The unparalleled wonder of silk for drug delivery and sensing inside the body

By Wilson da Silva

Nanodiamond embedded silk fibre membranes: healthy skin cells (green false colour growing on the membrane/scaffold). Credit: Asma Khalid

Dr Asma Khalid enjoys wearing silk dresses, and appreciates diamonds for their beauty —but she never expected both silk and diamonds to end up being the cornerstone of her work as a physicist. Yet they have, and opened up a whole new way to see deep in the body, sense infections on the skin and even deliver drugs in controlled amounts.

As a PhD student at the University of Melbourne, she had been working with nanodiamonds, particles of solid carbon arranged in a crystal structure that are less than one thousandth of a millimetre in size. Because they’re inert to biological structures and have excellent light-emitting properties, nanodiamonds are being widely explored in biology as sensitive tools for diagnostic imaging and sensing.

‘Nanodiamonds are fluorescent, they glow brightly when we excite them with a laser. However they have a rough surface and tend to clump together a lot,’ said Dr Khalid, now a Vice-Chancellor’s Postdoctoral Fellow at RMIT University and an Associate Investigator at the CNBP. ‘We wanted to improve their surface and optical properties by coating them with a material that was still biocompatible in the body. That’s when we tried silk.’

One of the advantages of silk is that it has great optical properties, such as being optically transparent. ‘Silk actually enhances the brightness of the nanodiamonds significantly. And when we implanted a silk-coated hybrid inside mice, we found the silk dissolved in the body without causing any inflammation.’

Her resulting paper in Biomedical Optics Express generated a lot of interest, leading Dr Khalid to a scholarship to visit Prof Fiorenzo Omenetto’s Silk Lab at Tufts University in Boston USA, which has pioneered the use of silk in photonics and biotechnology. There, she found the inspiration to work in the multidisciplinary field of silk optics.

‘I learned how to extract silk from cocoons and transform that liquid silk into a range of different structures, like implantable films, injectable nanoparticles, 2D and 3D printed silk, and several other structures and devices for biophotonics and biomedical applications,’ she recounted.

‘I also produced silk-coated nanodiamond spheres, which worked really well as super-bright cell imaging tools, and drug-loaded nanodiamond silk spheres that could be used as vehicles for controlled release of drugs in anticancer treatment,’ she said. ‘The hybrid spheres can release small amounts of drug over the period of weeks as the silk dissolves, and because the nanodiamonds fluoresce, we can track the release of the drug.’

The work led to two new scientific papers, and also piqued the interest of Tufts University in the optical properties of nanodiamonds as imaging and biosensing tools. When Dr Khalid returned to Australia and joined RMIT, she brought the Tufts and University of Melbourne collaboration with her. ‘So, we combined our work and got very interesting applications in imaging, sensing, drug release cell growth and tissue regenerating.’