a, Light-emitting fibres with low device density. (i) Photograph of light-emitting fibres containing blue-colour LEDs. The devices appear with a periodicity of 2,000 ± 110 mm. Scale bar, 50 cm. (ii) Illustration of the device density in the polymeric layer, which is placed in the middle of the preform, with a device separation of 1.25 mm. b, Higher density of devices in the fibre. (i) Photograph of light-emitting fibres containing blue-colour LEDs, where the devices appear every 370 ± 110 mm. This is the maximum linear density available with the given draw-down ratio (40) and device size for the single-layer (plane) architecture. Scale bar, 50 cm. (ii) Illustration of the maximal linear density of devices in the preform. The devices are placed side by side in a single plane. c, An alternative approach to increasing device density in fibres. (i) Illustration of the structure of the fibre cross-section, where light-emitting devices (blue shapes) are placed in two layers on top of each other, connected to metallic electrodes (red circles) for current delivery. The + and − signs represent the polarity of the wires when connected to the power supply. (ii) Photograph of the resulting fibre, in which the devices appear every 173 ± 92 mm. Scale bar, 20 cm. (iii) Side view of the light-emitting fibre, showing the presence of three electrode wires. Scale bar, 600 μm. d, Distance between devices in the fibre as a function of the distance between devices in the preform for a draw-down ratio of 40 and using LEDs. Solid lines show calculation results; black curve, single device layer in the preform; blue curve, two device layers in the preform; green curve, three device layers in the preform. Red circles represent measurements of inter-device spacing in the fibres. The dashed red line corresponds to the minimal distance between devices in the preform, which is equal to the size of the devices (170 μm). e, Distance between devices in the fibre as a function of the draw-down ratio. Solid curves show calculation results: black curve, single device layer with a spacing of 230 μm between devices in the preform; green curve, two device layers with a spacing of 230 μm; blue curve, single device layer with a spacing of 1.25 mm. Red circles, measurements of inter-device spacing in the fibres. Error bars represent one standard deviation.