'Micro-swimmers' could deliver drugs

WHEN you're just a few microns long, swimming can be difficult. At that size scale, the viscosity of water is more like that of honey, and momentum can't be relied upon to maintain forward motion. Now a team of researchers at the Georgia Institute of Technology has used complex computational models to design swimming micro-robots that could overcome these challenges to carry cargo and navigate in response to stimuli such as light. 

When they're actually built some day, these simple micro-swimmers could rely on volume changes in unique materials known as hydrogels to move tiny flaps that will propel the robots. The micro-devices could be used in drug delivery, lab-on-a-chip microfluidic systems – and even as micro-construction robots working in swarms. The simple micro-swimmers were described July 23 in the online advance edition of the journal Soft Matter, published by the Royal Society of Chemistry in the United Kingdom.

The simple swimmer designed by Alexander Alexeev and collaborators Hassan Masoud and Benjamin Bingham consists of a responsive gel body about ten microns long with two propulsive flaps attached to opposite sides. A steering flap sensitive to specific stimuli would be located at the front of the swimmer. The responsive gel body would undergo periodic expansions and contractions triggered by oscillatory chemical reactions, oscillating magnetic or electric fields, or by cycles of temperature change. These expansions and contractions – the chemical swelling and de-swelling of the material – would create a beating motion in the rigid propulsive flaps attached to each side of the micro-swimmer. Combined with the movement of the gel body, the beating motion would move the microswimmer forward.

A flexible steering flap on the front micro-swimmer's front would control its trajectory. The flap would be made of a material that deforms based on changes in light intensity, temperature or magnetic field. Key to the operation of the micro-swimmer would be the latest generation of hydrogels, materials whose volume changes in a cyclical way. The hydrogels would serve as "chemical engines" to provide the motion needed to move the device's propulsive flaps.