Researchers at MIT and several other institutions have developed a method for making photonic devices — similar to electronic devices but based on light rather than electricity — that can bend and stretch without damage. The devices could find uses in cables to connect computing devices, or in diagnostic and monitoring systems that could be attached to the skin or implanted in the body, flexing easily with the natural tissue.
The researchers were interested in the possibility of optical technologies that can stretch and bend, especially for applications such as skin-mounted monitoring devices that could directly sense optical signals. Such devices might, for example, simultaneously detect heart rate, blood oxygen levels, and even blood pressure. Their findings involve the use of a specialized kind of glass called chalcogenide.
Photonics devices process light beams directly, using systems of LEDs, lenses, and mirrors fabricated with the same kinds of processes used to manufacture electronic microchips. Using light beams rather than a flow of electrons can have advantages for many applications; if the original data is light-based, for example, optical processing avoids the need for a conversion process.
Most photonics devices are currently fabricated from rigid materials on rigid substrates and thus have an inherent mismatch for applications that should be soft like human skin. However, most soft materials, including most polymers, have a low refractive index, which leads to a poor ability to confine a light beam.
Instead of using this type of flexible material the team took a novel approach. They formed the stiff material — in this case a thin layer of a type of glass called chalcogenide — into a spring-like coil. Just as steel can be made to stretch and bend when formed into a spring, the architecture of this glass coil allows it to stretch and bend freely while maintaining its desirable optical properties.
The coil is as flexible as rubber, still has a high refractive index, and is very transparent. Tests have shown that such spring-like configurations, made directly on a polymer substrate, can undergo thousands of stretching cycles with no detectable degradation in their optical performance. The team produced a variety of photonic components, interconnected by the flexible, spring-like waveguides, all in an epoxy resin matrix, which was made stiffer near the optical components and more flexible around the waveguides.
Other kinds of stretchable photonics have been made by embedding nanorods of a stiffer material in a polymer base, but those require extra manufacturing steps and are not compatible with existing photonic systems.
Such flexible, stretchable photonic circuits could also be useful for applications where the devices need to conform to the uneven surfaces of some other material, such as in strain gauges. Optics technology is very sensitive to strain and could detect deformations of less than 0.01 percent.
The research is still in the early stages; the team has so far demonstrated only one device at a time. For it to be useful, all the components have to be integrated on a single device. Work is ongoing to develop the technology to that point so that it could be commercially applied, which could happen in two to three years.