Stretchable optical circuit enables wearable body sensor networks

Technology News | February 19, 2014

By eeNews Europe

Optoelectronics

The interconnections, made of a transparent poly-dimethylsiloxane (PDMS) material, are capable of guiding light along their path even when stretched up to 30% and when bent around an object the diameter of a human finger.

By integrating the stretchy interconnections into a circuit – with a light source on one end and a detector on the other – the researchers have created a miniature stretchable, bendable ‘link’ that could be incorporated into optical communications systems.

“To our knowledge, this is indeed the first truly bendable, stretchable optical link with these miniature dimensions,” said lead author Jeroen Missinne of Ghent University and imec, a micro- and nano-electronics research center, in Belgium.

Previously, researchers had created optical interconnections – also called lightguides or waveguides – from various rubbery materials. But until now, the researchers say, no one had discovered a way to enable these materials to carry light while stretched. Past efforts also included embedding waveguides made of semi-rigid glass fibers into a stretchable substance. In the new method, the stretchable substance itself is the waveguide.

The new connector consists of two materials, both made of PDMS: a transparent core through which the light travels, surrounded by another transparent layer of PDMS with a lower refractive index, a characteristic of the material that describes how light moves through it. The configuration traps light in the guide’s core, causing it to propagate along its length.

The Belgian team have tested how far they could bend and stretch their new optical connector before too much light escaped.

“We were surprised that stretching had so little influence on the waveguides and also that their mechanical performance was so good,” Missinne said. The guide’s reliability was also ‘remarkable’,” explained Missinne. The researchers did not see a degradation in the material even after mechanically stretching it to a 10 percent elongation 80,000 times.

"waveguides are useless if you cannot launch light into them and collect light on the other end. If you want to obtain a truly stretchable optical link, the light sources and detectors need to be integrated together with the stretchable waveguide,” said Missinne. In this case, a VCSEL (vertical-cavity surface-emitting laser), commonly used for fiber optic communications, served as the light source, and a photodiode was the detector. The configuration allowed the team to create the first truly stretchable optical interconnector.

Future uses for the new optical link might include building networks of wearable body sensors, moving machine parts such as robotic limbs, and deformable consumer electronics. Meanwhile, the team plans to make their waveguide smaller, down from 50 micrometers to just a few micrometers in diameter, which will also require a redesign of the parts of the waveguide where light enters and exits.

The researcch work was performed at the Centre for Microsystems Technology (CMST), a laboratory associated with imec and Ghent University.