“Unfortunately, silk cannot be obtained in large quantities from spiders,” says team leader Daniel Söderberg, “and recombinant production processes are still rather expensive. We have thus combined small amounts of functionalized spider-silk proteins with the most abundant structural component on Earth, cellulose nanofibrils (CNFs), to fabricate isotropic and anisotropic hierarchical structures.”

The composites produced have excellent hitherto unseen mechanical properties, including a stiffness of around 55 GPa, strength at break of around 1015 MPa and a toughness of 55 MJ/m3. Adding small amounts of silk fusion proteins to the CNF results in materials with advanced biofunctions that cannot be achieved in CNFs on their own. Such bio-based materials could be used to design novel, inexpensive and lightweight architectures with good mechanical and bio-properties, say the researchers.

Well-controlled process required

Söderberg and colleagues made their composites by adding small amounts of recombinant spider silk proteins to a nanocellulose dispersion. One concept was to form sheets from the dispersion (giving an isotropic structure – that is, in the plane of the sheet) through casting. But by processing the dispersion into filaments/fibres using a flow-focusing technique, making use of hydrodynamics, they created a material with a highly aligned internal structure on the micro- and nanoscales.

“We succeeded in retaining the mechanical performance of nanocellulose (strength and stiffness) and the bio-properties of the recombinant silk protein,” explains Söderberg. “Our work shows that this cannot be achieved by simply mixing the components but requires a well-controlled process to succeed.”

Material is biocompatible

The composite could be used as a biomaterial, given that the human body is inert to cellulose, he tells nanotechweb.org. “The spider-silk proteins, for their part, could provide functionality such as promoting cell growth and a very interesting possibility would be to try and use the fabricated filament materials as a building block for ligaments.”

The team, reporting its work in ACS Nano DOI: 10.1021/acsnano.7b02305, says that it is now looking to scale up its fabrication process and find real-world applications for the composite it has made.