Jun 17 2009
EU-funded researchers in France and the US studying stickers peeling from windows have found a new way to control the fabrication of stretchable electronics. The results, published in the Proceedings of the National Academy of Sciences, have implications for the development of electronic devices embedded into materials such as surgical gloves, flexible displays and electronic paper.
The results are an outcome of the Mechplant ('The role of mechanical instabilities in leaf development') project, which was funded with EUR 1.3 million under the 'New and emerging science and technologies' Theme of the Sixth Framework Programme (FP6). Mechplant partners studied shape formation during leaf development in order to better understand the interplay between a plant's genetic properties and mechanical instabilities.
In the current study, the team analysed the way that stickers attached to glass wrinkle, blister and delaminate. For example, one often sees air bubbles form between a sticker and a car window; this is usually caused by expansion of the sticker, usually caused by heat. Alternatively, compression of the surface causes the sticker, or film, to bend until it pops away from the surface, forming 'blisters'.
'It's something that's around you all the time, but if you look at it a different way you can see something new,' said co-investigator Pedro Reis of Massachusetts Institute of Technology in the US.
According to the authors, considerable efforts have gone into understanding how buckling, wrinkling and delamination of thin coatings can be avoided. Recent studies, however, have exploited these properties under compression in order to develop flexible electronic devices. 'A major technological challenge limiting the development of such devices is the requirement that the substrate be able to flex without stretching and damaging the wires that make up the circuit,' they explain.
'One way to overcome this challenge is to use a polymer substrate that is first stretched and then coated with wires according to the required pattern,' the study reads. When the strain on the substrate is released, the relatively stiff wires buckle out, leading to 'wrinkling instability' and subsequently to the formation of delamination blisters. The blisters can make it easier for the substrate to flex because the wires can bend, rather than stretch.
The researchers stretched and compressed surfaces with thin films attached to them, and measured the dimensions of resulting blisters. Specifically, they stuck a thin polypropylene film to a soft substrate, which was either thin (stretched prior to adhesion) or deep (unstretched). The thin and deep substrates were then compressed. At first, a single blister would appear; under further compression a series of almost identical blisters popped up. The dimensions of the first blister and the subsequent evolution of multiple blisters were then characterised.
The results enabled the scientists to develop a model of the formation, size and evolution of delamination blisters. Blister size, they found, depends on the elasticity of the film, the substrate and the strength of adhesion between them. The new model effectively predicts the size of the blisters that will form under specific conditions.
'Delamination blisters have a characteristic size that they try to choose for themselves,' said Dominic Vella of France's Centre National de la Recherche Scientifique. 'We've characterised this size so that in principle it can be determined just from the parameters of a given system.'
Intentionally creating delaminated surfaces could make the design of flexible devices much simpler, the team found, because the wires attached to a surface can move with the material without breaking. If the wires are already separated from the surface, the team reasoned, twisting and turning of the material will not cause them to break.
The researchers also suggest that ultra-thin and high-strength films such as graphene sheets 'may be the ideal material for applications of bendable circuitry at small scales using delamination structures'.
Previous work has relied on complex microfabrication techniques to force delamination blisters to appear. The new model offers engineers the opportunity to more precisely control delamination, and could greatly facilitate the development of stretchable electronics via blistering of the material.
Source : Cordis