[Image above] Example of a ceramic nanospring created using the coaxial electrospinning technique. Credit: Dong et al., ACS Nano (CC BY 4.0)
In the medical field, one method that is gaining renewed attention for creating nanoscale, composite bioceramics is the fairly simple and low-cost technique of electrospinning.
Electrospinning involves using an electric force to eject a polymeric solution or melt from a thin needle onto a solid platform called the collector, where it solidifies into thin fibers. Adjusting the electric field strength, needle diameter, and solution viscosity and flow rate, among other parameters, can change the characteristics of the resulting fibers.
The reliance on sol–gel solutions to spin ceramic fibers is one parameter that researchers have struggled to adjust, however.
To date, almost all electrospinning work to synthesize ceramic fibers uses viscous solutions containing sol–gel reactants (such as alkoxides and metallic salts), a polymer carrier, a hydrolyzing agent (alcohol or water), an additive, and a solvent or solvent mixture. These solutions are electrospun into precursor fibers and then calcined at high temperatures to remove the organic components and trigger the crystallization of ceramics.
The polymer carrier in the solution is necessary to provide it with sufficient viscosity and prevent it from breaking into droplets during ejection. However, the reliance on polymers comes with two main limitations.
- Although many well-established precursor solution systems exist, modifying them by introducing other ions or additives may trigger polymer cross-linking and phase segregation, making these solutions nonspinnable.
- Even in well-established precursor solutions, the polymer intermixes with the sol–gel reactants and occupies a significant volume of the materials (typically about 40–60 wt. %). Removing the polymer leaves inorganic fibers with rough surfaces and pores.
A few research groups have tried electrospinning alkoxide sols without polymers, but the papers describing the experiments report fibers with highly nonuniform diameters and inadequate structural homogeneity, among other unsatisfactory results.
In a recent open-access study, University of Oxford researchers led by professor of nanomaterials Nicole Grobert explored the potential of a modified electrospinning technique called coaxial electrospinning to overcome the limitations with conventional sol–gel solutions.
Coaxial electrospinning involves arranging multiple solution feed systems to simultaneously electrospin two or more solutions from coaxial capillaries. The spinnerets in this setup share an axis, allowing for the injection of one solution into the other at the needle tip. This arrangement results in the core fluid getting drawn within the outer one, thus producing continuous coated or hollow nanofibers.
As explained in a book chapter on coaxial electrospinning, “The beauty with [this] method is that the duo-phase fibers produced boast of a blend of their properties while each of them maintains their separate identities, which allows for the combination of elecrospinnable with nonspinnable materials in the form of polymer/polymer, polymer/inorganic, and inorganic/inorganic coaxial nanofibers.”
Despite the possibility of combining elecrospinnable with nonspinnable materials with this technique, “To our understanding, applying a coaxial method to electrospin nonspinnable ceramic sols to create high-quality ceramic fibers has not yet been realized,” the Oxford researchers write in their paper.
To test this possibility, they created a low-viscosity sol comprising titanium isopropoxide, acetic acid, and rhodamine B (the last ingredient dyed the sol a dark pink color for better visualization). They fed this sol through the inner core of a coaxial nozzle, while a viscous polyvinylpyrrolidone/ethanol solution was fed through the outer nozzle.
Thanks to rhodamine B, the researchers clearly observed a sharp boundary between the core and shell part during the electrospinning process. The as-obtained precursor fibers retained this sharp boundary. After calcination, the obtained titanium dioxide fibers had a continuous structure and uniform diameter, in addition to exceptional flexibility.
Surprisingly, the calcined fiber adopted different morphologies depending on the final heat treatment process. When attached to and supported by the substrate during calcination, the precursor fibers remained straight after calcination. But if the fibers were peeled off and calcined in a free-standing mode, they turned into ceramic micro- and nanosprings, which have not been reported before.
In addition to creating strong, stable ceramic fibers, the researchers note another benefit of the coaxial electrospinning technique is the option to prepare the core and shell solutions separately and then store them for months.
“…once the precursor and polymeric solutions are mixed, the dynamic sol–gel transition gradually alters the solution properties, causing the solutions to be only spinnable for a short period, e.g., several days or hours,” they write. “Preparing the two solutions separately saves a lot of time as mixing them homogeneously is no longer necessary.”
Ultimately, “This research introduces a universal electrospinning approach for nonspinning solutions, which is envisaged to lead to a great variety of fiber materials with improved mechanical properties for diverse advanced applications,” the researchers conclude.
The open-access paper, published in ACS Nano, is “Electrospinning nonspinnable sols to ceramic fibers and springs” (DOI: 10.1021/acsnano.3c12659).