Addressing the needs of nanofiber mass production

Addressing the needs of nanofiber mass production (Limitation of nozzle system)

Electrospinning has been widely used in laboratories to fabricate nanofibers due to its relatively ease of setting up and requirements. However, this process presents several challenges when scaling up to mass produce nanofibers. The unique challenge in electrospinning versus other fiber spinning technique is its use of electrical charges to stretch the polymer solution into fiber. Beyond this, it shares the same requirements to conventional fiber spinning process.

Challenges of mass production

Low output per spinneret

A main shortcoming with electrospinning is the low fiber output per spinneret. To produce nanofibers through electrospinning, it is essential to keep concentration of the solution to a minimum without causing beads to form. However, that means having the solvent making up more than 70% of the solution mass depending on the polymer and solvent system. Thus only a fraction of the solution that passes through the spinneret contributes to the mass of the fiber produced. As a result, several researchers are trying to electrospin polymer melt to increase the productivity and to eliminate the need to recover the vaporized solvents.

There is also a limit to the feed-rate per nozzle as higher feed-rate may result solution dripping out from the nozzle instead of spinning especially for the middle nozzles [Li et al 2006]. This is caused by insufficient applied electric field experienced by the middle nozzles resulting in inadequate solution drawing.

Clogging of spinneret

Clogging of the spinneret tip through gelation of the solution can be very disruptive to the spinning process as it causes production losses. This issue is more apparent when higher concentration solution is used which is likely due to higher viscosity [Li et al 2006]. The use of low boiling point solvent also contributes to the clogging of the spinneret tip [Liu et al 2015]. One method to reduce or eliminate clogging is by heating the spinneret tip [Fang and Reneker 1997]. Development of needleless electrospinning has also eliminated the problem of clogging although this process brings about its own set of challenges. If a low boiling point solvent is necessary for preparation of the solution, a high boiling point solvent may be added to reduce clogging [Liu et al 2015].

Electrospinnability of polystyrene solutions using different binary solvent systems (\, -, +, ++,+++, and ++++ represent no solution prepared, unelectrospinnable, severe needle clogging,medium needle clogging, occasional needle clogging, and no needle clogging, respectively) [Liu et al 2015. Nanoscale Research Letters, 10: 237. This work is licensed under a Creative Commons Attribution 4.0 International.]

Legend. Dichloromethane (DCM), acetone (ACE), tetrahydrofuran (THF), N,N-dimethylformamide (DMF), cyclohexanone (CYCo), 1-butanol (BuOH).

In melt electrospinning, clogging of the spinneret will occur when the polymer melt solidifies at the tip. A simple way to address this issue is to have the heating element located at the spinneret tip. This will ensure that the polymer at the spinneret tip is always molten [Koenig et al 2019]. Considering that a high potential difference between the tip and the collector is needed for electrospinning and that the high voltage cannot come into contact with the heating element, the arrangement of the setup needs to be modified. Application of heat to the spinneret tip may be through infra red instead of through direct contact. This will eliminate contact between the high voltage current and heating element. An alternative is to have the high voltage applied to the collector instead of the spinneret [Koenig et al 2019]. This too will create a potential difference to draw the polymer melt from the spinneret tip to the collector.

In many cases, having partially solidified material at the nozzle tip is inevitable especially after electrospinning for a long duration. When the nozzle is on top and the collector at the bottom, dislodging the material will cause it to drop onto the membrane below and affect its quality. However, when the position gets switched around, with the nozzle at the bottom instead, dislodged semi-solid material at the nozzle may fall on the ground under its weight and away from the electrospun membrane above. Using the same logic, the nozzle to collector placement may be horizontal instead of vertical. Similarly, dislodged material will fall onto the ground in the space between the nozzle and the collector instead of at the collector. It has been suggested that with the electrospinning source at the bottom, the polymer solution was made to travel against the gravitational pull. This may help to reduce the formation of beads and droplets due to excess solution feeding [Prabu et al 2020]. This simple adjustment in nozzle to collector position will help to achieve better quality membrane.

Electric field disturbance for multiple spinnerets

While conventional microfiber spinning process is able to increase the productivity by packing more spinnerets per spinning head, this is not possible for electrospinning. As the solution is ejected from the tip of the spinneret under the influence of the electrostatic charges, it spread outs to form an expanding cone [Theron et al 2001] which will interfere with neighboring jets if the spinnerets are placed too close to one another [Theron et al 2005]. This significantly reduces the number of spinnerets that can be placed in a spinning head. Uniformity of the nanofiber layer thickness deposited on a substrate is also compromised by inter-jet interference. When used as air filter membrane, this may led to a drop in its filtration efficiency [Li et al 2006] or large variance in its performance [Marton 2015]. Experiment by Theron et al showed that an inter-nozzle distance of 1 cm is required to achieve reasonable stability and uniformity [Theron et al 2005]. However, the influence of the electric field between electrospinning jets easily goes beyond 8 cm [Li et al 2006].

Health and environmental hazards

This hazard is mainly posed by the solvent used in forming the solution. Appropriate scrubbing to recover the solvent or proper removal of the volatile organic compound from the manufacturing chamber needs to be in place before being vented into the atmosphere to prevent contamination or pollution.

Fire hazard

Accumulation of solvent vapor if allowed poses a significant fire hazard. Proper detection mechanism should be in place to sound an alarm when the solvent vapor crosses a pre-determined level.

Meeting electrospinning mass production requirements

1. Increase output
- 1.1 Multiple spinnerets
  - 1.1.1 Increasing distance between spinneret tip and collector
    When more nozzles are used for electrospinning, a greater spinning distance between the tip and the collector is required to collect dry fibers. This is due to the reduced bending instability of the electrospinning jet as a result of smaller electric field slope (due to influence from the neighboring jet) leading to shorter flight distance [Varesano et al 2009; Yang et al 2010]. More spinning jets will also increase the density of solvent vapor in the vicinity of the individual jet leading to lowered evaporation rate. Thus a greater distance is required for sufficient drying of the fibers [Yang et al 2010]. Similarly, when spinning fiber on a thicker non-conducting substrate, a greater distance between the patches of deposited fiber from the nozzle is required [Varesano et al 2010].
  - 1.1.2 Using a shield ring
    A shield ring is a secondary electrode ring surrounding an array of electrospinning nozzles. Yang et al demonstrated the advantage of using a shield ring to maintain a relatively uniform electric field from all the nozzles within the shield ring. The diameter of the shield ring, its vertical position from the tip of the nozzle and the applied voltage needs to be optimized to obtain the most uniform electric field distribution from the nozzles. With this, they were able to spin PEO using a 37 needle system at a working distance of 35 cm at 0.5 ml/h per needle [Yang et al 2010]. Similarly, King et al (2017) used auxiliary electrodes placed at the sides of the spinning jets to create a well balanced electric field across the electrospinning surface. Without the auxiliary electrodes, electric field concentration can be seen at the ends of the electrospinning electrodes where electrospinning jets can be observed but not at the middle portion. With the balanced electric field, electrospinning jets can be seen across the whole electrospinning electrode. This significantly increases the output production of the setup.
  - 1.1.3 Multiple jets per spinneret
    Generation of multiple jets per spinneret is an interesting concept that has been demonstrated in a few publications. Although the production rate in terms of mass per unit time may change little, it will increase the volume of fibers produced in the nano-dimension.
- 1.2 Without spinnerets
  Electrospinning using needle spinnerets are restricted by the interference of the electric field between the charged needles thus this impose a limit on the number of spinnerets that can be packed together in a spinning head. This has led to the development of alternatives such as free surface eletrospinning and fiber spinning from solution droplets. With free-surface electrospinning, the density of jets that can erupt is not restricted by physical structures. There will also be self-organization of the jets for optimum spacing. In solution droplets spinning, the limitation is on the amount of droplets dispensed.
  Read
- 1.3 Increase feed rate
- 1.4 Increase ambient temperature
- 1.5 Polymer melt
2. Clogging of spinneret
- 2.1 Use higher boiling point solvent (eg. DMF)
  One of the factors that cause clogging of the tip is solidification of the solution at the Taylor cone when low boiling point solvent is used. Where possible, a higher boiling point solvent should be used instead.
- 2.2 Use physical device to remove clog
- 2.3 Use coaxial gas jacket.
  An inner tube dispenses the spinning solution while the outer capillary delivers a continuous flow of an inert gas saturated with the solvent of the solution [Larsen et al 2004]
- 2.4 Needleless electrospinning
  To eliminate clogging of spinneret, an obvious solution is to eliminate the source of the problem which is to electrospin without spinnerets. Several methods of electrospinning without the use of spinneret have been developed using concepts such as sharp edges or points, free surface, temporary surface disruption and droplets.
- 2.5 Heating element at needle tip for melt electrospinning
  Having the heating element directed at the tip of the needle will ensure that the polymer at the tip will always be molten. This will ensure that it does not solidifies and clog the needle tip.

3. Monitoring of electrospinnning process and fiber quality

3.1 Electrical current
To monitor the stability of the electrospinning process, a proposed method is to monitor the electrical current profile during fiber generation. During electrospinning, charges are carried by the electrospinning jet from the spinneret to the collector. If there is any disturbance to the electrospinning process, this will be reflected by fluctuation in the electrical current [Samatham and Kim 2006].
3.2 Assessing fiber layer uniformity using air jet
In some electrospun products such as in air filtration, the uniformity of the nanofiber layer on the substrate is critical so that air flow across the whole membrane during operation is consistent. One method to assess the fiber layer uniformity during production is to have sampling head that blows a jet of air at one side the membrane and a probe at the other side to measure the air velocity that pass through the membrane [Luzhansky 2003]. If the nanofiber layer is uniform, the air velocity detected by the probe at the various locations should be within a specific range.
3.3 Assessing fiber layer uniformity using light transmittance
Another method of ensuring electrospun membrane thickness uniformity is by monitoring light transmittance through the membrane. Ryu et al (2020) used diffused LED lights and their transmittance measured using a CCD camera. With real time feedback and a programmed, movable collector stage, the stage may shift the collector to collect more fibers at locations where the fibers are less. With their camera, the minimum resolution of light transmittance was about 0.5%. However, the real time measurement of the thickness resolution was 1% for a 10 µm thick nanofiber mat and reduced to 10% when the thickness was increased to 100 µm. This is due to exponential reduction in light transmittance with increasing nanofiber mat. Electrospinning polycaprolactone as the model polymer solution, the standard deviation of the collected membrane thickness using the feedback system was 8 times less than conventional electrospinning.

3.4 Monitoring Taylor Cone and Electrospinning Jet
The formation of Taylor cone and diameter electrospinning jet has a significant correlation with the quality of collected electrospun fibers. Without the use of an expensive zoom camera for visual monitoring of the electrospinning Taylor cone and jet diameter, Lunni et al (2020) proposed the use of light shining through the core of the electrospinning jet as a means of monitoring the process. A core-shell nozzle is needed for this setup where the core of the nozzle is inserted with an optical fiber and the outer orifice is where the solution for electrospinning was fed. As the light passes through the optical fiber, it illuminates the droplet and electrospinning jet and this emitted light can be captured on a light detector. By measuring the light emitted by the optical fiber and the light from the electrospinning jet, it is possible to find a correlation between the light emissions and process parameters. Changes in the process parameters can be detected real-time using this setup hence simplifying the monitoring task.

Experimental validation. (a,b) Two illuminated electrospun fibers (EFs) associated with different control parameters. Different control parameters develop effects on the droplet-jet transition region affecting the light waveguide length during the process (insets). (c,d) Resulting EFs obtained through different process parameters observed through SEM [Lunni et al 2020].

4. Fiber diameter consistency

4.1 Addition of salt
Conductivity of the solution can be improved through the addition of salt and this result in a more efficient transfer of charges to the solution. This has been shown to narrow the fiber diameter distribution [Lee et al 2005]. However, addition of salt will also have an effect on the solution viscosity and surface tension. Increase in solution viscosity as a result of salt addition will invariably increase the fiber diameter instead of reducing it. Conversely, if the viscosity drops below a critical point, the electrospinning jet may become unstable resulting in fiber splitting and a corresponding increase in the fiber diameter distribution [Lin et al 2010].
4.2 Vibrorheological effect
4.3 Using a shield ring
Yang et al demonstrated that with the help of a shield ring on a multiple spinnerets system (a ring electrode surrounding an array of nozzles), they were able to reduce the fiber diameter distribution compared to a system without a shield ring. Using the same concept, King et al (2017) used auxiliary electrodes to create a more uniform electric field across the electrospinning jets and this has been shown to significantly reduce fiber diameter variance.
4.4 Current fluctuation
The range of fiber diameter distribution may be related to the stability of the electrospinning solution at the tip. An indirect method of monitoring the spinning process stability is through the monitoring of the fluctuation of the current. When the fluctuation of the current is low, there is a corresponding reduction in the fiber diameter distribution [Yan and Gevelber 2009].
4.5 Using electroblowing or sheath gas
Electroblowing uses a complimentary external force application through the use of a gas jet surrounding the electrospinning nozzle. In a multiple nozzle electrospinning setup, the use of electroblowing helps to reduce the differences in electrospinning conditions of the jets coming from the inner nozzles and the outer nozzles. In multiple nozzle electrospinning setup, the inner nozzles experience a reduced electric field strength, reduced traveling distance of the electrospinning jet and higher residual charges at the center collector region. These will result in higher diameters in the fibers collected in the center compared to the edges. The addition of sheath gas helps to reduce the difference in the electrospinning conditions by providing a uniform drawing force from the sheath gas across all the nozzles, dispersing the residual charges from the collected fibers and restricting the area which the electrospinning jet can travel across all the nozzles. Studies by Zheng et al showed that a higher sheath gas pressure significantly reduces the differences in the fiber diameters from the fibers collected at the center and the edges.

4.6 Insulated-Cover for free-surface/slit/bowl electrospinning
The use of a cover for electrospinning off the edge of a bowl is more beneficial than just reduction of solvent lost through evaporation. By having a cover, the edge of the cover and the bowl effectively forms a slit dispenser. Xiong et al (2021) further refined the slit dispenser design by isolating the charge concentration to just a single edge by using a conductive metal bowl and a polytetrafluoroethylene (PTFE) mushroom-shaped cover to create a slit between the cover and the edge of the bowl. When compared with an uncovered metallic bowl setup, a lower voltage is required for jet initiation for the (PTFE) mushroom-shaped covered bowl. More jets were also formed on the mushroom-shaped covered bowl at the same voltage. The deviation of the electrospun polyacrylonitrile (PAN) fiber diameter was also reduced from a coefficient of variation of 20% to 10% for uncovered and covered bowls respectively. The size of the mushroom cover was also found to influence the charge concentration with a larger mushroom cover resulting in higher charge concentration at the edge of the metallic bowl.

Slit Dispenser
The distribution of electric field intensity of the mushroom-spinneret from (a) the front view and (b) the top view, the inset picture is the corresponding three-dimensional partial enlarged diagram. The distribution of electric field intensity of the metallic slit spinneret from (c) the front view and (d) the top view, the inset picture is the corresponding three-dimensional partial enlarged diagram. (e) Comparison of the distribution of electric field intensity of the mushroom-spinneret and the metallic slit spinneret along the same path in the diameter direction [Xiong et al 2021].

5. Uniformity of fiber mesh
- 5.1 Increase deposition area
- 5.2 Reducing jet repulsion in multiple spinnerets
  Apart from repulsion caused by the charges on the electrospinning jet, another source of repulsion when multiple spinnerets are used is the field lines emitted from the individual spinneret which is stronger than the surface charges of the spinning jet. Kumar et al (2010) uses a plastic spinning head with multiple spinning holes to reduce the electric field density from each outlet [Kumar et al 2010]. A more uniform fiber distribution was obtained compared to multiple spinneret configurations.
- 5.3 Reduce residual-charge concentration on collector surface
  Another method of increasing the uniformity of fiber deposition is to use a neutralizing charge. With electrospinning to deposit fibers on a frame, electrospraying of an opposing charged solvent can be used on the other side to neutralize the charges on the deposited fibers [Morozov et al 2007]. A charge neutralizer may be used to remove the residual charges on the electrospun mat and this has been demonstrated to reduce the inter-jet repulsion [Tsai et al 2003] thereby increasing fiber membrane uniformity.
  Hypothesis: An AC power supply may be used to spin the fiber instead so that the presence of both positive and negative charges can have a mutual neutralizing on the fabricated membrane.
- 5.4 Varying Electric Field
  Heterogeneous distribution of electrospun fibers in a multiple electrospinning jets scenario may be attributed to the concentration of electric field at various locations. To create a more uniform fiber layer, one means is to disrupt this electric field concentration profile. Chen et al {2019} disrupts these region of electric field concentration by changing the position of the region with the greatest electric field strength. In their setup, a ring electrode close to the spinneret was used to exert an electric field on the spinneret. By incorporating a louver mechanism to the ring electrode, the electric field strength and profile can be changed such that the electrospinning jet gets redirected. This way, they were able to construct a more uniform layer of electrospun fibers.
- 5.5 Collector movement
  Fabricating a uniform thickness electrospun membrane is a challenge for both mass production setup and single nozzle laboratory setup. Methods to overcome this issue from a single nozzle laboratory setup may be adapted for mass production setups. Pathalamuthu et al (2019) describes a spirograph-based collector movement for collection of electrospun polyacrylonitrile (PAN) fibres. The spiropath path comprises of multiple circular movement passing through a point centre. Movement speed of the collector need to be kept below 300 rpm or the air flow generated by the collector movement would disrupt the depositing jet. On a static collector, the coefficient of variation was 12.5%. However, the coefficient of variation with the spirograph-based collector movement was 2.4%. This demonstrated a significant improvement in electrospun mat thickness uniformity. In most electrospinning mass production setup, the fibers are deposited on a rolling belt. However, if the fiber deposition point on the collector can be varied throughout production as in the case of the spirograph path, a membrane with a more uniform thickness can be expected.

Video of FLUIDNATEK LE-100 electrospinning unit with integrated membrane thickness monitoring system.

6. Power Management

6.1 Reduction of power consumption in nozzle electrospinning
Reduction in the power consumption of the electrospinning process can be achieved by modifying the nozzle design. For initiation of electrospinning jet, energy is expended to overcome the surface tension and capillary action of the solution. Kang et al (2020) showed that by inserting a solid Teflon needle in the nozzle, they were able to reduce the power needed for electrospinning. To further reduce energy loss to the environment, much of the metal nozzle was covered with epoxy resin except a short section at the tip for connecting to incoming power. Using polyvinylpyrrolidone (PVP) as the model solution for testing the nozzle design, they found that having a solid Teflon needle in the nozzle for electrospinning and insulation of the tip, only uses 54% of the power of conventional metal nozzles. This was calculated from the reduced voltage required for steady electrospinning and lower current. Such power reduction has been attributed to the hydrophobic Teflon inner rod that reduces the capillary counter force and the concentration of the applied charges to the solution at the tip. Evidence of more efficient charging of the electrospinning jet can be seen by a longer stable jet phase and larger spiraling radius of the electrospinning jet with the modified nozzle.

Conceptualization of the concentric spinneret with a solid Teflon-core rod and a traditional stainless steel needle utilized as the control. [Kang et al 2020)]

Published date: 19 March 2013
Last updated: 14 September 2021

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