New Zealand scientists say Auckland-based research into artificial muscles may soon lead to "'soft generators" that give clothing the ability to power a phone.

Auckland Bioengineering Institute's research, published today in the journal Applied Physics Letters, details the early stages of development of a new class of variable capacitor generators known as dielectric elastomer generators.

Dielectric elastomers, often called "artificial muscles", are made of a flexible material that produces energy when deformed - in this case a rubber membrane coated with carbon grease.

The lead author on the paper, PhD student Thomas McKay, said the work eliminated the need for external circuitry by integrating flexible electronics directly onto the artificial muscles so that "wearable energy harvesters" could convert movement into battery power.

"Basically you have a stretchy bit of rubber and you supply some mechanical energy to it by stretching it," said Mr McKay.

"We've developed a low-cost power generator with an unprecedented combination of softness, flexibility, and low mass. These characteristics provide an opportunity to harvest energy from environmental sources with much greater simplicity than previously possible."

The Auckland researchers described building a 11cm-wide, plunger-shaped generator capable of producing 10 milliwatts of power, using new, flexible "dielectric elastomer switches", developed by mechatronics engineer Ben O'Brien.They estimated the hand-pumped generator cost $5 to build, and an additional $20 for its perspex case.

Mr McKay's academic supervisor, Iain Anderson, told Australia's ABC that it worked as a reverse charge pump, or stretchable capacitor.

The membrane did not conduct electricity, and the carbon became a stretchable electrode on the top and bottom surfaces, which could be used to convert mechanical energy into electrical energy.

Dr Anderson said by removing all the hard electrical components the team had gone a step further than other groups in the "soft generator" field.

Mr O'Brien has also invented a technique for producing simple dielectric elastomer switches.

A circuit typically contains capacitors and diodes, but previous research had shown how to get rid of the hard capacitors.

Mr O'Brien's switch worked by stretching the rubber until the carbon grease no longer conducted electricity.

"You take this carbon grease and you paint it on one of the surfaces. You stretch it and stretch it and stretch it and the grease will get thinner and thinner and thinner," said Dr Anderson. "The little particles of conductive carbon in there will get further and further apart.

"If you stretch it just a little bit more, the resistance will rocket's a stretch-dependent switch."

"You could create a shoe with a generator in it that doesn't sacrifice comfort," said Dr Anderson.

However, the technique was still a long way from being commercially viable for manufacturing.