Electrospun Materials: By Emma Klinkhamer

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Contents

Background

Process

Figure 1 Electrospinning process setup[4]
Figure 1 Electrospinning process setup[4]
Figure 2 Cellulose acetate fibers[4]
Figure 2 Cellulose acetate fibers[4]


Electrospinning is a simple technique in which a mat is fabricated from nonwoven fibers with diameters ranging from nano to micro sized. The electrospinning technique can be used to spin a variety of natural and synthetic polymers. Electrospun fibers can be used in a wide range of applications ranging from drug delivery, liquid filtration, nano-sensors and military clothing. The process to create these non-woven mats utilizes a syringe filled with the polymer solution of choice. The solution is dispensed from the needle tip at a very low feed rate. Simultaneously a voltage is applied to the needle tip as well as a copper plate (typically around 10 cm away from the needle tip). Once this voltage is applied, the polymer extends to copper plate in fiber form, while the solvent evaporates. This produces the electrospun fiber mat. Figure 1 illustrates the process schematic.[1] The electrospinning process has many parameters that can be manipulated in order to fine tune the desired mat. For example, the distance from the copper plate to the needle tip can be increased in order to decrease fiber diameter; spinning time can be increased to create a higher density fiber mat. Additionally there are myriad polymer solution choices. The choice of polymer is very application dependent. For instance, cellulose acetate can be spun to create mats that would optimize wound healing (Figure 2). Similarly chitosan can be spun in order to create anti-bacterial membranes.[4]Beside a user defined polymer choice, another benefit of electrospinning is the spinning method itself. For example a lot of research has gone into using a rotating drum to create a tube of fibers. There are some collector mechanisms that even produce an aligned arrangement of fibers as opposed to the non-woven matrix.[5]

Applications

Figure 3 Applications based on US patents[1]
Figure 3 Applications based on US patents[1]

Filtration

The diversity in electrospun material applications stems from the versatility and tunability of the system parameters (Figure 3). For example, a large application for these fiber mats is filtration. Electrospun materials are beneficial in the filtration field not only because the polymer solution choice can be application defined, but also because these solutions can be impregnated with bacteria killing nanoparticles such as silver.[4] Filtration is actually the only electrospinning application that has reached the industry level. Freudenberg Nonwovens is a manufacturer for electrospun filter media that has been in business for over 20 years. Other electrospun filtration companies include Donaldson and Finetex.[5]

Medical Prosthesis

Though filtration is the only application to reach industry level, judging by patents alone, medical prosthesis is the leading application for electrospinning.[1] This encapsulates a wide array of different grafts including skin, bone, and veins. Another application related to grafts are wound healing systems in which the nanofiber mat is used as a very powerful "band-aid" for large or chronic wounds. Research has also gone into drug delivery systems within these bandages. Drugs can be loaded in the polymer solution pre-spin or seeded into the fibers post spin. [4] The main reason for the large amount of research in the grafting and wound healing fields is due to the fact that the fibers are pours, non-woven, and have a large surface area to volume ratio. These qualities are all dominate in our own extracellular matrix, making these fibers a close replicate to our body's tissue.

Material Reinforcements

Another interesting application for these fibers are material reinforcements. Specifically, research has gone into using electrospun nanofibers along with carbon nanotubes to strengthen materials. One of the reasons this is a popular area of research is because of nanofibers outstanding mechanical properties.[1]

History

  • (1934-1944) Formalas gets patent for producing polymer fibers using an electrostatic force
  • (1952) Vonnegut and Neubauer develop apparatus in which a wire is put in contact with liquid contained within a glass capillary tube, thus electrifying liquid droplets.
  • (1955) Drozin finds that certain liquids can be dispersed as a uniform aerosol depending on conditions
  • (1966) Simons gets patent for apparatus that produces thin fibers using two electrodes
  • (1971) Baumgarten creates set-up that produces small diameter fibers using a stainless steel capillary tube, an infusion pump, a high voltage dc current and a metal screen to collect fibers
  • (1994) The term "electrospinning" is used [1]

Motivation

Wound Dressings

Figure 4 Publication in the electrospinning field [1]
Figure 4 Publication in the electrospinning field [1]

Perhaps one of the most researched applications is electrospun fibers used for wound dressings and wound healing (Figure 4). Non-fatal burn injuries are one of the major causes of infection and disease. Around 265,000 deaths each year are due to burn related injuries. Current wound dressings must be improved in order to better prevent infection and allow air diffusion through the bandage. A promising wound dressing alternative would be a bandage composed of electrospun fibers to treat burns and other related skin issues such as chronic ulcers. Electrospun fibers have many medical benefits including optimum porosity, and high surface-to-volume ratio. These properties, on a nanoscale basis, lead to enhanced natural skin healing, moisture retention and exudate removal. Therefore electrospun mats could provide wounds with an effective, protective barrier similar to the extracellular matrix (ECM). Previous work has shown that nanofiber mats increase cell proliferation, growth factors, and cellular ability to heal.[4] There is also research that suggest these wound dressings could not only aid in the healing process, but also serve as anti-microbial agents. In one study a solution of CA (0.5 and 5.0%) incorporated with chitosan/poly(ethylene oxide) (PEO) was successfully spun. These nanofibers demonstrated the ability to decrease activity rates of Escherichia coli and Pseudomonas aeruginosa. Thus electrospun fibers have the potential to combat bacterial infections.[6]

Vascular Grafts

1.4 million patients per year need arterial prosthesis grafts. Current treatments include allografts, xenografts and synthetic grafts; however allografts are in limited supply, xenografts have a very short life span and synthetic grafts can pose poor integration problems. Composite electrospun scaffolds are being continuously researched as an alternative method for arterial prosthesis grafts. Different types of polymers have been researched for this purpose, including polyurethanes, collagen and chitosan just to name a few. These types of scaffolds have great potential in the vascular graft field; however more research needs to be conducted detailing cell seeding conditions as well as physical properties of the scaffold.[3]

Bone Tissue Engineering

There is a great need to find bone tissue replacement alternatives instead of relying on autografts or allografts. Autografts are in limited supply and other foreign materials demonstrate poor integration with the patient's body. Electrospun materials have potential to be used as bone tissue scaffolds. Not only do electrospun fibers mimic the ECM, but these fibers can also be tuned and made into composite materials to optimize performance as bone scaffolds. In one study it was shown that a composite made from electrospun chitosan and hydroxyapatite nanoparticles could be crosslinked with genipin to create a very effective scaffold for non-weight bearing bone (cranial or maxillofacial). This composite material exhibited similar mechanical properties to bone tissue. Once seeded with osteoblast-like cells, the cells showed stellar proliferation, differentiation and maturation.[2]

Challenges

One of the biggest challenges with electrospinning is scale up. Since the process requires a small diameter needle, it would be difficult to scale this process up unless somehow hundreds of syringes were lined up and one large collector plate was used for fiber deposition. This method would increase cost greatly. Another issue with using electrospun fibers for tissue engineering is the pore size of the mats. While most human cells are upwards of 10 microns, poor size in an electrospun mat is typically less than 5 microns. This factor makes it difficult for cells to migrate and uniformly cover a surface. However to combat this obstacle certain materials (eg. pan-MMP) can be added to scaffolds to cleave sites so cells migrate.[1][5]

References

[1] Huang, Z. M., Zhang, Y. Z., Kotaki, M., & Ramakrishna, S. (2003). A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Composites science and technology, 63(15), 2223-2253.

[2] Frohbergh, M. E., Katsman, A., Botta, G. P., Lazarovici, P., Schauer, C. L., Wegst, U. G., & Lelkes, P. I. (2012). Electrospun hydroxyapatite-containing chitosan nanofibers crosslinked with genipin for bone tissue engineering. Biomaterials, 33(36), 9167-9178.

[3] Hasan, A., Memic, A., Annabi, N., Hossain, M., Paul, A., Dokmeci, M. R., ... & Khademhosseini, A. (2014). Electrospun scaffolds for tissue engineering of vascular grafts. Acta biomaterialia, 10(1), 11-25.

[4] Rieger, K. A., Birch, N. P., & Schiffman, J. D. (2013). Designing electrospun nanofiber mats to promote wound healing–a review. Journal of Materials Chemistry B, 1(36), 4531-4541.

[5] Teo, W. E., Inai, R., & Ramakrishna, S. (2016). Technological advances in electrospinning of nanofibers. Science and Technology of Advanced Materials.

[6]Rieger, K. A., & Schiffman, J. D. (2014). Electrospinning an essential oil: Cinnamaldehyde enhances the antimicrobial efficacy of chitosan/poly (ethylene oxide) nanofibers. Carbohydrate polymers, 113, 561-568.

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