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Electrospn 1 ramakrishna-full

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Electrospn 1 ramakrishna-full

  1. 1. The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey 1 SPEED BREAKERS FOR THE NEXT PHASE OF WORLDWIDE GROWTH OF ELECTROSPINNING & ELECTROSPRAYING D. PLISZKA1,2 , A. GÓRA2,3 and S. RAMAKRISHNA2,3,4 1 NUS Nanoscience & Nanotechnology Initiative, 117581, Singapore. 2 Center for Nanofibers and Nanotechnology, NUSCNN, National University of Singapore. 3 Department of Mechanical Engineering, National University of Singapore. 4 Department of Bioengineering, National University of Singapore seeram@nus.edu.sg Abstract: Due to their cost effectiveness and relative simplicity of the process they are widely researched worldwide providing results showing immense potential applications in various domains like environmental engineering, energy applications and bioengineering. Nanomaterials obtained by these techniques are attracting growing interest from the industry side to adopt them to the applications where their unique properties give them advantage over currently used solutions. Scaling up of the electrospinning and electrospraying for the mass production of functional nanomaterials has met numerous obstacles, to provide results consistent with experimental scale of the production. Here we present and discuss problems related with scaling up of the electrospinning and electrospraying processes for mass production as well as existing and possible solutions allowing solving the problems and making electrospun nanofibers based materials to be fully applicable. Keywords: Electrospinning, electrospraying, needle-less electrospinning, mass production 1. Introduction Electrospinning is a simple and cost effective process for the fabrication of the polymeric fibers with diameters in a micro and nanometer range by using an electrostatic field (Figure 1). Despite the fact it was patented in 1902 by J.F. Cooley its wider applications started in 1990s [1]. It is the only nanotechnology method for the production of continuous fibers at nanometer range [2]. Electrospun materials have proven applicability in microelectronics, energy technology, environmental engineering and bioengineering [2-6]. Electrospraying is a process of creation particles in micron and nanometer diameter ranges by using electrostatic atomization and dispersion of the nanoparticles colloidal suspensions or polymeric solutions [7]. Attractiveness of electrospraying as method of production and coating surfaces is their nanometers sizes range. Comparing to the other atomization techniques resulting particles sizes are significantly smaller due to effect of the continuous dispersion of the droplets, after leaving the nozzle, driven by electrostatic forces. Technically both electrospinning and electrospraying are using the same setup, and basic difference is properties of the processed solution. Electrospraying solution has smaller viscosity, so that continuous fibers are not emerging but initial jet is disrupted into droplets which due to electrostatic forces are further disrupted decreasing their dimensions.
  2. 2. The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey 2 Figure 1. Schematic of the electrospinning and electrospraying setup. Attractiveness of the electrospun and electrosprayed materials, as well as their composites [8] proven in the experimental works, created a need for scaling up manufacturing process for full scale industrial applications. One of the biggest drawbacks of the electrospinning is relatively low rate of production large mass or volume of the nanofibers. Earliest approaches were involving most obvious ideas like increasing of the spinneret numbers, but soon it has been found that there is a limit in the density of electrospinning jets that could be packed together, and problems with it maintenance. Electrostatic field interactions from different spinnerets may cause suppression of the jet electrospinning from the innermost nozzles due to decrease of the electric field intensity below the threshold, where electrostatic force overcomes solution viscosity and nanofibers emerging. In this case we will get electrospun fibers from the external spinnerets while from innermost polymeric solution will just drip [9]. Other approach is elimination of the spinneret or nozzles from the electrospinning set-up what leaded to development of the needleless electrospinning or free-surface electrospinning. It basic principle was coating of electrode with layer of solution or creating electrified free surface of polymeric solution. Once surface of solution is sufficiently charged electrospinning jets will self-emerge towards the collector. Apparently, in spite of the efforts of largest industry players, electrospinning and electrospraying are not yet widely used in the industry. Two main factors was recognized as the most apparent brakers. One is increasing of the production rate of the nanofibers allowing mass production of the membranes. Other important factor not widely discussed is durability of the deposited nanofibrous membranes and their attachment strength to the backing material. This factor is particularly important when deposited membranes are undergoing further fabrications steps. Below we are giving an overview of the current approaches in the mass manufacturing of the nanofibers with their drawbacks, problems related and possible solutions. 2. Multiple nozzles Standard laboratory based electrospinning setup is a needle connected to the high voltage power supply used to dispense and to apply charges to the electrospinning solution and grounded collector (Figure 1). Important advantage of the use of single spinnerets is good control and uniformity of the nanofibers diameters and relatively low voltage required. Unique feature of the nozzle based electrospinning is the possibility of fabricating fibers with different configurations like: core-shell or multi component materials. To increase the output of the setup, more needles can be used. However, the configuration of the needle arrangement needs to be optimized to reduce charge interference between them. Besides mentioned above supression of the electrospinning action, charged jets emerging from the multiple nozzles due to repulsive forces between them are deflected outer wards and inner jets are suppressed creating pattern of the separated nanofibers spots (Figure 2) [10]. That creates problems related with uniformity of the resulting membrane, what in turns for the mass production of the reliable membranes has a crucial importance. This
  3. 3. The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey 3 adverse effect may be eliminated or significantly decreased by using additional electrodes behind the spinnerets tip changing electric field distribution and causing overlapping of the deposition spots from the particular nozzles. Figure 2. Spots of the multi-needle electrospinning: a) without additional electrodes, b) with ring electrode placed behind the nozzles tips [10]. Mass production of the nanofibers requires stability and continuity of the process. For nozzle based electrospinning serious drawback is clogging of the orifices due to gelation of the solution. One method to reduce or eliminate clogging is by heating the spinneret tip [11]. Other solution is using higher boiling point solvent like DMF, what may have limited applicability due to polymer solution optimal composition. Larsen et al. [12] studied using coaxial spinneret with pressurized inert gas in the outer orifice to eliminate the clogging. Other but very problematic option for practical use due to disruption of the electrospinning process is design and use of the physical device to remove clogs periodically during nanofibers manufacturing. 2.1 Porous surface electrode Multiple orifices may be obtained by use of porous surfaces. The solution dispenser may come in the form of a cylinder with porous surface which solution may flow through the wall and to the outer surface [13]. Advantage of this setup is introducing many orifices for multiple electrospinning jet, it also have several critical disadvantages. Cleaning of clogged pores between electrospinning runs will be much more difficult although it can still be carried out by flushing with large amount of solvents. Uniform feed rate is difficult control – its excess will create a solution layer on the surface making this method more like free surface electrospinning. Other disadvantage is higher voltage applied than in case of spinneret based electrospinning Figure 3. Close-up photo of conical drops formed during electrospinning at the locations of the drilled holes on the porous wall tube manifold [13]. 3. Sharp edges needleless electrodes It has been found that surface with higher curvature (sharp features) will have a higher charge density of the electric field at these spots, increasing the probability of generating sufficient repulsive force to initiate the electrospinning process. Using this concept sharp point or edges needleless designs have was investigated a) b)
  4. 4. The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey 4 [14]. This designs typically consists of spikes [15], sharp edges [16] or ridges [17] rising just above the surface of a reservoir of solution or disks [18, 19] dipped in the polymer solution. Advantages of this approaches is easy maintenance of the setup without clogging of the eliminated orifices and self-organization of the spacing between large numbers of self-emerging electrospinning jets. Less specified electric field concentration points than in the needle based electrospinning this method requires much higher voltages applied. Disadvantage of this approach is worse nanofibers diameter distribution control than in needle based approaches with well controlled solution flow rate. Due to free emerging of the jets amount of the polymer solution available at these points is random and basically roughly controllable only by the polymer solution coating on the surface of the electrodes. Other important drawback is difficulty with maintaining consistent polymeric solution concentration and viscosity. Due to exposure of the free surface solution to the air it changes its properties and to maintain consistent nanofibers quality it is required to replenish the solution continuously. This leads to the large polymer solution wastage increasing significantly manufacturing costs. It is worth to note that over the time polymeric solution may solidify at the electrodes edges or tips of the spikes stopping the electrospinning process. Figure 3. Needleless disk (left) and spiral coil (right) electrode electrospinning process [14]. Interesting variant of this approach is use as an electrode a thin wire [20]. A constant supply of solution can be coated on the wire through an applicator or by rotating a coil of wire through a solution reservoir or applicator moving along the wire. The advantage of the second solution application method is that any solidified polymer can be cleaned off the wire through the sliding motion of the applicator. However, a drawback is that the spinning is periodically disrupted. It has also an advantage in non-changing in time properties of the polymeric solution by lack of exposure to air. Figure 4. Schematic of the wire based needleless electrospinning process [21].
  5. 5. The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey 5 4. Free surface needleless electrospinning The main difference between free surface needleless electrospinning approach and previously discussed is that there are no physical structures to create charge concentration and initialize electrospinning process. Without any preferred spots from which electrospinning jets may emerge their distribution is depending on their self-organization and localized charges distribution on the surface of the polymeric solution. This implies the need for applying higher voltage to initialize process. There are several variants of the free surface needleless electrospinning designs and methods of initialization of the electrospinning jets. Rotating drum with the surface constantly replenished with a thin layer of the solution is one of the most effective methods of electrospinning using charged free surface [14]. The electrospinning jets emerging from the drum surface are self-organizing when approaching the top surface, closest to the counter-electrode. Optional method of the polymeric solution deposition on the drum is use of droplets of solution deposited on the drum surface. With every droplet of the solution reaching to the drum new electrospinning jet would emerge [22]. Figure 4. Free surface needleless electrospinning with rotating drum [14]. 5. Summary Electrospinning has been widely used in laboratories to fabricate nanofibers due to its relatively ease of setting up. Wide range of electrospun materials and their performance in numerous applications create a need to scale up the process. However, this relative simple method presents several serious challenges when scaling up to mass produce nanofibers. Each presented approach has its advantages and drawbacks related with the specificity of the design. Taking into account characteristics of the mass production of the nanofibrous membranes most likely application domain would be, where there is no need for detachment membranes form backing material. These applications may be addressed to as environmental engineering in water and air filtration as well as electronics and catalytic. Other applications, like bioengineering in most of the cases require just nanofibrous membrane with thickness of few hundred microns. Detachment of the membrane from the backing support would create complications and problems with stability of the process in mass production. Other potential applications area is materials engineering with nanofibrous based membranes as strength enhancements for the composite materials Mass production of the electrospun nanofibers creates also challenges non-directly related with the design. Due to large number of the electrospinning jets and greatly enhancement of evaporation of the solvent it is required to provide effective ventilation system to avoid accidental ignition of the fumes and it recycling what may significantly reduce production cost. Important becomes also development of the quality control methods of the process to avoid situation when unaware to the user process has been interrupted. Acknowledgements This research is supported by the Singapore National Research Foundation under CREATE programme: The Regenerative Medicine Initiative in Cardiac Restoration Therapy (NRF-Technion).
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