Beating nanorods mimic lung defence

Hair-like polymer nanorods which wiggle and bend under magnetic fields are helping researchers understand how the lung defends itself against invaders. The rods could also provide a new way of mixing fluids in lab-on-a-chip devices, the researchers say.

The lung protects itself against foreign bodies with a filter of mucus, which is moved around the body by cilia: tiny appendages that hang off cell surfaces. Cilia beat together in a coordinated whiplash motion to propel mucus along; but scientists don’t understand how that mechanism works, or how the body maintains its mucus flow, said Richard Superfine, from the Virtual Lung Project at the University of North Carolina, US.

Biologists and computer simulators have some ideas, so Superfine used controllable mechanical models to test their assumptions. His team poured a liquid polymer, polydimethylsiloxane, into a mould and added iron nanoparticles before etching away the template to leave flexible magnetic nanorods the size of biological cilia - about 10 micrometres long and 200nm across. Moving magnetic fields below the rods caused them to bend and beat in a variety of predictable patterns.

Although the nanorods may not coordinate exactly as cilia do, Superfine has used them to help validate computer simulations, and even to propel mucus along. The tests provide useful studies of viscoelastic flow, said Superfine, but he has also incorporated the nanorods in microfluidic chips, to help mix and move liquids. If optical elements were poured into the mould, Superfine suggested, the rods might be moved around by magnets to control light reflection from a surface, or to act as dynamic diffraction gratings.

Superfine hopes his artificial cilia will shed light on many other functions of their biological counterparts. Cilia vibrate in the ear to enhance hearing, for example. They also control the placement of the heart in the developing embryo, and act as sensors in many organs, including the kidney and the brain. Although cilia can be cultured in a laboratory dish, Superfine says his synthetic equivalents are more easily varied and controlled for physical understanding. 

Richard Van Noorden