The idea of clothing that can communicate by light may sound like a fantasy but that’s exactly what scientists from the US and Switzerland have now achieved. Writing in Nature, Michael Rein and co-workers report how they have successfully found a way to integrate discrete semiconductor p–n-diode-based light-emitting diodes (LEDs) and photodetectors and their metal wire connections into optical fibres that can then be woven into a separate textile fabric (Nature 560, 214–218; 2018). In-fibre arrays of InGaN and AlGaAsP LEDs emitting blue, green and red light and GaAs p–i–n photodetectors have all been demonstrated (pictured), opening the door to fabrics that support visible-light fibre communications and sensing.

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Springer Nature Ltd

The approach works by first making a preform of the functional fibres from polycarbonate slabs that are sandwiched together. The outer slabs feature milled grooves running along their entire length to accommodate copper or tungsten wires that interface with the devices in the fibre. In the inner slab, numerous pockets on the order of 100 μm are drilled to accommodate a linear array of semiconductor miniature elements such as p–i–n diodes. The metallic wires are fed through the grooves of the preform and fibres are then fabricated by a thermal drawing process by placing the preform in a three-zone heating furnace, where the top, middle and bottom zones are heated to 150 °C, 270 °C and 110 °C, respectively.

The cross-section of the formed fibre was roughly quadrangular in shape (350 μm × 350 μm). Electrical connection to the in-fibre devices was achieved by stripping away the polycarbonate cladding at one end of the fibre. In principle, the fabrication method enables kilometres of functional fibre to be drawn down from a single preform with more than a hundred discrete devices connected in parallel throughout the entire fibre.

Incorporating these functional fibres into fabrics brings many opportunities for applications. As an example, the US and Swiss scientists demonstrated a textile-based photoplethysmography system for cardiac pulse measurement. A green LED fibre was embedded in a cotton fabric sock adjacent to a GaAs photodetecting fibre. A pulse measurement was implemented by placing an index finger on both fibres. The change of the light intensity recorded by the photodetecting fibre due to the change in the light reflectance from the skin was clearly observed. The measured signal directly correlated with volume changes in small blood vessels, which expanded and constricted with every heartbeat. The functional fibre withstood the strains and stresses of textile manufacturing techniques, and even machine washing. “We envision that this technology will enable new technological advances in the textile and apparel domains, telecommunications, as well as in biological and medical sciences”, Rein said.