Silk nanostructures have clean room potential

Engineers at Tufts University in the US have shown that it is possible to generate nanostructures from silk in a process that uses water as a developing agent and standard fabrication techniques. 

Accordning to Tufts, this approach provides an alternative to the toxic materials commonly used in nanofabrication – a process at the centre of semiconductor fabrication and manufacture of other electronic and photonic devices – while delivering fabrication quality comparable to conventional synthetic polymers.

A paper describing this work – All Water-based Electron Beam Lithography Using Silk as a Positive, Negative and Biofunctional Resist – appears in Nature Nanotechnology.

‘In a world that strives to reduce toxic footprints associated with manufacturing, our laboratory is exploring biopolymers, and silk in particular, as a candidate material to replace plastics in many high-technology applications,’ said Fiorenzo Omenetto, Ph.D., senior researcher on the work and Frank C. Doble Professor of Biomedical Engineering at Tufts.

Nanofabrication involves high-resolution patterning with features so small that they have at least one dimension no larger than 100nm. 

Nanoscale fabrication is usually obtained depositing thin films of customised polymers, called resists, onto silicon wafers. Each resist layer is successively patterned by using light or electrons (via electron beam lithography) to expose the part of the resist not covered by a mask.  Subsequently, positive resists are dissolved when subjected to a developer while negative resists remain behind after development.  The composition and configuration of the layers determine the properties of the structure.

Developing a resist typically requires toxic chemicals, which need careful, and costly, handling and disposal.  Significant advances have been made using more environmentally friendly resists that can be developed with water, but these techniques have so far lacked the desired precision and scalability. 

‘In contrast, our process is entirely water-based, starting with the silk aqueous solution and ending with simple development of the exposed silk film in water, and the resolution achieved was comparable to one of the commonly used synthetic polymers,’ Omenetto said in a statement. ‘A variety of manufacturing industries, high-tech companies and academic labs could ultimately benefit from clean rooms that are also green.’

For this work, the Tufts engineers fabricated nanoscale photonic lattices using neat silk and functionalised silk doped with quantum dots, green fluorescent proteins (GFPs) or horseradish peroxidase (HRP).

‘By showing that biomolecules of the enzyme HRP remained active after the electron beam nanofabrication process, we demonstrated the feasibility of fabricating biologically active silk sensing devices, something not currently available,’ said Benedetto Marelli, Ph.D. Marelli is a post-doctoral associate in Omenetto’s laboratory and a lead co-author on the paper.