“Swallow the surgeon,” Richard Feynman had declared, all the way back in 1959.
He had not, of course, been encouraging people to eat their doctors. The “surgeon” in question was in fact a tiny, hypothetical, machine, one capable of entering the human body and fixing what large, clumsy hands could not.
As a concept, it was bizarre – but also compelling in the extreme.
The idea has since resonated through the decades – today, nanotechnology remains a cutting-edge field with diverse research prospects and wide-ranging applications.
More recently, in a paper published earlier this year, Associate Professor Richard D. Webster of the Division of Chemistry and Biological Chemistry at SPMS has been working with recently graduated PhD student Tijana Maric, and the Director of the Center for Advanced Functional Nanorobots, Professor Martin Pumera, to create microrobots from pollen grains.
Professor Martin Pumera, formerly of NTU, now Director of the Center for Advanced Functional Nanorobots in the Czech Republic.
Microrobots, as the name suggests, are miniature self-propelling (through chemical reactions) devices. In the course of the project, the researchers worked to determine the potential environmental and biomedical applications of such pollen-based microrobots.
At first glance, it seems inconceivable that pollen, the powdery, seemingly ephemeral substance, could ever be considered a sturdy material, much less used to build microrobots.
Previously, “micro- and nanosized particles [have been] based on artificial metallic structures,” Assoc Prof Webster says. “But these are difficult to synthesize and do not survive long in the environment.”
In contrast, pollen grains have an outer layer made of sporopollenin, which is widely considered to be the “diamond” of biopolymers.
Accordingly, this makes pollen grains some of the most stable structures in the world – the “diamond” exterior ensures their survival in harsh environments, which has allowed them to be preserved in the fossil record, and of course, used as the building blocks for microrobot fabrication.
Dr Tijana Maric, former PhD student at NTU, one of the main researchers behind the project.
To begin with, the researchers first used a scanning electron microscope to characterise the morphology of the nine chosen pollen types, along with their corresponding micromotors counterparts, creating blueprints of sorts.
The microrobots were then fabricated via “sputtering”, a process where pollen grains are asymmetrically coated in thin films of platinum (Pt).
The resulting platinum-pollen hybrid, Pt-pollen, capitalizes on platinum’s characteristic as a catalyst for hydrogen peroxide, the medium used by the microrobots as fuel. As that fuel is broken down, the platinum on one side of the pollen facilitates the production of bubbles, allowing the microrobots to achieve self-propulsion.
But for all the nifty technology that has gone into the creation of the Pt-pollen microrobots, the question remains: what are they good for?
“We want to target representative environmental and health-based problems that are known to exist,” Assoc Prof Webster says.
Oceanic pollution is, of course, one such important health concern. Some of the more serious global hazards are water-soluble mercury ions (Hg2+), which frequently seep into the oceans, only to be ingested by seafood, transforming normally harmless fish and shellfish into harbingers of mercury poisoning.
A series of stunning up-close images of the nine different types of pollen grains produced by the scanning electron microscope used by the researchers.
Where microrobots come in is with their high capacity for adsorption, which causes the mercury ions to adhere to the Pt-pollen surface – with their high mobility in fluids, the microrobots can then remove the mercury. In testing, the researchers discovered that, across the board, the microrobots showed great adsorption towards the mercury ions in aqueous solutions.
In particular, the Pt-Lyc microrobots, which were fabricated with lycopodium pollen, achieved a roughly 90% removal efficiency, the highest amongst the nine different pollen types.
With other microrobots, this time fabricated from lotus pollen (Pt-Lot), the researchers also investigated their applicability in the targeted delivery of drugs to specific locations in the human body.
To do this, the anti-cancer drug Doxorubicin (DOX) was first loaded onto the Pt-Lot microrobots. Their large surface area, in comparison to the other pollen types, made it easier to bind DOX to the microrobot surface.
Associate Professor Webster working with samples in the lab.
In subsequent testing, the researchers found that the successful delivery of DOX via Pt-Lot to cancer cells caused the viability of said cancer cells to drop to almost 41%; incidentally, free-moving DOX (used as a positive control) had caused cancer cell viability to drop to 23%.
While such clear indicators of success might paint the Pt-pollen microrobots as promising weapons of cancer eradication, society-wide adoption of nanotechnology still remains far off.
“Despite the huge amount of research on newly synthesised metallic nanomaterials, there exist very few, if any, real-world applications,” Assoc Prof Webster explains. “Part of the reason for this is the difficulty in synthesising large quantities of the materials and keeping them in a dispersed state.”
But as the success of experimenting with pollen has shown, there now exist more pathways towards the future mass production, and incorporation, of microrobots into daily life. It helps, too, that pollen grains are already naturally abundant, relatively uniform in shape and size, and also environmentally friendly and non-toxic.
“Studies such as ours – that use naturally occurring micro- and nanosized particles – increase the likelihood of more widespread adoption for industrial scale applications,” Assoc Prof Webster concludes.