Stéphanie P. Lacour

Stretchable gold conductors on elastomeric substrates

Stripes of thin gold films are made on an elastomeric substrate with built-in compressive stress to form surface waves. Because these waves can be stretched flat they function as elastic electrical conductors. Surprisingly, we observe electrical continuity not only up to an external strain of ∼2% reached by stretching the films first flat (∼0.4%) and then to the fracture strain of free-standing gold films (∼1%), but up to ∼22%. Such large strains will permit making stretchable electric conductors that will be essential to three-dimensional electronic circuits.

Stretchable Interconnects for Elastic Electronic Surfaces

Elastic electronic surfaces will integrate stiff thin film devices onto compliant polymer substrates. These surfaces may be stretched once or many times, by up to tens of percent strain. One way to make such an elastic electronic surface is to distribute rigid subcircuit islands over the polymer surface, and then fabricate active devices on the islands. These islands need to be interconnected with stretchable metallization. We describe stretchable interconnects made of stripes of thin gold film patterned on elastomeric membranes. These membranes can be stretched by up to twice their initial length and maintain electrical conduction. We review the fabrication of these conductors, present their electrical and mechanical properties, and summarize our model for their extreme stretchability. Using such stretchable interconnects, we made the first elastic circuit, an inverter of thin film transistors. The circuit remains functional when stretched and relaxed by 12% strain.

Mechanisms of reversible stretchability of thin metal films on elastomeric substrates

Gold films on an elastomeric substrate can be stretched and relaxed reversibly by tens of percent. The films initially form in two different structures, one continuous and the other containing tribranched microcracks. We have identified the mechanism of elastic stretchability in the films with microcracks. The metal, which is much stiffer than the elastomer, forms a percolating network. To accommodate the large elongation of the elastomeric substrate, the metal network twists and deflects out of the plane but remains bonded to the soft substrate. Consequently, the metal film experiences only small strains and deforms elastically without suffering fatigue.

Design and performance of thin metal film interconnects for skin-like electronic circuits

We prepare stretchable electrical conductors of 25-nm-thick gold films on elastomeric substrates prestretched by 15%. When the substrates relax from the prestretch, the gold stripes form surface waves with /spl sim/8.4-/spl mu/m wavelength and /spl sim/1.2-/spl mu/m amplitude. When the strain is cycled between 0 and 15%, both the wave pattern and the electrical resistance of the gold stripes change in reproducible cycles. Such repeatedly stretchable metallization can serve as interconnects for skin-like, conformal, and electroactive polymer circuits.

Stretchability of thin metal films on elastomer substrates

Many flexible electronic surfaces comprise inorganic films on organic substrates. Mechanical failure of such integrated structures of stiff and compliant materials poses a significant challenge. This letter studies the stretchability of metal films on elastomer substrates. Our experiment shows that, when stretched, elastomer-supported metal films rupture at strains larger than those reported for freestanding films. We use a finite element code to simulate the rupture process of metal films. A freestanding metal film ruptures by forming a single neck. By contrast, a metal film on an elastomer substrate may develop an array of necks before rupture. While the pre-rupture necks do not change the electrical conductance appreciably, they elongate the metal film, leading to a large overall rupture strain.

Extended cyclic uniaxial loading of stretchable gold thin-films on elastomeric substrates

Gold thin-films (50 nm thick) on silicone membranes are reversibly stretchable. They exhibit continuous electrical conduction when pulled and relaxed by tens of percent. Here, we show that gold thin-film conductors on elastomeric substrates can withstand extensive uniaxial stretch cycling without electrical failure. The gold film develops into an interconnected network of islands, which reversibly move on the surface of the elastomer with applied strain. The resulting electrical resistance of the conductor remains finite and reproducible over 250 000 cycles to 20% applied strain.

A Multifunctional Capacitive Sensor for Stretchable Electronic Skins

We present a stretchable and multifunctional capacitive sensor made of gold thin films embedded in silicone rubber. The mechanical compliance of the gold films and silicone membranes allow the device to be bent, folded, or stretched without damage, making it a suitable candidate for electronic skin applications. The device can detect strains up to 20%, human touch, and pressure up to 160 kPa, and reliably function when it is held stretched or relaxed.

Stretchable wavy metal interconnects

Buckled, wavy metal stripes are promising candidates for interconnects in flexible and stretchable electronics. To obtain wavy metal films, 5 nm of Cr (for adhesion) and 20 nm of Au were evaporated on polydimethyl siloxane (PDMS) prestretched by 25%. The metals buckle to a wave upon release of the PDMS from the prestretched position. The electrical resistance of the Au was measured as a function of applied tensile strain. Results show the metal remains electrically conductive up to 100% strain and maintains electrical continuity under repeated mechanical deformation. Presented are the sample fabrication, surface topography, and results of experiments conducted on these stretchable wavy metals.

Flexible and stretchable micro-electrodes for in vitro and in vivo neural interfaces

Microelectrode arrays (MEAs) are designed to monitor and/or stimulate extracellularly neuronal activity. However, the biomechanical and structural mismatch between current MEAs and neural tissues remains a challenge for neural interfaces. This article describes a material strategy to prepare neural electrodes with improved mechanical compliance that relies on thin metal film electrodes embedded in polymeric substrates. The electrode impedance of micro-electrodes on polymer is comparable to that of MEA on glass substrates. Furthermore, MEAs on plastic can be flexed and rolled offering improved structural interface with brain and nerves in vivo. MEAs on elastomer can be stretched reversibly and provide in vitro unique platforms to simultaneously investigate the electrophysiological of neural cells and tissues to mechanical stimulation. Adding mechanical compliance to MEAs is a promising vehicle for robust and reliable neural interfaces.

Flexible ferroelectret field-effect transistor for large-area sensor skins and microphones

Ferroelectrets generate an electric field large enough to modulate the conductance of the source-drain channel of a thin-film field-effect transistor. Integrating a ferroelectret with a thin-film transistor produces a ferroelectret field-effect transistor. The authors made such transistors by laminating cellular polypropylene films and amorphous silicon thin-film transistors on polyimide substrates. They show that these ferrroelectret field-effect transistors respond in a static capacitive or dynamic piezoelectric mode. A touch sensor, a pressure-activated switch, and a microphone are demonstrated. The structure can be scaled up to large-area flexible transducer arrays, such as roll-up steerable compliant sensor skin.