A product that could monitor stresses on aircraft or a patient's vital signs has been developed by Belgian researchers by embedding optical links in a thin foil-like flexible substrate, creating pressure-sensitive films.

Dr Geert van Steenberge, a post-doctoral researcher at the Ghent University laboratory of the IMEC research centre, is leading European- funded research investigating the integration of active components into these ultra-thin foils.

'We believe that by integrating waveguides and active components — light sources and detecting units — we could pave the way towards low-cost optical sensing foils,' said van Steenberge. 'Using these sensing foils it would be possible to measure changes in pressure and deformation when attached to very irregular surfaces for a wide range of applications.'

IMEC is targeting two versions of the technology. The first, dubbed array waveguide sensors, features high densities of sensors on a smaller area. This is based on a so-called crossing waveguide approach. In the second method, which uses more established technology, less densely distributed optical fibre sensors provide the sensing foil functionality.

One of the aims of the project is to integrate the two technologies into one flexible sheet, which is stretchable, foldable and sensitive to touch, pressure, or deformation. 'Alternative flexible electrical pressure sensor solutions exist, mainly based on resistors and capacitors, but they could be made much cheaper using our technology,' said van Steenberge. 'The other reason optical sensors are better is that they are insensitive to electromagnetic interference, and can be made with a very high density.'

To create such thin foils, the researchers take commercially-available polymer waveguide optoelectronic components and use a mechanical thinning and polishing process to thin them down to 30 microns, allowing them to be embedded in the substrate.

The foil itself is made of optically transparent polymers, which are dedicated for optical instrument applications. 'They are optimised towards low loss, and are tuned to the relevant wavelength which, depending on the application, is near-infrared or infrared,' said van Steenberge.

The foils are fabricated using low-cost processes, such as standard lithography and stamping. They also need to be highly flexible, which is achieved by applying special processes to standard polymers.

The different processes create sensors for different purposes. The array waveguide sensors are used for small areas that need a high degree of sensitivity, such as the fingertip of a robot. This could be used to give feedback in applications such as minimally invasive surgery where robots need detailed feedback on the amount of pressure being applied. The sensors could also be attached to the footpad of robots that need to move around an unstructured environment to help them remain stable on unsteady surfaces.

The other approach suits larger areas, and combining the two technologies could lead to sophisticated optical sensor foils that could be used to create a fine artificial skin for industrial and medical applications. These foils could be used to continuously monitor the integrity of buildings and civil structures, examine stresses on aircraft wings and helicopter rotor blades, or to monitor respiration or heart rate in patients.

Working towards this aim, IMEC is collaborating on European 7th Framework Programme project PHOSFOS, co-ordinated by the Free University of Brussels. Van Steenberge's team is working with researchers from Poland on specialty microstructure fibres for whom they developed the technology to integrate them into the flexible foils. They are also collaborating with Aston University to target polymer fibre sensors that could have some advantages over silicon fibre sensors. A group from the University of Ghent is developing specialty polymers that could be more flexible and stretchable than commercially available polymers. There are also industrial partners from Belgium and the UK.

'Our latest breakthrough is the realisation of the fully-flexible optical link — the flexible waveguide integrated to the light sources and detectors,' said van Steenberge. 'Using this breakthrough it's become possible to realise a functional sensing force for the two approaches.'

The next steps for van Steenberge's team concern the crossing waveguide sensors. 'We have shown the proof of principle concept, now we need to integrate both light sources and detectors together with our sensor arrays. For the optical fibre sensor approach, the main goal there is to again connect the integrated flexible sources with the fibre sensors themselves.'

The project, which started in April 2008, runs for three years, when the team intends to have a fully-working proof of principle demonstrated.

Berenice Baker