Thin films can buckle round curved substrates to form gears for micromachines, say US researchers.

Many different shapes of gear are possible with self-assembly

Xi Chen and colleagues from Columbia University, New York, deposited a stiff thin polyvinyl chloride film on the surface of a polyurethane cylinder. Upon dehydration the polyurethane contracted more than the film, causing the film to buckle and form structures like the teeth on a gear.

Mismatched deformation is the key to their method, explains Chen. 'Cooling or dehydration is just an approach to introduce the mismatch,' he says. 'Heating the system also works, as long as the film expands more than the substrate.'

The researchers showed they could predict the number and depth of the teeth. 'The stiffer the film, the harder to buckle and the fewer teeth you get,' states Chen. He claims the method can be easily adapted to make microgears for flexible electronics and biological microelectromechanical systems.

Sami Yunus, who studies the self-organisation of thin films at the Catholic University of Louvain, Belgium, expresses interest in the method. 'It's a new way of thinking about how we use materials to build gears,' he comments. 'Using a natural phenomenon such as self assembly is very powerful because it avoids the complications of lithography and clean rooms [used in current methods].'

Until now, making microgears has required expensive etching and micromachining. Chen's method only requires a change in temperature - no external guidance is required. Chen demonstrated that he could make a variety of gears - helical and bevel, for example - by varying the shape of the polyurethane substrate or the type of film.

The team have made gears with diameters of 6 millimetres but they are keen to go smaller and make true microgears that could be used in biomedical engineering or aerospace. 'Future experiments will be extended to micrometre or sub-micrometre scales,' says Chen.

Christina Hodkinson

Enjoy this story? Spread the word using the 'tools' menu on the left or add a comment to the Chemistry World blog.