (2014) used polydimethylsiloxane to construct the mold by pouring the pre-polymer and cross-linker into a plastic cage containing electrospun poly(ethyl acrylate) fibers. After curing, the mold was separated from the nanofibrous template and cleaned to remove any residual fibers. The mold was successfully used to reproduce the nanofibrous surface using poly-L-lactide (PLLA) solution.

SEM images of molded PLLA [Ballester-Beltran et al. Biofabrication 2014; 6: 035009. This work is licensed under a Creative Commons Attribution 3.0 Unported.]

Seeding template

Electrospinning is a fast and effective way of forming continuous fiber on a substrate. In application where a continuous path is required, electrospinning is able to lay a continuous fiber which may subsequently be replaced by other material. Kim et al (2018) used this ability of electrospinning to create a flat copper nanofiber network with high conductivity and transparency. This copper network was formed by first electrospinning a polymer solution containing palladium ions. The electrospun layer was subsequently decomposed and calcinated at high temperature to form a seed layer. This seed layer was used for copper electroless deposition. The resultant electrode showed transparency of over 90% over the entire visible light range and a sheet resistance of 4.9 ohms/sq.

1-dimensional material organization

Fibers with very high surface roughness are known to demonstrate higher hypdrophobicity. Beaded fibers have been constructed from linear organization of silica beads with electrospinning. To achieve this, a beads colloidal dispersion was prepared and electrospinning is subsequently carried out. Silica beads in the sub-micron diameter have been arranged into fibers using this method [Lim et al 2007]. Sintering was used to remove the polymer matrix that was used to arrange the beads during electrospinning.

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Content [Microstructure Fabrication Methods]