Stretchable circuits for artificial skin

Materials World magazine
2 Apr 2014

Artificial skin and wearable sensors could benefit from the development of a stretchable optical circuit. A team of Belgian researchers has made a rubber-based optical waveguide – a transparent channel in which light can be launched, guided and detected from end to end.

According to lead researcher Jeroen Missinne, of the University of Ghent, other researchers have made waveguides from similar rubber materials, but these have not been used to exploit the material’s stretchy nature. The reason for this, he says, is that waveguides are useless if light cannot be guided all the way through them. The team uses polydimethylsiloxane (PDMS) to marry stretchiness with useful optical properties. Missinne explains, ‘PDMS can be stretched by up to 100% and has similar mechanical properties to human skin. It is, therefore, one of the materials of choice for artificial skin and stretchable electronics research.

To make the sensors, the team first creates the waveguides, before adding the light sources and detectors. Missinne says, ‘The waveguides are fabricated by casting a liquid PDMS against a master mould, which has ridges with the required dimensions of the waveguides. For the next step, a uniform, flat layer of the same type of PDMS is attached to the patterned side to obtain closed channels. These channels are then filled with another type of PDMS that forms the core of the waveguide.’

For the core, the team uses a type of PDMS with a higher refractive index to ensure light is trapped in the channel and transmits along the length of the material. The light sources and detectors are then integrated into flexible, 40µm-thick polymer foils to make the transition from stretchable waveguide to flexible foil more mechanically reliable.

After that, bare die chips are added to the polymer foil. Metallic pads are also appended so the foil can be driven by voltage. Finally, the flexible foils are attached to the stretchable waveguides.

According to Missine, the device can be stretched 80,000 times at 10% elongation without showing signs of degradation, and the optical loss is low enough for applications that require interconnections over a few tens of centimetres.

There are, however, modifications to be made. Missine notes, ‘We want to improve the mechanical stability of the input and output regions of the waveguides, where the light sources and detectors are coupled to the waveguide.’

The researchers are looking to conduct more reliability tests at higher elongation, and are hoping to make waveguides that are only a few micrometres in diameter.