Nanofibres, which can be used in a variety of applications including biotechnology, nanocomposites and nanodevices, are usually produced by techniques such as electrospinning, lithography and molecular self-assembly. However, certain biological materials also contain nanofibres. For example, spider and silkworm silks contain tens of thousands of nanofibres that are about 30 nm across, and Feng and colleagues have succeeded in extracting these using high-frequency ultrasound at 20 kHz.

The researchers began by immersing about 0.05 g of spider silk in around 100 ml of water. Next, they placed the mixture in an ordinary, off-the-shelf, sonifier apparatus containing an ultrasonic probe tip and left it for up to 45 minutes at powers ranging from 900 to 1000 W. After sonolysis, the team cooled the resulting mixture to room temperature and collected the fibres, which had gathered at the bottom of the vessel. The method also works for other types of fibres, including silkworm fibres, collagen from fish scales, chitin fibres from shrimps and crabs, and cellulose fibres of cotton, bamboo, wood and hemp.

Using scanning electron microscopy, Feng and co-workers observed that the natural materials had disassembled into fibres with diameters between 25 and 60 nm (figures 1 & 2). Moreover, the researchers found they could tune the size of the fibres from 20 to 100 nm by changing the duration and power of the ultrasound.

The team believes the nanofibres are produced by acoustic cavitation effects. High-frequency sound waves create microbubbles in solution that grow and then violently collapse. This collapse induces microjets and shock waves on the surfaces of the natural fibres so that they split away along their lengths. The sound waves effectively break the relatively weak interfaces among the nanofibres, which are bound together by interactions such as Van der Waals forces. The rate at which the fibres disassemble depends on the intensity and frequency of the ultrasonic waves – in general, the higher the intensity, the faster the process.

The researchers say that the new technique is a cost-effective, simple technique for producing bionanofibres from nature and could easily be scaled up for mass production by simply increasing the number of ultrasonic probes. The resulting nanofibres have many advantages over artificially produced fibres and are strong, flexible, biocompatible and biodegradable.

The work was published in Appl. Phys. Lett..