Reconstituted silk can be printed into tough, unnatural structures
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Semi-artificial silk many times stronger than natural silk has been developed at Massachusetts Institute of Technology (MIT). This material could have a range of commercial and biomedical applications.
Raw silk fibre is produced by some insects and other creatures. While silkworms and spiders are best known for their silk production, the larval forms of bees, ants, wasps and fleas also produce silk as they form cocoons.
Silk cloth was first developed in ancient China. Since then, humans have taken steps to produce greater quantities of silk or better quality silk, such as by breeding the domestic silkworm, genetically modifying silkworms such that they produce silk with particularly desirable qualities, or feeding silkworms an unusual diet to change the properties of their silk.
Silk, being very smooth, shiny, and among the strongest natural fibres, is a highly valuable material. While spider silk is renowned for its even greater strength, spiders cannot be made to produce silk in the volume that silkworms can, and no artificial version of silk can match the strength of silkworm or spider silk.
However, researchers based at MIT have now created a reconstituted silk – “regenerated silk fibre” (RSF) – which is many times stronger than natural silkworm silk.
They began by dissolving the cocoons built by silkworms with chemical processes, stopping just before the molecular structure of the silk breaks down. At this point, the material is composed of microfibrils: thread-like structures which retain some of the properties that give silk its strength. The researchers compared this to retaining bricks while demolishing a house, in order to use again.
“We’re not satisfied with what [silkworms] make,” said Professor Markus Buehler, who is head of the department of civil and environmental engineering at MIT. “We want to make our own new materials.”
“Nature is still better at making the microstructures […] in this case, we take advantage of what nature provides.”
The researchers extruded the dissolved material through a tiny opening, allowing them to produce a fibre twice as stiff as natural silk, and almost as stiff as spider dragline silk. This allowed Professor Buehler and his colleagues essentially to 3D-print the material to form silk tubes, meshes, thick fibres, coils, sheets and other shapes. Adjusting the speed at which the silk is extruded allows for the stiffness and toughness of the silk structures to be controlled.
As well as being stronger and more versatile than natural silk, RSF is sensitive to humidity, and by adding a layer of other material – such as carbon nanotubes – can be made electrically conductive. This could render RSF a useful and sensitive constituent of sensors. For instance, Professor Buehler suggests that this material could be worked into smart bedsheets, capable of monitoring the sedentary behaviour of bedridden patients in order to lower the risk of bedsores.
As well as sensors, RSF could be used as the basis for surgical tools such as sutures or scaffolding for the growth of new tissue, given that – like natural silk – it is biocompatible.