Benefits of Bead-on-String Electrospun fibers

Illustration of droplet migration from cylindrical portion of fiber to the beads.

Beads on fibers are generally considered as a defect for electrospun fibers to be removed by optimization of the spinning condition. Although there are numerous studies on the application of electrospun fibers, almost all of the studies are based on smooth fibers. Beads are usually unwelcomed as beaded membranes tend to exhibit a lower mechanical property [Gaharwar et al 2014]. This is due to stress concentration as a result of large dimensional variation between the beads and fiber region. Nevertheless, presence of beads may exhibit unique properties that benefits specific application.

Surface roughness

Electrospun membrane of non-water absorbent material is known to exhibit enhanced hydrophobic characteristics. This has been attributed to the surface roughness formed by the inter-connecting small diameter fibers. Beaded fibers are known to exhibit a greater water contact angle compared to smooth fibers [Ma et al 2005; Shao et al 2009] and this is probably due to the increased surface roughness caused by the presence of beads. Based on the Cassie-Baxter equation, water contact angle is higher for a droplet sitting on spheres compared to cylinders of comparable radius [Ma et al 2005]. Thus the presence of more beads will result in greater water contact angle in agreement with the equation.

Molecular Distribution

The relative ease of electrospinning mixtures have enabled researchers to construct fibers loaded with active agents. Drugs and other biomolecules are the most commonly encapsulated agents in electrospun fibers. In most drug release applications, it is preferable that the drugs are being released at a constant rate to maintain a stable dosage. Due to the small surface area to volume of electrospun fibers, it is common to encounter burst release of the drugs. Somvipart et al (2013) showed that with beaded silk fibroin/gelatin fibers, the release of methylene blue as model drug can be sustained over a longer duration compared to smooth fibers. Relatively larger volume of drug encapsulated in the beads may be the reason for a more prolonged release of drugs. Gaharwar et al (2014) demonstrated the sustained release of dexamethasone (Dex) within electrospun beaded poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT/PBT) fibers. Using a fluorescence microscope, it was observed that most of the Dex was found in the beads instead of the fibers thus the beads act as a reservoir for the drug. The release kinetic data of Dex correlates to the Korsmeyer-Peppas model for drug diffusion from a polymer matrix with a Fickian diffusion coefficient of 0.33. In the encapsulation of enzymes in emulsion (water-in-oil mixture) electrospinning, Dai et al (2013) found greater concentration of laccase within the beads of the resultant beaded fibers.

Chromophores distribution in beaded fibers was found to be dependent on the polymer used. Electrospinning of polyethylene oxide (PEO) loaded with 1,1',3,3,3',3'-hexamethylindodicarbocyanine (DiIC₁(5)) was found to be concentrated along the edges of PEO beads while the same chromophores was found to fill the volume beads of poly(methyl methacrylate) (PMMA) [Tomczak et al 2005]. It has been suggested that differences in crystallinity between PEO and PMMA as the reason behind the differences in DiIC₁(5) distribution. PEO being semi-crystalline to pushes DiIC₁(5) towards the edges of the beads while PMMA, being amorphous , retains DiIC₁(5)within the interior of the beads.

Preferential encapsulation of agents within the beads may be ideal for certain release applications. Park and Braun (2010) used the preferential entrapment of siloxane precursors, comprising of two separate parts, within poly(vinylpyrrolidone) beads to demonstrate self-healing capability of electrospun core-shell fibers on a substrate. The greater volume of the healing agents found within the beads enable rapid healing of the damaged site.

Droplet migration

Bead-on-string presents an interesting structure where there is a small radius cylindrical section which encourages fluid droplet to spread along its length and a large radius bead section that tends to hold the droplet. This provides a mechanism where fluid droplets are encouraged to move from the cylindrical section to the bead section. Polycyclic aromatic hydrocarbons (PAHs) adsorbed onto beaded poly(d,l-lactide-co-glycolide) (PDLGA)/laccase fibers were found to demonstrate migration towards the beads. Given the larger surface area of inter-beads fiber, it is expected that more PAHs will adsorbed onto the fiber surface instead of the beads surface. Niu et al (2013) hypothesized that given the surface of the fibers are smoother than the beads, the beads carries a higher surface energy than the fibers. This drives the PAHs droplets from the fiber towards the beads. Experimental observation does seem to suggest movement of the PAHs from the fibers to the beads. Given that more laccase was encapsulated within the beads, this enhances the rate of decomposition of PAHs [Niu et al 2013].

To further differentiate the bead properties from the smooth inter-beads cylindrical portion of the fiber, various techniques have been explored to introduce a second material. Core-shell electrospinning has been used to produce smooth inner fiber while the shell portion is encapsulated by low molecular weight solution which forms beads. This way, a hydrophobic fiber may be populated with hydrophilic beads [Tian et al 2011]. Another method is to disperse nanoparticles on the surface of smooth fibers followed by condensation of water droplets in a humidity chamber to coalesce the nanoparticles on the fiber to form beads. Zhao et al (2014) used this technique to form titanium tetrachloride (TiCl₄) beads on electrospun poly(methyl methacrylate) (PMMA) fibers. The length of exposure and the humidity level alters the size of the beads on the fiber. By varying the shape, properties and composition of the beads and the fibers, the bioinspired fibrous structure has the potential to improve the functional performance of electrospun membrane [Zhu et al 2012].

Contact points for microelectronics

Smooth nanofibers used in microelectronics and sensors may face a practical difficulty in establishing proper contact between the fiber and the probe. Where there are beads on the fiber, the beads offer a much bigger target for contact with the probe. Good contact between the probe and the beads on the fiber will facilitate transfer of signal from the fiber to the signal receiver. Xue et al (2013) deliberately fabricated beads on fibers by mixing polystyrene (PS) particles into poly(vinylpyrrolidone) (PVP) solution and electrospinning. The PS particles form aggregates which are distributed periodically along the fiber. The size of the aggregates and the distance between them gets smaller and longer respectively as the volume ratio of PS particles against PVP gets lower. Sensor made from these beads on PVP fibers loaded with FeCl₃ and coated with polypyrrole was shown to be highly sensitive with NH₃ detection limit down to 1 ppb level.

Air Filter Quality Factor

In air filtration, the air quality factor is a measure of the filter media air filtration efficiency over its pressure drop. The pressure drop relates to how difficult it is to pass air through it. One of the earliest commercial applications of electrospun fiber is in air filtration. While beads on fiber is generally regarded as an undesirable flaw, Akmal et al (2019) found that having beads of fibers may actually improve the air filter quality factor based on their electrospun acrylonitrile butadiene styrene (ABS) fiber membrane. In their tests with incense smoke, they found that the membrane with the highest air filter quality factor had the highest beads density. They hypothesized that the beads create a physical separation between fiber layers and this allows easier air flow through the membrane hence reducing the pressure drop without substantially reducing air filtration efficiency. However, it must be noted that the presence of beads reduces the mechanical strength of the membrane. They also found that membranes with beaded fibers clogged faster than smooth fiber membranes.

Bonding Points

Beads on electrospun fibers generally have a much larger diameter than the diameter of the fiber it sits on. This is useful when the material is used to mesh with another material. Guo et al (2021) showed that having a bead-on-string fiber to be used as a bonding agent is able to impart a stronger mechanical adhesion to another material compared to a smooth fiber structure. The composite structure is constructed by simultaneous electrospinning of polyethylene terephthalate (PET) and thermoplastic polyurethane (TPU) with the latter as the binding agent. Temperature of the TPU solution was kept high at 105 °C so that beaded fibers are produced. The beaded fibers help to form bonding points and increase the spacing between nanofibers. The tensile strength of PET/TPU nanofiber membrane was 4.33 MPa which is almost twice that of pure PET membrane with tensile strength of 2.33 MPa.

Release of entrapped fluid

In applications where the electrospun structure is being used to infuse a fluid and to be released when compressed, structures with beaded fibers may perform better compared to smooth fibers. Rubio-Valleet al (2022) explored the use of lignin/cellulose acetate (CA) fiber-bead electrospun structures as vegetable lubricating oil. The lignin/CA structure was first immersed in castor oil to form a uniform and gel-like dispersion. Due to the entanglement of the fibers, the structure is able to entrap castor oil within it, forming an oleogel. When this oleogel was used as a lubricant, the oleogel with beaded fibers released more oil compared to smooth fibers and this has been attributed to the smaller fiber diameter between beads and softer texture which makes it easier for the oil to be released. With smooth fibers, the structure is more robust and is better able to retain oil instead of releasing it as desired.

[+]

[-] comments powered by Disqus

Content [Fiber Physical Structure]