(Nanowerk Spotlight) Electrospinning, a versatile method for fabricating nano- and microfibers, has the capability to produce fibers with diameters ranging from nanometers to micrometers out of ceramic, polymer, and metallic materials. These fine fibers are highly sought after for a wide array of applications, from tissue engineering and filtration to fuel cells and lithium batteries, largely due to their unique high-aspect-ratio morphology and expansive surface area.
While traditional electrospinning has been adept at creating two-dimensional fiber mats, these structures have their limitations. The fibers in 2D mats are tightly packed, leading to smaller pore sizes and less space for materials to pass through or be stored. This makes them less ideal for applications requiring high porosity or mechanical flexibility, such as filtration systems or tissue scaffolds. Additionally, the 2D mats are prone to delamination, which can compromise their structural integrity over time.
In contrast, three-dimensional (3D) fiber macrostructures offer a host of advantages. These structures have significantly larger fiber-to-fiber spacing and pore sizes, which not only enhances their mechanical resilience but also increases their capacity for thermal insulation and pollutant absorption. The 3D configuration allows the fibers to entangle and build up layer-by-layer, creating a highly porous network that is more robust and versatile. This is particularly beneficial for applications like thermal insulation, where the larger air pockets within the 3D structure provide superior heat resistance compared to 2D mats.
Moreover, the high surface area and porous nature of the 3D fibers enable ultrahigh absorption capacities. When treated to be hydrophobic, these ceramic fibers can selectively absorb organic solvents from water, making them highly effective for environmental cleanup tasks such as oil spill remediation.
So far, producing true 3D porous fibrous structures has required complex modifications to the electrospinning setup and labor-intensive post-processing methods.
Researchers from the University of Oxford have now developed a simple yet powerful technique to produce 3D fibrous materials directly from solutions using a standard electrospinning setup. By tuning the composition and properties of the starting sol-gel solutions, the team achieved in situ formation of highly porous 3D fibrous networks.
Creation of 3D fiber assembly via sol−gel electrospinning. (A) Schematic illustration of the 3D fiber assembly electrospinning process for thermal insulation and oil−water separation; the chemical composition of sol−gel reactants and solid fibers are shown. (B) The evolution of solution parameters as functions of additive concentration. (C−E) Schematics and digital photos of the 2D, 2.5D, and 3D fiber macrostructures suggest the differences in their macropore shapes, fiber orientation, and fiber-to-fiber distance. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
“By incorporating an yttrium salt into a solution containing titanium and silicon alkoxide precursors, the solution’s conductivity and viscosity increase in a way that modifies the electrodynamic jet behavior and fiber assembly process,” Shiling Dong, the paper’s first author, explains to Nanowerk.
This new method, which doesn’t require complex post-processing, could enable more functional and economical next-generation materials for applications ranging from thermal insulation to environmental cleanup.
Using a high-speed camera, the team captured the differences between the jet dynamics and fiber collection comparing additive-free solution versus additive-containing solution. With higher conductivity, the whipping instability zone where the jets begin to bend, and whip starts closer to the nozzle. More interestingly, the jets develop spirals not just vertically but also parallel to the electric field due to charge redistribution along the jet.
This vertical collection is enabled by polarization effects between the incoming fibers and those already collected. As the fibers land perpendicularly and entangle, a highly porous 3D fibrous network is built up in situ layer-by-layer during electrospinning. The resulting 3D architecture has significantly larger fiber-to-fiber spacing and pore sizes compared to conventional tightly packed 2D mats.
As the fibers then approach the collector, the additive-containing fibers were observed to land vertically on top of deposited fibers instead of packing flat. This vertical collection is enabled by polarization effects between the incoming fibers and those already collected. Ultimately a highly porous 3D fibrous network is built up in situ during electrospinning.
Analyzing the stiffness of individual fibers using simulations revealed that fibers spun from more viscous solutions are thicker and more rigid. This mechanical resilience prevents collapse of the 3D architecture. By tuning the precursor concentrations and additive content, the team mapped out the optimal ‘3D region’ of solution properties, specifically viscosity and conductivity ranges, for achieving 3D fiber assemblies.
“Remarkably, this approach allows 3D fibrous materials to be produced directly using a classic electrospinning setup simply by tailoring the starting solution,” Barbara M. Maciejewska, the paper’s co-first author, points out. “After calcining to remove the polymer component, the result is ceramic fiber materials with ultralight density and exceptional porosity.”
As a demonstration, the researchers tested the insulating performance of flat 2D fiber mats versus 3D fiber assemblies. The 3D structures provided superior thermal insulation, maintaining a flower’s freshness for over 10 minutes on a hot 200 °C plate.
The high surface area and porous nature of the 3D fibers also enable ultrahigh absorption capacities. When made hydrophobic, the ceramic fibers could selectively absorb organic solvents from water, showcasing potential for cleaning up oil spills.
“This breakthrough tackles a longstanding challenge in electrospinning and fiber production,” Prof. Nicole Grobert, who led this work, concludes. “The ability to easily generate 3D fibrous networks expands possibilities for designing advanced materials. Coupling simple manufacturing with shape versatility could enable customizable insulation, absorbents, tissue scaffolds, battery electrodes, and more. Importantly, the method’s basis in universal solution properties opens the door to creating tailored 3D fiber structures from diverse material systems.”
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