Spider silk is well known for being incredibly strong, possessing ten times the tensile strength of steel wire and the ability to stretch up to four times its original length without damage. A strand a millimetre thick can lift a five kilogram mass, and it retains these properties even at minus forty degrees Celsius. Now, scientists in the national High Magnetic Field Laboratory in Tallahassee have managed to increase these properties further by adding carbon nanotubes to the strands.
Carbon nanotubes are miniscule cylinders of carbon molecules, the same thickness as DNA, but many times stronger than steel, and with excellent conductive properties for electricity and heat. Between these materials, the new mixture combines to one greater than the sum of its parts.
Taking the silk produced by the Golden Orb Web Spider (Nephila clavipes), which is the strongest known spider silk produced, the researchers found that the most effective method for covering the strands was to add a small quantity of water to a dry powder of carbon nanotubes, rubbed together between sheets of Teflon. The resultant coating over the silk was almost perfectly uniform at around 100 nanometres thick, with Scanning Electron Microscope (SEM) images showing that some nanotubes were even permeating into the silk itself. The nanotube coated silk was over triple the strength of the spider silk alone, and maintained its conductive properties even when stretched to twice its length.
The strength and flexibility of carbon nanotube spider silk (CNT SS) makes it of great interest to researchers in prosthetic development and in medical technology, for the production of artificial muscles or synthetic skin. There is a further property of CNT SS of novel use – the electrical conductivity changes based on humidity as well as physical strain. Using these properties, the team were able to develop a heart pulse sensor with the prototype strands they manufactured. By increasing the thickness of the carbon layering, the conductivity was preserved at the expense of reduced elasticity. As experimentation with different thicknesses and nanostructures continues, spiders may come to spin more than webs in times to come.