Selected Review Articles

• Wee-Eong Teo, Ryuji Inai and Seeram Ramakrishna. (2011) Technological advances in electrospinning of nanofibers. Sci. Technol. Adv. Mater. vol 12 pg. 013002. (Downloaded over 500 times within 2 months. Top 3% download in ALL IOP journals in first quarter of 2011) Download
• W.E. Teo, S. Ramakrishna (2009) Electrospun nanofibers as a platform for multifunctional, hierarchically organized Nanocomposite. Composites Science and Technology vol. 69 pg. 1804-1817.
• W.E. Teo and S. Ramakrishna (2006) A Review on Electrospinning Design and Nanofibre Assemblies. Nanotechnology vol. 17 pg. 89-106. (Downloaded over 250 times within a month; Top four most assessed articles in category of Materials: synthesis or self-assembly in the year 2006 in Nanotechnology journal)

Setup for constructing continuous twisted nanofibrous yarn

Published work:

1. Maryam Yousefzadeh, Masoud Latifi, Wee-Eong Teo, Mohammad Amani-Tehran, Seeram Ramakrishna (2011) Producing Continuous Twisted Yarn from Well-Aligned Nanofibers by Water Vortex. Polymer Engineering & Science vol 51 pg. 323-329. Link to abstract
2. Read on Plastics Research Online

Overview:

Generally, individual strand of yarn is usually made out of twisted fibers to give it strength and integrity. While various methods of fabricating continuous nanofibrous yarn using electrospinning has been proposed, the fibers in the yarn are generally untwisted. With the development of dynamic fluid flow as a method to modify nanofiber arrangement to form various nanofibrous structures, this method has been applied to fabricate continuous twisted nanofibrous yarn

To create a twist, one point of the fiber must be rotated relative to another point along the principal axis of the fiber. As nanofibers are relatively weak, water with its high surface tension is able to provide sufficient holding force to twist the fiber relative to a fixed point without breaking the yarn. To create a twisting effect, the water must be circulating about the point where the yarn is going to be drawn. The circulating water can be created using a vortex flowing down a central hole in the basin.

As the fibers are deposited at one corner of the basin, the flowing water down the vortex would draw the fibers toward it. The circulation of the water is greatest at the edge of the vortex and this is where the yarn would be drawn off the surface of the water. As the yarn is guided through a stationary guide, the circulating water below would provide the necessary twisting force on the yarn.

Schematic of the electrospinning setup

Twisted nanofiber yarn with high degree of fibers alignment, at (a) 3 m/min and (b) 4.5 m/min take up velocities.(Maryam Yousefzadeh et al. Polym. Eng. Sci. 2010; © John Wiley & Sons, Inc.).

See related work

1. Wee-Eong Teo, Renuga Gopal, Ramakrishnan Ramaseshan Kazutoshi Fujihara, Seeram Ramakrishna (2007) A dynamic liquid support system for continuous electrospun yarn fabrication. Polymer vol. 48 pg. 3400-3405.

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Setup for constructing 3D nanofibrous yarn structure

Published work:

1. W.E Teo, S. Liao, C.K. Chan, S. Ramakrishna (2008) Remodeling of Three-dimensional Hierarchically Organized Nanofibrous Assemblies. Current Nanoscience vol. 4 pg. 361-369. Download
2. Wee Eong Teo, Kazutoshi Fujihara, Casey Kwan-Ho Chan, Seeram Ramakrishna. Fiber Structures and Process for their preparation. WO 2008/036051. 27 March 2008. Singapore Patent No. 150798, 31 March 2010. Download
3. Wee Eong Teo. Natured Inspired Composite Nanofibers. 2009. Link to technology disclosure

Overview:

Fabricating a nanofibrous block with comparable length, width and height poses a significant challenge to the electrospinning process. In the electrospinning process, nanofibers are deposited in layers as they settle on top of one another. Due to the nano-dimension, it would take a very long time for the nanofibers to build up to a thick layer. Moreover, all the nanofibers deposited in this manner would have its length parallel to the plane of the flat collector. This would mean that the pore size through the thickness of the nanofibrous membrane would be significantly smaller to the pore sizes as seen from above. Therefore, the electrospinning setup needs to be modified such that the orientation of the nanofibers are not restricted to a single plane.

Using the dynamic fluid setup for yarn fabrication, a three-dimensional nanofibrous structure can be fabricated. Instead of collecting the yarn off a rotating drum, the consolidated nanofibrous yarn is allowed to fall into a basin of water below the reservoir above as shown in the figure below. Over time, clumps of nanofibrous yarn would be collected in the water basin. These can then be gathered and free-dried to give a block of nanofiber clump.

Schematic of the electrospinning setup

Left: Freeze-dried nanofibrous 3D structure. Right: View of the nanofibrous arrangement under SEM (W.E. Teo et al. Current Nanoscience 2008; 4: 361. © BSP).

See related work

1. Wee-Eong Teo, Renuga Gopal, Ramakrishnan Ramaseshan Kazutoshi Fujihara, Seeram Ramakrishna (2007) A dynamic liquid support system for continuous electrospun yarn fabrication. Polymer vol. 48 pg. 3400-3405.

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Single nanofiber layer membrane and 3D nanofibrous structure

Published work:

1. Wee Eong Teo, Kazutoshi Fujihara, Casey Kwan-Ho Chan, Seeram Ramakrishna. Fiber Structures and Process for their preparation. WO 2008/036051. 27 March 2008. Singapore Patent No. 150798, 31 March 2010. Link to technology disclosure

Overview:

Through modification of the interaction between the water flow and the deposited nanofibers, unique nanofibrous structures can be fabricated. The figure below shows an inclined water flow setup used for collecting nanofibers.

Schematic of the electrospinning setup.

In this setup, the nanofiber would be washed down the incline as soon as it hit the surface of the water. With sufficient water flow speed, none of the collected nanofiber will overlap with the preceding nanofiber thereby creating a unique nanofibrous structure where the membrane consists of just a single layer of nanofiber as it collects in a relatively stagnant pool of water at the base. The single nanofiber layer may be lifted off the surface of the water by placing a plate under it.

If a three-dimensional structure is preferred, the nanofibrous membrane may be picked off the water surface and the weight of the water on the membrane would just cause it to collapse into a clump. The clump of nanofibers may then be freeze-dried to give a block of nanofibers.

See related work

1. Wee-Eong Teo, Renuga Gopal, Ramakrishnan Ramaseshan Kazutoshi Fujihara, Seeram Ramakrishna (2007) A dynamic liquid support system for continuous electrospun yarn fabrication. Polymer vol. 48 pg. 3400-3405.

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Continuous nanofibrous yarn using dynamic liquid support system

Published work:

1. Wee-Eong Teo, Renuga Gopal, Ramakrishnan Ramaseshan Kazutoshi Fujihara, Seeram Ramakrishna (2007) A dynamic liquid support system for continuous electrospun yarn fabrication. Polymer vol. 48 pg. 3400-3405.
2. Wee Eong Teo, Kazutoshi Fujihara, Seeram Ramakrishna. Method & Apparatus for producing Fiber Yarn. WO 2007/013858 A1. 01 February 2007. Link to technology disclosure
3. Wee Eong Teo. Natured Inspired Composite Nanofibers. 2009. Link to Thesis

Overview:

In a typical electrospinning setup, the nanofibers are deposited on a solid substrate and the resultant nanofibrous structure will be difficult to modify. However, to alter the nanofibrous structure after collection, a dynamic system which would not cause nanofiber breakage will be desirable. With this in mind, a fluid such as water has been identified as a possible substrate for nanofiber collection such that it can be manipulated after deposition.

Key advantages of using water as a working substrate,
1. Nanofibers can potentially be oriented by movement on the water surface
2. Adhesion between the nanofibers and the water surface is sufficiently weak such that the fiber integrity can be maintained when it is being shifted by the water.

Using a dynamic fluid system, continuous nanofibrous yarn can be fabricated. The first criteria of the setup is that it should have sufficiently large surface area to capture the nanofibers. The second criteria is that the fluid which carries the nanofibers would have a flow profile that draws the nanofiber into a yarn. The setup shown in the figure below demonstrated how these two criteria can be achieved.

Left: Schematic of the electrospinning setup. Right: Mandrel rolling in the yarn.

The open surface of the water reservoir allows electrospun nanofibers to be deposited. At the base of the reservoir, a hole was created such that a vortex can be created by the draining water. The draining water would consolidate and pull the deposited nanofibers to form a yarn. Continuous nanofibrous yarn can be collected using this setup with speed up to 90 m/min. An important note for using this setup is that the rate of fiber deposition on the reservoir surface must be high enough such that the consolidated nanofibrous yarn has sufficient strength to withstand the drawing process.

SEM image of a single strand of yarn. (Wee-Eong Teo et al. Polymer 2007; 48: 3400. © Elsevier doi:10.1016/j.polymer.2007.04.044).

See related work

1. W.E. Teo and S. Ramakrishna (2006) A Review on Electrospinning Design and Nanofibre Assemblies. Nanotechnology vol. 17 pg. 89-106. (Downloaded over 250 times within a month; Top four most assessed articles in category of Materials: synthesis or self-assembly in the year 2006 in Nanotechnology journal)
2. Application of technology as intraluminal guidance channel in peripheral nerve regeneration
   H S Koh, T. Yong, W E Teo, C K Chan, M E Puhaindran, T C Tan, A Lim, B H Lim, S Ramakrishna. (2010) In vivo study on novel nanofibrous intra-luminal guidance channels to promote nerve regeneration. Journal of Neural Engineering vol. 7 pg. 046003 (14pp)

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Aligned Nanofibrous Bundle

Published work:

1. W E Teo and S Ramakrishna. (2005) Electrospun fibre bundle made of aligned nanofibres over two fixed points. Nanotechnology vol. 16 pg.1878 – 1884.

Overview:

While 2-dimensional nanofibrous membrane has been regularly constructed using electrospinning, fabricating nanofibrous bundle or yarn has yet to be reported in detail. Using knowledge of the recent reported use of parallel electrodes to generate aligned nanofibers, a pair of knife-edge electrodes was demonstrated to collect nanofibers aligned from point to point at the tip of the knife-edge as shown in the figure below.

Schematic of the electrospinning setup showing the fibers (red lines) spanning the gap between the electrodes.

However, due to the presence of residual charges on the fibers, they were spread apart in the middle of the gap although the ends were gathered at a point. To bundle the fibers tightly together, it was necessary to dip the fibers in water such that the water surface tension would bring them together.

SEM image of nanofibers bundle showing excellent alignment (Teo W E et al. Nanotechnology 2005; 16: 1878. © Copyright (2005) IOP Publishing Ltd. http://www.iop.org.).

See related work

1. W.E. Teo and S. Ramakrishna (2006) A Review on Electrospinning Design and Nanofibre Assemblies. Nanotechnology vol. 17 pg. 89-106. (Downloaded over 250 times within a month; Top four most assessed articles in category of Materials: synthesis or self-assembly in the year 2006 in Nanotechnology journal)

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Nanofibrous tube with controlled fiber orientation

Published work:

1. W E Teo, M Kotaki, X M Mo and S Ramakrishna. (2005) Porous tubular structures with controlled fibre orientation using a modified electrospinning method. Nanotechnology vol. 16 pg. 918 – 924.
2. Teo W.E., M. Kotaki and S. Ramakrishna. An Apparatus and Method for Producing Nanofibres. Singapore Patent No. 117506. 29-July-2004. National University of Singapore.

Overview:

Electrospinning has been used to create nanofibrous tubular scaffolds for biomedical applications such as vascular graft and nerve conduits. A functional vascular graft for example would require appriopriate mechanical properties which would depend on factors such as material used, post-electrospinning modifications (such as cross-linking) and nanofibers orientation. Due to the difficulty in controlling the electrospinning jet, most tubular scaffolds are made out of randomly organized nanofibers. Force distribution through the nanofibers will be limited as a result.

In this work, the electrospinning jet can be sufficiently controlled through the use of guidance electrodes to construct a tubular scaffold out of nanofibers aligned in the oblique direction. The schematic of the setup is shown in the figure below.

Schematic of electrospinning setup.

As the flight of the electrospinning jet follows the electric field line between the electrodes, the rotating rod between the electrodes would wind the fibers in an oblique orientation.

Portion of the nanofibrous tube showing oblique fiber alignment (Teo W E et al. Nanotechnology 2005; 16: 918. © Copyright (2005) IOP Publishing Ltd. http://www.iop.org.).

See related work

1. W.E. Teo and S. Ramakrishna (2006) A Review on Electrospinning Design and Nanofibre Assemblies. Nanotechnology vol. 17 pg. 89-106. (Downloaded over 250 times within a month; Top four most assessed articles in category of Materials: synthesis or self-assembly in the year 2006 in Nanotechnology journal)

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