Compliant thin film patterns of stiff materials as platforms for stretchable electronics
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Stéphanie P. Lacour and Sigurd Wagner Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544 (Received 16 July 2005; accepted 12 September 2005)
A thin film of a stiff material, patterned as a serpentine on a flat elastomeric substrate, can elongate substantially when the substrate is pulled. We showed that the film elongates by twisting out of plane, accommodated by the compliance of the substrate and the pattern of the film. Consequently, large elongations of the substrate induce small strains in the film, even when the width of the film is much larger than its thickness. Such a wide serpentine, or other compliant patterns of stiff materials, can serve as a platform on which electronic circuits can be fabricated. This architecture will make electronics elastically stretchable.
A new direction for integrated circuit technology is to develop deformable electronic surfaces.1–6 Of the various modes of deformation (e.g., bending, twisting, and stretching), stretching is typically the most demanding, easily inducing a large tensile strain on the surface of a substrate. While an elastomeric substrate can recover from a large strain, most electronic materials, such as metals, dielectrics, and semiconductors, fracture at small strains (less than about one percent).7–11 How to use these materials to make stretchable electronic circuits remains uncertain. We propose that electronic circuits can be fabricated on a platform of a stiff material but a compliant pattern lying on an elastomeric substrate. These circuits will function without appreciable fatigue when the substrate is pulled for many cycles. A helical spring can elongate substantially, even though the material of which it is made can sustain only a small strain. One could fabricate electronic circuits on a helical platform, but this approach would require microfabrication in three dimensions, a technology that requires substantial development itself. To be compatible with planar microfabrication technology, the platform must be planar. As an illustration of this principle, Fig. 1 shows a piece of paper cut into a serpentine and pulled at the two ends. While initially planar, the serpentine elongates by twisting out of plane so that a large elongation induces only small strains. The serpentine illustrates the principle that a film of a stiff material can be made compliant if the film is suitably patterned.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0422 3274
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J. Mater. Res., Vol. 20, No. 12, Dec 2005 Downloaded: 18 Mar 2015
It has been shown that serpentine metal interconnects on elastomeric substrates can sustain more than 200 cycles of elongation by 25%.12 An existing design for stretchable electronics is to fabricate on a polymeric substrate small functional islands of stiff materials, which are then linked by metal interconnects.4,6,13 While the metal interconnects can be made stretchable,12,14 the crossovers of the interconnects from the po
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