Fabricating Complex Geometries with Electrospinning

Electrospinning has emerged as a transformative technique for fabricating fibrous materials with highly controlled geometries. This method, characterized by its ability to produce ultrafine fibers from various materials, has garnered widespread attention across industries, including healthcare, energy, and materials science. A particularly exciting aspect of electrospinning is its capacity to create complex geometric structures composed of nanoscale and microscale fibers, which significantly enhance their functionality for advanced applications. Traditional electrospinning produces planar surfaces composed of randomly oriented fibers but advancements in the field have enabled the fabrication of more sophisticated structures. In this blog we will delve into some of the complex geometric structures that can be fabricated by electrospinning and their different applications.

Structures with Oriented Fibers

Fiber orientation refers to the alignment of fibers within a material and it plays a crucial role in determining sample mechanical properties and performance. A higher degree of fiber alignment always improves mechanical properties when compared to randomly oriented fibers as they can distribute the applied load along their length. This has been seen in multiple works including polymers like polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) and polyacrylonitrile (PAN) and polycaprolactone¹ (PCL).² In tissue engineering applications, both randomly oriented fibers and aligned fibers are used to mimic in vivo environment of native tissue according to the application of interest.^1,2,3 The most typical method to control fiber orientation is by using a rotating drum collector, wherein the fiber alignment is a function of the linear speed of the collector drum (Figure 1).

Bumpy Surfaces 

Mesoscale surface roughness, also known as rugose surface, can be obtained due to charge retention in the sample being electrospun. These types of fibers allow an increase in the overall surface area of the final sample at the macro-level. It can be helpful in applications like filtration, where the filtering capacity can be increased due to an increase in surface area. Figure 2 showcases a bumpy surface made of PCL. Different methods have been reported to fabricate these bumpy surfaces including using a small diameter grounded rotating collector at low rotational speeds.⁴

Researchers from the Institute of Agrochemistry and Food Technology (IATA) found that these rugose structures on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)can be induced and controlled based on the substrate, sample thickness and relative humidity during sample formation.⁵

Patterned surfaces

Patterned and super aligned fibers are additional possible microstructures generated with the electrospinning technique. Figure 3a shows the microstructure of PCL microfibers collected on a drum collector by manipulating the electric field. The induced fiber-fiber bonding improves mechanical properties of the resultant surface. Super aligned fibers can also be fabricated by increasing the speed of the drum collector. Super aligned polyhydroxybutyrate (PHB) fibers are shown on Figure 3b, where the rotational speed was increased to 3,000 rpm to obtain this structure.

Surfaces with Combined Fibers

Surfaces with combined fibers can be fabricated by co-electrospinning wherein two different solutions are simultaneously processed at independent voltages to collect complex structures (Figure 4a). These structures can be composed of two different types of fibers, fibers and particles, or even fibers and living cells or microorganisms. Figure 4b shows an example of two types of fibers, microscale thermoplastic polyurethane (TPU) and nanoscale polyethylene oxide (PEO), combined in the same structure.

Co-electrospun structures find uses in different applications especially where the properties of one material can either enhance or complement the other. For example, the structural properties of gelatin fibers used in wound healing can be enhanced by co-electrospinning with PCL. In the case of drug delivery, co-electrospinning with two materials, one that releases the drug fast and another slow, can enhance the efficacy of delivery.

                An additional example, in flow batteries and fuel cells involves the use of bipolar membranes (BPM). Normally, 2D BPM structures are developed by combining an anion and cation exchange layers adhered by casting or lamination. One of the disadvantages of 2D BPM is the lack of a distinguished interfacial or junction region in the BPM. Electrospinning offers the opportunity to solve these issues by taking advantage of co-electrospinning 3D BPM structures.⁶

Yarns

Electrospun fibers can also be used to fabricate continuous thread-like structures using a specialized yarn collector (Figure 5a) and representative microstructure a yarn composed of PHB fibers is shown on Figure 5b. Yarns can also be fabricated by co-electrospinning to combine multiple types of fibers. These electrospun based yarns offer many advantages when compared to traditional yarn structures in terms of mechanical and physical properties, along with functional performance.

Electrospun yarns can incorporate temperature sensitive drugs, amino acids, peptides, proteins and sensitive pharmaceutical ingredients within the nanoscale fibers in the yarn for drug delivery and wound healing applications. A drug contained within the yarn can be released at a more controlled rate, remain in an amorphous state, and prevent possible drug bursting that could cause toxic levels. A recent study showcased how the antibiotic ciprofloxacin hydrochloride (CPX) was incorporated into poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) electrospun yarns with varying 3HV content with a final application of biodegradable sutures.⁷

Multilayered Structures

Multilayered electrospun fibers is another form of structure that electrospinning offers as an advantage. These types of fibers can be used to control material mechanical properties, providing distinctive layers to mimic natural tissue, improve filtration efficiency and permeability, and target drug delivery. These types of fibers can be done in situ during the fiber formation by manipulating the electrical field or other mechanical components like the drum collector speed, or by post-processing the sample to the desired shape (Figure 6).

Conclusion

Electrospinning is a highly versatile and innovative technique for not just simple fibers but fabricating complex fiber and surface geometries. By leveraging advanced methods such as coaxial electrospinning, multi-jet systems, and tailored collector designs, researchers and engineers can create intricate fibrous structures that meet the demands of cutting-edge applications. Despite some challenges, ongoing advancements in this field continue to expand the possibilities, ensuring that electrospinning remains at the forefront of fabrication technologies.

References

1. Orkwis, J. A.; Wolf, A. K.; Shahid, S. M.; Smith, C.; Esfandiari, L.; Harris, G. M. Development of a piezoelectric PVDF‐TRFE fibrous scaffold to guide cell adhesion, proliferation, and alignment. Macromolecular Bioscience 2020, 20 (9). https://doi.org/10.1002/mabi.202000197 ↩︎
2. Malik, S.; Sundarrajan, S.; Hussain, T.; Nazir, A.; Ramakrishna, S. Fabrication of Highly Oriented Cylindrical Polyacrylonitrile, Poly(lactide-co-glycolide), Polycaprolactone and Poly(vinyl acetate) Nanofibers for Vascular Graft Applications. Polymers 2021, 13 (13), 2075. https://doi.org/10.3390/polym13132075 ↩︎
3. Gregory, H. N.; Guillemot-Legris, O.; Crouch, D.; Williams, G.; Phillips, J. B. Electrospun aligned tacrolimus-loaded polycaprolactone biomaterials for peripheral nerve repair. Regenerative Medicine 2023, 19 (4), 171–187. https://doi.org/10.2217/rme-2023-0151 ↩︎
4. Gregory, H. N.; Guillemot-Legris, O.; Crouch, D.; Williams, G.; Phillips, J. B. Electrospun aligned tacrolimus-loaded polycaprolactone biomaterials for peripheral nerve repair. Regenerative Medicine 2023, 19 (4), 171–187.https://doi.org/10.1016/j.polymer.2021.124120 ↩︎
5. Basar, A. O.; Prieto, C.; Pardo-Figuerez, M.; Lagaron, J. M. Poly(3-hydroxybutyrate-CO-3-hydroxyvalerate) electrospun nanofibers containing natural deep eutectic solvents exhibiting a 3D rugose morphology and charge retention properties. ACS Omega 2023, 8 (4), 3798–3811. https://doi.org/10.1021/acsomega.2c05838 ↩︎
6. Al-Dhubhani, E.; Swart, H.; Borneman, Z.; Nijmeijer, K.; Tedesco, M.; Post, J. W.; Saakes, M. Entanglement-Enhanced Water Dissociation in Bipolar Membranes with 3D Electrospun Junction and Polymeric Catalyst. ACS Applied Energy Materials 2021, 4 (4), 3724–3736. https://doi.org/10.1021/acsaem.1c00151 ↩︎
7. Teno, J.; Pardo-Figuerez, M.; Evtoski, Z.; Prieto, C.; Cabedo, L.; Lagaron, J. M. Development of Ciprofloxacin-Loaded electrospun yarns of application interest as antimicrobial surgical suture materials. Pharmaceutics 2024, 16 (2), 220. https://doi.org/10.3390/pharmaceutics16020220 ↩︎