21 April 2006
21 April 2006
Researchers at University of California, Berkeley, have found a way to use the electric-field process to make nanofibres in a direct, continuous and controllable manner.
The new technique, known as near-field electrospinning, offers the possibility of producing new, specialized materials out of nanofibres, with organized patterns that can be used for such applications as wound dressings, filtrations and bio-scaffolds.
Electrospinning was first patented in 1934, when scientists learned how to eject a thin stream of polymer mixed with a solvent out of a syringe into a charged field. As the solvent evaporates, electric forces pull at the polymer, accelerating and elongating it into a long fibre that forms a matted pile on a charged screen 10 to 30 centimetres away.
In the mid-1990s, the emerging field of nanotechnology rekindled interest in electrospinning. Since then, scientists have spun more than 100 synthetic and natural polymers into fibres with diameters ranging from tens of nanometres to a few microns..
When Daoheng Sun, a professor of mechanical and electrical engineering from China's Xiamen University came to Liwei Lin's laboratory at UC Berkeley for two years with the Berkeley Scholars Program in 2004, he looked around for a suitable research project. He and Lin, a professor of mechanical engineering, came up with the idea of trying to tame the electrospinning process to make orderly arrays of fibres.
""I'd been doing work with nanotechnology, but nothing on electrospinning before then,"" Lin said. ""We were really outsiders in the field, so we didn't have any preconceived notions. We just tried things that others may have never thought about. And in the end, they worked just fine.""
What they attained with their innovations are fibres ranging from 50 to 500 nanometers in diameter that are deposited onto a collector plate in a directed, controlled manner. In reference to the shortened distance between the ejector and collection points that it used, the team named the new process ""near-field electrospinning.""
""Conventional electrospinning is a random, chaotic process,"" Lin said. ""Our breakthrough is that we are now able to control fairly precisely the location and deposition of the nanofibres.""
Sun and Lin's method varies in four important ways from the conventional method of electrospinning.
Firstly, instead of applying the polymer solution into the electric field with a syringe, they used a fine-tipped tungsten electrode, which they dipped into the solution like a pen into ink. Then, positioning the electrode above a collection plate, they applied electrical voltage to it, creating the electric field and initiating the process of electrospinning with the tiny drop of polymer on the electrode's tip. This allowed the team to reduce the initial diameter of the polymer stream as it leaves the electrode far below the diameter of the stream produced by the conventional syringe.
Secondly, the researchers shortened the distance the polymer travels in the electric field from the conventional 10 to 30 centimetres to between one-half millimetre and three millimetres. This allowed them to take advantage of the brief period of stability that polymer fibres exhibit when the electrospinning process begins, when the fibres move in a relatively straight line for a brief moment when they enter the electric field. In Sun and Lin's near-field technique, the fibres are captured before their billowing begins.
The shortened distance also meant that Lin and Sun could dramatically reduce the voltage required from 30,000 volts to as low as 600 volts. Because the strength of an electric field is determined by voltage divided by distance, the shorter field maintains the same strength even with less applied voltage.
Finally, rather than using a screen fixed in place to capture the fibres, Sun and Lin let the fibres land on a plate that could be moved in various patterns at various speeds.
Lin said he foresees the possibility of two immediate directions for the new process. One is for device applications that require precise deposition of the nanofibres, such as making nanosensors for biological measurements - a glucose monitor, for instance. The other will be to make non-woven fabrics with organized patterns that can have many applications, such as scaffolds for living cells. Near-field electrospinning may also be useful in nanolithography for making next-generation microchips, Lin predicted. But, he said, this will require more effort to develop.
Lin is currently working on two improvements to the near-field process: an electrode that can provide a continuous supply of polymer and a movable stage with good planar control to capture the fibres.
The Berkeley Scholars Program is a privately funded program founded by the Tang Family Foundation. The program's goal is to enhance scientific and technological standards throughout the world by developing cooperative relationships between the best scholars at the threshold of their careers in China and established research leaders at UC Berkeley. Sun's program was sponsored by the Lee Foundation of Singapore.
The two other authors of this study are Chieh Chang and Sha Li, both graduate students in Lin's UC Berkeley laboratory.