SYDNEY: Scientists have found a way to make minute synthetic bubbles, known as microcapsules, move by mimicking the behaviour of biological cells. The technique brings us one step closer to nanomachines with applications ranging from drug delivery to fuel cells.

The team from the University of Pittsburgh in the U.S. have used computational modelling to demonstrate how the controlled motion of these microcapsules could be manipulated to do tasks at the microscopic scale.

The technique could eventually be applied to procedures such as targeted drug delivery, where the capsules would be directed though the body to a target site and then "exploded" by heat from lasers, releasing the particles of drugs within them.

Bold and exciting

"This is a very bold and exciting idea," enthused Michael Cortie, director of the Institute for Nanoscale Technology at the University of Technology Sydney (UTS), who was not involved in the research. "It goes one step further in the direction of trying to achieve a nanorobot."

To make the discovery, chemical engineer Anna Balazs, and her team used computer simulations to design a system of microscopic capsules that move by communicating in the same way that organic cells do. They report the find this week in the American Chemical Society journal ACSNano.

"Biological cells communicate through a complex chemical process where a signalling cell secretes molecules that are then detected by receptors on the target cell," says the study. "We show how one microcapsule 'signals' to another and thereby initiates the motion of both."

The team's simulation models two fluid-filled polymer capsules resting on a surface within a viscous fluid. One of these, the "signalling" capsule, contains nanoparticles – tiny particles less than 100 nanometres across (one nanometre is a billionth of a metre) - which diffuse through the porous shell and into the surrounding fluid.

These particles affect the adhesive properties of the surface, making parts of it less or more sticky. The second "target" capsule is attracted to the more sticky part of the surface, and so moves towards to it. This movement affects the fluid around the first capsule – think of the wake left by a boat moving in water – and drives it to follow.

The nanoparticles therefore act as a stimulus, causing first one, then both capsules to move. Continuous motion can be achieved by carefully altering some of the system variables, such as the size of the capsule, the density of the host fluid and the absorption properties of the surface.

Balazs describes the groundbreaking nature of their work: "It is the first study to predict how to get two inanimate microscopic objects – the microcapsules – to effectively communicate with each other and thereby carry out a concerted action – namely, motion."

Fundamental insights

The find also provides insight into the fundamental processes that control the movement of biological cells as they respond to chemicals in their environment. But the potential applications for moveable nanomachines are what the experts are excited about.

"These communicating microcapsules could be used in microfluidics, which are small-scale devices used to carry out rapid biological assays or to perform synthesis of minute quantities of chemicals," said Balazs. "It is bringing us closer to the goal of designing "smart materials" that can autonomously perform specific functions."

"The possibilities for this kind of small-scale technology are very optimistic," added Michael Cortie. "If everyone systematically works on the problems faced [in nanotechnology research], we will have breakthroughs that could make a tangible difference to human health and energy problems [facing civilisation.]"

"This is a really cool piece of work," agreed Mike Ford, an associate professor of nanotechnology also at UTS in Sydney. "The work is more at the level of basic science, however one could imagine a raft of applications that might spring out of this work later down the track, such as controlling the motion of micro-objects in liquids and enforcing collective motion."