The twist here is that by radially cladding an elastic core with thin alternating layers of two transparent elastomers of sufficiently different refractive indexes, the researchers were able to not only reproduce a Bragg reflector that strongly reflects visible light in a narrow spectral range, they figured out that by stretching the fibre, the natural mechanical deformation would make all the layers thinner, changing the Bragg reflector’s characteristics, hence the observable colour.
This is how by wrapping a transparent bilayer film of polydimethylsiloxane (PDMS, refractive index n ≈ 1.41) and polystyrene-polyisoprene triblock copolymer (PSPI, n ≈ 1.55) around a carbon-black coloured fibre core, they obtained elastic photonic fibres providing immediate and reversible visual feedback upon strain.
In a paper titled “Stretchable Optomechanical Fiber Sensors for Pressure Determination in Compressive Medical Textiles” published in the Advanced Healthcare Materials journal, the researchers expose the different construction approaches they envisaged and how such a strain sensor could be used in elastic bandages for compression therapy.
In their experiments, the bilayer film thickness comprised 100nm of PDMS and 300nm of PSPI, wrapped between 30 and 60 times around the carbon-black PDMS core. Because it is the nanoscale periodicity of the alternating thin transparent films that determines the peak reflection band in the visible light spectrum, the researchers were able to engineer fibres so as to adjust the zero strain colour. This was done by controlling the periodicity of the multilayer cladding during the fibre’s manufacture.
The fibres were tested with repetitive strains of over 100%, and responded to deformation with a predictable and reversible colour variation. As a proof-of-concept, the fibres were stitched onto single-component elastic compression bandages and simple trials have shown that they could visually help obtain prescribed levels of pressure for compression therapy.
Although the colour shift naturally occurs from red to blue as the fibres are stretched (with a diminishing Bragg period towards lower wavelengths), the authors note that such fibres could be engineered to have multiple carefully positioned reflection peaks to create an apparent shift to red hues. The idea here would be to get a visual transition from a blue or green coloration to a red or orange hue at some prescribed strain, in line with the human perception of signal colours, red hues indicating dangerous levels of strain.
The researchers envisage such strain sensors to find their way into compression medical bandage but also athletic apparel and other smart wearables. They also hint that because the transparent materials used to create the photonic fibres could be chosen to respond to different physical or chemical stimuli (the Bragg reflector expanding or contracting in response to thermal gradients or to the presence of solvent vapours for example), such fibres could be designed for many different sensing scenarios, always providing a clear colour-coded response.
Massachusetts Institute of Technology – www.mit.edu