Spinning a yarn, nano-style - more affordable fibres
A novel electrospinning system could make nanofibres more affordable, says Luis Velasquez Garcia, principal research scientist at MIT’s Microsystems Technology Laboratories.
Nanofibres are generated using electrospinning, a technique that involves applying high electric fields to a liquid plastic, generating a cone shape. A thin, liquid jet is then ejected from the tip of the cone, which dries as it advances, resulting in a thin, continuous fibre. These fibres are desirable for technological progress because they can increase surface area in certain electrical devices such as batteries, improving their efficiency.
Until now, nanofibres have been too expensive to make on a commercial scale, says Garcia, because of a limit in the drawing speed of the fibre. However, by using arrays of electrospinning emitters constructed using chip fabrication techniques, he claims to have found a method that could develop the technology for high volume nanofibre production by lowering the cost.
The method involves microfabricating the arrays from silicon wafers and immersing them in a fluid containing a dissolved plastic, polyethylene oxide (PEO), in a water/ethanol mixture. This results in a fibre that measures 22nm across. The cones used in the experiment improve on existing methods, says Garcia, which result in coarser fibres that do not have the same emitter density. ‘The [current methods] need more power and voltage to operate, and they cannot control the texture of the emitters the way we do, which feeds the liquid to the emitter tips.’
The researchers took inspiration from some of the fabrication processes used by the microchip industry. Garcia continues, ‘These techniques are used in the chip industry because they can be used to define very small features, are highly controllable, and have high throughput. The processing is done in parallel across the wafer, they are compatible with CMOS processing and are very sensitive to contamination.’
The new technique can produce a variety of lengths and sizes of fibres depending on the liquid. ‘Based on our results, nanofibre sources with as many as 100 emitters in one square centimetre should be straightforward to implement,’ says Garcia. ‘We typically get 100–200nm fibres using PEO dissolved in a mix of alcohols, but thinner and thicker fibres can be made by varying the liquid concentration. You need long polymer chains and a minimum concentration to create nanofibres using the electrospinning technique, but there is a range of materials, concentrations and extraction conditions that would produce nanofibres.’
The research could be used for a range of applications. The obvious benefits are to improve the performance of many energy devices, such as fuel cells, supercapacitors, and electrodes for batteries, by increasing their surface-to-volume ratio, but there are also many possibilities beyond energy uses, says Garcia. ‘Electrospun nanofibres can be biodegradable and used in tissue engineering. You could make yarns of nanofibres, and use them to produce textiles with enhanced properties, or to reinforce materials (i.e., nanostructured composites), which should represent important savings in weight. The nanofibres could also be used in flexible electronics.’
In the context of current concern over nanoparticle exposure potentially posing a danger to humans, Garcia admitted he was unsure about toxicity, but pointed out that it would depend on the material the nanofibres were made from, and that ‘the research community and the government should continue monitoring the matter and introduce modifications once new facts are established’.