Thin metal films coated on soft elastomeric foam substrates such as polyurethane have enhanced electromechanical performance, according to a paper published in Advanced Materials, a peer review journal.
Stéphanie Lacour, a professor at the Ecole Polytechnique Federale de Lausanne leads the team of researchers carrying out the work. They have found that when open-cell foams are stretched, the thin layer of capping elastomer which leads to cracks and folds localised above foam cells. The surrounding cell ligaments remain almost strain free allowing a stable electrical conductor to be embedded in the cell structure.
Her team showed that a metallic film placed on a polyurethane foam substrate can be stretched in a reversible manner, without disrupting its electrical conductivity. So it could be possible to produce flexible electronic circuits.
"Up to now, to find an appropriate substrate, we mainly worked on full and uniform elastomers," she explained. "Now, we're exploring new kinds of flexible, but heterogeneous, substrates in the form of foams. A more or less dense network of air bubbles inserted in an elastic matrix lets us modulate the elasticity of the substrate."
On a uniform elastomeric substrate, tension tests revealed the creation of micro-fissures in the metallic layer, which would eventually result in the rupture of the conducting network. But with foam substrates, these cracks only occurred above the air bubbles.
"Between the bubbles, the metal remained intact. The conducting network is thus maintained and can function," she explained. "Our measurements showed that we could achieve a level of elasticity over 100 percent without disrupting the network. These metallic pathways built upon foam could thus be used as electrodes, sensors or interconnections for the electronic skin that we're developing."